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Adsorption, aggregation and
phase separation in colloidal
systems
Jing Dai
KTH Royal Institute of Technology
School of Chemical Science and Engineering
Department of Chemistry
Applied Physical Chemistry
SE-100 44 Stockholm, Sweden
ii
Copyright © Jing Dai, 2017. All rights are reserved. No parts of this thesis can be
reproduced without the permission from the author.
Paper I © 2015 American Chemical Society
Paper II © 2017 American Chemical Society
TRITA CHE Report 2017:88
ISSN 1654-1081
ISNB 978-91-7729-647-8
Akademisk avhandling som med tillstånd av Kungliga Tekniska Högskolan i
Stockholm framlägges till offentlig granskning för avläggandet av teknologie
doktorsexamen fredagen den 9 feb kl 10:00 i sal F3, KTH, Lindstedtsvägen 26,
Stockholm. Avhandlingen försvaras på engelska.
Fakultetsopponent: Prof. Peter Griffiths, University of Greenwich, UK
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To my parents
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Abstract
The thesis presents work regarding amphiphilic molecules associated in aqueous
solution or at the liquid/solid interface. Two main topics are included: the temperature-
dependent behavior of micelles and the adsorption of dispersants on carbon nanotube
(CNT) surfaces. Various NMR methods were used to analyze those systems, such as
chemical shift detection, spectral intensity measurements, spin relaxation and, in
particular, self-diffusion experiments. Besides this, small angle X-ray scattering
(SAXS) was also applied for structural characterization.
A particular form of phase transition, core freezing, was detected as a function of
temperature in micelles composed by a single sort of Brij-type surfactants. In mixed
micelles, that phase transition still occurs accompanied by a reversible segregation of
different surfactants into distinct aggregates. Adding a hydrophobic solubilizate shifts
the core freezing point to a lower temperature. Upon lowering the temperature to the
core freezing point, the solubilizate is released. The temperature course of the release
curves with different initial solubilizate loadings is rationalized in terms of a
temperature-dependent loading capacity.
The behavior of amphiphilic dispersant molecules in aqueous dispersions of carbon
nanotubes (CNTs) has been investigated with a Pluronic-type block copolymer as
frequent model dispersant. Detailed dispersion curves were recorded and the
distribution of the dispersant among different available environments was analyzed.
The amount of dispersed CNT was shown to be defined by a complex interplay of
several factors during the dispersion process such as dispersant concentration,
sonication time, centrifugation and CNT loading. In the dispersion process, high
amphiphilic concentration is required because the pristine CNT surfaces made
available by sonication must be rapidly covered by dispersants to avoid their re-
attachment. In the prepared dispersions, the competitive adsorption of possible
dispersants was investigated that provided information about the relative strength of
the interaction of those with the nanotube surfaces. Anionic surfactants were found to
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have a strong tendency to replace Pluronics, which indicates a strong binding of those
surfactants.
CNTs were dispersed in an epoxy resin to prepare nanotube-polymer composites. The
molecular mobility of epoxy was investigated and the results demonstrated the
presence of loosely associated CNT aggregates within which the molecular transport
of epoxy is slow because of strong attractive intermolecular interactions between
epoxy and the CNT surface. The rheological behavior is dominated by aggregate-
aggregate jamming.
Keywords: NMR, chemical shift, spin relaxation, self-diffusion, micelle, core
freezing, segregation, solubilization, release, adsorption, binding, surfactant, carbon
nanotube, block copolymer, dispersion, competitive adsorption, nanocomposite.
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Sammanfattning
Avhandlingen innefattar studier av amfifila molekyler som associerar med varandra i
lösningar eller adsorberas vid gränsytor mellan vätska och fast material. Det finns två
huvudteman: det temperatur-beroende beteendet av miceller och adsorptionen av
dispergerande molekyler till ytorna av kolnanorör (CNT, carbon nanotubes). Olika
NMR spektroskopiska metoder har använts för att studera dessa system, bland annat
mätningar av kemiska skift och spektralintensitet, spinnrelaxation, och i synnerhet
självdiffusionsexperiment. Dessutom har small angle X-ray scattering (SAXS)
utnyttjats för strukturell karaktärisering.
En specifik form av fasövergångar, solidifiering av micellens inre, har undersökts vid
olika temperatur i miceller av enskilda Brij-typ surfaktanter. Denna fasövergång
uppkommer även i miceller som består av blandningar av sådana surfaktanter och
resulterar i reversibel segregering av olika molekyltyper i separata aggregat.
Hydrofoba solubilisater bidrar till att minska temperaturen för fasövergången. När
temperaturen minskar ner till övergångspunkten, släpps solubilisaten ut ur micellen till
vattenfasen på ett sätt som beror av micellens reducerade kapacitet att behålla dessa
molekyler.
Beteendet av amfilila dispersanter i vattenbaserade dispersioner av CNT har
undersökts med en Pluronic-typ block copolymer som den vanligaste dispersanten.
Detaljerade dispersionskurvor har uppmätts och distributionen av dispersanten mellan
olika molekylära miljöer analyserats. Mängden av dispergerade CNT beror av olika
faktorer i komplex samspel, såsom dispersantens koncentration, ultrasonikeringstiden,
centrifugeringen, och den initiala mängden av CNT. Hög amfifilkoncentration krävs
under dispersionsprocessen så att de nya interna ytor som bildas med hjälp av
ultrasonikering täcks av dispersanter och ytornas omedelbara slutning undviks.
Kompetitiv adsorption av olika dispersanter har undersökts för att ge information om
den relative bindningsstyrkan till CNT-ytor. Anjoniska surfaktanter som visat sig att
ha en hög kapacitet för att ersätta Pluronic binder starkt till CNT.
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Kolnanorör dispergerades även i epoxyresin med ändamålet att skapa polymer-nanorör
kompositer. Den uppmätta molekylära mobiliteten i epoxyn tyder på att det skapats
lösa nanorörsaggregat i dessa system. Inom dessa aggregat, starka attraktiva
intermolekylära krafter mellan epoxy och CNT leder till en långsam molekylär
transport av epoxy. De reologiska egenskaperna domineras av aggregatens ömsesidiga
obstruktion.
Nyckelord: NMR, kemisk skift, spinnrelaxation, självdiffusion, micell, solidifiering,
segregation, solubilisering, utsläpp, adsorption, bindning, surfaktant, kolnanorör,
block copolymer, dispersion, kompetitiv adsorption, nanokomposit.
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List of papers
I. Core freezing and size segregation in surfactant core-shell micelles
Beatrice Plazzotta, Jing Dai, Manja A. Behrens, Istvan Furo and Jan Skov Pedersen
J. Phys. Chem. B, 2015, 119 (33), 10798–10806
II. Release of solubilizate form micelle upon core freezing
Jing Dai, Zahra Alaei, Beatrice Plazzotta, Jan Skov Pedersen and István Furó
J. Phys. Chem. B, 2017, 121 (45), 10353–10363
III. Propofol adsorption at the air/water interface: a combined vibrational sum
frequency spectroscopy, nuclear magnetic resonance and neutron reflectometry
study
Petru Niga, Petra M. Hansson-Mille, Agne Swerin, Per M. Claesson, Joachim Schoelkopf, Patrick A.
C. Gane, Jing Dai, István Furó, Richard A. Campbell and C. Magnus Johnson
Submitted
IV. The dispersion process of carbon nanotubes sonicated in aqueous solutions of
a dispersant
Jing Dai, Ricardo M. F. Fernandes, Oren Regev, Eduardo F. Marques and István Furó
Manuscript
V. Block copolymers adsorbed on single-walled carbon nanotubes. Block
polydispersity and the modes of surface attachment
Ricardo M. F. Fernandes, Jing Dai, Oren Regev, Eduardo F. Marques and István Furó
Manuscript
VI. Assessing surfactant binding to carbon nanotubes via competitive adsorption:
binding strength and critical coverage
Ricardo M.F. Fernandes, Jing Dai, Oren Regev, Eduardo F. Marques and István Furó
Manuscript
VII. Polymer nanocomposites: insights on rheology, percolation, jamming and
molecular mobility
Roey Nadiv, Ricardo M. F. Fernandes, Guy Ochbaum, Jing Dai, Matat Buzaglo, Maxim Varenik, Ronit
Biton, Istvan Furo and Oren Regev
Submitted
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The author contribution to the appended papers
I. Initiated the project, planned and performed all the NMR measurements and data
analysis. Participated in the writing of the manuscript.
II. Planned and performed the majority of the experimental work and data analysis.
Wrote the preliminary version of the manuscript and participated in writing the final
version.
III. Performed the NMR experimental work and data analysis. Wrote the NMR related
text.
IV. Participated in the project planning. Performed the majority of the experimental
work. Contributed to the data analysis and to writing the manuscript.
V. Participated in the project planning. Performed roughly half of the experimental
work. Contributed to the data analysis and to writing the manuscript.
VI. Performed less than half of the experimental work. Contributed to the data analysis
and to writing the manuscript.
VII. Performed roughly half of the NMR-related experimental work. Contributed to
writing the manuscript.
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Table of contents
1. Introduction ........................................................................... 1
1.1. SURFACTANTS............................................................................................................. 1 1.2. AMPHIPHILIC BLOCK COPOLYMERS................................................................................ 2 1.3. MICELLES AND SOLUBILIZATION IN MICELLES ................................................................. 3 1.4. CARBON NANOTUBES AND CARBON NANOTUBE DISPERSIONS ......................................... 7
2. Experimental ....................................................................... 13
2.1. SAMPLE PREPARATION ...............................................................................................13 2.1.1. Micelles with and without solubilizates ...................................................................... 13
2.1.2. Propofol with different concentrations in D2O ............................................................ 15
2.1.3. CNT dispersions ........................................................................................................... 15
2.1.4. CNT-loaded epoxy ....................................................................................................... 17
2.2. CHARACTERIZATION BY NMR SPECTROSCOPY ............................................................17 2.2.1. Principles of NMR ........................................................................................................ 18
2.2.2. Chemical shift .............................................................................................................. 21
2.2.3. Spin relaxation ............................................................................................................ 22
2.2.4. NMR diffusion experiments ......................................................................................... 26
2.3. ADDITIONAL CHARACTERIZATION TECHNIQUES ............................................................29 2.3.1. Small-angle X-ray scattering (SAXS) ............................................................................ 29
3. Summary ............................................................................. 31
3.1. AMPHIPHILIC MOLECULES ASSOCIATED IN SOLUTION ....................................................31 3.1.1. Micelle core freezing ................................................................................................... 31
3.1.2. Release of solubilizate from micelle upon core freezing ............................................. 35
3.1.3. The state of propofol in aqueous solution .................................................................. 41
3.2. BLOCK COPOLYMERS ADSORBED AT CARBON NANOTUBE SURFACES.............................42 3.2.1. The dispersion process of carbon nanotubes .............................................................. 42
3.2.2. Competitive adsorption of amphiphilic molecules at CNT surfaces ............................ 45
3.2.3. Polymer-CNT composite: structure and mobility ........................................................ 47
4. Concluding remarks ........................................................... 50
5. List of abbreviations ........................................................... 52
6. Acknowledgements ............................................................ 53
7. References ........................................................................... 55
1
1. Introduction
In this thesis, some studies regarding the structure, composition, and preparation of
some colloidal systems are presented. First, the basics regarding the systems
investigated and the methods used are introduced. Then, we summarize the most
important findings in the papers and manuscripts included.
1.1. Surfactants
The word surfactant arises from “surface-active agent” that illustrates surfactant
behavior: namely, that it can accumulate at an interface to reduce the free energy of
the boundary between any two immiscible phases. Generally, a surfactant molecule
contains two parts, one is soluble in a particular fluid, called the lyophilic part, and the
other insoluble in that fluid, called the lyophobic part. When that special fluid is water,
those two parts are called hydrophilic head and hydrophobic tail (Figure 1.1).
Figure 1.1 Surfactants structure and accumulate at interfaces
Usually the hydrophobic tail is formed by one or several alkyl chains, typically
involving around 8 to 18 methylene and methyl groups, but sometimes it can be more
complex with branched or unsaturated groups or even aromatic groups.1 The
hydrophilic head can be charged or neutral. Depending on this latter property,
surfactants can be divided into two broad classes, ionic (charged) surfactants and non-
ionic (uncharged) surfactants. Ionic surfactant can be further classified as anionic
2
surfactant, (that have anionic functional groups as head groups, such as carboxylate,
sulfonate and phosphate) or cationic surfactant (which may, for example, have amine
or ammonium groups as head group) and zwitterionic surfactant (where the head group
contains both negative and positive charges). For a non-ionic surfactant, the polar head
group often consists of polyhydroxyls or polyoxyethylene. In polyoxyethylene-based
surfactants each oxyethylene group in the head is denoted as E and, if the hydrophobic
part is an alkyl chain of m methylene/methyl units, then this surfactant can be referred
to as CmEn. For an ionic surfactant, the volume of hydrophilic head group is usually
much smaller than that of the hydrophobic tail. For polyoxyethylene-based surfactants,
the hydrophilic and the hydrophobic groups often have similar sizes and therefore it
can also be considered as a short AB block copolymer.
1.2. Amphiphilic block copolymers
Polymers are a large molecule formed with a great number of small molecules
covalently bound in a repeating structure whose properties are insensitive to adding
one or a few of those repeating units to it.2 If the polymer contains only one type of
repeating unit (A), it is called a homopolymer; if the polymer is formed by two or more
sorts of repeating units (A,B, etc.), it is a copolymer and can be classified according to
the arrangements of the constituting units, such as block copolymers, graft copolymers,
alternating copolymers, and statistical copolymers (Figure 1.2). If the different blocks
in block copolymers have different polarities, such as one of them hydrophilic and the
other one hydrophobic, then that copolymer behaves as an amphiphilic
macromolecule.
Amphiphilic block copolymers containing poly(ethylene oxide) (PEO) have been
extensively studied because the PEO segment is environmentally friendly, non-toxic,
biodegradable, has a good biocompatibility and a wide range of solubility.3-8 Among
those polymers, we focused on the Brij series and Pluronics. The Brij polymer is
composed of an alkyl chain of particular length and a PEO chain of particular length.
The variation of those hydrophilic and hydrophobic segment lengths makes their
properties, such as molecular weight, melting point, hydrophilic/lipophilic balance,
etc., different that permits different applications.9-13 Pluronic triblock copolymers are
3
also one of the most commonly used amphiphilic copolymers. They consist of PEO
blocks as hydrophilic segments and poly(propylene oxide) (PPO) blocks as the
hydrophobic part. The PEO and PPO blocks are structured as: PEOx-PPOy-PEOx
where the number of PEO and PPO blocks (also known as PEO/PPO ratio) can be
varied to change the molecular properties and, consequently, the phase behavior that
may permit a variety of applications.14-19
Figure 1.2 A homopolymer and different kinds of copolymers.
1.3. Micelles and solubilization in micelles
As described previously, the fundamental property of a surfactant is packing at the
interface to lower the interfacial tension. Actually, surfactant molecules can also be
associated with each other in bulk. This process of forming aggregates is called
surfactant self-assembly. The driving force is to prevent hydrophobic groups being in
contact with water, thereby reducing the free energy of the system.20 Different
aggregates with various structures could be formed and the self-organized structure
depends primarily on the volume and length of the hydrophobic chain and the area of
hydrophilic head group, summarized in the form of the critical packing parameter
(cpp)21, 22
4
𝑐𝑝𝑝 =𝑉ℎ𝑐
𝑎ℎ𝑔𝑙ℎ𝑐 (1.1)
where 𝑉ℎ𝑐 and 𝑙ℎ𝑐 are the volume and length of the fully extended hydrocarbon chain,
and 𝑎ℎ𝑔 is the hydrated hydrophilic head group area.
Figure 1.3 The different schematic structures of surfactant aggregates that may form in
water.
In general, if the volume of the hydrocarbon chain is equal to the volume of the
cylinder that is defined by area 𝑎ℎ𝑔 and length 𝑙ℎ𝑐, one obtains cpp=1, for which a non-
curved hydrophilic-hydrophobic interface is favored, and lamellar bilayer aggregates
are formed. If cpp>1, the shape of the molecule is similar with an inverted wedge,
bicontinuous phases or reverse micelles are favored. If cpp<1, an interface curved
toward the hydrophobic domain is favored, yielding bilayer vesicles (1/2<cpp<1),
5
cylindrical micelles (1/3<cpp<1/2) or spherical micelles (cpp<1/3). Those different
structures are illustrated in Figure 1.3.
In this thesis, the aggregates are mainly micelles. The process they are formed by is
called micellization and the concentration at which this process commences is called
the critical micelle concentration, abbreviated as cmc. Below the cmc, surfactants are
monomers in the bulk. Once the concentration reaches the cmc, they start to be
aggregated into micelles. The number of surfactants within a micelle is referred to as
the aggregation number. Normally, the cmc and aggregation number can be
determined by many techniques, such as surface tension, dynamic light scattering,
fluorescence spectroscopy, small angle X-ray scattering (SAXS), sedimentation
velocity and nuclear magnetic resonance spectroscopy (NMR).23-28 Each surfactant has
its own cmc. For example, in a PEO-based surfactant (such as the Brij family), a larger
polar head group leads to a moderate increase of cmc. The micelle is not a rigid
aggregate, but is dynamic in several ways: the molecules move within the micelles,
the chains have high flexibility, and individual molecules exchange between the free
surfactant state in bulk and the micelles.
Amphiphilic macromolecules behave somewhat similarly to simple surfactants, that is
they form micelles in solution, but their micellization is not completely the same. The
difference is related to the molecular weight of the polymer, and its hydrophobic and
hydrophilic moiety lengths, which give rise to a larger micellar core as well as a larger
overall micelle size. An additional important factor is the polydispersity of the
polymer, which also makes the aggregation more complex. Importantly, for
amphiphilic block copolymers there often is no sharp cmc even if the polymer has a
narrow molecular weight distribution.29, 30 Instead aggregation commences in a certain
range of concentration, and this concentration range may vary depending on the
different experimental techniques and conditions. Besides the cmc behavior, there is
also a strong temperature dependence in micellization for many amphiphilic block
copolymers, and the characteristic temperature is called the critical micelle
temperature. For example, for Pluronics, micelle formation is not only molecular
weight and PEO/PPO ratio modulated, but also temperature dependent.31-33 In the case
of Pluronic F127, the cmc decreases dramatically by increasing temperature, because
6
the PPO moieties are more hydrophobic at higher temperatures, which leads to easier
formation of micelles and lower aggregation numbers.16
Micelles have a hydrophobic core part, which can act as a good container for
hydrophobic molecules. In other words, the solubility of hydrophobic compounds in
aqueous solution can be increased dramatically in the presence of micelles. When the
surfactant concentration is lower than the cmc, the solubility of hydrophobic molecules
is still very low as it is in pure aqueous solution. Above the cmc, hydrophobic
molecules can be loaded in micelle core part until one reaches a solubilization
equilibrium. Here we need to distinguish this kind of solubilization (or loading) from
forming emulsions or microemulsions. First, emulsion is a suspension, which is
heterogeneous with particle size often larger than 1000 nm, and furthermore, those big
droplets are not in equilibrium and undergo coalescence until one obtains macroscopic
phase separation. On the contrary, micelles and microemulsions are part of equilibrium
phases and are homogeneous with sizes generally less than 100 nm. When a
hydrophobic compound is solubilized in micelles, the size of particles in the system
will not change significantly.34-37 Although micelles and microemulsions have similar
core-shell structures, the composition of the core part is different. As shown in Figure
1.4a, the micelle core is the hydrophobic region with the amphiphile tails mixed with
the solubilizate (Figure 1.4b), while the core part of a micromulsion consists of a small-
molecular hydrophobic liquid of oil, which is surrounded by the amphiphilic film
(Figure 1.4c). There exist also nanocapsules that are typically bigger than micelles,
and have polymeric films around the inner oil droplet.38 Solubilization is one of the
most important phenomena for micellar solutions, which can be used as detergents and
for formulation of drug carriers, where micelles can act as stimuli responsive particles
that can release molecules as defined by the different functional groups of the
constituting amphiphilic molecules.
7
Figure 1.4 Illustration of (a) a micelle, (b) a micelle with solubilizate loaded in it and (c) a
microemulsion/nanocapsule, where red represents the solubilizate or oil phase.
1.4. Carbon nanotubes and carbon nanotube dispersions
The introduction of CNTs can be traced back to the year 1991 when Iijima discovered
CNTs having a hollow cylindrical nanostructure with nm-order diameter and with
sheets of hexagonally arranged carbon as its wall.39 Due to their cylindrical structure
and small diameter, CNTs have a very high aspect ratio, that is the length/diameter
ratio in the order of 1000. In CNT walls, each carbon atom binds to three other carbons
through sp2 carbon-carbon bonds. This bond is even stronger than the sp3 bonds in
diamond, and therefore CNTs can have extreme mechanical strength. Moreover, the
p-electrons in the sheet form a π-system40, 41 and therefore the CNTs can be electrically
conductive.42
CNTs can be classified into two major types, single-walled CNTs (SWNTs) and multi-
walled CNTs (MWNTs). Single-walled CNTs are essentially single graphene layers
rolled up to a seamless cylinder while multi-walled CNTs are composed by two or
more concentric graphene cylinders. In reality, CNTs are not formed by rolling up
sheets of graphene but through other approaches.
In the picture presented above, rolling can proceed along different directions (called
the chiral angle 𝜃) within graphene layer. There are three familiar directions and
thereby resulting structures are illustrated in Fig. 1.5. for CNTs: zig-zag, armchair and
chiral. The different hypothetical rolling directions are defined by so-called chiral
8
vector, Ch=na1+ma2, shown in Figure 1.5, where the integers n and m indicate the steps
along the graphene lattice real space unit vectors (a1, a2). The structure of CNTs can
be assigned by those integers. When m=0, the chiral angle is 0o, and the CNTs are zig-
zag type; when m=n, the chiral angle is 30o, and the CNTs are armchair type. In all
other cases, the CNTs are chiral. The electronic properties of CNTs depend on the
chiral angle: if the number (n-m) is a multiple of three, the CNTs are conductive,
otherwise semiconductive. MWNTs consist of two or more rolled-up graphene sheets,
each of which can has its own chirality, and therefore the properties of MWNTs are
more complex.
Figure 1.5. Classification of the wall structure of CNTs by their chirality.
CNTs have some unusual properties, such as outstanding mechanical and electrical
performance, and can therefore be applied in a lot of areas. Moreover, CNTs have
interesting magnetic and optical properties as well, which also have been explored in
many studies.42-48 However, to exploit the full potential of CNTs in various
applications, it is often required that the CNTs exist individually, that is separated from
each other. In other words, they should not be present in form of aggregates (such as
bundles, mats and ropes), but well dispersed instead.
9
Many methods were proposed and tested to achieve this. One possible approach is to
dissolve CNTs in some suitable organic solvents. Since the surface of CNTs is
hydrophobic, using an organic hydrophobic solvent would be the first possible
approach. Unfortunately, only a few solvents can dissolve/disperse a limited amount
of CNTs, such as chloroform, N,N-dimethylformamide (DMF), o-dichlorobenzene
(ODCB) and, the best among them, N-methyl-2-pyrrolidone (NMP).45, 48-50 Another
drawback for this method is that those organic solvents are neither environmentally
benign nor biocompatible, which would limit their applications.
To avoid this disadvantage and disperse more CNTs in water, another common method
is to chemically modify the surface of CNTs with some functional groups, which have:
1) favorable interactions with the water and 2) enhance the repulsions between CNTs
to avoid re-aggregation and keep them in individually dispersed state. Although a
considerable amount of CNTs can be dispersed on this manner in aqueous solution,
but one drawback of this method is that this kind of modification of the wall can change
the nature of the carbon-carbon bonds (functionalization should disrupt the π-system)
and thereby alter any of those advantageous properties of CNTs. Moreover, some
structural defects could also be introduced.41, 51, 52
An alternative and for some applications clearly better way to disperse CNTs is relying
on not covalently attached but merely adsorbed groups. In other words, one would use
a dispersant that spontaneously attaches itself to the outer surface of CNTs and provide
the two functions specified above. The dispersant needs to have affinity to water,
meanwhile, it needs to have some hydrophobic moieties, too, that may attach to the
CNTs without changing any chemical bonds. Both hydrophilic and hydrophobic
properties are required in the same molecule, which calls for an amphiphilic one.
Surfactants and amphiphilic polymers are prime candidates to act as the CNT
dispersants in water.
As described above, amphiphilic molecules can associate both in the bulk and at an
interface. Although both of those arrangements may permit hydrophobic moieties
avoiding contact with water, adsorption at a solid hydrophobic surface is different from
the self-association in solution. Typically, in solution, amphiphiles first adsorb at the
air-liquid interface until their concentration reaches the cmc, and then they start to self-
10
associate to form micelles in the bulk. If CNTs appear in solution, there is a stronger
attraction between the hydrophobic segments of amphiphiles and the surface of CNTs,
which induces the adsorption of amphiphilic molecules onto the CNTs surface.40, 41
Different from the micellization process, which requires a minimum critical
concentration, the adsorption of dispersants on CNTs can happen at a lower
concentration than cmc.53, 54 Normally, the initial adsorption of amphiphiles on CNTs
occurs in a random way, and at higher concentrations, aggregates, like hemimicelles
or cylindrical micelle, may form on the surface.54-57
To disperse pristine CNTs (always received as powder) in water, ultrasonication is also
needed in the amphiphilic solutions discussed above. During ultrasonication, high
shear forces are generated that, interacting with CNT bundles, may open up clefts in
the bundle. The newly exposed CNT surfaces in the cleft are hydrophobic and, if
nothing happens to them, they may quickly re-aggregate. However, if the dispersants
adsorb on those newly created CNT surfaces, re-aggregation between CNTs and re-
closing the cleft may be prevented.58, 59 A propagating cleft leads to complete
exfoliation. This process is known as the unzipping mechanism and is illustrated in
Figure 1.6.
Figure 1.6 The unzipping mechanism of exfoliation: (a) a bundle of CNTs; (b) a cleft at the
bundle end is opened by high local shear forces; (c) surfactant molecules adsorb on the new
surfaces created; (d) the cleft propagates and a CNT coated by surfactant is released and
remains dispersed in solution.
11
The main driving force for adsorption of a surfactant on CNTs is the hydrophobic
interaction. Besides that, π-π interactions can also play an important role in
adsorption.52, 60, 61 As mentioned above, electrons in the CNT wall can form a π system,
which is similar to that in aromatic molecules. Thus, a strong and specific interaction
can arise between CNTs and molecules containing aromatic moieties.
Similarly to surfactants, polymers can also adsorb on CNTs, with their hydrophobic
segments attached to the wall and hydrophilic segments interacting with water.59, 62
But there is some difference between surfactants and polymers. Polymers have high
molecular weight and different possible arrangement of the repeating units, which
likely will affect the interactions with nanotubes. Thus, polymers may not only loosely
adsorb on nanotube surface but can in some cases also wrap the tube by coiling around
it.63-65 The polymers in the wrapping mode of adsorption are clearly less dynamic
compared to those with loose adsorption. Hence, polymers with loose adsorption mode
can exchange between being attached on the CNTs surface and being free in bulk
solution, and achieve an equilibrium with a dynamic balance between those states.
Besides the structure of dispersants, their concentration is important as well for
dispersing CNTs. Initially, researchers prepared CNTs dispersions using surfactant
with concentrations higher than the cmc, but later it was found that it does not matter
whether the concentration is above cmc or not.66 In other words, the formation of
micelles is not a requirement for dispersibility of CNTs. Typically, the concentration
of dispersed CNTs increases as a function of dispersant concentration until a plateau
is obtained. When the dispersant concentration increased to an even higher value, the
dispersed CNT concentration may decrease.
Due to their unique mechanical,67-73 electrical74-78 and thermal73 properties, CNTs are
excellent fillers for polymers when creating composite material systems. Dispersing
CNTs in polymer melts is separate chapter regarding dispersibility. One connection to
aqueous dispersibility is that it might be advantageous, in order to achieve thin bundles
or individual CNTs, to pre-disperse CNTs in aqueous phase first, dry them, then
12
disperse this material instead of pristine CNTs in polymers melts or precursors. This
was the route chosen for the study described in Paper VII in this thesis.
13
2. Experimental
2.1. Sample preparation
To make experiments reproducible, the first step is to make the samples reproducible.
In this subsection, we focus on micelles containing solubilizates and CNT dispersions.
A number of features caught our attention and lead to a lot of work and delays even if
they did not make into the final publication - such as the choice of a suitable
solubilizate to be loaded in micelles or the sonication strength and time to disperse
CNTs. Some of those issues are going to be discussed in this section.
2.1.1. Micelles with and without solubilizates
The formation of micelles is simple and commences upon mixing and dissolution of
the surfactant/amphiphilic polymer in water at a concentration higher than its cmc. In
our research, neat Brij S20 (C18E20) or neat Brij S100 (C18E100) were dissolved in
heavy water D2O at a concentration of 1 wt%. A mixed micellar system of Brij S20
and Brij S100 was obtained by mixing the 1 wt% solution of the pure surfactants at a
ratio of 1:1. Among those three micellar solutions, only the one formed by Brij S20
(that exhibited the sharpest core freezing behavior) was used to solubilize small
hydrophobic (and a few amphiphilic) molecules.
There are two common methods employed to prepare micelles containing solubilizates
in them.35-37 The first one is direct dissolution where the solubilizate is added to a
micellar solution and partitions rapidly and spontaneously into the core part of
micelles. The second and more involved method is film casting where the
surfactant/amphiphilic polymer and solubilizate are both dissolved in an organic
solvent, which solvent is then evaporated leaving a film in the container wall. Water
is then added to dissolve the mixed film of surfactant and solubilizate. In our
experiments, the first and simpler method was sufficient and was thereby chosen to
prepare the samples.
A more complex point was to choose a good solubilizate for this model study (that is,
on contrast to applications where the solubilizate is given). Since the main
14
characterization method is 1H NMR, one important requirement was that the
solubilizate exhibited an easily distinguishable peak in the NMR spectrum with no
overlap with the micellar signal. Besides that, several physical properties had to be
considered. First, the density of solubilizate could not be too close to that of heavy
water, because in that case an excess oil phase did not phase separate sufficiently.
Secondly, the solubilizate had to be liquid in the experimental temperature range of 0-
20 °C to avoid signal loss by freezing. Moreover, the solubilizate could not be volatile
in the same temperature range that, in practice, meant that the boiling point had to be
far higher than 20 °C. Finally and importantly, the solubilizate was not permitted to
completely suppress core freezing since it was exactly the influence of that
phenomenon on solubilizate content that was the subject of our study.
A number of liquids were tested (such as toluene or hexamethylsilane) and among
those it was hexamethyldisiloxane (HMDSO) that fulfilled best all requirements and
was thereby selected. It presents a single 1H NMR peak at approx. 0 ppm chemical
shift and with no overlap with the micellar signals at approx. 1 ppm or above, a density
of 0.764g/mL that is far below that of heavy water, a boiling point at 100 °C and a
melting point at -59°C. While core freezing was shifted by it, it was not completely
suppressed.
A series of samples was prepared by adding different amount (from 0.5 µL to 80 µL)
of HMDSO to 1.5 mL micellar solution (Brij S20 1 wt%). After vortex mixing for 5
min, the sample was subsequently centrifuged for 30 min at 6500 rpm. This procedure
yielded a two-phase sample, where the upper part was the excess amount of HMDSO.
The transparent lower part, around 1 mL, was taken out and moved to a 5 mm NMR
tube yielding >7 cm sample column. Subsequently, a second centrifugation step was
applied to the sample in the NMR tube for 30 min at 1250 rpm in order to make sure
that any non-solubilized HMDSO that remained was moved to the sample top and
thereby out of the NMR detection range. All the samples were prepared at room
temperature and used in following NMR experiments.
15
2.1.2. Propofol with different concentrations in D2O
Propofol stock solution (0.89 mM) was received from a collaborating group. This stock
solution was then diluted with heavy water to obtain a series of propofol solutions with
different concentrations (from 0.02 mM to 0.89 mM). Propofol was also one of the
liquids we tested as solubilizate in Brij S20, but at high loading it has completely
suppressed core freezing.
2.1.3. CNT dispersions
In general, CNT dispersions were prepared by ultrasonicating the CNT powders mixed
into a dispersant solution. The role of the dispersant has already been discussed above
(see Fig. 1.6). Subsequent centrifugation removed the non-exfoliated CNT particles
and other impurities from the dispersions whose CNT content was evaluated with a
combination of thermogravimetric analysis (TGA) and Ultraviolet-visible (UV-vis)
spectroscopy.62, 79
During sonication, the cavitation bubbles can be created and also collapsed. The rapid
collapse of those bubbles generates very fast local solvent streams and pressure
gradient that exerts high shear forces on the CNTs bundles. Those forces can overcome
the van der Waals forces that keep the CNTs together and create temporary clefts.80, 81
The number and size of cavitation bubbles are dependent on sonication frequency and
power. Lower frequency ultrasound tends to create larger cavitation bubbles and the
larger the cavitation bubbles, the greater the forces generated upon collapse. Normally,
tip sonication is operated at a lower frequency than bath sonication, which is one
reason we chose tip sonication for exfoliation. In addition, tip sonication can also
deliver higher acoustic power that yields more bubbles. If high power is used for long
enough time, one can create dispersion with significant CNT content.82 Unfortunately,
sonication also leads to breaking the dispersed CNTs,83-86 and therefore the used time
and power is typically (in our case, to yield a total delivered acoustic energy density
of 1100 J/mL) a compromise between sufficiently high content of sufficiently intact
CNTs. In addition to CNT exfoliation and breakage, the other effect of ultrasonication
is to increase the temperature of the processed liquid mixture. This was shown to lead
to inferior and scattered results and therefore the sonication vial was immersed into a
16
home-made cooling bath that kept the temperature low enough during the whole
process. Other details, such as an exact and reproducible volume of the sonicated liquid,
an exact and reproducible position of the sonication tip relative to the vial were
important to ensure the reproducibility of the samples. Sonication also strongly
depended on the shape of the vials and therefore the exactly the same type of vials has
to be used for a given study.
After the sonication step, one has a suspension-dispersion containing individual CNTs,
thin CNT bundles and non-exfoliated CNT particles (and, if the starting pristine CNT
is not pure, impurities). These objects exhibit different sedimentation velocities and
therefore centrifugation is a good method to separate them. Hence, a mild
centrifugation (typically, 30 min at 4000 g) was sufficient to remove non-exfoliated
particles and thereby obtain as supernatant the desired dispersions of individual CNTs
and thin CNT bundles.83, 87-89
When the CNTs dispersions are done, their CNTs concentration has to be quantified.
The protocol for this was described in detail elsewhere.62, 79 Since CNTs absorb visible
light with a high absorption at the wavelength λ=660 nm and their apparent absorption
follows the Lambert-Beer law, their obtained absorbance A is proportional to their
concentration c:
𝐴 = 휀𝑐𝑙 (2.1)
where 휀 is the extinction coefficient, and 𝑙 is the length of light path.
With A and l measured/known, the last required parameter to obtain the CNT
concentration is the extinction coefficient 휀. This quantity is not a priori known and
has to be calibrated against CNT mass in a dispersion. The calibration method was the
following one.62 A certain volume 𝑉𝑠 of the CNTs dispersion was first freeze dried, the
mass 𝑀𝑠 was determined by weighing and then TGA was applied to get the mass
fractions of CNT and dispersant. Finally, the concentration of CNT can be calculated
as:
17
𝑐𝐶𝑁𝑇 = [𝑀𝑠 × (1 −𝜙𝑠
𝜙𝑑)] ×
1
𝑉𝑠 (2.2)
where 𝜙𝑠 is the mass fraction of dispersant in supernatant and 𝜙𝑑 is the dispersant
decomposition fraction (which was obtained in a separate TGA experiment). The
advantage of this combined procedure is that the CNT concentration can be obtained
by rapid spectroscopic experiments on readily accessible UV-vis spectrometers.
2.1.4. CNT-loaded epoxy
As described above, CNTs need to be pre-dispersed and then mixed with epoxy resin
to form a composite. The CNT pre-dispersions were prepared by sonication with
Pluronic F127 as dispersant. The process was as above, with multi-walled CNTs added
at 2 mg/mL to an aqueous F127 solution (1.5 mg/mL), followed by tip sonication with
approx. 310 J/mL delivered acoustic energy density. During this process, the
temperature was kept at 0 oC. After sonication, the dispersion was centrifuged for 20
min at 1000 g. After centrifugation, the supernatant was collected and freeze-dried to
remove the water. The final yield of cotton-like CNT-F127 powder was subsequently
mixed with epoxy resin by a planetary mixer for 10 min to obtain the composite.
2.2. Characterization by NMR spectroscopy
NMR spectroscopy was used in this thesis as the main tool to get the information about
the structure, dynamics and association of molecules. Well-established NMR
relaxation and diffusion methods were used for analyzing the state of
surfactants/amphiphilic polymers in aqueous solutions or at the surface of CNTs.
Besides NMR, the structural method of SAXS also played an important role in our
work.
18
2.2.1. Principles of NMR
NMR spectroscopy is based on the nuclear property of nuclear spin which makes that
nuclei can interact with an external magnetic field. Some nuclei lack this property and
they cannot thereby be used for NMR spectroscopy.90
The spin is defined as the intrinsic angular momentum and magnetic moment that exist
in all the elementary particles such as protons, neutrons and electrons. It is
characterized by spin quantum number I that is I=1/2 for protons, neutrons and
electrons. Nuclei with odd mass number have a spin I that is odd multiples of 1/2 (1/2,
3/2, 5/2…), while nuclei with even mass number but with odd numbers of protons and
neutrons have integer spin I (1, 3, 4, 5, 7, by chance there is no I=2 nucleus). Nuclei
with even mass number and even atomic number such as 12C and 16O have no spin.
A nuclear spin I can have 2I+1 possible states, that are indexed by the quantum number
m, m=−I, −I+1, −I+2, …, I. Hence, the component of angular momentum 𝜤 along any
spatial direction z can be quantized as:
𝛪𝑧 = 𝑚ℏ (2.3)
where ℏ is the Planck’s constant divided by 2𝜋. The simplest case is spin I=1/2, such
as for the 1H, 13C, 31P nuclei, where there are two possible states with m=1/2 and
m=−1/2.
The magnetic moment 𝝁 of a nucleus is connected to its angular momentum 𝜤 by
simple proportionality:
𝝁 = γ𝜤 (2.4)
where γ is the gyromagnetic ratio, specific for each nucleus. It is a scalar that can be
either positive or negative and therefore the nuclear magnetic moment can be parallel
or antiparallel, respectively, to its associated angular momentum.
In the absence of an external magnetic field, all the possible 2I+1 possible nuclear spin
states have the same energy. In a magnetic field B0 whose direction is assigned as z,
the nuclear magnetic moment interacts with the field as:
19
Em=−𝜇𝑧𝐵0 (2.5)
and the energy levels split as given by:
𝜇𝑧 = 𝑚ℏ𝛾 (2.6)
For the simplest case of spin I=1/2, there are two energy levels associated with m=1/2
and m=−1/2, with the energy difference between those two states given as:
Δ𝐸 = ℏ𝛾𝐵0 (2.7)
The two energy levels are populated by spins aligned either along or against the
external magnetic field. In thermal equilibrium, the Boltzmann distribution prevails
yielding:
𝑁ℎ𝑖𝑔ℎ
𝑁𝑙𝑜𝑤= 𝑒−Δ𝐸 𝑘𝐵𝑇⁄ (2.8)
where 𝑘𝐵 is the Boltzmann constant and T is the absolute temperature. while the
number of nuclei at the two energy levels are denoted as Nlow (at the lower energy
level) and Nhigh (at the higher energy level).
Without an external magnetic field, the nuclear spins have no preferred orientations,
and therefore the total nuclear magnetization, the vector sum of the individual nuclear
magnetic moments, is zero. In an external magnetic field, the number of spins aligned
along and against the magnetic field are different and a net nuclear magnetization
arises parallel to the direction z of the applied magnetic field. This net magnetization
is proportional to the population difference between different spin states, Δ𝑁 =
𝑁𝑙𝑜𝑤 − 𝑁ℎ𝑖𝑔ℎ . This population difference and thereby the equilibrium nuclear
magnetization is very small,91 Δ𝑁/𝑁 being in the order 10-4-10-8, depending on the
field strength and the gyromagnetic ratio. As one consequence, a large amount of
sample is required for NMR spectroscopy, performed with concentrations typically at
or above the mM range.
In NMR experiments, radiofrequency (rf) pulses, delivered by a coil arranged around
the sample, are applied to excite this net magnetization out of its equilibrium state.
20
This rf field can be seen as a time-dependent magnetic field that is aligned orthogonally
to 𝐵0. To be able to excite nuclear spin transitions, its frequency has to match the
energy difference between two different spin states, that is ℎ𝜈 = Δ𝐸 = ℏ𝛾𝐵0. Hence
arises the resonance condition:
𝜈0 = 𝛾𝐵0/2𝜋 (2.9)
This resonant frequency is called the Larmor frequency, sometimes also given in terms
of angular frequency 𝜔0 = 𝛾𝐵0.
In the simplest NMR experiment, shown in Figure 2.1, there is one rf pulse applied.
This pulse has a specific length and rf field strength so that its effect is to turn the
nuclear magnetization from its equilibrium state along the z axis to the perpendicular
xy-plane. Once in that plane, the magnetization precesses around the direction of 𝐵0
with its Larmor frequency 𝜔0. This motion changes the magnetic flux in the detection
coil (typically the same one as that for excitation) surrounding the sample and the
changing flux induces a time-dependent voltage in the coil. This is called the free
induction decay (FID), which is the primary time-dependent NMR signal. After being
Fourier transformed, it yields the NMR spectrum as a function of frequency.
Figure 2.1 A 90° rf pulse and the time-domain FID signal excited by it.
21
2.2.2. Chemical shift
The Larmor frequency as defined above is set by the external magnetic field. Yet, this
frequency is not exactly the same for the same nuclei either in different molecules or
at different positions in the same molecule. Namely, nuclei in atoms and molecules are
surrounded by electrons and one effect of 𝐵0 is to perturb slightly the electronic states.
This perturbation can be represented as if a small additional magnetic field 𝐵′ was
added to an external magnetic field 𝐵0, typically in the opposite direction. Hence, the
nuclei at different sites reside in a magnetic field that varies from site to site on a
manner that depends on the local electronic environment. This phenomenon is called
shielding and is represented as:
𝐵 = 𝐵0 − 𝐵′ = 𝐵0(1 − 𝜎) (2.10)
where 𝜎 is the shielding constant. Hence, the resonant frequency of a nucleus at a
particular site becomes:
𝜈 = 𝛾𝐵0(1 − 𝜎)/2𝜋 (2.11)
Instead of detecting the shielding constant, a more convenient way to represent it is by
the chemical shift 𝛿, that is defined as the difference in resonant frequencies between
a nucleus at a site of interest and a nucleus in a reference compound:
𝛿 = 106𝜈 − 𝜈𝑟𝑒𝑓
𝜈𝑟𝑒𝑓 (2.12)
typically expressed in units of ppm. Defined on this manner, 𝛿 is independent on the
external magnetic field. The chemical shift depends not only on the intramolecular but
also, to a lesser extent, on the intermolecular surrounding.27, 92-94
Because of the chemical shift, the NMR spectra typically contain many peaks. Separate
peaks with their respective individual chemical shifts are detected if there is no
exchange of the atoms between those sites or the exchange is slow relative to the
inverse of the frequency difference between the peaks belonging to those sites. If the
exchange is faster than that, one can only detect a single peak at the frequency given
by the average chemical shift.
22
Besides the chemical shift, there are additional spin interactions that may influence
NMR spectra. In liquids, the other relevant interaction is the spin-spin or J-coupling.
The effect of that is the various signals are split, depending of the arrangement of other
nuclei around a site in a molecule. This interaction also depends on the electronic
structure but the role of the electrons is to transfer, through their delocalization over
covalent bonds, the magnetic effect of a nuclear spin in a given atom to the spin
residing in another atom. There are additional and in magnitude much larger spin
interactions such as the dipole-dipole coupling. However, in a liquid their spectral
effect is averaged out by fast isotropic molecular motions and they only influence spin
relaxation, discussed in the next sub-section.
2.2.3. Spin relaxation
In thermal equilibrium state, the net nuclear magnetization is oriented along the z-axis
and there is no magnetization in the xy-plane. After a 90° rf pulse as in Figure 2.1, there
is net magnetization in the xy-plane, but the magnetization along the z-axis is zero. In
other words, rf excitation has put the sample spins into a non-equilibrium state. Hence,
like in any other non-equilibrium state, the system relaxes back to the equilibrium one.
For nuclear spins, this process is called spin relaxation that, for the simplest case of
spin I=1/2, includes two independent processes: the longitudinal relaxation and the
transverse relaxation, characterized by their respective relaxation time constants T1 and
T2. Longitudinal relaxation, also called spin-lattice relaxation, characterizes the return
of the longitudinal (that is, along the z-axis) magnetization to its equilibrium value.
Transverse relaxation, also called spin-spin relaxation, refers to the decay of transverse
magnetization to zero.
The driving force of spin relaxation is the fluctuation of the local field (or, more
generally, the fluctuations of the various nuclear spin couplings such as the dipolar
coupling, electric quadrupole, chemical shift anisotropy95) that arise from molecular
motions. The quantity that is often used to characterize the temporal properties of those
motions is the motional correlation time 𝜏𝑐. In itself, using a single parameter is a
simplification; in reality, the proper tool to analyze molecular motions is their auto-
and cross-correlation functions. Yet, in simple liquids the decay of those correlation
23
functions is well represented by a single 𝜏𝑐 which is then used to indicate the time
needed for a molecule or moiety to significantly change its orientation with respect to
the external magnetic field. When presenting a qualitative model of spin relaxation,
one can disregard the actual spin coupling that is modulated and can instead assume
that the correlation time represents the average time that is needed to experience a
significant change in a randomly fluctuating magnetic field.
Within the random local field model, the fluctuating field has components in all
different directions x, y and z. Of these components, it is only fluctuations in the x and
y directions that cause transitions among the involved energy levels, for the same
reason that it is only an rf field in the xy-plane can excite the nuclear spin system.
Because the longitudinal relaxation influences the total energy of the spin system, it
requires such transitions between energy levels. On the other hand, fluctuations in the
z-direction cause no transitions, but only change the precession speed of the spins
relative to each other which leads to a spread of spins in the xy-plane and the loss of
their sum, the total precessing nuclear magnetization. For this reason, both longitudinal
and transverse relaxation are influenced by the field fluctuations in the xy-direction,
while fluctuations in the z-direction only affect the latter process. Besides the direction,
the time scale of the fluctuations has also an effect. Namely, to be able to cause
transitions, the fluctuating field must possess temporal variation that is significant in
the range of the Larmor frequency; in that case, it can satisfy the resonance condition.
For that reason, the relation 𝜏𝑐 ~1/𝜔 must be satisfied. On the other hand, fluctuations
in the z-direction on any time scale can contribute to the spreading of magnetization
on the xy-plane and thereby to transverse relaxation. In summary, only fast fluctuations
have an effect on T1 while both fast and slow fluctuations can affect T2.95 Very fast
𝜏𝑐 ≪ 1/𝜔 fluctuations are ineffective to cause either longitudinal or transverse
relaxation.
To illustrate the consequences, we consider a typical experimental situation when one
performs temperature dependent spin relaxation experiment. Temperature changes the
molecular motions (often through an Arrhenius-type dependence). This leads to the
variation illustrated in Figure 2.2. At high temperatures with short correlation times,
both relaxation times are long and of approximately the same value because the
24
molecular motions are not effective; this regime is called extreme narrowing. Upon
decreasing temperature and increasing correlation times, both relaxation times
decrease. Yet, upon further change, the longitudinal relaxation time experiences a
minimum that signifies that the molecular motions reached the range where 𝜏𝑐 ~1/𝜔.
Upon further decrease of temperature, T1 starts to increase as the molecules motions
become less effective to cause transitions among the spin energy levels. Yet, T2
continues to decrease since it can be affected also by slow fluctuations. In the slow
motion regime of 𝜏𝑐 ≫ 1/𝜔, often experienced in large molecules or in the solid state,
a typical experimental situation is T1 ≫ T2.95, 96
Figure 2.2 The schematic dependence of the relaxation times T1 and T2 on the motional
correlation time 𝜏𝑐.
The transverse relaxation time T2 determines the decay of the precessing nuclear
magnetization and thereby the decay of the FID signal, too. Through the Fourier
transform between the FID and the spectrum, it defines the width of the spectral peaks;
the shorter the T2, the broader the peaks. However, the line width in the spectra is also
25
contributed by the static inhomogeneity of the magnetic field. The spin-echo pulse
sequence,97 shown in Figure 2.3, can be used to separate those two effects.
Figure 2.3 The spin echo pulse sequence.
In this pulse sequence, a 90° rf pulse turns the magnetization to the xy-plane where the
magnetization starts to precess. If the magnetic field is inhomogeneous, the nuclear
magnetization from the different regions precess with different speed, with some
magnetization components going ahead and some lagging behind. Individually, each
component is also affected by transverse relaxation. As a consequence of the
inhomogeneity, the magnetization fans out in the xy-plane. After a duration of τ, a 180°
rf pulse is applied that inverts the orientation of the magnetization components in the
xy-plane so that the relative order of those components changes: the slowly-precessing
components are placed ahead and fast ones will be put at the end. Yet, they retain their
relative precessing speed which means that the fast magnetization components
gradually catch up with the slow ones. In other words, the magnetization that was
originally fanning out during the first half of the pulse sequence is now gathered and
refocused. This apparent re-appearance of decaying transverse magnetization is called
the spin echo. If one detects the FID signal starting at the time τ, the resulting spectral
peaks are still broadened by the magnetic field inhomogeneity but if one records their
decay upon increasing τ, one obtains their true T2 relaxation time. Similarly, one has
to use a separate pulse sequence to measure T1. It should note that the separation
between fluctuating and static random fields is not clear-cut. Roughly speaking, the
spin echo experiment refocused that effect of those fields that fluctuate on the time
scale is slow as compared to τ.
26
2.2.4. NMR diffusion experiments
Self-diffusion, the Brownian motion without any applied potential gradient, is the
random translational motion of molecules. It is the most fundamental form of
molecular transport, taking part in all chemical phenomena. In an isotropic
homogeneous system, the effect of self-diffusion is typically formulated in terms of
the mean-square displacement by the Einstein relation:
⟨(𝒓𝑡 − 𝒓0)2⟩ = 6𝐷𝑡 (2.13)
where r0 is the initial position for a molecule and rt is its position after time t. In this
relation, D is the self-diffusion coefficient,92, 98, 99 expressed in units of m2s-1. In a
small-molecular liquid at room temperature, D is typically in the range of 10-9 – 10-12
m2s-1,92, 99 and is related to the molecular size as given by Stokes-Einstein equation:
𝐷 =𝑘𝐵𝑇
6𝜋𝜂𝑅ℎ (2.14)
where 𝑘𝐵 is the Boltzmann constant, T is the absolute temperature, 𝜂 is the viscosity
of the solution at the temperature T, and 𝑅ℎ is the hydrodynamic radius of the molecule
or particle. In this equation, the viscosity is another molecular property influenced
heavily by intermolecular interactions.
For a given molecule, adsorption (for example, to other molecules of larger size or to
surfaces) or self-association into an aggregate may change the detected diffusion
coefficient. Namely, under those conditions and assuming that the association is
persistent on the time scale of the experiment performed, the measured diffusion
coefficient is going to be that of the total associated system that, under the conditions
specified, is lower than that for molecules without association. Hence, one can for
example detect both micelle formation,92, 99 and the attachment of molecules to a CNT
surface.100, 101 The diffusion coefficient is also influenced if the molecules exchange
among the environments of different self-diffusion coefficients. In that case, one may
obtain a population average of that property.
NMR spectroscopy provides a non-invasive method to determine the self-diffusion
coefficient of native molecules. In other words, NMR-based diffusion measurements
27
do not require the samples to be chemically or physically disturbed, or labeled probe
molecules, or macroscopic potential gradients to initiate net transport. The reason for
that is that in NMR diffusion experiments the diffusing atoms/molecules are marked
with regard to their position by applying, during the course of suitable NMR
experiments, spatially dependent magnetic fields, also called magnetic field gradients.
In this manner, the Larmor frequency becomes spatially dependent. Since the nuclear
spin transitions involve very small energy differences, this has no influence on any
other relevant degrees of freedom.
Specifically, if the applied magnetic field gradient G is spatially independent (called
“linear gradient”), the resonant frequency at position r becomes:
𝜔𝑟 = 𝛾𝐵0 + 𝛾𝑮 ∙ 𝒓 (2.15)
The pulsed-field-gradient spin-echo (PGSE)102 experiment, based on the modification
of the spin-echo experiment, takes advantage of such labeling of the spins to determine
the self-diffusion coefficient.
In the PGSE pulse sequence shown Figure 2.4a, two identical gradient pulses of
magnitude g and duration 𝛿 are added to the spin echo experiment, one before and one
after the refocusing pulse. The two gradient pulses are separated by a period that is
called the diffusion time and is denoted by Δ. If the spin-bearing entities were
immobile and kept their position over Δ, the experiment would work just like the spin
echo experiment: the transverse magnetization spread out by the field inhomogeneity
caused by the first gradient pulse would be completely refocused. The condition for
such complete refocusing is that the resonance frequencies of the spins are identical at
the time of the two gradient pulses. If the spin-bearing entities randomly move, the
resonance frequencies before and after the refocusing pulses are not the same but differ
by a random factor. Hence, the magnetization components from the different nuclei do
not align with each other in the xy-plane but will be spread out in that plane depending
on the random factor to mark the change in their position by diffusion. Since the NMR
signal is proportional to the total transverse magnetization, if the magnetization
components are spread out, their vector sum decreases and so does the NMR signal.
28
Figure 2.4 The pulsed-field-gradient spin-echo (a) and stimulated-echo (b) experiments.
Detailed analysis shows that the decrease of the NMR signal acquired in the PGSE
experiment is related to the diffusion coefficient (because that governs the random
displacement of spin-bearing entities) as given by the so-called Stejskal-Tanner
equation102
𝑆
𝑆0= 𝑒𝑥𝑝 {−𝛾2𝛿2g2𝐷 (𝛥 −
𝛿
3)} (2.16)
where 𝑆 and 𝑆0 denote the signal intensities obtained with and without the gradient
pulses, respectively. The signal attenuation can be monitored upon increasing the
gradient, and the signal decay yields D. The huge advantage with PGSE NMR
diffusion experiments is that the detected spectrum is of high resolution with resolved
peaks for different chemical entities. Hence, by following the diffusional decay of a
particular peak in the spectrum, the diffusion coefficient of the corresponding
29
particular molecule can be selectively obtained, even in complex mixtures. This has
been exploited throughout the work in this thesis.
In practice, the different experimental parameters, such as the diffusion time Δ, the
gradient strength g and length 𝛿 have to be set so that the signal is detectable and the
decay is well-sampled. For a small molecule with a large diffusion coefficient, a small
gradient might be suitable to preserve the NMR signal; for a large molecule with a
small diffusion coefficient, larger and longer gradients and a long diffusion time must
be used to make sure that there is significant signal attenuation. However, for large
molecules or aggregates the transverse relaxation time can, as discussed above, be very
short and thereby the one cannot set suitably long diffusion time with gradient pulses
limited in strength because the signal would then be lost by transverse relaxation. To
alleviate this problem, the stimulated-echo pulse sequence (Figure 2.4b) has been
introduced. Here, the magnetization is stored not in the xy-plane but along the z axis
during the diffusion time. Since z-magnetization is influenced by longitudinal
relaxation and because, for large molecules and aggregates, the longitudinal relaxation
time is often much longer than the transverse relaxation time, much more signal is left
for a given diffusion time. Most of the diffusion experiments in this thesis work were
performed with the stimulated-echo pulse sequence or with one of its varieties.
2.3. Additional characterization techniques
2.3.1. Small-angle X-ray scattering (SAXS)
SAXS is a powerful method in colloidal science to investigate the size, shape and
internal structure of particles in the size range from nanometers up to several hundred
nanometers.103 Normally, it can be applied to systems consisting of randomly oriented
or/and statistically distributed structures of colloidal dimensions, such as proteins,104
polymer particles105 and molecular aggregates in solutions106 (e.g., micelles). During
a SAXS experiment, an X-ray beam irradiates the sample and electrons in the atoms
coherently scatter the incident radiation in directions away from the directions of the
incident beam. The intensity of scattered X-rays can be measured as a function of the
angle away from the direction of the incident radiation. In principle, the solvent in the
system provides a background scattering which is almost constant at small angles
30
where the scattering from larger structures is significant; since the particles of interest
in the system have different composition and thereby different electron density (that
is, as compared to the surrounding solvent) the scattering relative to the solvent
background is containing the sought structural information.
SAXS is based on the coherent scattering of X-rays. Such scattering events result in
scattered X-rays that are phase coherent with the incident radiation. Hence,
interference phenomena can arise; in crystalline lattices, this result in diffraction. For
randomly arranged objects, the result is that the scattered intensity becomes the
orientationally averaged Fourier transform of the autocorrelation of the electron
density. The latter quantity depends on the geometry (and, in sufficiently dense
systems, interparticle arrangement yielding the so-called structure factor) of the
scattering particles (often summarized as the so-called form factor).
To extract the structure from the angular dependence of the scattered radiation
intensity, the data can be analyzed by either model-independent approaches or by
direct modeling. The former procedure necessarily involves inverse Fourier
transformation.107 In direct modeling, a geometrical model of the objects is assumed
and the scattering intensity is calculated first, then the parameters of the model can be
optimized by, for example, least-squares fitting of the calculated function to the
experimental data.108 Nowadays, various models are available to provide information
on aggregate shape and aggregation number. In this thesis, the SAXS analysis relied
on a model based on a spherical core-shell structures.109, 110
31
3. Summary
In this thesis, the behavior of amphiphilic molecules associated in aqueous bulk
solution and to nanotube surfaces was detected and analyzed by NMR (chemical shift,
spin relaxation and self-diffusion coefficient) experiments, in some cases in
combination with SAXS experiments. In bulk solution, we focus on the temperature-
dependent properties of some micellar systems (formed by Brij surfactants) and the
solubilizate release from such micelles upon decreasing temperature (Papers I and II).
Whether or not a small amphiphilic molecule, propofol, self-associates in water
solution was also investigated (Paper III). A large part of the work concerned aqueous
dispersions of CNTs, where the adsorption behavior of the amphiphilic polymer
(Pluronic F127) and of different kinds of small surfactant molecules were studied with
emphasis on the question how does the adsorption affect the dispersibility (Papers IV,
V and VI). Furthermore, the CNT dispersions in an epoxy resin were prepared and the
underlying mechanism for the rheological behavior was investigated (Paper VII).
3.1. Amphiphilic molecules associated in solution
Core-shell (with alkyl cores and polyethylene oxide shell) micelles were formed by
dissolving either pure Brij S20 (C18E20), pure Brij S100 (C18E100) or their 1:1
mixture in water. Furthermore, the micelles formed by Brij S20 were used as a carrier
to be loaded by HMDSO.
3.1.1. Micelle core freezing
Brij micelles exhibit the interesting feature of having the hydrophobic chains in their
micellar core frozen below a given temperature. Core freezing was detected by us from
the temperature-dependent line-width data shown in Figure 3.1. Clearly, the core peaks
broadened a lot below 6-8 °C that is a consequence of the shortening of the transverse
relaxation time T2 because of the slowing down of molecular mobility and the loss of
segmental flexibility. That this is a local feature in the core and does not caused by,
32
for example, a large change in the overall micellar mobility is shown by the absence
of any change in the data for the shell moieties.
Figure 3.1 The temperature-dependent 1H NMR line width for moieties within the micellar
core and shell parts in pure Brij S20 (A), pure Brij S100 (B) and 1:1 mixture of those two
(C).
Micelles prepared with different Brij compositions behaved slightly differently from
each other (Figure 3.1), although the line width increased by decreasing temperatures
in all three systems. In Brij S20 micelles, the core freezing commenced sharply at
around 7 °C while in Brij S100 micelles the slowing down of dynamics was seemingly
more gradual but ending with a sharp transition below 3 °C. Since the hydrophobic
chains of Brij S20 and Brij S100 are the same, the observed differences indicate that
the longer PEO blocks in Brij S100 hinder the ordering of the core alkyl chains.
Considering the high mobility and strong hydration of PEO peaks, this is quite
plausible. In the case of the mixture, the presence of a second component has not made
33
core freezing more spread out but, if anything, contributed to sharpening that
transition.
The freezing of the micellar core was also detected in the longitudinal relaxation data,
see Figure 3.2. The relative change in those data upon freezing is smaller than that for
the line widths that can be explained by having smaller effect of the alkyl chain
ordering on fast and small-amplitude local segmental motions. The continuous change
of the data for the PEO chains signifies a conventional continuous change in the
thermally activated processes in the shell part.111
Figure 3.2 The temperature dependent longitudinal relaxation times for moieties within the
micellar core and shell parts in pure Brij S20 (A), pure Brij S100 (B) and 1:1 mixture of
those two (C).
There are some additional details to notice when comparing the shell data in Figures
3.1 and 3.2. Firstly, while the longitudinal relaxation time is decreasing by decreasing
temperature, the line width keeps constant. The reason for the insensitivity of the latter
34
parameter to a slow gradual change in dynamics is that for the very flexible PEO chain
the transverse relaxation is quite slow and therefore the line width is dominated other
effects like field inhomogeneity and spin-spin coupling. Secondly, the line width of
PEO moieties from Brij S20 micelles is larger than that in Brij100. This could be the
result of having shorter PEO blocks in Brij S20 and the PEO segments closest to
micellar core probably exhibit slower dynamics than the segments further out.
NMR spectroscopy was more sensitive to the core freezing features than SAXS.
However, SAXS experiment could reveal that, in the mixture, one obtains not only
core freezing but also a phase separation of the molecular constituents. Namely, the
SAXS data obtained for the mixture below the core freezing point were well
reproducible by assuming two different micellar aggregates with shell sizes dictated
by the different PEO block lengths.110 As shown by Figure 3.3, above the core freezing
point, the same structural model was clearly invalid. Yet, the data still revealed a core-
shell geometry. These observations are consistent with the two surfactants forming a
single mixed aggregate above core freezing.
Figure 3.3 The discrepancy (𝜒2) between the scattering data obtained in the 1:1 mixture of
Brij S20 and Brij S100 and the linear combination of the data for the two pure constituents.
The discrepancy is shown for repeated heating/cooling cycles.
35
3.1.2. Release of solubilizate from micelle upon core freezing
Micelles are widely used to solubilize hydrophobic molecules. As a particular aspect,
they can also act as carriers of hydrophobic drugs or other agents that can be released
under the stimuli of pH, light, or temperature, or the combination thereof.112-119 The
condition for release is that those stimuli change the micellar state so that it becomes
less favorable for holding the solubilizate. Hence, the core freezing of Brij micelles
discussed above seemed to be worth testing as a possible triggered release mechanism.
HMDSO was chosen as a hydrophobic model molecule to be loaded in Brij S20 (1 w%
in heavy water) micelles. An experimental protocol was developed to investigate the
saturation point of the micelles with HMDSO; since the uptake kinetics was slow, the
protocol involved an over-saturation of the solution with HMDSO and after which the
superfluous non-solubilized HMDSO was repeatedly removed via phase
separation/centrifugation.
Figure 3.4 The amount of solubilized HMDSO as a function of the initial HMDSO loading
in the micellar solution. The insert shows the initial part of the loading curve.
As shown by Fig. 3.4, the saturation level was approx. 0.25 mg/mL that corresponds
(see below) to approx. 10% HMDSO in the total core volume. Figure 3.5 displays that
the core freezing, as witnessed by the line width of the core methylene peaks, was
suppressed, even well below core saturation, from 7-8 °C to 2-3 °C.
36
Figure 3.5 The temperature-dependent line width of the core methylene peak in Brij S20
micelles with and without added HMDSO. The different symbols belong to samples
prepared by different initial loadings.
Core freezing does not only lead to slowing down the molecular motions of the alkyl
chains in the micellar core. In addition, the temperature dependence of the 1H chemical
shifts (see Fig. 5 in Paper II) shows that the core becomes denser and the alkyl chains
presumably attain all-trans conformations. Again, in line with what was discussed in
the previous section, the 1H chemical shift of the shell moieties shows no discernible
effect upon core freezing, thereby indicating that the shell remains in a well hydrated
and highly mobile state even if the core attains limited mobility.
Another 1H chemical shift effect permits us to assess the amount of HMDSO within
the micelles and being out in the aqueous phase. Namely, the 1H chemical shift is
different in those two environments. Hence, upon decreasing the temperature toward
the core freezing point, the HMDSO peak splits into two as shown by Figure 3.6, where
the peak around 0 ppm arises from HMDSO loaded in the micelles and the peak at
approx. -0.05 ppm belongs to HMDSO that has been released into aqueous
environment.
37
Figure 3.6 The 1H NMR spectra of HMDSO at saturation level in Brij S20 micelles (A) with
the spectral segment with HMDSO peak enlarged (see dashed delimiters in A) at 20 °C (B),
7 °C (C), 4 °C (D) and 1 °C (E).
Upon lowering the temperature, the intensity of the HMDSO peak at -0.05 ppm
becomes higher, indicating cumulative release all the way down to the core freezing
point, to which point we return later. The HMDSO for which this peak arises from
cannot be in a molecularly dispersed state, because the equilibrium HMDSO solubility
in water is approx. 0.0005 mg/mL. Two more observations, the slight turbidity of the
liquid at the temperatures where the split spectra are observed and the slow (hours)
38
decrease of the peak intensity by time, indicate that the corresponding HMDSO state
is small droplets dispersed in water. The decrease of signal intensity is a consequence
of those droplets floating to the top of the sample columns and out of the sensitive
probe volume.
Figure 3.7 Surfactant (from shell moieties) and HMDSO self-diffusion coefficients at
different initial loadings as obtained by 1H diffusion NMR experiments.
The fact that we see two HMDSO peaks in Figure 3.6 indicates that the HMDSO
exchange between micelles and droplets is slow on the spectral time scale (∼40 ms).
On the other hand, the self-diffusion coefficients of HMDSO and the micellized
surfactant (see Figure 3.7) indicated that, above the onset of the release, the exchange
between molecularly dissolved HMDSO and the micelles was fast on the diffusion
time scale (∼200 ms). This apparent controversy can be resolved (see the relevant
discussion in more detail in Paper II) by considering that the exchange between the
environments (A) HMDSO solubilized in the micelles and (B) HMDSO in droplets, is
proceeding through (C) HMDSO molecularly dissolved in water. The small size (recall
the solubility of 0.0005 mg/mL) of environment C makes it act as a bottleneck.
39
From the spectra as in Figure 3.6 that relative proportions of HMDSO in the two
dominant environments, solubilized and released, can be extracted by fitting two
Lorentzian peaks to the experimental points. The results are presented in Figure 3.8.
Figure 3.8 The HMDSO content (%) within () and out of () micelles at different
temperatures, at different initial loadings (A) 2.5 mg/mL, (B) 1.5 mg/mL and (C) 0.5
mg/mL.
In all cases, one detects a fairly stable situation at high temperatures but upon
approaching the core freezing point of unloaded Brij S20 micelles, released HMDSO
appears in the system. This release accelerates in the temperature region down to the
core freezing point below which the micelles retain HMDSO at approx. 15% of the
saturation level. In samples that were originally unsaturated (Figure 3.8 C) the initial
levels are more stable and the transition is sharper than those in saturated samples.
Polydispersity, both in size and in HMDSO content, might be the underlying reason
for this observation.
40
Additional insight into the release process can be obtained by expressing the HMDSO
content in the micelle core through its volume fraction 𝜙:
𝜙 =𝑉𝐻𝑀𝐷𝑆𝑂 ∙ 𝑐𝐻𝑀𝐷𝑆𝑂
𝑉𝐶18 ∙ 𝑐𝑠 + 𝑉𝐻𝑀𝐷𝑆𝑂 ∙ 𝑐𝐻𝑀𝐷𝑆𝑂 (3.1)
where 𝑉𝐻𝑀𝐷𝑆𝑂 and 𝑉𝐶18 are the molar volumes of HMDSO (0.212 L/mol) and C18
chains (0.314 L/mol), respectively and 𝑐𝐻𝑀𝐷𝑆𝑂 and 𝑐𝑠 are the molar concentrations of
HMDSO and surfactant, respectively. As shown in Figure 3.9, the volume fraction in
saturated micelles is approx. 9.5%. This value and the decrease of it upon decreasing
temperature has been confirmed by SAXS experiments, see details in Paper II.
Figure 3.9 The temperature dependence of the volume fraction of HMDSO within the
micellar core.
The most interesting feature in Figure 3.9 is that the HMDSO content during the
release process follows the same master curve irrespective of the initial HMDSO
content. This result suggests that the release of HMDSO from micelle is not only
related to core freezing but is also associated with a loading capacity, which is
temperature dependent. Once the loading capacity is lower than the solubilizate
content in the micelle, solubilizate is released.
41
3.1.3. The state of propofol in aqueous solution
As described in section 2.2.2, the chemical shift depends on both the intra- and
intermolecular environments for a particular atom. Hence, changes in the
intermolecular environment in form of self-association should be reflected in the
chemical shift. Propofol, an anaesthetic (see Fig. 3.10), is an amphiphilic molecule and
in gas phase it was indicated to form trimers and tetramers in association to water
molecules.120 Since the association of propofol at surfaces and in pores turned out to
be an open issue, it turned out that a question arose concerning its self-association in
bulk water. We note in passing that propofol was also one molecule we tested (and
ruled out) as solubilizate to be released upon core freezing as discussed in the previous
section.
Figure 3.10 The molecular structure and 1H NMR peak assignment of propofol. The
chemical shift was calibrated with the water peak at 4.78 ppm.
To investigate this point, the chemical shift of propofol was recorded in aqueous
solutions with different propofol concentrations. Since propofol contains an aromatic
ring, one would expect large changes in the chemical shift upon any conceivable self-
association. As shown in Figure 3.11, all chemical shifts within the propofol are
constant irrespective the concentration range which indicates that propofol remains a
single-molecular solute. Furthermore, the intensity of NMR peaks of propofol are
proportional to its concentration, excluding the presence of large aggregates (for which
the NMR signal could be lost because of large broadening).
42
Figure 3.11 The 1H NMR chemical shift recorded for propofol at different concentrations in
water, the symbols correspond to atom of the same color in Figure 3.10.
3.2. Block copolymers adsorbed at carbon nanotube surfaces
As described above, CNTs can be dispersed in aqueous media with the help of Pluronic
F127 as a dispersant. Several factors can markedly affect the dispersions created, such
as the initial loading of CNT and the sonication time. Dispersions can also be prepared
by other surfactants, and competitive adsorption between different surfactants and
F127 can reveal the relative strength of binding of those molecules to CNTs is another
factor that influences the dispersion process. These phenomena and some insights into
them via the work presented in this thesis are shortly discussed below. Furthermore,
the effect of dispersed CNTs on the rheology of epoxy resins and the molecular
features affecting this are deliberated as the final topic of this thesis.
3.2.1. The dispersion process of carbon nanotubes
In some previous studies, one has exploited the rather plausible and consistent
postulate that Pluronic F127 molecules that are attached to nanotube surfaces still
exhibit mobile sections and corresponding narrow NMR spectra.62, 100 Based on that
postulate, one analyzed the bound fraction of F127 molecules in SWNT dispersions
43
and investigated, for example, how does that bound fraction exchange with the F127
molecules free in solution.
In Paper V we investigate in detail whether or not the state of F127 molecules attached
to SWNT surfaces matches those earlier expectations. Indeed, we verify that the slow
tail in the diffusional decay of F127 molecules in SWNT dispersions arises from F127
molecules that are attached to the SWNT surfaces by their PPO block. Figure 3.12
summarizes the unambiguous evidence. To collect this evidence we, again, relied on
the chemical selectivity of NMR spectroscopy – namely, that the signal arises from the
PPO blocks presents a single-component diffusional decay while the same decay for
the signal from the PEO block is two-exponential. This can only happen if a strongly
attached and thereby dynamically strongly hindered PPO block fails to exhibit
sufficient motional averaging of spin couplings and thereby its 1H NMR signal suffers
from large spectral broadening so that it becomes undetectable.
Figure 3.12 The 1H NMR diffusional decays of the peaks arising from the different polymer
blocks of Pluronic F127, in both neat solution and in SWNT-F127 dispersion.
In addition to this confirmation, a careful analysis of the relative spectral intensities of
the different peaks in the F127 spectrum yielded that the block polydispersity of F127
played a role in the preparation of dispersions. Namely, we found that F127 polymers
44
with relatively large hydrophobic blocks dominantly adsorb on such bundles of
nanotubes that do not become exfoliated and subsequently dispersed. Assumedly, this
can be a consequence of competitive adsorption of different F127 varieties on the
hydrophobic bundle surfaces.
Similarly to a large group of other ionic surfactants,121 the amount of SWNTs in
dispersions exhibits a characteristic sigmoidal dispersion curve (see Figure 1 in Paper
IV) as a function of the logarithmic surfactant concentration. The question is what are
the physical/molecular mechanisms that provide this curve with its characteristic and
apparently generic form. Specifically, (i) what provides the apparent threshold (cdc,
critical dispersibility concentration) at which the SWNT concentration in the
dispersions starts to grow and (ii) what is the mechanism that limits the plateau value
(the maximum dispersed SWNT concentration, cSWNT,max) to far below the initial
amount of added SWNT.
Figure 3.13 (a) Initial F127 concentration vs the F127 concentration in the dispersion as
estimated from NMR intensity of the ethylene oxide signal in the 1H NMR spectrum. The
lines are exponential fits to guide the eye. (b) The dispersion curves plotted as a function of
the polymer content in the supernatant. Data in both (a) and (b) are provided at different
initial SWNT loadings cSWNT-init.
Regarding question (i), by following the dispersant concentration in the dispersions
via 1H NMR spectral intensities we could show that initially, the dispersants are
45
adsorbed to the pristine SWNT particles/bundles mixed into the solution. Apparently,
the concentration in the dispersant must then reach a threshold value. If the dispersion
curves are re-scaled with the surfactant concentration in the dispersant as the control
parameter, they seem to coincide (see Figure 3.13b) for different initial loadings of
SWNT. Moreover, 1H diffusion studies along the dispersion curves (see Figure 5 in
Paper IV) indicate that, in the dispersion, most of the dispersants remain dissolved in
the bulk, with only a few % providing surface coverage for the defoliated small bundles
and individual nanotubes. Hence, the threshold behavior in the dispersion curves
seems to originate from kinetic demands: that sufficient amount of surfactant must be
around to be able to provide coverage to freshly exposed surfaces in a sufficiently rapid
manner. If this not happens, the openings created by ultrasonication close back.
Regarding question (ii), we performed experiments with increasing sonication time
and found that the one obtains similar dispersion curves with the plateau value
increasing by increasing sonication time. The conclusion drawn was that cSWNT,max is
set on one hand by the particle/bundles size distribution that is moving to smaller sizes
upon increasing sonication time and, on the other hand, by the centrifugation step that
sets up a size limit above which the particles/bundles are precipitated. Yet, simply
increasing the ultrasonication time is not a necessarily recipe to follow to prepare
SWNT dispersions – besides making smaller bundles, the nanotubes are also become
broken by sonication which limits their applicability.
3.2.2. Competitive adsorption of amphiphilic molecules at CNT
surfaces
As mentioned earlier, the F127 PEO signal in SWNT dispersion exhibits a bi-
exponential diffusional decay from which the amount of F127 adsorbed at the SWNT
surfaces in dispersion can be estimated.62, 100 From those studies, one could also
conclude that the PPO groups of the adsorbed F127 molecules provide a complete
coverage of the available SWNT surfaces. In Paper VI, this feature is exploited to
assess the competitive adsorption of surfactants to SWNT. Namely, after having
prepared dispersions by F127 (as detailed in the previous sub-section), one can add
another sort of surfactant to the dispersion and record the diffusional decay of F127
46
PEO signal (Figure 3.14). As earlier, the slowly decaying component provides the
F127 adsorbed on SWNT surface. If this component decreases with the increasing of
concentration of the added second surfactant, it indicates that the added surfactant
replaces F127 on the SWNT surface.
Figure 3.14 The diffusional decay of the normalized 1H NMR signal from the PEO peak in
F127 as a function of concentration of SDS (a) and DTAB (b) added to SWNT-F127
dispersion. The lines are fits of bi-component decays described with the customary Stejskal-
Tanner equation with b = γ2δ2g2(Δ-δ/3), see eq 2.16.
As shown by Figure 3.14, different surfactants show markedly different abilities to
replace F127. In other words, they show markedly different strength of binding to the
SWNT surfaces. To provide a more quantitative analysis, the fraction of adsorbed
F127 on the SWNT surfaces, fF127 was extracted by fitting the experimental F127
diffusional decay with a bi-exponential expression.62, 100 From the obtained values
shown in Figure 3.15, the apparent surface coverage 𝜎F127 of SWNTs in the dispersion
can also be assessed. Both the fF127 and 𝜎F127 strongly decrease with adding SDS in the
dispersion, while the addition of DTAB hardly influences those two parameters.
Hence, SDS, an anionic surfactant, binds to SWNT surfaces significantly stronger than
F127. The binding of DTAB, a cationic surfactant, is on the other hand significantly
weaker than that of F127. A more quantitative analysis over a wider range of
surfactants is presented in detail in Paper VI.
47
Figure 3.15 Fraction fF127 of F127 molecules adsorbed on the SWNT surface and the
corresponding apparent surface coverage 𝜎F127 by F127 molecules as a function of the
concentration of SDS (a) and DTAB (b) added to system originally dispersed by F127. The
dashed lines are linear fits.
3.2.3. Polymer-CNT composite: structure and mobility
CNT-polymer composites represent one current and important applications of CNTs.
In one preparation pathway, CNTs are dispersed into a monomeric/oligomeric resin
and the resin is subsequently polymerized. One limiting factor for this process is the
very high viscosity the dispersion may exhibit. This feature and its structural/molecular
details constituted on important motivation for the experiments in Paper VII.
Figure 3.16 Light microscopy images of epoxy resin loaded with CNTs at (a) 0.05, (b) 0.10
and (c) 0.15 wt%.
Dominantly monomeric epoxy resin was loaded with different CNTs (the CNTs were
first pre-dispersed and then dried). Their large-scale structure consists of aggregates
48
as revealed by the light microscopy images in Figure 3.16. The aggregates, whose size
is around tens of microns, increase in volume fraction as the CNT concentration
increases. Yet, considering the composition (∼0.1 wt%), they must be rather loose with
most of the aggregate volume filled by epoxy.
To get more insight, 1H diffusion NMR experiments were performed on the epoxy
molecules. As shown by Figure 3.17, the diffusional decays were clearly not single-
exponential but seemed to exhibit a slowly-decaying tail that indicated the presence of
epoxy molecules that diffused much slower than those in the bulk. To extract the
fraction p of epoxy molecules that resided in the environment with slow diffusion, we
fitted the data by a simple bi-exponential decay.62, 100
Figure 3.17 The diffusional decay of the 1H NMR signal of the epoxy-specific aromatic
protons in CNT-loaded epoxy resin with various CNT concentrations. The lines are bi-
exponential fits, with the exception of CNT-free epoxy (single exponential fit).
In bulk epoxy without any CNT, the self-diffusion coefficient of the epoxy was
obtained to Dneat=5.05×10-12m2/s, a value that is more than an order of magnitude
lower than that in alkanes of comparable size. This points to strong intermolecular
interactions among the epoxy molecules. Regarding the slow component, in contrast
to the F127 molecules in aqueous dispersions,62, 100 this could not be identified as
surface-adsorbed epoxy. Namely, the small epoxy molecules could not be divided up
49
into CNT-attached and mobile blocks. Hence, the slow component was identified
instead as epoxy within the aggregates, where a strong attractive interaction with the
CNT surfaces leads to slowing down of molecular transport. The strong interaction
with the CNT surface was confirmed by SAXS data that indicated the intermolecular
order of epoxy changed in the presence of the CNTs. As a further support for this
hypothesis, the diffusional decays showed no discernible dependence on the diffusion
time that indicated no significant exchange of epoxy between the bulk and within-
aggregates states. This was consistent with the aggregate sizes as revealed by
microscopy.
Figure 3.18 (a) The variation of p, the fraction of epoxy within the CNT aggregates and (b)
Dslow/Dneat, the epoxy diffusion coefficient within the aggregates normalized with Dneat of
neat epoxy.
Upon increasing CNT content, the proportion of the aggregate-inhibited epoxy p
increased, first quickly and then toward an apparent saturation, see Figure 3.18. At the
same time, the within-aggregate diffusion coefficient slowed down which indicated
that at higher CNT content the assumed aggregates were becoming denser. As shown
in detail in Paper VII, the variation of p with increasing CNT content had some
similarities with the variation of various rheological parameters. This was rationalized
by positing that aggregate-aggregate interactions and jamming are the features that
control rheology. Such behavior is probably facilitated by having the aggregates
stabilized by the strong attractive interactions between the CNT scaffold and the epoxy
filler.
50
4. Concluding remarks
In this work, two types of colloidal systems, micelles and CNT dispersions, were
studied by various NMR spectroscopic methods in combination with other techniques
such as SAXS (micellar system) and TGA/UV-vis (CNT dispersions). The obtained
results reveal molecular-level information about phase transitions of micellar cores
and about dispersant-CNT interactions. In both cases, the chemical selectivity of NMR
spectroscopy was a crucial advantage: by exploiting it we could investigate particular
molecular components in complex mixtures.
In the Brij-type micellar systems studied here, we could reveal the structural details of
two separate phase transition phenomena, both of which appears in connection to the
freezing of the alkyl core of the micelles upon decreasing temperature. Regarding the
first of those phase transitions, the spontaneous separation of different sorts (that is,
Brij S100 and Brij S20) of surfactants into distinct aggregates, NMR spectroscopy
could accurately pinpoint freezing in the mixed surfactant system while SAXS
indicated the presence of one aggregate type above core freezing and two aggregate
types below core freezing. In pure micelles, we could show that the presence of longer
PEO block leads to a significant reduction in the core freezing temperature but also to
a more complex (and, assumedly, more heterogeneous) freezing behavior. In micelles
loaded with a hydrophobic solubilizate, the phase transition was again from a mixed
phase, namely the mixed micellar core, into two pure phases, the micelles practically
devoid of solubilizate and solubilizate droplets in water. Universal release curves
obtained at different initial solubilizate loadings were rationalized in terms of a
temperature-dependent loading capacity of micelles.
In most studies of CNT dispersions, Pluronic F127 was used as a model dispersant. As
we have argued above, this molecule adsorbs to CNT surfaces via its hydrophobic PPO
block but its terminal PEO blocks remain extended into water and highly mobile. For
that reason, they provide easily detectable NMR spectra even if the F127 molecule,
via its PPO block, is attached to a CNT. This feature could be exploited in NMR
diffusion experiments to obtain a quantitative measure of the amount of the adsorbed
F127 molecules within a dispersion. Hence, the distribution of the dispersant over
various molecular states could be followed in detail along the dispersibility curves that
51
recorded the amount of CNT that could be put into dispersion with at a given
concentration of the dispersant. The role and effect of the ultrasonication in the
dispersion process has been clarified and the results strengthened the notion that the
dispersant concentration required to disperse is a kinetic requirement: enough
surfactant must be around to be quickly able to cover newly explored CNT surfaces.
Yet, the strength of binding of different possible dispersants is still an important factor
that could be assessed by measuring the competitive adsorption of different dispersants
and F127 at the surface of CNTs.
NMR diffusion experiments were also helpful in another system, constituted by CNTs
dispersed in an epoxy resin. Surprisingly, we detected that a fraction of the epoxy
diffused much slower in such dispersions which was explained by the presence of
randomly associated CNT aggregates. Within those, the slow epoxy diffusion was
attributed to strong interactions between epoxy and the CNT surface. The interaction
of the aggregates with each other was found to be responsible for the observed
rheological behavior.
52
5. List of abbreviations
cdc Critical dispersibility concentration
cmc Critical micelle concentration
cpp Critical packing parameter
CNT Carbon nanotube
DMF N,N-dimethylformamide
DTAB Dodecyltrimethylammonium bromide
FID Free induction decay
HMDSO Hexamethyldisiloxane
MWNT Multi-walled carbon nanotube
NMP N-methyl-2-pyrrolidone
ODCB O-dichlorobenzene
PEO Poly(ethylene oxide)
PGSE Pulsed field gradient spin Echo
PPO Poly(propylene oxide)
rf Radiofrequency
SAXS Small-angle X-ray scattering
SDS Sodium dodecyl sulfate
SWNT Single-walled carbon nanotube
TGA Thermogravimetric analysis
UV-vis Ultraviolet-visible spectroscopy
53
6. Acknowledgements
This work would be impossible without lots of help and support from many people.
Therefore I would like to address my deeply thanks to:
Prof. István Furó for accepting me as a PhD student in your department. Thanks for
your excellent supervision, your patience and sharing your knowledge and great ideas.
Things were not always going well during last five years, but you always kept passion
and encouraged me to overcome all the difficulties. Thanks for your trust and help,
otherwise I could not reach the finish line. Thank you also for lots of help during the
writing process.
Prof. Sergey Dvinskikh for giving support whenever I got problems with the software
or spectrometers. Thank you for sharing your knowledge, answering all my questions
and helping me to improve my experimental skills.
Dr. Pavel Yushmanov for helping to deal with all the instrumental problems. Thank
you for building various useful things in the lab. Thanks for your intelligent
suggestions and improvements on many devices used in this work. I also thank you for
the nice time we shared in our office during the last but important period of my doctoral
study.
Prof. Mark Rutland for reviewing this thesis and making valuable comments.
Lena Skowron for your kind help not only in administrative stuffs but also in my daily
life. Vera Jovanovic and Dr. Ulla Jacobsson for your kind assistance in the
administrative matters. Johanna Hagerman, Alexandra Rudyk Kinnander and
Daniel Tavast for answering the questions related to thesis preparation.
Dr. Ricardo M. F. Fernandes for your guidance of all the carbon nanotube work.
Thank you for sharing your knowledge and ideas, answering my questions with the
greatest patience. Thanks for discussing and giving valuable suggestions in this work.
Thanks for your encouragement all the way.
54
Dr. Yuan Fang for your kind help from the beginning of my PhD days, both in studies
and daily life. Thank you for encouraging me to go through the very hard time before.
Thanks for supporting me whenever I need it.
My dear friends and colleagues in our corridor for creating a very nice working
atmosphere. Thank you Marianne, Erik, Fredrik, Evgeny, Bulat, Boris, Zoltan,
Tore, Joakim, Björn, Camilla G., Max, Ge, Himanshu, Christofer, Rena, Malisa,
Vasantha, Maria, Kristina, Camilla T., Michal, Mika for all the good time we
shared in fika and after-works. Thank you Dr. Robert Corkery for inspiring
discussions when I got stuck on some difficulties in my work. Thank you for sharing
your knowledge, experiences and giving helpful suggestions. Thank you also Zahra
Alaei for the support and companionship. Thanks for the time we spent on discussing
and working in the lab. I really enjoyed the time we worked together.
My friends in Sweden: Zhizhi, Hongduo, Miaomiao, Hao, Qiong, Hui, Bin, Yujie,
Wei, Duo, Dongming,Weifeng, Xiaoyan, Fan, Geng for all the good time we shared
and all the helps you gave me.
My parents for your love and support all the time. I love you!
55
7. References
1. Holmberg, K., et al., Surfactants and Polymers in Aqueous Solution, Second Edition.
2002: John Wiley & Sons.
2. Rubinstein, M. and R.H. Colby, Polymer physics 2003: Oxford University Press.
3. Fruijtier-Pölloth, C., Safety assessment on polyethylene glycols (PEGs) and their
derivatives as used in cosmetic products. Toxicology, 2005. 214(1): p. 1-38.
4. Harris, J.M., Poly(Ethylene Glycol) Chemistry: Biotechnical and Biomedical
Applications. 2013: Springer Science & Business Media.
5. Lee, J.H., H.B. Lee, and J.D. Andrade, Blood compatibility of polyethylene oxide
surfaces. Progress in Polymer Science, 1995. 20(6): p. 1043-1079.
6. Smyth, H.F., C.P. Carpenter, and C.S. Weil, The toxicology of the polyethylene
glycols. Journal of the American Pharmaceutical Association, 1950. 39(6): p. 349-354.
7. Zalipsky, S., Chemistry of polyethylene glycol conjugates with biologically active
molecules. Advanced Drug Delivery Reviews, 1995. 16(2): p. 157-182.
8. Zhu, J., Bioactive modification of poly(ethylene glycol) hydrogels for tissue
engineering. Biomaterials, 2010. 31(17): p. 4639-4656.
9. Behrens, M.A., J. Bergenholtz, and J.S. Pedersen, Temperature-Induced Attractive
Interactions of PEO-Containing Block Copolymer Micelles. Langmuir, 2014. 30(21): p. 6021-
6029.
10. Jensen, G.V., et al., Structures of PEP–PEO Block Copolymer Micelles: Effects of
Changing Solvent and PEO Length and Comparison to a Thermodynamic Model.
Macromolecules, 2012. 45(1): p. 430-440.
11. Ribeiro, M.E.N.P., et al., Solubilisation capacity of Brij surfactants. International
Journal of Pharmaceutics, 2012. 436(1): p. 631-635.
12. Sommer, C., J.S. Pedersen, and V.M. Garamus, Structure and Interactions of Block
Copolymer Micelles of Brij 700 Studied by Combining Small-Angle X-ray and Neutron
Scattering. Langmuir, 2005. 21(6): p. 2137-2149.
13. Tang, J., et al., Key Structure of Brij for Overcoming Multidrug Resistance in Cancer.
Biomacromolecules, 2013. 14(2): p. 424-430.
14. Alexandridis, P., Amphiphilic copolymers and their applications. Current Opinion in
Colloid & Interface Science, 1996. 1(4): p. 490-501.
15. Batrakova, E.V. and A.V. Kabanov, Pluronic block copolymers: Evolution of drug
delivery concept from inert nanocarriers to biological response modifiers. Journal of
Controlled Release, 2008. 130(2): p. 98-106.
56
16. Bromberg, L.E. and E.S. Ron, Temperature-responsive gels and thermogelling
polymer matrices for protein and peptide delivery. Advanced Drug Delivery Reviews, 1998.
31(3): p. 197-221.
17. Kabanov, A.V., E.V. Batrakova, and V.Y. Alakhov, Pluronic® block copolymers as
novel polymer therapeutics for drug and gene delivery. Journal of Controlled Release, 2002.
82(2): p. 189-212.
18. Kabanov, A.V., et al., Pluronic® block copolymers: novel functional molecules for
gene therapy. Advanced Drug Delivery Reviews, 2002. 54(2): p. 223-233.
19. Alexandridis, P., Poly(ethylene oxide) poly(propylene oxide) block copolymer
surfactants. Current Opinion in Colloid & Interface Science, 1997. 2(5): p. 478-489.
20. Nagarajan, R., Micellization, mixed micellization and solubilization: The role of
interfacial interactions. Advances in Colloid and Interface Science, 1986. 26(Supplement C):
p. 205-264.
21. Lombardo, D., et al., Amphiphiles Self-Assembly: Basic Concepts and Future
Perspectives of Supramolecular Approaches. Advances in Condensed Matter Physics, 2015.
22. Ramanathan, M., et al., Amphiphile nanoarchitectonics: from basic physical
chemistry to advanced applications. Physical Chemistry Chemical Physics, 2013. 15(26): p.
10580-10611.
23. Chattopadhyay, A. and E. London, Fluorimetric determination of critical micelle
concentration avoiding interference from detergent charge. Analytical Biochemistry, 1984.
139(2): p. 408-412.
24. Ebel, C., Sedimentation velocity to characterize surfactants and solubilized membrane
proteins. Methods, 2011. 54(1): p. 56-66.
25. Lipfert, J., et al., Size and shape of detergent micelles determined by small-angle x-
ray scattering. Journal of Physical Chemistry B, 2007. 111(43): p. 12427-12438.
26. Rosen, M.J. and S. Aronson, Standard Free-Energies of Adsorption of Surfactants at
the Aqueous Solution-Air Interface from Surface-Tension Data in the Vicinity of the Critical
Micelle Concentration. Colloids and Surfaces, 1981. 3(3): p. 201-208.
27. Söderman, O., P. Stilbs, and W.S. Price, NMR studies of surfactants. Concepts in
Magnetic Resonance Part A, 2004. 23A(2): p. 121-135.
28. Xu, R.L., et al., Light-Scattering Study of the Association Behavior of Styrene
Ethylene-Oxide Block Copolymers in Aqueous-Solution. Macromolecules, 1991. 24(1): p. 87-
93.
29. Jones, M.C. and J.C. Leroux, Polymeric micelles - a new generation of colloidal drug
carriers. European Journal of Pharmaceutics and Biopharmaceutics, 1999. 48(2): p. 101-111.
30. Alexandridis, P. and T.A. Hatton, Poly(Ethylene Oxide)-Poly(Propylene Oxide)-
Poly(Ethylene Oxide) Block-Copolymer Surfactants in Aqueous-Solutions and at Interfaces -
Thermodynamics, Structure, Dynamics, and Modeling. Colloids and Surfaces a-
Physicochemical and Engineering Aspects, 1995. 96(1-2): p. 1-46.
57
31. Bohorquez, M., et al., A Study of the Temperature-Dependent Micellization of
Pluronic F127. Journal of Colloid and Interface Science, 1999. 216(1): p. 34-40.
32. Kulthe, S.S., et al., Mixed micelle formation with hydrophobic and hydrophilic
Pluronic block copolymers: Implications for controlled and targeted drug delivery. Colloids
and Surfaces B: Biointerfaces, 2011. 88(2): p. 691-696.
33. Perry, C.C., et al., Fluorescence of commercial Pluronic F127 samples: Temperature-
dependent micellization. Journal of Colloid and Interface Science, 2011. 354(2): p. 662-669.
34. Gaucher, G., et al., Block copolymer micelles: preparation, characterization and
application in drug delivery. Journal of Controlled Release, 2005. 109(1-3): p. 169-188.
35. Kataoka, K., A. Harada, and Y. Nagasaki, Block copolymer micelles for drug delivery:
design, characterization and biological significance. Advanced Drug Delivery Reviews, 2001.
47(1): p. 113-131.
36. Kwon, G., et al., Block copolymer micelles for drug delivery: loading and release of
doxorubicin. Journal of Controlled Release, 1997. 48(2-3): p. 195-201.
37. Christian, S.D. and J.F. Scamehorn, Solubilization in Surfactant Aggregates. 1995:
CRC Press.
38. Letchford, K. and H. Burt, A review of the formation and classification of amphiphilic
block copolymer nanoparticulate structures: micelles, nanospheres, nanocapsules and
polymersomes. European Journal of Pharmaceutics and Biopharmaceutics, 2007. 65(3): p.
259-269.
39. Iijima, S., Helical Microtubules of Graphitic Carbon. Nature, 1991. 354(6348): p. 56-
58.
40. Britz, D.A. and A.N. Khlobystov, Noncovalent interactions of molecules with single
walled carbon nanotubes. Chemical Society Reviews, 2006. 35(7): p. 637-659.
41. Vaisman, L., H.D. Wagner, and G. Marom, The role of surfactants in dispersion of
carbon nanotubes. Advances in Colloid and Interface Science, 2006. 128: p. 37-46.
42. Grady, B.P., Carbon Nanotube-Polymer Composites: Manufacture, Properties, and
Applications. 2011: Wiley.
43. Baughman, R.H., A.A. Zakhidov, and W.A. de Heer, Carbon nanotubes - the route
toward applications. Science, 2002. 297(5582): p. 787-792.
44. De Volder, M.F.L., et al., Carbon Nanotubes: Present and Future Commercial
Applications. Science, 2013. 339(6119): p. 535-539.
45. Popov, V.N., Carbon nanotubes: properties and application. Materials Science and
Engineering: R: Reports, 2004. 43(3): p. 61-102.
46. Wu, Z., et al., Transparent, Conductive Carbon Nanotube Films. Science, 2004.
305(5688): p. 1273-1276.
58
47. Spitalsky, Z., et al., Carbon nanotube–polymer composites: Chemistry, processing,
mechanical and electrical properties. Progress in Polymer Science, 2010. 35(3): p. 357-401.
48. Terrones, M., Science and technology of the twenty-first century: Synthesis, properties
and applications of carbon nanotubes. Annual Review of Materials Research, 2003. 33: p.
419-501.
49. Bergin, S.D., et al., Towards solutions of single-walled carbon nanotubes in common
solvents. Advanced Materials, 2008. 20(10): p. 1876-1881.
50. Kim, K.K., et al., Design of dispersants for the dispersion of carbon nanotubes in an
organic solvent. Advanced Functional Materials, 2007. 17(11): p. 1775-1783.
51. Balasubramanian, K. and M. Burghard, Chemically Functionalized Carbon
Nanotubes. Small, 2005. 1(2): p. 180-192.
52. Kharissova, O.V., B.I. Kharisov, and E.G.D. Ortiz, Dispersion of carbon nanotubes
in water and non-aqueous solvents. Rsc Advances, 2013. 3(47): p. 24812-24852.
53. Backes, C., Noncovalent Functionalization of Carbon Nanotubes: Fundamental
Aspects of Dispersion and Separation in Water. 2012: Springer-Verlag.
54. Wang, H., Dispersing carbon nanotubes using surfactants. Current Opinion in Colloid
& Interface Science, 2009. 14(5): p. 364-371.
55. Islam, M.F., et al., High weight fraction surfactant solubilization of single-wall carbon
nanotubes in water. Nano Letters, 2003. 3(2): p. 269-273.
56. Poulin, P., B. Vigolo, and P. Launois, Films and fibers of oriented single wall
nanotubes. Carbon, 2002. 40(10): p. 1741-1749.
57. Wallace, E.J. and M.S.P. Sansom, Carbon nanotube self-assembly with lipids and
detergent: a molecular dynamics study. Nanotechnology, 2009. 20(4).
58. Strano, M.S., et al., The role of surfactant adsorption during ultrasonication in the
dispersion of single-walled carbon nanotubes. Journal of Nanoscience and Nanotechnology,
2003. 3(1-2): p. 81-86.
59. Nativ-Roth, E., et al., Physical adsorption of block copolymers to SWNT and MWNT:
A nonwrapping mechanism. Macromolecules, 2007. 40(10): p. 3676-3685.
60. Backes, C., et al., High Population of Individualized SWCNTs through the Adsorption
of Water-Soluble Perylenes. Journal of the American Chemical Society, 2009. 131(6): p. 2172-
2184.
61. Hunter, C.A. and J.K.M. Sanders, The Nature of Pi-Pi Interactions. Journal of the
American Chemical Society, 1990. 112(14): p. 5525-5534.
62. Fernandes, R.M.F., et al., Surface Coverage and Competitive Adsorption on Carbon
Nanotubes. Journal of Physical Chemistry C, 2015. 119(38): p. 22190-22197.
63. O'Connell, M.J., et al., Reversible water-solubilization of single-walled carbon
nanotubes by polymer wrapping. Chemical Physics Letters, 2001. 342(3-4): p. 265-271.
59
64. Star, A., et al., Preparation and properties of polymer-wrapped single-walled carbon
nanotubes. Angewandte Chemie-International Edition, 2001. 40(9): p. 1721-1725.
65. Gao, J., et al., Selective Wrapping and Supramolecular Structures of Polyfluorene-
Carbon Nanotube Hybrids. Acs Nano, 2011. 5(5): p. 3993-3999.
66. Moore, V.C., et al., Individually suspended single-walled carbon nanotubes in various
surfactants. Nano Letters, 2003. 3(10): p. 1379-1382.
67. Domun, N., et al., Improving the fracture toughness and the strength of epoxy using
nanomaterials - a review of the current status. Nanoscale, 2015. 7(23): p. 10294-10329.
68. Kim, J.A., et al., Effects of surface modification on rheological and mechanical
properties of CNT/epoxy composites. Carbon, 2006. 44(10): p. 1898-1905.
69. Nadiv, R., et al., Optimal nanomaterial concentration: harnessing percolation theory
to enhance polymer nanocomposite performance. Nanotechnology, 2017. 28(30).
70. Nadiv, R., et al., The multiple roles of a dispersant in nanocomposite systems.
Composites Science and Technology, 2016. 133: p. 192-199.
71. Reales, O.A.M. and R.D. Toledo, A review on the chemical, mechanical and
microstructural characterization of carbon nanotubes-cement based composites. Construction
and Building Materials, 2017. 154: p. 697-710.
72. Shtein, M., et al., Fracture behavior of nanotube-polymer composites: Insights on
surface roughness and failure mechanism. Composites Science and Technology, 2013. 87: p.
157-163.
73. Yang, K., et al., Effects of carbon nanotube functionalization on the mechanical and
thermal properties of epoxy composites. Carbon, 2009. 47(7): p. 1723-1737.
74. Allaoui, A. and N. El Bounia, Rheological and Electrical Transitions in Carbon
Nanotube/Epoxy Suspensions. Current Nanoscience, 2010. 6(2): p. 158-162.
75. Bauhofer, W. and J.Z. Kovacs, A review and analysis of electrical percolation in
carbon nanotube polymer composites. Composites Science and Technology, 2009. 69(10): p.
1486-1498.
76. Du, F.M., et al., Nanotube networks in polymer nanocomposites: Rheology and
electrical conductivity. Macromolecules, 2004. 37(24): p. 9048-9055.
77. Hu, G.J., et al., Low percolation thresholds of electrical conductivity and rheology in
poly(ethylene terephthalate) through the networks of multi-walled carbon nanotubes.
Polymer, 2006. 47(1): p. 480-488.
78. Sandler, J.K.W., et al., Ultra-low electrical percolation threshold in carbon-
nanotube-epoxy composites. Polymer, 2003. 44(19): p. 5893-5899.
79. Shtein, M., I. Pri-bar, and O. Regev, A simple solution for the determination of pristine
carbon nanotube concentration. Analyst, 2013. 138(5): p. 1490-1496.
60
80. Matarredona, O., et al., Dispersion of single-walled carbon nanotubes in aqueous
solutions of the anionic surfactant NaDDBS. Journal of Physical Chemistry B, 2003. 107(48):
p. 13357-13367.
81. Xu, H.X., B.W. Zeiger, and K.S. Suslick, Sonochemical synthesis of nanomaterials.
Chemical Society Reviews, 2013. 42(7): p. 2555-2567.
82. Grossiord, N., et al., Time-dependent study of the exfoliation process of carbon
nanotubes in aqueous dispersions by using UV-visible spectroscopy. Analytical Chemistry,
2005. 77(16): p. 5135-5139.
83. Blanch, A.J., C.E. Lenehan, and J.S. Quinton, Optimizing Surfactant Concentrations
for Dispersion of Single-Walled Carbon Nanotubes in Aqueous Solution. Journal of Physical
Chemistry B, 2010. 114(30): p. 9805-9811.
84. Hennrich, F., et al., The mechanism of cavitation-induced scission of single-walled
carbon nanotubes. Journal of Physical Chemistry B, 2007. 111(8): p. 1932-1937.
85. Lucas, A., et al., Kinetics of Nanotube and Microfiber Scission under Sonication.
Journal of Physical Chemistry C, 2009. 113(48): p. 20599-20605.
86. Pagani, G., et al., Competing mechanisms and scaling laws for carbon nanotube
scission by ultrasonication. Proceedings of the National Academy of Sciences of the United
States of America, 2012. 109(29): p. 11599-11604.
87. Blanch, A.J., C.E. Lenehan, and J.S. Quinton, Parametric analysis of sonication and
centrifugation variables for dispersion of single walled carbon nanotubes in aqueous solutions
of sodium dodecylbenzene sulfonate. Carbon, 2011. 49(15): p. 5213-5228.
88. Fagan, J.A., et al., Length fractionation of carbon nanotubes using centrifugation.
Advanced Materials, 2008. 20(9): p. 1609-1613.
89. Zhang, R. and P. Somasundaran, Advances in adsorption of surfactants and their
mixtures at solid/solution interfaces. Advances in Colloid and Interface Science, 2006. 123: p.
213-229.
90. Keeler, J., Understanding NMR spectroscopy. 2end ed. 2010: John Wiley and Sons,
Ltd.
91. Hore, P.J., Nuclear Magnetic Resonance. 1995: Oxford University Press.
92. Furo, I., NMR spectroscopy of micelles and related systems. Journal of Molecular
Liquids, 2005. 117(1-3): p. 117-137.
93. Nordstierna, L., I. Furó, and P. Stilbs, Mixed Micelles of Fluorinated and
Hydrogenated Surfactants. Journal of the American Chemical Society, 2006. 128(20): p.
6704-6712.
94. Szutkowski, K., P. Stilbs, and S. Jurga, Proton Chemical Exchange in Aqueous
Solutions of Dodecylammonium Chloride: Effects of Micellar Aggregation. The Journal of
Physical Chemistry C, 2007. 111(43): p. 15613-15619.
61
95. Farrar, T.C. and E.D. Becker, Pulse and Fourier transform NMR: introduction to
theory and methods. 1971: Academic Press.
96. Kowalewski, J. and L. Maler, Nuclear Spin Relaxation in Liquids: Theory,
Experiments, and Applications. 2006: CRC Press.
97. Hahn, E.L., Spin Echoes. Physical Review, 1950. 80(4): p. 580-594.
98. Price, W.S., NMR Studies of Translational Motion: Principles and Applications. 2009:
Cambridge University Press.
99. Stilbs, P., Fourier transform pulsed-gradient spin-echo studies of molecular diffusion.
Progress in Nuclear Magnetic Resonance Spectroscopy, 1987. 19: p. 1-45.
100. Fernandes, R.M.F., et al., Lateral Diffusion of Dispersing Molecules On Nanotubes
As Probed by NMR. Journal of Physical Chemistry C, 2014. 118(1): p. 582-589.
101. Frise, A.E., et al., Polymer Binding to Carbon Nanotubes in Aqueous Dispersions:
Residence Time on the Nanotube Surface As Obtained by NMR Diffusometry. Journal of
Physical Chemistry B, 2012. 116(9): p. 2635-2642.
102. Stejskal, E.O. and J.E. Tanner, Spin diffusion measurements: spin echoes in the
presence of a time-dependent field gradient. Journal of Chemical Physics, 1965. 42: p. 288-
292.
103. Brumberger, H., Modern Aspects of Small-Angle Scattering. 2010: Springer
Netherlands.
104. Lipfert, J. and S. Doniach, Small-angle X-ray scattering from RNA, proteins, and
protein complexes. Annual Review of Biophysics and Biomolecular Structure, 2007. 36: p.
307-327.
105. Ballauff, M., et al., Small-angle X-ray scattering on latexes. Macromolecular
Chemistry and Physics, 1996. 197(10): p. 3043-3066.
106. Ballauff, M., SAXS and SANS studies of polymer colloids. Current Opinion in Colloid
& Interface Science, 2001. 6(2): p. 132-139.
107. Pedersen, J.S., Small-Angle Scattering from Surfactants and Block Copolymer
Micelles, in Soft Matter Characterization, R. Borsali and R. Pecora, Editors. 2008, Springer
Netherlands. p. 191-233.
108. Pedersen, J.S., Analysis of small-angle scattering data from colloids and polymer
solutions: modeling and least-squares fitting. Advances in Colloid and Interface Science,
1997. 70: p. 171-210.
109. Pedersen, J.S. and C. Svaneborg, Scattering from block copolymer micelles. Current
Opinion in Colloid & Interface Science, 2002. 7(3-4): p. 158-166.
110. Plazzotta, B., et al., Core Freezing and Size Segregation in Surfactant Core-Shell
Micelles. Journal of Physical Chemistry B, 2015. 119(33): p. 10798-10806.
62
111. Nelson, T.R. and S.M. Tung, Temperature dependence of proton relaxation times in
vitro. Magnetic Resonance Imaging, 1987. 5(3): p. 189-199.
112. Nakayama, M., J. Akimoto, and T. Okano, Polymeric micelles with stimuli-triggering
systems for advanced cancer drug targeting. Journal of Drug Targeting, 2014. 22(7): p. 584-
599.
113. Neradovic, D., C.F. van Nostrum, and W.E. Hennink, Thermoresponsive polymeric
micelles with controlled instability based on hydrolytically sensitive N-isopropylacrylamide
copolymers. Macromolecules, 2001. 34(22): p. 7589-7591.
114. Oerlemans, C., et al., Polymeric Micelles in Anticancer Therapy: Targeting, Imaging
and Triggered Release. Pharmaceutical Research, 2010. 27(12): p. 2569-2589.
115. Phillips, D.J. and M.I. Gibson, Towards being genuinely smart: 'isothermally-
responsive' polymers as versatile, programmable scaffolds for biologically-adaptable
materials. Polymer Chemistry, 2015. 6(7): p. 1033-1043.
116. Qin, S.H., et al., Temperature-controlled assembly and release from polymer vesicles
of poly(ethylene oxide)-block-poly(N-isopropylacrylamide). Advanced Materials, 2006.
18(21): p. 2905-2909.
117. Rapoport, N., Physical stimuli-responsive polymeric micelles for anti-cancer drug
delivery. Progress in Polymer Science, 2007. 32(8-9): p. 962-990.
118. Rijcken, C.J.F., et al., Triggered destabilisation of polymeric micelles and vesicles by
changing polymers polarity: An attractive tool for drug delivery. Journal of Controlled
Release, 2007. 120(3): p. 131-148.
119. Wohl, B.M. and J.F.J. Engbersen, Responsive layer-by-layer materials for drug
delivery. Journal of Controlled Release, 2012. 158(1): p. 2-14.
120. Leon, I., et al., Water Encapsulation by Nanomicelles. Angewandte Chemie-
International Edition, 2014. 53(46): p. 12480-12483.
121. Fernandes, R.M.F., et al., Dispersing Carbon Nanotubes with Ionic Surfactants under
Controlled Conditions: Comparisons and Insight. Langmuir, 2015. 31(40): p. 10955-10965.