ISSN 1744-683X

www.rsc.org/softmatter Volume 7 | Number 22 | 21 November 2011 | Pages 10485–11032

1744-683X(2011)7:22;1-I

REVIEWLei Zhao and Zhiqun LinSelf-assembly of non-linear polymers at the air/water interface: the eff ect of molecular architecture

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Cite this: Soft Matter, 2011, 7, 10520

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Self-assembly of non-linear polymers at the air/water interface: the effect ofmolecular architecture

Lei Zhaoab and Zhiqun Lin*ab

Received 23rd April 2011, Accepted 1st June 2011

DOI: 10.1039/c1sm05745f

In comparison with conventional linear polymers, due to the presence of joints, branches, end groups,

etc, non-linear polymers possess more variables that allow for the tailoring of steric constraints,

stacking interactions, and hydrogen bonding of these polymers at the air/water interface to precisely

control the self-assembly process to yield functional Langmuir–Blodgett (LB) films for various

applications. This Review summarizes recent developments in the field of self-assembly of non-linear

polymers at the air/water interface, focusing on the influence of molecular architecture. Four classes of

non-linear systems (polymer brushes, star-like polymers, dendritic polymers, and linear–dendritic

polymers) with representative examples are highlighted to elaborate their interfacial behaviors

originating from different molecular architectures.

Introduction

Due to the surface effect, polymer thin films and micro- or

nanostructured patterns usually adopt different configurations

aDepartment of Materials Science and Engineering, Iowa State University,Ames, IA, 50011, USA; Fax: +1-515-294-7202; Tel: +1-515-294-9967bSchool of Materials Science and Engineering, Georgia Institute ofTechnology, Atlanta, GA, 30332, USA. E-mail: zqlin@iastate.edu; Web:http://zqlin.public.iastate.edu

Lei Zhao

Lei Zhao received a B.S. degree

in Materials Science and Engi-

neering from Xidian University,

Xi’an, China in 2005, and

a Masters degree in Materials

Science and Engineering from

the University of Science and

Technology of China, Hefei,

China in 2008. He is currently

a PhD student under the super-

vision of Prof. Zhiqun Lin in the

Department of Materials

Science and Engineering at Iowa

State University. His doctoral

research mainly focuses on the

self-assembly of block polymers

at the air/water interface. His research interests also include

synthesis and characterization of multifunctional nanocrystals,

block polymers, and semiconductor organic–inorganic nano-

composites for energy applications.

10520 | Soft Matter, 2011, 7, 10520–10535

or self-assembled states from those in bulk, resulting in unique

surface structures and chemical compositions, and thus,

intriguing properties, including friction, shearing, lubrication,

abrasion, wetting, adhesion, adsorption, and indentation.1–5 In

addition, hierarchical structures composed of polymers that

exhibit controlled ordering at different length scales are highly

desirable for many applications in optical, electronic, optoelec-

tronic, and magnetic materials and devices.6 Self-assembly has

been widely recognized as a most promising route to organizing

Zhiqun Lin

Zhiqun Lin received a B.S.

degree in Materials Chemistry

from Xiamen University in 1995,

a Masters degree in Macromo-

lecular Science from Fudan

University, Shanghai, China in

1998, and a PhD degree in

Polymer Science and Engi-

neering from UMass, Amherst

in 2002. He joined Iowa State

University as an Assistant

Professor in 2004 and was

promoted to Associate Professor

in 2010. His research interests

include multi-arm star-like

functional block copolymers,

semiconductor organic/inorganic nanocomposites, block copoly-

mers, hierarchical structure formation and assembly, phase equi-

librium and phase separation kinetics, and multifunctional

nanocrystals. He is a recipient of an NSF Career Award and a 3M

Non-Tenured Faculty Award.

This journal is ª The Royal Society of Chemistry 2011

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pre-programmed building blocks into hierarchically ordered

structures.7–9 In this context, a fundamental understanding of the

supramolecular structure and dynamic self-assembly process at

the surface is of great importance in designing and engineering

new generation polymer films and patterns with novel functional

properties. Recent research has witnessed rapid advances in

polymer synthesis techniques that yield a myriad of polymers

with unique molecular architectures and properties, and their

self-assembly in bulk form are fairly well understood.10–12 In

stark contrast, their self-assembly in ultrathin films and the

resulting supramolecular organization at surfaces and interfaces

are far less researched.12

One of the most commonly used methods to scrutinize the

supramolecular structure and dynamic self-assembly of polymers

in ultrathin films or at an interface is the Langmuir–Blodgett

(LB) technique, which renders the self-assembly of polymers at

an air/water interface under well controlled conditions.13–15 The

Langmuir–Blodgett (LB) method, named after Irving Langmuir

and Katharine Blodgett, is perhaps the earliest approach to

realize what is now called ‘supramolecular assembly’,16 providing

the opportunities to exercise molecular level control over the

structure of organic thin films.16 Various characterization tech-

niques, including high-resolution X-ray diffraction,17–20 scanning

probe microscopy,15,21–24 electron microscopy,25,26 and Brewster

angle optical microscopy27–30 can be integrated with the LB

trough and have proven to be effective at addressing many

fundamental questions regarding the LB film. Several compre-

hensive books31–33 and reviews15,34,35 describing the state-of-the-

art LB method are available, offering many aspects of the

background science, from LB film deposition and characteriza-

tion to their applications.

To date, a great diversity of molecules and polymers, primarily

linear, have been investigated with the LB technique, and the

resulting films have been widely used in the areas of micro-

lithography,36–39 thin film devices,13,15,40–45 and biomimetic

films.46–49 Among them, linear amphiphilic block copolymers

(BCP) are the most well studied systems. Ever since the seminal

work by Eisenberg and Lennox,50 the self-assembly of different

linear BCPs, e.g., polystyrene-b-poly(ethylene oxide) (PS-b-

PEO)36,51,52 and polystyrene-b-poly(methyl methacrylate) (PS-b-

PMMA),53–56 have been intensively investigated, and different

models, including ‘‘pancake’’ and ‘‘brush’’ models, have been

proposed to successfully illustrate the self-assembly

process.36,57–60 Upon deposition of these polymers on the water

surface, the unfavorable interfacial interaction between the

hydrophobic block and water induces the aggregation of

hydrophobic blocks, forming different morphologies to reduce

the overall free energy of the system. On the other hand, the

hydrophilic block tends to adsorb on the water surface or

dissolve in the water subphase depending on the surface pressure,

forming so-called surface micelles.36,51 During this process, the

hydrophobic block acts as a buoy, anchoring the polymer chain

at the air/water interface and preventing the hydrophilic blocks

from dispersing into the water subphase. Under the applied

surface pressure, the hydrophilic blocks are desorbed from the

water surface and pushed into the water subphase, yielding

the so-called ‘‘brush’’ structure (i.e., transitioning from the

‘‘pancake’’ to the ‘‘brush’’61). During this transition, the surface

area decreases dramatically while the surface pressure is

This journal is ª The Royal Society of Chemistry 2011

maintained constant, resulting in a plateau region in the

isotherm. The length of the plateau region is determined by

several parameters, including the ratio of hydrophilic block to

hydrophobic block and the chemical composition of the BCP.

Consequently, three characteristic regions (i.e., liquid, plateau,

and condensed regions) are yielded for amphiphilic linear BCPs.

For amphiphilic linear BCPs, the resulting monolayer

(supramolecular organization) depends heavily on the charac-

teristics of the blocks (e.g., amphiphilicity, solubility, molecular

weight, block ratio, etc.) and processing conditions (e.g., solvent

used, concentration of spreading solution, temperature,

compression speed, etc.).36,50,57,59 Among these variables, the

shape of the surface micelles of linear BCPs is found to depend

mainly on the relative size of the two blocks. Circular micelles are

usually formed at high hydrophilic block composition and rod-

shaped micelles are preferentially formed when the proportion of

the hydrophilic block is reduced, while a uniform structure of

surface micelles is no longer observed if the hydrophilic block

content is too low. Therefore, the structure and chemical

composition of LB films can be readily tuned by designing the

polymer chains and modifying the relative block ratio. These LB

films, however, are still limited for use in the areas of surface

modifcation and patterning where self-assembled structures with

higher complexity and larger variety are needed.

In order to obtain surface morphologies other than dots and

rods (or ribbons), a rational molecular design that allows for the

manipulation of steric constraints, stacking interactions due to

crystallization and p–p stacking, and hydrogen bonding is

postulated to be critical to precisely control the self-assembly

process. The variation in the shape of molecules, architecture of

the polymer backbone and specific intermolecular interactions

have been proven to be very effective in tailoring the air/water

interfacial behavior of polymeric materials.12 Thus, polymers

with more complex structures have been exploited to modify

their self-assembly process and ultimately well controlled

morphology. This activity has in turn stimulated the proper

design of a large number of polymers with new structures and

chemical compositions. To date, self-assembly of many non-

linear polymers, including polymer brushes, star copolymers,62–66

and dendritic polymers,67–69 at the air/water interface have been

studied, in which a broad range of morphologies, such as

dots,51,66,69,70 spaghetti,36,59,70,71 ribbons,50,68,69 islands,66,68,69

continents,70,71 wormlike,72 twister,73 and bicontinuous

networks,65 were observed.

For non-linear BCPs, the presence of joints, branches, and

a low degree of entanglement significantly alters the physical

properties compared to their linear counterparts, resulting in

a totally different but more controllable self-assembly process.74

In addition to structural influences (both chemical composition

and chemical structure), the functional end groups in non-linear

BCPs play a much more important role in determining their self-

assembly behaviors. Simple modification of the end group can

dramatically affect the physical properties of polymers. As noted

above, changing the hydrophobic/hydrophilic ratio is the most

viable means of controlling the morphology. In contrast to linear

BCPs, in which alteration of the ratio can only be realized by

changing the percentage of hydrophobic and hydrophilic blocks,

the branched structures in non-linear BCPs offer more variables

to modify the ratio, including the architecture (e.g., the number

Soft Matter, 2011, 7, 10520–10535 | 10521

Fig. 1 Schematic illustration of phase transition of polymer brushes

from (a) a rod-like to (b) a globule state caused by partial desorption of

side chains. The fraction of adsorbed side chains fa is a function of

spreading coefficient S, the length of side chains n, and the grafting

density h. Reprinted with permission from ref. 81, (2001 American

Chemical Society).

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of arms in the star-like polymers, grafting density in polymer

brushes) and functional end groups because of increased ends at

the periphery of the chain, which in turn enable the self-assembly

of much more complex yet well controlled structures.

Over the past decade, the self-assembly of non-linear polymers

has received much attention due to increased synthesis

capabilities. The influence of the molecular architecture and the

existence of end groups on the self-assembly and supramolecular

organization of non-linear BCPs has not yet been systematically

outlined in the literature regardless of some models proposed for

specific polymers.68,75 This Review seeks to summarize the recent

developments in the field of self-assembly of non-linear polymers

at the air/water interface, focusing primarily on the influence of

molecular architecture. Four categories of non-linear systems,

i.e., polymer brushes, star-like polymers, dendritic polymers, and

linear–dendritic polymers are highlighted. Within each category,

we make no attempt to thoroughly cover the literature but rather

select a number of representative examples to elaborate the

interfacial behavior of these intriguing polymers originating

from their different molecular architectures.

Fig. 2 Scanning force microscopy (SFM) images of monolayers of

PnBA brushes transferred onto mica at different degrees of compression,

(a) at low pressure, (b) at medium pressure, (c) at high pressure. Image

size ¼ 600 � 400 nm2 in (a)–(c). (d) AFM image showing molecular

resolution of dense monolayer of gradient brushes at a higher degree of

compression. Image size ¼ 1200 � 800 nm2. Reprinted with permission

from ref. 81, (2001 American Chemical Society).

Polymer brushes

One the most studied non-linear polymers at the air/water

interface are graft polymers,76 also known as molecular brush or

comb-like polymers. They are typically synthesized by attaching

polymer chains along a flexible backbone with different grafting

densities.77–80 Significant advance in synthesis techniques make it

feasible to prepare various novel brush-like polymers, including

comb-like (identical type of side chains),81 centipede (two

different types of side chains),82 and barbed wire (multiple types

of side chains)82,83 that are named based on the presence of side

chains, possessing different kinds of architectures (e.g., tadpole,84

tablet-like,85 and bottle-shaped brushes10which are defined based

on the shape of polymer as whole). Depending on the distribu-

tion of side chains along the backbone, polymer brushes are also

classified as regular,86 random,78 and gradient brushes.87

Because of the steric repulsion of densely grafted side chains,

polymer brushes usually adopt a wormlike conformation in

solution.87–90 When adsorbed on the surface (water or solid

substrate), the interaction between the side chain and the surface

changes the orientation of the side chain relative to the backbone

and breaks the symmetry and the dimensionality of the

system,91,92 thus allowing for more conformations that depend on

the fraction of adsorbed side chains, fa (Fig. 1).75 Two distinct

conformations are usually observed, resulting from a competi-

tion between the energetically favorable interaction of side chains

with the substrate and entropically unfavorable extension of

adsorbed side chains. A brush with a high fa adopts a rod-like

conformation (Fig. 1a). Conversely, a brush with a relatively

small fa prefers a globular conformation favored by the aggre-

gation of desorbed side chains (Fig. 1b).87

As the free energy of these two conformations varies with

respect to the contact area and the interaction strength, the brush

was shown to undergo a phase transition at a certain surface

pressure or at a certain spreading coefficient.93 Such a confor-

mational transition was reported for cylindrical brushes of poly

(n-butyl acrylate) (PnBA) due to lateral compression at the air/

water interface.75,87,93 A series of three PnBA brushes with

10522 | Soft Matter, 2011, 7, 10520–10535

different lengths and grafting densities of the side chains were

investigated and clear morphological changes induced by the

lateral compression were observed (Fig. 2a–c). As the surface

pressure was increased, the polymer underwent a transition from

a rod-like to a globular conformation with a coexistence range

clearly present at medium pressure (Fig. 2b). This transition was

fully reversible. In addition, the side chain length and grafting

density, which determine fa, are key parameters governing this

transition. The transition became less distinct as the length of

side chain decreased and eventually vanished below a certain

critical value. The denser brushes exhibited a rod-globule tran-

sition, while the loose brushes (i.e., with a low grafting density)

retained their extended conformation under surface pressure. To

confirm the model, cylindrical molecular brushes with a gradient

grafting density along the backbone at the air/water interface

were investigated, revealing a transition from rod-like to the so-

called ‘‘tadpole’’ conformation: upon compression, the rod–

This journal is ª The Royal Society of Chemistry 2011

Fig. 4 (a) Chemical structure of the CBCP, which can be viewed as

a triblock polymer. (b) Schematic illustration of the packing of micro-

structures in the low-pressure region. (c–d) AFM height images of the

CBCP LB monolayer obtained after solvent evaporation for 2 h; (c) from

chloroform solution and (d) from toluene solution. The deposition

pressure, p ¼ 0. Scan size ¼ 10 � 10 m2. Reprinted with permission from

ref. 95, (2009 Royal Society of Chemistry).

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globule transition took place at the end where the brush was

densely grafted, leaving behind a molecule with a globular

‘‘head’’ and an extended ‘‘tail’’ (Fig. 2d).

In addition to the rod–globule transition observed within the

molecule, certain modifications of molecular architecture can

also lead to more complicated self-assembly processes, forming

various novel surface morphologies. Recently, the interfacial

behavior of a novel bottlebrush block copolymer (BBCP; i.e.,

polystyrene–polylactide (PS–PLA)) with ultrahigh molecular

weight was reported.94 Both PS and PLA chains are hydrophobic

and densely grafted along the polymethacrylate backbone

through the ester group. Because of steric crowding of the PLA

and PS arms, the BBCP adopted a relatively rigid cylindrical

morphology in solution. When placed on the water surface, the

PS part of the BBCP aggregated, forming dot-like domains to

reduce its surface energy (Fig. 3a). The presence of certain

hydrophilic moiety (i.e., ester group) led to an attractive inter-

action between PLA and the water subphase in comparison with

PS. As a result, PLA formed cylindrical arms surrounding the PS

domain (Fig. 3a). Rather than undergoing a ‘‘pancake’’ to

‘‘brush’’ transition as commonly seen in amphiphilic BCPs, the

pancake-like morphology of BBCP formed at low pressure

transformed into an interdigitated, and eventually an island-like

morphology at high pressure (Fig. 3b).

Recently, self-assembly of a newly synthesized, amphiphilic

comb block copolymer (CBCP) at the air/water interface was

systematically explored using the LB technique.95 The CBCP had

an ultra high molecular weight with PS arms grafted along one

block of the long hydrophilic backbone (Fig. 4a). The block with

PS arms assumed a rigid rod shape due to the steric crowding

between the arms, resulting in elongation of the backbone

polymer in that block. After being placed on the water surface,

ribbon-like structures were observed at zero surface pressure,

Fig. 3 (a) AFM images of the BBCP Langmuir monolayer obtained

after scratching off the top portion of the monolayer (left) and the

domain enclosed with a red dashed line and arms with a white dashed line

(right). Scan size ¼ 1 mm � 1 mm (left) and 0.3 mm � 0.3 mm (right). (b)

Schematic stepwise representation of the packing of microstructures of

BBCP from low to high pressures. Reprinted with permission from ref.

94, (2009 American Chemical Society).

This journal is ª The Royal Society of Chemistry 2011

and an increase in self-assembly time led to an increase in the

ribbon length (Fig. 4c). It was found that two CBCP molecules

aggregated head to head to yield a pair (i.e., red B blocks on the

top and the bottom with blue C blocks in the middle). Then this

polymeric pair assembled side by side with adjacent pairs to form

a long ribbon (for example, ribbon 1 in Fig. 4b). The long

hydrophilic B chains were adsorbed on the water surface around

the chemically linked PS arms (i.e., C blocks), keeping the

neighboring ribbons apart. Since the tip of ribbon was not

shielded by B blocks, free molecules could diffuse to the tip site

and self-assemble along the axis, resulting in the increase in

ribbon length over time. The morphological evolution under

surface pressure was explored. Upon compression, the

‘‘pancake’’ to ‘‘brush’’ transition occurred: hydrophilic B blocks

partially submerged into the water subphase to form a brush-like

structure, and consequently, the ribbons associated side by side.

At extremely high surface pressure, the hydrophilic chains were

completely submerged into water, and the PS chains aggregated

to form an island-like morphology. By using a less volatile

solvent, a state relatively closer to the final equilibrium state was

achieved by providing CBCP molecules with a longer time to self

assemble, yielding a cellular pattern (i.e., interconnected network

structures; Fig. 4d) composed of ribbons of the same width and

height as those shown in Fig. 4c. This phenomenon can be

attributed to the dewetting of the thin film. During the solvent

evaporation, the concentration of CBCP was greatly increased,

leading to the aggregation of PS chains (i.e., C blocks) and thus

the formation of ribbon-like structures to reduce the overall free

energy of the system. Because the formation of a cellular pattern

could eliminate the existence of ribbon tips, a position possessing

high free energy, the ribbons tended to be connected, forming

a continuous cellular pattern.

In addition to carbon-based polymers, the monolayer

assembly of comb polymers containing fluorocarbon side chains

Soft Matter, 2011, 7, 10520–10535 | 10523

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have also been extensively investigated by several groups. These

fluorinated polymers possess unique tribological behavior due to

the low energy surface96 and negative surface potential.97 They

also have excellent thermal stability and chemical resistance for

practical use as lubricants at low temperature.98 Recently,

Langmuir monolayer of comb polymers containing (per-

fluoroalkyl)ethyl and (partialfluoroalkyl) side chains with

various chain lengths have been explored by AFM, friction force

microscopy (FFM), and out-of-plane and in-plane X-ray

diffraction.99 For all the comb polymers studied, the fluoro-

carbon side chains were found to pack into ordered two

dimensional lattice structures in the condensed state. As shown

in Fig. 5, hexagonal, orthorhombic, and triclinic packing of

fluorocarbon side chains were revealed by in-plane X ray

diffraction measurements. The characteristic friction behavior of

the multilayer films composed of comb polymers prepared by the

LB technique depended strongly on the chemical constituents of

the outermost layer on the film surface.99

Notably, various other fluorocarbon comb copolymers (e.g.,

binary copolymers100–102 and ternary comb copolymers103) were

also studied. The fine structure in the solid state and phase

transition behavior of methacrylate comb copolymers with

fluorocarbon and hydrocarbon side chains obtained from

immiscible monomers of octadecyl methacrylate and 2-(per-

fluorodecyl) ethyl methacrylate (FF10EMA) at different ratios

were reported.101 The results showed that fluorinated and

hydrogenated side chain crystals of these comb copolymers were

independently packed and formed hexagonal subcells in a two

dimensional lattice. The phase transition independently occurred

at different temperature regions between the fluorocarbon and

hydrocarbon side chain crystals during the heating process, as

measured by differential scanning calorimetry (DSC).101 AFM

Fig. 5 The in-plane XRD patterns (a) for X-type multilayers of the

fluorinated comb polymers, together with (b) the estimated lattice

structures for molecular packing of the fluorocarbon side chains in the

X-type films. Reprinted with permission from ref. 99, (2004 Wiley).

10524 | Soft Matter, 2011, 7, 10520–10535

measurements showed that the size of hydrogenated domains in

these phase-separated, methacrylate monolayer structures were

at the submicron scale (i.e., island-like structures as illustrated in

Fig. 6b), whereas corresponding acrylate copolymer monolayers

formed a surface morphology on the 10–30 nm scale (i.e., island-

like structures in Fig. 6a). This was caused by the hindrance of

packing of fluorinated side chains due to the arrangement of the

low mobility main chains. Recently, self-assembly of ternary

comb polymers containingN-vinylcarbazole (NVCz) in the main

chain obtained by copolymerizing hydrogenated and fluorinated

long-chain vinyl compounds was also reported (Fig. 7).104 It was

found that monolayers of these ternary copolymers on the water

surface were highly condensed, and side chains and side-chain

crystals cannot form phase-separated structures in two-dimen-

sional films due to the enhancement of p–p interactions between

the arranged carbazole rings.104 The side chains of the copolymer

in the film were in a miscible state, and monolayers formed

a homogeneous amorphous surface because of the cancellation

of the difference in van derWaals forces between the two types of

side chains (i.e., octadecyl acrylate and 2-(perfluorodecyl)ethyl

acrylate).104

Due to a large electric field associated with aligned helix

dipoles, polymer brushes of a-helical synthetic polypeptides are

of particular interest for use in colloid stabilization, actuation

and sensing, nanotechnoloy, and biotechnology.105–107 Recently,

a series of studies have focused on the brush BCP consisting of

a hydrophobic block of hairy-rod poly(g-methyl-L-glutamate-

ran-g-stearyl-L-glutamate) with stearyl substituents

(PMLGSLG) and a hydrophilic block of rigid-rod poly-

(R,L-glutamic acid) (PLGA) (Fig. 8).107–110 The monolayers of

these diblock copolypeptides exhibited a brush structure

composed of densely packed parallel a-helices that were tilted at

an angle with respect to the water surface normal (Fig. 8).108 The

helix orientation of such diblock copolypeptides was strongly

affected by the chain length.111 The average tilt angle between the

helices and the surface normal decreased upon compression due

to an increase in chain density. The hydrophobic block length

was found to significantly affect the maximum surface chain

density and thus the helix orientation of the diblock copoly-

peptides. The helix tilt in the monolayer can be attributed most

likely to the off-axis interactions of the unscreened peptide

Fig. 6 Schematic illustration of molecular arrangement and surface

morphology of fluorinated acrylate (left) and fluorinated methacrylate

(right) copolymer films. Reprinted with permission from ref. 101, (2007

Wiley).

This journal is ª The Royal Society of Chemistry 2011

Fig. 7 Chemical structure of N-vinylcarbazole (NVCz): octadecyl

acrylate (OA): 2-(perfluorodecyl)ethyl acrylate (FF10EA) ternary comb

polymer. Reprinted with permission from ref. 104, (2010 American

Chemical Society).

Fig. 8 Chemical structure of PLGA-b-PMLGSLG and simple sche-

matic representation of the double-brush structure of the PLGA-b-

PMLGSLG LB monolayer. Reprinted with permission from ref. 108,

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dipoles between parallel a-helices. The correlation of the helix tilt

angle and the PMLGSLG block length arose from the change in

the tilting potential as a function of the number of dipoles.107 The

influence of the dipping position relative to the previous transfer

position on the azimuthal helix orientation was also investi-

gated.110 The dipping position can be manipulated, yielding an

azimuthal alignment of a-helices in the transferred films parallel

to or tilted by an angle of 45� with respect to the dipping direc-

tion.110 In addition, the effects of annealing on the structure and

helix orientation in the monolayer were also explored.108 Upon

annealing at 100 �C for 24 h, a-helices became less tilted toward

the surface normal, and the surface area reduced, forming more

compact and uniform films. Moreover, the annealing process

facilitated the removal of the bound water, leading to significant

interchain interactions between the carboxylic group of the

PLGA segments. The melting of the side chain mantle into

a liquid-like state of the PMLGSLG block during annealing, and

then favored the helices to be further oriented toward the surface

normal.

Star-like polymers

Star-like polymers are also an important class of non-linear

polymers, in which several linear polymer chains (i.e., arms) are

attached to one compact core. These chains can be chemically

similar or different, thus producing heteroarm or miktoarm

This journal is ª The Royal Society of Chemistry 2011

star-like polymers.112 Star-like polymers with compact, low-

generation dendrimers as a core are usually called dendritic

stars.113,114 The self-assembly of star-like polymers at the air/

water interface was only studied rather recently due mainly to the

difficulty in their synthesis with high structural uniformity.76

Star-like BCP can behave like either a linear BCP or a uni-

molecular micelle (i.e., composed of single copolymer molecule),

or somewhere in between, depending heavily on the number of

arms and physical properties of the arms. With an increased

number of arms, the molecules become more compact, resulting

in less entanglement and more defined supramolecular organi-

zation. When absorbing on the surface, the unfavorable inter-

action with the surface leads to the formation of globular

structures,115 while the favorable interaction usually allows the

arm to spread, adopting an extended conformation.116–118

One of the earliest studies on star-like polymers was reported

in 1991.119 An amphiphilic star-shaped copolymer composed of

a hydrophobic styrene–divinylbenzene (St–DVB) microgel core

and hydrophilic poly(methacrylic acid) arms was investigated for

the purpose of improving the thermal and mechanical stabilities

of LB films which were rather poor when produced from linear

BCPs.119 These amphiphilic macromolecules were spread on the

water surface in aggregates, which finally dissolved in the water

subphase with the increase of the hydrophilic components of the

polymer. Due to limited availability of characterization tech-

niques at that time, the comprehensive understanding of the self-

assembly process was not possible.

Recently, the self-assembly of a series of amphiphilic three-

arm star BCP with different molecular structure and chemical

composition at the air/water interface has been explored. The

interfacial behavior of a three-arm star amphiphilic BCP, PEO3-

b-PS3 (inner PEO block and outer PS block), was investigated by

performing AFM measurements on the monolayer deposited on

mica.62 Spontaneous aggregation was observed after spreading

the polymer solution on the water surface. Hexagonally packed

spherical domains were seen at low compression pressures. It was

then transformed into a rod-rich morphology at intermediate

pressures, and clusters and collapse of the film occurred at high

pressures. Later on, the surface morphology of the three-arm star

BCP, (PS-b-PEO)3 (inner PS block and outer PEO block) con-

taining the same inner PS core but with different lengths of PEO

block at the air/water interface was examined.120 They possessed

circular domains at low pressure. Longer PEO chains were found

to lead to greater intermolecular separation, resulting in smaller

circular domains. While for shorter PEO chains, closely packed,

larger domains were yielded. The number of domains increased

as surface pressure increased. Eventually, the domains were

pushed close enough to undergo chaining in which both the

domain area and aggregation number increased dramatically.

Such micelle chaining occurred more readily in stars than in

linear systems of similar chemical composition. The surface

micelle chaining process was carefully explored. The circular

domains were composed of aggregated PS blocks separated by

PEO coronas (Fig. 9a and b).120 Upon compression, the PS cores

were brought closer together. The PEO chains between the PS

cores were thus forced to shift to the side, forming a higher local

density. Eventually, the PS cores moved close enough to chain

with one another, and the PEO chains were pushed to either side

of elongated PS aggregation.120 Thus further chaining was more

Soft Matter, 2011, 7, 10520–10535 | 10525

Fig. 9 Schematic illustration of surface micelle chaining process. (a) Side

view, (b) top view, and (c) the distorted hexagonal geometry around one

domain when a dimer forms. Reprinted with permission from ref. 120,

(2005 American Chemical Society).

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likely to occur in the area of low PEO density, and the most

probable location for additional aggregation was the position

a as illustrated in Fig. 9c.120

Subsequently, the influence of the chemical composition and

the molecular architecture on the self-assembly of both conven-

tional and dendritic-like star BCPs containing either a PS core

and PEO corona or vice versa were systematically explored

(Fig. 10).121 Three distinct regions in the isotherm were seen when

the stars contained 20% or more PEO, which absorbed at the air/

water interface, forming pancake-like structures. The length and

the pressure of the pseudoplateau region increased with PEO

content, indicating that the molecular arrangement can be

controlled by varying the relative ratio of the two blocks.

Additionally, the architecture as reflected through the placement

Fig. 10 The model depicting the effect of architecture on the self-

assembly process. When PEO is the core, PS tends to dominate the

process, leading to greater intra- and intermolecular PS aggregation

(top). By contrast, when PEO resides in the corona, more spreading

occurs (bottom). The hydrophobic PS aggregates in either structure.

Reprinted with permission from ref. 121, (2005 American Chemical

Society).

10526 | Soft Matter, 2011, 7, 10520–10535

of PEO in either the core or corona and the number of branches

also greatly influenced the self-assembly process (Fig. 10).121 The

PEO-core star BCPs were more compact while the PS-core star

BCPs spread more. A larger number of branches in the core led

to greater spreading for PEO but higher compactness for PS; and

PEO chain length determined the surface film behavior more

than the number of PEO branching.121

Another interesting molecular structure explored at the air/

water interface is the so-called ‘‘Janus’’ structure, which contains

two types of polymer chain grafted on each side of a core.

Recently, a ‘‘Janus micelle’’, an asymmetric star BCP, was crafted

by cross-linking the spherical domains of the polybutadiene (PB)

middle block of polystyrene-block-polybutadiene-block-poly

(methyl methacrylate) triblock copolymers (inset in Fig. 11).122

The ‘‘Janus micelle’’ can be divided into a ‘‘northern’’ hemisphere

composed of PS and a ‘‘southern’’ hemisphere composed of

PMMA.122 The ‘‘Janus micelle’’ formed arrays of circular

domains (Fig. 11). By contrast, the monolayer of the precursor

triblock copolymer was mainly composed of significantly smaller

elongated domains due to their much lower molecular weight. In

the ‘‘Janus micelle’’, PMMA chains spread out on the water

surface, while the hydrophobic PS and PB chains dewetted from

the water surface. As the number of Janus micelles forming

a domain in the monolayer was similar to the number of Janus

micelles associating in solution, it was concluded that association

already occurring in the spreading solvent influenced the

domains finally formed in the monolayer.122

It is worth noting that in addition to chemical composition and

molecular architecture, the functional end group also exerted

a profound influence on the self-assembly of star-like BCPs at the

air/water interface. For example, the role of combined functional

terminal groups in (X-PEO)2-(PS-Y)2 heteroarm star copolymers

was investigated with respect to their interfacial behavior and

surface morphology.66 Various terminal functional groups (i.e.,

X and Y), including bromine, amine, tert-butyldiphenylsilane

(TBDPS), hydroxyl, and carboxylic groups, were combined and

explored.66 The results showed that the opposite nature of

polymer chains and their end groups (e.g., hydrophobic chains–

hydrophilic ends or hydrophilic chains–hydrophobic ends) were

crucial in the formation of a stable circular morphology, rather

than a cylindrical morphology expected for the given chemical

Fig. 11 Scanning force microscopy image of Janus micelle monolayer

deposited on mica at higher lateral pressure. Insert shows the architecture

of the ‘‘Janus micelle’’ generated by cross-linking the spherical domains of

the polybutadiene middle block followed by dissolution. Reprinted with

permission from ref. 122, (2001 American Chemical Society).

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composition of star BCPs.66 The hydrophobic terminal groups

linked to hydrophilic PEO chains anchored them on the water

surface, preventing neighboring micelles from merging. Mean-

while, adding hydrophilic amine terminal groups to the PS chains

changed the aggregation behavior and also prevented the lateral

aggregation of PS domains.66 Additionally, it was found to be

more effective in producing a stable and very fine circular

nanodomain morphology when the functional end groups of

hydrophobic chains were modified.66

It is noteworthy that the number of arms in the amphiphilic

star-like BCPs discussed above was usually very limited and did

not exceed more than six, thereby making their self-assembly at

the air/water interface similar to that of linear BCPs (i.e.,

hydrophilic chains form a partially submerged pancake-like

structure as the hydrophobic chains collapse into segregated

globules). To reveal the effect of the crowding of arms attached

to a single core on the interfacial behavior and surface

morphology, a series of PEOnPSn star BCPs with a large number

of arms and different chemical compositions were studied.123 The

LB monolayer morphology was found to be determined by the

fraction of hydrophilic PEO. With an increase of PEO content,

the morphology changed from cylindrical domains to circular

domains, and finally to a bicontinuous network. At low PEO

content, the shorter PEO chains limited contact between PEO

arms of neighboring molecules, thereby allowing the PS chains to

form continuous one-dimensional structures via lateral aggre-

gation.123 As the PEO content increased, the cylindrical structure

collapsed into densely packed circular micelles due to increased

repulsion originating from neighboring PEO arms. With the

highest PEO content, the star polymer formed a virtually

uniform morphology with an underlying PEO layer covered with

a PS phase, leading to a bicontinous morphology.123 Besides the

PEO content, the number of arms also had a crucial influence on

the morphology of monolayers.123 By reducing the crowding of

PS and PEO chains, for polymers with the highest PEO content,

a novel dendritic superstructure (branched textures) was

observed upon compression, which was caused by microphase

segregation at the air/water interface and PEO crystallization due

to reduced constraints from the lowered number of arms in the

molecules.123

Recently, the self-assembly of an amphiphilic heteroarm star

polymer containing 12 alternating hydrophobic/hydrophilic

arms of PS and poly(acrylic acid) (PAA) connected to a well-

defined rigid aromatic core air/water interface was studied

Fig. 12 (a) Chemical structure and (b) molecular model of (PAA25)6-s-

(PS25)6. Reprinted with permission from ref. 65, (2004 American

Chemical Society).

This journal is ª The Royal Society of Chemistry 2011

(Fig. 12).65 Under compression, confined phase separation of

dissimilar polymer arms occurred upon their segregation on the

opposite sides of the rigid disk-like aromatic core, with PAA

phase dissolved in water and PS aggregating laterally in air,

forcing the rigid cores to adopt a face-on orientation with respect

to the interface.65 As a result, pancake-like micellar aggregates

formed in the gas state and gradually merged into a dense

monolayer as the surface pressure increased.65

Dendritic polymers

Due to the promising properties stemming from functionalized

polymer chains and nanoparticle-like compact molecular archi-

tecture, dendritic polymers have received considerable attention

over the past decades.124–131 Two major classes of dendritic

polymers are hyperbranched polymers and dendrimers.132 Den-

drimers have regular, tree-like architectures, divergent from

a single, point-like core with very regular branches radially

extending from a single center.76 Because of their well-defined

globular architecture, the dendrimers at the air/water interface

have been extensively investigated.69 The ‘‘edge on’’ and ‘‘face

on’’ models have been widely used to explain the self-assembly of

a variety of dendrimers at the air/water interface.69 We refer the

reader to several comprehensive reviews on dendrimers and their

assemblies.133–136 Here we only focus on the hyperbranched

polymers with tree-like architectures, which are quite similar to

dendrimers but have a lower degree of branching and less regular

architecture.76 The morphology and overall shape of hyper-

branched polymers as well as their interfacial behavior can also

be altered by modifying the internal chemical architecture, the

nature and distribution of terminal groups, and the strength of

the polymer/surface interaction.69,137 Compared to dendrimers,

despite the similarity in architecture, hyperbranched polymers

show a very different self-assembly process, which is due to their

much higher molecular weight and less defined architectures.

Recently, several second-generation hyperbranched polyesters

with a variable composition of alkyl-terminated groups have

been synthesized (Fig. 13).132 Chemical modification of the

hyperbranched cores by substituting a controlled fraction of the

terminal hydroxyl groups with hydrophobic alkyl chains was

found to be an effective method to control amphiphilic balance

of hyperbranched cores, and thus their self-assembly at the air/

water interface.132 Microstructural analysis showed that the

organized monolayers formed at the air/water interface only

Fig. 13 Chemical structure of hyperbranched polyesters. Reprinted with

permission from ref. 132, (2003 American Chemical Society).

Soft Matter, 2011, 7, 10520–10535 | 10527

Fig. 15 Molecular structure of a dendritic PS–PEO block copolymer.

Reprinted with permission from ref. 68, (2008 American Chemical

Society).

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when the number of alkyl tails was higher than two per core. In

this case, the alkyl tails of hyperbranched molecules at high

surface pressure formed intramonolayer ordering of the quasi-

hexagonal type, which was similar to regular dendrimers

(Fig. 14).132 Due to irregular branching and random attachments

of the terminal alkyl tails in the hyperbranched molecules, the

alkyl tails became arranged in an up-right orientation at high

pressure, instead of crystallizing with regular lateral ordering

usually observed for modified dendrimers (Fig. 14).132

Because of their compact structure (i.e., smaller hydrodynamic

volume and radius of gyration) and high concentration of

functional terminal groups, dendritic polymers usually have

much less interchain entanglement as compared with linear

polymer counterparts; this promotes the formation of ‘‘unim-

icelle’’ (i.e., surface micelles composed of only a single molecule).

Recently, the interfacial behavior of dendritic PS–PEO copoly-

mers was researched (Fig. 15).68 Such a dendritic copolymer

formed a ‘‘unmicelle’’ that possessed a large hydrophobic PS core

surrounded by a thin hydrophilic PEO corona. The dendritic PS–

PEO unimolecular micelles displayed properties comparable to

sterically stabilized colloids (Fig. 16a). Their self-assembly was

governed by a competition between van der Waals attractive

forces between adjacent PS cores and the repulsive steric inter-

action between the coronas due to the elastic deformation of

PEO chains.69 The interaction of dendritic copolymers can also

be modified by varying the branching functionality (i.e., gener-

ation) and composition, as well as by adjusting the length of the

chains in the core, to control the structural rigidity of the

molecule.

For low PS chain branching samples, the film topology was

greatly influenced by composition for copolymers incorporating

a relatively low number of long, flexible PS segments. With an

increase of the PEO content from 15% (Fig. 16b) to 31%

Fig. 14 Top: models of hyperbranched polymer with a random orien-

tation of alkyl terminal tails (left) and in a compressed state with

a standing-off orientation of alkyl tails and flattened polar core (right).

Center: illustration of molecular conformation in condensed monolayer

state for molecules with low (left) and high (right) number of alkyl tails.

Bottom: illustration of molecular packing for Langmuir monolayer at

low (left) and high (right) surface pressures. Reprinted with permission

from ref. 132, (2003 American Chemical Society).

10528 | Soft Matter, 2011, 7, 10520–10535

(Fig. 16c), the topology evolved from large circular island-like

domains to ribbon-like superstructures. For low PEO content

samples, van der Waals interactions between the PS cores were

more significant, leading to the coalescence of molecules into

large island-like clusters (Fig. 16b).69 With an increase of the

PEO content, repulsive forces became dominant to prevent

coalescence, the islands gradually collapsed into ribbons

(Fig. 16c).69 The branching of PS core also governed the polymer

association. Highly branched samples were more rigid and the

deformability of the PEO chains in hyperbranched samples was

lower than that in low branched samples, leading to a lower

degree of association. Consequently, the hybranched molecules

remained isolated or formed low-order aggregates with the

decrease of the PEO content (Fig. 16d).69 When aggregated,

individual molecules due to their very stiff structure are clearly

visible and are mainly arranged into a regular, hexagonally

packed array.

Later on, the influence of surface pressure and subphase

temperature on the association of these copolymers was

Fig. 16 AFM amplitude images of (a) G1-5PS-HB-74 copolymer; (b)

G1-30PS-LB-15 copolymer; (c) G1-30PS-LB-31 copolymer; and (d) G1-

30PS-LB-22 copolymer. G1: twice-grafted (generation G1). LB: low

branching, and HB: high branching. Reprinted with permission from ref.

69, (2008 Wiley).

This journal is ª The Royal Society of Chemistry 2011

Fig. 18 Chemical structure of amphiphilic hyperbranched polyester.

Reprinted with permission from ref. 63, (2004 Wiley).

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investigated.68 Under surface pressure, the isolated molecules can

associate into short ribbon-like structures through the side-by-

side assembly. As shown in Fig. 17, the molecules in the gas state

formed a ‘‘unimicelle’’ consisting of a PS-rich rigid core sur-

rounded by adsorbed flexible PEO chains. During compression,

the increased van der Waals attractive forces between the PS

cores became larger than the elastic deformation forces of the

flexible PEO chains, resulting in the association of molecules.68 In

the meantime, sideways displacement of the PEO chains led to an

increased lateral PEO chain density, rendering the exclusive

formation of ribbons with a width and thickness comparable to

the size of the individual molecules.68 When the subphase

temperature increased from 12 �C to 27 �C, the same effect was

also observed even in the absence of compression. The temper-

ature-induced association was attributed to increased van der

Waals attractive forces between the PS cores relative to the steric

repulsive forces between PEO chains in the coronas, which was

induced by decreased solvent quality of PEO segments at higher

temperature.68

It is well known that only dendrimers with peculiar symmetry

architecture can form self-assembled, one-dimensional supra-

molecular structures, such as rods, fibers, ribbons, and

helices.138–140 Recently, one-dimensional supramolecular struc-

tures were shown, for the first time, to be assembled from

irregular, highly branched polymer.63 The molecule used was

a hyperbranched polyester modified with terminal alkyl and

amino groups (Fig. 18). The isolated individual nanofiber was

achieved at low surface pressure (Fig. 19a). With an increase in

surface pressure, individual nanofibers became more uniform

and well-defined. In the condensed state, they formed densely

packed bundles weaving across large surface areas with overall

lengths exceeding several micrometres (Fig. 19c,d).63 The mole-

cules possessed a hemispherical conformation in which the

Fig. 17 Influence of pressure on the self-assembly of high PEO content,

low branching functionality copolymers at (a) low pressure and (b) high

pressure; Scanning size ¼ 1.5 � 1.5 mm2. (c) Top view of the reversible

association model proposed for the reversible formation of ribbon-like

superstructures. Reprinted with permission from ref. 68, (2008 American

Chemical Society).

This journal is ª The Royal Society of Chemistry 2011

hydrophilic cores were compressed against the solid surface. The

dense lateral stacking caused by directional crystallization of

alkyl tails was proposed as a means of assembling these branched

molecules in one-dimensional continuous rows resembling

hemicylindrical micelles.63

Recently, the surface morphology of novel hyperbranched

amphiphilic (PEO–PS)n copolymers obtained by controlled

radical polymerization was reported.141 The molecular structures

of brush-like and hyperbranched polymers are shown in Fig. 20a.

In stark contrast to conventional linear and star PEO–PS BCPs,

dissimilar segments of these hyperbranched polymers were not

clearly separated into/out of water/air but rather formed a mixed

uniform phase at the surface (Fig. 20b).141The random chemical

architecture of relatively short hydrophilic and hydrophilic

Fig. 19 AFM phase images of nanofibrillar structures formed from low

pressure to extremely high pressure, p. (a) p¼ 0.2 mNm�1, (b) p¼ 5 mN

m�1, and (c, d) p ¼ 30 mN m�1. Reprinted with permission from ref. 63,

(2004 Wiley).

Soft Matter, 2011, 7, 10520–10535 | 10529

Fig. 20 (a) Chemical structure of two kinds of hyperbranched polymers.

(b) Two possible scenarios of state of the macromolecules at the air/water

interface in comparison with linear block copolymer (center). Left:

separated hydrophobic and hydrophilic segments. Right: mixed hydro-

phobic and hydrophilic segments confined to the water surface. Terminal

chains added to reflect the presence of terminal alkyl ends. Reprinted with

permission from ref.141, (2006 American Chemical Society).

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fragments was believed to be responsible for such mixed inter-

facial structure.

Linear–dendritic polymers

As noted previously, dendritic polymers are monodisperse, tree-

like macromolecules with a regular and highly branched archi-

tecture consisting of a dendritic core and peripheral sites. This

peculiar structural feature opens many opportunities for their use

in drug delivery (serving as host for foreign molecules),142 inter-

facial liquid membranes for stabilizing aqueous–organic emul-

sions, catalysis and reaction sites,143 etc. However, such compact

structure often leads to little or no entanglements, thus limiting

certain applications where highly robust and cohesive films are

needed. One way to overcome this problem is to capitalize on

a dendrimeric polymer containing a linear block or segment

(Fig. 21). The linear block increases entanglements, thus

enhancing the mechanical integrity, whereas the self-assembly

nature of BCPs can be exploited to assist the assembly of

Fig. 21 Molecular structure of a linear–dendritic polymer.

10530 | Soft Matter, 2011, 7, 10520–10535

organized films.144 Most importantly, the unique properties of

dendritic polymers can be easily retained in linear–dendritic

polymers. Because of such a novel hybrid structure, intriguing

interfacial behavior may be expected when self-assembled at the

air/water interface. Compared with the non-linear polymers

discussed above, linear–dendritic polymers cover a wider range

of materials, but have received far less attention to date.

Due to the compact structure, the dendritic components

usually assume a rigid rod-like morphology, thereby forming

a rod (semi-rod)–coil structure when a flexible linear chain is

attached to the dentritic block. Recently, the self-assembly of

PEO-dendronized polymethacrylate (PDMA) linear–dendritic

polymer (PEO–PDMA) was studied (Fig. 22).73 PDMA assumed

a coil–semirod molecular morphology. The interfacial behavior

of this macromolecule was very unique although the isotherm

was typical of an amphilphilic polymer with three distinct

regions. Wormlike surface micelles were first observed after an

increase in surface pressure (Fig. 23A). These micelles were

rather flexible and can be extended to accommodate the reduced

surface area under compression. As a result, when further

compressed, the micelles were orderly packed, and their long axis

was nearly perpendicular to the compression direction

(Fig. 23B).73 During this process, the short PEO chains were

immersed into the water subphase, and in the meantime the

PDMA blocks became close enough to assemble into the core of

the surface micelles. As the compression progressed, two typical

morphologies were observed in the plateau region of the

isotherm: the cylindical micelles became bent and broken

(Fig. 23C) and later twisted and even interwound into bundles

(Fig. 23D) with further increase in surface pressure.73 The

micelles can be twisted with both right- and left-handed sense

(Fig. 23D*). It was proposed that micelles packed in a parallel

fashion were forced to rotate along their long axis with the

increased compression, and the friction between the neighboring

micelles may cause such a micelle twisting.73 The surface

morphology of the LB film after collapse was also investigated.

Two types of characteristic 3D aggregates, springlike (Fig. 23E

and E*) and disklike (Fig. 23F) were obtained. These morpho-

logies were believed to originate from the rolled packing of the

micelles, i.e., compression at the final stage can squash the

‘‘springs’’ to give away the space therein, and the micelles then

collapsed and even joined together to form ‘‘disks’’.73

Fig. 22 Chemical structure of PEO–PDMA linear–dendritic block

polymer. Reprinted with permission from ref. 73, (2006 American

Chemical Society).

This journal is ª The Royal Society of Chemistry 2011

Fig. 23 A series of SEM images of LB films obtained at various surface pressures. Reprinted with permission from ref. 73, (2006 American Chemical

Society).

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Recently, a new kind of dendron–coil molecule with various

dendron structures and chemical functionalities was reported

(Fig. 24a).144,145 The molecule has a linear hydrophilic PEO block

and a dendritic hydrophobic polyamidoamine (PAMAM) block.

The dendrimer generations can be modified from one to four,

and the end group can be functionalized with a variety of groups.

Among these end groups, stearate terminated linear–dendritic

diblocks were found to be highly amphiphilic, and produced the

most stable monolayer at the air/water interface. Additionally,

dendrimer generation was found to be another key factor

determining the self-assembly process. Surface micelle formation

did not occur for generations 2 and higher, indicating a totally

different self-assembly process from linear block

copolymers.144,145

Using specular neutron reflectivity, the structure of the

monolayers formed from linear–dendritic diblock copolymers

Fig. 24 (a) Schematic illustration of linear–dendritic diblock copolymer

amphiphiles. (b–c) Schematic diagram of layered model used to calculate

reflectivity for the third generation stearate-functionalized linear–

dendritic diblock copolymer monolayer at (b) low pressure and (c)

extremely high pressure. (d) Schematic diagram of layered model used to

calculate reflectivity for the fourth generation stearate-functionalized

linear–dendritic diblock copolymer monolayer. Reprinted with permis-

sion from ref. 144 and 145, (1995 and 2002, American Chemical Society).

This journal is ª The Royal Society of Chemistry 2011

with third or fourth generation dendrons was measured. For the

third generation case (Fig. 24b), at low surface pressure, stearate

groups existed primarily on the top of the monolayer, minimizing

the interfacial energy at the air/water interface. They spread as

much as possible across the water phase to lower interfacial

tension, leading to a thinner monolayer with PEO being situated

on the subphase.144,145 Under extremely high pressure, the

monolayer was at its most compressed state (Fig. 24c). The

stearate groups can form a well-ordered phase oriented perpen-

dicular to the interface, yielding a thicker monolayer.144,145

Different from the third generation, where the stearate layer was

composed of stearate groups and located above the PAMAM

layer, the stearate layer, for the fourth generation case (Fig. 25),

was a mixture of stearate and PAMAM (Fig. 24d), whose

composition varied with the surface concentration. The inter-

mixing of stearate groups and PAMAM occurred since the outer

edge of the fourth generation dendrimer had significant curva-

ture.144,145 Thus the ordering of the stearate groups on the den-

drimer can be controlled by tuning the dendrimer generation and

surface pressure.

In comparison to the dendron–coil structure, dendron–rod

structure has been less researched. It is not surprising that more

novel self-assembly behaviors may be observed due to the rigid

structure of dendron–rod polymers. Recently, amphiphilic den-

dron–rod molecules with three hydrophilic PEO branches

Fig. 25 Chemical structure of the PEO–PAMAM linear–dedndritic

polymer. Reprinted with permission from ref. 145, (2002 American

Chemical Society).

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attached to a hydrophobic octa-p-phenylene rod stem at inter-

faces was investigated (Fig. 26a).72 Two kinds of amphiphilic

molecules, one terminated with methyl (molecule A) and one

with hydroxyl group (molecule B), were used in the study. The

terminal groups determined the self-assembly process although

both molecules had nearly identical architecture (i.e., a rod-like

hydrophobic fragment with highly branched hydrophilic side

chains). For molecule A, ring-shaped surface nanostructures

were found to randomly pack within the topmost surface of the

outer layer, and ribbon-like surface structure can be formed from

these molecules under extremely high lateral compression.72 In

the ring-shaped structure, the rod-like fragments were placed in

the interior due to steric interactions, and the bulkier hydrophilic

branches constituted the outer edge of the ring.72 Such structures

can be stabilized by strong hydrophobic interactions between

polyphenylene chains (right in Fig. 26b). Under further

compression, the bulkier hydrophilic branches were displaced

from the interface and submerged while rod-like fragments

interdigitated to compensate for remaining cross-sectional

mismatch, thereby yielding ribbon-like structures (left in

Fig. 26b).72

The presence of hydrophobic terminal groups in hydrophilic

PEO branches was considered to be a critical factor in the

formation of these surface structures as it stabilized the forma-

tion of organized surface structures. Replacement of the hydro-

phobic terminal groups in molecule A with hydroxyl groups in

molecule B led to the disappearance of these surface nano-

structures, leaving behind a uniform surface topography without

any characteristic surface micellar structures. In this case, the

amphiphilic balance, by adding 12 hydroxyl terminal groups,

was shifted toward highly hydrophilic branched PEO, resulting

in a much less stable monolayer as the molecules tended to

Fig. 26 (a) Chemical structures of molecule A, methyl-terminated

branches and molecule B, hydroxyl-terminated branches. (b) Molecular

model of the packing of molecule A, showing interdigitated layering (left)

and a circular planar structure (right). Reprinted with permission from

ref. 72, (2005 American Chemical Society).

10532 | Soft Matter, 2011, 7, 10520–10535

submerge into the water subphase at higher surface pressure

without forming organized 2D structures.72

Conclusions and outlook

In this Review, the interfacial behavior of four classes of non-

linear polymers, i.e., polymer brushes, star-like polymers,

dendritic polymers, and linear–dendritic polymers, have been

highlighted. However, they only represent a small fraction of

non-linear polymers, and many other nonlinear systems (e.g.,

crosslinked, arborescent, etc.) are not covered. Within the four

systems reviewed, the self-assembly of some intriguing architec-

tures at the air/water interface has not yet been explored (for

example, random and barbed wire brushes, although the polymer

brushes have been the most well studied non-linear system to

date). On the other hand, little work was reported on novel

linear–dendritic polymers. In sharp contrast to the intensive

research on the bulk system, the progress in the field of LB

monolayers of non-linear polymers is comparatively slow, which

may be due primarily to the availability of powerful characteri-

zation techniques to explore the much reduced dimensions of LB

films. Synchrotron-based techniques (scattering and reflectivity)

are of key importance to probe the configuration and crystalli-

zation of LB film due to the limited amount of materials to be

detected, and more interfacial assembly research will be per-

formed using this technique. With more knowledge and funda-

mental understanding gained from the comprephesive study, we

anticipate elucidation of the complex self-assembly process of

non-linear polymers at the air/water interface, which in turn will

provide the guidance to design new generation untra-thin films

and polymer patterns with novel properties and enhanced

performance.

We note that only a few surface micelles produced from the air/

water interface can be arrayed into well ordered structures, which

are of technological interest in the areas of microelectronics,

information processing and storage, and nano/micro-

fluidics.146,147 For example, ribbon-like surface micelles can be

readily prepared from polymer brushes. However, such ribbons

are usually randomly dispersed within the LB film. Several

techniques have emerged to arrange surface micelles within the

frame of LB methodology. First, the dewetting of a polymer

solution on the LB trough during the solvent evaporation

process can be harnessed to assemble surface micelles into

cellular patterns or parallel wires. Moreover, the shear force

induced by vertically withdrawing the solid substrate from the

water subphase can also be utilized mainly through the control of

the withdrawing speed and surface pressure to pattern surface

micelles.148–153 A number of ordered patterns (e.g., dendritic

structures148 and parallel stripes146,149–154) have already been

made possible. However, such successes are limited to several

polymer systems, such as L-a-dipalmitoylphosphatidylcholine

(DPPC),149–153 colloidal particles,75 and comb-block copolymer.

Non-linear polymers, especially those having extremely compact

structures (e.g., densely grafted brushes, star-like polymers with

more arms, dendritic polymers with more generations), have

much less intermolecular entanglements, which would facilitate

the arrangement of the molecules. For example, some hyper-

branched dendritic polymers, which behave like colloidal parti-

cles, are highly likely to be organized into parallel polymer stripes

This journal is ª The Royal Society of Chemistry 2011

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(or micochannels) using the strategy developed for colloidal

particles.75 Furthermore, ordered stripes with specific perfor-

mance can be readily designed by integrating different kinds of

polymer chain into the core of the dendritic polymers.

Another important area that may demandmuch research is the

specific applications of the resulting LB films. Current work on

non-linear systems is centered on conventional polymer chains,

such as PS and PEO, for fundamental research. A wide range of

non-linear polymers with advantageous optical, electrical,

optoelectronic properties can be synthesized. Of particular

interest is the non-linear polymer composed of luminescent

conjugated polymer chain,155 for example, poly(3-hexyl-

thiophene) (P3HT)156–158 and poly[2-methoxy,5-(20-ethyl-hexoxy)-1,4-phenylenevinylene] (MEH-PPV).159,160 The

application of the LB technique to conjugated polymers has

produced a variety of electrical and optical ultra-thin film

devices, such as light emitting diodes,161–166 thin film conduc-

tors,167–171 memory devices,172 ferroelectric thin films,173–175

nonlinear optical devices,176,177 sensors,178–182 and field-effect

transistors.183,184 The ability to pattern the conjugated non-linear

polymers may promise new opportunities to fabricate flexible

integrated circuits. The other technologically important func-

tional polymers are adaptive polymers. By integrating the

adaptive chains, which respond to very subtle changes in the

surrounding environment, such as pH,185–193 temperature,194–197

and light,198–200 into non-linear polymer systems, the resulting

surface structures may hold the promise of numerous applica-

tions in colloid stabilization,201 drug delivery and biomimetic

materials,46,202 chemical gates,203 and tuning lubrication, friction,

adhesion, and wettability for tailored surfaces.204–206

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