sonication induced morphological transformation between 3d gel

Colloids and Surfaces A: Physicochem. Eng. Aspects 452 (2014) 165172

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

journa l h om epage: www.elsev ier .com/ locate /co lsur fa

Sonicat tionetwor on

Feng Ye, College of Chem 31001

h i g h l i g h t s

Two-component gelator based on theinteraction between dendritic branchand diamin

The exchbetween globular st

Self-assemcomponen

g r a p h i c a l a b s t r a c t

Schematic diagram of the ultrasonic effects on the gelation process.

a r t i c l

Article history:Received 5 NoReceived in reAccepted 22 MAvailable onlin

Keywords:Two-componeMMASonicationMorphology

1. Introdu

There hators (LMWGinto a 3D gmolecular nVanderwaa[16]. The

CorresponE-mail add

http://dx.doi.o0927-7757/ ododecane.ange of morphology

a 3D gel network andructure.bly mechanism of two-t gelator.

e i n f o

vember 2013vised form 18 March 2014arch 2014e 3 April 2014

nt gelator

a b s t r a c t

A two-component gelator (B1 C12 B1) based on interaction between dendritic branch (Branch1(COOH)) and diaminododecane is designed and synthesized, and its self-assembly behavior in methylmethacrylate (MMA) is rstly investigated. It is interesting to nd that the gel formation depends onthe concentration and sonication during cooling of the B1 C12 B1/MMA solution. Sonication playsas a switch simulator for exchanging morphology between a 3D gel network and globular structure ina certain concentration range. Thermal stability, rheological properties, morphology and self-assemblymechanism of the gels were investigated with tube inversion methodology, Dynamic oscillatory measure-ment, SEM and FTIR, respectively. The results indicated that Sonication-induced-gel (S-gel) exhibits muchhigher gelsol temperature, storage modulus and lower critical gelator concentration compared withTemperature-induced-gel (T-gel), while sonication could partially destroy the hydrogen bond networkand benet the formation of the organogels.

2014 Elsevier B.V. All rights reserved.

ction

s been a surge of interest in low-molecular-weight gela-s), a family of organic molecules that can self-assembleel network in organic solvent through intra or inter-on-covalent interaction including hydrogen bonding,ls force, stacking and charge transfer interactionsubject of gels have attracted considerable attention

ding author. Tel.: +86 0571 88320855; fax: +86 0571 88320855.ress: [email protected] (X. Wang).

because of their special properties and potential applications ineld such as drug delivery, catalysis, cosmetics, food, tissue engi-neering, textile, the recovery of spilled crude oil and lubrication[712]. In order to obtain suitable organogels, numerous studieshave been carried out over the past several decades [1315]. Amongthose studies, development of smart or adaptive gels which can bemanipulated or switched by external responsive stimulus is verystill a challenge.

Stimuli-responsive gels have been attracted widespread atten-tion in are as ranging from chemistry and biology to materialsscience because of their potential applications in sensors, actuators,shape memories, drug delivery, hydrophilicity/hydrophobicity

rg/10.1016/j.colsurfa.2014.03.0842014 Elsevier B.V. All rights reserved.ion induced morphological transformak and globular structure in a two-comp

Si Chen, GuoDong Tang, Xu Wang

ical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou n between 3D gelent gelation system

4, China

166 F. Ye et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 452 (2014) 165172

modulation and so on [1620]. It is generally accepted that exter-nal stimuli such as light, electric, temperature and PH have a greateffect on switching of molecular aggregations involved in the for-mation of gel, micelles, vesicles and membranes [2124]. However,a simple physical method of controlling that provides not only areversible switching of original molecular aggregation but also thekinetic control of the hierarchical assembly of molecules remains achallenge. Recently, sonication as one of the external stimulationshas begun to play a signicant role in the organogel eld [2527]. Ingeneral, sonication is usually used to disrupt the weak non-covalentinteractions or to disintegrate aggregated particles but seldom tobe in favor of the assembly formation [3337].But recent reportsshowed that sonication could act as a stimulus to induce gela-tion of organic liquids with LMWGs. The Naota and Zhang groupsindependently observed sonication-induced gelation in hydrogen-bonded gelators for the rst time in 2005 [28,29]. More recently,Sijbesma and associates reported that sonication could cause rheo-logy switching and conformational changes, subsequently inducedsupramolecular self-assembly leading to gelation of the solvent[30,31]. Furthermore, Bardelang and associates have shown thatsonication may reshape sheet-like dipeptide particles into elon-gated molecular assemblies and that sonocrystallisation is at theorigin of gelation [32]. Although the studies on the sonication-induced reversible gelation have been reported stage by stage,effective manipulation of the morphology change in a gel systemby sonication still remains rarely.

In our present work, our research group has designed andsynthesized a two-component gelator (B1 C12 B1) based oninteraction between dendritic branch (Branch1 (COOH)) anddiaminododecane and rstly investigated the inuence of ultra-sonic stimulation for their self-assembly behavior in MMA. It isinteresting to nd that, during cooling of the B1 C12 B1/MMAsolution in a certain concentration range, gelation is observedexclusively when sonication is used as an external stimulus, while

deposit is obtained without sonication. The SEM results conrmedthat B1 C12 B1 deposit from MMA resulting in 2545 mglobular structure, while B1 C12 B1 xerogel from MMA undersonication leads to the gel network among 50150 nm formation.The results of tube inversion methodology, Dynamic oscillatorymeasurement and Fourier transform infrared (FTIR) indicated thatsonication acting as a simple physical stimulus could signicantchanges the self-assembly properties of gels. This paper provides anew way for the exchanging morphology between a 3D gel networkand globular structure, which are consequently able to achievethe supramolecular functional materials with controllable macro-scopic properties.

2. Materials and methods

2.1. Materials

l-Lysine methyl ester dihydrochloride (98%, Aladdin, China),Boc-protected l-lysine (99%, Aladdin, China), 1,3-dicyclohexyl car-bodiimide (DCC, 99%, Shanghai Covalent Chemical TechnologyCo., Ltd., China), 1,12-diaminododecane (98%, Aladdin, China),1-hydroxybenzo-triazole (HOBt, 99%, Shanghai Covalent Chem-ical Technology Co., Ltd., China), triethylamine (NEt3, Aladdin,China), sodium hydroxide (NaOH, 96%, Aladdin, China) wereused as received. Solvents including dichloromethane, methanol,ethanol, petroleum ether (PE) and ethyl acetate (EtOAc) (ARgrade, Aladdin China) were used as received. MMA (99%, Aladdin,China).

2.2. Syntheses of compounds

The l-lysine-based dendritic branch about Branch1 (COOH)was synthesized using standard amide coupling methodology(Scheme 1) according to Smith and co-workers [33]. Boc-protected

H

ture o+DCC ,HOBt

NEt3,EtOA c

NaOH

Ethanol

H3COOCNH2

NH2

HOOCNH

NHO

O

O

O

HOOCNH

NHO

O

HNO

O

NHO

O

NHO

O

HNO

O

Branch 1(COOH )

Scheme 1. Synthetic route and molecular struc3COOCNH

NHO

O

HNO

O

NHO

O

NHO

O

HNO

O

Branch1( COOMe)

f Branch1 (COOH).

F. Ye et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 452 (2014) 165172 167

OONH

OO

action

l-lysine wain the prescompound converted NaOH in mchromatogrfrom Aldricof two-comon interactdiaminodod

2.3. Charac

2.3.1. GelatThe gela

by using a 120 C and ture. If the of time in Mdenoted a Tsample treacalled an S-

2.4. Rheolo

Rheologgels by usinAustria). Thand sonicaof 25 mm ddynamic ossample wasmize solvenup to 100% 25 C.

2.5. FESEM

A Hitach(FESEM, HitThe samplegel formedformed by pared B1 to dry undegel and drygold before

IR sp

rier outtivelyples

reme

ults

l for

th12 h1 (C2, resere nd th

repino

in Ml fortion late once5 mMge os B1n-gethe rmatble Tled NH

NHO

O

NHO

O

NHO

O

NHO

O

NH2

CO

OH

B1C12B1

Scheme 2. Structure of two-component gelator (B1 C12 B1) based on inter

s reacted with l-lysinemethyl ester using DCC and HOBtence of Net3as base and EtOAc as solvent to yieldBranch1 (COOMe). The methyl ester was subsequentlyto Branch1 (COOH) by saponication with aqueousethanol. The target products were puried by silica gelaphy. In addition, the diaminododecane was purchasedh and used without further purication. The structureponent dendritic gelation system (B1 C12 B1) basedion between dendritic branch (Branch1 (COOH)) andecane is showed in Scheme 2.

terization

ion experimentstion tests on B1 C12 B1 was carried out with MMAtest-tube-inversion method. The tube was heated tothen put into a thermostat to cool to room tempera-prepared sample can form a gel when left for a periodMA (for example 10 min) at ambient conditions, it was-gel. The gel which was obtained when the preparedted by sonication (50 W, 40 kHz) for 10 min at 25 C wasgel.

gical measurements

ical measurements were carried out on freshly preparedg a controlled-stress rheometer (MCR302, Anton Paar,ese gels were obtained by a heating-cooling processtion irradiation, respectively. Parallel-plate geometryiameter and 1 mm gap was employed throughout thecillatory work. The following tests were performed: The

submitted to this parallel-plate very quickly to mini-t evaporation. Then increasing amplitude of oscillation

2.6. FT

Foucarriedrespecthe sammeasu

3. Res

3.1. Ge

For(B1 C(Branc1 and here wratio agelatorof diamgelatorthe gesonicanot geat a cabout the rancontainin a nowhen (gel foreversiassembapparent strain shear (kept a frequency of 1 rad s1) at

measurements

i S-4700 eld emission scanning electron microscopeachi, Japan) was used for the morphological analysis.s were prepared as follow: The B1 C12 B1/MMA

in a glass vial by sonication irradiation and deposita heating-cooling process, respectively. And the pre-C12 B1/MMA gel and deposit were severally allowedr vacuum to a constant weight. Then the resulting xero-

deposit were respectively coated with a thin layer of investigation.

shown in Fthis two-coof 1.59 mMcould also about 5 mMHowever, thication can in MMA [37

In addititemperaturtion upon hsolution wasonication. tens of timeNH

NHO

O

HN

NHO

O

NHO

O

HNO

O

H2N C

O

HO

between Branch1 (COOH) and diaminododecane.

ectra measurements

transform infrared (FTIR) spectra measurements were on a Nicolet 6700.The deposit and gel sample were

listed on a surface of glass sheet and the solvent of evaporated spontaneously at room temperature beforent.

and discussion

mation

e two-component dendritic gelation system B1), dendritic peptides based on the amino l-lysineOOH)) and diaminododecane were used as componentpectively [34,35]. All the gel-phase materials reportedgenerated by using a dendron/diaminododecane (2:1)e molar concentration of the two-component dendriticorted here was equivalent to the molar concentrationdodecane. The gelating ability of the two-componentMA was rst examined. It is interesting to nd that

mation in MMA depends on the concentration andirradiation [36]. We found that B1 C12 B1 couldin MMA by a traditionally heating-cooling processntration lower than 9 mM (self-assembled process

B1 C12 B1/MMA mixture is shown in Fig. 1b). Inf 19 mM, the cooling of a hot solution of a mixture C12 B1 and MMA at quiescent condition resultedlled liquids with numerous white deposit. However,concentration of B1 C12 B1 exceeded than 9 mMion concentration, CGC), a stable, opaque and thermal-gel was formed after a heating-cooling process (self-process about 10 mM B1 C12 B1/MMA mixture is

ig. 1c). Unexpectedly, when sonication was used inmponent gelation system in the concentration range

for 2 min during heating-cooling process, the S-gelbe formed (self-assembled process with sonication

B1 C12 B1/MMA mixture is shown in Fig. 1a).e CGC is reduced to 1.5 mM[46]. This implied that son-inuence the self-assembly properties of B1 C12 B1,38].on, it is interesting that the S-gel could be stable at roome, but was readily converted to the original hot solu-eating. The deposit was obtained again when this hots subsequently cooled to room temperature withoutThe heating-cooling cycle could well be performed fors without any tiredness [39].


Fig. 1. Images of the exchange process for two-component gelation system (B1 C12 B1) in MMA with or without ultrasound.

3.2. The thermal stability of organogels

In order to evaluate the effect of sonication on the thermal stabil-ity of two-component organogels in MMA, the gel-to-sol transitiontemperature (Tgel) of B1 C12 B1/MMA gel under different con-centrations were analyzed by a tube inversion methodology andthe results were showed in Fig. 2a. It can be observed that boththe Tgel values of S-gel and T-gel increase with the increasing ofB1 C12 B1 molar concentration and reach a horizontal regionabove a cerplateau reg(20 mM), wgel (42 C) agelsol tranHoff relatio

ln c = HRTge

where c is tsition enthatransition t

From thcentration, from the slFig. 2b, andTable 1. Thetion enthalpFrom the abmal stabilithas a signithe gels forto sonicatio

Table 1Gelation ability for B1 C12 B1 in MMA with or without ultrasound.

Gelation CGC (mM) Tgel (C) Hg (kJ mol1)

With ultrasound 1.5 95.5 38.5Without ultrasound 9 42 29.9

Note: CGC, critical gelation concentration in molar concentration of Branch (COOH);Tgel, gelsol transition temperature of the gels in the plateau region; Hg, enthalpyof gelsol transition of B1 C12 B1 in MMA.

osm

e mo

is ce o12 olingnditt (Fign wt and

invetinglroge C1eter

cleauting C1d entnneclly ex

Fig. 2. Gel-to-ultrasound antain concentration. Interestingly, the S-gel reaches theion at a lower gelator concentration (7 mM) than T-gelhilst S-gel shows the much higher Tgel (95.5 C) than T-t the plateau region. The thermodynamic analysis forsition of T-gel and S-gel were carried out using a vantnship [40,41].

l+ constant

he gelator molar concentration, H is the gelsol tran-lpy, R is the gas constant and Tgel is the gelsol phaseemperature.e relationship between Tgel and corresponding con-the gelsol transition enthalpy (H) was determinedope of ln c versus (Tgel)1.The plots are represented in

the resulting enthalpy for T-gel and S-gel are listed in results indicate that S-gel shows larger gelsol transi-y (H = 38.5 kJ mol1) than T-gel (H = 29.9 kJ mol1).ove analysis, it suggests that S-gel reveals higher ther-y than T-gel [42,43]. These results imply that sonicationcant inuence on the self-assembly of molecules, andmed by a thermal process and that formed in responsen process have different molecular aggregation mode at

microcerty.

3.3. Th

SEMinuenB1 CThe cocent codeposisolutiodeposiple areInteresand xeThe B1a diamgraphsdistribthe B1deneintercomaticasol phase transition temperature (Tg) versus concentration of Branch (COOH) in MMAd () without ultrasound, respectively.ic stratums thus generating distinct macroscopic prop-

rphology of the self-assembly

used to certicate that sonication has a signicantn the self-assembly of molecules [25,26]. Take 5 mMB1/MMA as a typical example, which is shown in Fig. 3.

of a hot solution of 5 mM B1 C12 B1/MMA at quies-ion results in a non-gelled liquid with numerous white. 3A) whilst a stable gel formed after cooling of the hotith ultrasonic treatment (Fig. 3B). The morphology of

xerogel obtained from 5 mM B1 C12 B1/MMA sam-stigated by SEM and the results are showed in Fig. 3.y, we nd that the morphology of B1 C12 B1 depositl strongly depend on the external sonication stimuli.2 B1 deposit from MMA gives ower-like ball with

in the range of 2545 m (Fig. 3a) [32]. The micro-rly indicate that there are vast tiny holes and wrinkles

on the surface of the ball (Fig. 3b and c). However,2 B1 xerogel from MMA under sonication gives a wellangled network with a diameter of about 50 nm that areted with each other and also shows the majority of dra-pansile holes in the range of 50150 nm among the gel (a) and vant Hoff plots (b) for corresponding organogels () with


Fig. 3. 5 mM B tively

network[32able enhancof the B1 of ower-linetwork byber netwoball in 5 mMporosity bin one owelike ball areoccurring unected fromS-gel showsgel for B1 plays a switgel networklar self-asse

3.4. Rheolo

Elasticityproperty ocal measurdynamic osof the gel

to 112 and ith sodulug-coo

of t12 sticiplert ftion-itch fion ce of

the tronystem-gel.1 C12 B1 samples formed (A) without and (B) with ultrasound stimulus, respec

]. As indicated by the aforesaid two cases, the remark-ement for the gelation capability and thermal stability

C12 B1 in MMA is correlated to the transformationke ball network to well dened interconnected ber

sonication [36,44]. Compared with the interconnectedrk occurring in the sonication induced gels, ower-like

B1 C12 B1/MMA shows more compact and lessers network on the surface. Probably because ber armr-like ball hardly interpenetrate to one another, ower-

spatially detached whilst interconnected ber networknder sonication have branched bers that intercon-

one network to the adjacent networks [44], therefore higher gelation capability and thermal stability than T-C12 B1/MMA system [32]. Moreover, sonication alsoch simulator for exchanging morphology between a 3D

and globular structure in MMA through supramolecu-mbly in a certain concentration range.

gical properties

is an important structural-directing macroscopic

0.01% B1 Cment gels wage mheatinolutionB1 Cthe elathe sam

Apasonicathe swformatthe casulus oftimes sS-gel swith Tf a gel network [45]. For comparison, the rheologi-ements for the above samples are studied by usingcillatory measurements. The linear viscoelastic regionsis determined by strain amplitudes ranging from

3.5. FTIR sp

It is wellin the form

Fig. 4. Storage modulus G of the two-component or. The corresponding micro/nano-structures were shown by SEM.

00% at 1 rad s1.The storage modulus of the 5 mMB1/MMA samples which obtained by sonication treat-heatingcooling process are showed in Fig. 4a. Theonication treatment shows the plateau of value stor-s G 6700 pa, whereas the non-gelled sample withling process shows the magnitude of G below the res-he instrument (


Fig. 5. FTIR spectra of the xerogel and deposit formed from 5 mM B1 C12 B1/MMA solution with and without ultrasound, respectively.

in the assembly aggregates is that the gel sample and deposit ofB1 C12 B1 changed into clear solution immediately by additionof a few drops of methanol, a solvent known to be capable of break-ing hydrogen bonds [46]. In order to clarify the effect of sonicationon gel formation, we measured the IR spectrum of B1 C12 B1deposit from MMA without sonication and B1 C12 B1 xerogelfrom MMA with sonication, respectively [41]. The results are shownin Fig. 5. It 3296.69, 16shifted in co1648.47. Thtions in the

clear solution at elevated temperature was cooled down withoutexternal sonication stimuli, strong H-bonds between the moleculeswould occur and nally caused the deposit of the compounds insolvent. However, if the solution was subjected to the sonication,some of the hydrogen bond would be destroyed and an organogelwas formed [26,43]. In order to further conrm this viewpoint,we put the B1 C12 B1 deposit in MMA solvent to sonicate and

at orent s

undartiagel fcan be observed that three strong bands appeared at86.51, 1640.27 cm1 for the deposit, while there is red-mparison with that of the xerogel at 3330.66, 1692.40,is indicates that there are stronger H-bond interac-

precipitate than in the xerogel sample [26]. When the

nd thsuremcoolingcould porganoScheme 3. Schematic diagram of the ultrasonic effects oganogel is also formed after several minutes. FTIR mea-howed the same spectrum as that of the gel formed byer sonication. Therefore, we speculate that sonicationlly destroy the hydrogen bond network beneting theormation.n the gelation process.


3.6. Possible process for the hierarchical self-assembly oftwo-component gelator self-assembly in MMA with or withoutsonication stimulus

On basis[47,34,48,4two-composonication wesized that (Branch1 (Ccomplex (Bbrous seehydrogen bkinetic pathmode [50] During thethe strong tion and hetgrowth of gintroduced This can be brous seednucleation-nucleation aof gel ber suppressingbetween a in turn chan

4. Conclus

A two-cobetween dewas designegel networkand a heatininversion mand FTIR, wical stimuluthermal stasonication cbenets thenew approatrollable ma

Acknowled

Financiaof China (GInnovative acknowledg

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Sonication induced morphological transformation between 3D gel network and globular structure in a two-component gelation ...1 Introduction2 Materials and methods2.1 Materials2.2 Syntheses of compounds2.3 Characterization2.3.1 Gelation experiments

2.4 Rheological measurements2.5 FESEM measurements2.6 FTIR spectra measurements

3 Results and discussion3.1 Gel formation3.2 The thermal stability of organogels3.3 The morphology of the self-assembly3.4 Rheological properties3.5 FTIR spectra3.6 Possible process for the hierarchical self-assembly of two-component gelator self-assembly in MMA with or without soni...

4 ConclusionsAcknowledgementsReferences

sonication induced morphological transformation between 3d gel

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