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Colloids and Surfaces A: Physicochem. Eng. Aspects 346 (2009) 184–194

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

journa l homepage: www.e lsev ier .com/ locate /co lsur fa

he synthesis of P(MAn-co-St)-b-PS-b-P(MAn-co-St) block copolymers by RAFTolymerization and the nanostructure of their self-assembly aggregates

ui Ma a, Rongming Ma b, Lingling Feng c, Liren Fan a, Yan Liu c, Bin Xing c, Yubang Hou c, Feng Bao c,∗

Engineering Research Center of Nano-Geomaterials of Ministry of Education, China University of Geosciences, Wuhan 430074, ChinaDepartment of Chemistry and Life Science, Xianning College, Xianning 437005, ChinaDepartment of Chemistry, Central China Normal University, Wuhan 430079, China

r t i c l e i n f o

rticle history:eceived 23 January 2009eceived in revised form 10 June 2009ccepted 10 June 2009vailable online 18 June 2009

a b s t r a c t

A series of block copolymers of styrene, maleic anhydride and acrylic acid were synthesized by the reverseaddition–fragmentation chain transfer (RAFT) process. The structure, molecular weight and polydisper-sity index were determined by FTIR, 1H NMR, SEC&MALLS and DSC analysis. The results showed thatthe polymerization occurred in a living and controlled manner. Multiple self-assembled nanostructuresof these block copolymers were investigated by transmission electron microscopy (TEM). Tetrahydro-furan (THF), N,N-dimethylformamide (DMF) and 1,4-dioxane were used as the common solvents and

eywords:iving radical polymerizationeversible addition–fragmentation chainransfer (RAFT process)elective solventanostructure

twice-distilled water as the selective solvent to clarify the effects of the solvent. The results revealed thatwith the increase of the extension degree of the core, non-spherical aggregates were easily formed, thecomposition of the copolymers influences the aggregation behavior, and other factors also influence theself-assembly, such as hydrolysis, temperature, annealing time, molecular architecture etc. A mechanismis proposed to illustrate the formation of the various aggregates of P(MAn-co-St)-b-PS-b-P(MAn-co-St)

onfirm

ggregateself-assembly

copolymer, which were c

. Introduction

Living polymerization has recently emerged as one of the mostffective synthetic routes to well-defined polymers [1–4]. Whatistinguishes the Reversible Addition–Fragmentation chain Trans-

er (RAFT process) polymerization is that it can be used with aide range of monomers and reaction conditions and in each case

t provides controlled molecular weight polymers with very nar-ow polydispersities [5–9]. Therefore RAFT polymerization is byar one of the most valuable practical synthetic tools. The controlf RAFT polymerization is achieved by performing it in the pres-nce of a suitable thiocarbonylthio compound that acts as a highlyfficient reversible addition–fragmentation chain transfer agent. Aimplified mechanism elucidated by Rizzardo et al. [10], involves aeversible chain transfer process in which a thiocarbonylthio agent

species able to initiate polymerization. The thiocarbonylthio isransferred between the active and dormant chains, thus main-aining the controlled/living character of polymerization. Recently,

dditional aspects of the mechanism and kinetics of RAFT have beennvestigated [11–16], such as RAFT polymerization at ambient tem-erature by adjusting the structure of thiocarbonylthio compound,

n emulsion polymerization technique and under irradiation.

∗ Corresponding author. Tel.: +86 27 63233058; fax: +86 27 67867955.E-mail address: polymerbaofeng@yahoo.com.cn (F. Bao).

927-7757/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2009.06.012

ed by TEM results.© 2009 Elsevier B.V. All rights reserved.

It has been well known that when block copolymers are dis-solved in a solvent that is selective for one of the blocks, colloidalsize aggregates or micelles can form as a result of the associationof the insoluble blocks. Self-assembly has become one of the majortopics in the colloid and interface field [17–18]. Block copolymersare able to self-assemble into various ordered nanostructures thatare of wide scientific and technological interest as a consequenceof the incompatibility of the constituent blocks [19–23]. In the 21stcentury, the design and controlled fabrication of materials withstructures on the nanoscale is a challenge for the modern materialsciences. The interplay between the different kinds of molecules cangive rise to exciting phenomena [24–26], leading to various proper-ties by designing the molecular arrangement. Recently, numeroustypes of morphologies have been found in dilute solutions of blockcopolymers, that is, spherical micelles, rods, vesicles, lamellae,tubes, large compound micelles (LCM) and hollow spheres [27–34].The study of these special aggregates may throw light on the under-standing of the self-assembly of biological systems. What is more,the aggregates could have potential applications in drug deliverysystems, stable dispersions, foaming, the construction of soft bio-materials, etc.

Recently, polymers containing maleic anhydride and styreneco-polymerized blocks have attracted interest for their appli-cations in improving the compatibility of polymer blends andbinding intensifier for metals on polar substrates [35,36]. More-over, well-defined block copolymers containing maleic anhydride

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R. Ma et al. / Colloids and Surfaces A: Ph

ay have an effect on the aggregation behavior under given con-ition. Herein, a series of block copolymers of styrene and maleicnhydride P(MAn-co-St)-b-PS-b-P(MAn-co-St) was obtained first,sing dibenzyl trithiocarbonate (DBTTC) as the chain transfergent (CTA). Then multi-block copolymer P(MAn-co-St)-b-PS--P(MAn-co-St)-b-PAA-b-P(MAn-co-St)-b-PS-b-P(MAn-co-St) wasynthesized using the above P(MAn-co-St)-b-PS-b-P(MAn-co-St)s the macro-CTA. The nanostructures of the block copolymersn selective solvents were analyzed by TEM. We hope this work

ay contribute some meaningful information to understanding theontrol factors of the self-assembly of block copolymers in selec-ive solvents. So far, the polymerization system, tailored polymersnd their self-assembly aggregation behaviors have not been thor-ughly reported.

. Experimental

.1. Materials

Styrene was purchased from Tianjin Chemical Plant, was stirredver CaH2 overnight and distilled under reduced pressure beforese. Maleic anhydride and 2,2-azoisobutyronitrile (AIBN) wereurchased from Shanghai Chemical Plant and were recrystallizedefore use. Unless specified, all other reagents purchased from com-ercial sources were of analytical purity and used without further

urification

.2. Synthesis of DBTTC

The DBTTC was synthesized by reacting CS2 and 33% aque-us NaOH with benzyl chloride using a phase transfer catalystn-Bu4HSO4) [37].

.3. Polymer synthesis

.3.1. Synthesis of P(MAn-co-St)-b-PS-b-P(MAn-co-St)All polymerizations were carried out in a 100-mL flask with

rubber stopper and a magnetic stir bar. To start our experi-ents, 0.0773 g DBTTC, 0.0212 g AIBN, 10 mL 1,4-dioxane and theonomers styrene and maleic anhydride (with the molar ratio of

:1) were added to the flask under a nitrogen atmosphere and thenufficiently degassed. The sealed flask was immersed in an oil-bathhermostat at 60 ◦C. After the set reaction time, the polymerizationas stopped by cooling. The polymer was precipitated by methanol

nd dried in vacuum for 24 h to give yellow powders. Then theonversion of the polymerization was determined gravimetrically.he polymerization conditions of the polymers are summarized inable 1.

.3.2. Synthesis of P(MAn-co-St)-b-PS-b-P(MAn-co-St)-b-PAA-b-(MAn-co-St)-b-PS-b-P(MAn-co-St)

An appropriate amount of P(MAn-co-St)-b-PS-b-P(MAn-co-St)s the macro-CTA, 0.0203 g AIBN, 1 mL acrylic acid and 10 mL

able 1olymerization conditions of polymers synthesized by RAFT.

olymers Styrene(mL)

Maleic anhydride(g)

RAFT agent(g)

AIBN

(MAn-co-St)67PS201P(MAn-co-St)67

11.2 1.2011 0.0773a 0.021

(MAn-co-St)44PS132P(MAn-co-St)44

11.2 1.2015 0.0775a 0.02

(MAn-co-St)-b-PS-b-PAA-b-PS-b-P(MAn-co-St)

11.2 1.2011 0.0770b 0.021

a Dibenzyl trithiocarbonate as RAFT agents.b P(MAn-co-St)67PS201P(MAn-co-St)67 as macro-RAFT agent.

hem. Eng. Aspects 346 (2009) 184–194 185

1,4-dioxane were added to the 100-mL flask under a nitrogenatmosphere and then sufficiently degassed. The sealed flask wasimmersed in an oil-bath thermostat at 60 ◦C. After the set reac-tion time, the polymerization was stopped by cooling. The polymerwas precipitated by n-hexane and dried in vacuum for 24 h to giveyellow products.

2.4. Preparation of solutions and aggregates

The block copolymer P(MAn-co-St)-b-PS-b-P(MAn-co-St) solu-tions were prepared by dissolving the copolymers, in tetrahydrofu-ran (THF), 1,4-dioxane and N,N-dimethyllformamide (DMF), whichare common solvents for both PS and P(MAn-co-St) blocks, withstirring overnight at room temperature. The initial polymer con-centration was 2 wt %. To prepare their aggregates, twice-distilledwater, which is a precipitant for the PS block, was added dropwiseto the copolymer solutions with vigorous stirring, with one dropadded every 10–15 s. When the precipitating agent content reachedca. 90 wt%, the solutions were placed in a dialysis tube (regener-ated cellulose tube, Mw cut-off 8000, USA) and dialyzed againsttwice-distilled water for at least 3 days to remove the common sol-vents. The twice-distilled water for dialysis was changed twice aday. Additionally, the samples were stored for 3–7 days, dependingon the water content, to ensure that true equilibria were reached.For the multiblock copolymer P(MAn-co-St)-b-PS-b-P(MAn-co-St)-b-PAA-b-P(MAn-co-St)-b-PS-b-P(MAn-co-St), the self-assembledcondition is similar to P(MAn-co-St)-b-PS-b-P(MAn-co-St). THF,1,4-dioxane and DMF are used as common solvents; twice-distilledwater is a precipitant for the P(MAn-co-St)-b-PS-b-P(MAn-co-St)block.

2.5. Measurements

Infrared spectra were recorded on a PerkinElmer Spectrum OneFTIR instruments using KBr tablets. The spectra were obtained overa frequency range of 4000–400 cm−1 at a resolution of 4 cm−1. 1HNMR spectra were obtained on a Mercury 600 NB NMR spectrome-ter (Varian Inc., USA) with 600 MHz at room temperature in CDCl3(for copolymer) solution using tetramethylsilane as the standard.

The light scattering intensities of the block copolymer solu-tion were determined with a multi-angle laser light scattering(MALLS) instrument equipped with a He–Ne laser (� = 633 nm;DAWN®DSP, Wyatt Technology Co., Santa Barbara, CA, USA) in theangles of 43◦, 49◦, 56◦, 63◦, 71◦, 81◦, 90◦, 99◦, 109◦, 118◦, 127◦,136◦ and 152◦ at 25 ◦C. The redistilled THF was used as solvent.The polymer solutions of desired concentrations were prepared,and optical clarification of the solution was achieved by filtra-tion through a 0.45 �m pore size filter (PTFE, Puradisc 13 mm

syringe Filters, Whattman, England) into the scattering cell. Therefractive index increment (dn/dc) of the styrene–maleic anhydrideblock copolymer was measured by using a double-bleam differ-ential refractometer (DRM-1020 Otsuka Electronics Co., Japan) at633 nm and 25 ◦C. Then the obtained dn/dc value was set as the

(g) Acrylic acid(mL)

1,4-Dioxane(mL)

Reaction time(h)

Temperature(◦C)

Conversion(%)

2 – 10 10 60 53

08 – 10 14 60 64

2 1 10 24 60 77

1 ysicochem. Eng. Aspects 346 (2009) 184–194

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86 R. Ma et al. / Colloids and Surfaces A: Ph

arameter during size exclusion chromatography combined withALLS (SEC-LS) measurement. SEC-LS was carried out on a size

xclusion chromatograph combined with multiangular laser pho-ometer, mentioned above, combined with a P100 pump (Thermoeparation Products, San Jose, USA). It was equipped with columnsf G4000H8 (MicroPak, TSK) combined with G3000H8 (MicroPak,SK) and the differential refractometer, and run at 25 ◦C. The car-ier solution was redistilled, dust-free treated and degassed THF.he THF and polymer solution were filtrated and degassed as men-ioned above before use. The injection volume was 200 �L withhe concentration of 0.2% for each sample, and the flow rate was.0 mL/min. Astra software was utilized for data acquisition andnalysis. Their polydispersity indices are very narrow, which is con-enient for the study of self-assembly.

Differential scanning calorimetry (DSC) measurements werearried out on a DSC-204 apparatus (Netzsch Co., Germany) undernitrogen atmosphere at a heating rate of 20 ◦C min−1 from 0 to

00 ◦C. Before the test, samples of the desired weight (10 ± 1 mg)ere heated from room temperature to 100 ◦C to remove the

olatile components, and then they were cooled by liquid nitrogen.The aggregates were characterized with transmission elec-

ron microscopy (TEM) techniques. For TEM measurement a smallroplet of the dilute solution with the concentration of 2‰ waseposited onto a copper EM grid. Before sample deposition, the EMrid was precoated with a thin film of Formvar and then coated witharbon. The sample was dried in air for a certain time, and then wasacuum dried. The aggregates were observed on a H-7000 (Hitachi,apan) transmission electron microscope at an accelerating voltagef 60 kV.

. Results and discussions

.1. Structure of the block copolymers

The structure of the block copolymers was confirmed by FTIRnd 1H NMR spectra. Fig. 1a shows the FTIR spectrum of the blockopolymer of styrene and maleic anhydride P(MAn-co-St)-b-PS--P(MAn-co-St). In the spectrum, the strong absorptions at 1601,494 and 1454 cm−1, are characteristic of the phenyl ring. Theeak at 700 cm−1 corresponds to the signal of single substitutedhenyl rings. The peaks at 3026 and 2850 cm−1 are assigned tohe absorption of the proton of the phenyl ring and the methy-ene of the PS chain, respectively. Furthermore, there are two strongbsorption peaks around 1780 and 1850 cm−1, characteristic ofhe anhydride. In addition, the peaks at 920 and 1060 cm−1 aressigned to the characteristic IR band of the carbon carbon doubleond and carbon sulfur double bond, respectively [38]. Using theynthesized P(MAn-co-St)-b-PS-b-P(MAn-co-St) block copolymers the macro-CTA, the multi-block copolymer P(MAn-co-St)-b-PS--P(MAn-co-St)-b-PAA-b- P(MAn-co-St)-b-PS-b-P(MAn-co-St) wasrepared by adding acrylic acid to the polymerization system viahe RAFT process; the FTIR spectrum is shown in Fig. 1b. Comparedith Fig. 1a, besides the other similar absorption peaks, there is a

eak around 1700 cm−1, corresponding to the absorption charac-eristic of polyacrylic acid, which confirmed the synthesis of the

ulti-block copolymer.The 1H NMR spectrum of P(MAn-co-St)-b-PS-b-P(MAn-co-St) is

hown in Fig. 2a. In the spectrum, the peak at ı around 7.2 ppmelongs to the signals of the aromatic ring of the polystyrenelock. The peak at ı = 1.7 ppm is assigned to the vinyl groups ofolystyrene. In addition, there is a characteristic signal at around

= 6.6 ppm of maleic anhydride. Moreover, the peak at ı = 3.77 ppm

s assigned to the signal of the junction units methine of P(MAn-o-St) and PS blocks [39]. The contents of PS and P(MAn-co-St)locks in the copolymers were calculated from the peak intensi-ies. According to Fig. 2b, similar chemical shifts appeared at ı = 7.2,

Fig. 1. The FTIR spectra of the block copolymers, (a) P(MAn-co-St)-b-PS-b-P(MAn-co-St), (b) P(MAn-co-St)-b-PS-b-PAA-b-PS-b-P(MAn-co-St).

1.7 and 6.6 ppm. Additionally, the peaks at ı around 6.3–6.5 ppmbelongs to the signals of the protons of the polyacrylic acid. Con-sequently, combining the FTIR and 1H NMR results, the structuresof the block copolymers of P(MAn-co-St)-b-PS-b-P(MAn-co-St) andP(MAn-co-St)-b-PS-b-P(MAn-co-St)-b-PAA-b- P(MAn-co-St)-b-PS-b-P(MAn-co-St) can be confirmed.

3.2. Molecular weights and molecular weight distributions

The dn/dc value of the styrene–maleic anhydride block copoly-mer in THF solution by light scattering analysis was determined tobe 0.20 mL g−1. And the dn/dc value was set as the parameter duringSEC-LS measurement. Moreover, the value of the second virial coef-ficient, A2, was calculated to be 2.328 × 10−3 mol mL g−1 by usingthe Astra software. It suggests that THF was a good solvent for thepolymer samples, and the polymer existed as expanded coils in thedilute solution. Fig. 3 shows the SEC chromatograms of the P(MAn-co-St)-b-PS-b-P(MAn-co-St) block copolymers from Section 2.3.The SEC traces of the samples show sharp and monomodal distri-butions (Mw/Mn = 1.19–1.23), indicating that they are pure withoutany homopolymer. Using the Astra software, the number-averagemolecular weight (M ) was estimated from the M data (M ) of

n i w

many fractions detected by MALLS, leading to the rise in Mn values,and the reduction of the molecular weight distribution (Mw/Mn).Herein, according to the references, we have calibrated the Mw/Mn

values of the polymers through calculation [40,41]. The values of

R. Ma et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 346 (2009) 184–194 187

Fig. 2. 1H NMR spectra of the block copolymers in CDCl3, (a) P(MAn-co-St)-b-PS-b-P(MAn-co-St),(b) P(MAn-co-St)-b-PS-b-P(MAn-co-St)-b-PAA-b-P(MAn-co-St)-b-PS-b-P(MAn-co-St).

Fig. 3. The SEC chromatograms of the styrene (a) and maleic anhydride block copolymers (b).

188 R. Ma et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 346 (2009) 184–194

Table 2Molecular weights, molecular weight distribution, and compositions of polymerssynthesized by RAFT.

Polymers Mwa Mw/Mn PS content (%)b

P(MAn-co-St)67PS201P(MAn-co-St)67 48,140 1.23 55P

mtapi

cnriaipaoftcmsabsmttsbwoStcMmadw

3

misrqtacHpsTPb

(MAn-co-St)44PS132P(MAn-co-St)44 31,990 1.22 44

a Molecular weight determined from SEC-LS.b PS contents were obtained from 1H NMR.

olecular weights, molecular weight distribution, and composi-ions of the polymers are summarized in Table 2. The light scatteringnd SEC-LS results clearly indicate that the samples were monodis-ersed polymers having narrow molecular weight distribution; that

s, the polymerization occurred in a living and controlled manner.Styrene and maleic anhydride can form a charge transfer pair

omplex; then alternating co-polymerization will take place underormal radical polymerization conditions. Herein, when the feedatio is 1:8 (molar ratio of maleic anhydride to styrene), the alternat-ng co-polymerization will occur first, namely, styrene and maleicnhydride will be simultaneously consumed. Because styrene isn excess, the propagation of the styrene/MAn chain will takelace till the exhaustion of MAn [42]. According to RAFT mech-nism, the trithiocarbonate functionality is located in the centerf the polymer chains, and the middle location of the activeunction afford the ability to insert the styrene/MAn chain athis site, thus forming P(MAn-co-St)-b-PS-b-P(MAn-co-St) blockopolymers. Consequently, combined by the RAFT polymerizationechanism, the obtained block copolymers have two kinds of

egments, namely, two co-polymerized blocks of styrene–maleicnhydride and the pure polystyrene block in the middle, which cane confirmed by the glass transitions in the DSC results. Fig. 4ahows the DSC curve of the styrene–maleic anhydride block copoly-ers polymerized with the chain transfer agent DBTTC. There are

wo distinguishable glass transitions at 100 and 165 ◦C, respec-ively, which correspond to the glass transition temperatures of thetyrene block and the styrene–maleic anhydride co-polymerizedlock. Fig. 4b shows the DSC curve of the St-MAn copolymer thatas only initiated with AIBN without adding DBTTC. There is only

ne glass transition at around 165 ◦C, which corresponds well to thet-MAn co-polymerized chain. Consequently, it is further confirmedhat the polymerization products by the RAFT process are the blockopolymers that are composed of a polystyrene block and two St-An co-polymerized blocks. Finally, the combination of the RAFTechanism and various confirmations from the FTIR, 1H NMR, DSC

nd SEC results indicated that the polymers obtained were well-efined block copolymers of St-MAn and St and the polymerizationsere controlled.

.3. Aggregation behavior

To understand the aggregation behaviors of the block copoly-ers in the selective solvents, the preparation of the aggregates

s most important. Surfactants usually dissolve easily in the goodolvents for both the hydrophobic and hydrophilic segments, andelatively quick morphology equilibrium can be established by theuick exchange between the unimer and the micelle. However,his is not the case for macromolecular surfactants, such as blocknd graft copolymers, because of the undissolved hydrophobichains, especially for the relatively longer hydrophobic segments.erein, the solvent induced method was used to conveniently pre-

are the aggregates, and the morphologies of the aggregates wereuccessfully controlled by changing their preparation conditions.wice-distilled water is a selective solvent for the block copolymer(MAn-co-St)-b-PS-b-P(MAn-co-St) in the self-assembly process,ecause water is a good solvent only for the hydrophilic P(MAn-co-

Fig. 4. The DSC curve of the styrene and maleic anhydride block copolymers, (a)with DBTTC, (b) without DBTTC.

St) blocks and a precipitant for the hydrophobic PS blocks. THF, DMFand 1,4-dioxane are good solvents for both P(MAn-co-St) and PSblocks, During water addition to the initial copolymer solution, thequality of the solvent for the PS block gradually decreased. There-fore, the hydrophilic P(MAn-co-St) blocks formed the corona of theaggregate and the hydrophobic PS blocks were involved in the coreof the aggregates.

3.3.1. Aggregation behavior ofP(MAn-co-St)-b-PS-b-P(MAn-co-St) block copolymers3.3.1.1. Common solvent effect. Fig. 5 shows the TEM photos ofaggregates for P(MAn-co-St)44PS132P(MAn-co-St)44 block copoly-mers obtained by using DMF, THF, 1,4-dioxane and THF/H2O(6 wt%)as common solvents, respectively, and then adding the twice-distilled water to induce the self-assembly. The initial polymerconcentrations were all 0.5 wt)%. As shown in Fig. 5a, the aggregatesadopted a spherical shape when DMF was the common solvent. Thespheres were relatively uniform and most of their diameters areabout 50 nm, the largest are 80 nm. Moreover, further aggregationcan be observed between the spheres, as shown by the faint edge ofthe two spheres interface. The spheres could touch and join whileswollen, then shrink during drying to produce the necks. Thus the

aggregation among the sphere aggregates may be an intermediatestate. Under more careful observation, a few smaller, irregular andfaint aggregates can be seen in the photo background, which maybe the primary aggregates in the self-assembly system.

R. Ma et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 346 (2009) 184–194 189

F n-co-T

FoTtopwosatgctdasosttttsiTob

ig. 5. TEM photographs of self-assembled aggregates of P(MAn-co-St)44PS132P(MAHF, (c) 1,4-dioxane, (d) THF/H2O (6 wt%).

When using THF as the common solvent (Fig. 5b), compared withig. 5a, continuous further coalescence of the spherical aggregatesccurred, and rod-like aggregates can be found among the spheres.hese aggregates were polydispersed with various diameters. Theransformation of spherical micelles to rod-like aggregates mayccur through a two-step mechanism as others have observed in theolystyrene-b-poly(acrylic acid) (PS-b-PAA) system [43]; it beginsith the adhesive collisions of spheres resulting in the formation

f irregular “pearl necklace” structures. Therefore, the observedimultaneous multiple morphology of spheres and rods is actuallyn intermediate state. Similarly, sphere aggregates were found inhe 1,4-dioxane system (Fig. 5c). Interestingly, tubule-like aggre-ates were observed among the spheres. These spheres were alsooalesced and polydispersed. The results from Fig. 5a–c indicatedhat the quality of the common solvents play important roles inetermining the morphologies of the aggregates, owing to the inter-ction between the polymer and the solvent. When the commonolvent was changed to THF/H2O (6 wt%) denser sphere aggregatesccurred with a wide distribution of sizes, due to the change of theolvent quality (Fig. 5d). The volume distributions of the solvents inhe core affect the morphologies of the aggregates [44]. The size ofhe core increases with the increase of the quantity of the solvent inhe core, since the core-forming center block continuously extendso the interface of the core and corona. As a result, the degree of

tretching of the core-forming chains has to increase with increas-ng solvent content, which is thermodynamically unfavourable.hus, the extension of the core-forming block will lead to the lossf conformational entropy as the size of the spherical aggregateecome larger. Moreover, from the formula �G = �H − T�S, we

St)44. The aggregates were made from a 0.5 wt % copolymer solution in (a) DMF, (b)

know that as a result of the loss of conformational entropy, thesystem will be forced to adopt aggregation state with smaller corein order to be stable. Namely, the morphology will change fromspheres to non-spherical aggregates, such as rods, tubules, lamellaand vesicles. On the other hand, it is well known that the quantityof the solvent in the core is related to the solubility parameter ofthe core-forming chain. The smaller difference between the solu-bility parameter of the chain and the solvent, the larger quantityof solvent there is in the core. And the interaction between thecore-forming chain and solvent becomes stronger, which leads toa large extension of the core-forming block. Similarly, due to theneed of the Gibbs energy of micellization, the aggregates will trans-form from spheres to rods, tubules, lamella and other non-sphericalstate. Accordingly, the results revealed that for a certain composi-tion block copolymer the morphologies of the aggregates could betuned by selecting various common solvents. In order to investi-gate the effects of the solvent quality and the microenvironmentsof the hydrophilic chains on self-assembly, unlike Fig. 5a–c, Fig. 5dused a THF/water mixed solvent system with 6 wt% water. Similarto Fig. 5b, there are polydispersed spheres, with better spheres thanFig. 5b. Moreover, coalescence and aggregation occurred, which canbe evidenced by the darker and irregular sphere interfaces. It canbe imagined that the more obvious coalesced aggregation state iscaused by the introduction of the small quantity of water, which

changes the microenvironments of the hydrophilic chains and thequality of the solvent [45], such as the solubility parameter, polarityand dielectric constant, and accordingly changes the extension ofthe core-forming blocks, leading to the continuous transformationto the non-spherical states.

190 R. Ma et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 346 (2009) 184–194

rom t

amFetdaTcsaafd

F(

Fig. 6. Scheme illustration of the formation of the aggregates made f

A schematic process describing the formation of the variousggregates of P(MAn-co-St)-b-PS-b-P(MAn-co-St) block copoly-ers in different common solvent systems is presented in Fig. 6.

irst, the P(MAn-co-St)-b-PS-b-P(MAn-co-St) block copolymerxists as random coils in the bulk state, which expand after addinghe common solvents. When the precipitant for PS, the twice-istilled water, was added, the looping of the hydrophobic PS blocknd the stretching of the hydrophilic P(MAn-co-St) block occurred.he formation of various self-assembled morphologies in differentommon solvents is attributed to the interaction of polymer and

olvent. PS block has better solubility in 1,4-dioxane than in THF,nd better solubility in THF than in DMF. Then the dimensions ofggregate cores increased with the change of the common solventrom DMF, to THF and then 1,4-dioxane. As a result, the extensionegree of the core-forming block PS increased, correspondingly,

ig. 7. TEM photographs of self-assembled aggregates of P(MAn-co-St)67PS201P(MAn-co-Stb) THF.

he P(MAn-co-St)-b-PS-b-P(MAn-co-St) copolymer solution in water.

which is entropically unfavorable. Therefore, as a result of solventeffect mentioned above, the aggregates transformed from spheresto rods and then to tubules, in order to decrease the free energy ofmicellization, when the solvent changed from DMF to THF and then1,4-dioxane.

3.3.1.2. Composition effect. A similar transformation rule can befound for P(MAn-co-St)67PS201P(MAn-co-St)67 sample, that is, the1,4-dioxane system forms larger size cores than DMF and THFsystems and simultaneous multiple morphologies can be easily

obtained. Furthermore, as shown in Fig. 7, for a given commonsolvent, such as 1,4-dioxane and THF, and given other conditions,P(MAn-co-St)67PS201P(MAn-co-St)67 more easily tends to formnon-sphere aggregates, such as tubules and rods, than P(MAn-co-St)44PS132P(MAn-co-St)44, which may due to the different

)67. The aggregates were made from a 0.5 wt % copolymer solution in (a) 1,4-dioxane,

R. Ma et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 346 (2009) 184–194 191

F 67PS20

(

ec

3SacsuTttct

Fu

ig. 8. TEM photographs of self-assembled aggregates of hydrolyzed P(MAn-co-St)a) DMF, (b)THF.

xtension of the core-forming segment caused by the differentomposition of the block copolymers.

.3.1.3. Hydrolysis effect. When the sample P(MAn-co-t)67PS201P(MAn-co-St)67 was hydrolyzed under strong acidnd stirring conditions, the aggregation morphology became moreomplex and fragmentized when using DMF and THF as commonolvent with a given initial concentration, as shown in Fig. 8, irreg-lar dispersed spheres and worm rod-like aggregates formed. The

EM results indicated that the different degree of the hydrolysis ledo the change of the end group structure, accordingly, the change ofhe quality of the hydrophilic and hydrophobic segments; then thehain exchange of the block copolymer was limited. Moreover, dueo the complex interaction between the polymer and the solvent,

ig. 9. TEM photographs of self-assembled aggregates of P(MAn-co-St)44PS132P(MAn-co-nder (a) 30 ◦C, (b) 60 ◦C.

1P(MAn-co-St)67. The aggregates were made from a 0.5 wt % copolymer solution in

the aggregation behavior also became more complex. In addition,in THF, non-spherical aggregates more easily formed than in DMF,which well accorded the conclusion from the solvent effect.

3.3.1.4. Temperature effect. Fig. 9 shows the morphologies ofP(MAn-co-St)44PS132P(MAn-co-St)44 self-assembled at 30 ◦C and60◦C with 1,4-dioxane as common solvent and 0.5 wt% initial con-centration. Small rough spherical aggregates self-assembled at30 ◦C (Fig. 9a), while as the temperature increased to 60 ◦C, more

irregular eggshell-like aggregates formed (Fig. 9b) [46]. Meanwhile,during the experimental process, we found that a stable colloidsolution could hardly be prepared at higher temperatures than60 ◦C, and precipitation occurred, which may due to the corre-sponding change of the amphiphilicity with the increase of the

St)44. The aggregates were made from a 0.5 wt % copolymer solution in 1,4-dioxane

192 R. Ma et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 346 (2009) 184–194

F An-coa

th

3bc

mScsdo

3

cr

Fg

ig. 10. TEM photographs of self-assembled aggregates of P(MAn-co-St)-b-PS-b-P(M0.5 wt % copolymer solution in (a) DMF, (b) THF, (C) 1,4-dioxane.

emperature, leading to the decrease of the hydrophilicity of theydrophilic chain.

.3.2. Architecture effect: aggregation behavior of P(MAn-co-St)--PS-b-P(MAn-co-St)-b-PAA-b-P(MAn-co-St)-b-PS-b-P(MAn-o-St) multi-block copolymer

Using the synthesized P(MAn-co-St)-b-PS-b-P(MAn-co-St) as aacro chain transfer agent, the multi-block copolymer P(MAn-co-

t)-b-PS-b-P(MAn-co-St)-b-PAA-b- P(MAn-co-St)-b-PS-b-P(MAn-o-St) was obtained by adding acrylic acid to the polymerizationystem. Similarly, its aggregation behavior in selective solvent isiscussed below for the purpose of clarifying the architecture effectn the self-assembly of the block copolymers.

.3.3. Common solvent effectFig. 10 shows TEM photos of the aggregates of the multi-block

opolymer using DMF, THF and 1,4-dioxane as common solvents,espectively, under the same self-assembly conditions. An analo-

ig. 11. TEM photographs of self-assembled aggregates of P(MAn-co-St)-b-PS-b-P(MAn-cates were made from a 0.5 wt % copolymer solution in 1,4-dioxane.

-St)-b-PAA-b-P(MAn-co-St)-b-PS-b-P(MAn-co-St). The aggregates were made from

gous rule could be found as mentioned above, that is, the aggregatesgradually transformed from spheres to large compound rods and toregular branched continuous rods. While compared with Fig. 5, itis obvious that more regular aggregation states have been obtainedwith the increase of the number of the blocks and with the com-plexity of the chain architecture, indicating that the strengthenedconformation restriction of copolymer chain helps to the regularself-assembly of the block copolymer.

3.3.3.1. Annealing effect. Usually, annealing time will influence themorphologies of the aggregates. It has been referred as the lengthof the time between copolymer dissolution in the solvent and thesubsequent precipitant addition [47]. To this end, increasing the

dissolution time of the copolymer in the common solvent or pro-longing the addition time of the selective solvent is operable. Inour investigation, the annealing effect has been considered by stir-ring the copolymer 1,4-dioxane solution 12 h and 72 h before theaddition of the twice-distilled water. As shown in Fig. 11, when

o-St)-b-PAA-b-P(MAn-co-St)-b-PS-b-P(MAn-co-St) for 12 h (a), 72 h (b). The aggre-

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(MAn-co-St)-b-PS-b-P(MAn-co-St)-b-PAA-b- P(MAn-co-St)-b-PS--P(MAn-co-St) was dissolved in 1,4-dioxane for 12 h before addingater, quasi tubule-like aggregates were formed (Fig. 11a). Besides

he quasi tubules, there are a few rough spheres and short-branchedods. Meanwhile, with further increasing of the dissolution time to2 h, regular tubule aggregates with clear outline formed (Fig. 11b).he results indicated that prolonging the dissolved process timeelped the establishment of the chain of the self-assembly system.herefore, annealing helped the formation of the non-sphericalggregates, such as tubules, that not well formed in Fig. 10c.

. Conclusion

A series of block copolymers of styrene, maleic anhydride andcrylic acid have been synthesized by RAFT process. They wereharacterized by FTIR, 1H NMR and GPC&MALLS analysis. The TEMesults revealed that for a certain composition block copolymer the

orphologies of the aggregates could be well controlled by select-ng various common solvents; the composition of the copolymersnfluences the aggregation behavior; the factors that increase theegree of extension of the core lead to the formation of the non-pherical aggregates; increased temperature influences the regularelf-assembly; the delicate molecular architecture will strengthenonformation restriction of the copolymer chain, which helps theegular self-assembly of the block copolymer; furthermore, anneal-ng helped the formation of the non-spherical aggregates.

cknowledgements

The supports by the Research Foundation for Outstanding Youngeachers, China University of Geosciences (Wuhan), the Educationureau of Hubei Province (project D2006-28004), the Natu-al Science Foundation of Hubei Province (project 2007ABA295,008CDZ065) and the National Natural Science Foundation of Chinaproject 20872044) are gratefully acknowledged.

eferences

[1] K. Matyjaszewski, Controlled/Living Radical Polymerization: Progress in ATRPNMP and RAFT AN S C Symposium Series, vol. 768, American Chemical Society,Washington, DC, 2000, p. 211.

[2] M.K. Georges, R.P.N. Veregin, P.M. Kazmaier, G.K. Hamer, Narrow molecularweight resins by a free-radical polymerization process, Macromolecules 26(1993) 2987–2988.

[3] W. Devenport, L. Michalak, E. Malmstorm, M. Mate, B. Kurdi, C.J. Hawker, G.G.Barclay, R.F. Sinta, “Living” free-radical polymerizations in the absence of ini-tiators: controlled autopolymerization, Macromolecules 30 (1997) 1929–1934.

[4] D. Benoit, V. Chaplinski, R. Braslau, C.J. Hawker, Development of a universalalkoxyamine for “living” free radical polymerizations, J. Am. Chem. Soc. 121(1999) 3904–3920.

[5] D.G. Hawthorne, G. Moad, E. Rizzardo, S.H. Thang, Living radical polymeriza-tion with reversible addition-fragmentation chain transfer (RAFT): direct ESRobservation of intermediate radicals, Macromolecules 32 (1999) 5457–5459.

[6] E. Rizzardo, S.H. Thang, G. Moad, Living free-radical polymerization byreversible addition-fragmentation chain transfer: the RAFT process, Macro-molecules 31 (1998) 5559–5562.

[7] T.A. Roshan, E. Rizzardo, G. Moad, Living radical polymerization with reversibleaddition-fragmentation chain transfer (RAFT Polymerization) using dithiocar-bamates as chain transfer agents, Macromolecules 32 (1999) 6977–6980.

[8] G.Y. Xu, W.T. Wu, Y.S. Wang, W.M. Pang, Q.R. Zhu, P.H. Wang, Functionalizedcarbon nanotubes with polystyrene-block-poly (N-isopropylacrylamide) by insitu RAFT polymerization, Nanotechnology 18 (2007) 145606–145612.

[9] Y.Z. You, C.Y. Hong, C.Y. Pan, Directly growing ionic polymers on multi-walledcarbon nanotubes via surface RAFT polymerization, Nanotechnology 17 (2006)2350–2354.

10] E. Rizzardo, J. Chiefari, R. Mayadunne, G. Moad, S. Thang, Tailored polymer archi-tectures by reversible addition–fragmentation chain transfer, Macromol. Sym.174 (2001) 209–212.

11] A. Goto, K. Sato, Y. Tsujii, T. Fukuda, G. Moad, E. Rizzardo, S.H. Thang, Mechanism

and finetics of RAFT-based living radical polymerizations of styrene and methylmethacrylate, Macromolecules 34 (2001) 402–408.

12] C. Barner-Kowollik, J.F. Quinn, T.L.U. Nguyen, J.P.A. Heuts, T.P. Davis, Kineticinvestigations of reversible addition fragmentation chain transfer polymeriza-tions: cumyl phenyldithioacetate mediated homopolymerizations of styreneand methyl methacrylate, Macromolecules 34 (2001) 7849–7857;

[

[

hem. Eng. Aspects 346 (2009) 184–194 193

C. Barner-Kowollik, J.F. Quinn, D.R. Morsley, T.P. Davis, Modeling the reversibleaddition-fragmentation chain transfer process in cumyl dithiobenzoate-mediated styrene homopolymerizations: assessing rate coefficients for theaddition-fragmentation equilibrium, J. Polym. Sci. Part A: Polym. Chem. 39(2001) 1353–1365.

13] J.F. Quinn, E. Rizzardo, T.P. Davis, Ambient temperature reversibleaddition–fragmentation chain transfer polymerisation, Chem. Commun.(2001) 1044–1045.

14] J.G. Tsavalas, F.J. Schork, H. de Brouwer, M.J. Monteiro, Living radical polymeriza-tion by reversible addition–fragmentation chain transfer in ionically stabilizedminiemulsions, Macromolecules 34 (2001) 3938–3946;M.J. Monteiro, J. de Barbeyrac, Free-radical polymerization of styrene in emul-sion using a reversible addition–fragmentation chain transfer agent with a lowtransfer constant: effect on rate, particle size, and molecular weight, Macro-molecules 34 (2001) 4416–4423.

15] A. Butte, G. Storti, M. Morbidelli, Miniemulsion living free radical polymeriza-tion by RAFT, Macromolecules 34 (2001) 5885–5896.

16] R.K. Bai, Y.Z. You, C.Y. Pan, Co-60 gamma-irradiation-initiated “living” free-radical polymerization in the presence of dibenzyl trithiocarbonate, Macromol.Rapid Commun. 22 (2001) 315–319.

17] Z. Tuzar, P. Kratochvil, Micelles of Block and Graft Copolymers in Solution Sur-face and Colloid Science, Plenum Press, New York, 1993.

18] S.E. Webber, P. Munk, Z. Tuzar, Solvents and self-organization of polymer, in:NATO ASI Series, Series E: Applied Sciences, Kluwer Academic Publisher, Dor-drecht, 1996.

19] P. Alexandridis, B. Lindman, Amphiphilic Block Copolymers: Self-assembly andApplications, Elsevier, Amsterdam, 2000.

20] G. Riess, P.H. Dumas, G. Hurtrez, Block copolymer Micelles And Assemblies,MML, Series 5, Citus Books, London, 2002.

21] J.C. Tao, X. Chen, Y. Sun, Y. Shen, N. Dai, Controllable preparation of ZnO hollowmicrospheres by self-assembled block copolymer, Colloids Surf. A: Physiochem.Eng. Aspects 330 (2008) 67–71.

22] Z.W. Jin, X.D. Wang, X.G. Cui, Synthesis and morphological investigation ofordered SBA-15-type mesoporous silica with an amphiphilic triblock copoly-mer template under various conditions, Colloids Surf. A: Physiochem. Eng.Aspects 316 (2008) 27–36.

23] H.J. Jeon, D.H. Go, S. Choi, K.M. Kima, J.Y. Lee, D.J. Choo, H.O. Yoo, J.M. Kim, J.Kim, Synthesis of poly(ethylene oxide)-based thermoresponsive block copoly-mers by RAFT radical polymerization and their uses for preparation of goldnanoparticles, Colloids Surf. A: Physiochem. Eng. Aspects 317 (2008) 496–503.

24] J. RodrÍguez-Hernández, F. Chécot, Y. Gnanou, S. Lecommandoux, Stimuli-responsive A-b-(B-co-C) diblock terpolymers bearing polyampholytesequences, Prog. Polym. Sci. 30 (2005) 691–724.

25] S. Lecommandoux, O. Sandre, F. Chécot, J. RodrÍguez-Hernández, R. Perzin-sky, Magnetic nanocomposite micelles and vesicles, Adv. Mater. 17 (2005)712–718.

26] I. Olli, G. Brinke, Functional materials based on self-assembly of polymericsupramolecules, Science 295 (2002) 2407–2409.

27] D. Liu, C. Zhong, Novel two-compartment micelles formed by self-assembly oflinear pentablock copolymers in selective solvents, Macromol. Rapid Commun.28 (2007) 292–297.

28] Y. Zhang, Z. Zhang, Q. Wang, C. Xu, Z. Xie, Multi-morphological com-plex aggregates formed from amphiphilic poly(ethylene oxide)-block-polydimethylsiloxane-block-poly(ethylene oxide) triblock copolymers, Macro-mol. Rapid Commun. 27 (2006) 1476–1482.

29] X. Li, J. Ji, J. Shen, Spontaneous vesicle formation in aqueous solutions of comb-like PEG, Macromol. Rapid Commun. 27 (2006) 214–218.

30] A. Tsimelzon, D. Deamer, R. Braslau, Synthesis and self-assembly of amphiphilicdiblock copolymers using a fluorescently labeled N-alkoxyamine initiator,Macromol. Rapid Commun. 26 (2005) 1872–1877.

31] H.W. Shen, L.F. Zhang, A. Eisenberg, Multiple pH-induced morphologicalchanges in aggregates of polystyrene-block-poly(4-vinylpyridine) in DMF/H2Omixtures, J. Am. Chem. Soc. 121 (1999) 2728–2740.

32] A. Samson, X. Jenekhe, C. Linda, Self-assembled aggregates of rod-coil blockcopolymers and their solubilization and encapsulation of fullerenes, Science279 (1998) 1903–1907.

33] J.Q. Zhao, E.M. Pearce, T.K. Kwei, H.S. Jeon, P.K. Kesani, N.P. Balsara, Micellesformed by a model hydrogen-bonding block copolymer, Macromolecules 28(1995) 1972–1978.

34] D.Y. Yan, Y.F. Zhou, J. Hou, Supramolecular self-assembly of macroscopic tubes,Science 303 (2004) 65–67.

35] B.C. Trivedi, B.M. Culbertson, Maleic Anhydride, Plenum, New York, 1982, p. 364(Chapter 3).

36] C.W. Lin, W.L. Lee, An investigation on the modification of polypropylene bygrafting of maleic anhydride based on the aspect of adhesion, J. Appl. Polym.Sci. 70 (1998) 383–387.

37] J.J. Yuan, R. Ma, Q. Gao, Y.F. Wang, S.Y. Cheng, L.X. Feng, Z.Q. Fan, L. Jiang, Synthesisand characterization of polystyrene/poly(4-vinylpyridine) triblock copolymersby reversible addition-fragmentation chain transfer polymerization and theirself-assembled aggregates in water, J. Appl. Polym. Sci. 8 (2003) 1017–1025.

38] H.M. Li, D.W. Zhou, P.S. Liu, Synthesis of random styrene–maleic anhydriedcopolymer by solution polymerization, China Synth Resin Plast. 15 (1998)64–65.

39] E.-S. Park, M.N. Kim, I.-M. Lee, H.S. Lee, Y.S. Yoon, Living radical copolymerizationof styrene/maleic anhydride, J. Polym. Sci.: Part A: Polym. Chem. 38 (2000)2239–2244.

1 ysicoc

[

[

[

[

[

[

94 R. Ma et al. / Colloids and Surfaces A: Ph

40] M. Netopillk, S. Podzimek, P. Kratochvll, Estimation of width of narrowmolecular-weight distributions by size-exclusion chromatography with con-centration and light scattering detection, J. Chromatogr. A. 922 (2001) 25–36.

41] Z.D. He, X.C. Zhang, R.S. Cheng, Simultaneous calibration of molecular weightseparation and column dispersion of GPC by coupling with lallis, J. Liq. Chro-matogr. 5 (1982) 1209–1222.

42] M.Q. Zhu, L.H. Wei, P. Zhou, F.S. Du, Z.C. Li, F.M. Li, Radical reverse addition-

fragmentation chain transfer polymerization of maleic anhydride with styreneand a novel functional block copolymer prepared therefrom, Acta. Polym. Sinica6 (2001) 415–417.

43] S.E. Burke, A. Eisenberg, Kinetics and mechanisms of the sphere-to-rod androd-to-sphere transitions in the ternary system PS310-b-PAA52/Dioxane/Water,Langmuir 17 (2001) 6705–6714.

[

[

hem. Eng. Aspects 346 (2009) 184–194

44] L. Zhang, A. Eisenberg, Multiple morphologies and characteristics of “Crew-Cut” micelle-like aggregates of polystyrene-b-poly(acrylic acid) diblockcopolymers in aqueous solutions, J. Am. Chem. Soc. 118 (1999) 3168–3181.

45] L. Zhang, A. Eisenberg, Morphogenic effect of added ions on crew-cut aggregatesof polystyrene-b-poly(acrylic acid) block copolymers in solutions, Macro-molecules 29 (1996) 8805–8815.

46] N.S. Cameron, M.K. Corbierre, A. Eisenberg, Asymmetric amphiphilic blockcopolymers in solution: a morphological wonderland, Can. J. Chem. 77 (1999)1311–1326.

47] Y. Yu, A. Eisenberg, Bilayer morphologies of self-assembled crew-out aggregatesof amphiphilic PS-b-PEO diblock copolymers in solution, Macromolecules 31(1998) 3509–3518.