Novel Thermoresponsive and pH-Responsive Aggregates from Self-Assembly of TriblockCopolymer PSMA-b-PNIPAAm-b-PSMA

Junfeng Zhou, Li Wang,* Qiang Yang, Qingquan Liu, Haojie Yu, and Zhenrong ZhaoState Key Laboratory of Chemical Engineering, Zhejiang UniVersity,Hangzhou 310027, People’s Republic of China

ReceiVed: January 19, 2007; In Final Form: March 4, 2007

A series of novel triblock copolymers of poly(stearyl methacrylate)-b-poly(N-isopropylacrylamide)-b-poly-(stearyl methacrylate) (PSMA-b-PNIPAAm-b-PSMA) with different molecular weights was synthesized throughcarboxyl-terminated trithiocarbonates as a highly efficient RAFT agent via reversible addition-fragmentationchain transfer (RAFT) polymerization. The resultant polymers were characterized by1H NMR, FT-IRspectroscopy, and GPC. By varying the organic solvent used in the self-assembly procedure and adjustingthe copolymer composition, multiple morphologies ranging from vesicles and core-shell spherical aggregateswith different dimensions to pearl-necklace-like aggregates were obtained. The aggregates showed thermo-responsive and pH-responsive properties through the lower critical solution temperature (LCST) of PNIPAAmand the two carboxyl end groups of the copolymer.

1. Introduction

In recent years, considerable efforts have been devoted topreparing stimuli-responsive polymers. This kind of “smart”materials exhibits dramatic changes in their properties inresponse to the application of environmental stimuli, such astemperature, pH, ionic strength, electric or magnetic fields, andso on.1-3 Since their excellent physical and chemical propertiescan be adjusted by external stimuli, stimuli-responsive polymershave widespread applications in drug delivery systems and indevices such as actuators, artificial muscles, and controlledmolecular gates and switches.4

Amphiphilic macromolecules can self-assemble into eithercore-shell morphology or cavity-containing structures withnanometer or submicrometer scales through weak noncovalentinteractions (e.g., hydrogen-bonding interaction,π-π interac-tion, charge-transfer interaction, van der Waals forces, host-guest interaction, and electrostatic forces). The self-assembledaggregates have attracted great interest in recent years due totheir excellent characteristics and broad potential applicationssuch as microreactors, microcapsules, drug delivery systems,and encapsulation of various kinds of guest molecules.5-9

Among the reported aggregates with various sizes, giantaggregates (up to 1µm) have been an active topic of researchbecause their curvatures are much more similar to those of cellsand these aggregates can be used as attractive models for cellsand organelles.10,11 Moreover, giant aggregates have a uniqueproperty: visibility under the light microscope.12

Poly(N-isopropylacrylamide) (PNIPAAm) is one of the moststudied thermoresponsive polymers and exhibits a reversiblethermoresponsive phase transition in aqueous solution.13,14

Below a specific lower critical solution temperature (LCST),PNIPAAm is water-soluble, is hydrophilic, and exists in anextended chain form, while PNIPAAm changes to an insolubleand hydrophobic aggregate due to its coil-to-globule transitionabove the LCST. Up to now, the self-assembly behaviors of

thermoresponsive diblock copolymers have been investigatedwidely because the LCST of PNIPAAm is close to physiologicaltemperature and the self-assembled aggregates have enormouspotential in technology and in biomedical application.15 Forexample, Shi et al. have investigated the thermoresponsivemicellization of poly(ethylene glycol)-b-poly(N-isopropylacry-lamide) (PEG110-b-PNIPAAm44) in water by static light scat-tering and dynamic light scattering.16

However, to our knowledge, the studies on the self-assemblybehavior of triblock copolymers are not sufficient. Moreover,if the thermoresponsive polymers combine with carboxyl groups,they will create new systems that can respond to complexexternal stimuli. The novel thermoresponsive or pH-sensitiveaggregates self-assembled from the above polymers can beconjugated to drugs to be applied in drug delivery throughexternal stimuli.17 With these in mind, a novel triblockcopolymer capped with two carboxyl end groups (PSMA-b-PNIPAAm-b-PSMA; where PSMA is poly(stearyl methacry-late)) is designed, and the resultant polymer can exhibit pH-responsive and thermoresponsive characteristics. Moreover,these carboxyl groups and acrylamide groups can provide theself-assembly driving forces (intermolecular hydrogen bond) toform giant aggregates. The “intelligent” polymeric aggregatesformed from the polymers are of interest in further chemical orbiological modification due to carboxyl groups on the surfaceof the aggregates. Furthermore, the resultant copolymer is abrush-type triblock copolymer and possesses unique character-istics and potential applicable foreground because stearylmethacrylate has a pendent long alkyl side chain.18

In this study, we synthesize and characterize novel brush-type triblock copolymers capped with two carboxyl groups,PSMA-b-PNIPAAm-b-PSMA, which are prepared convenientlythrough carboxyl-terminated trithiocarbonates as a highly ef-ficient RAFT (reversible addition-fragmentation chain transfer)agent via RAFT polymerization in 1,4-dioxane solution, andmultiple morphologies and different dimensions of these ag-gregates are obtained by the self-assembly process from thesetriblock copolymers. The thermoresponsive and pH-responsive

* Corresponding author. Telephone: (86)-571-87953200. Fax: (86)-571-87951612. E-mail: opl_wl@zju.edu.cn.

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10.1021/jp070480h CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 05/02/2007

characteristics of the aggregates are also studied by dynamiclight scattering (DLS).

2. Experimental Section

2.1. Materials. N-Isopropylacrylamide (NIPAAm; AldrichOrganics) was recrystallized from hexane-acetone solutionbefore use. 1,1′-Azobis(isobutyronitrile) (AIBN; Shanghai No.1Chemical Reagent Factory) was purified by recrystallizationfrom methanol. Stearyl methacrylate (SMA; Tianjing TianjiaoChemical Reagent Factory) was treated according to theliterature method.19 SMA was purified by dissolution in hexaneand extraction three times with 5.0% aqueous NaOH. After theorganic phase was dried over magnesium sulfate, the solutionwas passed through neutral alumina and solvent was removedunder reduced pressure. All other reagents were of analyticalgrade and used as received.

2.2. Synthesis ofS,S′-Bis(r,r-dimethyl-r′′-acetic acid)-trithiocarbonate BDAAT. 20 Carbon disulfide (6.8501 g, 0.09mol), chloroform (26.9008 g, 0.225mol), acetone (13.08 g, 0.225mol), and tetrabutylammonium hydrogen sulfate (phase transfercatalysis, PTC) (0.6025 g, 1.775 mmol) were mixed with 30mL of 1,4-dioxane in a 250 mL jacketed reactor cooled withtap water under nitrogen. Sodium hydroxide aqueous solution(50.0 wt %) (50.4002 g, 0.63 mol) was added dropwise over90 min in order to keep the temperature below 25°C. Thereaction was stirred overnight. A 100 mL volume of water wasthen added to dissolve the solid, followed by 60 mL ofconcentrated HCl to acidify the aqueous layer and stirring for30 min with nitrogen purge. The solution was filtered, and thesolid was rinsed thoroughly with water. It was dried to constantweight to collect a earth-colored product. It was further purified

by stirring in toluene/acetone (v/v 4/1) to afford 2.3048 g of ayellow crystalline solid.1H NMR (CDCl3, ppm from TMS):1.69 (s, 12H), 12.91 (s, 2H).

2.3. RAFT Polymerization of Macro-Chain-Transfer AgentPSMA-SC(S)S-PSMA.A single-neck round-bottom flask wascharged with SMA (5.0786 g, 15mmol), BDAAT (0.1860 g,0.74mmol), AIBN (0.0246 g, 0.15mmol), and 1,4-dioxane (6.0mL). The flask was degassed by three consecutive freeze-pump-thaw cycles and then immersed in an oil bath thermo-stated at 60°C for 7 h. The polymerization was terminated byrapid cooling and freezing. The crude product was precipitatedthree times in anhydrous ethanol to remove SMA monomer and1,4-dioxane.

2.4. RAFT Polymerization of Block Copolymer PSMA-b-PNIPAAm-b-PSMA. A single-neck round-bottom flask wascharged with macro-chain-transfer agent PSMA-SC(S)S-PSMA(0.8002 g, 0.1201mmol), AIBN (0.0039, 0.0237mmol), NIPAAm(0.2720 g, 2.40mmol), and 1,4-dioxane (3.0 mL). The flask wasdegassed by three freeze-pump-thaw cycles and then im-mersed in an oil bath thermostated at 60°C. After several hours(5 or 11 h), the polymerization flask was rapidly cooled to roomtemperature; the crude product was precipitated three times inanhydrous ethanol to remove NIPAAm monomer and 1,4-dioxane.

2.5. Self-Assembly Procedure of PSMA-b-PNIPAAm-b-PSMAMolecules.ThetriblockcopolymerPSMA10-b-PNIPAAm68-b-PSMA10 was first dissolved in organic solvents (THF or 1,4-dioxane) at a desired concentration before use at roomtemperature. Deionized water was then added slowly to eachof the copolymer solutions under stirring with a magnetic bar.In the system of the polymer/organic solvent, the final water

SCHEME 1: Synthesis Procedures of BDAAT and PSMA-b-PNIPAAm-b-PSMA

TABLE 1: Morphologies Obtained from Triblock Copolymer of PSMA- b-PNIPAAm-b-PSMA under Different Conditions

polymer concn (wt %) solvent water content (wt %) morphology

PSMA10-b-PNIPAAm68-b-PSMA10 0.1 THF 10.0 giant vesiclesPSMA10-b-PNIPAAm68-b-PSMA10 0.1 THF 30.0 giant spheresPSMA10-b-PNIPAAm68-b-PSMA10 0.1 THF 50.0 associated spheresPSMA10-b-PNIPAAm68-b-PSMA10 0.1 dioxane 10.0 vesiclesPSMA10-b-PNIPAAm68-b-PSMA10 0.1 dioxane 30.0 spheresPSMA10-b-PNIPAAm68-b-PSMA10 0.1 dioxane 50.0 pearl necklacesPSMA10-b-PNIPAAm26-b-PSMA10 0.1 THF 30.0 vesicles

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content was varied between 10.0 and 50.0 wt % while thepolymer concentration of each of samples was maintained at0.1 wt % in the resultant solution. A similar procedure wasperformed to prepare aggregates of PSMA10-b-PNIPAAm26-b-PSMA10.

Samples for TEM measurement were prepared by mountinga drop (ca. 10µL) of the above solution on the carbon-coatedCu grids and allowing the samples to dry in air.

2.6. Characterization. 1H NMR experiments were carriedout on a 400 MHz AVANCE NMR spectrometer (ModelDMX400) using CDCl3 as solvent and tetramethylsilane asinternal reference. Fourier transform infrared (FT-IR) spectrawere obtained on a Nicolet 5700 infrared spectrometer. Mo-lecular weight and molecular weight distribution were deter-mined on a Waters 201 gel permeation chromatograph (GPC)equipped with UltraStyragel columns with pore sizes of 103-105 Å, using monodispersed polystyrene as calibration standard.The eluent was tetrahydrofuran (THF) at a flow rate of 1.0 mL/min. A detection wavelength of 632.8 nm and the refractionindex increment value of the polymer solutions dn/dc ) 0.20were used for laser scattering detection. Transmission electronmicroscopy (TEM) studies were performed with a JEOL Model1200EX instrument at a voltage of 160 kV. Samples were

prepared by drop-casting self-assembled solutions onto carbon-coated copper grids and air-dried at room temperature beforemeasurement. Dynamic light scattering (DLS) measurementswere performed in THF/water using a Zetasizer 3000HSAapparatus (Malvern Instruments Ltd.) equipped with a 125 mWlaser operating atλ ) 633 nm. All the DLS measurements wereat a scattering angle of 90°. Energy-dispersive X-ray analysis(EDAX) was carried out on a Hitachi S-4800 scanning electronmicroscope (SEM) equipped with an energy-dispersive X-rayanalyzer (EDAX 350, Horiba) at a voltage of 15 kV.

3. Results and Discussion

3.1. Preparation of PSMA-b-PNIPAAm-b-PSMA. It hasbeen reported that the RAFT polymerization of NIPAAm is aliving free radical polymerization in nature.23,24 Therefore, theRAFT polymerization of NIPAAm with macromolecules con-taining trithiocarbonate groups as macro-chain-transfer agentcan be applied to prepare a series of novel triblock copolymerscapped with two carboxyl groups (PSMA-PNIPAAm-PS-MA).21,22The synthesis of macro-chain-transfer agent BDAATand the synthetic strategy followed for the triblock copolymervia the RAFT route are depicted in Scheme 1. The homopolymerPSMA-SC(S)S-PSMA was prepared using BDAAT as theRAFT agent in 1,4-dioxane solution at 60°C. Subsequently,amphiphilic triblock copolymer PSMA-b-PNIPAAm-b-PSMAwas synthesized through the RAFT polymerization of NIPAAmwith PSMA-SC(S)S-PSMA as macro-chain-transfer agent.

The chemical structure of the homopolymer PSMA-SC(S)S-PSMA was studied first by1H NMR spectroscopy. The1H NMRspectrum of PSMA-SC(S)S-PSMA shows peaks with thefollowing shifts: the H of the methylene group adjacent to theoxygen atom of stearyl shows a chemical shift of 4.10-3.82ppm, the H of the methylene in the backbone shows a chemicalshift of 1.99-1.72 ppm, the H of the methylene adjacent to themethylene connected to the oxygen atom of stearyl shows achemical shift of 1.66-1.52 ppm, the H in the long-chainmethylene bands exhibits a chemical shift of 1.40-1.20 ppm,and the chemical shifts for the methyl groups are ranged within0.90-0.78 ppm. The1H NMR result is thus consistent with thestructure of the homopolymer PSMA-SC(S)S-PSMA.

To verify the existence of carboxyl groups and trithiocar-bonate groups in the homopolymer PSMA-SC(S)S-PSMA, FT-IR spectroscopy was used. The FT-IR spectrum of PSMA-SC(S)S-PSMA is shown in Figure 2A. As well as thecharacteristic absorption band for the long alkyl chain of PSMAat 2923, 2852 cm-1 in Figure 2A, the characteristic absorptionbands for the trithiocarbonate groups (>CdS, 1062 cm-1) andfor the carboxyl groups (-COOH, 3448 cm-1) also appear inthis figure. Energy-dispersive X-ray analysis (EDAX) was usedto further characterize PSMA-SC(S)S-PSMA. Figure 3 shows

Figure 1. 1H NMR spectra of PSMA10-SC(S)S-PSMA10 (A) andPSMA10-b-PNIPAAm68-b-PSMA10 (B).

Figure 2. FT-IR spectra of PSMA10-SC(S)S-PSMA10 (A) and PSMA10-b-PNIPAAm68-b-PSMA10 (B).

Figure 3. EDAX spectrum of PSMA10-SC(S)S-PSMA10.

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the EDAX spectrum of PSMA-SC(S)S-PSMA. An obvioussulfur peak could be observed in the sample. The above resultsdemonstrate that BDAAT participates in the RAFT polymeri-zation of SMA. Consequently, PSMA-SC(S)S-PSMA wassuccessfully prepared by the RAFT polymerization in thepresence of BDAAT.

For confirmation of the triblock polymer structure, a typical1H NMR spectrum was measured and is shown in Figure 1B.Compared with the1H NMR spectrum of PSMA-SC(S)S-PSMAin Figure 1A, the characteristic signals of PNIPAAm can beseen atδ ) 1.14 (e) and 6.38 (c) , corresponding to the methylprotons of isopropyl units and the imido group adjacent to thecarbonyl group, respectively. This result reveals the successfulpreparation of the triblock copolymer (PSMA-PNIPAAm-PSMA).

The chemical structure of PSMA-b-PNIPAAm-b-PSMA wasalso determined from FT-IR, and the spectrum is shown inFigure 2B. In its FT-IR spectrum, the characteristic bands forPSMA and PNIPAAm (in KBr: C-H at 2923, 2852, 1468cm-1; CdO at 1731 cm-1; and N-H at 3367, 1654 cm-1) areclearly observed.

The molecular weights and their distribution of PSMA-SC-(S)S-PSMA and PSMA-b-PNIPAAm-b-PSMA were character-ized by GPC in THF using polystyrene calibration. The narrowlydistributed PSMA-SC(S)S-PSMA (Mn ) 6663,Mw/Mn ) 1.25)and PSMA-b-PNIPAAm-b-PSMA (Mn ) 9515,Mw/Mn ) 1.47andMn ) 14 296,Mw/Mn ) 1.55, respectively) are obtained.The triblock copolymer composition is calculated from theGPC data; PSMA10-b-PNIPAAm26-b-PSMA10 and PSMA10-b-PNIPAAm68-b-PSMA10 were prepared. The typical mol-ecular weight distributions for PSMA-SC(S)S-PSMA and theformer triblock copolymer are shown in Figure 4. A comparisonwith the GPC curve of PSMA-SC(S)S-PSMA in Figure 4Ashows that a single GPC peak of PSMA-b-PNIPAAm-b-PSMA appears at a high-molecular-weight position and exhibitsno appreciable tailing at the lower molecular weight side inFigure 4B, indicating the participation of almost all PSMA-SC(S)S-PSMA in the radical polymerization of NIPAAm.

3.2. Self-Assembly Behaviors of PSMA-b-PNIPAAm-b-PSMA in Selective Solvents.It is known that the morphologyand dimension of aggregates via self-assembly principallydepend on several factors, including the copolymer concentra-tion, the copolymer composition, the nature of common solvent,temperature, pH, and many other conditions.5-8 All of thesefactors and the interplay between them influence the morphol-ogies of these aggregates. In the present paper, we discussmainly the effects of the common solvent and the copolymercomposition on the morphologies of PSMA-b-PNIPAAm-b-PSMA aggregates.

Due to the higher composition of the hydrophobic PSMAblocks, the triblock copolymer cannot be dissolved in waterdirectly. However, the copolymer can be first dissolved in anorganic solvent that is good for both blocks. THF and 1,4-dioxane were tentatively used as good solvents for both PSMAand PNIPAAm. After that, deionized water was added slowlyto the solution to predetermined contents. Unless stated other-wise, all the aggregates were prepared and characterized at25 °C.

3.2.1. Effect of Various Common Organic SolVents on theSelf-Assembly BehaVior. As shown in Figure 5, a set of typicalTEM micrographs demonstrates the morphologies of aggregatesself-assembled from PSMA10-b-PNIPAAm68-b-PSMA10 at thepolymer concentration of 0.1 wt % in THF/water with variouswater contents. From Figure 5A, we can see that the dominantmorphology is a giant vesicle with the average diameter of ca.1.5 µm when the water content is maintained at 10.0 wt %.Figure 5B is the magnification of Figure 5A, from which wecan see that it is clear that a transmission around the peripheryof the aggregates is lower than in their center and the wallthickness of the giant vesicle is about 80 nm. The stretchingand structure of the vesicles from this triblock copolymer aredifferent from those of diblock copolymer and other types oftriblock copolymer (e.g., the hydrophilic segments on both endsof hydrophobic block). A schematic representation of PSMA10-b-PNIPAAm68-b-PSMA10 vesicular aggregates in THF/water isgiven in Figure 6A. The PSMA blocks associate with each otherand form the vesicle wall, while the soluble PNIPAAm blocksextend from the inner and outer surfaces into the selectivesolvents (Figure 6A).25

When the water content becomes higher, to 30.0 wt %, it isfound that the triblock copolymer self-assembles into giantspheres with average outer diameters of 1.0µm (Figure 5C).Figure 5D is the magnification of Figure 5C, from which wecan see that it is obvious that a transmission in the center ofthe aggregates is lower than around the periphery. Clearly, thesphere has a typical core-shell structure with a core radius about1000 nm and a shell about 80 nm. The looping of the hydrophilicPNIPAAm middle block should form the corona of theaggregates and the tailing of two hydrophobic PSMA end blocksshould associate with each other in the core of the aggregates.Considering the chemical structure of PSMA10-b-PNIPAAm68-b-PSMA10, the dimension of these aggregates is very large. Ifthe PSMA10-b-PNIPAAm68-b-PSMA10 chains stretched out inthe solution, the diameter of these aggregates should not exceed22.5 nm) 10 × 2 × 0.25 (the PSMA blocks)+ 2 × 0.25 nm(the trithiocarbonate group)+ 68 × 0.25 nm (the PNIPAAmblocks). Thus, these resultant aggregates are not simple spheresand some PNIPAAm blocks must be located inside the core ofthe aggregates. Consequently, PSMA10-b-PNIPAAm68-b-PS-MA10 molecules form mainly a structure analogous to largecompound micelles. Yan et al. have reported large sphericalaggregates from hyperbranched multiarm copolyethers of PEHO-star-PPO and explained the formation of the large aggregatesby the model of multimicelle aggregates.26 Eisenberg et al. havesuccessfully explained the large micelles (up to 1200 nm)formed from linear copolymers of PS-b-PAA by the “LCM”model.27 According to the above studies, a tentative molecularpacking model for the self-assembly of PSMA10-b-PNIPAAm68-b-PSMA10 molecules is presented in Figure 6B. On the self-assembly of amphiphilic triblock copolymers, as water is addedto the initial solution, the solubility of the hydrophobic block(the PSMA block) is decreased; they would associate to formsmall spherical aggregates driven by the attractive forces

Figure 4. GPC curves of PSMA10-SC(S)S-PSMA10 (A) and PSMA10-b-PNIPAAm26-b-PSMA10 (B).

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between the molecules and the repulsive forces that prevent theinitial growth of the aggregate.28 It should be stressed that smallspheres presented here are similar to the conventional core-shell-type aggregates with small size (less than 22.5 nm).Meanwhile, the corona of the aggregates should be composedof the hydrophilic PNIPAAm blocks and some carboxyl groups,and the core of the aggregates is formed from the hydrophobicPSMA blocks. However, these small spheres are just an

intermediate and would collide, fuse, and undergo a secondaryaggregation to form large spherical aggregates with core-shellstructure by intermicellar interactions such as hydrogen bondsbetween the carboxyl groups, acrylamide groups, and van derWaals interactions.26

When the water content is raised to 50.0 wt %, it is foundthat these giant core-shell spheres are fused together (Figure5E). This result is ascribed to the fact that the repulsive

Figure 5. TEM micrographs of the aggregates made from PSMA10-b-PNIPAAm68-b-PSMA10 at various water contents in THF/water at polymerconcentration of 0.1 wt %. Water content: 10.0 (A), 10.0 (B), 30.0 (C), 30.0 (D), and 50.0 wt % (E), respectively.

Figure 6. Proposed molecular packing models for the self-assembly of PSMA10-b-PNIPAAm68-b-PSMA10 molecules at various water contents inTHF/water at polymer concentration of 0.1 wt %. Water content: 10.0 (A), 30.0 (B), and 50.0 wt % (C), respectively. The blue zones presentPSMA blocks, the red ones express carboxyl groups, and the green ones denote PNIPAAm blocks.

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interaction of the corona (the PNIPAAm blocks) can nolonger stabilize giant spherical aggregates with the watercontent increasing. It will result in the flocculation of thecorona-chain and the fusion of these giant spheres. The possiblemechanism of the morphological transition is shown in Fig-ure 6C.

Simultaneously, the self-assembly behavior of the triblockcopolymer is investigated in 1,4-dioxane/water. As shown inFigure 7, a series of typical TEM micrographs demonstratesthe morphologies of aggregates self-assembled from PSMA10-b-PNIPAAm68-b-PSMA10 at the polymer concentration of 0.1wt % in 1,4-dioxane/water with various water contents. FromFigure 7A, we can see that the dominant morphology is vesicleswith average diameters of ca. 250-500 nm when the watercontent is maintained at 10.0 wt %. When the water contentbecomes as high as 30.0 wt %, it is found that the triblockcopolymers self-assemble into these core-shell spheres withaverage diameters of about 200 nm (Figure 7B). As the watercontent further increases to 50.0 wt %, pearl-necklace-likeaggregates form from the triblock copolymer as shown inFigure 7C.

Depending on the nature of organic solvent employed, it canbe found that the morphologies of aggregates formed in dioxane/water are similar to ones formed in THF/water except for theirdimensions. The stretching of the core, the interfacial tensionbetween the core and the solvent, and the intercorona repulsionare believed to be the main parameters dominating the formationof multimorphological aggregates.29 In the case of the triblockcopolymer studied here, it seems that the nature of the organicsolvent mainly controls the morphologies owing to the samecopolymer and water content. The swelling degree of the PSMAblocks in THF more than that in dioxane reflects their solubility.Therefore, the stretching degree of the PSMA cores at the onsetof self-assembly is higher for THF as the common solvent thanfor dioxane. Moreover, when water is used as the precipitant,it is known that the number of the aggregates increases as thecommon solvent is changed from dioxane to THF.30,31 On thebasis of the self-assembly mechanism (Figure 6), the dimensionof the resultant aggregates should be larger for THF than fordioxane.

3.2.2. Effect of Copolymer Composition on Self-AssemblyBehaVior. The copolymer composition is another effectiveapproach to affect the morphology of aggregates. As shown inFigure 8, PSMA10-b-PNIPAAm68-b-PSMA10 molecules yieldalone large core-shell spheres with average diameters about500-1000 nm (Figure 8A) at the polymer concentration of 0.1wt%inTHF/water70/30(w/w),whereasPSMA10-b-PNIPAAm26-b-PSMA10 molecules form a lot of vesicles with averagedimension about 250 nm in the same self-assembly condition(Figure 8B). This alteration of the morphology, as the hydro-philic block is decreased, is similar to previous reports on otherdiblock copolymers.32

Eisenberg et al. have pointed out that three sources are themajor contributions to the thermodynamics in micelles, namely,the core chain stretching, the interfacial tension, and the coronarepulsion.29 In this case, it is seemed that the hydrophilic block(the PNIPAAm block), that is, the hydrophilic/hydrophobicbalance, mainly controls the morphologies since the hydrophobicblock length (the PSMA block) in block copolymers is constant.With the PNIPAAm block length increasing, the corona

Figure 7. TEM micrographs of aggregates made from PSMA10-b-PNIPAAm68-b-PSMA10 at various water contents in 1,4-dioxane/water at polymerconcentration of 0.1 wt %. Water content: 10.0 (A), 30.0 (B), and 50.0 wt % (C), respectively.

Figure 8. TEM micrographs of aggregates made from PSMA-b-PNIPAAm-b-PSMA with different copolymer compositions at polymer concentrationof 0.1 wt % in THF/water 70/30 (w/w). (A) PSMA10-b-PNIPAAm68-b-PSMA10 and (B) PSMA10-b-PNIPAAm26-b-PSMA10.

Figure 9. Thermoresponsive behaviors of aggregates made fromPSMA10-b-PNIPAAm68-b-PSMA10 at polymer concentration of 0.1 wt% in THF/H2O 70/30 (w/w) upon temperature changes. Diameterchanges as a function of temperature.

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repulsion must be improved, which favors a decrease in theradius of curvature of the aggregates. Consequently, themorphologies should change in the direction of structures tovesicles in order to reduce the free energy of the system at thesame water content.33

The morphologies of aggregates from different polymersobtained under a variety of preparative conditions are sum-marized in Table 1.

3.3. Double-Responsive Characteristics of PSMA-b-PNIPAAm-b-PSMA Aggregates.The combination of thermo-responsive PNIPAAm and pH-responsive carboxyl groups inthe triblock copolymer results in a system that responds to bothtemperature and pH. Here, we use the DLS technique toinvestigate the change of the average diameter of polymericaggregates as the function of temperature or pH.

3.3.1. ThermoresponsiVe Structural Change of PSMA-b-PNIPAAm-b-PSMA Aggregates.It is known that the conforma-tion and solubility of the PNIPAAm chains in water can changewith temperature. The hydrophilic PNIPAAm chain is highlyhydrated and adopts an extended conformation below its LCST,while it may be dehydrated and therefore collapse near or aboveits phase transformation temperature.34 To determine whetherthese PSMA10-b-PNIPAAm68-b-PSMA10 aggregates exhibit athermal response in selective solvents, the change of the averagediameter of the polymeric aggregates as a function of temper-ature was examined at the polymer concentration of 0.1 wt %in THF/water 70/30 (w/w) by DLS. It is worth noting that eachdata point was obtained after the dispersion reached thermalequilibrium. A thermoresponsive characteristic of the PSMA-b-PNIPAAm-b-PSMA aggregates is shown in Figure 9. It isobvious that the diameter of the aggregates undergoes a changefrom ca. 800 nm to ca.1550 nm at a temperature correspondingto the LCST of the PNIPAAm. The phenomenon is ascribed tothe change of the hydrophobic/hydrophilic property of PNIPAAm.When the temperature is raised to above the LCST, thePNIPAAm chains turn more hydrophobic and collapse on theaggregates. More aggregates are needed to associate togetherin order to improve the shell-solvent interactions and therepulsion of the corona. Finally, these aggregates form largeraggregates with the multicore structure and the intercoreconnections via the intramolecular bridging between thePNIPAAm blocks, and avoid precipitation in selective sol-vents.35 The schematic model for the variation of diameter ofthe aggregates upon temperature changes is shown inFigure 10.

3.3.2. pH-ResponsiVe Structural Change of PSMA-b-PNIPAAm-b-PSMA Aggregates.To determine whether these PSMA10-b-PNIPAAm68-b-PSMA10 aggregates exhibit a pH response, thechange of the average diameter as a function of pH was alsoconfirmed at the polymer concentration of 0.1 wt % in THF/water 70/30 (w/w) by DLS at 25°C. It is worth stressing thatthe pH value of the original solution without HCl or NaOH isabout 5.40. A pH-responsive characteristic of the PSMA-b-

PNIPAAm-b-PSMA aggregates is shown in Figure 11. It isobvious that the diameter of the aggregates undergoes changeswith the variation of pH. The triblock copolymer yields giantspheres with average diameters of 960 nm without the additionof HCl or NaOH to the solution (Figure 11A). When HCl isadded to the solution, the pH value is changed to 2.0 and thediameter of the aggregates is changed to ca. 1600 nm (Figure11B). However, when NaOH is used and the pH value ofthe solution is converted into 9.0, it is found that the diameterof the aggregates is altered to ca. 1200 nm (Figure 11C).The conversion may be due to the fact that carboxyl groups areprotonated or deprotonated and intermolecular hydrogenbonds are changed with the variation of pH in selectivesolvents.33,36

4. Conclusion

A series of novel triblock copolymers of PSMA-b-PNIPAAm-b-PSMA with different molecular weights were syn-thesized through carboxyl-terminated trithiocarbonates as ahighly efficient RAFT agent via RAFT polymerization. Theformation of aggregates of various structures from PSMA-b-PNIPAAm-b-PSMA triblock copolymers in solvent mix-tures was studied by varying the organic solvent used in theself-assembly procedure and adjusting the copolymer composi-tion. It was found that, as the water content increases,the morphology of the aggregates in THF/water changesfrom giant vesicles to giant spheres, and then to ass-ociated spheres, while it changes from small vesiclesto small spheres and then to “pearl necklaces” in dioxane/water. As the PNIPAAm block length increases, the morph-ology of the aggregates in THF/water changes from giantspheres to small vesicles. Moreover, the resultant aggregatesshowed thermoresponsive and pH-responsive propertiesthrough the lower critical solution temperature (LCST) ofPNIPAAm and the two carboxyl end groups. The resultant

Figure 10. Schematic model for the variation of diameter of the aggregates upon temperature changes.

Figure 11. pH-responsive behaviors of aggregates made from PSMA10-b-PNIPAAm68-b-PSMA10 at polymer concentration of 0.1 wt % in THF/H2O 70/30 (w/w) upon pH changes. pH 5.40 (A), 2.00 (B), and 9.00(C). Diameter changes as a function of pH.

PSMA-b-PNIPAAm-b-PSMA Aggregates J. Phys. Chem. B, Vol. 111, No. 20, 20075579

aggregates might become important in new opportunities forapplications in thefield of stimuli-responsive sensors, micro-capsules, and drug delivery.

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