Novel Thermoresponsive and pH-Responsive Aggregates from Self

May 2, 2007 - ReceiVed: January 19, 2007; In Final Form: March 4, 2007. A series of ... methacrylate has a pendent long alkyl side chain.18. In this s...
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J. Phys. Chem. B 2007, 111, 5573-5580

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Novel Thermoresponsive and pH-Responsive Aggregates from Self-Assembly of Triblock Copolymer PSMA-b-PNIPAAm-b-PSMA Junfeng Zhou, Li Wang,* Qiang Yang, Qingquan Liu, Haojie Yu, and Zhenrong Zhao State 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 through carboxyl-terminated trithiocarbonates as a highly efficient RAFT agent via reversible addition-fragmentation chain transfer (RAFT) polymerization. The resultant polymers were characterized by 1H NMR, FT-IR spectroscopy, and GPC. By varying the organic solvent used in the self-assembly procedure and adjusting the copolymer composition, multiple morphologies ranging from vesicles and core-shell spherical aggregates with different dimensions to pearl-necklace-like aggregates were obtained. The aggregates showed thermoresponsive and pH-responsive properties through the lower critical solution temperature (LCST) of PNIPAAm and the two carboxyl end groups of the copolymer.

1. Introduction In recent years, considerable efforts have been devoted to preparing stimuli-responsive polymers. This kind of “smart” materials exhibits dramatic changes in their properties in response to the application of environmental stimuli, such as temperature, pH, ionic strength, electric or magnetic fields, and so on.1-3 Since their excellent physical and chemical properties can be adjusted by external stimuli, stimuli-responsive polymers have widespread applications in drug delivery systems and in devices such as actuators, artificial muscles, and controlled molecular gates and switches.4 Amphiphilic macromolecules can self-assemble into either core-shell morphology or cavity-containing structures with nanometer or submicrometer scales through weak noncovalent interactions (e.g., hydrogen-bonding interaction, π-π interaction, charge-transfer interaction, van der Waals forces, hostguest interaction, and electrostatic forces). The self-assembled aggregates have attracted great interest in recent years due to their excellent characteristics and broad potential applications such as microreactors, microcapsules, drug delivery systems, and encapsulation of various kinds of guest molecules.5-9 Among the reported aggregates with various sizes, giant aggregates (up to 1 µm) have been an active topic of research because their curvatures are much more similar to those of cells and these aggregates can be used as attractive models for cells and organelles.10,11 Moreover, giant aggregates have a unique property: visibility under the light microscope.12 Poly(N-isopropylacrylamide) (PNIPAAm) is one of the most studied thermoresponsive polymers and exhibits a reversible thermoresponsive phase transition in aqueous solution.13,14 Below a specific lower critical solution temperature (LCST), PNIPAAm is water-soluble, is hydrophilic, and exists in an extended chain form, while PNIPAAm changes to an insoluble and hydrophobic aggregate due to its coil-to-globule transition above the LCST. Up to now, the self-assembly behaviors of * Corresponding author. Telephone: (86)-571-87953200. Fax: (86)-57187951612. E-mail: [email protected].

thermoresponsive diblock copolymers have been investigated widely because the LCST of PNIPAAm is close to physiological temperature and the self-assembled aggregates have enormous potential in technology and in biomedical application.15 For example, Shi et al. have investigated the thermoresponsive micellization of poly(ethylene glycol)-b-poly(N-isopropylacrylamide) (PEG110-b-PNIPAAm44) in water by static light scattering and dynamic light scattering.16 However, to our knowledge, the studies on the self-assembly behavior of triblock copolymers are not sufficient. Moreover, if the thermoresponsive polymers combine with carboxyl groups, they will create new systems that can respond to complex external stimuli. The novel thermoresponsive or pH-sensitive aggregates self-assembled from the above polymers can be conjugated to drugs to be applied in drug delivery through external stimuli.17 With these in mind, a novel triblock copolymer capped with two carboxyl end groups (PSMA-bPNIPAAm-b-PSMA; where PSMA is poly(stearyl methacrylate)) is designed, and the resultant polymer can exhibit pHresponsive and thermoresponsive characteristics. Moreover, these carboxyl groups and acrylamide groups can provide the self-assembly driving forces (intermolecular hydrogen bond) to form giant aggregates. The “intelligent” polymeric aggregates formed from the polymers are of interest in further chemical or biological modification due to carboxyl groups on the surface of the aggregates. Furthermore, the resultant copolymer is a brush-type triblock copolymer and possesses unique characteristics and potential applicable foreground because stearyl methacrylate has a pendent long alkyl side chain.18 In this study, we synthesize and characterize novel brushtype triblock copolymers capped with two carboxyl groups, PSMA-b-PNIPAAm-b-PSMA, which are prepared conveniently through carboxyl-terminated trithiocarbonates as a highly efficient RAFT (reversible addition-fragmentation chain transfer) agent via RAFT polymerization in 1,4-dioxane solution, and multiple morphologies and different dimensions of these aggregates are obtained by the self-assembly process from these triblock copolymers. The thermoresponsive and pH-responsive

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

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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 PSMA10-b-PNIPAAm68-b-PSMA10 PSMA10-b-PNIPAAm68-b-PSMA10 PSMA10-b-PNIPAAm68-b-PSMA10 PSMA10-b-PNIPAAm68-b-PSMA10 PSMA10-b-PNIPAAm68-b-PSMA10 PSMA10-b-PNIPAAm26-b-PSMA10

0.1 0.1 0.1 0.1 0.1 0.1 0.1

THF THF THF dioxane dioxane dioxane THF

10.0 30.0 50.0 10.0 30.0 50.0 30.0

giant vesicles giant spheres associated spheres vesicles spheres pearl necklaces vesicles

characteristics of the aggregates are also studied by dynamic light scattering (DLS). 2. Experimental Section 2.1. Materials. N-Isopropylacrylamide (NIPAAm; Aldrich Organics) was recrystallized from hexane-acetone solution before use. 1,1′-Azobis(isobutyronitrile) (AIBN; Shanghai No.1 Chemical Reagent Factory) was purified by recrystallization from methanol. Stearyl methacrylate (SMA; Tianjing Tianjiao Chemical Reagent Factory) was treated according to the literature method.19 SMA was purified by dissolution in hexane and extraction three times with 5.0% aqueous NaOH. After the organic phase was dried over magnesium sulfate, the solution was passed through neutral alumina and solvent was removed under reduced pressure. All other reagents were of analytical grade and used as received. 2.2. Synthesis of S,S′-Bis(r,r-dimethyl-r′′-acetic acid)trithiocarbonate BDAAT.20 Carbon disulfide (6.8501 g, 0.09 mol), chloroform (26.9008 g, 0.225mol), acetone (13.08 g, 0.225 mol), and tetrabutylammonium hydrogen sulfate (phase transfer catalysis, PTC) (0.6025 g, 1.775 mmol) were mixed with 30 mL of 1,4-dioxane in a 250 mL jacketed reactor cooled with tap water under nitrogen. Sodium hydroxide aqueous solution (50.0 wt %) (50.4002 g, 0.63 mol) was added dropwise over 90 min in order to keep the temperature below 25 °C. The reaction was stirred overnight. A 100 mL volume of water was then added to dissolve the solid, followed by 60 mL of concentrated HCl to acidify the aqueous layer and stirring for 30 min with nitrogen purge. The solution was filtered, and the solid was rinsed thoroughly with water. It was dried to constant weight 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 a yellow crystalline solid. 1H NMR (CDCl3, ppm from TMS): 1.69 (s, 12H), 12.91 (s, 2H). 2.3. RAFT Polymerization of Macro-Chain-Transfer Agent PSMA-SC(S)S-PSMA. A single-neck round-bottom flask was charged with SMA (5.0786 g, 15mmol), BDAAT (0.1860 g, 0.74mmol), AIBN (0.0246 g, 0.15mmol), and 1,4-dioxane (6.0 mL). The flask was degassed by three consecutive freezepump-thaw cycles and then immersed in an oil bath thermostated at 60 °C for 7 h. The polymerization was terminated by rapid cooling and freezing. The crude product was precipitated three times in anhydrous ethanol to remove SMA monomer and 1,4-dioxane. 2.4. RAFT Polymerization of Block Copolymer PSMAb-PNIPAAm-b-PSMA. A single-neck round-bottom flask was charged 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 was degassed by three freeze-pump-thaw cycles and then immersed in an oil bath thermostated at 60 °C. After several hours (5 or 11 h), the polymerization flask was rapidly cooled to room temperature; the crude product was precipitated three times in anhydrous ethanol to remove NIPAAm monomer and 1,4dioxane. 2.5. Self-Assembly Procedure of PSMA-b-PNIPAAm-bPSMAMolecules.ThetriblockcopolymerPSMA10-b-PNIPAAm68b-PSMA10 was first dissolved in organic solvents (THF or 1,4dioxane) at a desired concentration before use at room temperature. Deionized water was then added slowly to each of the copolymer solutions under stirring with a magnetic bar. In the system of the polymer/organic solvent, the final water

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

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

prepared by drop-casting self-assembled solutions onto carboncoated copper grids and air-dried at room temperature before measurement. Dynamic light scattering (DLS) measurements were performed in THF/water using a Zetasizer 3000HSA apparatus (Malvern Instruments Ltd.) equipped with a 125 mW laser operating at λ ) 633 nm. All the DLS measurements were at a scattering angle of 90°. Energy-dispersive X-ray analysis (EDAX) was carried out on a Hitachi S-4800 scanning electron microscope (SEM) equipped with an energy-dispersive X-ray analyzer (EDAX 350, Horiba) at a voltage of 15 kV. 3. Results and Discussion

Figure 2. FT-IR spectra of PSMA10-SC(S)S-PSMA10 (A) and PSMA10b-PNIPAAm68-b-PSMA10 (B).

content was varied between 10.0 and 50.0 wt % while the polymer concentration of each of samples was maintained at 0.1 wt % in the resultant solution. A similar procedure was performed to prepare aggregates of PSMA10-b-PNIPAAm26-bPSMA10. Samples for TEM measurement were prepared by mounting a drop (ca. 10 µL) of the above solution on the carbon-coated Cu grids and allowing the samples to dry in air. 2.6. Characterization. 1H NMR experiments were carried out on a 400 MHz AVANCE NMR spectrometer (Model DMX400) using CDCl3 as solvent and tetramethylsilane as internal reference. Fourier transform infrared (FT-IR) spectra were obtained on a Nicolet 5700 infrared spectrometer. Molecular weight and molecular weight distribution were determined on a Waters 201 gel permeation chromatograph (GPC) equipped with UltraStyragel columns with pore sizes of 103105 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 refraction index increment value of the polymer solutions dn/dc ) 0.20 were used for laser scattering detection. Transmission electron microscopy (TEM) studies were performed with a JEOL Model 1200EX instrument at a voltage of 160 kV. Samples were

3.1. Preparation of PSMA-b-PNIPAAm-b-PSMA. It has been reported that the RAFT polymerization of NIPAAm is a living free radical polymerization in nature.23,24 Therefore, the RAFT polymerization of NIPAAm with macromolecules containing trithiocarbonate groups as macro-chain-transfer agent can be applied to prepare a series of novel triblock copolymers capped with two carboxyl groups (PSMA-PNIPAAm-PSMA).21,22 The synthesis of macro-chain-transfer agent BDAAT and the synthetic strategy followed for the triblock copolymer via the RAFT route are depicted in Scheme 1. The homopolymer PSMA-SC(S)S-PSMA was prepared using BDAAT as the RAFT agent in 1,4-dioxane solution at 60 °C. Subsequently, amphiphilic triblock copolymer PSMA-b-PNIPAAm-b-PSMA was synthesized through the RAFT polymerization of NIPAAm with PSMA-SC(S)S-PSMA as macro-chain-transfer agent. The chemical structure of the homopolymer PSMA-SC(S)SPSMA was studied first by 1H NMR spectroscopy. The 1H NMR spectrum of PSMA-SC(S)S-PSMA shows peaks with the following shifts: the H of the methylene group adjacent to the oxygen atom of stearyl shows a chemical shift of 4.10-3.82 ppm, the H of the methylene in the backbone shows a chemical shift of 1.99-1.72 ppm, the H of the methylene adjacent to the methylene connected to the oxygen atom of stearyl shows a chemical shift of 1.66-1.52 ppm, the H in the long-chain methylene bands exhibits a chemical shift of 1.40-1.20 ppm, and the chemical shifts for the methyl groups are ranged within 0.90-0.78 ppm. The 1H NMR result is thus consistent with the structure of the homopolymer PSMA-SC(S)S-PSMA. To verify the existence of carboxyl groups and trithiocarbonate groups in the homopolymer PSMA-SC(S)S-PSMA, FTIR spectroscopy was used. The FT-IR spectrum of PSMASC(S)S-PSMA is shown in Figure 2A. As well as the characteristic absorption band for the long alkyl chain of PSMA at 2923, 2852 cm-1 in Figure 2A, the characteristic absorption bands for the trithiocarbonate groups (>CdS, 1062 cm-1) and for the carboxyl groups (-COOH, 3448 cm-1) also appear in this figure. Energy-dispersive X-ray analysis (EDAX) was used to further characterize PSMA-SC(S)S-PSMA. Figure 3 shows

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Figure 4. GPC curves of PSMA10-SC(S)S-PSMA10 (A) and PSMA10b-PNIPAAm26-b-PSMA10 (B).

the EDAX spectrum of PSMA-SC(S)S-PSMA. An obvious sulfur peak could be observed in the sample. The above results demonstrate that BDAAT participates in the RAFT polymerization of SMA. Consequently, PSMA-SC(S)S-PSMA was successfully prepared by the RAFT polymerization in the presence of BDAAT. For confirmation of the triblock polymer structure, a typical 1H NMR spectrum was measured and is shown in Figure 1B. Compared with the 1H NMR spectrum of PSMA-SC(S)S-PSMA in Figure 1A, the characteristic signals of PNIPAAm can be seen at δ ) 1.14 (e) and 6.38 (c) , corresponding to the methyl protons of isopropyl units and the imido group adjacent to the carbonyl group, respectively. This result reveals the successful preparation of the triblock copolymer (PSMA-PNIPAAmPSMA). The chemical structure of PSMA-b-PNIPAAm-b-PSMA was also determined from FT-IR, and the spectrum is shown in Figure 2B. In its FT-IR spectrum, the characteristic bands for PSMA and PNIPAAm (in KBr: C-H at 2923, 2852, 1468 cm-1; CdO at 1731 cm-1; and N-H at 3367, 1654 cm-1) are clearly observed. The molecular weights and their distribution of PSMA-SC(S)S-PSMA and PSMA-b-PNIPAAm-b-PSMA were characterized by GPC in THF using polystyrene calibration. The narrowly distributed PSMA-SC(S)S-PSMA (Mn ) 6663, Mw/Mn ) 1.25) and PSMA-b-PNIPAAm-b-PSMA (Mn ) 9515, Mw/Mn ) 1.47 and Mn ) 14 296, Mw/Mn ) 1.55, respectively) are obtained. The triblock copolymer composition is calculated from the GPC data; PSMA10-b-PNIPAAm26-b-PSMA10 and PSMA10-bPNIPAAm68-b-PSMA10 were prepared. The typical molecular weight distributions for PSMA-SC(S)S-PSMA and the former triblock copolymer are shown in Figure 4. A comparison with the GPC curve of PSMA-SC(S)S-PSMA in Figure 4A shows that a single GPC peak of PSMA-b-PNIPAAm-bPSMA appears at a high-molecular-weight position and exhibits no appreciable tailing at the lower molecular weight side in Figure 4B, indicating the participation of almost all PSMASC(S)S-PSMA in the radical polymerization of NIPAAm. 3.2. Self-Assembly Behaviors of PSMA-b-PNIPAAm-bPSMA in Selective Solvents. It is known that the morphology and dimension of aggregates via self-assembly principally depend on several factors, including the copolymer concentration, the copolymer composition, the nature of common solvent, temperature, pH, and many other conditions.5-8 All of these factors and the interplay between them influence the morphologies of these aggregates. In the present paper, we discuss mainly the effects of the common solvent and the copolymer composition on the morphologies of PSMA-b-PNIPAAm-bPSMA aggregates.

Zhou et al. Due to the higher composition of the hydrophobic PSMA blocks, the triblock copolymer cannot be dissolved in water directly. However, the copolymer can be first dissolved in an organic solvent that is good for both blocks. THF and 1,4dioxane were tentatively used as good solvents for both PSMA and PNIPAAm. After that, deionized water was added slowly to the solution to predetermined contents. Unless stated otherwise, all the aggregates were prepared and characterized at 25 °C. 3.2.1. Effect of Various Common Organic SolVents on the Self-Assembly BehaVior. As shown in Figure 5, a set of typical TEM micrographs demonstrates the morphologies of aggregates self-assembled from PSMA10-b-PNIPAAm68-b-PSMA10 at the polymer concentration of 0.1 wt % in THF/water with various water contents. From Figure 5A, we can see that the dominant morphology 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 we can see that it is clear that a transmission around the periphery of the aggregates is lower than in their center and the wall thickness of the giant vesicle is about 80 nm. The stretching and structure of the vesicles from this triblock copolymer are different from those of diblock copolymer and other types of triblock copolymer (e.g., the hydrophilic segments on both ends of hydrophobic block). A schematic representation of PSMA10b-PNIPAAm68-b-PSMA10 vesicular aggregates in THF/water is given in Figure 6A. The PSMA blocks associate with each other and form the vesicle wall, while the soluble PNIPAAm blocks extend from the inner and outer surfaces into the selective solvents (Figure 6A).25 When the water content becomes higher, to 30.0 wt %, it is found that the triblock copolymer self-assembles into giant spheres with average outer diameters of 1.0 µm (Figure 5C). Figure 5D is the magnification of Figure 5C, from which we can see that it is obvious that a transmission in the center of the aggregates is lower than around the periphery. Clearly, the sphere has a typical core-shell structure with a core radius about 1000 nm and a shell about 80 nm. The looping of the hydrophilic PNIPAAm middle block should form the corona of the aggregates and the tailing of two hydrophobic PSMA end blocks should associate with each other in the core of the aggregates. Considering the chemical structure of PSMA10-b-PNIPAAm68b-PSMA10, the dimension of these aggregates is very large. If the PSMA10-b-PNIPAAm68-b-PSMA10 chains stretched out in the solution, the diameter of these aggregates should not exceed 22.5 nm ) 10 × 2 × 0.25 (the PSMA blocks) + 2 × 0.25 nm (the trithiocarbonate group) + 68 × 0.25 nm (the PNIPAAm blocks). Thus, these resultant aggregates are not simple spheres and some PNIPAAm blocks must be located inside the core of the aggregates. Consequently, PSMA10-b-PNIPAAm68-b-PSMA10 molecules form mainly a structure analogous to large compound micelles. Yan et al. have reported large spherical aggregates from hyperbranched multiarm copolyethers of PEHOstar-PPO and explained the formation of the large aggregates by the model of multimicelle aggregates.26 Eisenberg et al. have successfully 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 molecular packing model for the self-assembly of PSMA10-b-PNIPAAm68b-PSMA10 molecules is presented in Figure 6B. On the selfassembly of amphiphilic triblock copolymers, as water is added to the initial solution, the solubility of the hydrophobic block (the PSMA block) is decreased; they would associate to form small spherical aggregates driven by the attractive forces

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Figure 5. TEM micrographs of the aggregates made from PSMA10-b-PNIPAAm68-b-PSMA10 at various water contents in THF/water at polymer concentration 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 in THF/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 present PSMA blocks, the red ones express carboxyl groups, and the green ones denote PNIPAAm blocks.

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

intermediate and would collide, fuse, and undergo a secondary aggregation to form large spherical aggregates with core-shell structure by intermicellar interactions such as hydrogen bonds between the carboxyl groups, acrylamide groups, and van der Waals interactions.26 When the water content is raised to 50.0 wt %, it is found that these giant core-shell spheres are fused together (Figure 5E). This result is ascribed to the fact that the repulsive

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Figure 7. TEM micrographs of aggregates made from PSMA10-b-PNIPAAm68-b-PSMA10 at various water contents in 1,4-dioxane/water at polymer concentration 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 concentration of 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 from PSMA10-b-PNIPAAm68-b-PSMA10 at polymer concentration of 0.1 wt % in THF/H2O 70/30 (w/w) upon temperature changes. Diameter changes as a function of temperature.

interaction of the corona (the PNIPAAm blocks) can no longer stabilize giant spherical aggregates with the water content increasing. It will result in the flocculation of the corona-chain and the fusion of these giant spheres. The possible mechanism of the morphological transition is shown in Figure 6C. Simultaneously, the self-assembly behavior of the triblock copolymer is investigated in 1,4-dioxane/water. As shown in Figure 7, a series of typical TEM micrographs demonstrates the morphologies of aggregates self-assembled from PSMA10b-PNIPAAm68-b-PSMA10 at the polymer concentration of 0.1 wt % in 1,4-dioxane/water with various water contents. From Figure 7A, we can see that the dominant morphology is vesicles with average diameters of ca. 250-500 nm when the water content is maintained at 10.0 wt %. When the water content becomes as high as 30.0 wt %, it is found that the triblock copolymers self-assemble into these core-shell spheres with average diameters of about 200 nm (Figure 7B). As the water content further increases to 50.0 wt %, pearl-necklace-like aggregates form from the triblock copolymer as shown in Figure 7C.

Depending on the nature of organic solvent employed, it can be found that the morphologies of aggregates formed in dioxane/ water are similar to ones formed in THF/water except for their dimensions. The stretching of the core, the interfacial tension between the core and the solvent, and the intercorona repulsion are believed to be the main parameters dominating the formation of multimorphological aggregates.29 In the case of the triblock copolymer studied here, it seems that the nature of the organic solvent mainly controls the morphologies owing to the same copolymer and water content. The swelling degree of the PSMA blocks in THF more than that in dioxane reflects their solubility. Therefore, the stretching degree of the PSMA cores at the onset of self-assembly is higher for THF as the common solvent than for dioxane. Moreover, when water is used as the precipitant, it is known that the number of the aggregates increases as the common solvent is changed from dioxane to THF.30,31 On the basis of the self-assembly mechanism (Figure 6), the dimension of the resultant aggregates should be larger for THF than for dioxane. 3.2.2. Effect of Copolymer Composition on Self-Assembly BehaVior. The copolymer composition is another effective approach to affect the morphology of aggregates. As shown in Figure 8, PSMA10-b-PNIPAAm68-b-PSMA10 molecules yield alone large core-shell spheres with average diameters about 500-1000 nm (Figure 8A) at the polymer concentration of 0.1 wt % in THF/water 70/30 (w/w), whereas PSMA10-b-PNIPAAm26b-PSMA10 molecules form a lot of vesicles with average dimension about 250 nm in the same self-assembly condition (Figure 8B). This alteration of the morphology, as the hydrophilic block is decreased, is similar to previous reports on other diblock copolymers.32 Eisenberg et al. have pointed out that three sources are the major contributions to the thermodynamics in micelles, namely, the core chain stretching, the interfacial tension, and the corona repulsion.29 In this case, it is seemed that the hydrophilic block (the PNIPAAm block), that is, the hydrophilic/hydrophobic balance, mainly controls the morphologies since the hydrophobic block length (the PSMA block) in block copolymers is constant. With the PNIPAAm block length increasing, the corona

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

repulsion must be improved, which favors a decrease in the radius of curvature of the aggregates. Consequently, the morphologies should change in the direction of structures to vesicles in order to reduce the free energy of the system at the same water content.33 The morphologies of aggregates from different polymers obtained under a variety of preparative conditions are summarized in Table 1. 3.3. Double-Responsive Characteristics of PSMA-bPNIPAAm-b-PSMA Aggregates. The combination of thermoresponsive PNIPAAm and pH-responsive carboxyl groups in the triblock copolymer results in a system that responds to both temperature and pH. Here, we use the DLS technique to investigate the change of the average diameter of polymeric aggregates as the function of temperature or pH. 3.3.1. ThermoresponsiVe Structural Change of PSMA-bPNIPAAm-b-PSMA Aggregates. It is known that the conformation and solubility of the PNIPAAm chains in water can change with temperature. The hydrophilic PNIPAAm chain is highly hydrated and adopts an extended conformation below its LCST, while it may be dehydrated and therefore collapse near or above its phase transformation temperature.34 To determine whether these PSMA10-b-PNIPAAm68-b-PSMA10 aggregates exhibit a thermal response in selective solvents, the change of the average diameter of the polymeric aggregates as a function of temperature was examined at the polymer concentration of 0.1 wt % in THF/water 70/30 (w/w) by DLS. It is worth noting that each data point was obtained after the dispersion reached thermal equilibrium. A thermoresponsive characteristic of the PSMAb-PNIPAAm-b-PSMA aggregates is shown in Figure 9. It is obvious that the diameter of the aggregates undergoes a change from ca. 800 nm to ca.1550 nm at a temperature corresponding to the LCST of the PNIPAAm. The phenomenon is ascribed to the change of the hydrophobic/hydrophilic property of PNIPAAm. When the temperature is raised to above the LCST, the PNIPAAm chains turn more hydrophobic and collapse on the aggregates. More aggregates are needed to associate together in order to improve the shell-solvent interactions and the repulsion of the corona. Finally, these aggregates form larger aggregates with the multicore structure and the intercore connections via the intramolecular bridging between the PNIPAAm blocks, and avoid precipitation in selective solvents.35 The schematic model for the variation of diameter of the aggregates upon temperature changes is shown in Figure 10. 3.3.2. pH-ResponsiVe Structural Change of PSMA-b-PNIPAAmb-PSMA Aggregates. To determine whether these PSMA10-bPNIPAAm68-b-PSMA10 aggregates exhibit a pH response, the change of the average diameter as a function of pH was also confirmed at the polymer concentration of 0.1 wt % in THF/ water 70/30 (w/w) by DLS at 25 °C. It is worth stressing that the pH value of the original solution without HCl or NaOH is about 5.40. A pH-responsive characteristic of the PSMA-b-

Figure 11. pH-responsive behaviors of aggregates made from PSMA10b-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.

PNIPAAm-b-PSMA aggregates is shown in Figure 11. It is obvious that the diameter of the aggregates undergoes changes with the variation of pH. The triblock copolymer yields giant spheres with average diameters of 960 nm without the addition of HCl or NaOH to the solution (Figure 11A). When HCl is added to the solution, the pH value is changed to 2.0 and the diameter of the aggregates is changed to ca. 1600 nm (Figure 11B). However, when NaOH is used and the pH value of the solution is converted into 9.0, it is found that the diameter of the aggregates is altered to ca. 1200 nm (Figure 11C). The conversion may be due to the fact that carboxyl groups are protonated or deprotonated and intermolecular hydrogen bonds are changed with the variation of pH in selective solvents.33,36 4. Conclusion A series of novel triblock copolymers of PSMA-b-PNIPAAmb-PSMA with different molecular weights were synthesized through carboxyl-terminated trithiocarbonates as a highly efficient RAFT agent via RAFT polymerization. The formation of aggregates of various structures from PSMA-bPNIPAAm-b-PSMA triblock copolymers in solvent mixtures was studied by varying the organic solvent used in the self-assembly procedure and adjusting the copolymer composition. It was found that, as the water content increases, the morphology of the aggregates in THF/water changes from giant vesicles to giant spheres, and then to associated spheres, while it changes from small vesicles to small spheres and then to “pearl necklaces” in dioxane/ water. As the PNIPAAm block length increases, the morphology of the aggregates in THF/water changes from giant spheres to small vesicles. Moreover, the resultant aggregates showed thermoresponsive and pH-responsive properties through the lower critical solution temperature (LCST) of PNIPAAm and the two carboxyl end groups. The resultant

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