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Synthesis of Multicompartment Nanoparticles of ABC Triblock Copolymers through Intramolecular Interactions of Two Solvophilic Blocks Habib Khan,† Shengli Chen,† Heng Zhou,† Shuang Wang,† and Wangqing Zhang*,†,‡ †

Key Laboratory of Functional Polymer Materials of the Ministry of Education, Institute of Polymer Chemistry, and ‡Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China S Supporting Information *

ABSTRACT: A new strategy to synthesize multicompartment block copolymer nanoparticles (MCBNs) of poly(4-vinylpyridine)-block-polystyrene-block-poly(4-hydroxystyrene) (P4VP-b-PS-b-P4HS) combining the polymerization-induced self-assembly (PISA) of linear ABC triblock copolymers and the intramolecular complexation through hydrogen bonding between the poly(4-vinylpyridine) (P4VP) and poly(4hydroxystyrene) (P4HS) blocks is proposed. These MCBNs contain a polystyrene (PS) core of about 30 nm and the dispersed 3−5 nm microdomains of the P4VP/P4HS complexes on the solvophobic PS core, in which the size of the dispersed P4VP/P4HS microdomains increases with the polymerization degree (DP) of the P4HS block. The successful synthesis of MCBNs is ascribed to the intramolecular interactions via hydrogen bonding between the P4HS and P4VP blocks confined within the triblock copolymer nanoparticles. The strategy affords the efficient synthesis of MCBNs and avoids the application of the ABC miktoarm terpolymers and the fluorinated ABC triblock copolymers.

1. INTRODUCTION Multicompartment block copolymer nanoparticles (MCBNs) are those composing of phase-separated microdomains in the nanoparticles,1−3 which endow them with promising applications in scheduled delivery of multi drugs, selective catalysis, and nanotechnology. Because of the special structure, these MCBNs are usually prepared through two strategies.2−37 The first strategy is through the micellization of linear fluorinated ABC triblock copolymers and ABC miktoarm star terpolymers,4−11 in which A denotes the solvophilic block and B and C are the two incompatible solvophobic blocks. Through proper choice of the polymer architecture and the processing conditions, a wide diversity of MCBNs including cylinders, segmented worms, disks, plates, toroids, and raspberry-like nanoparticles have been prepared.12−23 The second strategy to prepare MCBNs is through comicellization or blending of two or more presynthesized block copolymers, e.g., the mixture of AB diblock copolymer and ABC miktoarm star terpolymer or the mixture of AB and BC diblock copolymers.24−37 Although various MCBNs have been prepared, in comparison with synthesis of general block copolymer nanoparticles, synthesis of well-defined MCBNs suffers great inconvenience or difficulty. This inconvenience of synthesis of well-defined MCBNs arouses from great difficulty to make phase-separated microdomains in the particle body. For this reason, ABC miktoarm terpolymers or fluorinated ABC triblock copolymers, which have suitable conformation or chemical composition to arouse © XXXX American Chemical Society

phase separation in the particle body, are usually involved in synthesis of MCBNs.4−14,34−37 It is known that desirable interaction between two different polymers may result in polymer−polymer complexes.14−16,31−52 Intermolecular complexation between two polymers driven by electrostatic interactions or hydrogen bonding can lead to either associative phase separation or segregative phase separation,14−16,31−52 which is represented by a phase being enriched in both components or each phase being enriched in one polymer, respectively. As an important intermolecular interaction, hydrogen bonding plays a key role to form phase-separated structure of block copolymers,14,15,37−52 and much effort has been made to tune the morphologies of block copolymer by addition of complex forming metal salts,40−42 homopolymers,43−46 and small molecules.47−50 Up to now, intermolecular complexation between two polymers or between a polymer and a small molecule is usually studied.14,15,37−50 However, intramolecular complexation within a unitary block copolymer is rarely studied.51,52 In this contribution, a new strategy to synthesize multicompartment nanoparticles of linear ABC triblock copolymer through intramolecular interactions of two solvophilic blocks of Received: February 1, 2017 Revised: March 1, 2017

A

DOI: 10.1021/acs.macromol.7b00242 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Summary of the Synthesis of P4VP-TTC, P4VP-b-PS-TTC, and P4VP-b-PS-b-P4AS Mn (kg/mol) entry

polymer

[M]0:[CTA]0:[I]0

time (h)

conva (%)

Mn,thb

Mn,GPCc

Mn,NMRd

Đe

1 2 3 4 5

P4VP58-TTC P4VP58-b-PS242-TTC P4VP58-b-PS242-b-P4AS96 P4VP58-b-PS242-b-P4AS67 P4VP58-b-PS242-b-P4AS30

60:1:1/4 270:1:1/3 120:1:1/3 70:1:1/3 35:1:1/3

15 8 12 12 10

96.9 89.9 80.3 96.3 89.2

6.52 31.74 47.98 42.96 36.74

7.14 23.95 43.26 42.02 34.86

6.13 49.93 56.72 47.39 39.85

1.01 1.08 1.13 1.11 1.10

a

Monomer conversion detected via 1H NMR. bTheoretical molecular weight based on monomer conversion. cNumber-average molecular weight found by GPC. dMolecular weight calculated via 1H NMR. eĐ or the Mw/Mn values observed by GPC. the P4VP58-b-PS242-b-P4AS nanoparticles by seeded RAFT polymerization.56−61 Into a 50 mL Schlenk flask containing a magnetic bar, DDMAT (0.2894 g, 0.7929 mmol), 4VP (5.00 g, 47.5556 mmol), AIBN (0.0325 g, 0.1979 mmol), and ethanol (10.00 g) were fed. The polymerization mixture was first allowed to deoxygenation at 0 °C and then conducted to the solution RAFT polymerization at 70 °C. After 15 h, the flask was immediately immersed in iced water to quench the polymerization. The monomer conversion of 96.9% was identified by 1H NMR. The prepared polymer with DP at 58, P4VP58-TTC, was precipitated in cold diethyl ether, dried under vacuum overnight, and collected. Into a 50 mL Schlenk flask equipped with a magnetic bar, P4VP58TTC (1.00 g, 0.1548 mmol), St (4.3516 g, 0.0418 mol), AIBN (8.4704 mg, 0.0516 mmol), the methanol/water mixture (21.7580 g, 80/20 by weight) were weighed. The polymerization mixture was deoxygenated at 0 °C and then conducted to dispersion RAFT polymerization at 70 °C with vigorous stirring. After 8 h of polymerization, the polymerization was quenched by immediately cooling to 0 °C, and the colloidal dispersion of P4VP58-b-PS242-TTC was obtained. The 89.9% monomer conversion was identified by UV−vis analysis at 245 nm as mentioned elsewhere.64 The colloidal dispersion was dialyzed against the 80/20 methanol/water mixture to remove the residual styrene monomer (cutoff 7000 Da) and then diluted with a suitable solvent of the 80/20 methanol/water mixture to make 19.60 wt % dispersion for the next use. Into a 50 mL Schlenk flask having a magnetic bar, the colloidal dispersion of P4VP58-b-PS242-TTC (2.4636 g, containing 0.4827 g or 0.0153 mmol of P4VP58-b-PS242-TTC), AIBN (0.8344 mg, 5.0813 mmol) dissolved in the 80/20 methanol/water (4.0192 g), and 4AS (0.1509−0.3096 g) were weighed (see the detailed recipe in Table 1). The flask content was allowed to deoxygenation at 0 °C and conducted to seeded RAFT polymerization at 70 °C. After a specific time, the seeded RAFT polymerization was quenched, and conversion of the 4AS monomer was detected by 1H NMR following eq S1. The resultant P4VP-b-PS-b-P4AS colloidal dispersion was dialyzed against the 80/20 MeOH/H2O mixture (molecular weight cutoff 7000 Da) and diluted with the 80/20 MeOH/H2O mixture to make 10.13 wt % working solution of the P4VP-b-PS-b-P4AS nanoparticles. The P4VPb-PS-b-P4AS triblock copolymer was collected by centrifugation of the colloidal dispersion at 12500 rpm and then dried at room temperature under vacuum for further characterization. 2.3. Preparation of the MCBNs of P4VP-b-PS-b-P4HS. The MCBNs of P4VP-b-PS-b-P4HS were prepared through hydrazinolysis of the P4VP-b-PS-b-P4AS nanoparticles. Herein, a typical synthesis of MCBNs of P4VP58-b-PS242-b-P4HS96 was introduced. Into a two-neck flask sealed with a rubber septum and equipped with a condenser, the P4VP-b-PS-b-P4AS nanoparticles dispersed in the 80/20 methanol/ water mixture (3.30 g, 10.13 wt %) and hydrazine aqueous solution (0.83 g, 80 wt %) were added. Hydrazinolysis was allowed to continue at room temperature (20−25 °C) for about 12 h under a nitrogen atmosphere and with vigorous stirring to afford the MCBNs of P4VP58-b-PS242-b-P4HS96. 2.4. Characterization. 1H NMR analysis was accomplished on a Bruker Avance III 400 MHz NMR spectrometer, in which CDCl3 or DMSO-d6 was used as solvent. Fourier transform infrared (FTIR)

A and C within the nanoparticles is proposed. For the purpose of the synthesis of multicompartment block copolymer nanoparticles, the linear triblock copolymer nanospheres of poly(4-vinylpyridine)-block-polystyrene-block-poly(4-acetoxystyrene) (P4VP-b-PS-b-P4AS) with a hydrophobic core of the polystyrene (PS) and poly(4-acetoxystyrene) (P4AS) mixture and a solvophilic corona of poly(4-vinylpyridine) (P4VP) were synthesized by seeded RAFT polymerization,54−62 which is a valid formulation to prepare ABC triblock copolymer nanoassemblies. And then, the P4AS block was converted into poly(4-hydroxystyrene) (P4HS) by hydrazinolysis, and the triblock copolymer nanospheres of poly(4-vinylpyridine)-blockpolystyrene-block-poly(4-hydroxystyrene) (P4VP-b-PS-bP4HS) are formed. Since P4HS and P4VP are proton donor and proton acceptor, respectively, and strong intramolecular interactions via hydrogen bonding between the P4HS and P4VP blocks confined within the P4VP-b-PS-b-P4AS nanospheres take place during hydrazinolysis. The strong intramolecular interaction between the P4HS and P4VP blocks leads to the P4HS/P4VP complexes, which is much immiscible with the PS core, and then these P4HS/P4VP complexes deposit on the PS core to form segregative P4HS/P4VP microdomains; therefore, the MCBNs of P4VP-b-PS-b-P4HS are formed. Herein, this synthesis of MCBNs combines the polymerization-induced self-assembly (PISA)53−55 to prepare block copolymer nanoassemblies and the hydrogen bonding promoted intramolecular complexation,51,52 and it affords efficient synthesis of MCBNs and avoids application of ABC miktoarm terpolymers and fluorinated ABC triblock copolymers.

2. EXPERIMENTAL SECTION 2.1. Materials. Styrene (St, >98%, Tianjin Chemical Company, China) and 4-vinylpyridine (4VP, 96%, Alfa, China) were purified by distillation in reduced pressure. 4-Acetoxystyrene (4AS, >98.00%, Tianjin Heowns Biochem LLC, China) was passed through a column with activated Al2O3 (Tianjin Chemical Company, neutral, standard grade, 100−200 mesh, 58 Å). S-1-Dodecyl-S′-(α,α′-dimethyl-α″-acetic acid) trithiocarbonate (DDMAT, Scheme S1) was prepared as reported.62 2,2′-Azobis(2-methylpropionitrile) (AIBN, >99% Tianjin Chemical Company, China) was purified by recrystallization from ethanol. Deionized water, methanol (>99.00%, Tianjin Chemical Company, China), 1,3,5-trioxane (98.00%, Alfa) employed as inner standard for 1H NMR analysis, and the rest reagents of being analytical grading were used as acquired. 2.2. Synthesis of the P4VP-b-PS-b-P4AS Nanoparticles. The P4VP-b-PS-b-P4AS triblock copolymer nanoparticles were prepared following three steps including (1) RAFT synthesis of poly(4vinylpyridine) trithiocarbonate (P4VP58-TTC, in which TTC indicates the RAFT moiety of trithiocarbonate and the subscript of 58 represents the polymerization degree of monomer (DP), (2) preparation of the P4VP58-b-PS242-TTC diblock copolymer nanoparticles by dispersion RAFT polymerization,63 and (3) preparation of B


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Macromolecules Scheme 1. Synthesis of the P4VP-TTC Macro-RAFT Agent, the Seed Nanoparticles of the P4VP-b-PS-TTC Diblock Copolymer, the Triblock Copolymer Nanoparticles of P4VP-b-PS-b-P4AS, and the MCBNs of P4VP-b-PS-b-P4HS

analysis was achieved at diffuse reflection mode on an FTS6000 spectrometer. The molecular weight and polydispersity index (Đ = Mw/Mn) were determined by gel permeation chromatography (GPC) equipped with three SHODEX columns and a RL 2000 refractive index detector, where DMF containing LiBr (0.012 mol/L) at 50.0 °C with a flow rate of 0.8 mL/min was used as eluent and the samples of narrow-polydispersity poly(methyl methacrylate) (PMMA) with different molecular weight were used as calibration standard. Differential scanning calorimetry (DSC) analysis was accomplished on a Mettler-Toledo DSC1 differential scanning calorimeter under the nitrogen atmosphere. About 5−10 mg of polymer was encapsulated in the 40 μL aluminum pans and then was heated from 25 to 225 °C at a heating/cooling rate of 10 °C min−1 . UV−vis analysis was accomplished on a Varian 100 UV−vis spectrophotometer. Transmission electron microscope (TEM) observation was performed using a Tecnai G2 F20 electron microscope at 200 kV. In the case of the P4VP-b-PS-TTC and P4VP-b-PS-b-P4AS nanoparticles, a small drop of the diluted colloidal dispersion was deposited onto a piece of copper grid, dried at room temperature, and last observed by TEM. For MCBNs of P4VP58-b-PS242-b-P4HS96, besides the unstained nanoparticles were checked, the nanoparticles were also negatively stained with phosphotungstic acid (PTA) and jointly stained with PTA and RuO4 vapors as discussed elsewhere (seeing the staining procedures in the Supporting Information)65,66 and then checked by TEM. The average size of MCBNs or microdomains was obtained by analyzing more than 100 nanoparticles with the ImageJ software. Zetapotential measurements were conducted on a NanoBrook Omni (Brookhaven) laser light scattering spectrometer at 25 °C.

The solution RAFT polymerization of 4VP was run under [4VP]0:[DDMAT]0:[AIBN]0 = 60:1:1/4. After 15 h of polymerization, P4VP58-TTC was obtained at 96.9% monomer conversion. As indicated in Figure 1A, the molecular weight of

Figure 1. GPC traces of P4VP58-TTC (A), P4VP58-b-PS242-TTC (B), and P4VP58-b-PS242-b-P4AS96 (C).

P4VP58-TTC by GPC, Mn,GPC, at 7.14 kg/mol is very close to the theoretical molecular weight Mn,th of 6.52 kg/mol calculated by eq S2. Besides, the molecular weight of P4VP58-TTC has a low distribution (Table 1). P4VP58-TTC was also checked by NMR (Figure 2A). By comparing the characteristic chemical shifts at δ = 0.86 ppm (the RAFT terminal in P4VP58-TTC) and at δ = 8.33 ppm (the polymeric backbone), the molecular weight of P4VP 58 -TTC, Mn,NMR , at 6.13 kg/mol was determined. Clearly, the three cases of molecular weight of P4VP58-TTC, Mn,GPC, Mn,NMR, and Mn,th, are close to each other, confirming the controlled synthesis of P4VP58-TTC. The P4VP58-b-PS242-TTC nanoparticles were prepared by dispersion RAFT polymerization under [St]0:[P4VP58-TTC]0: [AIBN]0 = 270:1:1/3. This PISA formulation of dispersion RAFT polymerization was applied to prepare the P4VP58-bPS242-TTC nanoparticles, since this PISA method has been proved to be very effective for synthesis of block copolymer nanoparticles.53−55 After 8 h polymerization with 89.9% conversion of the St monomer, the P4VP58-b-PS242-TTC nanoparticles were prepared. As shown in Figure 3A and Figure S2, the P4VP58-b-PS242-TTC nanoparticles have a uniform size distribution centered at 28 ± 3 nm. The P4VP58-b-PS242-TTC diblock copolymer was confirmed by GPC (Figure 1B) and NMR (Figure 2B), and the well-defined molecular structure as summarized in Table 1 is demonstrated (see the calculation of Mn,NMR in eq S3). It is expected that these P4VP58-b-PS242-TTC nanoparticles have a core−corona

3. RESULTS AND DISCUSSION 3.1. Synthesis of the P4VP-b-PS-b-P4AS Nanoparticles. To prepare MCBNs, P4VP-b-PS-b-P4AS nanoparticles are initially synthesized. These triblock copolymer nanoparticles are composed of a core formed by the two solvophobic blocks of PS and P4AS and a corona of the solvophilic block of P4VP. Choosing the P4VP-b-PS-b-P4AS triblock copolymer to prepare MCBNs is based on two concerns. First, the solvophobic P4AS block can be converted into the solvophilic block of P4HS by hydrazinolysis at ambient conditions as discussed elsewhere.67−69 Second, the P4HS and P4VP are typical proton donor and proton acceptor,70−72 and therefore the newly formed P4HS block/segment via hydrazinolysis and the P4VP block can form hydrogen-bonded complexes of P4HS/P4VP. The P4VP-b-PS-b-P4AS nanoparticles are synthesized by seeded RAFT polymerization,56−61 which includes three procedures (Scheme 1), e.g., (1) the solution RAFT polymerization to synthesize P4VP58-TTC, (2) the dispersion RAFT polymerization to produce the P4VP58-bPS242-TTC core−corona nanoparticles, and (3) the seeded RAFT polymerization to produce the P4VP58-b-PS242-b-P4AS core−corona nanoparticles. C


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obtained. By tuning the ratio of [4AS]0:[P4VP58-b-PS242TTC]0:[AIBN]0, three P4VP58-b-PS242-b-P4AS nanoparticles with different DP of the P4AS block were synthesized. These P4VP58-b-PS242-b-P4AS triblock copolymers were characterized by GPC (Figure 1) and NMR (Figure 2 and Figure S1), and the results are summarized in Table 1. As shown in Figures 3B−D, all the P4VP58-b-PS242-b-P4AS nanoparticles have spherical morphology, and the nanoparticle size is correlative to the DP of the P4AS block. For instance, with the DP of the P4AS block increases from 0 to 96, the nanoparticle size increases from 28 to 40 nm. The increased size of the P4VP58b-PS242 -b-P4AS nanoparticles is due to the increasing dimension of the P4AS block. In the P4VP58-b-PS242-b-P4AS triblock copolymer nanoparticles, the solvophilic P4VP block forms the solvated corona to keep the nanoparticles suspending in the solvent, which is somewhat similar to those in the P4VP58-b-PS242-TTC diblock copolymer nanoparticles. Since the newly introduced P4AS block is solvophobic, the P4AS block is expected to be situated in the inner core of the P4VP58b-PS242-b-P4AS nanoparticles (see the inset in Figure 3C). The P4VP58-b-PS242-b-P4AS96 triblock copolymer as well as the corresponding homopolymers was analyzed by DSC. As indicated in Figure 4, two glass transition temperatures, Tg, at

Figure 2. 1H NMR spectra of P4VP58-TTC (A), P4VP58-b-PS242-TTC (B) in CDCl3 and P4VP58-b-PS242-b-P4AS96 (C), and P4VP58-b-PS242b-P4HS96 in DMSO-d6 (D).

Figure 4. DSC thermograms of P4VP58-TTC (A), P4AS110-TTC (B), P4HS110-TTC (C), PS300-TTC (D), P4VP58-b-PS242-b-P4AS96 (E), and P4VP58-b-PS242-b-P4HS96 (F). Note: see the synthesis of P4AS110TTC, P4HS110-TTC, and PS300-TTC in the Supporting Information.

103.7 and 123.2 °C are observed for the P4VP58-b-PS242-bP4AS96 triblock copolymer. By comparing the DSC thermograms of the P4VP58-b-PS242-b-P4AS96 triblock copolymer and the reference homopolymers, it is concluded that the Tg at 123.2 °C is corresponding to the P4VP58 block. The Tg at 103.7 °C locates between those of the homopolymers of P4AS110TTC (89.3 °C) and PS300-TTC (105.5 °C), suggesting the Tgs of the P4AS96 and PS242 blocks are coalesced to a single Tg. The reason is due to the P4AS96 and PS242 blocks being miscible, which is further confirmed by the DSC thermograms of the reference diblock copolymer of PS300-b-P4AS144 (Figure S3). On the basis of the DSC analysis, we prefer to conclude that the P4AS96 and PS242 blocks are homogeneously distributed in the core of the P4VP58-b-PS242-b-P4AS nanoparticles as sketched out by the inset in Figure 3C. 3.2. Preparation of MCBNs of P4VP-b-PS-b-P4HS. The MCBNs of P4VP-b-PS-b-P4HS were prepared by hydrazinolysis of the P4VP-b-PS-b-P4AS nanoparticles. Conversion of P4AS into P4HS by hydrazinolysis was widely reported,67−69 and this conversion was successfully achieved due to the

Figure 3. TEM images of the nanoparticles of P4VP58-b-PS242-TTC (A), P4VP58-b-PS242-b-P4AS30 (B), P4VP58-b-PS242-b-P4AS67 (C), and P4VP58-b-PS242-b-P4AS96 (D).

structure, in which the core is constituted by the solvophobic PS block and the corona is constituted by the solvophilic P4VP block as sketched out by the inset in Figure 3A. Thanks to the solvophilic P4VP block acting as stabilizer, these P4VP58-bPS242-TTC nanoparticles can be uniformly dispersed in the alcoholic solvent. Into colloidal dispersion of the P4VP58-b-PS242-TTC nanoparticles, the monomer of 4AS and the initiator of AIBN were added. After the 4AS monomer being molecularly dissolved, seeded dispersion RAFT polymerization was conducted at 70 °C. Different from the dispersion RAFT polymerization employing a soluble P4VP58-TTC macro-RAFT agent, herein the P4VP58-b-PS242-TTC nanoparticles containing the RAFT functional moiety were used as macro-RAFT seeds, onto which the third solvophobic block of P4AS was extended and the P4VP58-b-PS242-b-P4AS triblock copolymer nanoparticles were D


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4.5 ± 0.7 nm are discerned (Figure 5C,D and Figure S4). With the DP of the P4HS decreasing to 67, the size of the P4VP/ P4HS microdomains slightly decreases to 3.0 ± 0.3 nm (Figure 5B), and finally the P4VP/P4HS microdomains in the MCBNs of P4VP58-b-PS242-b-P4HS30 are too small to be discerned (Figure 5A). The formation of MCBNs of P4VP-b-PS-b-P4HS was further checked by DSC. As shown in Figure 4F, by comparing the DSC thermograms of P4VP58-b-PS242-b-P4HS96 and its precursor of P4VP58-b-PS242-b-P4AS96, the Tg at 123.2 °C disappeared, and a new Tg at 185.5 °C higher than that of the homopolymers of P4HS (178.1 °C) and P4VP (133.5 °C) is found. This high Tg is due to the strong interaction exists between these two block components as similar as those discussed previously,70−72 and therefore it confirms the complexation of the P4VP58 and P4HS96 blocks to form MCBNs of P4VP-b-PS-b-P4HS. The formation of MCBNs of P4VP-b-PS-b-P4HS was further revealed by FTIR. As displayed in Figure 6, by comparing the

cleavable acetoxy group. After 12 h of hydrazinolysis at room temperature, the P4VP-b-PS-b-P4AS nanoparticles were converted into MCBNs of P4VP-b-PS-b-P4HS. Figures 2C,D show the 1H NMR spectra of the representative P4VP58-bPS242-b-P4AS96 and P4VP58-b-PS242-b-P4HS96 triblock copolymers before and after hydrazinolysis, from which the signal at δ = 2.2 ppm belonging to the methyl substituent in P4AS is disappeared, confirming full conversion of P4AS into P4HS. Figure 5 displays the TEM images of the P4VP58-b-PS242-bP4HS MCBNs with different DP of the P4HS block. In

Figure 5. TEM images and schematic structure of the triblock copolymer MCBNs: P4VP58-b-PS242-b-P4HS30 (A), P4VP58-b-PS242-bP4HS67 (B), and P4VP58-b-PS242-b-P4HS96 (C, D). All particles were stained with PTA and RuO4.

Figure 6. FTIR spectra of P4VP58-b-PS242-b-P4AS96 (A), P4VP58-bPS242-b-P4HS96 (B), P4HS110-TTC (C), and P4VP58-TTC (D). Insets on the right side: the FTIR spectra of the region between 1025 and 970 cm−1 (range II) of P4VP58-b-PS242-b-P4HS96 (B), P4HS110-TTC (C), and P4VP58-TTC (D). All the samples were fully dried before the FT-IR experiments.

comparison with the P4VP58-b-PS242-b-P4AS nanoparticles, these MCBNs of the P4VP58-b-PS242-b-P4HS nanoparticles have similar size. However, by carefully checking the MCBNs of P4VP58-b-PS242-b-P4HS, the difference is observed. That is some dispersed microdomains indicated by black dots in the periphery of the MCBNs have been observed. These dispersed microdomains are formed by the complexation of the P4VP58 and P4HS blocks, and such complexation between P4VP and P4HS has been verified previously.70−72 During the hydrazinolysis, the solvophobic P4AS block locating in the inner core of the triblock copolymer nanoparticles gradually converts into P4HS, and then the P4HS block transfers from the inner core to the outer corona of the triblock copolymer nanoparticles since P4HS is solvophilic. Once the P4HS block moves to the outer corona, complexation between the P4VP and P4HS blocks occurs, and the resultant P4VP/P4HS complexes are insoluble in the solvent and therefore form the dispersed P4VP/P4HS microdomains on the PS core. Besides, since the P4VP/P4HS complexes are immiscible with the PS core as shown by the DSC analysis (which will be discussed subsequently), which also helps formation of the dispersed P4VP/P4HS microdomains on the PS core. Furthermore, it is found that the dimension of the P4VP/P4HS microdomains is positively correlative to the DP of P4HS block. For instance, in MCBNs of P4VP58-b-PS242-b-P4HS96 containing the longest P4HS96 block, the P4VP/P4HS microdomains with the size of

FTIR spectra of P4VP58-b-PS242-b-P4HS96 and its precursor of P4VP58-b-PS242-b-P4AS96, the characteristic absorption at 1761 cm−1 corresponding to the CO stretching vibration is disappeared (Figure 6A), suggesting the conversion of P4VP58b-PS242-b-P4AS96 into P4VP58-b-PS242-b-P4HS96. Besides, from the FTIR spectra of P4VP-b-PS-b-P4HS (Figure 6B), complexation of the P4VP58 and P4HS96 blocks within the MCBNs of P4VP-b-PS-b-P4HS through hydrogen bonding was further confirmed, as indicated by the shift of characteristic absorption of the phenolic hydroxyl group (range I, 3600−3100 cm−1) and the pyridine ring (range II, 1025−970 cm−1) in the P4VP/ P4HS complexes. In range I (see the insets on the right side of Figure 6), the free P4HS shows two vibrations corresponding to the free and intra-associated O−H groups at 3542 and 3323 cm−1 (Figure 6C). Whereas, for the P4HS block in MCBNs, the intensity of the free hydroxyl band at 3542 cm−1 decreases, and the center of the broad hydrogen band shifts from 3323 to 3313 cm−1, which indicates that the intramolecular hydrogen bonding of the P4HS/P4VP blocks is stronger than the intermolecular hydrogen bonding in the free P4HS. In range II, the characteristic bands at 993 cm−1 related to the aryl C−H bend absorption of the pyridine ring in free P4VP and at 1014 cm−1 corresponding to the benzene ring in free P4HS can be clearly identified (Figures 6C,D). In the triblock copolymer of E


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Macromolecules

positive value (3.63 mV) with the DP of P4HS block increasing from 30 to 96. Note: the present MCBNs were dispersed in alcoholic solvent, and therefore the zeta potential was relatively smaller than those dispersed in water in the presence of inorganic salt as discussed elsewhere.73,74 From above-mentioned discussion, synthesis of MCBNs of P4VP-b-PS-b-P4HS can be schematically summarized in Figure 8. That is, following seeded dispersion RAFT polymerization, the triblock copolymer nanoparticles of P4VP-b-PS-b-P4AS with a core formed by the solvophobic and miscible PS and P4AS blocks and a solvated corona of the P4VP block are initially synthesized. Subsequently, upon the hydrazinolysis of the triblock copolymer nanoparticles, the solvophobic P4AS block was converted into the solvophilic P4HS block, and then the resultant solvophilic P4HS block transfers from the core to the corona. Finally, the solvophilic P4HS block and the P4VP block, both of which locate at the corona and one is a proton donor and the other is a proton acceptor, form the P4VP/ P4HS microdomains through the strong intramolecular interactions via hydrogen bonding to form the MCBNs of P4VP-b-PS-b-P4HS. In the MCBNs, the particle surface is firmly dependent on the DP of the P4HS and P4VP blocks. That is, in the case of DPP4VP > DPP4HS MCBNs having a P4VP corona and in the case of DPP4VP < DPP4HS MCBNs having a P4HS corona are formed, respectively.

P4VP58-b-PS242-b-P4HS96, the aryl C−H bend absorption shifts to 1005 cm−1 due to the hydrogen-bonding interaction between the pyridine ring in the P4VP blocks and the phenol group in the P4HS block. The shifts in both wavenumber ranges (I and II) suggest strong hydrogen bonding within P4VP-b-PS-bP4HS and therefore the synthesis of MCBNs. The P4VP-b-PS-b-P4HS MCBNs with different DP of the P4HS block are deemed to have different surfaces. That is, when the DP of the P4HS block is smaller than that of the P4VP block, part of the P4VP chains accompanied by the P4HS chains form the P4VP/P4HS complexes; then the P4VP/P4HS complexes further deposit on the PS core to form dispersed P4VP/P4HS microdomains, and the excess P4VP segments are solvated in the solvent to form the corona to keep the MCBNs suspending in the solvent. When the DP of the P4HS block is equal to that of the P4VP block, all the P4VP and P4HS chains form the P4VP/P4HS microdomains on the PS core. When the DP of the P4HS block is larger than that of the P4VP block, the excess P4HS chains and/or segments form the solvated corona, which are schematically shown by the insets in Figure 7. This

4. CONCLUSIONS A new strategy to synthesize linear ABC triblock copolymer MCBNs through intramolecular interaction of the A and C blocks is proposed. This strategy involves (1) synthesis of the P4VP-b-PS-TTC diblock copolymer nanoparticles through dispersion RAFT polymerization under PISA conditions, (2) synthesis of the P4VP-b-PS-b-P4AS triblock copolymer nanoparticles by seeded RAFT polymerization, and (3) conversion of the P4VP-b-PS-b-P4AS nanoparticles into MCBNs of P4VPb-PS-b-P4HS by hydrazinolysis. The successful synthesis of MCBNs is ascribed to the strong intramolecular interactions via hydrogen bonding between the P4HS and P4VP blocks confined within the P4VP-b-PS-b-P4AS nanospheres during hydrazinolysis, which forms the dispersed microdomains of the P4VP/P4HS complexes on the solvophobic PS core. It is found that there is a positive correlation between the size of the P4VP/P4HS microdomains and the DP of P4HS block; that is,

Figure 7. Zeta potential of the P4VP-b-PS-b-P4HS MCBNs with increasing DP of the P4HS block. Note: MCBNs were dispersed in the 80/20 methanol/water mixture, and the insets show the schematic structure of the MCBNs of P4VP58-b-PS242-b-P4HS30, P4VP58-b-PS242b-P4HS67, and P4VP58-b-PS242-b-P4HS97.

hypothesis is confirmed by checking the zeta potential of the MCBNs of P4VP-b-PS-b-P4HS (Figure 7), in which the zeta potential changes from a negative value (−3.54 mV) to a

Figure 8. Formation of MCBNs of P4VP-b-PS-b-P4HS by hydrazinolysis of P4VP-b-PS-b-P4AS nanoparticles. F


Article

Macromolecules it increases to ∼5 nm with the increasing DP of the P4HS block from 30 to 96. The present synthesis of MCBNs combines the PISA to synthesize block copolymer nanoassemblies and the hydrogen bonding promoted intramolecular complexation, and it affords efficient synthesis of MCBNs and avoids application of ABC miktoarm terpolymers and the fluorinated ABC triblock copolymers.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00242. Supplementary characterization and experimental details (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], Tel 86-22-23509794, Fax 8622-23503510 (W.Z.). ORCID

Wangqing Zhang: 0000-0003-2005-6856 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support by the National Science Foundation for Distinguished Young Scholars (No. 21525419), the National Science Foundation of China (No. 21474054), and the Ministry of Science and Technology of the People’s Republic of China (2016YFA0202503) is gratefully acknowledged.

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