In Situ Synthesis of Coil–Coil Diblock Copolymer Nanotubes and

Sep 27, 2017 - ... Institute of Polymer Chemistry, College of Chemistry, and ‡Collaborative Innovation Center of Chemical Science and Engineering (T...
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In Situ Synthesis of Coil−Coil Diblock Copolymer Nanotubes and Tubular Ag/Polymer Nanocomposites by RAFT Dispersion Polymerization in Poly(ethylene glycol) Zhonglin Ding,† Mingdu Ding,† Chengqiang Gao,†,‡ Cyrille Boyer,§ and Wangqing Zhang*,†,‡ †

Key Laboratory of Functional Polymer Materials of the Ministry of Education, Institute of Polymer Chemistry, College of Chemistry, and ‡Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China § Centre for Advanced Macromolecular Design, School of Chemical Engineering, University of New South Wales, Sydney, New South Wales 2052, Australia S Supporting Information *

ABSTRACT: Hollow polymeric nanotubes have the potential to be employed as advanced nanomaterials in a variety of applications; however, their synthesis from the assembly of coil−coil diblock copolymers (DBCPs) has typically been limited. Herein, we report a novel method for synthesis of coil−coil DBCP nanotubes by implementing RAFT dispersion polymerization in low molecular weight poly(ethylene glycol) (PEG). This method for the in situ synthesis of coil−coil DBCP nanotubes is particularly versatile and can be achieved with a range of block copolymers including poly(N-isopropylacrylamide)-blockpolystyrene (PNIPAM-b-PS), poly(4-vinylpyridine)-block-polystyrene (P4VP-b-PS), and poly(methyl methacrylate)-blockpolystyrene (PMMA-b-PS). Using this approach, several interesting coil−coil DBCP tubular morphologies are observed, including single-wall nanotubes, multiwall nanotubes, and porous nanotubes. Furthermore, RAFT dispersion polymerization conducted in the presence of Ag nanoparticles can be used to yield complex tubular nanocomposites. The structure of these nanotubes such as the wall thickness and surface roughness can be tuned by varying the degree of polymerization (DP) of the solvophobic polystyrene block and/or the Ag fraction in the Ag/DBCP nanocomposites. tions.16 In contrast to other DBCP systems, most coil−coil DBCPs show higher chain flexibility and fewer anisotropic intermolecular interactions possibly restricting the stabilization of the nanotube morphology. The second method to prepare polymeric nanotubes is through template synthesis either using a sacrificial core or via a process of molecular sculpting.17,18 For example, Liu and co-workers prepared polymeric nanotubes by molecular sculpting of ABC triblock copolymer coaxial micelles.17 The triblock copolymer of polyisoprene-blockpoly(2-cinnamoyloxyethyl methacrylate)-block-poly(tert-butyl acrylate) (PI-b-PCEMA-b-PtBA) was initially self-assembled into coaxial micelles in a block selective solvent, followed by

1. INTRODUCTION One-dimensional (1-D) nanostructures such as polymeric nanotubes have attracted increasing interest in recent years.1 However, compared with other nanostructures,2,3 such as 0dimensional nanospheres and 1-D nanorods, synthetic reports of polymeric nanotubes are relatively scarce.4−18 Generally, two main methods are used to prepare polymeric nanotubes. The first method is via the self-assembly of amphiphilic block copolymers, usually chiral, rod−coil, or linear−dendritic diblock copolymers (DBCPs), in a block-selective solvent.4−16 For general coil−coil DBCPs, various nanoassemblies such as spherical micelles, worms/nanorods, and vesicles have been reported.2,3,19 Nanotube formation by self-assembly, however, is typically limited to chiral DBCPs,13 rod−coil DBCPs,14 and linear−dendritic DBCPs.15,16 The reason for this limitation is that nanotube formation generally requires highly ordered molecular packing and anisotropic intermolecular interac© XXXX American Chemical Society

Received: June 26, 2017 Revised: September 8, 2017

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Macromolecules Table 1. Synthesis and Characterization of Macro-RAFT Agentsa Mn (g/mol)

a

macro-RAFT

RAFT

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

time (h)

conv (%)

Mn,th

Mn,GPC

Mn,NMR

Đ

PNIPAM18-TTC P4VP27-TTC PMMA46-TTC

ECT DDMAT ECT

80:4:1 120:4:1 500:10:1

2 5.5 4

88 90 92

2.3 × 103 3.2 × 103 4.9 × 103

3.7 × 103 3.4 × 103 5.0 × 103

1.8 × 103 3.6 × 103 4.4 × 103

1.04 1.05 1.05

The synthesis of PMMA46-TTC and P4VP27-TTC is described in the Supporting Information with the experimental details summarized in Table 1.

photo-cross-linking of the PCEMA shell and finally removal of the PI core by ozonolysis to yield hollow nanotubes. Nanocomposites composed of inorganic nanoparticles (NPs) and organic polymers represent a new class of materials exhibiting improved performance compared with their individual counterparts.20−23 Polymer−inorganic nanocomposites may be prepared by grafting synthetic polymers on inorganic particles or by adding modified nanoparticles (NPs) into polymer matrices.20 Typically, dispersed metal-NPs/DBCP nanocomposites are prepared by a multistep procedure: (1) synthesis of amphiphilic DBCPs usually by controlled radical polymerization, (2) preparation of DBCP nanoassemblies in block-selective solvents, (3) loading metal ions onto DBCP nanoassemblies usually through coordination or ionic interactions between the chelate moiety in DBCP and metal ions, and (4) reducing the loaded metal ions into metal NPs.24−33 Considering the large surface area and hollow structure afforded by DBCP nanotubes, the synthesis of tubular metalNPs/DBCPs nanocomposites is highly desirable.33 For example, Winnik and co-workers prepared DBCP nanotubes through solution self-assembly of poly(ferrocenylsilane)-blockpoly(methylvinylsiloxane) (PFS-b-PMVS), and then Ag nanoparticles were loaded into the nanotubes through redoxinduced encapsulation to prepare tubular Ag/PFS-b-PMVS nanocomposites.33 However, this multistep synthesis of metalNPs/DBCP nanocomposites was limited at relatively low solids content usually below 1 wt %. Polymerization-induced self-assembly (PISA) especially via RAFT dispersion polymerization mediated with a macromolecular RAFT (macro-RAFT) agent has been proven to an effective strategy to synthesize DBCP nanoassemblies with controlled morphologies and at relatively high polymer concentrations (up to 50 wt %).34−36 The PISA approach is particularly attractive since synthesis of concentrated DBCP nanoassemblies can be performed in one pot and the morphology and/or size can be readily tuned via changes in either the DBCP composition or the polymerization conditions. This approach has been successfully employed for synthesis of various DBCPs nanoassemblies such as spheres, worms or nanorods, lamellae, and vesicles.37−62 However, to date there have been scarce reports on the synthesis of DBCP nanotubes.62 Recently, our group reported a novel RAFT dispersion polymerization performed in poly(ethylene glycol) (PEG), and nanotubes of a coil−coil DBCP of poly(Nisopropylacrylamide)-block-polystyrene were prepared.63 However, the detailed synthesis of coil−coil DBCP nanotubes and methods to tune the nanotube structure were not explored. In this study, we discuss the synthesis of coil−coil DBCP nanotubes of poly(N-isopropylacrylamide)-block-polystyrene (PNIPAM-b-PS), poly(4-vinylpyridine)-block-polystyrene (P4VP-b-PS), and poly(methyl methacrylate)-block-polystyrene (PMMA-b-PS) by RAFT dispersion polymerization in PEG, and the change in nanotube structure is investigated with respect to the DBCP composition. Further, since the PEG

polymerization medium is also a good stabilizer for Ag NPs, we also explore the synthesis of 1-D tubular Ag/DBCP nanocomposites through RAFT dispersion polymerization in PEG and in the presence of Ag nanoparticles. We expect that these tubular Ag/DBCP nanocomposites will find potential usage in catalytic or antimicrobial applications.

2. EXPERIMENTAL SECTION 2.1. Materials. Styrene (St, >98%, Tianjin Chemical Company, China), 4-vinylpyridine (4VP, 96%, Alfa), and methyl methacrylate (MMA, >99%, Tianjin Chemical Company, China) were distilled under reduced pressure prior to use. N-Isopropylacrylamide (NIPAM, >99%, Acros Organics) was purified by recrystallization in the nhexane/acetone mixture (1:1, v/v). 4-Cyano-4-(ethylsulfanylthiocarbonyl)sulfanylpentanoic acid (ECT) and S-1-dodecyl-S′-(α,α′dimethyl-α″-acetic acid) trithiocarbonate (DDMAT) were synthesized as discussed elsewhere,64,65 and their NMR spectra are shown in Figure S1. 2,2′-Azobis(2-methylpropionitrile) (AIBN, >99%, Tianjin Chemical Company, China) was recrystallized twice from ethanol before being used. Poly(ethylene glycol) (PEG, Mn at 400 or 800 Da, Alfa), abbreviated as PEG400 or PEG800, respectively, was used as received. All other chemical reagents were analytical grade and used as received. 2.2. Synthesis of the Macro-RAFT Agents. Three macro-RAFT agents (Table 1), poly(4-vinylpyridine) trithiocarbonate (P4VP-TTC), poly(methyl methacrylate) trithiocarbonate (PMMA-TTC), and poly(N-isopropylacrylamide) trithiocarbonate (PNIPAM-TTC), in which TTC refers to the RAFT moiety of trithiocarbonate, were prepared by homogeneous RAFT polymerization.66 Herein, the synthesis of PNIPAM18-TTC employing ECT as the RAFT agent and AIBN as the initiator under [NIPAM]0:[ECT]0:[AIBN]0 = 80:4:1 is described. Into a 25 mL Schlenk flask, NIPAM (2.00 g, 0.0202 mol), ECT (0.266 g, 1.01 mmol), AIBN (33.1 mg, 0.202 mmol), and 1,4dioxane (4.00 g) were weighed. The flask contents were degassed with nitrogen at 0 °C, and RAFT polymerization was conducted at 70 °C for 2 h and quenched in iced water. The monomer conversion of 87.5% was determined by 1H NMR through comparing the signal of N,N-dimethylacrylamide at δ = 5.64−5.69 ppm with δ = 5.16 ppm assigned to the 1,3,5-trioxane internal standard. The synthesized PNIPAM18-TTC was precipitated in iced diethyl ether and then dried in vacuo overnight. 2.3. Synthesis of Ag Nanoparticles Dispersed in PEG. Into a flask equipped with a magnetic stirrer bar, AgNO3 (0.2024 g, 0.001 19 mol) and PEG400 (80.0 g) were fed. The mixture was put in an ultrasonication bath for 15 min at room temperature to promote AgNO3 dissolving in PEG400. The solution was heated at 50 °C, and then hydrogen was bubbled for 8 h to give a yellowish PEG400 dispersion of Ag NPs, which is abbreviated to PEG/Ag-NPs. In the PEG/Ag-NPs dispersion, the concentration of Ag NPs is 0.015 mmol/ g. 2.4. Synthesis of Coil−Coil DBCP Nanotubes. Three coil−coil DBCP nanotubes of PNIPAM-b-PS, P4VP-b-PS, and PMMA-b-PS were synthesized by RAFT dispersion polymerization in PEG. Herein, a typical synthesis of the PNIPAM-b-PS nanotubes using a [St]0: [macro-RAFT]0:[AIBN]0 = 900:3:1 and at a solid content of 20 wt % is described. Into a 25 mL Schlenk flask equipped with a magnetic stirrer bar, PNIPAM18-TTC (0.0442 g, 0.019 mmol), AIBN (1.05 mg, 0.0063 mmol), St (0.600 g, 0.0058 mmol), and PEG400 (2.576 g) were weighed. The mixture was stirred until homogeneous solution B

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Figure 1. (A) UV−vis absorption spectra of the Ag/PNIPAM18-b-PS144 nanocomposites (1), the PNIPAM18-b-PS128 nanotubes (2), and the PEG/ Ag-NPs dispersion (3). (B) TEM image of Ag NPs.

Figure 2. Monomer conversion−time plot (A) and the ln([M]0/[M])−time plot (B) for RAFT dispersion polymerization in PEG400 using a [St]0: [PNIPAM18-TTC]0:[AIBN]0 = 900:3:1 with and without Ag nanoparticles, the GPC traces (C), and the molecular weight and Đ (Mw/Mn) of PNIPAM18-b-PS prepared through RAFT dispersion polymerization without Ag nanoparticles (D). was observed. The flask contents were degassed with nitrogen, and then polymerization was initiated by immersing the flask into a preheated oil bath at 70 °C under gentle stirring. After a given time, polymerization was quenched by rapidly cooling the flask into iced water. The St monomer conversion was detected by UV−vis analysis at 245 nm as discussed previously.67 To detect the morphology of the resulting block copolymer colloids, a small drop of colloidal dispersion was diluted with methanol and then deposited onto a piece of a copper grid, dried at room temperature, and finally observed by a transmission electron microscope (TEM). To collect the polymer for gel permeation chromatography (GPC) and 1H NMR analysis, the resultant colloidal dispersion of PNIPAM18-b-PS was precipitated into methanol, separated by centrifugation (10 000 rpm, 10 min), washed with methanol, and finally dried at room temperature under vacuum. The P4VP-b-PS and PMMA-b-PS nanotubes were synthesized in a similar manner with further experimental details in the Supporting Information. 2.5. Synthesis of the Ag/PNIPAM-b-PS Nanocomposites. The Ag/PNIPAM-b-PS nanocomposites were synthesized using a [St]0: [PNIPAM18-TTC]0:[AIBN]0 = 900:3:1 similar to those of the PNIPAM-b-PS nanotubes, except that the RAFT dispersion polymerization was performed in the presence of the PEG/Ag-NPs dispersion. By changing the molar ratio of [Ag]0:[PNIPAM18-TTC]0 from 0:1 to

1:1, the Ag/PNIPAM-b-PS nanocomposites containing different Ag fraction were prepared. To separate PNIPAM-b-PS from the Ag/PNIPAM-b-PS nanocomposites, the Ag/PNIPAM-b-PS nanocomposites were dispersed in toluene, kept for about 1 h, and separated by centrifugation (10 000 rpm, 10 min), the organic phase was filtered with filter paper to remove Ag NPs, and finally the block copolymer of PNIPAM18-b-PS was precipitated into methanol. After three repeated cycles of dissolution/separation/precipitation, PNIPAM18-b-PS was obtained. 2.6. Characterization. 1H NMR analysis was performed on a Bruker Avance III 400 MHz NMR spectrometer using CDCl3 as solvent. GPC analysis was performed on a Waters 600E GPC system equipped with three TSK-GEL columns and a Waters 2414 refractive index detector, where THF was used as eluent at a flow rate of 0.6 mL min−1 at 30.0 °C and samples of narrow-polydispersity polystyrene with different molecular weight were used as calibration standard. UV−vis analysis was performed on a Varian 100 UV−vis spectrophotometer. TEM observation was performed using a Tecnai G2 F20 electron microscope at an acceleration of 200 kV. C

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Figure 3. TEM/SEM images of the PNIPAM18-b-PS nanoassemblies synthesized at polymerization times of 0.5 (A), 4 (B), 6 (C, D, and E), 8 (F), 14 (G), and 24 h (H). Note: part D shows SEM images of nanotubes, and part E shows the TEM images of the stained nanotubes.

3. RESULTS AND DISCUSSION 3.1. Synthesis of the Macro-RAFT Agents. All the three macro-RAFT agents of PNIPAM18-TTC, P4VP27-TTC, and PMMA46-TTC were synthesized by homogeneous RAFT polymerization. The purified macro-RAFT agents were characterized by NMR (Figure S2) and GPC (Figure S3). As summarized in Table 1, the experimental molecular weights determined by GPC and NMR are in good agreement with the theoretical values calculated by eq S1 (see Supporting Information). Furthermore, narrow molecular weight distributions (Đ below 1.1) were obtained for the three macro-RAFT

agents, which confirms that the RAFT polymerizations were well controlled. As the morphology of linear amphiphilic DBCP nanoassemblies is determined according to a dimensionless packing parameter p, i.e., spherical micelles are formed when p ≤ 1/3, cylindrical micelles or rods when 1/3 ≤ p ≤ 1/2, and vesicles when 1/2 ≤ p ≤ 1,68 we decided to target relatively low DPs for the three macro-RAFT agents in order to synthesize coil−coil DBCPs nanotubes. 3.2. Synthesis of Ag NPs Dispersed in PEG400. In the solution-based synthesis of metal nanoparticles, two parameD

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Figure 4. TEM images of the P4VP27-b-PS385 (A), PMMA46-b-PS286 (B), and PNIPAM18-b-PS206 (C) nanotubes prepared through the RAFT dispersion polymerization.

slightly increases from 23 to 31 nm as the DP of the PS block increases from 128 to 206 (Figure 3B,C). Extension of the PS block to 236 leads to multiwall nanotubes (Figure 3F). In these multiwall nanotubes, the width is larger than the single-wall nanotubes (118 nm vs 80 nm), and the wall thickness becomes slightly thinner. Further extension of the PS block leads to nanorods (Figure 3G), and at the end of the RAFT dispersion polymerization, some warped nanorods along with membranelike aggregates are formed (Figure 3G). Interestingly, it is found that the surface of the PNIPAM18-b-PS nanotubes/nanorods varies with the length of the PS block. At low or moderate DP of the PS block, the PNIPAM18-b-PS nanoassemblies have a smooth surface, whereas as the DP of the PS block increases, TEM indicates that a rough topology is found. Just as reported,2 there exists a balance between nanoassemblies and single polymer chains. It is thought that a rough surface of the DBCP nanoassemblies is possibly due to the deposition of single polymer chains on the DBCP nanoassemblies in the RAFT dispersion polymerization with the DP of the PS block increasing, although exact reason needs further study. Following this work using PNIPAM18-TTC as a macroRAFT agent, nanotubes of other two coil−coil DBCPs, i.e., P4VP27-b-PS385 (Figures 4A) and PMMA46-b-PS286 (Figure 4B), were also formed. It is found that nanotubes can be formed only when a high DP (∼300) of the PS block is obtained. When a low DP of the PS block is targeted in the RAFT dispersion polymerization, vesicles or lamella-like assemblies of coil−coil DBCPs are instead formed (Figure S5). This is different than the coil−coil DBCPs of PNIPAM18-b-PS, which form nanotubes at a lower DP of the PS block (Figure 3). This is attributed to the relatively low DP of the PNIPAM18-TTC macro-RAFT agent compared to the P4VP27-TTC or PMMA46TTC macro-RAFT agents. It should be noted that the molecular weights of the coil−coil DBCPs assessed by GPC and NMR (Figures S6 and S7) are in good agreement with the theoretical values. Despites the difference, the solvophilic block in all these three DBCPs is short and the solvophobic PS block is relatively long, which leads to a high packing parameter p and is essential for synthesis of DBCP nanotubes. Figure 4 also indicates that these three coil−coil DBCP nanotubes are slightly different. That is, the nanotubes of P4VP27-b-PS385 (∼1600 nm × 250 nm, Figure 4A) and PMMA46-b-PS286 (∼800 nm × 200 nm, Figure 4B) are short in length and broad in width, whereas the PNIPAM18-b-PS206 nanotubes (∼2400 nm × 80 nm, Figure 4C) are long and thin, and the reason needs further study. The DBCP nanoassemblies of PNIPAM18-b-PS282 and P4VP27-b-PS353 were also synthesized by RAFT dispersion polymerization in an 80/20 methanol/water mixture; however,

tersthe stabilizer and the reducing agentare highly important for a controlled synthesis.69 First, the polymerization medium of PEG400 is used as both stabilizer and solvent for the Ag NPs. Second, H2 was used as the reducing agent. This avoids the unwanted byproducts introduced in the RAFT dispersion polymerization and therefore affords a clear synthesis of Ag NPs.69 Figure 1 shows the UV−vis absorption spectra and the TEM image of the Ag NPs dispersed in PEG400. As expected, the PEG/Ag-NPs dispersion is yellowish and shows a characteristic absorption at around 421 nm (Figure 1A), which is a typical absorption of Ag NPs.70 The TEM image indicates that the size of Ag NPs is 2.1 ± 0.4 nm (Figure 1B), and no aggregation of Ag NPs is found. These data indicate the successful synthesis of Ag NPs in PEG400. 3.3. Synthesis of Coil−Coil DBCP Nanotubes. The nanotubes of coil−coil DBCPs including PNIPAM-b-PS, P4VPb-PS, and PMMA-b-PS were prepared by RAFT dispersion polymerization of St employing the macro-RAFT agent, e.g., PNIPAM18-TTC, P4VP27-TTC, and PMMA46-TTC, in PEG. Since the PS block is insoluble in PEG, and P4VP, PMMA, or PNIPAM blocks are soluble, these DBCPs form nanoassemblies in PEG. Herein, the typical RAFT dispersion polymerization in PEG400 employing PNIPAM18-TTC and the synthesis of the PNIPAM18-b-PS nanotubes are introduced. Figures 2A and 2B summarize the polymerization kinetics of the RAFT dispersion polymerization of St in PEG400 employing PNIPAM18-TTC as macro-RAFT agent. The DBCPs of PNIPAM18-b-PS synthesized at different polymerization times are characterized by GPC analysis and 1H NMR analysis (Figure 2 and Figure S4). The RAFT dispersion polymerization follows pseudo-first-order kinetics as indicated by the linear ln([M]0/[M])−time plot (Figure 2B). As summarized in Figure 2D, three cases of the molecular weight, Mn,NMR which is calculated by comparing the signal at δ = 6.31−7.25 ppm (h, i, j) to that at δ = 3.88−4.20 ppm (g), Mn,GPC by GPC, and the theoretical molecular weight Mn,th calculated by eq S1, linearly increase with monomer conversion, and Đ maintains relatively low values at 1.29. Taken together, these data suggest that the RAFT dispersion polymerization in PEG400 has the characteristics of a controlled/living radical polymerization. Figure 3 shows the TEM/SEM images of the PNIPAM18-bPS nanoassemblies synthesized at different polymerization times. When the DP of the PS block is as low as 18, the DBCP of PNIPAM18-b-PS18 forms nanorods (Figure 3A). When DP of the PS block increases above 128, nanotubes are formed (Figure 3B−E). The width of the DBCP nanotubes keeps relatively constant at about 80 nm, whereas the wall thickness E

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Macromolecules consistent with previous reports only nanospheres or vesicles were formed (Figure S8). This suggests that the polymerization medium of PEG plays a key role in the DBCP nanotubes formation. Note: PMMA46-TTC is insoluble in the 80/20 methanol/water mixture, and therefore the PMMA46-b-PS nanoassemblies cannot be synthesized by RAFT dispersion polymerization in alcoholic solvent. 3.4. Synthesis of the Tubular Ag/PNIPAM18-b-PS Nanocomposites. DBCP nanotubes have a hollow cavity and are therefore suitable scaffolds for encapsulation of metal NPs. For example, Winnik and co-workers reported the encapsulation of Ag NPs into preprepared DBCP nanotubes leading to nanotube/Ag nanocomposites.33 As demonstrated above, PEG acts as both the medium for RAFT dispersion polymerization to afford the direct synthesis of coil−coil DBCP nanotubes and also as the stabilizer for the Ag NPs. Therefore, RAFT dispersion polymerization in the presence of the PEG/ Ag-NPs dispersion may be a suitable method to synthesize tubular Ag/DBCP nanocomposites. In this synthesis of the tubular Ag/DBCP nanocomposites, we considered three main points:(1) whether the Ag NPs can affect the kinetics and control of the RAFT dispersion polymerization, (2) whether the presence of Ag NPs can affect the morphology of the DBCP nanoassemblies, and (3) where the Ag NPs are located within the tubular Ag/DBCP nanocomposites. Herein, we first performed the RAFT dispersion polymerization in PEG/Ag-NPs dispersion using a [St]0:[PNIPAM18TTC]0:[AIBN]0:[Ag]0 = 1800:6:2:3. In this manner, the polymerization is performed under similar conditions as for the synthesis of the pure PNIPAM18-b-PS nanotubes except that it is performed in the presence of Ag NPs. As shown in Figure 2, the rate of polymerization in the RAFT dispersion polymerization is the same whether in the presence or absence of Ag NPs. Importantly, GPC and NMR (Figures S9 and S10) indicate that the experimental molecular weights of PNIPAM18b-PS DBCPs are close to the theoretical values. Furthermore, narrow molecular weight distributions confirm a good control of the polymerizations. As shown by the TEM images in Figure 5, the Ag/PNIPAM18-b-PS nanocomposites maintain their elongated 1-D morphology; however, some structural changes are observed with extension of the PS block. Initially, multiwall nanotubes of Ag/PNIPAM18-b-PS49 (Figure 5A) were formed, followed by porous nanotubes of Ag/PNIPAM18-b-PS144 (Figure 5B), and finally spica-like nanorods of Ag/ PNIPAM18-b-PS241 and Ag/PNIPAM18-b-PS287 (Figure 5C,D) were formed. By comparing the morphology of the pure PNIPAM18-b-PS nanoassemblies (Figure 3) with the Ag/ PNIPAM18-b-PS nanocomposites, it is evident that the presence of the Ag NPs significantly affects the DBCP morphology. Herein, the possible reason is discussed. At the beginning of the RAFT dispersion polymerization, all reactants except Ag NPs are molecularly soluble in PEG400. When the DP of PS exceeds a critical point, nucleation of PNIPAM18-b-PS occurs. It is expected that Ag NPs, which are stabilized by or wrapped with the hydrophilic PEG chains, change the nucleation behavior of PNIPAM18-b-PS and therefore change the morphology of the Ag/PNIPAM18-b-PS nanocomposites, although the exact reason needs further study. From Figure 5A, Ag NPs encapsulated within the multiwall nanotubes of Ag/PNIPAM18-b-PS49 are clearly observed, and no aggregation of Ag NPs is found. However, when longer PS blocks are targeted, it is difficult to discern the individual Ag NPs within the Ag/PNIPAM18-b-PS nanocomposites. In contrast, the EDX

Figure 5. TEM images of the tubular Ag/PNIPAM18-b-PS nanocomposites prepared by RAFT dispersion polymerization at polymerization times of 2 (A), 4 (B), 6 (C), and 15 h (D).

spectrum (Figure S11) and subsequent Ag mapping (Figure 6) of the typical nanotubes of Ag/PNIPAM18-b-PS144 indicate the uniform distribution of the Ag NPs within the 1-D tubular Ag/ PNIPAM18-b-PS nanocomposites. The tubular Ag/PNIPAM18-b-PS144 nanocomposites were typically characterized by UV−vis (Figure 1A). In comparison with the reference PNIPAM18-b-PS128 nanotubes, the tubular nanocomposites of Ag/PNIPAM18-b-PS144 have a slight absorption around 420 nm. The absorption at around 420 F

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4. CONCLUSIONS Herein, we discuss the synthesis of coil−coil DBCP nanotubes of PNIPAM-b-PS, P4VP-b-PS, and PMMA-b-PS and 1-D tubular Ag/DBCP nanocomposites via RAFT dispersion polymerization. In contrast to conventional RAFT dispersion polymerizations performed in aqueous or alcoholic solvent, PEG or the PEG/Ag-NPs dispersion was employed as a polymerization medium for the synthesis of coil−coil DBCP nanotubes and tubular Ag/DBCP nanocomposites. The RAFT dispersion polymerization in PEG proceeds with good control over the molecular weight and molecular weight distribution of the DBCPs as indicated by the linearly increasing molecular weight with monomer conversion and the low Đ. Single-wall nanotubes and multiwall nanotubes are synthesized by targeting a suitable DP of the PS block, and the nanotube structure including the wall thickness and the nanotube surface changes with the DP of the PS block. In the synthesis of 1-D tubular Ag/DBCP nanocomposites by RAFT dispersion polymerization in the PEG/Ag-NPs dispersion, it is found that the Ag NPs have little effect on the polymerization kinetics or degree of control but have a significant effect on the morphology of the Ag/DBCP nanocomposites. At a constant molar ratio of Ag/PNIPAM18-b-PS, the Ag/DBCP nanocomposites change from multiwall nanotubes to porous nanotubes and finally to spica-like nanorods with the extension of the PS block. At a similar DP of the PS block, the morphology of the Ag/PNIPAM18-b-PS nanocomposites changes from single-wall nanotubes to a mixture of singlewall nanotubes and multiwall nanotubes and finally to porous nanotubes bended porous short nanotubes as the mass fraction of Ag increases. This study demonstrates that RAFT dispersion polymerization is an efficient method for synthesis of coil−coil DBCP nanotubes, which may find application as antimicrobial materials or as catalytic nanoreactors.

Figure 6. Ag mapping in the tubular nanocomposites of Ag/ PNIPAM18-b-PS144.

nm is not as clear as in the PEG/Ag-NPs dispersion, and the reason is due to the low Ag fraction (about 0.5 wt %) in the tubular Ag/PNIPAM18-b-PS144 nanocomposites. By tuning the molar ratio of [Ag]0:[PNIPAM18-TTC]0 from 0:1 to 1:1 in the RAFT dispersion polymerization and quenching the polymerization at a monomer conversion of around 40%, Ag/PNIPAM18-b-PS nanocomposites with different mass fractions of Ag NPs are obtained. It should be noted that in these Ag/PNIPAM18-b-PS nanocomposites all DBCPs in the Ag/PNIPAM18-b-PS nanocomposites have similar length PS blocks of around 140 (Figure S12) but vary only in the mass fraction of Ag NPs. As shown in Figure 7, as the molar ratio of Ag/PNIPAM18-b-PS is increased from 0:1 to 1:1, the morphology of the Ag/PNIPAM18-b-PS nanocomposites changes from nanotubes (Figure 7A) to a mixture of nanotubes and multiwall nanotubes (Figure 7B,C), to porous nanotubes (Figure 7D,E), and finally to irregular porous short nanotubes. In these 1-D tubular Ag/PNIPAM18-b-PS nanocomposites, Ag NPs with a diameter of about 2 nm as indicated by red circles have been discerned, and no aggregation of Ag NPs is found. Note: when the ratio of [Ag]0:[PNIPAM18-TTC]0 is above 1, macroscopic precipitation occurs in the during the RAFT dispersion polymerization.

Figure 7. TEM images of the Ag/PNIPAM18-b-PS nanocomposites with the molar ratio of Ag/PNIPAM18-b-PS at 0:1 (A), 1/4:1 (B, C), 1/3:1 (D), 1/2:1 (E), and 1:1 (F). G

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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.7b01363. Characterization and experimental details (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (W.Z.). ORCID

Cyrille Boyer: 0000-0002-4564-4702 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 National Key Research and Development Program of China (2016YFA0202503) is gratefully acknowledged.

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