Letter pubs.acs.org/macroletters
Polymerization-Induced Self-Assembly of Homopolymer and Diblock Copolymer: A Facile Approach for Preparing Polymer Nano-Objects with Higher-Order Morphologies Jianbo Tan,*,†,‡ Chundong Huang,† Dongdong Liu,† Xueliang Li,† Jun He,† Qin Xu,† and Li Zhang*,†,‡ †
Department of Polymeric Materials and Engineering, School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, China ‡ Guangdong Provincial Key Laboratory of Functional Soft Condensed Matter, Guangzhou 510006, China S Supporting Information *
ABSTRACT: Polymerization-induced self-assembly of homopolymer and diblock copolymer using a binary mixture of small chain transfer agent (CTA) and macromolecular chain transfer agent (macro-CTA) is reported. With this system, homopolymer and diblock copolymer were formed and chain extended at the same time to form polymer nano-objects. The molar ratio of homopolymer and diblock copolymer had a significant effect on the morphology of the polymer nanoobjects. Porous vesicles, porous nanospheres, and micron-sized particles with highly porous inner structure were prepared by this method. We expect that this method will greatly expand the promise of polymerization-induced self-assembly for the synthesis of a range of polymer nano-objects with unique morphologies.
B
assembly of poly(acrylic acid)-block-polystyrene (PAA-b-PSt) formed various morphologies, and adding PSt into the formulation changed the morphologies from vesicles or worms to spheres. Qiu et al.37 reported the preparation of uniform rectangular platelet micelles of controlled sized via solution self-assembly of crystalline homopolymer and crystalline-coil block copolymers. They further tuned the morphology of micelles by cross-linking the corona of the block copolymers. Cambridge et al.38 reported a systematic study of the assembly of a polyferrocenylsilane-block-polyisoprene sample (PFS-b-PI) with two different PFS homopolymer samples. Lamellae with fiber-like protrusions from the ends were prepared by this strategy. Cai et al.39 reported a discovery on the self-assembly of poly(g-benzyl-L-glutamate)block-poly(ethylene glycol) (PBLG-b-PEG) and homopoly(gbenzyl-L-glutamate) in water, with hybrid helical rods and rings being obtained. Polymerization-induced self-assembly using a binary mixture of macro-RAFT agents has recently been explored by several research groups.40−43 However, the polymerization-induced self-assembly of blends of homopolymer and diblock copolymer has been rarely reported. Typically, a macroRAFT agent is employed to mediate PISA, and a diblock (or
lock copolymer nano-objects with unique morphologies have a number of useful and important applications including catalysis, coating, bioimaging, nanoreactor, biomineralization, and drug delivery.1−4 Generally, two approaches for preparing block copolymer nano-objects are considered. The first one is the most commonly used method called solution self-assembly of block copolymer, which usually requires a multiple-step procedure (e.g., dialysis, pH adjustment). In addition, the solids content in this approach is relatively low (95%) was achieved as confirmed by 1H NMR spectroscopy. Styrene (St), an extensively studied monomer in PISA, was employed as the core-forming monomer in this paper. During the PISA of St at 70 °C in a methanol−water mixture (80/20, w/w), blends of PSt homopolymer and mPEG45-PSt diblock copolymer form at the same time. Precipitation occurs as the PSt homopolymer propagates to the critical chain length. The block copolymer formed in situ acts as a stabilizer and is adsorbed by the PSt homopolymer via the PSt block, with the mPEG block stretching into the reaction medium to stabilize the homopolymer. The molar ratio of PSt and mPEG45-PSt is determined by the feeding molar ratio of DDMAT and mPEG45-DDMAT. Figure 1 shows TEM images of polymer nano-objects with different PSt/mPEG45-PSt molar ratios. The target degree of polymerization (DP) was 200, and the concentration of St was kept at 15% w/w. In all formulations, high monomer conversions (>90%) were achieved within 24 h at 70 °C. Similar gel permeation chromatography (GPC) profiles were obtained with narrow molecular weight distributions (Mw/Mn < 1.15), as shown in Figure S1 and Table S1. In the absence of PSt homopolymer, well-defined vesicular morphology was observed (Figure 1a) with a membrane thickness of 35.5 nm, which is similar to literature.44 When the molar ratio of PSt and mPEG45-PSt was 1/3, vesicular morphology was still obtained, and pores were distributed in the membrane of some vesicles (Figure 1b). In this case, the membrane thickness of vesicles became not so uniform due to the presence of homopolymer.
triblock) copolymer will form in situ during the polymerization. As the insoluble block grows to the critical chain length, phase separation occurs, and polymer nano-objects form as the polymerization proceeds. Herein, we report a facile PISA formulation by using a binary mixture of small molecular RAFT agent and macro-RAFT agent. The presence of small molecular RAFT agent and macro-RAFT agent will lead to the in situ formation of blends of homopolymer and diblock copolymer during PISA. As shown in Scheme 1, S-1-dodecyl-S′-(α,α′-dimethyl-α″acetic acid) trithiocarbonate (DDMAT) was utilized as the Scheme 1. RAFT Dispersion Polymerization of Styrene Mediated by DDMAT and mPEG45-DDMAT in a Methanol−Water (80/20, w/w) Mixture at 70 °C
small molecular RAFT agent. The macro-RAFT agent was prepared via the esterification of monomethoxy poly(ethylene glycol) (mPEG45) and DDMAT in anhydrous dichloro-
Figure 1. TEM images of polymer nano-objects prepared via PISA of St (15% w/w St concentration) in methanol−water at different DDMAT/ mPEG45-DDMAT molar ratios: (a) [DDMAT]/[mPEG45-DDMAT] = 0/1, (b) [DDMAT]/[mPEG45-DDMAT] = 1/3, (c) [DDMAT]/[mPEG45DDMAT] = 1/2, and (d) [DDMAT]/[mPEG45-DDMAT] = 1/1. 299
DOI: 10.1021/acsmacrolett.7b00134 ACS Macro Lett. 2017, 6, 298−303
Letter
ACS Macro Letters
Figure 2. (a) Kinetic data derived from 1H NMR studies of the RAFT dispersion polymerization of styrene (target DP of 200) at 70 °C using [DDMAT]/[mPEG45-DDMAT] = 1/1 in a methanol−water (80/20, w/w) mixture. (b) semilogarithmic plot according to the data in (a). (c) THF GPC traces obtained for the samples during the kinetic study. (d) Evolution of Mp of two GPC peaks (derived from (c)) with monomer conversion.
Figure 3. TEM images of polymer nano-objects prepared by RAFT dispersion polymerization of styrene (15% w/w) with [DDMAT]/[mPEG45DDMAT] = 1/1 in a methanol−water (80/20, w/w) mixture at different DPs: (a) DP = 49, (b) DP = 99, (c) DP = 146, (d) DP = 218, (e) DP = 278. (f) The corresponding GPC traces as determined by THF GPC.
corresponds to the precipitation of homopolymer, subsequently solubilized with diblock copolymer, and the occurrence of nucleation. The final dispersion was milky white with no precipitates being observed. As shown in Figure 2a, high monomer conversion (>90%) was achieved after 14 h of reaction. The semilogarithmic plot in Figure 2b shows three distinct regimes. The first regime is from 0 to 3.2 h, corresponding to the formation of dissolved polymer chains. In the second regime, a higher rate of polymerization was observed, which corresponds to the onset of nucleation as the polymer chains (both PSt and mPEG45-PSt) grow to a certain length. A third regime was also observed in the present case, which may correspond to the transformation of morphologies.45 TEM measurement indicated that the morphology changed from lamellae at 7 h to vesicles at 11 h (Figure S3). Samples extracted during the kinetic study were further characterized by tetrahydrofuran (THF) GPC, as shown in Figure 2c. During the early stage, two separated GPC peaks were observed with narrow molecular weight distributions (Mw/Mn < 1.10), corresponding to PSt homopolymer (the right one) and mPEG45-PSt diblock copolymer (the left one).
When the molar ratio of PSt and mPEG45-PSt was increased to 1/2, a significantly larger amount of porous vesicles were present (Figure 1c). Further increasing the molar ratio of PSt and mPEG45-PSt to 1/1 led to porous vesicles and porous nanospheres with a high density of pores (Figure 1d). These pores should be distributed in the inner of particles according to scanning electron microscopy (SEM) characterization (Figure S2) since no pores were observed form the particle surface. TEM images obtained at different tilting angles (from −60 to +60°) further confirmed the distribution of pores in the polymer nano-objects (see the video in the Supporting Information and Figure S8). These results indicated that the presence of homopolymer had a significant impact on the morphology of polymer nano-objects prepared via PISA of blends of homopolymer and block copolymer. In order to elucidate the effect of DDMAT on the polymerization, a kinetic study was performed with the target DP of 200 at 15% w/w monomer concentration ([DDMAT]/ [mPEG45-DDMAT] = 1/1). During the kinetic study, it was found that the reaction mixture was turbid at 1 h, subsequently changed to transparent, and became bluish at around 3 h. This 300
DOI: 10.1021/acsmacrolett.7b00134 ACS Macro Lett. 2017, 6, 298−303
Letter
ACS Macro Letters
S5), which is similar to the case of DDMAT. Zou et al.44 reported that the formation of porous vesicles or porous nanospheres could be attributed to the vesicles being swelled with monomer and thus the occurrence of phase segregation. To prove that this is the exact reason for the formation of porous polymer nano-objects, we swelled the mPEG45-PSt192 vesicles with styrene or toluene in a methanol−water mixture (40% w/w water). After this swelling procedure, the vesicles were converted into porous vesicles and porous nanospheres (Figure S7). These results demonstrated that the formation of porous vesicles and porous nanospheres was attributed to the vesicles being swelled with styrene. In the present case, PSt homopolymer was formed within the solvophobic regions of the nanoparticles, and this contributes to the hydrophobic volume fraction. Moreover, the precipitation of PSt homopolymer from the reaction medium was very fast, allowing the solvophobic polymers to be swelled with St monomer for extended time. Therefore, the PSt/mPEG45-PSt vesicles can be swelled with St monomer effectively, leading to the formation of porous vesicles and porous vesicles and nanospheres. Monomer swellability. Monomer swellability of vesicles is the key factor to promote the formation of higher-order morphologies via PISA of homopolymer and block copolymer. Thus, maybe it is possible to prepare novel morphologies by changing the monomer swellability of vesicles (e.g., increasing the monomer concentration). We then conducted the PISA at high St concentrations (20% and 25%) with a molar ratio of DDMAT/mPEG45-DDMAT of 1/1. Monomer conversions were relatively low (90%), the GPC profiles changed to unimodal. This can be explained by the reason that the molecular weight difference between PSt and mPEG45-PSt is too small to be distinguished by the GPC equipment when the molecular weight is high enough. Figure 2d shows the evolution of Mp values of two GPC peaks with monomer conversion. It should be noted that the GPC traces eventually overlap at high monomer conversions (>80%). Thus, we are only able to get separated Mp values of PSt and mPEG45-PSt at relatively low monomer conversions (
