Letter pubs.acs.org/macroletters
Alcoholic Photoinitiated Polymerization-Induced Self-Assembly (Photo-PISA): A Fast Route toward Poly(isobornyl acrylate)-Based Diblock Copolymer Nano-Objects Jianbo Tan,*,†,‡ Chundong Huang,† Dongdong Liu,† Xuechao Zhang,† Yuhao Bai,† 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: We report a fast alcoholic photoinitiated polymerization-induced self-assembly (photo-PISA) formulation via photoinitiated RAFT dispersion polymerization of isobornyl acrylate (IBOA) in an ethanol/water mixture at 40 °C using a monomethoxy poly(ethylene glycol) (mPEG) based chain transfer agent. Polymerization proceeded rapidly via the exposure to visible light irradiation (405 nm, 0.5 mW/cm2), and high monomer conversion (>95%) was achieved within 30 min. Kinetic studies confirmed that good control was maintained during the photo-PISA process, and the polymerization can be activated or deactivated by light. Finally, we demonstrated that a diverse set of complex morphologies (spheres, worms, or vesicles) could be achieved by varying reaction parameters, and a phase diagram was constructed.
P
conversions.17,20,23−27 For example, Pan et al.17 reported RAFT dispersion polymerization of styrene in methanol at 80 °C, and only 30−70% styrene conversions were observed within 48 h. The reason for the slow polymerization can be attributed to the relatively low reactivity of styrene. As an alternative, Armes et al.20 and Charleux et al.27 reported the alcoholic RAFT dispersion polymerization of benzyl methacrylate (BzMA), and near quantitative monomer conversions were achieved in most studied cases within 24 h at 70 °C. Jones et al. reported an alcoholic PISA formulation in ethanol−water mixtures, and they found that the addition of water led to faster polymerizations. For a target degree of polymerization (DP) of 200, 90% monomer conversion was achieved within 6 h at 70 °C with the addition of 20% w/w water. However, the faster kinetic limited the evolution of polymer morphology, and only spheres were obtained under these conditions. Recently, our group28 reported an aqueous photoinitiated polymerizationinduced self-assembly (photo-PISA) formulation, and a dramatic increase in the rate of polymerization was observed compared to thermally initiated aqueous PISA. This gives us a clue that one may be able to speed up the alcoholic PISA by introducing photoinitiation in PISA. Another limitation of alcoholic RAFT dispersion polymerization is that most core-forming monomers are methacrylates
olymer nano-objects have attracted considerable attention due to their broad applications in coating, gas adsorption, Pickering emulsion, biomineralization, nanoreactors, bioimaging, and drug/gene delivery.1−6 Solution self-assembly of block copolymer is one of the most commonly used methods to prepare polymer nano-objects, and a wide range of morphologies have been reported, including spheres, worms, vesicles, lamellae, sun-flowers, and butterflies.7,8 Typically, the solution self-assembly method is conducted at dilute copolymer concentrations (420 min at 15% w/w. Figure 2e and f show the plots of ln([M]0/[M]) versus reaction time for thermally initiated PISA of IBOA at 30 and 15% w/w IBOA concentrations, respectively. There is clear evidence that the introduction of photoinitiation in PISA significantly enhances the rate of polymerization both at the homogeneous stage and heterogeneous stage. The fast polymerization behavior of photo-PISA can be attributed to the rapid decomposition of TPO, under which conditions the half-life of TPO is around 30 min (see Figure S2). Samples extracted during the kinetic study of photo-PISA at 30% w/w were also analyzed by tetrahydrofuran (THF) gel permeation chromatography (GPC; Figure 3a), which confirmed the linear evolution of Mn with monomer conversion. Polydispersities were low (Mw/Mn < 1.30) during the whole process. Moreover, GPC traces shifted to a higher molecular weight with the increase of time, and each GPC curve proved to be unimodal and symmetric (see Figure 3b). These results indicated that good control was maintained during photo-PISA of IBOA in ethanol−water. To demonstrate that the polymerization can be activated and deactivated by switching the light, we conducted polymerizations exposed to an alternating sequence of visible light (“ON” and “OFF”), and the samples were extracted and analyzed immediately by 1H NMR spectroscopy. As shown in Figure 3c, in the absence of light, no polymerization was observed. When the light was on, the polymerization continued to proceed as expected. Figure 3d shows the GPC data obtained for a series of mPEG45-PIBOAn diblock copolymers synthesized by photo-PISA of IBOA at a concentration of 30% w/w. In each case, more than 90% monomer conversion was observed. There is some evidence for a low molecular weight shoulder corresponding to unreacted mPEG45-DDMAT (or mPEG45). Systematic variation of the target DP of the core-forming block (PIBOA in this case) led to a monotonic increase in the GPC curves of the diblock copolymer. Moreover, molecular weight distributions were low in all cases (Mw/Mn < 1.30), regardless of whether the obtained nano-objects were worms or vesicles. One of the main advantages of the PISA approach is its ability to prepare a diverse set of complex morphologies (e.g., 897
DOI: 10.1021/acsmacrolett.6b00439 ACS Macro Lett. 2016, 5, 894−899
Letter
ACS Macro Letters
Figure 5. (a) TEM image of mPEG45-PIBOA40 diblock copolymer nano-objects prepared by photo-PISA at 30% w/w; (b) digital photos representing a reversible transition of sample (a) from a free-standing physical gel to a free-flowing dispersion upon heating.
initiated RAFT dispersion polymerization of IBOA in an ethanol/water (85/15, w/w) mixture at 40 °C. Kinetic experiments confirmed that high monomer conversions (>95%) were achieved within 30 min of 405 nm light irradiation, and GPC results indicated well-controlled polymerizations. The polymerization can be activated or deactivated by turning on/off the light source, which is particularly beneficial for the mechanistic study and the construction of novel PISA formulations. Finally, a phase diagram was constructed for this particular photo-PISA formulation, which can be used as a roadmap for reproducible synthesis of various mPEG45-PIBOA diblock copolymer nano-objects. Our group is currently working on expanding this approach to other core-forming monomers and macro-RAFT agents.
worms, vesicles) by varying reaction parameters, for example, the degree of polymerization and the monomer concentration. For reproducible synthesis of mPEG45-PIBOAn diblock copolymer nano-objects, it is desirable to construct a detailed phase diagram of photo-PISA. A phase diagram for a series of mPEG45-PIBOAn diblock copolymer nano-objects determined by TEM is shown in Figure 4. The final copolymer compositions were determined by 1H NMR spectroscopy. The mPEG45-PIBOAn morphology is strongly concentrationdependent, of which lower concentrations favor the formation of spherical micelles. At the lowest IBOA concentration (15% w/w), only spherical micelles were formed even as the target DP of PIBOA increased to 188. Presumably, this phenomenon reflects the reduced probability of sphere−sphere fusion at low monomer concentrations, which is critical to the generation of higher order morphologies.38 At the concentration of 20% w/w, all the obtained diblock copolymer nano-objects were contaminated with spheres, and only mixed phases were observed. Varying the target DP of the PIBOA block at a fixed IBOA concentration of 30% w/w allows access to pure worms and vesicles. For example, targeting mPEG45-PIBOA69 leads to the formation of pure worms. Increasing the target DP of the PIBOA block to 99 forms pure vesicles, and the visual inspection of this sample confirms an increase in turbidity. The phase diagram reported here can be used as a roadmap for reproducible synthesis of mPEG45-PIBOAn diblock copolymer nano-objects (e.g., spheres, worms, or vesicles) via alcoholic photo-PISA. A free-standing physical gel was formed when the nanoobjects were pure worms, as shown in Figure 5. This phenomenon can be ascribed to interworm entanglements, which has proved to be common in PISA.36,37,39,40 The physical gel could be transformed to a free-flowing dispersion when the temperature increased to around 70 °C, and this process was fully reversible, which is similar to the case reported by Yeow et al.36 The main difference between photo-PISA and thermally initiated PISA is the relatively low reaction temperature of photo-PISA (40 °C in this case). The low reaction temperature made it possible to observe the increase of viscosity during the polymerization (see the video in Supporting Information) corresponding to the formation of worm-like micelles. That means one may be able to prepare pure worm-like micelles quickly by just monitoring the viscosity change during the alcoholic photo-PISA. It should be noted that the Boyer group has successful prepared worm-like micelles using this methodology, but long reaction time is usually required.37 In conclusion, this work demonstrates a fast alcoholic photoPISA formulation using an acrylate monomer as the coreforming monomer. A monomethoxy poly(ethylene glycol) (mPEG45) macro-RAFT agent was chain-extended via photo-
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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/acsmacrolett.6b00439. Full experimental section and additional results (PDF). Video of alcoholic photo-PISA at 30% w/w (from 6.5 to 7.8 min), demonstrating the viscosity change (AVI).
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge support from the National Natural Science Foundation of China (Grant 21504017), Guangdong Natural Science Foundation (Grant 2016A030310339), and the Innovation Project of Education Department in Guangdong (Grant 2015KTSCX029). Prof. Zhaohua Zeng (Sun Yat-sen University) is thanked for the great support he provided for this work.
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