Photoinitiated Polymerization-Induced Microphase Separation for the

Oct 26, 2015 - We report on the use of photoinitiated reversible addition–fragmentation chain transfer (RAFT) polymerization for the facile fabricat...
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Photoinitiated Polymerization-Induced Microphase Separation for the Preparation of Nanoporous Polymer Films Jaehoon Oh and Myungeun Seo* Graduate School of Nanoscience and Technology, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea S Supporting Information *

ABSTRACT: We report on the use of photoinitiated reversible addition−fragmentation chain transfer (RAFT) polymerization for the facile fabrication of cross-linked nanoporous polymer films with three-dimensionally (3D) continuous pore structure. The photoinitiated polymerization of isobornyl acrylate (IBA) in the presence of 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (CTA) and 2,2dimethoxy-2-phenylacetophenone as a photoinitiator proceeded in a controlled manner, yet more rapidly compared to thermally initiated polymerization. When polylactidemacroCTA (PLA-CTA) was used, PLA-b-PIBA with high molar mass was obtained after several minutes of irradiation at room temperature. We confirmed that microphase separation occurs in the PLA-b-PIBA and that nanoporous PIBA can be derived from the PLA-b-PIBA precursor by selective PLA etching. To fabricate the cross-linked nanoporous polymer, IBA was copolymerized with ethylene glycol diacrylate (EGDA) in the presence of PLA-CTA to produce a cross-linked block polymer precursor consisting of bicontinuous PLA and P(IBA-co-EGDA) microdomains, via polymerization-induced microphase separation. We demonstrated that nanoporous P(IBA-co-EGDA) monoliths and films with 3D continuous pores can be readily obtained via this approach.

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advantages have been well demonstrated with polystyrene-bpolylactide (PS-b-PLA) systems, where PLA etching in basic conditions produces nanoporous PS.4−6 For applications such as filtration, where efficient transport through the pores is important, a nanoporous polymer film with three-dimensionally (3D) continuous pore structure would be beneficial since the percolating structure renders alignment of the pores unnecessary.7 Such a nanoporous polymer can be derived from a block polymer precursor in a bicontinuous gyroid phase.4d,8,9 However, the gyroid phase suffers from a narrow window of stability, which limits control over porosity. We recently reported that a disordered bicontinuous morphology can be spontaneously generated during the copolymerization of styrene and divinylbenzene (DVB) in the presence of a PLA macrochain transfer agent (PLA-CTA) via the thermally initiated reversible addition−fragmentation chain transfer (RAFT) process.5 As polymerization proceeded, the segregation strength of the forming block polymer PLA-b-P(Sco-DVB) increased and induced microphase separation into the bicontinuous morphology. PLA etching converted the crosslinked monolith into nanoporous PS with robust 3D

anoporous polymers with well-defined pore structure and narrow pore size distribution can be derived from

Scheme 1. Synthesis of PIBA or PLA-b-PIBA via Photoinitiated RAFT Polymerization of IBA in the Presence of CTA or PLA-CTA and DMPA

microphase-separated block polymer precursors by selectively etching a sacrificial component.1−3 Combined with advanced polymerization techniques that offer high precision and excellent tunability for the synthesis of the block polymer precursors, robust control can be achieved over pore size and functionality of the resulting nanoporous polymers. Such © XXXX American Chemical Society

Received: October 15, 2015 Accepted: October 20, 2015

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Figure 1. (a−c) Polymerization kinetics of IBA under long UV radiation in the presence of CTA, DMPA, and DMF at room temperature. The molar ratio was [DMF]:[IBA]:[CTA]:[DMPA] = 300:150:1:0.1. (a) Kinetic plot. The dashed line indicates linear regression fitting of the data from 6 min. (b) Conversion vs Mn and Đ plot. (c) SEC traces of PIBA over polymerization time. (d) SEC traces of PLA-b-PIBA over polymerization time.

the photoinitiated RAFT polymerization of IBA for synthesizing PIBA and PLA-b-PIBA. We confirmed that polymerization of IBA is controlled by the RAFT process yet proceeds faster than thermally initiated polymerization at room temperature. We also found that microphase separation occurs in PLA-bPIBA and that nanoporous PIBA can be generated by selective etching of PLA. We then applied this process for the copolymerization of IBA and EGDA in the presence of PLACTA to form a cross-linked block polymer precursor. The feasibility of the photoinitiated PIMS process was demonstrated by fabricating nanoporous monoliths and free-standing films. The route for the synthesis of PIBA and PLA-b-PIBA is presented in Scheme 1. PIBA has a high glass transition temperature (Tg), 112 °C, and we expected the large difference in the chemical structure of PLA and PIBA would induce sufficient immiscibility to produce microphase separation. 2(Dodecylthiocarbonothioylthio)-2-methylpropionic acid20 was chosen as the CTA, and PLA-CTA containing the trithiocarbonate moiety at the chain end was synthesized and used (see Supporting Information, Table S1 and Figure S1).4c,5,21,22 We note that RAFT polymerization of IBA in the presence of a thermal radical initiator and the CTA has been reported previously.23 To achieve well-controlled photoinitiated RAFT polymerization at ambient temperature, we selected 2,2dimethoxy-2-phenylacetophenone (DMPA) as the photoradical initiator and utilized long UV radiation to generate radicals via the photolysis of DMPA and to avoid decomposition of the CTA.17a,b

continuous pore structure. This polymerization-induced microphase separation (PIMS) method greatly reduces the effort needed to prepare bicontinuous block polymer precursors6 and can be utilized in other block polymer systems.10,11 However, fabrication of the precursor films via thermally initiated polymerization has been challenging because the monomers evaporate at the high polymerization temperature (e.g., 120 °C for 5 h in the case of styrene and DVB). To develop a facile method for fabricating nanoporous polymer films with 3D continuous pore structure, we explored the PIMS process, utilizing photoinitiated RAFT polymerization. Photoinitiated polymerization and curing processes are used industrially for polymer coatings, printing, and film fabrication owing to their advantages, which include great speed, spatial resolution, ambient temperature operation, and low energy consumption.12 Photoinitiation has been also used for controlled radical polymerizations13 including RAFT.14−17 Synthesis of block copolymers15b,16,17c−d and cross-linked polymer networks18 via photoinitiated RAFT polymerization has been reported. We also note that formation of phaseseparated polymer networks via photoinitiated (free radical) polymerization-induced phase separation (PIPS) has been documented.19 To investigate the potential of employing photoinitiated RAFT polymerization in the PIMS process, we designed a block polymer system consisting of PLA as the sacrificial block and poly(isobornyl acrylate-co-ethylene glycol diacrylate) (P(IBA-co-EGDA)) as the matrix block. We first investigated 1245

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synthesized by thermally initiated RAFT polymerization which took several hours at 60 °C. The microphase separation behaviors of PLA-b-PIBA were investigated by differential scanning calorimetry (DSC) and small-angle X-ray scattering (SAXS). The DSC thermograms of PLA-b-PIBA during the second heating cycle indicated two different endothermic transitions at 53 and 111 °C, corresponding to the Tgs of PLA and PIBA and suggesting incompatibility between PLA and PIBA (Figure S4). SAXS data of the PLA-b-PIBA confirmed microphase separation indeed occurred in PLA-b-PIBA (Figure 2a). On the basis of the position of the scattering peaks and considering the volume fraction of PLA (f PLA) = 0.21, we identified the microphaseseparated structure as hexagonally packed cylinders of PLA with the diameter of 16 nm in the PIBA matrix. The domain spacing (d) was estimated as 39 nm. We further confirmed that it was possible to selectively etch PLA microdomains by treating PLA-b-PIBA with 0.5 M NaOH solution in a mixture of water and methanol (6/4 (v/v)). When monitored by attenuated total reflection infrared (ATR-IR) spectroscopy, the vibrational frequency at 1754 cm −1 corresponding to the CO stretching of PLA diminished after immersing PLA-b-PIBA in the NaOH solution at 70 °C for 2 days (Figure S5). However, the frequency at 1729 cm−1 corresponding to the CO stretching of PIBA remained intact, indicating the stability of the ester linkages in PIBA under this condition. However, it was difficult to accurately determine the amount of residual PLA due to the overlap between the vibrational bands. The extent of PLA removal was quantified by 1H NMR analysis, which indicated 84% of PLA was removed after 4 days of treatment (Figure S6). We posit that some fraction of PLA remained inaccessible by the base, as the PLA cylinders in the precursor were not aligned. Nonetheless, a scanning electron micrograph of the polymer after PLA etching revealed hexagonally packed cylindrical nanopores with diameters of ca. 18 ± 3 nm, corroborating the formation of nanoporous PIBA by PLA etching (Figure 2b). Inspired by the results, we explored the formation of crosslinked PIBA containing 3D continuous nanopores via the photoinitiated PIMS process. Using EGDA as a photopolymerizable cross-linker, we formulated the polymerization mixture to contain PLA-CTA (18.3 wt %), IBA, EGDA (IBA/ EGDA = 5/1 (v/v), 44.1 wt % altogether), DMPA, and 1,4dioxane (37.7 wt %) which is a good solvent for both PLA and PIBA (Figure 3a). The polymerization mixture was highly transparent in the long UV regime (Figure S7). When the degassed polymerization mixture was irradiated for 6 min at room temperature, the mixture became gelled. A fully cross-linked and transparent monolith swollen with 1,4-dioxane was obtained after 30 min of exposure. By thermogravimetric analysis (TGA), we determined that the monolith contained 37.9% of 1,4-dioxane (Table S3 and Figure S8). A broad principal peak at q* accompanied by a shoulder at 2q* was observed in the SAXS data of the PLA-b-P(IBA-co-EGDA) monolith, suggesting formation of a disordered bicontinuous morphology consisting of PLA and P(IBA-co-EGDA) microdomains consistent with the PIMS mechanism (Figure S9a). Treatment of the monolith with NaOH solution at 70 °C for 4 days removed PLA, as evidenced by the increased SAXS intensity (Figure S9b) and the decreased peak intensity at 1754 cm−1 in the FTIR spectra (Figure S10a and b), resulting in nanoporous P(IBA-co-EGDA). A weight loss of 52.9% after

Figure 2. (a) SAXS data of PLA-b-PIBA (Mn,NMR,PLA = 14 kg mol−1, Mn,NMR,PIBA = 39 kg mol−1, f PLA = 0.21). Triangles show higher-order peak positions calculated for hexagonal symmetry based on the primary peak position. (b) SEM image of nanoporous PIBA derived from the PLA-b-PIBA by PLA etching. The image was obtained after Os coating.

Figure 1 depicts the polymerization kinetics of IBA under long UV radiation in the presence of CTA, DMPA, and N,Ndimethylformamide (DMF) as a solvent at room temperature. Consistent with the RAFT mechanism, a 6 min period of inhibition was observed at the beginning of polymerization, and then the conversion increased rapidly, following first-order kinetics with respect to [IBA] (Figure 1a). The number-average molar mass (Mn) of PIBA, as determined by 1H nuclear magnetic resonance (NMR) spectroscopy (Mn,NMR), also increased proportional to the conversion (Figure 1b and Figure S2). The size exclusion chromatography (SEC) traces of PIBA were generally unimodal except for the early stage of the polymerization (Figure 1c), and the dispersity (Đ) approached 1.15 at 54% conversion (Figure 1b), indicating that photoinitiated IBA polymerization was well-controlled via the RAFT process. Consistent with the literature, the Mn of PIBA estimated by the SEC analysis based on the linear polystyrene standards (Mn,SEC) was much smaller than the Mn,NMR.24 Even at room temperature, the photoinitiated polymerization proceeded approximately two times faster than the thermally initiated polymerization at 60 °C, using azobis(isobutyronitrile) (AIBN) as a thermal radical initiator. In the case of IBA polymerization in the presence of PLA-CTA, well-defined and high molar mass PLA-b-PIBA was obtained after only 4 min of exposure (Figure 1d, Figure S3 and Table S2). Mn and Đ values of the PLA-b-PIBAs were very comparable with those 1246

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Figure 3. (a) Synthetic route to nanoporous P(IBA-co-EGDA) via photoinitiated RAFT copolymerization of IBA and EGDA in the presence of PLA-CTA and DMPA and then selective PLA etching. (b,c) Cross-sectional SEM images of nanoporous P(IBA-co-EGDA) film derived from PLA-bP(IBA-co-EGDA). (b) Low magnification image showing the whole cross-section. (c) High magnification image close to the surface.

structures. Compared to the thermally initiated RAFT process, the photoinitiated polymerization proceeded much faster, yet under milder conditions, to generate the equivalent bicontinuous nanostructure via the PIMS mechanism. The feasibility of this approach for the synthesis of cross-linked nanoporous polymer films will be beneficial for practical applications.

PLA etching is consistent with the summed weight fractions of PLA and 1,4-dioxane. A 3D continuous pore structure was clearly visible by SEM and very comparable to those obtained by the thermally initiated PIMS process (Figure S11). PLA etching after vacuum drying of the monolith produced a pore structure that was indistinguishable from the previous sample (Figure S12). To demonstrate the feasibility of the photoinitiated PIMS methodology for the fabrication of nanoporous polymer films, PLA-b-P(IBA-co-EGDA) films were produced by exposing the polymerization mixture, sandwiched between two glass substrates, to long UV irradiation. Exposure for 10 min was sufficient to generate a free-standing, transparent, and flexible PLA-b-P(IBA-co-EGDA) film with dimensions of 2 cm × 2 cm × 100 μm. After washing with methanol and water, the freezefractured surface of the film revealed a nonporous texture, indicating that removal of the solvent does not generate voids (Figure S13).25 PLA and 1,4-dioxane were readily removed from the film by immersing it in 0.05 M NaOH solution for 7 h at room temperature (Figure S10c), which also induced a 26.3% shrinkage in the area of the film (Table S4). Formation of reticulated nanopores throughout the film thickness is apparent in the cross-sectional and surface SEM images (Figure 3b and c and Figure S14). The pore structure was retained after several swelling/deswelling cycles in 1,4-dioxane (Table S5 and Figure S15). We further demonstrated that a nanoporous P(IBA-co-EGDA) film with dimensions of 20 cm × 20 cm × 120 μm can be readily produced when an UV tape curing system was used for photoirradiation (Figure S16). In summary, the photoinitiated RAFT copolymerization of IBA with EGDA in the presence of PLA-CTA was found to be a facile method for generating a cross-linked block polymer precursor to a nanoporous polymer with 3D continuous pore

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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.5b00734. Materials and methods, Tables S1−S5, Figures S1−S16, and Supporting references (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Chul Jong Han for the use of the UV tape curing system. This research was supported by KAIST (Research Grant No. G04130012). Experiments at Pohang Accelerator Laboratory (PAL) were supported in part by Ministry of Science, ICT, and Future Planning of Korea and POSTECH.

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