Microphase Separation of P3HT-Containing Miktoarm Star

Jan 11, 2016 - Kyu Seong LeeJaeyong LeeChungryong ChoiYeseong SeoHong Chul MoonJin Kon Kim. Macromolecules 2018 51 (13), 4956-4965...
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Microphase Separation of P3HT-Containing Miktoarm Star Copolymers Jicheol Park,† Chungryong Choi,† Seung Hyun,† Hong Chul Moon,§ Kanniyambatti L. Vincent Joseph,† and Jin Kon Kim*,† †

National Creative Research Initiative Center for Smart Block Copolymers, Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, Kyungbuk 790-784, Republic of Korea § Department of Chemical Engineering, University of Seoul, Seoul 130-743, Republic of Korea S Supporting Information *

ABSTRACT: Well-defined [poly(methyl methacrylate)]2poly(3hexylthiophene) miktoarm star copolymers (PMMA2P3HT) were successfully synthesized via anionic coupling reaction. P3HT with two bromine groups at one chain end (P3HT-Br2) was synthesized by Williamson reaction between excess amount of tris(bromomethyl)benzene and hydroxyl-terminated P3HT. From anionic coupling reaction between living PMMA anions and P3HT-Br2, we prepared a series of PMMA2P3HTs having narrow molecular weight distribution (polydispersity index < 1.21) with various block compositions. While most P3HT-containing linear rod−coil block copolymers show only fibril structure, PMMA2P3HT shows conventional block copolymer self-assembled structures. Namely, spherical, hexagonally packed cylindrical, and lamellar microdomains including fibril structure were formed, confirmed by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS), depending on the weight fraction of P3HT (wP3HT). Even at a wP3HT = 0.72, lamellar microdomains were observed because of the curvature effect resulting from miktoarm architecture at the interface between two blocks. The result implies that the macromolecular architecture is one of the important factors for adjusting self-assembled morphology of P3HT-containing block copolymers. Moreover, the melting temperature of P3HT in PMMA2P3HT having lamellar or cylindrical morphology does not decrease compared with neat P3HT homopolymer, which means that the rod/rod interaction of P3HT was well-maintained under miktoarm architecture.

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INTRODUCTION Regioregular poly(3-hexylthiophene) (P3HT) has been extensively investigated due to its good solubility in organic solvents, high charge carrier mobility, and relatively facile synthesis using quasi-living Kumada catalyst transfer polymerization compared to other conjugated polymers.1−3 To enhance the performance of P3HT-based electronic devices, periodic P3HT structures with nanometer length scale are needed.4,5 For this purpose, numerous block copolymers consisting of P3HT and coil blocks have been synthesized6−15 to use their self-assembly ability.16−18 However, most P3HT-based rod− coil block copolymers have shown only fibril morphology due to strong rod/rod interaction of P3HT,18−22 although some nonfibril morphologies of P3HT-block-poly(2-vinylpyridine) (P2VP) and P3HT-block-poly(ethylene oxide) induced by specific solvent casting8,23 and incorporation of lithium salt,24 respectively, have been reported. The fibril structure, however, may not be an efficient nanopatterns, owing to poor ordering and difficulty in alignment of fibril along a desired direction. Many attempts to adjust two competitive driving forces (namely, rod/rod interaction of poly(3-alkylthiophene) (P3AT) block versus block copolymer phase separation) have © XXXX American Chemical Society

been tried. For example, modification of a side alkyl chain of P3AT has been reported to form ordered nanostructure because branched or longer alkyl side chains can effectively reduce rod/rod interaction of P3AT. Ho et al.25 reported that poly(3-(2′-ethyl)-hexylthiophene)-block-polylactide copolymer (P3EHT-b-PLA) forms lamellar morphology at wide ranges of the weight fraction of P3EHT block (wP3EHT) (0.25 ≤ wP3EHT ≤ 0.5). When wP3EHT was increased to 0.75, hexagonally packed (HEX) cylinders of PLA in the P3EHT matrix were obtained. Previously, we reported that poly(3-dodecylthiophene)-blockpoly(methyl methacrylate) copolymer (P3DDT-b-PMMA) exhibited various nanostructures depending on weight fraction of P3DDT (wP3DDT) for wP3DDT less than 0.7.26 But, P3DDT-bPMMA with wP3DDT = 0.76 showed fibril structure because the rod/rod interaction became dominant over the segregation power at high w P3DDT. In general, however, chemical modification of P3HT with longer (P3DDT) or bulkier Received: December 1, 2015 Revised: December 23, 2015

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Macromolecules Scheme 1. Synthetic Routes for (a) P3HT-Br2 and (b) PMMA2P3HT Miktoarm Star Copolymer

separation. However, only fibril structure was observed for P3HT2PMMA miktoarm star copolymer35 because rigid P3HT arms strongly interact with each other and thus prevent the formation of microphase separation of the block copolymer. It is well-known that for AnBm miktoarm star copolymers the interactions between the arms and the ability of stretching of the chains from the interface are significantly affected by the number of each arm.36 Thus, compared with P3HT2PMMA (rod2coil), another miktoarm star copolymer composed of two coil blocks and one P3HT block (coil2rod) might be more appropriate to induce various self-assembled microdomains, since two coil arms prefer to form a curved interface to avoid the overcrowding at the interface.37,38 In this study, we synthesized well-defined coil2rod [poly(methyl methacrylate)]2poly(3-hexylthiophene) miktoarm star copolymer (PMMA2P3HT) by using the anionic coupling reaction.11,12 First, we prepared P3HT functionalized with two bromine groups at the one end (P3HT-Br2) by Williamson reaction between excess amount of tris(bromomethyl)benzene and hydroxyl-terminated P3HT (P3HT-OH). After anionic coupling between excess living PMMA anions and P3HT-Br2, unreacted homopolymers in a crude product were removed by column chromatography, which gave PMMA2P3HTs having narrow molecular weight distribution (polydispersity index < 1.21). Then, we investigated self-assembled morphologies of PMMA2P3HTs having different wP3HT by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). PMMA 2P3HT showed nonfibril self-assembled morphologies, such as spheres, cylinders of P3HT, and lamellar microdomains. Although PMMA2P3HT having small molecular weight and high wP3HT still showed fibril structure, lamellar

(P3EHT) side chains decreases the electrical performance of P3AT-based electronic devices.27 One important factor for controlling the self-assembled structure of a block copolymer is macromolecular architecture.28−33 Some research groups modified the macromolecular architecture of P3HT-containing block copolymers to induce microphase separation. Kim et al. employed P3HT-graft-P2VP copolymer to adjust rod/rod interaction of P3HT block and showed that HEX cylinders of P3HT, lamellar microdomains were formed depending on weight fraction of P3HT block (wP3HT).30 However, the exact control of branching points between the P3HT and P2VP blocks is difficult, although the position of branched chain would be very important to control the self-assembled morphology of graft copolymers.31 In addition, the melting point of P3HT block was decreased arising from reduced rod/rod interaction, which might reduce the charge carrier mobility of P3HT.34 Kalow et al.32 reported that miktoarm H-shaped ABCBA copolymer, where the central C block is P3HT (other two blocks are different kinds of polynorbornene), showed lamellar morphology when the sample was solvent-annealed to reduce crystallization-driven fibril structure. Therefore, it is strongly needed to prepare welldefined P3HT-containing block copolymer exhibiting selfassembled morphology while maintaining good crystalline property of P3HT block. Since the miktoarm architecture of P3DDT 2 PMMA successfully induced self-assembled block copolymer microdomains,28 we applied this strategy to P3HT-containing block copolymer because P3HT showed better charge carrier mobility than P3DDT.27 That is we have synthesized P3HT2PMMA miktoarm star copolymer to induce microphase B

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Macromolecules Table 1. Molecular Characteristics of All Polymers Employed in This Study Polymera

Mn,PMMAb (g/mol)

P3HT-Br2-L PMMA2P3HT(0.70)-L PMMA2P3HT(0.53)-L PMMA2P3HT(0.38)-L PMMA2P3HT(0.22)-L P3HT-Br2-H PMMA2P3HT(0.79)-H PMMA2P3HT(0.72)-H PMMA2P3HT(0.51)-H

1000 2100 3900 7800 900 1400 3100

Mnc (g/mol)

Mw/Mnb

wP3HTc

4500 6400 8500 11800 20500 6600 8400 9200 12900

1.17 1.20 1.20 1.19 1.20 1.18 1.21 1.20 1.17

1.00 0.70 0.53 0.38 0.22 1.00 0.79 0.72 0.51

Tm,P3HTd (°C) 200, 203, 207 206 152 210, 205, 206, 214

208 210

218 219 218

morphologye fibril lamellae cylinder sphere fibril lamellae lamellae

a

Miktoarm star copolymers denoted by PMMA2P3HT(X)-L or PMMA2P3HT(X)-H, where X indicates weight fraction of P3HT; L and H are lower and higher molecular weights of P3HT block. bDetermined by SEC based on PMMA standards. cMeasured by 1H NMR. dDetermined by DSC at a heating rate of 10 °C/min. eDetermined by SAXS and TEM at room temperature after the samples were first annealed at 260 °C for 10 min followed by reannealed at 200 °C for 24 h and then slowly cooled to room temperature. Synthesis of PMMA2P3HT. The synthetic routes for P3HT-Br2 and PMMA2P3HT miktoarm star copolymer are shown in Scheme 1. P3HT-Br2. First, hydroxyl-functionalized P3HT (P3HT-OH) was synthesized according to the previous literature.7 Then, P3HT-OH (0.62 g, 0.138 mmol) and sodium hydride (48.2 mg, 2.01 mmol) were dissolved in anhydrous THF (20 mL) in a glovebox at room temperature. The resulting solution was allowed to warm to 50 °C. After stirring for 18 h, tris(bromomethyl)benzene (2.3 g, 6.44 mmol) in THF (10 mL) was injected, followed by stirring for another 24 h. The solution was quenched with distilled water (1 mL) and then precipitated in a large excess amount of methanol. P3HT-Br2 was purified by sequential Soxhlet extraction using methanol and acetone. Chloroform was used to extract desired P3HT-Br2. The chloroform fractionated part was precipitated into methanol, filtered, and dried under vacuum (0.58 g, yield 93.5%). 1H NMR (400 MHz, CDCl3): δ 6.98 (s, 1H), 4.45 (m, 6H) 3.58 (t, 2H), 2.80 (t, 2H), 1.71 (m, 2H), 1.43 (m, 2H), 1.34 (m, 4H), 0.91 (t, 3H); Mn: 4500 (1H NMR), PDI: 1.17 (SEC). PMMA2P3HT Miktoarm Star Copolymer. To prepare an initiator for the MMA monomer, sec-butyllithium (1.4 M solution in cyclohexane, 6.41 mL, 8.98 mmol) was reacted with diphenylethylene (2 mL, 11.33 mmol) in 30.0 mL of THF at 0 °C for 1 h. Lithium chloride (LiCl) (0.3 g, 7.08 mmol) in the reactor was vacuum-dried at 170 °C for 24 h, and 40 mL of THF was distilled. After LiCl was completely dissolved in THF, the reactor was put into 2-propanol/dry ice bath (−78 °C). Then, the prepared initiator solution was injected slowly into the reactor, followed by injection of purified MMA (0.9 g, 8.99 mmol). After 1 h, an aliquot was extracted for SEC characterization. For the coupling reaction, another THF solution (10 mL) containing P3HT-Br2 (0.12 g, 0.0267 mmol) was injected into the solution of living PMMA anions. The solution was allowed to stir at −78 °C for 4 h, and then temperature was increased to −30 °C. After 24 h, 2 mL of anhydrous 2-propanol was injected to terminate living anions. The crude product was precipitated into large amount of methanol. The unreacted homopolymer in the crude product was removed by column chromatography with acetic acid/methanol and toluene/hexane mixture as eluent. Weight fraction of P3HT block in miktoarm star copolymer was controlled by using different molecular weight of PMMA. Yield: 0.133 g (42.2%), PDI: 1.19 (SEC); weight fraction of P3HT (1H NMR) in the block copolymer: 0.38. The molecular characteristics of all polymers synthesized in this study are summarized in Table 1.

microdomains at high wP3HT were obtained by increasing the molecular weight. Moreover, the melting temperature of lamellar or P3HT cylindrical microdomains in PMMA2P3HT did not decrease compared with the pristine P3HT homopolymer, implying the rod/rod interaction of P3HT was preserved even in the microdomains.

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EXPERIMENTAL SECTION

Materials. Chemicals were purchased from Sigma-Aldrich and used without further purification except tetrahydrofuran (THF) and methyl methacrylate (MMA). THF and MMA were stirred with CaH2 overnight. Then, MMA was vacuum-distilled and stirred with trioctylaluminum (TOA) at room temperature for 20 min. THF was vacuum-distilled into benzophenone/sodium mixture and stirred until its color changed to deep violet, indicating oxygen-free state. MMA and THF were vacuum-distilled again before use. Silica gel (Merck silica gel 60, mesh size 0.2−0.5 μm) was used for column chromatography. Molecular Characterization. Polydispersity index (PDI) and molecular weight of PMMA precursor were measured by size exclusion chromatography (SEC: Waters 2414 refractive index detector) with two 300 mm (length) × 7.5 mm (inner diameter) columns including particle size of 5 μm (PLgel 5 μm MIXED-C: Polymer Laboratories). THF was the eluent, and the flow rate was 1 mL/min at 33 °C. 1H nuclear magnetic resonance spectra (1H NMR) were recorded on Bruker digital Avance III 300 to determine the molecular weight of P3HT and the weight fraction of P3HT in miktoarm star copolymer because the molecular weight of P3HT measured by SEC is overestimated.26 The solvent used in 1H NMR for all polymers was chloroform-d (CDCl3). Differential Scanning Calorimetry (DSC). The thermal properties of polymers were measured by using a PerkinElmer DSC 4000. Calibration was conducted with indium and zinc, and thermograms of samples (∼3 mg) were recorded under a nitrogen atmosphere during heating from 30 to 250 °C at a scan rate of 10 °C/min. Small-Angle X-ray Scattering (SAXS). All the samples for SAXS were prepared by pressing PMMA2P3HTs into 0.7 mm thick cell. Samples were first annealed at 260 °C for 10 min followed by reannealed at 200 °C for 24 h and then slowly cooled to room temperature. SAXS profiles were obtained on beamline 4C at the Pohang Accelerator Laboratory (South Korea), where the energy was set at 16.63 keV. The sample-to-detector distance was 4 m. The scattered X-rays were collected on a 2-D CCD detector (Princeton Instruments, SCX-TE/CCD-1242). Transmission Electron Microscopy (TEM). The samples annealed under the identical conditions for SAXS were ultramicrotomed (thickness ∼30 nm) by using a Leica Ultracut microtome (EM UV6 Leica Ltd.). Then, ultramicrotomed samples were exposed to RuO4 vapor to selectively stain P3HT domain. The TEM images were obtained by bright-field mode (S-7600 Hitachi Ltd.).

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RESULTS AND DISCUSSION PMMA2P3HT was synthesized via anionic coupling reaction between living PMMA anion and P3HT-Br2 (Scheme 1). First, P3HT-OH was prepared according to previous literature.7 Then, P3HT-Br2-L was synthesized via Williamson reaction between P3HT-OH and excess amount of tris(bromomethyl)benzene.39 1H NMR spectra of P3HT-OH and P3HT-Br2-L are C

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is consistent with the expected one (0.37) when we consider the molecular weights of P3HT-Br2 (Mn = 4500) and PMMA precursor (Mn = 3900). This result also implies that PMMA2P3HT(0.38)-L is successfully synthesized by anionic coupling reaction. The molecular characteristics of all PMMA2P3HTs synthesized are summarized in Table 1. Figures 3a and 3b show SAXS profile and TEM image for PMMA2P3HT(0.22)-L, respectively. SAXS profile show higher order peaks at positions of 2q* and √6q*, indicating spherical microdomains. The spherical microdomains consisting of P3HT chains were also observed by TEM. Dark and bright regions in the TEM image correspond to P3HT and PMMA microdomains, respectively, since P3HT was selectively stained by RuO4. In addition, the crystalline peak corresponding to (100) diffractions in WAXS patterns was shown, even though peak was broadened compared with (100) diffraction peak of other PMMA2P3HTs (see Figure S3). A schematic of schematic morphology with reduced crystallinity of P3HT block is shown in the inset of Figure 3a. Reduced crystallinity is because higher curvature at the interface could hinder the packing of P3HT chains.25,30 Figures 4a and 4b show SAXS profile and TEM image for PMMA2P3HT(0.38)-L, respectively. The SAXS profile shows higher order peaks of √3q* and √4q*, indicating HEX cylindrical morphology. The HEX cylindrical microdomains were also verified by TEM, in which both perpendicularly and parallel aligned cylindrical microdomains were clearly observed. In addition, a distinct (100), (200), and (020) diffraction peak in WAXS patterns (Figure S3) revealed that P3HT crystals were confined in the HEX cylindrical microdomains, as schematically shown in inset of Figure 4a. The formation of HEX cylindrical microdomains is attributed to the chain architecture of A2B miktoarm where flat interface is avoided to reduce the overcrowding and stretching of the two PMMA chains near the interface. A similar result was reported for polyhedral oligomeric silsesquioxane (POSS)-containing block copolymer, where miktoarm star copolymer formed cylindrical morphology, which has not been observed in the corresponding linear diblock copolymer because of large size mismatch between POSS and coil block.28 Also, PMMA2P3HT(0.38)-L showed HEX cylindrical microdomains in molten state above Tm of P3HT block (see Figure S4). This indicated that the crystallization of P3HT block could not break the preformed microdomains. Previously, we reported that linear P3HT-bPMMA having wP3HT = 0.3 and total molecular weight of 16 300 showed fibril morphology, not HEX cylindrical microdomains.12 Thus, the observation of self-assembled HEX cylindrical microdomains in PMMA2P3HT(0.38)-L is attributed to the miktoarm macromolecular architecture. Figures 5a and 5b are SAXS profile and TEM image for PMMA2P3HT(0.53)-L, respectively. Lamellar morphology was confirmed by higher order peak of 2q* and 3q* in the SAXS profile. The TEM image also supports lamellar morphology, where the domain spacing (15.4 nm) of the lamellae determined by TEM is close to that measured by SAXS (15.1 nm). A schematic of lamellar morphology is shown in the inset of Figure 5a. More importantly, the WAXS profile shows distinct (100), (200), (300), and (020) diffraction peaks, which reveals that P3HT crystals are confined in lamellar microdomains (Figure S3). On the other hand, as wP3HT increased, PMMA2P3HT(0.70)L exhibited fibril structure evidenced by SAXS profile and TEM image (Figure S5). One possible explanation for the origin of

given in Figure S1 of the Supporting Information. After Williamson reaction, the peak position of methylene protons adjacent to hydroxyl group was shifted from δ ∼ 3.78 ppm to δ ∼ 3.58 ppm. In addition, new peaks of methylene protons adjacent to the bromine end groups in P3HT-Br2 appeared at δ ∼ 4.45 ppm, indicating the successful introduction of two bromine groups to the end of P3HT. P3HT-Br2-H having different molecular weight was also synthesized similarly. Figure 1 show SEC traces of PMMA precursor, P3HT-Br2-L, and pure PMMA2P3HT(0.38)-L after the removal of unreacted

Figure 1. SEC traces for PMMA precursor, P3HT-Br2-L, and pure PMMA2P3HT(0.38)-L.

homopolymers by column chromatography. The PMMA precursor and P3HT-Br2-L have a narrow molecular weight distribution of PDI = 1.07 and 1.17, respectively. The SEC peak for pure PMMA2P3HT(0.38)-L was clearly shifted to earlier elution time compared with PMMA precursor and P3HT-Br2, which supports a successful coupling reaction. SEC traces for other PMMA2P3HTs are given in Figure S2. All of the synthesized PMMA2P3HTs showed narrow molecular weight distribution (PDI < 1.21). Figure 2 shows 1H NMR spectra for PMMA2P3HT(0.38)-L. The weight fraction of P3HT block was calculated as 0.38 from the ratio of characteristic peaks b and c in Figure 2. This value

Figure 2. 1H NMR spectra of PMMA2P3HT(0.38)-L. D

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Figure 3. (a) SAXS profile and (b) TEM image for PMMA2P3HT(0.22)-L. Inset of (a) is a schematic of spherical morphology (blue and green colors indicate P3HT and PMMA chains, respectively).

Figure 4. (a) SAXS profile and (b) TEM image for PMMA2P3HT(0.38)-L. Inset of (a) is a schematic of HEX cylindrical morphology (blue and green colors indicate P3HT and PMMA chains, respectively).

the fibril structure shown in PMMA2P3HT(0.70)-L is that block copolymer segregation power (χN) is weaker than the rod/rod interaction, since the total molecular weight (Mn = 6400) becomes smaller. Therefore, we synthesized new PMMA2P3HT-H having higher molecular weight of P3HT (Mn of P3HT-Br2-H = 6600) to induce microphase separation at higher wP3HT and investigate the effect of molecular weight on self-assembled morphology. When P3HT-Br2-H was used, PMMA2P3HT(0.51)-H showed lamellar morphology supported by a clear 3q* peak in the SAXS profile and TEM image (Figure S6), which is quite expected. Interestingly, PMMA2P3HT(0.72)-H also forms lamellar microdomains (Figure 6). Because the lamellar microdomains are clearly observed at molten state (250 °C), the crystallization only occurred inside the P3HT lamellar microdomains (see Figures S3 and S7). This result is different from the fibril structure of most other P3HT-containing rod−

coil block copolymer with similar wP3HT.4,8,30 The different phase behavior at high wP3HT may originate from two factors: (1) higher molecular weight of PMMA2P3HT (and thus larger χN) and (2) coil2rod macromolecular architecture. Higher molecular weight of PMMA2P3HT(0.72)-H causes stronger segregation power (χN) which induces block copolymer selfassembled morphology. Also, lamellar morphology instead of HEX cylinders at this composition (wP3HT = 0.72) arises from the AB2 miktoarm architecture where lamellar morphology is obtainable even at higher weight fraction of A.37 The size of dark P3HT microdomains was ∼15 nm in TEM image, which is close to the contour length of P3HT-Br2-H (15.4 nm).40 This implies that P3HT chains were stacked perpendicular to lamellar microdomains as fully extended structure (inset of Figure 6a).35,41 However, PMMA2P3HT(0.79)-H showed fibril structure, similar to the PMMA2P3HT(0.70)-L (Figure S8), which E

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Figure 5. (a) SAXS profile and (b) TEM image for PMMA2P3HT(0.53)-L. Inset of (a) is schematic of lamellar morphology (blue and green colors indicate P3HT and PMMA chains, respectively).

Figure 6. (a) SAXS profile and (b) TEM image for PMMA2P3HT(0.72)-H. Inset of (a) is a schematic of lamellar morphology (blue and green colors indicate P3HT and PMMA chains, respectively).

longer length of PMMA and high curvature at interface, which severely hinders the packing of P3HT chains.30 P3HT-bpoly(4-vinylpyridine) (P4VP) having a higher molecular weight of P3HT block (Mn,P3HT = 12 300 g/mol) showed melting temperature depression for wP3HT less than 0.23.10 On the other hand, P3HT-b-P2VP having a smaller molecular weight of P3HT block (Mn,P3HT = 6800 g/mol) showed decreased melting temperature even at wP3HT of 0.54.23 Therefore, the maintained melting temperature of P3HT even in PMMA2P3HT(0.38)-L (wP3HT of 0.38 and Mn,P3HT = 4500 g/mol) might be attributed to the low entanglement of P3HT chains in star-shaped macromolecular architecture,42 which permits local reorganization of P3HT chains to become well ordered during the crystallization.43 Recently, Ho et al.44 investigated the kinetics of formation of the microdomains for linear P3DDT-bPMMA copolymer. Thus, to understand in detail the formation of the microdomains of PMMA2P3HT miktoarm block

indicates that the molecular weight of PMMA2P3HT(0.79)-H would not be high enough for the microphase separation. The thermal properties of PMMA2P3HTs were investigated by differential scanning calorimetry (DSC), and the results are given in Table 1 and Figure S9. P3HT-Br2-H and P3HT-Br2-L show two endothermic peaks at 210, 218 °C and 200, 208 °C, respectively. Two neighboring peaks are related to main chain melting. The multiple peaks present the transitions for phase I → nematic mesophase → isotropic phase.23,26 When wP3HT is higher than 0.7, two endothermic peaks are maintained. These two endothermic peaks became one peak at 0.38 ≤ wP3HT ≤ 0.53. However, the melting temperature did not decrease compared with P3HT homopolymer, which indicated that rod/ rod interaction and well-ordered packing of P3HT chains were mostly maintained even in miktoarm star copolymer.10,34 In contrast, PMMA2P3HT(0.22)-L, having spherical microdomains, shows a significant drop in melting temperature due to F

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copolymers, a systematic study of the ordering kinetics is strongly needed, which would be a future investigation.

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CONCLUSIONS In this study, we successfully synthesized PMMA2P3HT miktoarm star copolymers with narrow molecular weight distribution by anionic coupling between P3HT-Br2 and living PMMA anion. Typical self-assembled block copolymer microdomains such as sphere, HEX cylinders, and lamellae were obtained depending on wP3HT. Especially, lamellar microdomains were observed even at wP3HT of ∼0.72 because of miktoarm architecture and high molecular weight. Moreover, melting temperature of P3HT in PMMA2P3HTs with HEX cylindrical and lamellar microdomains barely changed compared with P3HT homopolymer, indicating rod/rod interaction and ordered packing structure of the P3HT block in PMMA2P3HTs are similar to that of P3HT homopolymer.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02600. 1 H NMR for P3HT-OH and P3HT-Br2-L, SEC traces, WAXS profiles and thermograms of differential scanning calorimetry for PMMA2P3HTs, SAXS profiles and TEM images for PMMA2P3HT(0.7)-L, PMMA2P3HT(0.51)H, and PMMA2P3HT(0.79)-H, SAXS profiles for PMMA2P3HT(0.38)-L and PMMA2P3HT(0.72)-H at 250 °C (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.K.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Creative Research Initiative Program supported by the National Research Foundation of Korea (NRF). SAXS and WAXS experiments were performed at the beamline 4C of the Pohang Accelerator Laboratory (PAL).

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REFERENCES

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DOI: 10.1021/acs.macromol.5b02600 Macromolecules XXXX, XXX, XXX−XXX