Synthesis and Characterization of ABC-Type Asymmetric Star

Article pubs.acs.org/Macromolecules

Synthesis and Characterization of ABC-Type Asymmetric Star Polymers Comprised of Poly(3-hexylthiophene), Polystyrene, and Poly(2-vinylpyridine) Segments Tomoya Higashihara,*,† Shotaro Ito,‡ Seijiro Fukuta,† Tomoyuki Koganezawa,§ Mitsuru Ueda,†,∥ Takashi Ishizone,‡ and Akira Hirao‡,∥,⊥ †

Department of Polymer Science and Engineering, Graduate School of Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan ‡ Division of Soft Material Chemistry, Polymeric and Organic Materials Department, Graduate School of Science and Engineering, Tokyo Institute of Technology, S1-13, 2-12-1, Ohokayama, Meguro-ku, Tokyo 152-8552, Japan § Japan Synchrotron Radiation Research Institute, 1-1-1, Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan ∥ Institute of Polymer Science and Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan ⊥ College of Chemistry, Chemical Engineering and Materials Science, Soochow University, 199 Ren Ai Road, Suzhou Industrial Park, Suzhou 215123, China S Supporting Information *

ABSTRACT: ABC-type asymmetric star polymers containing a P3HT segment were successfully synthesized by polymer coupling/ linking reactions by combining the Kumada catalyst-transfer polymerization and living anionic polymerization for the first time. The synthetic methodology involves the following three stage reactions: (a) the anionic linking reaction between ω-chain-endfunctionalized P3HT with a bromobutyl moiety (P3HT-C4-Br) and living anionic PS end-capped with 1-(3-tert-butyldimethylsilyloxymethylphenyl)-1-phenylethylene (1), (b) the transformation reaction of the 3-tert-butyldimethylsilyloxymethylphenyl (TBDMS) moiety at the junction between the P3HT and PS segments into the α-phenyl acrylate (PA) moiety through the hydroxymethylphenyl moiety, and (c) the anionic linking reaction between PA-in-chainfunctionalized P3HT-b-PS and living anionic P2VP. Indeed, the well-defined ABC star polymers in which A, B and C are poly(3hexylthiophene) (P3HT), polystyrene (PS), and poly(2-vinylpyridine) (P2VP), respectively, could be synthesized. The molecular weights and compositions of the star polymers were controllable by possessing extremely low Đ values (Đ < 1.05). Two distinct transition temperatures (Tg,PS+P2VP and Tm,P3HT) were clearly observed in the DSC thermograms of the ABC star polymers, indicating the phase separation between the (PS+P2VP) and P3HT domains. The vibronic absorption of the ABC star polymer films based on the UV−vis spectroscopy indicated a high degree of ordering of the P3HT crystalline structures, supporting the isolation of the P3HT domains although the PS and P2VP segments are connected to P3HT at the core. In the AFM phase images of the ABC star polymer thin film surface, continuous fibril structures were clearly seen. GISAXS experiments confirmed the orientation of the fibril structures with the mean period distances depending on the P2VP arm lengths. The GIWAXS results showed that the P3HT crystalline domains in the microphase-separated P3HT domains align with an “edge-on” rich orientation and the π−π stacking distance in the range of 0.380−0.393 nm also depending on the length of P2VP segments.

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versatile synthesis of chain-end-functionalized P3HTs6 as well as block copolymers having P3HT segment(s).5,7 Star-branched polymers are a class of branched macromolecules with more than three arm segments emanating from a core. The exclusive hydrodynamic volume of such polymers is restricted depending on the arm numbers and molecular

INTRODUCTION

Poly(3-hexylthiophene) (P3HT) has received much attention in the field of polymer electronic devices, such as organic fieldeffect transistors1 and organic photovoltaic cells,2 because P3HT is one of the best balanced high-performance materials as p-type semiconductors in terms of solubility, chemical stability, charge mobility, and commercial availability. The breakthrough of the quasi-living Kumada catalyst transfer polymerization (KCTP) system, independently achieved by Yokozawa et al.3 and McCullough et al.,4,5 has encouraged the © XXXX American Chemical Society

Received: November 25, 2014 Revised: December 16, 2014

A

DOI: 10.1021/ma5023814 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Scheme 1. Synthetic Routes for ABC-Type Asymmetric Star Polymers

copolymers containing polystyrene(PS)-b-P3HT arm segments based on the atom transfer radical polymerization and 1,3dipolar cycloaddition click reaction.13 The characteristic and detailed physical properties of such well-defined P3HT-based star-branched polymers may possibly be investigated in the near future. Asymmetric star polymers (also called “miktoarm star polymers”), in which different types of arm segments are linked at the core, have the capability to create novel morphological nanostructures different from linear block copolymers or regular star polymers comprised of the same arm segments.14 However, it has been more difficult to synthesize asymmetric star polymers than regular ones due to the following reasons: (1) there are strict requirements of the multistep reactions corresponding to chemically different arms, (2) each polymer−polymer reaction should be nearly

weights. Indeed, less entanglement of the star-branched polymers causes a lower viscosity compared to linear analogues in the melt and solution states.8 However, examples of the synthesis of star P3HTs have rarely been reported due to the synthetic difficulty.9 Kim et al. reported the multiarm star P3HTs based on the divinyl compound technique and their unique crystalline behavior in comparison to linear P3HT.10 However, the quantitative relationship of the primary structure and physical properties remains unclear due to the use of star P3HTs with ill-defined arm numbers. Recently, Kiriy et al. reported the core-first method based on the externally initiated KCTP to synthesize a 6-arm star P3HT.11 Soon after, Luscombe and co-worker succeeded in synthesizing the welldefined 3-arm star P3HT with a low dispersity (Đ = 1.15) using a specially designed core compound.12 On the other hand, Zhao and co-workers reported the synthesis of 6-arm star-block B


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Macromolecules

pressure. Finally, it was distilled from its (C8H17)3Al solution (ca. 3 mol %) under high-vacuum conditions. LiCl was dried with stirring at 130 °C for 72 h under high-vacuum conditions. All monomers, DPE, and LiCl were diluted with dry THF and then divided into ampules equipped with break-seals under high-vacuum conditions. α-Phenylacrylic acid and 1-(3-tert-butyldimethylsilyloxymethylphenyl)-1-phenylethylene (1) were synthesized according to the procedures previously reported.17 P3HT-C4-Br was synthesized according to the reported procedure.18 Synthesis of TBDMS-in-Chain-Functionalized P3HT-b-PS. Anionic polymerizations and subsequent linking reactions were carried out under high-vacuum conditions (10−6 Torr) in sealed handmade glass reactors equipped with break-seals. The reactor was sealed off from the vacuum-line and prewashed with red 1,1-diphenylhexyllithium (ca. 0.05 M) in heptane solution prior to the polymerization. All operations were performed according to the usual high-vacuum technique with break-seals.15d Styrene (5.22 mmol) in THF solution (5.64 mL) was polymerized with sec-BuLi (0.0490 mmol) in heptane solution (1.08 mL) at −78 °C for 20 min, followed by addition of 1 (0.0899 mmol) in THF solution (1.66 mL) at −78 °C for 20 min to end-cap the living PS. The solution was then added to the THF solution (80.0 mL) of P3HT-C4Br (0.0161 mmol) at −78 °C, and the suspended solution was allowed to stand at −78 °C for 36 h. Prior to the linking reaction, P3HT-C4-Br solution was purified with Bu2Mg (0.110 mmol) at 25 °C for 10 min. The reaction was quenched with degassed methanol, the polymer was precipitated in methanol, and it was freeze-dried from its benzene solution (0.58 g, 85%). Target P3HT-b-PS was isolated in 40% yield (0.12 g) by HPLC fractionation. Mn (SEC-RALLS) = 19.5 kg/mol and Đ = 1.04 (SEC). 1H NMR (CDCl3) (300 MHz): δ = −0.01−0.06 (Si−(CH3)2, m, 6H), 0.51−0.78 (2CH3− (sec-Bu), m, 6H), 0.92 (−CH3, t), 1.02−2.33 (PS main chain and ThCH2−(CH2)4−, m), 2.49 (tolyl, s, 3H), 2.82 (ThCH2−, t), 4.55−4.68 (Ph−CH2− OTBDMS, m, 2H), 6.28−7.21 (Ph, m), 6.98 (Th (1H), s, overlapped with Ph group). Transformation Reaction of Functional Groups. Under an atmosphere of nitrogen, the resulting TBDMS-in-chain-functionalized P3HT-b-PS (0.00615 mmol) dissolved in THF (8 mL) was treated with (C4H9)4NF (0.32 mmol) in THF solution (0.32 mL) at 0 °C for 18 h. The reaction mixture was poured into a large amount of methanol to precipitate the polymer. The polymer was purified by reprecipitation from THF to methanol and freeze-dried from its absolute benzene solution (0.12 g, 100%). 1H NMR (CDCl3) (300 MHz): δ = 0.50−0.79 (2CH3− (sec-Bu), m, 6H), 0.92 (−CH3, t), 1.01−2.30 (PS main chain and ThCH2−(CH2)4−, m), 2.49 (tolyl, s, 3H), 2.81 (ThCH2−, t), 4.42−4.57 (Ph−CH2−OH, m, 2H), 6.27− 7.21 (Ph, m), 6.98 (Th (1H), s, overlapped with Ph group). Under an atmosphere of nitrogen, the OH-in-chain-functionalized P3HT-b-PS (0.00615 mmol) dissolved in THF (8 mL) was mixed with PPh3 (0.307 mmol), α-phenylacrylic acid (0.307 mmol), and diisopropyl azodicarboxylate (0.307 mmol) at 0 °C. The reaction mixture was allowed to react at 25 °C for 16 h, and then poured into methanol to precipitate the polymer. The polymer was purified by reprecipitation twice from the THF solution to methanol, and freezedried from its absolute benzene solution (0.11 g, 92%).1H NMR (CDCl3) (300 MHz): δ = 0.50−0.78 (2CH3- (sec-Bu), m, 6H), 0.92 (−CH3, t), 1.02−2.30 (PS main chain and ThCH2−(CH2)4-, m), 2.49 (tolyl, s, 3H), 2.81 (ThCH2-, t), 5.07−5.17 (Ph−CH2-O-, m, 2H), 5.82−5.86 (CHH, m, 1H), 6.24−7.22 (Ph, m), 6.98 (Th (1H), s, overlapped with Ph group). Synthesis of ABC-Type Asymmetric Star Polymers. In the representative experiment for Star 1, living P2VP was prepared at −78 °C in THF solution by the polymerization of 2VP (5.27 mmol) in THF solution (10.4 mL) for 30 min with the initiator prepared from sec-BuLi (0.108 mmol) and DPE (0.205 mmol) in the presence of LiCl (0.600 mmol). Thus, prepared living P2VP was in situ reacted with PAin-chain-functionalized P3HT-b-PS (0.00564 mmol) in THF solution (10.5 mL) at −78 °C for 18 h. The reaction was quenched with degassed methanol, the polymer was precipitated in hexane, and it was freeze-dried from its benzene solution (0.69 g, 100%). The objective

quantitative, and (3) isolation and purification of intermediate polymers are often required. Nevertheless, many types of asymmetric star polymers with conventional coil segments, such as polystyrene, poly(1,3-diene), and poly(meth)acrylate, have been successfully synthesized with a controlled arm number, molecular weights, composition, and low Đ, the details of which are well reviewed elsewhere.8f,15 The successful establishment of asymmetric star polymers is mainly due to the combination of the living/controlled polymerization system and quantitative linking reactions, the so-called arm-first method. However, there have been few examples of the synthesis of asymmetric star polymers with P3HT segment(s). To the best of our knowledge, Kim and co-workers first reported a well-defined AB2-type asymmetric star polymer, where A and B are the poly(methyl methacrylate) (PMMA) and P3HT segments, respectively, by the copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddition click reaction of chain-endfunctionalized PMMA with two azide moieties with monoethynyl-chain-end-functionalized P3HT.16 Since the PMMA and P3HT segments were prepared by the living anionic polymerization and KCTP, respectively, the resulting AB2-type asymmetric star polymer possess well-defined structures in terms of their molecular weight, Đ, and composition. However, the prepared star polymer consisted of only two different segments, and an example of the ABC-type asymmetric star polymer comprised of three different arm segments has never been reported. In this paper, we first report the synthesis of well-defined ABC-type asymmetric star polymers, where A, B, and C are P3HT, polystyrene (PS), and poly(2-vinylpyridine) (P2VP), respectively, with controlled molecular weights, composition, and extremely low Đ (Đ < 1.05) based on the catalyst-free anionic linking reactions in conjunction with the living anionic polymerization and KCTP. The synthetic methodology involves the following three stage reactions (Scheme 1): (a) an anionic linking reaction between ω-chain-end-functionalized P3HT with a bromobutyl moiety (P3HT-C4-Br) and living anionic PS end-capped with 1-(3-tert-butyldimethylsilyloxymethylphenyl)-1-phenylethylene (1), (b) a transformation reaction of the 3-tert-butyldimethylsilyloxymethylphenyl (TBDMS) moiety at the junction between the P3HT and PS segments into the α-phenyl acrylate (PA) moiety through the hydroxymethylphenyl moiety, and (c) an anionic linking reaction between PA-in-chain-functionalized P3HT-b-PS and living anionic P2VP. The thermal, optical, and morphological properties of the ABC-type star polymer are also investigated.

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EXPERIMENTAL SECTION

Materials. The reagents (>98% purities) were purchased from Aldrich, Japan, and used as received unless otherwise stated. Tetrahydrofuran (THF) was refluxed over sodium, distilled over LiAlH4 under nitrogen, and then distilled from its sodium naphthalenide solution under high-vacuum conditions (10−6 Torr). Heptane was washed with concentrated H2SO4, water, and aqueous NaHCO3, dried over P2O5, and finally distilled from its 1,1diphenylhexyllithium solution under high-vacuum conditions. Styrene and 1,1-diphenylethylene (DPE) were washed with 5% NaOH solution, water, and then dried over MgSO4. After filtration of MgSO4, they were distilled over CaH2 under reduced pressure. Finally, styrene was distilled from its Bu2Mg solution (ca. 3 mol %) under high-vacuum conditions. DPE was finally distilled from its 1,1diphenylhexyllithium solution (ca. 3 mol %) under high-vacuum conditions. 2-Vinylpyridine (2VP) was stirred over KOH overnight. After filtration of KOH, 2VP was distilled over CaH2 under reduced C


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Macromolecules

patterns were recorded with a 2D image detector (Pilatus 2M) with the sample-to-detector distances of 3022 mm.

star polymer, Star 1, was isolated by HPLC fractionation in 57% yield (0.08 g). Mn (SEC-RALLS) = 24.9 kg/mol and Đ = 1.02 (SEC). 1H NMR (CDCl3) (300 MHz): δ =0.41−0.78 (4CH3- (sec-Bu), m, 12H), 0.92 (−CH3, t), 1.03−2.46 (PS and P2VP main chains, and ThCH2− (CH2)4−, m), 2.49 (tolyl, s, 3H), 2.82 (ThCH2−, t), 6.13−7.22 (Ph and pyridine (3H) groups, m), 6.98 (Th (1H), s, overlapped with Ph and pyridine groups), 8.08−8.46 (pyridine ring (1H), m). The other star polymers with different compositions were synthesized in similar manners. Star 2: Yield: 0.06 g (17%). Mn (SEC-RALLS) = 30.9 kg/mol and Đ = 1.04 (SEC). 1H NMR (CDCl3) (300 MHz): δ =0.41−0.78 (4CH3− (sec-Bu), m, 12H), 0.92 (−CH3, t), 1.06−2.58 (PS and P2VP main chains, and ThCH2−(CH2)4−, m), 2.49 (tolyl, s, 3H), 2.82 (ThCH2−, t), 6.09−7.21 (Ph and pyridine (3H) groups, m), 6.98 (Th (1H), s, overlapped with Ph and pyridine groups), 8.06−8.46 (pyridine ring (1H), m). Star 3: Yield: 0.08 g (30%). Mn (SEC-RALLS) = 38.4 kg/mol and Đ = 1.04 (SEC). 1H NMR (CDCl3) (300 MHz): δ =0.39−0.79 (4CH3− (sec-Bu), m, 12H), 0.92 (−CH3, t), 1.15−2.47 (PS and P2VP main chains, and ThCH2−(CH2)4−, m), 2.49 (tolyl, s, 3H), 2.81 (ThCH2−, t), 6.15−7.24 (Ph and pyridine (3H) groups, m), 6.98 (Th (1H), s, overlapped with Ph and pyridine groups), 8.05−8.47 (pyridine ring (1H), m). Instruments. Molecular weights and dispersities (Đ) were measured on an Asahi Techneion AT-2002 equipped with a Viscotek TDA model 302 triple detector array using THF as a carrier solvent at a flow rate of 1.0 mL/min at 40 °C. Three PS gel columns (pore size (bead size)) were used: 650 (9 μm), 200 (5 μm), and 75 Å (5 μm). The relative molecular weights were determined by SEC with RI detection, using a standard PS calibration curve. The combination of viscometer, right angle laser light scattering detection (SEC-RALLS), and RI detection was applied for the online SEC system in order to determine the absolute molecular weights of polymers. Polymer mixtures were separated by using a preparative SEC (KNAUER, smartline) equipped with RI detector using THF as a carrier solvent at a flow rate of 5.0 mL/min at room temperature. Four columns with 30 cm in length and 21.5 mm in diameter were used. The polymer mixture of ca. 0.5 g can be separated. 1H NMR spectra were measured on a Bruker DPX300 in CDCl3. Chemical shifts were recorded in ppm downfield relative to CHCl3 (δ = 7.26) as standard. Thermal analysis was performed on a Seiko EXSTAR 6000 TG/DTA 6300 thermal analyzer at a heating rate of 10 °C/min for thermogravimetry analysis (TGA) and a TA Instruments Q-100 connected to a cooling system at a heating rate of 20 °C min−1 for differential scanning calorimetry (DSC). UV−vis absorption spectra were recorded using a Hitachi U4100 spectrophotometer. For the thin film spectra, polymers were first dissolved in toluene, followed with filtering through a 0.45 μm pore size PTFE membrane syringe filter, and then drop-casted onto quartz substrate. Tapping mode AFM observation was performed with an Agilent AFM 5500, using microfabricated cantilevers with a force constant of 34 N/m. The polymer samples were drop-cast onto Si wafers from toluene solutions. GIXS Experiment. The polymer samples for grazing incidence Xray scattering (GIXS) experiments were prepared by drop-casting onto Si wafers from toluene solutions followed by annealing at 150 °C for 1 h. GIWAXS measurements were conducted at the beamline BL46XU of SPring-8, Japan. The sample was irradiated at a fixed incident angle (αI) on the order of 0.12° through a Huber diffractometer with an Xray energy of 12.398 keV (X-ray wavelength λ = 0.10002 nm), and the GIWAXS patterns were recorded with a 2D image detector (Pilatus 300 K) with the sample-to-detector distances of 173.8 mm. The scattering vectors qy and qz for GIWAXS are defined in eq 1.19 (qy , qz) = (2π(sin ψ cos αf )/λ , 2π(sin αf sin αi)/λ)

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RESULTS AND DISCUSSION Synthesis of TBDMS-in-Chain-Functionalized P3HT-bPS. The TBDMS moiety has been used as a typical protecting group of the OH-functionality during anionic polymerizations.20 Hirao and co-workers further succeeded in the quantitative transformation reaction from the TBDMS moiety into the benzyl bromide moiety introduced at the chain-end or within-chain by treating with trimetylsilyl chloride/lithium bromide or trimetylsilyl bromide.21 On the basis of the iterative approach involving such a transformation reaction, a variety of regular and asymmetric star polymers could be synthesized, indicating the outstanding utility of the TBDMS moiety.22 According to Scheme 1a, sec-BuLi-initiated polystyryllithium was end-capped with 1 in THF at −78 °C for 20 min and reacted with P3HT-C4-Br, which was successfully synthesized as reported elsewhere,17 at −78 °C for 36 h to afford TBDMSin-chain-functionalized P3HT-b-PS. Parts a−c of Figure 1 show

Figure 1. SEC curves of (a) P3HT-C4-Br, (b) crude P3HT-b-PS, (c) isolated P3HT-b-PS, (d) crude Star 1, and (e) isolated Star 1.

the SEC RI traces of P3HT-C4-Br, the crude product after the linking reaction, and the isolated P3HT-b-PS by HPLC fractionation, respectively. Although a large amount of the deactivated PS used in excess remains in the crude product, it could be completely discarded by HPLC fractionation to obtain the desired P3HT-b-PS. The characteristic results of the number-average molecular weights (Mns), Đ values, and composition are summarized in Table 1. The Mn value of P3HT-b-PS determined by SEC-RALLS is in good agreement with that calculated based on the sum of the Mn values of the precursory polymers. The use of the narrowly distributed P3HT-C4-Br (Đ = 1.08) and living anionic PS leads the low Đ (1.04) value of P3HT-b-PS. The 1H NMR spectrum of P3HTb-PS is shown in Figure 2a. The methylene protons next to the thiophene ring and the thiophene protons at the 4-position in the P3HT segments appeared at 2.82 and 6.98 ppm, respectively. The broad signals corresponding to the PS segments are also found in the ranges of 1.02−2.33 ppm and 6.28−7.21 ppm for the methylene protons and aromatic protons, respectively. By comparing the intensity of these

(1)

where ψ is the out-of-plane angle and αf is the exit angle. GISAXS patterns were obtained at beamline BL46XU of SPring-8, Japan. The monochromated energy of the X-ray source was 10.314 keV (λ = 0.12022 nm) and the incidence angle αi was 0.15°. GISAXS D


Article

Macromolecules Table 1. Synthetic Results for P3HT-C4-Br, P3HT-b-PS, and ABC-type Asymmetric Star Polymers Mn (kDa) polymer d

P3HT-C4-Br PS P2VP P3HT-b-PS Star 1 PS P2VP P3HT-b-PS Star 2 PS P2VP P3HT-b-PS Star 3

calcd

SEC-RALLS

− 11.5 5.36 18.9 24.9 12.4 10.5 19.8 31.8 11.9 17.0 19.3 36.9

7.44 10.6 5.12 19.5 24.9 13.5 10.0 21.3 30.9 12.9 18.8 19.9 38.4

composition (wt %) (P3HT/PS/P2VP) 1

H NMRa

Đ SECb

calcd

SEC-RALLSc

− 12.1 6.16 19.1 25.7 15.7 14.1 27.6 37.2 11.7 20.1 24.0 39.1

1.08 1.04 1.13 1.04 1.02 1.03 1.06 1.04 1.04 1.03 1.05 1.04 1.04

100/0/0 0/100/0 0/0/100 39/61/0 30/48/22 0/100/0 0/0/100 39/61/0 23/43/34 0/100/0 0/0/100 38/62/0 19/31/50

100/0/0 0/100/0 0/0/100 38/62/0 30/48/22 0/100/0 0/0/100 35/65/0 24/45/31 0/100/0 0/0/100 37/63/0 20/32/48

1

H NMR

100/0/0 0/100/0 0/0/100 39/61/0 29/47/24 0/100/0 0/0/100 27/73/0 20/42/38 0/100/0 0/0/100 31/69/0 19/30/51

a

Calculated from the absolute number-average molecular weight of P3HT-C4-Br (Mn = 7.44 kDa) and the composition determined by 1H NMR. Determined by SEC using a PS calibration curve. cCalculated from the absolute number-average molecular weight of each arm segment determined by SEC-RALLS. dThe exactly same P3HT-C4-Br was used for the synthesis of all star polymers. b

deprotection and the two methylene protons (−CH2− OTBDMS) at 4.61 ppm were shifted to 4.50 ppm (−CH2− OH). After the esterification reaction, the new signals for the vinyl protons (5.84 ppm) appeared and the methylene protons (−CH2−OH) further shifted to 5.12 ppm (−CH2−O(CO) (Ph)CCH2). All the results confirmed the quantitative transformation reactions of the functional groups introduced at the junction of the P3HT and PS segments. Synthesis of ABC-Type Asymmetric Star Polymers. In order to synthesize ABC-type asymmetric star polymers, the combination of P3HT, PS, and P2VP segments has been selected, because it is well-known that the block copolymers containing P3HT and PS segments24 as well as ones containing P3HT and P2VP segments25 tend to well phase separate in the film states, showing periodical and clear morphology on a nano scale. In addition, each segmented component among P3HT, PS, and P2VP could be probes in a 1H NMR spectrum of the ABC type star polymer, making it possible to confirm the composition of the star polymer. Since the PA group could be readily and quantitatively reacted with low nucleophilic living anionic polymers as mentioned in the previous section, the linking reaction between PA-in-chain-functionalized P3HT-bPS and living anionic P2VP should be a promising strategy. Indeed, the linking reaction of PA-in-chain-functionalized P3HT-b-PS with a large excess of living anionic P2VP was performed in THF at −78 °C for 18 h (Scheme 1c). Figure 1d shows the SEC curve of the crude linked product, possessing the two main peaks corresponding to the expected ABC star polymer and P2VP arm used in excess. After the HPLC fractionation, the isolated ABC star polymer (Star 1) showed the sharp unimodal peak in the SEC curve (Figure 1e). The isolation yield of Star 1 is 57%. The characteristic results of the Mns, Đ values, and composition are summarized in Table 1. The Mn values of Star 1 determined by SEC-RALLS and 1H NMR (calculated based on the segment composition) agree well with that calculated one. Star 1 possesses an extremely low Đ value of 1.02. In the 1H NMR spectrum of Star 1, there are identical peaks related to three different segments of P3HT, PS, and P2VP (Figure 3). The compositions determined by SECRALLS and 1H NMR agree well with the calculated one. All the results indicate that the final polymer linking reaction between PA-in-chain-functionalized P3HT-b-PS and living anionic P2VP

Figure 2. 1H NMR spectra of (a) TBDMS-in-chain-functionalized P3HT-b-PS, (b) the deprotected product of TBDMS-in-chainfunctionalized P3HT-b-PS, and (c) PA-in-chain-functionalized P3HT-b-PS.

peaks, the composition of P3HT and PS was determined to be 39:61 by wt. which is almost consistent with both the calculated value and the value determined by SEC-RALLS. Transformation Reaction of Functional Groups. The introduction of PA moieties into the polymers has become an established methodology for the synthesis of block and star polymers by linking approaches based on the living anionic polymerization, especially when living anionic polymers with a relatively lower nucleophilicity, such as poly(methacrylate)s and poly(2-vinylpyridine), are used.23 According to Scheme 1b, the TBDMS groups of P3HT-b-PS were completely deprotected with tetrabutylammonium fluoride (Bu4NF) in THF at 0 °C for 18 h, as monitored by 1H NMR, to afford OH-in-chain-functionalized P3HT-b-PS. The Mitsunobu esterification reaction of OH-in-chain-functionalized P3HT-b-PS with α-phenylacrylic acid in THF at 25 °C for 16 h provided the target PA-in-chain-functionalized P3HTb-PS without any problems. Figure 2 shows the 1H NMR spectra of the in-chainfunctionalized P3HT-b-PS series. The characteristic signal for the TBDMS groups at 0.03 ppm completely disappeared after E


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Macromolecules

calorimetry (DSC) thermogram of Star 1. In the first cooling scan, the characteristic exothermic peak corresponds to the Tc of the P3HT segment. The fused Tg = 94.4 °C can be seen corresponding to the PS and P2VP segments in the second heating scan. There is also a distinct endothermic peak of Tm = 237 °C for the P3HT segment. The DSC curve indicates the phase separation between the P3HT crystalline and PS+P2VP amorphous domains, although the phase separation between the PS and P2VP domains cannot be seen by DSC due to their similar Tg values. Star 2 and Star 3 show similar DSC curves (Figure S2 in Supporting Information) except for the somewhat reduced heat flow values corresponding to Tm,P3HT and Tc,P3HT, compared to Star 1, probably due to the interruption of crystalline growth in the star polymer having higher molecular weights for P2VP segments, as discussed in detail in the section for the GIXS Experiments. The thermal properties of all the samples are summarized in Table 2. Optical Property. The optical property of the ABC star polymers was studied by ultraviolet−visible (UV−vis) spectroscopy. The solution-state UV−vis spectrum of Star 1 in chloroform shows the maximum absorption (λmax) at 447 nm (Figure 5a). In the film state, the λmax of Star 1 is

Figure 3. 1H NMR spectrum of Star 1.

proceeded to successfully synthesize the ABC-type star polymer, Star 1, comprised of P3HT = 7.44 kDa, PS = 12.1 kDa, and P2VP = 6.16 kDa (P3HT/PS/P2VP = 29/47/24 by weight (1H NMR)). In addition, the characteristic results of two other ABC-type star polymers with different molecular weights of the P2VP segment (Star 2 (P3HT = 7.44 kDa, PS = 15.7 kDa, and P2VP = 14.1 kDa, P3HT/PS/P2VP = 20/42/38 by weight (1H NMR)) and Star 3 (P3HT = 7.44 kDa, PS = 11.7 kDa, and P2VP = 20.0 kDa, P3HT/PS/P2VP = 19/30/51 by weight (1H NMR)) are also listed in Table 1. Thermal Property. The thermal stability of the ABC star polymers was investigated by a thermal gravimetric analysis (TGA), as shown in Figure S1 (Supporting Information). The onset decomposition temperatures of Star 1, Star 2, and Star 3 for a 5% weight loss are 335, 368, and 354 °C, respectively, indicating that the thermal stability of all the ABC star polymers is sufficient for application in future optoelectronic devices. Figure 4 shows the representative differential scanning

Figure 5. UV−vis spectra of Star 1 (a) in conform solution and (b) in the film state (as-cast).

bathochromically shifted to 557 nm compared with that in the solution state (Figure 5b). Figure 5b also shows a shoulder at 603 nm related to a vibronic absorption, indicating a high degree of ordering of the P3HT crystalline structures in the phase-separated thin film even when the PS and P2VP segments are covalently linked to the P3HT segment at the core. The other star polymers, Star 2 and Star 3, show a bathochromic shift similar to Star 1 (Figure S3 in Supporting Information). The similar results for block copolymers containing P3HT segments have been previously reported.26 Morphological Study. The surface morphology of the star polymer thin films was observed by atomic force microscopy (AFM) in the tapping mode. The as-cast film of Star 1 with the shortest P2VP arm segment shows clear nanofibril structures

Figure 4. DSC thermograms of Star 1 at a heating/cooling rate of 20 °C/min.

Table 2. Thermal Properties of Star 1, Star 2, and Star 3 Measured by TGA and DSC

a

polymera

Td,5% (°C)

Tg,PS+P2VP (°C)b

Tm,P3HT (°C)

Tc,P3HT (°C)

ΔH(Tm,P3HT) (J/g)

ΔH(Tc,P3HT) (J/g)

Star 1 Star 2 Star 3

335 368 354

94.4 94.9 97.6

237 236 234

134 133 131

1.81 0.840 0.810

−1.18 −0.500 −0.510

Tg of P3HT was not prominent enough to be observed. bCalculated from the midpoints. F


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Figure 6. Tapping mode AFM images of (a) Star 1 as-cast film (topography image), (b) Star 1 as-cast film (phase image), (c) Star 1 film annealed at 150 °C for 1 h (topography image), (d) Star 1 film annealed at 150 °C for 1 h (phase image), (e) Star 2 as-cast film (topography image), (f) Star 2 as-cast film (phase image), (g) Star 2 film annealed at 150 °C for 1 h (topography image), and (h) Star 2 film annealed at 150 °C for 1 h (phase image).

(Figure 6, parts a and b); however, this texture disappeared after annealing at 150 °C for 1 h (Figure 6, parts c and d). Taking into consideration the high surface roughness (∼22.5 nm) of the Star 1 film even after annealing, the aggregation of the P3HT domains would be induced by the high crystallinity. The relatively higher content of the P3HT segments in Star 1 prevents the microphase separation due to the strong interaction between the P3HT domains. In contrast, the Star 2 film shows a smooth surface roughness (∼5.5 nm, see Figure 6g) after annealing at 150 °C for 1 h and continuous nanofibril structures were observed in the phase image of the thin film (Figure 6h), in which the bright and dark areas represent the P3HT stiff-rod and PS+P2VP amorphous domains, respectively. The as-cast Star 2 film also shows nanofibril structures, although their resolution is not very high due to the high

surface roughness without annealing. The formation of the nanofibril structures should be dictated by the immiscibility of P3HT and other segments as reported in the literature describing the nanofibril structures of the P3HT-b-PS diblock copolymer.27 The Star 3 film with the longest P2VP arm segment also shows a smooth surface (Figure S4(c)) after annealing at 150 °C for 1 h; however, the texture of the fibril structures in the phase image (Figure S4(d)) becomes less clear than that of the Star 2 film (Figure 6h). It should be mentioned that the AFM images only reflect the surface morphology of the films and such an unclear morphology might be caused by surface enrichment with P3HT domains having a relatively low surface energy,28 induced by the long flexible P2VP segments. In order to investigate the inner morphology of the ABCtype asymmetric star polymer films in detail, their thin films G


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Figure 7. 2D GISAXS patterns of (a) as-cast film of Star 1, (b) annealed film of Star 1 at 150 °C for 1 h, (c) as-cast film of Star 2, (d) annealed film of Star 2 at 150 °C for 1 h, (e) in-plane/out-of-plane GISAXS profile extracted along qy/qz direction at qz/qy = qzo/qyo from the 2D image measured for the Star 1 film, respectively, and (f) in-plane/out-of-plane GISAXS profiles extracted along qy/qz direction at qz/qy = qzo/qyo nm−1 from the 2D image measured for the Star 2 film, respectively. Here, qyo and qzo are defined as 0.086 and 0.238 nm−1, respectively.

of the fibril structure. Indeed, there are scattering peaks at qy = 0.229 nm−1 in the in-plane GISAXS profile as well as at qz = 0.444 nm−1 in the out-of-plane GISAXS profile. The relatively long PS+P2VP arm segments covalently connected to the P3HT segments may disrupt the specific orientation of the nanofibril structures at higher temperature. The Star 3 film shows a trend similar to that of Star 2 (see Figure S5(a−c)). The mean fibril period distances for all samples are summarized in Table 3. It was found that the mean fibril period distance

were further investigated by GIXS using synchrotron radiation sources. Parts a−d of Figure 7 show the 2D grazing incidence small-angle X-ray scattering (GISAXS) patterns, which were measured using the Star 1 (as-cast), Star 1 (annealed at 150 °C for 1 h), Star 2 (as-cast), and Star 2 (annealed at 150 °C for 1 h) films coated on silicon substrates from toluene solutions. On the basis of these patterns, the out-of-plane and in-plane scattering profiles have been extracted and the resulting scattering profiles for Star 1 and Star 2 are shown in Figure 7, parts e and f, respectively. As can be seen in parts a, b, and e of Figure 7, there are only weak diffraction peaks in the as-cast and annealed Star 1 films which indicates the less ordered structure of the microphase separation independent of the annealing treatment. In contrast, the in-plane GISAXS profile of the as-cast Star 2 film (Figure 7f) clearly shows a scattering spot at qy = 0.258 nm−1. The scattering peak was not observed in the out-of-plane GISAXS profile for the as-cast Star 2 film. These scattering results indicate that in the Star 2 film, the ordered microphase separation across the film plane occurs. The distinct scattering spot in the in-plane GISAXS profile corresponds to the mean fibril period distance of 23.5 nm. This ordering length scale is almost coincident with the interspacing of the morphological textures observed in the AFM image as already described. Thus, the scattering spot is assigned as the first order reflection of the fibril structure formed in the film, although the second or higher order reflection could not be observed. Interestingly, the annealing at 150 °C for 1 h provided the isotropic orientation

Table 3. Mean Fibril Period Distances of Star 1, Star 2, and Star 3 in the In-plane Direction Measured by GISAXS mean fibril period distance (in-plane) (nm) polymer

as-cast film

annealed film at 150 °C for 1 h

Star 1 Star 2 Star 3

16.4 23.5 29.0

15.7 26.6 30.0

increased from 16.4/15.7 nm (for the as-cast/annealed Star 1 film) to 29.0/30.0 nm (for the as-cast/annealed Star 3 film), with the increasing length of the P2VP arm segment. On the other hand, the measured 2D GIWAXS patterns of all the star polymers after annealing at 150 °C for 1 h revealed three periodic scattering arcs along the qz direction (Figure 8a). These scattering features are indicative of a well-ordered multilayer structure of P3HT whose layers stack along a direction perpendicular to the film plane. Obviously, the Star 1 H


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Macromolecules

Figure 8. 2D GIWAXS patterns of (a) as-cast film of Star 1, annealed film of Star 1 at 150 °C for 1 h, as-cast film of Star 2, annealed film of Star 2 at 150 °C for 1 h, as-cast film of Star 3, annealed film of Star 3 at 150 °C for 1 h and (b) in-plane/out-of-plane GIWAXS profiles extracted along qy/qz direction from the 2D image measured for the Star 2 film, respectively.

film with the highest content of P3HT segments shows the sharpest periodic scattering arcs with the high intensity among all the samples, indicating the high crystallinity of the P3HT domains, which is consistent with the results of the DSC measurements. For the typical sample of Star 2 (annealed at 150 °C for 1 h), the first order peak appears at qz = 3.77 nm−1, giving a dspacing of 1.67 nm (Figure 8b). The d-spacing value is equivalent to the layer thickness of the multilayer structure formed in the P3HT films.29 The scattering peak qy = 16.3 nm−1, corresponding to 0.385 nm, can be assigned as a reflection from the π−π stacked thiophene backbones in the phase separated P3HT domains.29 The π−π stacking length became slightly longer for the Star 3 film (0.393 nm), probably because of the interruption of the π−π stacking due to the longer P2VP arm segments (see Figure S6); however, the “edge-on” orientation of the P3HT crystalline is still dominant, independent of the length of the P2VP segments, taking into consideration the fact that the reflection from the π−π stacked thiophene backbones appears only in the in-plane direction for all samples. Thus, it was concluded that there is a hierarchical nanostructure in the ABC-type asymmetric star polymers in which the multilayer crystalline structures were formed with an “edge-on” rich orientation in the nanofibril structures of the phase-separated P3HT domains (Figure 9). Unfortunately, the microphase separation between the PS and P2VP domains has not been clarified by GIXS due to the absence of the related patterns. Such an unclear phase separation may come from the strong aggregation of the P3HT domains to reduce the opportunity of phase separation between the PS and P2VP domains in the limited exclusive space from the P3HT aggregated domains in the films. Further detailed morphological studies are currently under investigation.

Figure 9. Illustration of a hierarchical nanostructure in the ABC-type asymmetric star polymers.

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CONCLUSIONS The synthesis of well-defined ABC asymmetric star polymers comprised of P3HT, PS, and P2VP segments based on the KCTP and living anionic polymerization was reported for the first time. The ABC star polymers possessed predictable Mns, compositions, and extremely low Đ values (Đ < 1.05). The synthetic methodology involves the following three stage reactions: (a) the anionic linking reaction between P3HT-C4Br and living anionic PS end-capped with 1, (b) the transformation reaction of the 3-TBDMS moiety at the junction between the P3HT and PS segments into the PA moiety via the hydroxymethylphenyl moiety, and (c) the anionic linking reaction between PA-in-chain-functionalized P3HT-b-PS and living anionic P2VP. The representative DSC thermogram of Star 1 showed two distinct transition temperaturea at 94.4 °C for the fused Tg value of PS+P2VP I


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Macromolecules and at 237 °C for the Tm value of P3HT, indicating the existence of a phase separation between the (PS+P2VP) and P3HT domains. The UV−vis spectroscopy supported the vibronic absorption of the ABC star polymer films, indicating a high degree of ordering of the P3HT crystalline structures in the isolated P3HT domains. From a tapping mode AFM observation, continuous nanofibril phase-separated structures were clearly seen in the phase images of the star polymer thin films. The GISAXS and GIWAXS results confirmed the hierarchical nanofibril structures, in which the P3HT crystalline domains align with an “edge-on” orientation in the exclusive area of the microphase-separated domains from the PS+P2VP ones in the thin films. It was also found that the fibril period distance as well as π−π stacking distance of P3HT crystallines depended on the length of the P2VP segments.

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ASSOCIATED CONTENT

S Supporting Information *

TGA themograms of ABC star polymer, DSC thermograms, UV−vis spectra, tapping-mode AFM images of ABC star polymer thin films annealed at 150 and 200 °C, TEM images of ABC star polymers, and GISAXS profiles. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(T.H.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Japan Science and Technology Agency (JST), PRESTO program (JY 220176). GIWAXS and GISAXS experiments were performed at the BL46XU of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2013A1196 and 2014B1590), respectively. We thank Prof. Itaru Osaka (RIKEN) for partly operating the GIWAXS experiments. S.F. thanks the Innovative Flex Course for Frontier Organic Material Systems and Graduate School of Science and Engineering, Yamagata University for his financial support. S.I. appreciates the support by Grant-in-Aid for JSPS fellows from Japan Society for the Promotion of Science (JSPS).

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Synthesis and Characterization of ABC-Type Asymmetric Star

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