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May 21, 2018 - ABSTRACT: We synthesized star-shaped 18-arm polystyrene-block-poly(3- dodecylthiophene) copolymers ((PS-b-P3DDT)18) having lamellar ...
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Vertically Oriented Nanostructures of Poly(3-dodecylthiophene)Containing Rod−Coil Block Copolymers Kyu Seong Lee,† Jaeyong Lee,† Chungryong Choi,† Yeseong Seo,† Hong Chul Moon,‡ and Jin Kon Kim*,† †

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National Creative Research Initiative Center for Smart Block Copolymers, Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, Kyungbuk 37673, Republic of Korea ‡ Department of Chemical Engineering, University of Seoul, Seoul 02504, Republic of Korea S Supporting Information *

ABSTRACT: We synthesized star-shaped 18-arm polystyrene-block-poly(3dodecylthiophene) copolymers ((PS-b-P3DDT)18) having lamellar microdomains, based on the cyclodextrin (CD) core. Without any surface modifications on a silicon substrate, thin films of (PS-b-P3DDT)18 showed vertically oriented lamellar microdomains by thermal annealing. This is attributed to the entropic penalty induced by star-shaped chain architecture. In contrast, a parallel orientation was observed in the film morphology of the corresponding linear PS-b-P3DDT. We also found that the P3DDT crystals were confined in the lamellar microdomains of (PS-b-P3DDT)18 as edge-on structure.

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By adjusting rod/rod interaction, a few research groups19−23 successfully prepared well-ordered nanostructures similar to conventional block copolymer self-assembled nanostructures.19−24 However, a critical issue still remains regarding the orientation of P3AT nanodomains in thin film. This is because regioregular P3ATs have relatively low surface tension (16.8−21.0 mN/m for P3HT25,26 and 19.8 mN/m for poly(3dodecylthiophene) (P3DDT)25,26) compared to commonly used coil polymers (for instance, 40.7 mN/m for polystyrene (PS)26,27 and 41.2 mN/m for poly(methyl methacrylate) (PMMA)27). Also, the Flory−Huggins interaction parameter (χ) between P3AT and conventionally used coil polymers is exceptionally high (e.g., 0.37 for χP3DDT/PMMA20 and 0.48 for χP3HT/PS28 at 25 °C), which is higher than that (0.1929) of a representative high χ block copolymer, PS-b-poly(dimethylsiloxane) (PDMS). A thin film of P3AT-coil block copolymer could show vertically oriented microdomains when it was prepared by drop casting using a very dilute concentration.30 However, the morphology corresponded to the kinetically driven morphology obtained during the deposition, not to the thermodynamic equilibrium. Thus, vertically oriented microdomains in thermal equilibrium are desirable for P3AT-coil block copolymer. Recently, we reported that 18-arm star-shaped PS-b-PMMA showed vertically oriented lamellar and cylindrical micro-

INTRODUCTION Regioregular poly(3-alkylthiophene) (P3AT) has been extensively investigated due to its good solubility in organic solvents, high charge carrier mobility, and relatively facile synthetic process via quasi-living Kumada catalyst transfer polymerization.1,2 To enhance the performance of P3AT-based electronic devices, the orientation of ordered nanostructures of P3AT should be carefully controlled.3−5 For instance, in bulk heterojunction organic photovoltaics (OPVs), a vertically ordered nanostructure is promising for efficient generation, diffusion, and dissociation of excitons.6,7 Several attempts have been made to fabricate a vertically oriented P3AT structure by top-down techniques. Kim et al.8 fabricated poly(3-hexylthiophene) (P3HT) nanopillar structure imprinted by nanoporous anodic aluminum oxide (AAO) templates and demonstrated improved device performance in comparison with flat bilayer structure. Russell and co-workers9 fabricated nanoimprinted P3HT structure by using AAO with grafted PDMS chains for easy removal of AAO. In addition, various vertically oriented P3HT structures were fabricated using various molds such as prepatterned silicon templates.10−13 These P3HT nanostructures showed improved device performance of OPVs.8−13 However, those imprinting methods should use sophisticated nanomolds. P3AT-containing rod−coil block copolymers have been considered as promising bottom-up protocols to obtain wellordered nanostructures.14−18 But, most P3HT-containing block copolymers showed a fibril structure, instead of ordered nanostructure due to the strong rod/rod interaction of P3HT. © XXXX American Chemical Society

Received: December 26, 2017 Revised: May 21, 2018

A

DOI: 10.1021/acs.macromol.7b02739 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthetic Routes for (a) (PS-N3)18 (1), (b) Ethynyl-Capped P3DDT (2), and (c) (PS-b-P3DDT)18 (3)

purified by silica column with n-hexane. Other chemicals including αcyclodextrin, α-bromoisobutyryl bromide, 4-(N,N-dimethylamino)pyridine, 2,2′-bipyridyl, and N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA) were used as received from Aldrich Chem. Co. Synthesis of Azido End-Functionalized Star-Shaped 18-Arm Polystyrene [(PS-N3)18, 1]. Star-shaped 18-arm polystyrene [(PSBr)18] was synthesized from bromine-functionalized cyclodextrin macroinitiator (Br-CD) according to the literature.34 Br-CD (64.2 mg, 0.3 mmol), copper bromide(I) (CuBr) (42 mg, 0.3 mmol), and 2,2′-bipyridyl (93 mg, 0.6 mmol) were placed in a round-bottom twoneck flask with a stopcock. Distilled styrene (25 mL) was added to the flask and degassed by three freeze−thaw cycles. The reaction was carried out at 100 °C for 18 h. After the reaction, the flask was dipped in liquid nitrogen to terminate the reaction. The reacted solution was diluted with tetrahydrofuran (THF) and then precipitated in methanol. The product was fully dried in a vacuum at 50 °C for 24 h. Azido end-functionalized star-shaped polystyrene [(PS-N3)18, 1] was synthesized from (PS-Br)18. (PS-Br)18 (1.0 g, 0.004 mmol) and sodium azide (NaN3, 0.25 g, 3.84 mmol) were placed in three-neck round-bottom flask and dissolved in 50 mL of dimethylformamide (DMF) with nitrogen purging. The flask was placed in an oil bath at 80 °C and stirred overnight. The reacted solution was passed through a neutral alumina column to remove remaining NaN3 and the other impurities and precipitated in methanol. Synthesis of Star-Shaped 18-Arm PS-b-P3DDT (STAR-SDDT) [(PS-b-P3DDT)18, 3]. Ethynyl-capped P3DDT (2) was synthesized according to the literature,22,23 and star-shaped 18-arm PS-b-P3DDT (STAR-SDDT) was synthesized via click reaction between 1 and 2, as shown in Scheme 1. 1 (0.13 g, 0.01 mmol; based on azide functional group, 0.055 mmol × 18-arm, 1 equiv), 2 (0.195 g, 0.015 mmol, 1.5 equiv), and copper(I) bromide (0.0204 g, 0.142 mmol) were introduced in a reactor under a nitrogen atmosphere. After anhydrous THF (40 mL) and PMDETA (46 μL, 0.22 mmol) were injected, the reactor was purged with nitrogen for 30 min. After the solution was stirred for 24 h at 50 °C, it passed through neutral alumina to remove copper(I) bromide. The solution was concentrated and dropped into methanol for precipitation. The remaining homopolymers in crude product were removed by column chromatography. Synthesis of Linear PS-b-P3DDT (LINEAR-SDDT) [4]. Linear PS-b-P3DDT (LINEAR-SDDT) was synthesized via click reaction

domains, irrespective of substrates without any surface modification.31 Ho and co-workers also demonstrated that three (or four)-arm star-shaped PS-b-PDMS exhibited a significantly enhanced vertical orientation of microdomains on silicon substrates.32,33 The vertical orientation of cylindrical (or lamellar) microdomains for star-shaped block copolymers on a substrate is attributed to a huge entropic penalty which overcomes the favorable interaction between each block and air (or substrate).31−33 In this study, we introduced a star-shaped molecular structure to P3AT-containing block copolymers (i.e., 18-arm PS-b-P3DDT copolymers (STAR-SDDT)) to overcome the large difference in the surface tension. The STAR-SDDT with 47% weight fraction of P3DDT block (wP3DDT = 0.47) was synthesized by a combination of atom-transfer radical polymerization (ATRP) and click reaction. Both transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) revealed lamellar morphology of STAR-SDDT in bulk. When the STAR-SDDT was spin-coated on a silicon substrate, vertically oriented lamellar microdomains were observed by tapping mode atomic force microscopy (AFM) and grazingincidence small-angle X-ray scattering (GISAXS). On the other hand, linear PS-b-P3DDT diblock copolymer (LINEARSDDT) exhibiting lamellar microdomains only showed parallel orientation of lamellar microdomains including P3DDT fibrils at the top surface of the film. Also, we confirmed that the P3DDT crystals within vertically oriented lamellar microdomains are packed with edge-on structure, verified by twodimensional (2D) grazing-incidence wide-angle X-ray scattering (GIWAXS) experiments.

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EXPERIMENTAL SECTION

Materials. Styrene (Aldrich, 99%) was distilled under reduced pressure before use. Copper(I) bromide (Aldrich, 98+%) was purified by sequential washing with acetic acid, ethanol, and diethyl ether, followed by fully drying in a vacuum. 2,5-Dibromo-3-dodecylthiophene was purchased from Tokyo Chemical Industry Co., Ltd., and B

DOI: 10.1021/acs.macromol.7b02739 Macromolecules XXXX, XXX, XXX−XXX

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Then, the samples were annealed at 100 °C for 24 h. Eventually, all samples were cooled to room temperature and ultrasectioned by using a Leica Ultracut Microtome (EM UC6 Leica Ltd.) with a thickness of ∼30 nm. Then, they were stained by exposure to RuO4 vapor to selectively stain P3DDT microdomains. The TEM images were obtained at room temperature by using bright-field TEM (S-7600 Hitachi Ltd.) at 80 kV. To prepare the sample for cross-sectional TEM of thin film, the film was embedded in an epoxy resin after carbon coating, and then the epoxy/carbon/film was removed from the silicon substrate by placing the specimen in liquid nitrogen. The film was microtomed at room temperature and stained with RuO4. Differential Scanning Calorimetry (DSC). The thermal properties of all polymers after thermal annealing were measured by using the PerkinElmer DSC 4000. Calibration was conducted with zinc and indium. The sample (∼5 mg) was first heated to 200 °C (first heating) and annealed at 200 °C for 10 min to remove thermal history. Then, it was cooled to 50 °C and crystallized at this temperature for 30 min. Finally, DSC thermograms were obtained by the second heating from 50 to 200 °C. All heating and cooling cycles were performed at a rate of 10 °C/min. Thin Film Sample Preparation. Thin films were prepared by spin-coating of 0.5−2.0 wt % toluene solution by changing rotating speed (rpm) to adjust film thickness on a pristine silicon wafer or indium tin oxide (ITO) wafer. ITO-coated glass substrate was used after ultrasonically cleaning for 10 min in deionized water, ethanol, and acetone sequentially. The thickness of all block copolymer thin films was measured by an ellipsometer (M-2000V, J.A. Woollam Co.). Films were dried in a vacuum at room temperature and then annealed at 200 °C for 1 h and followed by quenching using liquid nitrogen. The UV−vis absorption spectra of STAR-SDDT and LINEAR-SDDT thin films on ITO-coated glass substrate were obtained by a UV−vis spectrometer (Varian Cary-5000).

between azide end-functionalized PS (PS-N3, 1′) and 2, as shown in Scheme S1 of the Supporting Information. 1′ was synthesized according to the literature.35,36 In brief, we synthesized bromineterminated PS (PS-Br) by ATRP using 1-bromoethylbenzene as an initiator. Then, PS-Br (1.0 g, 0.077 mmol) and sodium azide (NaN3, 0.25 g, 3.84 mmol) were placed in three-neck round-bottom flask and dissolved in 50 mL of dimethylformamide (DMF) under nitrogen purging. The flask was placed in an oil bath and stirred at 80 °C overnight. The synthesis of LINEAR-SDDT proceeded in the same way as STAR-SDDT, except that excess PS was used relative to P3DDT. To synthesize LINEAR-SDDT, 1′ (0.26 g, 0.02 mmol), 2 (0.13 g, 0.01 mmol, 0.5 equiv), copper(I) bromide (0.0204 g, 0.142 mmol), and PMDETA (46 μL, 0.22 mmol) were reacted. After the click reaction, the remaining homopolymers in the crude product were removed by column chromatography using acetone as an eluent. Characterization. The number-average molecular weight (Mn) and polydispersity index (PDI) of all polymers 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) with THF as an eluent and a flow rate of 1 mL/min at 35 °C. End functionalization of all polymers was checked by 1H nuclear magnetic resonance spectra (1H NMR: Bruker digital AVANCE III 400 MHz), in which chloroform-d (CDCl3) was used as a solvent. The weight fraction of P3DDT (wP3DDT) was determined by 1 H NMR based on the characteristic peaks of PS and P3DDT (see Figure S1 in the Supporting Information). Then, P3DDT molecular weight was estimated from wP3DDT and molecular weight of PS predetermined by SEC. The molecular characteristics of all the samples are summarized in Table 1.

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Table 1. Molecular Characteristics of Polymers Employed in This Study sample (PS-N3)18 (1) Cut-(PSN3)18 Ethynylcapped P3DDT (2) STAR-SDDT (3) PS-N3 (1′) LINEARSDDT (4)

Mn,SECa (g/mol)

Mw/Mna

100000 13000

1.09 1.10

18000

1.09

12000

181000

1.10

434000

0.47

lamellae

13000 28000

1.09 1.10

25000

0.48

lamellae

Mn,NMRb

wP3DDT

b

RESULTS AND DISCUSSION Figure 1a shows SEC traces of (PS-N3)18 (1), ethynyl-capped P3DDT (2), crude STAR-SDDT, and purified STAR-SDDT (3). After the click reaction between 1 and 2, a peak corresponding to 3 appeared in addition to the unreacted homo P3DDT peak because we reacted an excess amount of 2 (1.5 equiv) to ensure full conversion of PS-N3 arms grafting from the α-CD core. When this remaining P3DDT homopolymer was completely removed by column chromatography, pure STAR-SDDT (3) was obtained. We investigated the molecular weight of block copolymers after cleaving ester linkage between α-cyclodextrin and PS chain. As a result, cut-SDDT (namely, PS-b-P3DDT arm of STAR-SDDT) does not contain any free PS arm without P3DDT block (Figure 1b). These results support complete reactions between 18 PS arms and 18 P3DDT chains. In addition, we examined whether P3DDT blocks were attached to all 18 PS chains using Fourier-transform infrared spectroscopy (FT-IR). From the variation in FT-IR spectra, we detected that the characteristic peak at 2200 cm−1, arising from the azide functional group of PS chain end, completely disappeared after the click reaction. Figure 2 displays SAXS profiles and TEM image of STARSDDT in bulk. The SAXS profile of STAR-SDDT at 200 °C shows scattering peaks of the lamellar structure at the position of 1:2 relatives to q* (0.213 nm−1) and corresponding domain size (L0) of 29.5 nm. The lamellar microdomains of STARSDDT were well maintained even after crystallization of P3DDT blocks at 100 °C (see characteristic scattering peaks of 1q* and 2q* in Figure 2), although the lamellar domain spacing was slightly increased (q* = 0.192 nm−1, L0 = 32.7 nm). Also, the TEM image (inset of Figure 2) supports the lamellar morphology for STAR-SDDT, in which the bright and

c

microdomains

a

Determined by SEC based on PS standards. bEstimated from 1H NMR. cDetermined by SAXS and TEM images. Small-Angle X-ray Scattering (SAXS). SAXS profiles (I(q) vs q (= (4π/λ) sin θ), where q and 2θ are the scattering vector and scattering angle, respectively) were obtained on beamline 4C at the Pohang Accelerator Laboratory (South Korea). The wavelength and beam size were 0.675 Å and 0.2 (H) × 0.6 (W) mm2, respectively. A two-dimensional charge-coupled detector (Mar USA, Inc.) was employed. The sample-to-detector distance was 3 m. Grazing-Incidence Small-Angle (and Wide-Angle) X-ray Scattering (GISAXS and GIWAXS). GISAXS experiments were performed at room temperature on beamline 3C at the PAL to investigate the morphology of thin film through the entire thickness. The operating wavelength was 0.15 nm, and a sample-to-detector distance was 3 m. The incident angle (αi) was set at 0.16°, which is above the critical angle (0.14°) of STAR-SDDT thin film. GIWAXS profiles were also obtained on the same line, but the sample-todetector distance was reduced to 0.2 m. Transmission Electron Microscopy (TEM). For TEM image of bulk samples, the samples were first annealed at 200 °C for 30 min followed by cooling to 100 °C within 10 min in a vacuum chamber. C

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Figure 1. (a) SEC traces before and after click reaction for STAR-SDDT. (b) SEC traces for each arm of star-shaped polymers after hydrolysis of ester linkage between cyclodextrin core and polymer chain. (c) FT-IR before and after click reaction for STAR-SDDT.

which is essentially the same as L0 of STAR-SDDT measured by SAXS and TEM. These results are consistent in the previous study of star-shaped block copolymers. When the film thickness of (PS-b-PMMA)18 thin film was above 2.0L0, vertical oriented lamellar was not observed.31 In addition, we checked how the casting condition (solvent) and postannealing (thermal annealing) conditions affected the microstructure of STAR-SDDT. We used three different solvents for spin coating: tetrahydrofuran (THF), chloroform (CF), and toluene (TL). The concentration of STAR-SDDT in each solvent was adjusted so that all samples had a thickness of 1.0L0. The AFM images of the spin-coated thin films from different solvents showed different morphologies (Figure 4a,c,e). Namely, when thin films were spin coated from both THF and CF solutions, line structures with dots were observed. But, those structures were not seen for another film spin-coated from TL. The morphology of as-cast STARSDDT thin film before thermal annealing corresponds to kinetical trapped one during spin-coating rather than thermal equilibrium. Therefore, the morphology depends on different swelling ratios between PS and P3DDT blocks to each solvent as well as different boiling points of a solvent.26,39,40 However, unlike the as-cast film, after thermal annealing, the same morphology was observed (Figure 4b,d,f). This is because thermal annealing was performed at 200 °C, which is higher than the glass transition temperature of PS (∼105 °C) and melting temperature (Tm) of P3DDT (∼140 °C) as well as the boiling point of each solvent. We also studied the morphology of STAR-SDDT thin film depending on the annealing temperature. Figure S3 shows the morphology as-cast STAR-SDDT from TL and after annealed at three difference temperatures (140, 170, and 240 °C). When the sample was annealed at 140 °C for 1 h (Figure S3b), STAR-SDDT exhibited similar morphology to the as-cast film, although some fibrils appeared to be formed. When we increased the annealing temperature to 170 °C (Figure S3c), film shows well-defined fingerprint patterns corresponding to vertically oriented morphology, which is similar morphology of the sample annealed at 200 °C. At 140 °C, close to the Tm of P3DDT (∼140 °C), the crystal structure in the P3DDT block remained and interfered the microphase separation of STARSDDT. However, at higher annealing temperatures (170−200 °C), the P3DDT crystals are fully molten and do not interfere

Figure 2. SAXS profiles for STAR-SDDT at temperatures higher (200 °C) and lower (100 °C) than melting temperature (140 °C) of P3DDT. Inset is TEM image for STAR-SDDT. The scale bar in the inset image is 200 nm.

dark regions correspond to PS and P3DDT microdomains, respectively, due to the selective staining of P3DDT by RuO4. The lamellar domain spacing (32 nm) measured from TEM image is well consistent with L0 estimated from the SAXS profile (2π/q*). We examined film morphology of this lamellar forming STAR-SDDT on a silicon substrate. Figure 3 shows AFM images of STAR-SDDT on a silicon wafer after annealing at 200 °C for 1 h followed by quenching using liquid nitrogen. Fingerprint patterns were observed at both thickness of 0.5L0 and 1.0L0, which implies vertically oriented lamellar microdomains (Figure 3a,b).37,38 However, as the film thickness increased, the morphology of thin film STAR-SDDT was changed. At 1.5L0 film thickness (Figure 3c), fingerprint patterns were still observed, although the long-range ordering of lamellae was poor. When the film thickness increased to 2.0L0 (Figure 3d), the fingerprint structure was no longer observed. The distance between two neighboring microdomains in all AFM images was 29.8 ± 2.3 nm (Figure S2), D

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Figure 3. Height (upper panels) and phase contrast (lower panels) AFM images of STAR-SDDT at different film thicknesses (a: 0.5L0 (15.2 ± 0.1 nm); b: 1.0L0 (29.4 ± 0.1 nm); c: 1.5L0 (46.1 ± 0.2 nm); d: 2.0L0 (61.1 ± 0.2 nm)) after the samples were annealed at 200 °C for 1 h and followed by quenching using liquid nitrogen. The scale bar in all images is 400 nm.

Figure 4. Before (a, c, e) and after (b, d, f) thermal annealing AFM images of STAR-SDDT spin-coated using chloroform (a, b), tetrahydrofuran (c, d), and toluene (e, f) [(a1−f1): height; (a2−f2): phase contrast]. Each sample has 1.0L0 film thickness (CF: 30.4 ± 0.1 nm; THF: 31.2 ± 0.2 nm; TL: 29.4 ± 0.1 nm) and annealed at 200 °C for 1 h. The scale bar in the images is 200 nm.

Since the AFM images give the surface morphology of thin film, we investigated the film morphology through the crosssectional TEM experiment to confirm the vertical orientation along the film thickness direction more briefly. Figure 5a gives cross-sectional TEM image of STAR-SDDT thin film with 1.0L0 thickness. The vertical orientations of PS (bright region) and P3DDT (dark region) lamellae are clearly seen throughout the entire film. The vertical orientation of the microdomains in thin films of the STAR-SDDT was further confirmed by GISAXS profile (Figure 5b). A sharp vertical streak (Bragg

with the formation of the microphase separation of block copolymers. Thus, the vertical orientation of lamellae seen in STAR-SDDT is attributed to the star architecture. However, when the annealing temperature becomes much higher (at 240 °C), the degree of the vertical orientation of lamellar microdomains becomes poor (Figure S3d). This is because the cyclodextrin core could be thermally unstable at high temperature;41 thus, the star-shaped chain architecture of STAR-SDDT would not be maintained. E

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Figure 5. (a) Cross-sectional TEM image (scale bar of 30 nm) for STAR-SDDT thin film with 1.0L0 thickness. (b) GISAXS pattern for STARSDDT with 1.0L0 thickness at an incident angle of 0.16° on a pristine silicon wafer. (c) The in-plane scan of GISAXS profile at qz = 0.285 nm−1. (d) GIWAXS pattern for STAR-SDDT at an incident angle of 0.16° on a pristine silicon wafer. (e) The in-plane scan of GIWAXS at qxy = 0 nm−1.

Figure 6. Height (upper panels) and phase contrast (lower panels) AFM images of LINEAR-SDDT at different film thicknesses (a: 0.5L0 (15.1 ± 0.1 nm); b: 1.0L0 (27.8 ± 0.1 nm); c: 1.5L0 (42.0 ± 0.2 nm); d: 2.0L0 (54.4 ± 0.2 nm)) after the samples were annealed at 200 °C for 1 h and followed by quenching using liquid nitrogen. The scale bar in the images is 400 nm.

rod) was observed at the first-order reflection (qxy = 0.207 nm−1), which implies lamellar microdomains oriented normal to the surface across the entire film thickness. The lamellar domain spacing of vertically oriented lamellae was 30.4 nm. Inplane scattering profile extracted at qz = 0.285 nm−1 shows a second peak (2q*), supporting lamellar phases (Figure 5c). We also conducted GIWAXS experiment to elucidate the orientation of P3DDT crystals within the vertically oriented lamellae (Figure 5d,e). From the GIWAXS pattern (Figure 5d), in addition to an isotropic ring at q ∼ 15 nm−1 originating

from the amorphous PS chains, three distinct spots were observed in the out-of-plane direction (qz). The domain spacing calculated from the first peak position (qz = 2.33 nm−1, as shown in Figure 5e) was 2.69 nm, which is consistent with previously reported domain spacing of the dodecyl side chain in P3DDT.42 Thus, three peaks in Figure 5e should be (100), (200), and (300) diffractions, representing the edge-on structure of P3DDT crystals. This result indicates that the P3DDT chains in the vertically oriented lamellar microF

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Figure 7. Chain arrangement of (a) STAR-SDDT and (b) LINEAR-SDDT on a substrate with thickness of 1.0L0. Inset of (a) shows the crystalline structure (edge-on) of P3DDT microdomain.

lamellae are alternately stacked having PS-P3DDT/P3DDTPS/PS-P3DDT/P3DDT-PS/PS-P3DDT sequence along the thickness direction from the substrate to air. Thus, three distinct PS (bright stripes) and P3DDT (dark stripes) lamellae are clearly seen in the cross-sectional TEM image. Also, PS and P3DDT are located at substrate/polymer interface and polymer/air interface, respectively. For LINEAR-SDDT, PS chains are more favorable to the silicon substrate than P3DDT chains. However, due to the existence of covalently linked junction point in STAR-SDDT, the chain arrangement on the substrate is different from that of LINEAR-SDDT. When the STAR-SDDT exhibits parallel oriented lamellae, PS chains should be stretched significantly at the polymer/substrate interface, resulting in a large entropic penalty. Thus, STAR-SDDT shows vertically oriented lamellae in thin film, as described in detail in a previous paper.31 On the other hand, the crystallization of P3DDT has little effect on the formation of the nanostructure because the sample was annealed at molten state (namely, above the Tm of P3DDT block). Furthermore, we found that when the sample was quenched from molten state to a temperature lower than Tm of P3DDT block, the lamellar morphology was well maintained, as shown in Figure 3. The chain arrangements of STAR-SDDT with vertical orientation and LINEAR-SDDT with hole−island structure are schematically depicted in Figure 7. To confirm whether this vertical orientation occurs on other substrates which are more suitable for photovoltaic devices, the experiment was carried out by changing the substrate to indium tin oxide (ITO)-coated glass substrate. Figure S8 shows AFM images of STAR-SDDT on ITO-coated glass substrate after thermally annealed at 200 °C for 1 h. Similar to the silicon substrate, the vertical orientation was well observed at 0.5L0 and 1.0L0, while the vertically oriented morphology was not seen as the thickness increased. This is because vertically orientation of STAR-SDDT is not greatly affected by the interaction (or enthalpic contribution) between the substrate and each block of STAR-SDDT. Rather, the vertical orientation was driven by entropic contribution of the starshaped molecule architecture. To investigate the effect of star-shaped chain architecture on the degree of crystallinity of P3DDT block, we performed DSC experiments for STAR-SDDT (3), LINEAR-SDDT (4), and homo P3DDT (2) (see Figure S9). Because the degree of crystallinity of P3AT is intimately related to the hole mobility, a higher degree of crystallinity is preferred to fabricate high

domains of STAR-SDDT form crystals with the edge-on structure. To investigate the effect of chain architecture on lamellar orientation, we synthesized linear PS-b-P3DDT (LINEARSDDT, 4) whose total molecular weight and weight fraction of P3DDT block were similar to those of each arm of STARSDDT (Figure S4). LINEAR-SDDT also showed lamellar microdomains, confirmed by SAXS and TEM (Figure S5). The lamellar domain spacing of LINEAR-SDDT obtained from SAXS profile (27.6 nm at 200 °C) was also similar to that of STAR-SDDT. Figure 6 gives AFM images for LINEAR-SDDT at four different film thicknesses. The LINEAR-SDDT showed distinctly different film morphology from STAR-SDDT, despite the same microdomains (lamellae) and similar lamellar domain spacing. When the film thickness was 0.5L0, the top surface of the film showed P3DDT fibrils the same as previously reported on P3AT-containing block copolymers (Figure 6a).43−47 However, LINEAR-SDDT thin film with 1.0L0 thickness showed hole−island structure, and a short P3DDT fibril was shown at the top surface (Figure 6b). The hole−island structure was also observed at a film thickness of 2.0L0, but another film with 1.5L0 thickness did not show this structure (Figures 6c,d). When lamellar microdomains of a block copolymer thin film are parallel oriented to the substrate, the hole−island structure is formed if the film thickness (t) is incommensurable with L0.48,49 In addition, since PS has much higher surface energy (40.7 mN/m)26,27 than P3DDT (19.8 mN/m),25,26 PS chains are preferentially placed at the substrate side because of higher surface energy (71.9 mN/ m) of a silicon substrate, while the P3DDT chains with lower surface energy are located at the air side (∼0 mN/m).48 Therefore, the formation of the hole−island structure depending on film thickness, as shown in Figure 6, can be explained by asymmetric wetting of PS and P3DDT lamellae. In this situation, the commensurability holds for t = (n + 1/2)L0, where n is the integer. Furthermore, the thickness difference between hole and island areas measured in AFM image well matched with L0 of LINEAR-SDDT determined by SAXS profile (Figures S6a,b). GISAXS of LINEAR-SDDT does not show any characteristic feature of vertically oriented lamellae (Figure S7). Figure S6c gives the cross-sectional TEM image for LINEAR-SDDT thin film with 2.5L0 thickness. Because of the commensurability of this film thickness, PS and P3DDT G

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performance devices.50,51 Although the chain architecture of crystalline polymer could affect the crystallinity,52 there is no significant difference in the crystallinity of P3DDT chains between STAR-SDDT and homo P3DDT. Because PS chains are located at the core region and the molecular weight of the PS is sufficiently large, the chain orientation of P3DDT chains located at the shell is less affected when P3DDT is crystallized. This means that with star-shaped P3AT containing block copolymer it is advantageous to obtain a vertical orientation without loss of degree of crystallinity. On the other hand, LINEAR-SDDT showed a slightly higher degree of crystallinity than that of homo P3DDT. The PS coil polymers that can move freely facilitate P3DDT chain mobility, giving increase of crystallinity of P3DDTs.50 Figure S10 gives UV−vis absorption spectra of STAR-SDDT and LINEAR-SDDT thin films on an ITO-coated glass substrate spin-coated from toluene solution followed by thermal annealing. Both thin films exhibited almost the same spectra: strong absorption peaks at ∼610 nm corresponding to π−π* transition and weak absorptions at ∼530 and ∼560 nm originating from transitions of the intrachain excitons.53 This indicates that the star-shaped architecture in STAR-SDDT does not much affect the electronic transition of P3DDT itself. We also fabricated hole-only space-charge limited current (SCLC) devices to compare hole mobility (μ) of STAR-SDDT with LINEAR-SDDT (Figure S11). μ was extracted by fitting with Mott−Gurney equation from current density−voltage (J− V) curves.54 μ of STAR-SDDT was 2.02 × 10−5 cm2 V−1 s−1, which is much higher than that (8.95 × 10−7 cm2 V−1 s−1) of LINEAR-SDDT, although UV−vis absorption spectra and crystallinity measured by DSC for STAR-SDDT and LINEARSDDT were very similar. STAR-SDDT thin film with vertical alignment of P3DDT microdomains provided an efficient pathway for the hole transport because both top and bottom electrodes are directly connected to P3DDT chains. On the other hand, LINEAR-SDDT thin film showed parallel oriented lamellae in which PS layer greatly blocked the hole transport. The thin insulating PS layer could allow the transport of holes generated in P3DDT layer through tunneling, but the tunneling across PS layer should be very low.55,56 Therefore, the hole mobility of LINEAR-SDDT measured along the vertical direction is much lower than that of STAR-SDDT.

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Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02739. Synthetic routs for LINEAR-SDDT, 1H NMR spectra of STAR-SDDT, height profiles of AFM images, AFM images of STAR-SDDT annealed at different temperature, SEC traces, SAXS profile, GISAXS profile, AFM image, cross-sectional TEM image for LINEAR-SDDT, AFM images of STAR-SDDT on ITO-coated glass substrate, DSC thermograms, UV−vis absorption spectra, and J−V curves of the SCLC device (PDF)

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AUTHOR INFORMATION

Corresponding Author

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

Hong Chul Moon: 0000-0003-2598-0925 Jin Kon Kim: 0000-0002-3872-2004 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 (2013R1A3A2042196). SAXS, GISAXS and GIWAXS experiments were done at 3C and 4C beamline of PAL (Korea).

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REFERENCES

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CONCLUSIONS

We synthesized 18-arm star-shaped PS-b-P3DDT (STARSDDT) by a combination of the ATRP and click reaction. Every PS arm was fully reacted with ethynyl-capped P3DDT. The synthesized STAR-SDDT showed well-ordered lamellar structure in bulk state, confirmed by TEM and SAXS. In the thin film, vertically oriented lamellar structure of the STARSDDT was observed, although there was a large difference of surface tension between PS and P3DDT blocks. Vertically oriented P3AT lamellae from P3AT-containing rod−coil block copolymers in thermal equilibrium were not reported in the literature. We also found that the vertically oriented lamellae obtained from STAR-SDDT thin film were not affected by deposition conditions and substrate. The crystallinity of the STAR-SDDT was also maintained even in film structure. Furthermore, the P3DDT crystalline also showed edge on morphology confirmed by GIWAXS. H

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Macromolecules

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