Regioregularity-Driven Morphological Transition of Poly(3

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Regioregularity-Driven Morphological Transition of Poly(3hexylthiophene)-Based Block Copolymers Jin-Seong Kim,† Yongjoo Kim,‡ Hyun-Jeong Kim,† Hyeong Jun Kim,† Hyunseung Yang,† Yeon Sik Jung,§ Gila E. Stein,∥ and Bumjoon J. Kim*,† †

Department of Chemical and Biomolecular Engineering,‡KAIST Institute for the Nanocentury, and §Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Korea ∥ Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States S Supporting Information *

ABSTRACT: Conjugated polymer-based block copolymers (BCPs) offer great potential to provide beneficial nanostructures for efficient organic optoelectronics. However, their complicated self-assembly behavior, which is attributed to the strong crystallization of conjugated blocks, is still not well understood due to the lack of a model BCP system. Herein, we develop a series of novel conjugated polymer-based BCPs, poly(3-hexylthiophene)-block-poly(2-vinylpyridine) (P3HT-b-P2VP), in which the regioregularity (RR) of the P3HT block was varied from 95 to 73%. The tunable RR content allows for precise regulation of P3HT crystallization with minimal influence on the microphase-separation force between the P3HT and P2VP blocks. When RR is high (i.e., 95 or 85%), structure formation is controlled by crystallization of P3HT, and the ultimate structure is characterized by nanoscale P3HT fibrils in an amorphous matrix. In contrast, as RR decreases to 78 and 73%, P3HT crystallization is suppressed. The self-assembly is controlled by the enthalpic interaction between P3HT and P2VP blocks, much like typical BCPs having two flexible blocks, and thermal annealing drives the formation of well-ordered lamellar or cylindrical phases. This morphological behavior is consistent with a Monte Carlo simulation based on a newly developed coarse-grained model. Significantly, this novel class of RR-controlled P3HT-based BCPs provides a simple method to tune bulk and thin film morphology for a variety of applications in nanostructured organic electronics.

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INTRODUCTION Conjugated polymers (CPs) can be engineered to deliver excellent opto-electrical properties1−6 and unique biological functionalities,7−10 so these systems are widely investigated for applications in plastic electronics and sensors. One area that has attracted a great deal of attention is the use of conductive block copolymers (BCPs), which are based on semiflexible πconjugated polymers11,12 linked to flexible amorphous polymers, for nanostructured optoelectronics.13−15 The strong intermolecular interactions between conjugated polymer chains (i.e., π−π interaction or supramolecular interaction) produce the unique optoelectronic properties and can also drive a strong crystallization.16−19 Self-assembly in these conjugated polymerbased BCP (CP-BCP) systems is controlled by competition between the microphase-separation process driven by the Flory−Huggins enthalpic interaction and chain entropy (χN, © XXXX American Chemical Society

segregation strength between two different segments) and the strong crystallization driven by the intermolecular interactions of conjugated chains.20,21 A well-documented challenge with this class of CP-BCPs is that strong crystallization in the conjugated polymer domains often dominates over the microphase-separation process, which slows down the kinetics for assembly and produces kinetically trapped, disordered morphologies.13,22−24 For example, most poly(3-hexylthiophene) (P3HT)-based BCPs form P3HT fibrils in an amorphous matrix because of the strong tendency for crystallization and high melting temperature (∼220 °C).23,25−28 The inability to control nanoscale morphology Received: January 17, 2017 Revised: February 17, 2017

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DOI: 10.1021/acs.macromol.7b00128 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules and orientation of these semiconducting fibrils has led to limited utility. On the other hand, poly(alkoxyphenylene− vinylene) (PPV)-based BCPs often produced better-ordered nanostructures due to their weaker tendency to crystallize, but their charge mobility is much lower.29,30 To address this issue, many attempts have been dedicated to control the self-assembly behavior of the CP-BCPs. Modifying the conjugated polymer to include side chains with longer lengths and/or branched structures can significantly reduce the crystallization temperature of the semiflexible, conjugated blocks and enable selfassembly into lamellar and cylindrical phases.31−33 Different copolymer architectures, such as graft and star copolymers, can produce high interfacial curvature and suppress crystallization, offering greater control over morphological behavior.34−36 While these approaches provide an effective route to control the self-assembly, the introduction of different alkyl chains or new polymer architecture often degrades the optical and electrical properties of the conjugated copolymers. A model CP-BCP system with systematically tuned crystallization is urgent and crucial to expanding the current understanding of self-assembly in these complex systems.21,37 Regioregularity (RR), which is defined as the content of head-to-tail (H−T) bonds in conjugated polymers, can be an important factor that determines the degree of crystallization.38,39 Numerous studies have demonstrated that high RR values in the P3HT polymers are important in producing long, highly crystalline fibrils, leading to high charge carrier mobility.40−42 Importantly, the crystalline properties of the P3HTs can be controlled by modulating the RR values. For example, Frechet et al. showed that the P3HT polymers with controlled RR value can produce both high photovoltaic performance and much enhanced thermal stability due to modulated crystalline and aggregated behavior of the P3HTs.43 Recently, we successfully developed a series of monodispersed P3HTs that have a wide range of RR from 64 to 98% and investigated the RR effect on the polymer crystallinity and mechanical properties.44 It was seen that the excessive crystallization of high RR P3HTs can cause problems for organic electronic applications in terms of processability and mechanical resiliences. Therefore, the RR represents the most effective factor to control the crystallization behavior of the polymers while maintaining their chemical structure, which can serve as an excellent model system for CP-BCP studies. To our knowledge, the self-assembly of BCPs based on conjugated polymers with tunable RR values have been never studied. Herein, we designed a novel series of CP-BCPs based on poly(3-hexylthiophene)-block-poly(2-vinylpyridine) (P3HT-bP2VP), in which the RR value of the P3HT block was systematically varied in a wide range from 95 to 73%. The methodology generates different RR values without affecting the molecular weight and polydispersity of the P3HT blocks, which allows for regulation of P3HT crystallization behavior. We observed that reducing RR suppressed the crystallization of P3HT and induced a remarkable transition of the P3HT-bP2VP structure from fibril-like to well-ordered lamellar and cylindrical (hexagonally packed) nanostructures. This RRcontrolled morphological transition was confirmed with different P2VP volume fractions ( f P2VP) using transmission electron microscopy (TEM) and grazing-incidence small-angle X-ray scattering (GI-SAXS). Experimental observations were validated with Monte Carlo simulations based on a coarsegrained model that captures the effects of RR. Our simulation results confirm that crystallization behavior of P3HT block is

greatly suppressed as the RR value is decreased, enabling the self-assembly of P3HT-b-P2VP into highly ordered nanostructures that are typical of equilibrium BCP phase behavior.

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RESULTS AND DISCUSSION The RR-controlled P3HT-b-P2VP copolymers were synthesized by “click reaction” between alkyne-terminated P3HT (P3HT-alkyne) and azide-terminated P2VP (P2VP-N3), which are polymerized via two different controlled polymerization methods (Figure 1a). First, a series of the RR-controlled P3HT-

Figure 1. (a) Synthesis route for RR-controlled P3HT-b-P2VP using modified GRIM polymerization and “click” reaction. (b) Normalized 1 H NMR spectra for a series of RR-controlled P3HT-alkynes. (c) SEC traces of RR 73-alkyne and derived block copolymers.

alkynes were synthesized by the method based on modified Grignard metathesis (GRIM) polymerization, in which H−H coupled bithiophene and thiophene monomer are copolymerized with different feed ratios. We previously reported that this method yielded well-defined P3HT with tunable RRs and controlled molecular weights.44 To decouple the RR effect on phase behavior from other effects of the molecular weight and its polydispersity, we fixed the amount of Ni-catalyst but controlled the feed ratio of H−H coupled bithiophene and thiophene monomer from 0.00 to 0.18. As a result, four P3HTalkynes were synthesized with different RR values, determined from 1H NMR spectra by comparing the integrated area of the peaks at δ 2.78 (H−T bonds) and δ 2.54 (H−H bonds). These polymers are labeled as RR 95-, 85-, 78-, and 73-alkyne, respectively (Figure 1b). All of the P3HT polymers have similar number-average molecular weight (Mn = 11−12 kg mol−1) and narrow dispersity (Đ = 1.1−1.3). Then, a series of P2VP-N3 polymers with narrow Đ of ∼1.1 were polymerized through reversible addition−fragmentation transfer (RAFT) polymerization using azide-terminated chain transfer agent (CTAN3).45 These P2VPs were designed to have different chain lengths to investigate the phase behavior of the P3HT-b-P2VP copolymers as a function of the volume fractions of P2VP ( f P2VP). Subsequently, the RR-controlled P3HT-alkyne polymers and the P2VP-N3 polymers were coupled by a copper(I)catalyzed azide−alkyne cycloaddition (CuAAC) “click reaction” between alkyne and azido groups of each polymer, yielding P3HT-b-P2VP copolymers with different RR and f P2VP values. B

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Macromolecules Table 1. Characteristics of Block Copolymers Used in the Study polymer RR RR RR RR RR RR RR RR

95-b-P2VP 95-b-P2VP 85-b-P2VP 85-b-P2VP 78-b-P2VP 78-b-P2VP 73-b-P2VP 73-b-P2VP

(0.35) (0.45) (0.27) (0.46) (0.31) (0.44) (0.30) (0.45)

RRa [% H−T linkages]

P3HT Mnb [kg mol−1]

P3HT Đb

f P2VPc

Tm,P3HTd [°C]

structuree

Df [nm]

95

11.4

1.14

85

12.1

1.16

78

11.7

1.33

73

12.1

1.35

0.35 0.45 0.27 0.46 0.31 0.44 0.30 0.45

218.0 223.0 211.8 208.9 193.4 189.5 184.6 186.8

fibril-like fibril-like fibril-like fibril-like HPC LAM HPC LAM

22 28 27 30 26 29 24 34

a

Determined from 1H NMR data (Figure 1b). bObtained from SEC calibrated with polystyrene standard (Figure 1c and Figure S1). cThe f P2VP values were calculated based on the integration of 1H NMR (Figures S2 and S3). dDetermined by DSC second heating cycle with a heating rate of 10 °C min−1 (Figure S4). eDetermined from TEM and GI-SAXS measurements. fDetermined from GI-SAXS analysis.

Figure 2. Cross-sectional TEM images represent phase transitions of P3HT-b-P2VP copolymers depending on their RR values in the bulk state. The first and second rows of the figures show the morphologies of RR-controlled P3HT-b-P2VP copolymers at two different f P2VP values of ∼0.45 and ∼0.30, respectively. (a) RR 95-b-P2VP (0.45); (b) RR 85-b-P2VP (0.46); (c) RR 78-b-P2VP (0.44); (d) RR 73-b-P2VP (0.45); (e) RR 95-b-P2VP (0.35); (f) RR 85-b-P2VP (0.27); (g) RR 78-b-P2VP (0.31); and (h) RR 73-b-P2VP (0.30). The inset of each figure shows the corresponding FFT images. Exposure to iodine vapor selectively stained dark P2VP domains. The scale bars are 100 nm.

were strongly dependent on the RR values, the Tm values of the P3HT block of the BCPs were somewhat higher than that of the parent P3HT homopolymer. For example, RR 85-b-P2VP series had approximately 10 °C higher Tm than that of the RR 85-alkyne. This might be attributed to the increase of lamellar crystal thickness and a partial chain extension of P3HT near the block interface.33,46 Overall, the crystallization behavior of P3HT-b-P2VP was strongly influenced by the RR value. Therefore, RR-controlled P3HT-b-P2VP copolymers were prepared to generate a model CP-BCP system: the choice of monomers is identical in all materials, so the system has very similar χ values, but crystallization of the P3HT blocks is highly tunable through RR content. Furthermore, the f P2VP values can be varied to explore different self-assembled morphologies. Table 1 summarizes the characteristics of the BCPs used in this study. We first investigated the morphology of RR-controlled P3HT-b-P2VP BCPs employing cross-sectional TEM. The bulk polymer samples were prepared by drop-casting from chloroform solutions and thermally annealed at 240 °C for 3 days under a highly pure argon atmosphere. Figure 2 compares

After the reaction, the copolymers were purified with acetone and tetrahydrofuran/hexane cosolvent to remove unreacted P2VP and P3HT homopolymers, respectively. Successful synthesis and purification of the P3HT-b-P2VP copolymers were confirmed by the peak shift toward higher molecular weight and a monomodal distribution in size exclusion chromatography (SEC) (Figure 1c and Figure S1) and 1H NMR spectroscopy (Figures S2 and S3). For convenience, polymers are denoted by RR X-b-P2VP (Y), where X is the RR of the P3HT segment and Y corresponds to the f P2VP value. To investigate the crystallization behavior of the RRcontrolled P3HT-b-P2VP copolymers, the thermal properties were examined by differential scanning calorimetry (DSC) (Figure S4 and Table S1). The melting temperature (Tm) and crystallization temperature (Tc) of the BCPs, and the degree of crystallinity of P3HT block (Dc,P3HT), indicative of crystallization of P3HT segment, were reduced as the RR decreased, which agreed with our previous report on RR-controlled P3HT homopolymers.44 For example, the Tm of BCPs with similar f P2VP of 0.45 was reduced from 223.0 to 186.8 °C as RR decreased from 95 to 73%. Interestingly, although the Tm values C

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Figure 3. Monte Carlo simulation snapshots of self-assembled P3HT-b-P2VP copolymers depending on their RR values. Pink line represents P3HT while blue line represents P2VP. The first and second rows of figures show the morphologies of RR controlled P3HT-b-P2VP copolymers at two different f P2VP values of 0.50, and 0.25, respectively. (a) RR 94-b-P2VP (0.50); (b) RR 83-b-P2VP (0.50); (c) RR 78-b-P2VP (0.50); (d) RR 72-bP2VP (0.50); (e) RR 94-b-P2VP (0.25); (f) RR 83-b-P2VP (0.25); (g) RR 78-b-P2VP (0.25); and (h) RR 72-b-P2VP (0.25).

morphologies of RR 95-b-P2VP (0.35) (Figure 2e) and the RR 85-b-P2VP (0.27) (Figure 2f). Note that all of the polymers have similar f P2VP value of ∼0.30. The D values were determined to be 26 and 24 nm for RR 78-b-P2VP (0.31) and RR 73-b-P2VP (0.30), respectively. The FFT images confirms this transition as a function of the RR values. The FFT images of RR 95-b-P2VP (0.35) and RR 85-b-P2VP (0.27) had broad ring patterns characteristic of disordered structures. In contrast, as the RR decreased to 78 and 73%, 6-fold peaks were clearly apparent, indicative of well-ordered HPC phases. This HPC morphology, which contains a higher fraction of the electroactive P3HT phase, is of crucial importance due to its potential in controlling conducting pathways and enhancing light absorption in organic electronics.33,47 Consequently, the P3HT-b-P2VP with low RR values resembles the equilibrium phase behavior of classical coil−coil BCPs, which also show a transition from LAM to HPC with increasing volume asymmetry. To clarify the effects of RR and f P2VP on morphology, we performed Monte Carlo simulations of coarse-grained P3HT-bP2VP with controlled RR values in the P3HT blocks. To simulate P3HT with different RR values, we modeled P3HT composed of two different types of beads. One bead contains a regioregular 3-hexylthiophene trimer with three head-to-tail bonds (HT−HT−HT) that increases crystallinity, while the other bead contains head-to-tail, head-to-head, and tail-to-tail bonds in series (HT−HH−TT) that suppresses crystallinity of P3HT. To realize this, we put a strong potential between two HT−HT−HT beads to promote crystallization between two beads based on previous coarse-grained simulation studies.48,49 On the other hand, the potential between two beads containing at least one HT−HH−TT bead is designed to be relatively weak to suppress the crystallinity between the beads. This potential is based on the glass transition temperature (−10 °C) of regiorandom P3HT (see Supporting Information for simulation details). This model was developed to investigate the competition between local crystallization and microphase separation, when quenched from homogeneous melt. We modeled P3HT with 12 beads (NP3HT = NHT−HT−HT + NHT−HH−TT = 12) to fix an Mn value of 12.0 kg mol−1 for P3HT block, but with different P2VP lengths to match the volume fractions f P2VP (0.50 with NP2VP = 12 and 0.25 with NP2VP = 4) in experiment. The RR of P3HT is defined as the

the bulk morphologies of the P3HT-b-P2VP BCPs with different RR values of 95, 85, 78, and 73%. The BCP samples with high RR P3HT content (95%) exhibited fibril-like phases of P3HT irrespective of the f P2VP values (Figures 2a,e). However, the domain spacing (D) increased from 22 (RR 95-bP2VP (0.35)) to 28 nm (RR 95-b-P2VP (0.45)) due to the extended length of the P2VP chain. As previously discussed in the literatures, these are commonly observed morphologies in P3HT-based BCPs driven by strong crystallization of P3HT.23,25−28 When the RR was reduced to 85%, the BCPs showed similar but more disordered structures. The RR 85-bP2VP (0.46) (Figure 2b) and RR 85-b-P2VP (0.27) (Figure 2f) exhibited disordered fibril-like and micelle phases with D values of 27 and 31 nm, respectively. Despite the reduction in RR value, the morphology of the BCPs was still dominated by the crystallization of P3HT, which inhibited the self-assembly into ordered nanostructures. A stark contrast in the self-assembly behavior was observed when RR was further decreased to values of 78 and 73%. The RR 78-b-P2VP (0.44) (Figure 2c) and RR 73-b-P2VP (0.45) (Figure 2d) copolymers, which had nearly identical f P2VP, were self-assembled into a well-ordered lamellar phase (LAM) with D values of 29 and 34 nm, respectively. The corresponding fast Fourier transform (FFT) images confirm this morphological transition, exhibiting sharp peaks normal to LAM structures of RR 78-b-P2VP (0.44) and RR 73-b-P2VP (0.45), compared to the broad halo of RR 95-b-P2VP (0.45) and RR 85-b-P2VP (0.46). Overall, there was a remarkable transition from disordered to ordered morphologies when the RR value was decreased from 85 to 78%. This phenomenon was attributed to the suppressed crystallization of P3HT with decreasing RR values. Therefore, the Flory−Huggins enthalpic interaction begins to dominate phase behavior at some point between RR values of 85 and 78%, allowing assembly of P3HT-b-P2VP BCPs into well-ordered nanostructures. To the best of our knowledge, this is the first demonstration where RR is used to control morphology in a CP-BCP. The dramatic morphological transition induced by changes in RRs (suppressed crystallinity) was also observed for different f P2VP values. For example, the RR 78-b-P2VP (0.31) (Figure 2g) and RR 73-b-P2VP (0.30) (Figure 2h) exhibited hexagonally packed cylinders (HPCs) of P2VP embedded in a P3HT matrix, in sharp contrast to the disordered D

DOI: 10.1021/acs.macromol.7b00128 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules fraction of (H−T) bonds along the polymer chain (RR = 100NHT/(NHT + NHH + NTT)), and it is expressed as 100(NP3HT + 2NHT−HT−HT)/(3NP3HT) in our model. In this way, we model P3HT with four different NHT−HT−HT (7, 8, 9, and 11) to match four RR values (72, 78, 83, and 94%). Positions of (HT−HT−HT) beads are randomly distributed along the chain to mimic the random polymerization process in experiment. To match the annealing condition for the samples in TEM and X-ray analysis, we ran 107 Monte Carlo movement steps (each step corresponds to accepted movements of entire molecules simultaneously) at an effective temperature of 240 °C. Figure 3 shows snapshots of simulated self-assembled morphologies of the P3HT-b-P2VP model polymers with four different RR (94, 83, 78, and 72%) and two f P2VP (0.50 and 0.25) values. Consistent with experimental results, the simulated systems having high RR (94 and 83%) show a poorly ordered phase-separated morphology, which resembles the disordered morphology observed in TEM. The polymer systems with higher RR have a higher probability of intermolecular stacking, which can increase the number of domains of crystallized P3HTs. In our simulated condition (P3HT-b-P2VP with a Mn value of 12.0 kg mol−1 for P3HT block, annealed at 240 °C), crystallized domains of P3HTs induced by the intermolecular interaction become unstable when the RR value is lower than 83%. For example, at the RR values of 78 and 72%, polymers are self-assembled into LAM phase for f P2VP = 0.50 and HPC phase for f P2VP = 0.25, which are well-known morphologies for coil−coil BCPs with minority block volume fraction of 0.50 and 0.25 (Figures 3c,d,g,h). The transition point in terms of the RR values is independent of the f P2VP values because the transition of fibril-to-ordered morphology is mainly affected by the local crystallization of P3HT block. It is worthwhile to note that the RR value (between 83 and 78%) for the phase transition are in excellent agreement with the RR value from our experimental observations. Thus, the phase behavior of BCPs with RR lower than this transition point depends mainly on the enthalpic interaction of BCPs. Thin films of conjugated polymers are used for polymerbased optoelectronic devices. It is well-known that confinement of the BCPs in a thin film geometry can drive a preferred domain orientation, such as layering of cylindrical or lamellar domains parallel to the interfaces, and also impact the phase behavior.50,51 We used GI-SAXS to evaluate the morphology of the P3HT-b-P2VP polymers in 100 nm films subjected to the same annealing protocol as bulk samples. Figure 4 presents the GI-SAXS patterns from RR 95-b-P2VP (0.35), RR 85-b-P2VP (0.27), RR 78-b-P2VP (0.31), and RR 73-b-P2VP (0.30) with f P2VP value of ∼0.3. Following the methods detailed elsewhere,52 the experimental patterns are compared with predicted peak positions for different packing symmetries, such as the parallel orientation of the HPC phase. Two peaks are observed for scattering from each plane, and these are designated by “+” and “−” symbols in Figure 4. The GI-SAXS data for samples with f P2VP ≈ 0.3 show a dramatic morphology transition as a function of RR value. Samples with 95% RR exhibit both in-plane and out-of-plane order with a domain periodicity of D = 22 nm but are poorly ordered, showing broad and extremely weak peaks (Figure 4a). These scattering patterns are consistent with mixed-oriented fibril-like domain structure. The Supporting Information includes additional details that pertain to this conclusion (Figure S5). Samples with 85% RR are highly disordered along

Figure 4. 2D GI-SAXS patterns, superimposed with predicted positions of Bragg peaks based on D in Table 1 and the HPC symmetry. (a) RR 95-b-P2VP (0.35), (b) RR 85-b-P2VP (0.27), (c) RR 78-b-P2VP (0.31), and (d) RR 73-b-P2VP (0.30). Angle of incidence ≈0.13°.

both in-plane and out-of-plane directions (Figure 4b), and the scattering cannot be assigned to a typical BCP lattice symmetry. Instead, we propose the domains are micelle-like with a correlation length near D ≈ 27 nm. In contrast, the GI-SAXS patterns for samples with low RR (78 and 73%) content are consistent with highly ordered HPCs, confirmed by comparing the predicted and observed Bragg peak positions for D ≈ 25 nm (Figures 4c,d). The cylindrical domains are oriented with the long axis in the plane of the film and stacked with hexagonal symmetry through the film thickness. Interestingly, from inspection of the in-plane line shapes, as well as the number of out-of-plane Bragg peaks, we conclude that ordering is much higher in polymers with 73% RR compared with 78% RR. For example, the RR 73-b-P2VP (0.30) film showed highly ordered HPC phases with very sharp and distinct peaks, which is typically observed in well-ordered coil−coil BCP structure (Figure 4d and Figure S6). This enhanced ordering of RR 73% BCP is due to a weaker tendency to crystallize. Therefore, the GI-SAXS results confirm a dramatic morphological transition from disordered fibril-like to well-ordered HPC phases with decreasing RR values, which are consistent with bulk TEM and simulation results. When f P2VP ≈ 0.45, all films exhibit a lamellar structure with layering parallel to the interfaces (Figure S7). Compared to bulk TEM images, the improvement in lamellar ordering of 95 and 85% RR BCPs is attributed to geometric confinement in thin film assembly.50,51 The beam stop blocks most of the signal from lamellar out-of-plane Bragg peaks, so it is difficult to comment on the extent of ordering. However, the intensity profiles near the beam stop are much stronger for samples with low RR (78 and 73%), which suggests that ordering is significantly improved in these samples. Figure S8 includes outE

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of-plane line cuts that offer additional evidence of the structural assignments proposed here. In summary, the GI-SAXS results provide clear evidence that RR can be manipulated to suppress crystallization and allow for BCP assembly into highly ordered nanostructures.

CONCLUSIONS We successfully demonstrated that the morphology of P3HT-bP2VP block copolymers can be tuned through control of P3HT regioregularity. A series of model P3HT-b-P2VPs were synthesized with polymerization techniques that afford independent control of Mn, composition f P2VP, and regioregularity RR with low dispersity. Bulk samples of P3HT-bP2VP with low RR (78 and 73%) assembled into well-ordered LAM and HPC structures, depending on f P2VP values, whereas those of high RR (95 and 85%) P3HT-b-P2VP only showed disordered fibril-like structures. These morphologies are explained in the context of crystallization versus equilibrium BCP self-assembly, using a simple coarse-grained model as a guide: When RR is high, crystallization dominates and controls the ultimate structure. However, when RR is low, the BCP can self-assemble into the highly ordered nanostructures that are typical of equilibrium BCP phase behavior. In addition, we investigated the thin film structures of P3HT-b-P2VP using GISAXS and confirmed that highly ordered thin film structures are achieved at low RR for all polymer compositions. The RR controlled P3HT-b-P2VP polymers provide a novel model system for fundamental studies on the self-assembly of conjugated polymers and opportunities to explore the functionality in a broader class of nanomorphologies for optoelectronic applications. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00128.

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Materials and methods, detailed experimental procedures, and additional characterization data (PDF)

AUTHOR INFORMATION

Corresponding Author

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

Yeon Sik Jung: 0000-0002-7709-8347 Bumjoon J. Kim: 0000-0001-7783-9689 Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Thompson, B. C.; Frechet, J. M. J. Organic Photovoltaics Polymer-Fullerene Composite Solar Cells. Angew. Chem., Int. Ed. 2008, 47, 58−77. (2) Sirringhaus, H. 25th Anniversary Article: Organic Field-Effect Transistors: The Path Beyond Amorphous Silicon. Adv. Mater. 2014, 26, 1319−1335. (3) Ku, S. Y.; Brady, M. A.; Treat, N. D.; Cochran, J. E.; Robb, M. J.; Kramer, E. J.; Chabinyc, M. L.; Hawker, C. J. A Modular Strategy for Fully Conjugated Donor-Acceptor Block Copolymers. J. Am. Chem. Soc. 2012, 134, 16040−16046. (4) Torsi, L.; Magliulo, M.; Manoli, K.; Palazzo, G. Organic FieldEffect Transistor Sensors: A Tutorial Review. Chem. Soc. Rev. 2013, 42, 8612−8628. (5) Kang, H.; Lee, W.; Oh, J.; Kim, T.; Lee, C.; Kim, B. J. From Fullerene−Polymer to All-Polymer Solar Cells: The Importance of Molecular Packing, Orientation, and Morphology Control. Acc. Chem. Res. 2016, 49, 2424−2434. (6) Kang, T. E.; Kim, K. H.; Kim, B. J. Design of Terpolymers as Electron Donors for Highly Efficient Polymer Solar Cells. J. Mater. Chem. A 2014, 2, 15252−15267. (7) Lee, M.; Cho, B. K.; Zin, W. C. Supramolecular Structures from Rod-Coil Block Copolymers. Chem. Rev. 2001, 101, 3869−3892. (8) Long, Y. Z.; Li, M. M.; Gu, C. Z.; Wan, M. X.; Duvail, J. L.; Liu, Z. W.; Fan, Z. Y. Recent Advances in Synthesis, Physical Properties and Applications of Conducting Polymer Nanotubes and Nanofibers. Prog. Polym. Sci. 2011, 36, 1415−1442. (9) Xie, H.; Luo, S. C.; Yu, H. H. Electric-Field-Assisted Growth of Functionalized Poly(3,4-ethylenedioxythiophene) Nanowires for Label-Free Protein Detection. Small 2009, 5, 2611−2617. (10) Wu, C. F.; Chiu, D. T. Highly Fluorescent Semiconducting Polymer Dots for Biology and Medicine. Angew. Chem., Int. Ed. 2013, 52, 3086−3109. (11) Zhang, W. L.; Gomez, E. D.; Milner, S. T. Predicting Chain Dimensions of Semiflexible Polymers from Dihedral Potentials. Macromolecules 2014, 47, 6453−6461. (12) McCulloch, B.; Ho, V.; Hoarfrost, M.; Stanley, C.; Do, C.; Heller, W. T.; Segalman, R. A. Polymer Chain Shape of Poly(3alkylthiophenes) in Solution Using Small-Angle Neutron Scattering. Macromolecules 2013, 46, 1899−1907. (13) Sary, N.; Richard, F.; Brochon, C.; Leclerc, N.; Leveque, P.; Audinot, J. N.; Berson, S.; Heiser, T.; Hadziioannou, G.; Mezzenga, R. A New Supramolecular Route for Using Rod-Coil Block Copolymers in Photovoltaic Applications. Adv. Mater. 2010, 22, 763−768. (14) Lee, E.; Hammer, B.; Kim, J. K.; Page, Z.; Emrick, T.; Hayward, R. C. Hierarchical Helical Assembly of Conjugated Poly(3hexylthiophene)-block-poly(3-triethylene glycol thiophene) Diblock Copolymers. J. Am. Chem. Soc. 2011, 133, 10390−10393. (15) Dai, C. A.; Yen, W. C.; Lee, Y. H.; Ho, C. C.; Su, W. F. Facile Synthesis of Well-Defined Block Copolymers Containing Regioregular Ppoly(3-Hexyl Thiophene) via Anionic Macroinitiation Method and Their Self-Assembly Behavior. J. Am. Chem. Soc. 2007, 129, 11036− 11038. (16) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Electroluminescent Conjugated Polymers - Seeing Polymers in a New Light. Angew. Chem., Int. Ed. 1998, 37, 402−428. (17) Salleo, A. Charge Transport in Polymeric Transistors. Mater. Today 2007, 10, 38−45. (18) Noriega, R.; Rivnay, J.; Vandewal, K.; Koch, F. P. V.; Stingelin, N.; Smith, P.; Toney, M. F.; Salleo, A. A General Relationship Between Disorder, Aggregation and Charge Transport in Conjugated Polymers. Nat. Mater. 2013, 12, 1038−1044. (19) Darling, S. B. Block Copolymers for Photovoltaics. Energy Environ. Sci. 2009, 2, 1266−1273. (20) Olsen, B. D.; Segalman, R. A. Self-Assembly of Rod-Coil Block Copolymers. Mater. Sci. Eng., R 2008, 62, 37−66. (21) Pryamitsyn, V.; Ganesan, V. Self-Assembly of Rod-Coil Block Copolymers. J. Chem. Phys. 2004, 120, 5824−5838.

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ACKNOWLEDGMENTS

This research was supported by the National Research Foundation Grant (2012M3A6A7055540, 2015M1A2A2057509), funded by the Korean Government. This research was also supported by the Research Projects of the KAIST-KUSTAR and the CRH (Climate Change Research Hub) of KAIST. G.E.S. acknowledges the support from NSFDMR-1151468. F

DOI: 10.1021/acs.macromol.7b00128 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules (22) Kong, X. X.; Jenekhe, S. A. Block Copolymers Containing Conjugated Polymer and Polypeptide Sequences: Synthesis and SelfAssembly of Electroactive and Photoactive Nanostructures. Macromolecules 2004, 37, 8180−8183. (23) Lohwasser, R. H.; Thelakkat, M. Synthesis of Amphiphilic RodCoil P3HT-b-P4VP Carrying a Long Conjugated Block Using NMRP and Click Chemstry. Macromolecules 2012, 45, 3070−3077. (24) Yassar, A.; Miozzo, L.; Gironda, R.; Horowitz, G. Rod-Coil and All-Conjugated Block Copolymers for Photovoltaic Applications. Prog. Polym. Sci. 2013, 38, 791−844. (25) Kaul, E.; Senkovskyy, V.; Tkachov, R.; Bocharova, V.; Komber, H.; Stamm, M.; Kiriy, A. Synthesis of a Bifunctional Initiator for Controlled Kumada Catalyst-Transfer Polycondensation/NitroxideMediated Polymerization and Preparation of Poly(3-hexylthiophene)Polystyrene Block Copolymer Therefrom. Macromolecules 2010, 43, 77−81. (26) Sauve, G.; McCullough, R. D. High Field-Effect Mobilities for Diblock Copolymers of Poly(3-Hexylthiophene) and Poly(Methyl Acrylate). Adv. Mater. 2007, 19, 1822−1825. (27) Boudouris, B. W.; Frisbie, C. D.; Hillmyer, M. A. Nanoporous Poly(3-Alkylthiophene) Thin Films Generated from Block Copolymer Templates. Macromolecules 2008, 41, 67−75. (28) Moon, H. C.; Anthonysamy, A.; Lee, Y.; Kim, J. K. Facile Synthesis of Well-Defined Coil-Rod-Coil Block Copolymer Composed of Regioregular Poly(3-hexylthiophene) via Anionic Coupling Reaction. Macromolecules 2010, 43, 1747−1752. (29) Tao, Y. F.; Zohar, H.; Olsen, B. D.; Segalman, R. A. Hierarchical Nanostructure Control in Rod-Coil Block Copolymers with Magnetic Fields. Nano Lett. 2007, 7, 2742−2746. (30) Sary, N.; Rubatat, L.; Brochon, C.; Hadziioannou, G.; Ruokolainen, J.; Mezzenga, R. Self-Assembly of Poly(Diethylhexyloxy-p-Phenylenevinylene)-b-Poly(4-Vinylpyridine) RodCoil Block Copolymer systems. Macromolecules 2007, 40, 6990−6997. (31) Lin, S. H.; Ho, C. C.; Su, W. F. Cylinder-to-Gyroid Phase Transition in a Rod-Coil Diblock Copolymer. Soft Matter 2012, 8, 4890−4893. (32) Moon, H. C.; Bae, D.; Kim, J. K. Self-Assembly of Poly(3dodecylthiophene)-block-poly(methyl methacrylate) Copolymers Driven by Competition between Microphase Separation and Crystallization. Macromolecules 2012, 45, 5201−5207. (33) Ho, V.; Boudouris, B. W.; McCulloch, B. L.; Shuttle, C. G.; Burkhardt, M.; Chabinyc, M. L.; Segalman, R. A. Poly(3alkylthiophene) Diblock Copolymers with Ordered Microstructures and Continuous Semiconducting Pathways. J. Am. Chem. Soc. 2011, 133, 9270−9273. (34) Kim, H. J.; Paek, K.; Yang, H.; Cho, C. H.; Kim, J. S.; Lee, W.; Kim, B. J. Molecular Design of “Graft” Assembly for Ordered Microphase Separation of P3HT-Based Rod-Coil Copolymers. Macromolecules 2013, 46, 8472−8478. (35) Park, J.; Choi, C.; Hyun, S.; Moon, H. C.; Joseph, K. L. V.; Kim, J. K. Microphase Separation of P3HT-Containing Miktoarm Star Copolymers. Macromolecules 2016, 49, 616−623. (36) Lee, W.; Kim, J. S.; Kim, H. J.; Shin, J. M.; Ku, K. H.; Yang, H.; Lee, J.; Bae, J. G.; Lee, W. B.; Kim, B. J. Graft Architectured Rod-Coil Copolymers Based on Alternating Conjugated Backbone: Morphological and Optical Properties. Macromolecules 2015, 48, 5563−5569. (37) Wang, Q. A. Theory and Simulation of the Self-Assembly of Rod-Coil Block Copolymer Melts: Recent Progress. Soft Matter 2011, 7, 3711−3716. (38) Snyder, C. R.; Henry, J. S.; DeLongchamp, D. M. Effect of Regioregularity on the Semicrystalline Structure of Poly(3-hexylthiophene). Macromolecules 2011, 44, 7088−7091. (39) Kohn, P.; Huettner, S.; Komber, H.; Senkovskyy, V.; Tkachov, R.; Kiriy, A.; Friend, R. H.; Steiner, U.; Huck, W. T. S.; Sommer, J. U.; Sommer, M. On the Role of Single Regiodefects and Polydispersity in Regioregular Poly(3-hexylthiophene): Defect Distribution, Synthesis of Defect-Free Chains, and a Simple Model for the Determination of Crystallinity. J. Am. Chem. Soc. 2012, 134, 4790−4805.

(40) Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Giles, M.; Mcculloch, I.; Ha, C. S.; Ree, M. A Strong Regioregularity Effect in Self-Organizing Conjugated Polymer Films and High-Efficiency Polythiophene: Fullerene Solar Cells. Nat. Mater. 2006, 5, 197−203. (41) Mauer, R.; Kastler, M.; Laquai, F. The Impact of Polymer Regioregularity on Charge Transport and Efficiency of P3HT:PCBM Photovoltaic Devices. Adv. Funct. Mater. 2010, 20, 2085−2092. (42) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Two-Dimensional Charge Transport in Self-Organized, High-Mobility Conjugated Polymers. Nature 1999, 401, 685−688. (43) Woo, C. H.; Thompson, B. C.; Kim, B. J.; Toney, M. F.; Frechet, J. M. J. The Influence of Poly(3-hexylthiophene) Regioregularity on Fullerene-Composite Solar Cell Performance. J. Am. Chem. Soc. 2008, 130, 16324−16329. (44) Kim, J. S.; Kim, J. H.; Lee, W.; Yu, H.; Kim, H. J.; Song, I.; Shin, M.; Oh, J. H.; Jeong, U.; Kim, T. S.; Kim, B. J. Tuning Mechanical and Optoelectrical Properties of Poly(3-hexylthiophene) through Systematic Regioregularity Control. Macromolecules 2015, 48, 4339−4346. (45) Erothu, H.; Kolomanska, J.; Johnston, P.; Schumann, S.; Deribew, D.; Toolan, D. T. W.; Gregori, A.; Dagron-Lartigau, C.; Portale, G.; Bras, W.; Arnold, T.; Distler, A.; Hiorns, R. C.; MokarianTabari, P.; Collins, T. W.; Howse, J. R.; Topham, P. D. Synthesis, Thermal Processing, and Thin Film Morphology of Poly(3hexylthiophene)-Poly(styrenesulfonate) Block Copolymers. Macromolecules 2015, 48, 2107−2117. (46) Muller, C.; Zhigadlo, N. D.; Kumar, A.; Baklar, M. A.; Karpinski, J.; Smith, P.; Kreouzis, T.; Stingelin, N. Enhanced Charge-Carrier Mobility in High-Pressure-Crystallized Poly(3-hexylthiophene). Macromolecules 2011, 44, 1221−1225. (47) Park, J.; Moon, H. C.; Choi, C.; Kim, J. K. Synthesis and Characterization of [Poly(3-dodecylthiophene)](2)Poly(methyl methacrylate) Miktoarm Star Copolymer. Macromolecules 2015, 48, 3523− 3530. (48) Schwarz, K. N.; Kee, T. W.; Huang, D. M. Coarse-Grained Simulations of the Solution-Phase Self-Assembly of Poly(3-Hexylthiophene) Nanostructures. Nanoscale 2013, 5, 2017−2027. (49) Jankowski, E.; Marsh, H. S.; Jayaraman, A. Computationally Linking Molecular Features of Conjugated Polymers and Fullerene Derivatives to Bulk Heterojunction Morphology. Macromolecules 2013, 46, 5775−5785. (50) Albert, J. N. L.; Epps, T. H. Self-assembly of block copolymer thin films. Mater. Today 2010, 13, 24−33. (51) Xu, T.; Hawker, C. J.; Russell, T. P. Interfacial Interaction Dependence of Microdomain Orientation in Diblock Copolymer Thin Films. Macromolecules 2005, 38, 2802−2805. (52) Vogt, B. D.; Chavez, V. L.; Dai, M. Z.; Arreola, M. R. C.; Song, L. Y.; Feng, D.; Zhao, D. Y.; Perera, G. M.; Stein, G. E. Impact of Film Thickness on the Morphology of Mesoporous Carbon Films Using Organic-Organic Self-Assembly. Langmuir 2011, 27, 5607−5615.

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DOI: 10.1021/acs.macromol.7b00128 Macromolecules XXXX, XXX, XXX−XXX