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Multiple Chain Packing and Phase Composition in Regioregular Poly(3-butylthiophene) Films Yuan Yuan,†,‡ Jie Shu,‡,§ Krzysztof Kolman,∥,⊥ Adam Kiersnowski,‡,∥ Christoph Bubeck,‡ Jianming Zhang,*,†,‡ and Michael Ryan Hansen*,‡,# †

Key Laboratory of Rubber-Plastics, Ministry of Education/Shandong Provincial Key Laboratory of Rubber-plastics, Qingdao University of Science & Technology, Qingdao 266042, China ‡ Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany § Analysis and Testing Center, Suzhou University, Renai Road 199, 215123 Suzhou, China ∥ Faculty of Chemistry, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland ⊥ Division of Applied Chemistry, Department of Chemistry and Chemical Engineering, Chalmers University of Technology, SE 412 96 Gothenburg, Sweden # Institute of Physical Chemistry, Westfälische Wilhelms-Universität Münster, Münster, Germany S Supporting Information *

ABSTRACT: On the basis of different film preparation conditions, we address the phase composition and thiophene main-chain and butyl side-chain organization of three regioregular poly(3-butylthiophene) (P3BT) samples prepared by drop-casting. These samples are readily identified by wideangle X-ray diffraction (WAXD) with the structural P3BT polymorphs commonly referred to as forms I, I′, and II. Here, we show by means of WAXD, solid-state nuclear magnetic resonance (NMR), and Fourier-transform infrared spectroscopy (FTIR) that the P3BT samples identified with these polymeric forms (I, I′, and II) do not contain a single, unique crystalline phase and an amorphous phase, but additionally consist of one (form II) or two (forms I and I′) distinct crystalline phases. Each of the crystalline phases is associated with specific thiophene main-chain packing arrangements and butyl side-chain organizations. We provide a spectroscopic identification for each of the P3BT crystalline phases, enabling a detailed and consistent picture of molecular order and polymer main-chain and side-chain packing. Thus, our results are expected to be valuable in future studies of poly(3-alkylthiophene) (P3AT) thin films, where in particular FTIR studies will permit quick and easy access to both phase composition and identification of multiple thiophene main-chain packing structures. crystallization, showing a layer distance of 1.26 nm.9 Compared to form I, form I′ is thought to be more ordered on the basis of thermal analysis and structural modeling.9,10 Films with form II are obtained by solvent annealing method in carbon disulfide (CS2).8,11 However, the structural parameters of form II are quite different from those of forms I and I′, since it exhibits a shorter layer distance of 0.96 nm and a longer lateral stacking distance of 0.42 nm. This indicates that form II represents a P3BT structure with a looser main-chain stacking and tighter side-chain packing. The characteristic unit cell dimensions of P3BT polymorphs are summarized in Table S1 of the Supporting Information. Although the structural characteristics of the different P3BT polymorphs have been revealed using Xray diffraction techniques, a complete and detailed picture of their phase composition as well as thiophene main-chain and

1. INTRODUCTION Regioregular poly(3-alkylthiophenes) (P3ATs) belong to the most studied p-type semiconducting polymers due to their widespread applications in field-effect transistors,1,2 polymer solar cells,3 chemical sensors,4 etc. The crystalline structure of P3ATs receives continuous attention because of its key role in the performance of such devices. P3ATs can be described as comb-like polymers with rigid thiophene main chains and flexible side chains. The layer structure model, with side chains acting as spacers between adjacent main chains, is widely accepted.5−7 Up to now, based on this layered structure, three types of polymorphs termed form I, I′, and II have been identified in one of the P3ATs family members, regioregular poly(3-butylthiophene) (P3BT).8−11 Form I of P3BT is the most commonly found modification. Films with form I are in general prepared by annealing a solution-cast sample at high temperature or by melt crystallization.9 Form I has a layer distance of 1.29 nm and a lateral stacking distance of 0.38 nm. Films of form I′, which is closely related to form I, are produced from solution © 2016 American Chemical Society

Received: August 29, 2016 Revised: December 1, 2016 Published: December 14, 2016 9493

DOI: 10.1021/acs.macromol.6b01828 Macromolecules 2016, 49, 9493−9506

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Macromolecules

performed using X-ray diffraction. However, such experiments have shortages when analyzing polymer crystal structures, since (i) the diffraction signals from polymer samples typically show a limited number of resolved peaks, so that several structural models can simultaneously match the diffraction result, and (ii) wide-angle X-ray diffraction does not provide direct (or complete) information about packing of molecules in the crystalline unit cell. For these reasons, we have combined WAXD with solid-state NMR and FTIR spectroscopies to provide additional and complementary information about these important structurally features for P3BT.30−34 Thus, from a broader perspective, our current study on P3BT demonstrates the potential of using a multitechnique approach to study phase composition and crystallography of polycrystalline materials. Our approach is particularly useful for the study of conjugated polymer films, where no single technique is able to provide the level of detail needed for a complete picture of molecular selfassembly and order in such materials. On the basis of the results of FTIR and solid-state NMR spectroscopy, we discuss and assign characteristic structural features of the different P3BT crystalline phases in relation to the known polymorphs, forms I, I′ and II. In particular, it will be shown that each of the polymeric forms does not contain a single, unique crystalline phase but instead contains several crystalline main-chain packing types, combined with different distinct side-chain organizations. We derive structural models for each of these P3BT crystalline phases, which are characterized by a unique set of solid-state NMR and FTIR spectroscopic fingerprints of specific packing types.

butyl side-chain organization is missing, leaving the following two key questions open. The first question is what type of the interchain π−π stacking mode is taken by the P3BT backbones? The π−π stacking mode is one of the important research topics in regioregular P3ATs, since it is the key driving force in the ordered selfassembly of such π-conjugated polymers.12 Moreover, it is also considered to be an important factor that affects the charge transport properties.1,13 Two types of configurations have been proposed for the P3ATs main chains. One is the so-called “aligned” configuration in which the neighboring thiophene rings are perfectly aligned on top of each other.14 The second type is the “staggered” configuration where the molecules are shifted by one thiophene unit along the backbones.5 Note that the effective π−π stacking distance is different in these two configurations. More recently, the form I structure of regioregular poly(3-hexylthiophene) (P3HT) was reported to crystallize in the space group P21/c from selected area electron diffraction (SAED) on epitaxial grown thin films and from multitechnique crystallography, combining solid-state NMR, powder XRD, and DFT calculations.15,16 The main difference between these two reports is that the P3HT structure based on SAED is characterized by a very short π−π stacking distance of 0.34 nm, requiring a 26° tilt of the P3HT main chains with respect to the crystallographic b-axis in addition to a shift of 0.19 nm along the crystallographic c-axis,15 whereas the P3HT structure from multitechnique crystallography had a π−π stacking distance of 0.39 nm with nontilted and nonshifted P3HT main chains.16 The second question is how the butyl side chains organize in P3BT polymorphs? It is generally thought that side-chain crystallization occurs in P3ATs containing more than ten alkyl carbons.17−20 However, several packing structures have been suggested for P3ATs with shorter side chains.6,10,17,21−24 Some researchers have considered that the alkyl side chains pack with ordered all-trans conformation,17,21,22 whereas the side-chain crystallization was suggested in the recent work by Wu et al.6 and Pankaj et al.23 On the other hand, Raos et al. concluded that the butyl side-chain organization of P3BT contains a certain degree of conformational disorder on the basis of molecular dynamics simulations.10 Furthermore, Gurau et al. reported that the hexyl chains of P3HT are liquid-like and highly disordered.24 It should be pointed out that these studies from various groups were based on P3ATs samples with different molecular weights and prepared under different conditions. Such differences can result in different P3BT polymorphs, since forms I and I′ are structurally very similar and were not differentiated before. In form II of P3ATs an interdigitated side-chain packing has often been proposed.25,26 However, some groups suggested that side-chain interdigitation is unlikely, in part because of the side-chain density,27,28 or the obstruction coming from the vibrational motion of the alkyl chains.29 These inconsistent reports on the form II side-chain structure request a clarification. We focus on the open questions mentioned above by our joint study of phase composition and molecular organization of thiophene main-chain and butyl side-chain for three P3BT samples, which from wide-angle X-ray diffraction (WAXD) are identified as being the polymorphs commonly termed form I, I′, and II. We have prepared these three types of samples with identical molecular parameters of P3BT (molecular weight, PDI, regioregularity) using specific film preparation procedures. The structural analysis of P3BT samples has often been

2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. Regioregular poly(3butylthiophene) (regioregularity is 93−95%, Mw = 5.85 × 104 g/mol from GPC against polystyrene standards with a PDI of 2.57) was purchased from Rieke Metals, Inc., and used as received. Three types of thin film samples were prepared by drop-casting on KBr pellets using different solvents and drying conditions. Samples 1 and 2 were prepared by dissolving P3BT in chloroform (CHCl3) with a concentration of 20 mg/mL. Solutions were first stirred on a hot stage at 40 °C until complete dissolution, filtered through a 0.45 μm PTFE syringe filter, and drop-cast at room temperature on KBr pellets used as substrates. Sample 1 was obtained by annealing the drop-cast film on a hot stage at 150 °C for 30 min under nitrogen. Sample 2 was prepared by slow solvent evaporation: after drop-casting, it was immediately placed in a closed weighing bottle with drops of CHCl3 at the bottom. The container was sealed immediately and kept at room temperature. Because of the slow evaporation rate, it took ca. 24 h for the film to dry. For the preparation of sample 3, P3BT was dissolved in carbon disulfide (CS2) with a concentration of 20 mg/mL, stirring at room temperature until complete dissolution. The solution was filtered through a 0.45 μm PTFE syringe filter before use. Subsequently, the solution was cast onto a KBr pellet and placed in a closed jar with drops of CS2 at the bottom. The film was dried by slow solvent evaporation. All samples 1, 2, and 3 were further dried under vacuum overnight at room temperature to remove any residual solvent prior to further characterization. Typical film thicknesses were in the order of 20−30 μm, measured using a thickness gauge (ID-S112M, Mitutoyo Corp. Japan). 2.2. Wide-Angle X-ray Diffraction (WAXD). The film samples 1, 2, and 3 of P3BT were removed from the KBr pellets, rolled, and put in 2 mm glass capillary tubes. X-ray diffraction measurements were performed in a transmission geometry using the Rigaku MicroMax 007 rotating copper anode source (Cu Kα, λ = 0.1542 nm) operated at 35 kV and 10 mA, Osmic confocal Max-Flux optics, and the Mar345 image plate detector. The exposure time was 30 min. Sample-to9494


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Macromolecules detector distance (320 mm) and the range of diffraction angle (2θ) were calibrated using silver behenate standard. One-dimensional plots of diffraction intensity I versus scattering vector s = λ−1 × 2 sin θ were obtained by integration of 2D diffraction patterns over the full azimuthal angle. The packing of P3BT chains in unit cells was modeled and visualized using Accelrys Materials Studio. The unit cells were built on the basis of experimental WAXD and NMR data. The simulated diffraction patterns for the unit cells were generated using the Mercury computer program, version 3.1. 2.3. Solid-State Nuclear Magnetic Resonance Spectroscopy. The P3BT films of samples 1, 2, and 3 were peeled off from the KBr pellet substrates, cut into small pieces, and packed firmly into the NMR rotors. All solid-state NMR experiments were performed on a Bruker Avance III 850 spectrometer operating at a Larmor frequency of 850.27 MHz for 1H and 213.85 MHz for 13C and equipped with a double-resonance magic-angle spinning (MAS) probe, supporting MAS rotors of 2.5 mm outer diameter. The rf nutation frequencies for 1 H and 13C were 100.0 kHz, corresponding to 2.5 and 5.0 μs for 90° and 180° pulses, respectively. 1D 13C cross-polarization/magic-angle spinning (CP/MAS) spectra were recorded using a CP time of 2 ms, a recycle delay of 3 s, and 1024 scans with SPINAL-64 decoupling applied during acquisition.35 The 2D 1H−13C frequency-switched Lee−Goldburg heteronuclear correlation (FSLG-HECTOR)36 experiments used a MAS frequency of 15.0 kHz, a recycle delay of 2 s, a cross-polarization (CP) time of 2.0 ms, and 512 scans for a total of 38 t1 increments. Each t1 increment had a span of five basic FSLG blocks (81.6 μs),37,38 and high-power 1H SPINAL-64 decoupling was used during acquisition. The CP step utilized Lee−Goldburg-CP (LG-CP) conditions39 to suppress 1H spin diffusion during CP. The Hartmann− Hahn matching condition was preoptimized on L-alanine. A scaling factor of 0.570 was determined by recording a 2D spectrum for adamantane using the identical experimental conditions and used to rescale the indirect dimension for the investigated samples. 2D 1H−1H double quantum-single quantum (DQ-SQ) correlation spectra were recorded at MAS frequency of 29762 Hz. DQ excitation and reconversion were achieved using the back-to-back (BaBa) sequence40,41 with 16 scans and 64 t1 increments. All 2D 1H−1H DQ-SQ spectra were plotted on the same intensity scale with 25 contour lines from 45 to 95% of the maximum signal intensity. All spectra were referenced with respect to tetramethylsilane (TMS) using solid adamantane (13C, 29.46 ppm; 1H, 1.85 ppm) as a secondary reference.42,43 2.4. FTIR Spectroscopy. FTIR spectra of the drop-cast P3BT films on KBr pellets were measured in the transmission mode with a Bruker tensor 27 spectrometer equipped with a DTG detector. The spectra were obtained by coadding 16 scans at 2 cm−1 resolution. The low-temperature FTIR spectra of P3BT samples were collected by means of a Harrick variable temperature cell, which was placed into the sample compartment of the spectrometer. First, the sample was cooled from room temperature to −160 °C by liquid nitrogen. FTIR spectra were recorded only during warm-up at 10 °C intervals from −100 up to 20 °C.

Figure 1. Experimental WAXD (black lines) and corresponding simulated I(s) patterns (s = 2 sin θ λ−1) with assigned diffraction peaks for samples 1, 2, and 3. Simulations are based on packing a (blue), packing b (green) and packing c (red) (see Figure 2). Note the reversed appearance of diffraction peaks 020 and 120 of packings a and b, respectively. Asterisks (∗) indicate unassigned diffraction features while crosses (×) indicate peaks related to lamellar assemblies of disordered main chains.

requires a correct assignment of Bragg peaks occurring at 2.3 < s < 2.8 nm−1. The WAXD profiles of samples 1, 2, and 3 agree with earlier reports for P3BT polymorphs, i.e., form I (= sample 1), form I′ (= sample 2), and form II (= sample 3) (see Table S1). However, due to the limited number of prominent diffraction peaks, in addition to the broad intensity distribution of peaks at higher s-values, it is not reasonable to resolve the structure using conventional methods, e.g., Rietveld refinement. Therefore, in our approach, we have compared experimental diffraction patterns with those calculated for specific packings of P3BT backbones as displayed in Figure 2. We used monoclinic (P21/c; packing a) and triclinic (P1; packing b and c) unit cells, with geometries similar to those reported earlier for P3BT10,11 and P3HT.16 Each periodic packing of molecules is paired with a unique, in terms of positions and relative intensities of peaks, X-ray diffraction pattern (Figure 1). In our study, positions and integral intensities of peaks were simulated directly from main

3. RESULTS AND DISCUSSION 3.1. Determination of P3BT Main Chain Packing Schemes by WAXD. Figure 1 shows the WAXD patterns of the three P3BT samples 1, 2, and 3. The most characteristic features of the patterns are the intense Bragg peaks visible at the scattering vectors between 0.75 and 1.05 nm−1, originating from diffraction of X-rays on (100) crystallographic planes of P3BT crystals.44 The d(100) spacing corresponding to these peaks is related to average distances between subsequent layers of π-stacked polythiophene backbones, which are separated by alkyl chains. The lateral stacking distance, identical to d(020) crystallographic distance, and d(100) spacing are main geometrical features that define packing of P3BT backbones in different polymorphs.8,9,45,46 Calculation of d(020) distance 9495


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operations (shift and tilt) caused a loss of the initial P21/c packing symmetry and resulted in the two earlier-mentioned P1 geometries. The final structures for packing a, b, and c are included in the Supporting Information in CIF format. We further note that the unassigned reflections marked by asterisks in Figure 1 most likely are due to packing effects related to the butyl side chains, which we have disregarded in our packing models (see above). Our simulations further made clear that the experimental patterns of samples 1 and 2 can be equally well interpreted assuming two different main-chain packing geometries: packing a with nontilted thiophene backbones or packing b in which the thiophene backbones are tilted as shown in Figure 2. After assuming specific d(100) of either sample 1 or 2 (Table 1) the diffraction patterns simulated for the both packing geometries gave identical agreement with experimental results. The peak at s = 2.6 nm−1 may either result from diffraction of X-rays on (020) planes of the packing a or from (120) of the packing b. At the same time the peak at s = 2.7 nm−1 may be connected with the (120) planes of packing a or (020) planes of packing b. It is also possible that the samples 1 and 2 are mixtures of the two packing configurations a and b. This open question will be resolved by the solid-state NMR and IR investigations in the following sections. Simulations of the WAXD pattern for sample 3 indicate that its crystalline phase can be fitted with only one model of chain packing, denoted here as c (Figure 2). This packing, with unit cell dimensions of a = 1.03 nm, b = 0.85 nm, c = 0.76 nm and the γ-angle 73°, shows geometric similarity to the form II crystalline structure recently reported for P3BT,11 with the most prominent exception that the symmetry relations imposed are less strict due the space group P1 (see above). Similar to the case of samples 1 and 2, we based our model on the structural information from solid-state NMR and FTIR experiments. The characteristic feature of sample 3 is according to the spectroscopic data, that the ordered thiophene units of P3BT macromolecules display remarkable torsional oscillations around the main-chain axis of the polymer backbone (i.e., dynamic twisting; see sections 3.2 and 3.3). However, due to distinctly dissimilar time scales of these molecular oscillations and the time frame of the conventional diffraction experiments performed in this work, all experimental information about dynamics is lost due to the time averaging.49 Hence, the WAXD patterns only provide information about the averaged structure appearing as static; i.e., we have for this reason used a static model for the crystalline structure of sample 3, where the subsequent modeling was performed analogically as in the case of samples 1 and 2. The experimental WAXD pattern of sample 3 suggests that in addition to the crystalline packing of P3BT it also includes a

Figure 2. Packing schemes of P3BT main chains (gray bars) in the monoclinic (P21/c; packing a) and triclinic (P1; packing b and c) unit cells. The c-projections of the unit cells are outlined with dotted red lines. Solid black lines indicate positions of Miller (hkl) planes while arrows mark interplanar distances (d(hkl)).

chain packing schemes. In order to obtain realistic X-ray diffraction patterns, we have included two additional parameters: (i) Scherrer broadening of peaks to mimic realistic sizes of crystals (scattering domains) and (ii) the amorphous halos corresponding to the portion of radiation scattered on the disordered fraction of P3BT. In this way, we obtained satisfying agreement between experimental and simulated WAXD data. We note that although a unit cell can be fitted with only one diffraction pattern, simple diffraction patterns are nonunique and can result from more than one crystal structure or packing scheme.47,48 In general, less complex patterns decrease the determination accuracy and allow more freedom in interpretation. In our analysis of the diffraction patterns we have only included effects that are clearly related to real-space features of the polymer. These include the (h00), (0k0), and (hk0) planes, which can be attributed to the packing motifs of thiophene main chains in the crystalline phases; i.e., we do not include any contributions from the butyl side chains to the calculated diffraction patterns. Since we observed similar π-stacking distances in both form I of P3HT and samples 1 and 2 of P3BT,6,16 the starting point in our reasoning was an assumption that the P3BT backbones can be packed in a monoclinic unit cell with P21/c symmetry similar to that reported for P3HT16 (with a correction for packing of the alkyl chains different in P3BT and P3HT). After a WAXD-based verification of the unit cell parameters, all our considerations were based on the geometry of packing motifs of the polythiophene backbones in P3BT. On the basis of the solid-state NMR results below, we shifted and tilted the P3BT backbones against each other until the point, where the positions of the h00, 0k0, and hk0 reflections were consistent with the corresponding experimental reflections; i.e., the local packing of thiophene units was consistent with the solid-state NMR results. These geometric

Table 1. Crystalline Main-Chain Packing Types of Regioregular P3BT and Their Assignment to Experimental Data WAXD packinga

a

NMR

d(020)

d(120)

d(100)

γ

(nm)

(nm)

(nm)

(deg)

a

0.38

0.37

b

0.37

0.39

c

0.40

0.42

b

1.27 1.31c 1.27b 1.31c 0.98

1

H(Hb)

(ppm)

13

C(Cb)

(ppm)

90

6.1

125.8

73

6.6

126.8

73

6.6

128.1

IR 13

C(Ch)

(ppm) c

14.6 15.9b 14.6c 15.9b 17.2d

δ(Cb−H) (cm−1) 819.1 811.1 825.0

See Figure 2. bSide chains in trans conformation. cSide chains in an average gauche conformation. dInterdigitated side chains. 9496


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Macromolecules lamellar phase with a 1.2 nm thiophene-layer spacing. The firstand second-order diffraction peaks related to this phase are marked with an “×” in Figure 1. The broad intensity distribution and significant integral intensity of these diffraction peaks suggest a low perfection and a considerable weight fraction of this lamellar phase, respectively. Despite the lack of other characteristic WAXD features, this may indicate that the packing of P3BT macromolecules in this phase is ordered in a manner characteristic of a liquid-crystalline phase (e.g., smectic phase) rather than a crystalline phase. So far, the formation of smectic phases was reported for P3HT only at elevated temperatures,50,6,51,52 while to the best of our knowledge, the presence of a smectic phase at room temperature in any of the other linear poly(3-alkylthiophenes) has not been reported. Hence, due to the limited experimental evidence, the presence of a stable smectic phase in sample 3 is a hypothesis that requires further experimental insights. 3.2. Molecular Packing in P3BT Characterized by Solid-State NMR Spectroscopy. To further disclose on the local polymer chain arrangements in the three differently prepared P3BT samples, 1D and 2D solid-state NMR experiments have been performed. Recent examples taking advantage of this kind of strategy include supramolecular systems,53−56 pharmaceuticals,45,46 and semicrystalline polymers.57−62 In the following we summarize the results from 13 C{1H} CP/MAS in combination with 2D 1H−1H and 1 H−13C NMR correlation spectroscopy to characterize both the P3BT main-chain arrangements and the butyl side-chain organization in all three P3BT samples. Main-Chain Arrangements. Figure 3 shows the 2D 1H−1H double quantum-single quantum (DQ-SQ) correlation spectra of all three P3BT samples. From Figures 3b and 3c, two resolved 1H signals at 6.6 and 6.1 ppm along with a broader resonance located above 7.0 ppm are detected from the thiophene protons in samples 1 and 2. The broad signal at 7.0 ppm can be assigned to the noncrystalline fraction of the samples due to its similarity with the 1H chemical shift in solution (see Figure S1 in the Supporting Information). In both 2D 1H−1H DQ-SQ correlation spectra of samples 1 and 2, the resonance at 6.6 ppm exhibits a more intense auto correlation peak (located at the diagonal) than the signal at 6.1 ppm without any cross-correlation between each other. This observation implies that the resonances at 6.6 and 6.1 ppm originate from two different P3BT main-chain arrangements, where the thiophene protons have an internuclear distance of less than 0.4 nm.64 Furthermore, such a short distance can only arise from a close contact between P3BT polymer chains along the π−π stacking direction; i.e., the shortest distance between thiophene protons along the P3BT main chain is ∼0.6 nm, while the distance between P3BT layer stacks is more than 1.2 nm. Thus, the 1H MAS NMR experiments point toward two different types of π−π stacking modes in both samples 1 and 2. The first, with a thiophene proton chemical shift of 6.1 ppm, includes a large overlap between the thiophene groups, since the aromatic ring currents of neighboring P3BT chains are the source of the low chemical shift.16 The second structure, with a thiophene proton chemical shift of 6.6 ppm, has a limited overlap with the thiophene groups of the neighboring P3BT chains, while maintaining a distance below 0.4 nm between thiophene protons. For sample 3 only a single 1H thiophene resonance is observed in the 2D 1H−1H DQ-SQ correlation spectrum shown in Figure 3d. This signal, located at 6.6 ppm,

Figure 3. 2D solid-state 1H−1H DQ-SQ NMR spectra recorded at 20.0 T (850.27 MHz for 1H) for the three differently prepared P3BT samples. The assignments follow the color code given in (a). Filled and open blue circles refer to 1H resonances of carbon b (Cb) at 6.6 and 6.1 ppm of P3BT chains, respectively. Orange colors indicate the butyl groups. The excitation and reconversion of DQ coherences employed the back-to-back (BaBa) NMR pulse sequence for one rotor period.63 All spectra were recorded at ambient temperature using a MAS frequency of 29 762 Hz.

also includes an autocorrelation peak, illustrating that only one crystalline main-chain arrangement exists in sample 3. Figure 4 displays the 1D 13C{1H} CP/MAS and 2D 13C{1H} FSLG-HETCOR spectra of the thiophene main-chain region for the three P3BT samples. The 13C chemical shifts are listed in Table S2 following the assignment scheme as shown in Figure 3a. As is apparent from Figures 4b and 4c, the broad resonance located above 139.0 ppm in samples 1 and 2 can be assigned to carbon c (anchoring point of the butyl side chains) of the amorphous fraction for these P3BT samples; i.e., both signals are correlated to the 1H chemical shift at 7.0 ppm (see above). Consistent with the conclusions drawn from the 1 H−1H DQ-SQ correlation spectra, both 2D 1H−13C FSLGHETCOR spectra of samples 1 and 2 include two types of P3BT main-chain arrangements. Notably, sample 2 exhibits two resolved 13C signals for site b (Figure 4c) with chemical shifts of 125.8 and 126.8 ppm that correlate with the 1H signals at 6.1 and 6.6 ppm, respectively. This observation of multiple resonances for carbon site b in sample 2 further supports the conclusion that this P3BT sample contains two different thiophene main-chain arrangements. The 2D 1H−13C FSLGHETCOR spectrum of sample 1 (Figure 4b) gives similar information as that for sample 2; i.e., the presence of two types of thiophene main-chain organizations is also clearly detected. However, as shown in Figure 4a, the ratios between the two crystalline fractions are unequal in samples 1 and 2. By deconvoluting the 1H MAS NMR spectra, we obtained the relative weight for two crystalline phases of 25.5% (6.1 ppm) and 18.4% (6.6 ppm) in sample 1, while 29.6% (6.1 ppm) and 30.4% (6.6 ppm) in sample 2 (see Figure S5 and Table S7). 9497


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Figure 4. (a) Solid-state 13C{1H} CP/MAS spectra and (b−d) 2D 13C{1H} FSLG-HETCOR NMR spectra of the three differently prepared P3BT samples, showing the thiophene main-chain region only. All spectra were recorded at ambient temperature using 15.0 kHz MAS and a CP time of 2.0 ms. The assignment of carbon and protons follows the scheme in Figure 3a with Table S2 summarizing the 13C chemical shifts. Filled and open blue circles refer to 1H(Cb) resonances at 6.6 and 6.1 ppm of P3BT chains, respectively. Signals from the amorphous fraction are marked with gray color.

Figure 5. (a) Solid-state 13C{1H} CP/MAS spectra and (b−d) 2D 13C{1H} FSLG-HETCOR NMR spectra of the three differently prepared P3BT samples, showing the butyl side-chain region only. Assignments and experimental details are as given in Figure 3.

deconvolution. It is worth to mention that the significantly different 13C{1H} CP/MAS NMR spectrum of sample 3 clearly demonstrates that this sample contains a different P3BT mainchain structure as those present in samples 1 and 2. As can be identified from Table S2, the 13C chemical shifts of sample 3 are very similar to those determined from solution 13C NMR, where limited contact between P3BT polymer chains is present. This similarity suggests that the interaction in terms of π−π overlap between the stacked P3BT chains in sample 3 is weaker than that found in samples 1 and 2. Therefore, the P3BT main chains in sample 3 have less restriction for a twisting motion of thiophene rings. This assumption is further supported by the results of site-specific molecular dynamics as probed by 2D

Based on further studies (to be published), a mesophase is indeed found in sample 1 and 2, where the main chains are in disordered conformation exhibiting restricted local motion in accordance with FTIR. Compared to samples 1 and 2, sample 3 shows a remarkably different 13C{1H} CP/MAS NMR spectrum as it includes four sharp 13C resonances associated with the four thiophene carbons (Figure 4a), indicating that only one P3BT main-chain stacking organization exists. The broad signal, which is spread over the entire thiophene region, comes from the amorphous fraction (marked with a gray color in Figure 4d). The relative contents of 57.8% and 42.2% for crystalline phase (packing c) and amorphous phase were deduced based on spectrum 9498


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C{1H} REREDOR sideband patterns (Figure S6). These patterns yield the 13C−1H dipole−dipole couplings for carbon b and accordingly the dynamic order parameter (SCH). SCH equals 0 for fast isotropic motion and 1 if the CH entity is rigid. On this basis, the dynamical order parameters for carbon site b in samples 1, 2, and 3 demonstrate that the thiophene backbones in the crystalline phases are rather rigid for samples 1 and 2, where SCH is equal to 1.0, while those in sample 3 have a SCH of 0.8. This difference in dynamic order parameters illustrate that the P3BT main chains in sample 3 are less densely packed compared to those in samples 1 and 2, allowing for small out-of-plane fluctuations of the thiophene groups. Such out-of-plane fluctuations correspond to a dynamically twisted P3BT main-chain conformation, which is a planar conformation on time-average only. Using the NMRWEBLAB,65 a maximum out-of-plane angle of approximately ±30° can be deduced for this kind of motion, assuming that the fluctuations are a result of a Gaussian distribution of displacement angles.32 Side-Chain Organization. The 13C{1H} CP/MAS and 2D 1 H−13C FSLG-HETCOR NMR spectra of the butyl side chains for all P3BT samples are presented in Figure 5. These spectra are used to infer about the butyl side chain conformations and the proximity of the end methyl groups to the P3BT main chains, respectively. The 13C resonances are assigned using the scheme in Figure 3a with their positions listed in Table S2. The assignments to the specific main-chain packing types are given in Table 1, and Figure 6 shows a schematic representation of the butyl side chain organization in the P3BT samples. For sample 1, Figure 5a includes a single and broad 13C resonance for the methyl group (site h) at 14.6 ppm. This resonance correlates to the thiophene 1H signal centered at 6.9 ppm (see Figure 5b); i.e., only the noncrystalline or amorphous fraction of the P3BT main chains shows proximity to the methyl group of the butyl side chains. It has been shown that the methyl group from alkyl-chain ends shows distinct 13C chemical shifts for gauche and trans conformations of the alkyl chain. A high-field shift up to 5 ppm could be observed when alkyl carbons in all-trans conformation are converted to threebond gauche conformation.66,67 Thus, the SSNMR results here show that the butyl side chains in sample 1 are predominantly in an average gauche conformation68 in both the crystalline and amorphous phases, where only the amorphous phase show a close proximity between the P3BT main chains and the end methyl group of the butyl side chains. For sample 2, Figure 5a shows that the methyl group (site h) includes two 13C signals at 14.6 and 15.9 ppm. These are associated with average gauche conformation and all-trans conformation of the butyl side chains, respectively.69−71 Furthermore, Figure 5c demonstrates that the 13C signal at 15.9 ppm correlates to the thiophene 1H signal centered at 6.4 ppm with a spread of 0.3 ppm. This is the average position of the two ordered and stacked P3BT main chains at 6.1 and 6.6 ppm (packing a and b, see Table 1 and section 2.4). On this basis, the methyl group (site h) at 15.9 ppm, representing butyl side chains in all-trans conformations, can be seen to have a close spatial proximity to both of the two ordered P3BT mainchain arrangements (packing types a and b). Meanwhile, the 13 C signal of carbon site h at 14.6 ppm correlates to thiophene protons from 6.4 to 7.5 ppm. This shows that the gauche conformation contributes to both the amorphous phase of sample 2 and the crystalline fraction with a 1H chemical shift of

Figure 6. Schematic of side-chain order types for P3BT derived from the NMR analysis (not drawn to scale, coordinates a and c refer to unit cell axes). Adjacent chains of P3BT are represented in red and blue. Alkyl chains can be (a) disordered, (b) ordered, or (c) ordered and interdigitated. The distance between the chains is marked by arrows and refer to d(100) (see Table 1).

6.6 ppm (packing b). Taking the above results for samples 1 and 2 together show that ordered and disordered side chains are observed for both packing types a and b as displayed in Table 1. Sharply different from samples 1 and 2 are the butyl side chains in sample 3 (see Figures 5a and 5d). Here, each butyl carbon site includes one signal with similar 13C chemical shifts as observed for samples 1 and 2 (cf. Figure 5a), in addition to a much narrower set of resonances at higher 13C chemical shift (marked by the blue regions and letters with primes in Figure 5d). According to the 2D 1H−13C FSLG-HETCOR spectrum in Figure 5d, the narrow set of signals, in particular the methyl signal at 17.2 ppm, has a much stronger correlation to the thiophene protons at 6.6 ppm; i.e., the methyl group from the 9499


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deconvolution are presented in the Supporting Information. The spectra of samples 1 and 2 are fitted by four curves, and the spectrum of sample 3 is composed of three curves. The fits are shown in Figure 7, and the corresponding parameters are presented in Tables S3 and S4. On the basis of earlier reports,76,77 we assign the band at 832 cm−1 to the disordered/amorphous phase and the other bands at 825, 819, and 811 cm−1 to the ordered phases of P3BT. Besides the amorphous characteristic band, the band at 825 cm−1 is observed in all samples, with a relative area of ca. 20% in samples 1 and 2 while more than 34% in sample 3. The bands at 811 and 819 cm−1 exist in both sample 1 and sample 2. However, the relative areas show a significantly different ratio. In sample 1, we obtain the ratio of area(811 cm−1)/area(819 cm−1) = 13%/14% = 0.93. It is important to note that this ratio is much larger in sample 2 where we get area(811 cm−1)/ area(819 cm−1) = 20%/4% = 5. These significantly different ratios in sample 1 versus sample 2 will enable the correlation between spectral features in solid-state NMR and FTIR (see section 3.4). The antisymmetric stretching mode νas(CC) in the region 1490−1540 cm−1 indicates structural order of the conjugated thiophene main chain, which is ascribed to the effective conjugation length (ECL).72,73 Decreasing chain lengths of thiophene oligomers causes a reduction of ECL.73 Similarly, increasing twist of thiophene rings leads to reduced planarity of the main chain of P3ATs and reduced ECL. The term twist here means that subsequent thiophene rings are not coplanar, as proposed by Chen et al.80 A frequency increase of the νas(CC) mode always indicates a shortening of the corresponding ECL.72,73,81 To analyze the impact of sample preparation on the main chain structural order of P3BT films, we present the IR spectra of all samples 1, 2, and 3 in the 1540−1490 cm−1 region (see Figure 8). Curve fitting and deconvolution of the spectra were performed, and the resulting optimized fit parameters are summarized in Table S5. The FTIR spectrum of sample 1 is fitted using three lines at 1518, 1512.5, and 1510 cm−1, whose frequencies are similarly seen in sample 2 (1518, 1512.5, and 1509 cm−1). Sample 3, however, shows only two sub-bands at 1518 and 1512.5 cm−1. The relative intensities, widths, and area percentages depend on sample preparation, which allows their specific assignments. We assign the sub-band at 1518 cm−1 to the amorphous phase, which exists in all samples. The high frequency of the observed νas(CC) modes is related to the shortest ECL, usually observed in solutions or in highly disordered regions, which are typical for the amorphous phase. The sub-band at 1512.5 cm−1 dominates the spectrum of sample 3. However, it is also observed in the FTIR spectra of samples 1 and 2 in smaller, but still significant area percentages of 43% and 16%, respectively. We conclude that the νas(CC) mode at 1512.5 cm−1 indicates a backbone conformation of P3BT chains in an ordered packing, which is clearly distinguishable from an amorphous phase. We further assume that the mode at 1512.5 cm−1 correlates with either a static twist, or a dynamically twisted state of the thiophene units as will be elucidated in section 3.4. The sub-band at 1509−1510 cm−1 exists in the IR spectra of samples 1 and 2 only. It has the lowest frequency of all observed νas(CC) modes. Therefore, it relates to the largest ECL, which is typical for a planar backbone conformation of regioregular P3BT in a crystalline packing. Further discussion and assignments of δ(Cb−H) and

narrow set of signals is closer to the thiophene main chain in the crystalline phase. Thus, sample 3 includes two major butyl side chain packing structures: disordered butyl side chains (see Figure 6a) and interdigitated butyl side chains as sketched in Figure 6c, corresponding to the narrow set of 13C resonances in Figure 5d. 3.3. Molecular Packing of P3BT Investigated by FTIR Spectroscopy. The main-chain configurations of P3BT show characteristic spectral features in the Cb−H out-of-plane deformation mode δ(Cb−H) at 860−790 cm−1 and in the antisymmetric stretching mode νas(CC) at 1540−1490 cm−1, respectively.72,73 The side-chain organization is indicated by characteristic alkyl stretching vibrations ν(CH2) and ν(CH3) in the region of 3000−2800 cm−1.21,74−79 These spectral regions were studied to find correlations with WAXD and NMR data, respectively. Main-Chain Conformations. Figure 7 shows the roomtemperature FTIR spectra of the δ(Cb−H) mode for all three

Figure 7. Room-temperature FTIR spectra of the δ(Cb−H) vibration mode for the three P3BT samples. Measured spectra are indicated by black filled circles and fitted spectra are shown by solid lines. Dashed lines, serving as guides to the eye, indicate peak positions. The inset shows the chemical structure of P3BT, where the out-of-plane vibration direction of the δ(Cb−H) mode is indicated by the blue−red circle.

P3BT samples. This vibration mode (its vibrational direction is shown as inset in Figure 7a) of P3ATs main chains receives continuous attention because of its sensitivity to the phase state.76,77 We observed several peaks (the positions are listed in Table S2) in the spectrum of the δ(Cb−H) mode for each of the P3BT samples by using the deconvolution analysis of the spectral region 860−790 cm−1. Details of the spectrum 9500


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Figure 8. Room-temperature FTIR spectra of the υas(CC) vibration mode in the region of 1540−1495 cm−1 for the three P3BT samples. All spectra were obtained from drop-cast films on KBr pellets in transmission mode, indicated by the filled circles. The fitted spectra are shown by solid lines; see Table S5 for fit parameters. Dashed lines, serving as guides to the eye, indicate peak positions.

Figure 9. Temperature-dependent FTIR spectra of the three P3BT samples and their corresponding second derivatives in the region of 3000−2800 cm−1. All spectra were obtained from drop-cast films on KBr pellets in transmission mode. Dashed lines, serving as guides to the eye, indicate peak positions and their assignment along with wavenumbers determined from the room-temperature spectra.

νas(CC) modes to specific packing schemes are presented in section 3.4. Side-Chain Organization. Figure 9 displays the FTIR spectra of the P3BT samples in the region of the symmetric and antisymmetric υ(CH2) and υ(CH3) bands at 3000−2800 cm−1, which are often used to probe the order of polymethylene chains in condensed materials.78,79 Cooling the samples from 20 to −100 °C helps to visualize characteristic spectral features more clearly. Samples 1 and 2 exhibit rather similar line positions as shown in Figures 9a and 9b. Only the spectral band widths in the case of sample 2 are significantly narrower as compared to sample 1, which is visible particularly at low temperatures. The observation of different line widths correlates well with the solid-state NMR results presented above, which specify more ordered butyl side chains in sample 2 as compared to sample 1, because disorder usually causes larger line widths in the IR spectra. Consistent conclusion is drawn from the spectra of samples 1 and 2 in the region of 1400−1270 cm−1 which shows the bands sensitive to the sidechain conformation.21 As shown in Figure S4, the ttt bands located at 1322 and 1298 cm−1 of sample 2 exhibit much higher intensities than that of sample 1, whereas the intensities of the gtg′ band at 1360 cm−1 and the gtt band at 1340 cm−1 of sample

2 are lower than that of sample 1. This observation also proves that sample 2 has more ordered side chains than sample 1. In contrast to samples 1 and 2, the CH2 vibrations of sample 3 show a band splitting into two components at 2862 and 2852 cm−1 (υs(CH2)) and at 2933 and 2920 cm−1 (υas(CH2)). This band splitting is even better visible in the second-derivative spectra shown in the lower panel of Figure 9c. Only the spectral positions of the υs(CH3) and υas(CH3) vibrations are very similar in all three samples. The IR band assignments of thiophene main chains and butyl side chains for the three P3BT samples are summarized in Table S2. 3.4. Appraisal of P3BT Polymorphs and Crystalline Packing Models by SSNMR Combined with WAXD and FTIR. We focus on the remaining open questions in the correlation between the data of WAXD, solid-state NMR, and FTIR by means of different sample preparation routes, which cause individual crystalline backbone packing modes to appear prominently at specific sample types. According to the current view of forms I, I′, and II, only one crystalline phase has been considered in each of these P3BT polymorphs. However, from our results presented above, two types of P3BT main-chain stacking structures coexist in both samples 1 and 2, while one ordered main chain structure is probed in sample 3. Direct 9501


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the 1H(Hb) resonance at 6.6 ppm. Thus, we assign the 825 cm−1 band in sample 3 to packing c. In previous works, a rigorously trans-planar main-chain conformation for form II of P3HT is derived, corresponding to packing c.11,84 For our sample 3, packing c is also considered to adopt a time-averaged planar conformation, leading to the very narrow and distinct 13 C resonances associated with the four thiophene carbons as indicated in Figure 4a. However, the fact that the P3BT main chains in sample 3 are less densely packed compared to those in samples 1 and 2 allows for small out-of-plane fluctuations of up to ±30° for the thiophene groups as revealed by SSNMR. Accordingly, such a time-averaged planar conformation is dynamically twisted, which corresponds to the νas(CC) mode at 1512.5 cm−1. The 832 cm−1 band, which appears in all samples, is assigned undoubtly to the amorphous phase. Additionally, we observe a weak sub-band at 845 cm−1 in sample 3 (see Figure 7c). At the current stage, we cannot assign this band clearly. It may be associated with a smectic phase in sample 3, as suggested by WAXD results. When dealing with the side-chain structures in different samples, SSNMR identifies that sample 1 contains disordered butyl side chains in the crystalline phase and sample 2 contains side chains in all-trans conformation. With the help of FTIR, we consider different side-chain order for these packings a and b further. Packing b has more ordered side-chain organization than packing a. This assignment may explain the origin of the low-temperature IR spectral changes of the 811 and 819 cm−1 bands (see Figures S2a and S2b). That is the exchange between more- and less-ordered conformations of butyl side chains with temperature causes the opposite intensity changes. It is known from literature that the P3ATs form II in general includes interdigitated side chains organization.25,26,35,36,53−55 Based on this information, the narrow set of unique aliphatic 13C resonances at 31.2, 34.5, 25.1, and 17.2 ppm, corresponding to sites Ce, Cf, Cg, and Ch, respectively (see Table S2 and Figure 5a), for sample 3 can be assigned to the interdigitated butyl side chains of P3BT. These specific NMR signals are visible in sample 3 only, which is in line with the observation of band splitting in the IR spectra of the υas(CH2) and υs(CH2) modes, which are solely observed in sample C (see Figure 9c). It is noticed that the 825 cm−1 band also appears in the IR spectra of samples 1 and 2. But the existence of this band cannot be explained with packing c because the WAXD data unambiguously show that the crystalline packing c does not exist in samples 1 and 2 (see Figure 1) in a corresponding and significant amount. By analyzing the fitting results of the δ(Cb− H) vibration further (see Figure 7 and Table S4), we notice the 825 cm−1 peak width of sample 3 is much narrower than that of samples 1 and 2. Moreover, the low-temperature IR spectral changes of the 825 cm−1 band for P3BT samples (see Figure S2) show different temperature dependence. Accordingly, we consider it likely that a mesophase also exists in samples 1 and 2, which could be attributed to the broad band (fwhm >7 cm−1) at 825 cm−1. In contrast to packing c, which has dynamically twisted main chains but highly ordered and interdigitated side chains, this mesophase is expected to adopt a main chain conformation with thiophene rings that are frozen with various twisting angles as concluded by Yazawa et al.77 for the band at 825 cm−1. This mesophase also has disordered side chains as summarized in Table 2. It should be noted that such a mesophase is hard to detect by WAXD and solid-state NMR due to its lack of packing ordering.

evidence comes from the fact that all samples showed distinct 1 H−1H autocorrelation signals between stacked thiophene groups (Figures 3b and 3c), demonstrating that the thiophene protons are spatially close with an internuclear distance of less than 4.0 Å. In our recent study of form I for P3HT, we have shown that this thiophene−thiophene distance constraint can only be consistent with a thiophene chain-packing model that includes flipping every second polymer backbone in the thiophene layer.16 Furthermore, the thiophene protons of P3HT resonate at 6.0 ppm, which we could show was a result of nontilted P3HT main chains with respect to the crystallographic b-axis.16 Taking this information into account in our current study and the WAXD data presented in this work for the three different P3BT samples (Figure 1), it becomes evident that the two different thiophene proton chemical shifts at 6.1 and 6.6 ppm are likely to represent nontilted (termed packing a) and tilted (termed packing b and c) P3BT main chains, respectively. Specifically, the main-chain organizations of both packing types a and b exist in the crystalline phase of sample 1 and sample 2, whereas sample 3 exhibits a single packing c. The packing schemes are confirmed further by assigning the observed δ(Cb−H) at 811, 819, and 825 cm−1 in FTIR spectra. The assignments are summarized in Tables 1 and 2. Table 2. Structure Correlation of IR Bands

a

δ(Cb−H) (cm−1)

fwhm (cm−1)

811 819 825

10−12 5−9 7 18−22

mesophase amorphous phase

structurea

main-chain conformation planar planar dynamically twisted twisted disordered

side-chain order more ordered less ordered interdigitated disordered disordered

The different packing schemes a, b, and c are shown in Figure 2.

The assignments of the IR bands at 811 and 819 cm−1 are verified by their relative band intensities in samples 1 and 2, in comparison to the relative signal intensities of the 13C NMR resonances for site Cb at 125.8 and 126.8 ppm, respectively. The relative intensity of the 811 cm−1 band is significantly larger in sample 2, which relates well to the signal intensity behavior of the 13C NMR signals at 126.8 ppm. Similarly, the 819 cm−1 band appears relatively stronger in sample 1, which justifies its assignment to the 13C NMR signal at 125.8 ppm. Accordingly, we conclude that the 811 and 819 cm−1 bands represent packing schemes b and a, respectively. The assignments of the δ(Cb−H) modes for P3ATs have been suggested by Yazawa et al. and Brambilla et al.77,81−83 (see Table S6 for details). Yazawa et al. considered the contributions of both main-chain conformation and chain dynamics, while Brambilla et al. emphasized the effect of side-chain ordering on the frequency of δ(Cb−H). Combined with the conclusions above and our spectrum fitting on the νas(CC) vibrations as shown in Figure 8, it is considered that the main chains in packings a and b take the planar conformation with the largest effective conjugation length, which induces the lowest frequency of the νas(CC) mode. On the other hand, only the δ(Cb−H) mode at 825 cm−1 is observed in the crystalline phase of sample 3, which correlates with NMR signals of the 13C(Cb) resonance at 128.1 ppm and 9502


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4. CONCLUSIONS Using different film preparation conditions, we have obtained three types of P3BT samples. Conventional analysis based solely on WAXD measurements would lead to a conclusion that these samples are semicrystalline and contain pure polymorphs traditionally referred to as form I, I′, and II. However, our solid-state NMR and FTIR results reveal that each of these forms is not phase pure. The different film casting conditions cause individual crystalline backbone packing types and side chain organizations to appear prominently for specific sample types and enable their spectroscopic identification. Our experimental WAXD profiles of samples 1, 2, and 3 agree with earlier reports for P3BT polymorphs, i.e., form I (= sample 1), form I′ (= sample 2), and form II (= sample 3). In our analytical approach, we use computer simulations to reveal if the picture of a reciprocal space given in X-ray diffractograms can be considered unique enough to conclude about structure of P3BT crystals. The WAXD patterns are simulated using specific crystalline unit cells: one monoclinic cell (P21/c, nontilted thiophene main chains, packing a), and two triclinic cells (P1, tilted thiophene main chains; packing b and c, which differ in their unit cell dimensions). Simulation of the WAXD data for samples 1 and 2 indicates that both packing types a and b can provide nearly identical diffraction patterns; hence, it is not possible to distinguish between packing schemes a and b by means of WAXD only. Sample 3, on the other hand, gives rise to a diffraction pattern that can be simulated using one specific crystalline unit cell only, corresponding to packing c. Our quantitative evaluation of WAXD data provides the cell dimensions of all three packing types. Application of solidstate NMR and IR spectroscopies enables us to correlate specific spectral features with regional chain environments and packing schemes, allowing us to establish spectroscopic fingerprints for the different solid crystalline phases of regioregular P3BT. Sample 1 (predominantly form I) and sample 2 (predominantly form I′) contain both thiophene main-chain packing types a and b with planar main-chain conformation and ordered side-chain structure. The coexistence of both packing a and b types was not recognized earlier because each of them causes nearly identical WAXD pattern. Additionally, samples 1 and 2 contain an amorphous phase and most likely also a mesophase characteristic of twisted main chains as well as disordered side chains. In contrast, sample 3 (predominantly form II) contains only one type of crystalline packing of main chain, namely packing c with interdigitated butyl side chains characteristic of form II. It has a dynamically twisted main chain, whose timeaverage is a planar conformation. We also observe an amorphous phase and, likely, a semiordered lamellar phase, which resembles a liquid-crystalline modification (which we did not analyze further). Our correlation between WAXD, solid-state NMR, and IR is beneficial for an improved understanding of organic field-effect transistors and other optoelectronics applications, since optimized design of such devices urgently needs wellunderstood structure−property relations. A unique advantage of IR spectroscopy is its easy application for thin-film characterization, providing quantitative information on relative amounts and orientations of crystalline packing types. In contrast, solid-state NMR intensities often depend on the specific measurements conditions, whose sensitivity is inferior to IR methods. Thus, our IR fingerprints of specific packing

schemes of regioregular P3BT as summarized in Table 2 are highly useful for improved understanding of morphology issues of related poly(3-alkylthiophenes).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01828. FTIR spectrum deconvolution; low-temperature FTIR spectra analysis; SI references; Tables S1−S7; Figures S1−S6; packings a, b, and c in CIF format (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (J.Z.). *E-mail [email protected] (M.R.H.). ORCID

Jianming Zhang: 0000-0003-1602-5010 Michael Ryan Hansen: 0000-0001-7114-8051 Author Contributions

Y.Y. and J.S. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support to initiate our collaboration at the Max Planck Institute for Polymer Research was provided by the International Max Planck Research School (Y.Y.), Max Planck Society (J.S.), Alexander von Humboldt Foundation (J.Z.), and European Commission (Marie Curie Intra-European Fellowship; A.K.). A.K. thanks National Science Centre Poland for funding through the grant NCN 2013/08/M/ST5/00914. M.R.H. acknowledges financial support from the Villum Foundation, Denmark, under the Young Investigator Programme (VKR023122). Y.Y. thanks the financial support from National Natural Science Foundation of China (21604047), Natural Science Foundation of Shandong Province (ZR2016BB28). J.S. thanks the financial support of National Natural Science Foundation of China (21303111, 21673148). J. Z. thanks the financial support from National Natural Science Foundation of China (21274071), Taishan Mountain Scholar Constructive Engineering Foundation (tshw20110510), and Natural Science Fund for Distinguished Young Scholars of Shandong Province (JQ200905). Polish countrywide license for Accelrys Materials Studio (ICM, University of Warsaw) provided by Wroclaw Centre for Networking and Supercomputing is acknowledged by K.K. and A.K. The authors thank Dr. B. Hou for technical support with WAXD measurements.

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REFERENCES

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