Crystalline Structural Evolution in Polythiophene‐Based Conjugated Polymer Blends During Rubbing

ABSTRACT

The ability to tailor the multilevel crystalline structures of conjugated polymers is highly desirable to establish the structure‐property relationship. However, investigations into conjugated polymer blends are somewhat limited. Herein, we report crystalline structural evolution in a series of poly(3‐alkylthiophene) (P3AT)‐based P3AT/P3AT blends containing various alkyl side chains by mechanical rubbing, including cocrystalline and phase‐separated structures, crystal orientations (i.e., edge‐on and face‐on), and large‐scale anisotropic crystalline structures. The as‐cast poly(3‐hexylthiophene)/poly(3‐octylthiophene) (P3HT/P3OT) and P3HT/poly(3‐dodecylthiophene) (P3HT/P3DDT) blended films show phase‐separated structures. Upon rubbing at increased temperatures, the P3HT/P3OT blend transforms into cocrystals at 60°C and 120°C, while P3HT/P3DDT blend becomes partially cocrystal at 60°C and changes back to phase‐separated structures at 120°C. Simultaneously, both P3HT/P3OT and P3HT/P3DDT blends transition from the edge‐on orientation in the as‐cast state to mixed edge‐on and face‐on orientations after rubbing at 60°C, and then back to the edge‐on orientation after rubbing at 120°C. On a larger scale, both blends become anisotropically aligned crystalline films along the rubbing direction. Formation mechanisms of these multilevel crystalline structures in P3AT/P3AT blends are elucidated.

The evolution of crystalline structures, including crystal orientations (i.e., edge‐on and face‐on), cocrystalline and phase‐separated structures, and large‐scale aligned structures is scrutinized in a series of poly(3‐alkylthiophene) (P3AT)‐based P3AT/P3AT blends during mechanical rubbing. Formation mechanisms of these different crystalline structures are elucidated.

1. Introduction

Since the discovery of conductive polyacetylene, conjugated polymers have attracted considerable interest over the past decades [1, 2, 3]. Due to their unique molecular structures, i.e., the combination of rigid backbones and flexible alkyl side chains, diverse chain conformations, and complex intra‐ and intermolecular interactions, conjugated polymers exhibit multiscale complex microstructures [4, 5]. At the nano/microscale, distinct crystal orientations (edge‐on, face‐on, and flat‐on) greatly influence the charge transport in various optoelectronic devices [6, 7, 8, 9]. Recent work showed that the bimodal edge‐on and face‐on orientations demonstrated superior charge transport owing to the formation of a three‐dimensional network structure [10]. At the macroscale, uniaxially aligned crystalline films facilitate the anisotropic charge transport to further improve the device performance [11, 12]. Currently, some processing strategies have been demonstrated to regulate the multiscale microstructures of conjugated polymers effectively, such as epitaxial orientation [13, 14], thermal and solvent vapor annealing [15, 16], rubbing [17], etc. Mechanical rubbing is an efficient strategy to produce large‐scale oriented crystalline films of conjugated polymers. For example, it showed that rubbing effectively transformed the poly(2,5‐bis(3‐dodecyl‐2‐yl)‐thieno[3,2‐b]thiophene) (C12‐pBTTT) films from the edge‐on to face‐on crystals, as well as produced uniaxial alignment of face‐on crystals in a large area [18].

Compared to single‐component conjugated polymers, the blending of conjugated polymers provides the possibility to render new structures and collective properties to the blends over the single component [19, 20]. Moreover, phase separation between different components and their cocrystallization, representing two competitive processes, can be involved in the conjugated polymer blends. Compared to the cocrystals of small molecules formed via noncovalent interactions, it is challenging to form polymer cocrystals as they need to meet three prerequisites, including structural similarity, close crystallization kinetics, and potential energies [21, 22]. Thus, only a few conjugated polymer cocrystals have ever been reported [23, 24, 25]. For example, our recent work demonstrated that the cocrystallization in polythiophene‐based blends and block copolymers leads to increased charge mobilities compared with their homopolymers [22, 26, 27]. Surprisingly, in spite of the effective characteristics of mechanical rubbing, utilizing it to control the cocrystals and crystal orientations in blends of conjugated polymers is rather limited.

In this work, we report the crystalline structural evolution in poly(3‐alkylthiophene) (P3AT)‐based P3AT/P3AT blends during mechanical rubbing. Poly(3‐hexylthiophene) (P3HT), poly(3‐octylthiophene) (P3OT), and poly(3‐dodecylthiophene) (P3DDT) were chosen to form two blends, i.e., P3HT/P3OT and P3HT/P3DDT blends. Both blends phase separated into respective crystalline domains in the as‐cast state. After rubbing at increased temperatures, the P3HT/P3OT blend changed to a cocrystalline structure. The P3HT/P3DDT blend became a partial cocrystalline structure after rubbing at 60°C, then back to a phase‐separated structure at 120°C. Simultaneously, both P3HT/P3OT and P3HT/P3DDT blends transitioned from the edge‐on (as‐cast state) to the mixed edge‐on and face‐on (rubbing at 60°C) and then back to the edge‐on orientation (rubbing at 120°C). On a large scale, they displayed anisotropically aligned crystals rendered by rubbing. Formation mechanisms of these multilevel crystalline structures in P3AT/P3AT blends are elucidated. Overall, this simple and robust rubbing strategy demonstrates the crystalline structural evolution of P3AT/P3AT blends, including cocrystalline and phase‐separated structures, crystal orientations (i.e., edge‐on and face‐on), and large‐scale anisotropic crystalline structures, which provide a deep fundamental understanding on the conjugated polymer blends.

2. Results and Discussion

2.1. Crystalline Structures of P3AT Homopolymers in the As‐Cast State

P3HT (M _n = 5.7 kg/mol, Ð = 1.17), P3OT (M _n = 4.5 kg/mol, Ð = 1.12), and P3DDT (M _n = 8.3 kg/mol, Ð = 1.14) were prepared through Kumada catalyst‐transfer polycondensation (KCTP) (Figures S1–S3 and Table S1) [28, 29]. The chain packing and crystalline structures of three P3AT single homopolymers (P3HT, P3OT, and P3DDT) before blending and rubbing were investigated by synchrotron 2D grazing incidence wide angle X‐ray scattering (2D‐GIWAXS) (Figure 1a). The as‐cast P3HT, P3OT, and P3DDT films showed their respective (100) diffraction peaks in the out‐of‐plane (q _z) direction at 4.0, 3.3, and 2.5 nm⁻¹, corresponding to the interlayer spacing (d ₁₀₀) of 1.57, 1.90, and 2.51 nm, respectively (Figure 1b,c). The d ₁₀₀ values increased monotonically with the extended alkyl side chains of P3ATs. All three P3ATs displayed a major edge‐on mode, in which their alkyl chains (100) direction was perpendicular and the π–π stacking (010) direction was parallel to the substrate, respectively (Figure 1d). Compared to the arc‐like (100) peaks of P3HT and P3DDT, the P3OT film showed a ring‐like (100) peak, suggesting more randomly oriented crystals of P3OT than P3HT and P3DDT.

FIGURE 1.

(a) Schematic illustration of the 2D‐GIWAXS measurement. (b) 2D‐GIWAXS images and their (c) 1D‐GIWAXS images of P3HT, P3OT and P3DDT single homopolymers in the as‐cast state. (d) Schematic illustration of the edge‐on orientation in the as‐cast P3HT film as a representative example. a, b, c‐axis: alkyl stacking (100), π–π stacking (010), and main backbone (001) directions, respectively.

2.2. Crystalline Structures of P3AT/P3AT Blends in the As‐Cast State and After Rubbing

Subsequently, these three P3AT homopolymers were blended with each other to form two blends (P3HT/P3OT and P3HT/P3DDT blends). These two as‐cast blended films were then subjected to a rubbing process at increased temperatures (Figure 2a). Their crystalline structures were investigated by 2D‐GIWAXS in both directions, that is, the X‐ray was parallel (∥) or perpendicular (⊥) to the rubbing direction (Figure 2b). The P3HT/P3OT blended films were first analyzed in parallel images (Figure 2c,d). The as‐cast P3HT/P3OT blended film before rubbing formed phase‐separated structures (Figure 2g, left panel), as evidenced by the (100) diffraction peaks of both P3HT and P3OT appearing in the q _z direction (q _z = 3.9 nm⁻¹ for P3HT and q _z = 3.3 nm⁻¹ for P3OT) (Figure 2c,d). Both P3HT and P3OT showed the edge‐on orientation, similar to their homopolymers (Figure 2h, left panel). After rubbing at 60°C, a new (100) peak (q = 3.6 nm⁻¹, d ₁₀₀ = 1.75 nm) appeared in both q _z and q _x,y directions (Figure 2c,d). Because P3HT and P3OT had similar molecular structures and this peak was situated between that of P3HT and P3OT, it was ascribed to the P3HT/P3OT cocrystals (Figure 2g, right panel). The hexyl and octyl side chains of P3HT and P3OT interdigitated with each other in the (100) direction and yielded a single and balanced d ₁₀₀. The (100) peak of cocrystals was positioned in both q _z and q _x,y directions, implying the coexistence of both edge‐on and face‐on cocrystals under this condition, in which the π–π stacking was perpendicular to the substrate and alkyl side chains were parallel to the substrate in the latter (Figure 2h, middle panel).

FIGURE 2.

(a) Schematic illustration of the rubbing process. A rotating cylinder, covered with microfiber nylon fabric, moves on the sample surface at increased temperatures. (b) Description of the 2D‐GIWAXS measurement. (c, e) 2D‐GIWAXS images and their (d,f) 1D‐GIWAXS images of P3HT/P3OT blends in the as‐cast film before rubbing, and after rubbing at 60°C and 120°C. The X‐ray beam was (c, d) parallel (∥) or (e,f) perpendicular (⊥) to the rubbing direction, respectively. (g, h) Schematic illustration of the chain packing and crystalline structures in P3HT/P3OT blended films via rubbing. In (g), phase‐separated structures of P3HT/P3OT blends and their cocrystalline structures are described. In (h), the P3HT/P3OT crystals are isotropically aligned in the edge‐on orientation before rubbing. After rubbing at increased temperatures, they formed cocrystals with anisotropically coexisted face‐on and edge‐on orientation at 60°C, and then changed into anisotropic edge‐on cocrystals at 120°C. In (h), the purple, pink, and yellow lamellae represent P3HT crystal, P3OT crystal, and the P3HT/P3OT cocrystals, respectively.

Interestingly, the (100) peak of P3HT/P3OT cocrystals in the q _x,y direction disappeared when further increasing the rubbing temperature to 120°C, leaving behind the (100) peak of P3HT/P3OT cocrystals with stronger signal in the q _z direction (Figure 2c,d). It indicated the face‐on cocrystals formed at 60°C were transformed into the edge‐on orientation after rubbing at higher 120°C (Figure 2h, right panel). For comparison, the P3HT/P3OT blended films were also thermal‐annealed at 60°C and 120°C without rubbing. However, they remained phase‐separated structure in the edge‐on orientation all the time (Figure S4). It proved the formation of P3HT/P3OT cocrystals and their crystal orientation changes were rendered by rubbing.

The content of edge‐on and face‐on crystals in the P3HT/P3OT blended films was calculated by the polar figures (see detailed calculation in the Supporting Information, Figure S5a). The edge‐on content of the respective P3HT and P3OT in the as‐cast state is 69.8% and 58.4%, respectively. After rubbing, the P3HT/P3OT cocrystals had the edge‐on content of 6.3% at 60 °C and 96.3% at 120 °C, respectively (Figure S5b). The (010) diffraction peaks of P3HT/P3OT blends (q = 16.2–16.4 nm⁻¹, d ₀₁₀ = 0.38–0.39 nm) concentrated along the in‐plane (q _x,y) direction in the as‐cast state before rubbing (Figure S6). After rubbing, the (010) peak concentrated along the out‐of‐plane (q _z) direction at 60°C, and then reverted back to the q _x,y direction at 120°C. These changes indicated the crystal orientation evolution from the edge‐on orientation (as‐cast) to mixed edge‐on and face‐on orientation (after rubbing at 60°C), and back to the edge‐on orientation (after rubbing at 120°C), which agreed well with the analysis obtained from the (100) diffraction peaks.

Interestingly, the (100) peak of P3HT/P3OT cocrystals in the q _x,y direction disappeared when further increasing the rubbing temperature to 120°C, leaving behind the (100) peak of P3HT/P3OT cocrystals with a stronger signal in the q _z direction (Figure 2c,d). It indicated the face‐on cocrystals formed at 60°C were transformed into the edge‐on orientation after rubbing at a higher temperature of 120°C (Figure 2h, right panel). For comparison, the P3HT/P3OT blended films were also thermal‐annealed at 60°C and 120°C without rubbing. However, they remained phase‐separated structure in the edge‐on orientation all the time (Figure S4). It proved the formation of P3HT/P3OT cocrystals, and their crystal orientation changes were rendered by rubbing.

Subsequently, the P3HT/P3OT blended films were analyzed when the X‐ray was perpendicular to the rubbing direction, which can reflect the large‐scale crystal alignment in the blended films (Figure 2e,f). The as‐cast film was the same as the parallel image, proving isotropic crystals in the P3HT/P3OT blended film before rubbing (Figure 2h, left panel). However, the (100) cocrystal peak (q = 3.6 nm⁻¹) of 60°C‐rubbed film was only observed in the q _z direction in the perpendicular image, in sharp contrast to the parallel image exhibiting the (100) cocrystal peak at both q _z and q _x,y directions. It was because the X‐ray beam was not scattered by the periodic side‐chain lamellae of P3HT/P3OT in the q _x,y direction (Figure 2b) [30]. It indicated the formation of highly anisotropically aligned crystals after rubbing at 60°C (Figure 2h, middle panel). As evidenced by the obviously narrowed (100) peak in the perpendicular image compared to the parallel image, such a highly anisotropically aligned crystalline structure remained in the 120°C‐rubbed P3HT/P3OT sample (Figure 2h, right panel).

Subsequently, the second P3HT/P3DDT blend films in the parallel direction were analyzed (Figure 3a,b). Similar to the P3HT/P3OT blend, the as‐cast P3HT/P3DDT blended film also showed phase‐separated structures of P3HT (q _z = 3.9 nm⁻¹, d ₁₀₀ = 1.61 nm) and P3DDT (q _z = 2.6 nm⁻¹, d ₁₀₀ = 2.42 nm) and both of them showed the edge‐on orientation (Figure 3e,f, left panel). However, this blend behaved differently during the rubbing process. After rubbing at 60°C, the (100) peak of P3HT remained, accompanied by the appearance of a new (100) peak (q _z = 3.0 nm⁻¹, d ₁₀₀ = 2.09 nm) in both q _z and q _x,y directions. This peak was ascribed to the cocrystals of P3HT and P3DDT as its lamellar spacing d ₁₀₀ was between that of P3HT homopolymer (d ₁₀₀ = 1.61 nm) and P3DDT homopolymer (d ₁₀₀ = 2.42 nm) (Figure 3e, right panel). It implied that after rubbing at 60°C, some P3HT chains cocrystallized with P3DDT in mixed edge‐on and face‐on orientations, and the other part of the P3HT chains remained in sole crystallization (Figure 3f, middle panel). Compared with the formation of complete cocrystals in P3HT/P3OT blend after rubbing, the partial cocrystals in P3HT/P3DDT blend indicated that P3HT and P3DDT had a weaker cocrystallization ability than P3HT and P3OT.

FIGURE 3.

(a, c) 2D‐GIWAXS images and the corresponding (b, d) 1D‐GIWAXS images of P3HT/P3DDT blended films in the as‐cast state before rubbing, and after rubbing at 60°C and 120°C. The X‐ray beam was (a, b) parallel (∥) or (c, d) perpendicular (⊥) to the rubbing direction, respectively. (e, f) Schematic illustration of the chain packing and crystalline structures in P3HT/P3DDT blended films via rubbing. In (e), phase‐separated structures of P3HT/P3DDT blends and their partial cocrystalline structures are described. In (f), the P3HT/P3DDT crystals are isotropically aligned in the edge‐on orientation before rubbing. After rubbing at increased temperatures, they formed partial cocrystals with anisotropically coexisted face‐on and edge‐on orientation at 60°C, and then changed into phase‐separated structures with anisotropic edge‐on orientation at 120°C. The purple, red, and yellow lamellae represent P3HT crystal, P3DDT crystal, and the P3HT/P3DDT cocrystals, respectively.

When increasing the rubbing temperature to 120°C, the P3HT/P3DDT cocrystals formed at 60°C were unstable and changed into respective P3HT and P3DDT crystal domains, as evidenced by two (100) peaks of P3HT (q _z = 3.9 nm⁻¹) and P3DDT (q _z = 2.6 nm⁻¹), respectively. At the same time, the previous (100) peak of cocrystals at q _x,y direction disappeared. It indicated the P3HT/P3DDT cocrystals in mixed edge‐on and face‐on orientations transitioned to respective P3HT and P3DDT crystals in the edge‐on orientation after rubbing at 120°C (Figure 3f, right panel). As for the content of edge‐on and face‐on crystals, and the (010) diffraction peaks of the P3HT/P3DDT blended films, they displayed similar trends to the P3HT/P3OT blend (Figures S7 and S8).

Compared to the parallel images, the main difference in the perpendicular images of the P3HT/P3DDT blend existed in their state after rubbing at 60°C. We can see the two (100) peaks of P3HT crystals and P3HT/P3DDT cocrystals only existed in the q _z direction in the perpendicular image, in sharp contrast to their parallel image displaying them in both q _z and q _x,y directions (Figure 3c,d). It proved anisotropically aligned crystals of P3HT/P3DDT along the rubbing direction.

2.3. Morphologies of P3AT/P3AT Blends After Rubbing

After elucidating how the crystalline structures of P3HT/P3OT and P3HT/P3DDT blends change during the rubbing process, their morphologies were further examined by optical microscopy (OM), cross‐polarized optical microscopy (CPOM), as well as atomic force microscopy (AFM) at different scales. Taking the P3HT/P3OT blended films as an example, after rubbing at 120°C, it changed from red to light brown as the transmissive axis of the polarizer rotated from parallel to perpendicular to the rubbing direction (Figure 4a). The color change indicated the presence of highly oriented crystalline structures in a large area. By CPOM, the rubbed P3HT/P3OT film at 120°C showed microgrooves along the rubbing direction. Moreover, it displayed strong birefringence when the rotational angle relative to the crossed polarizer was 45° and 135°, but extincted completely at 0°, 90°, and 180° (Figure 4c), which differed greatly from the unrubbed P3HT/P3OT film (always dark by CPOM) (Figure S9). The luminous intensity had fourfold symmetry (Figure 4b). The CPOM results are in line with the 2D‐GIWAXS results and again proved highly oriented crystalline structures in the blended films rendered by rubbing. By AFM observation, oriented stripe‐like morphology was observed in the rubbed P3HT/P3OT blended film, significantly different from the isotropic nodular morphology in the as‐cast state (Figure 4d,e). Similarly, periodic light on and extinction and microgroove morphology on a large scale were observed in the rubbed P3HT/P3DDT blended film (Figures S10 and S11). The combination of OM, CPOM, and AFM measurements clearly displays large‐scale anisotropic crystal alignment of the P3AT/P3AT blends after rubbing.

FIGURE 4.

(a) Optical micrograph of the P3HT/P3OT film rubbed at 120°C, imaged with the optical‐transmission axis of the polarizer parallel (∥) and perpendicular (⊥) to the rubbing direction, respectively. The rubbing direction (R) was marked by a white arrow in (a). (b) Polar plot depicting the gray scale from (c). (c) CPOM images of the P3HT/P3OT film rubbed at 120°C, captured by rotating the film at 15° intervals between the polarizer and the rubbing direction. (d, e) AFM images of the P3HT/P3OT film in the (d) as‐cast state and (e) after rubbing at 120°C. The rubbing direction (R) was marked by a white arrow in (e).

2.4. Formation Mechanisms of Multilevel Crystalline Structures of P3AT/P3AT Blends After Rubbing

The above results clearly demonstrate that the rubbing not only induced large‐area anisotropically aligned crystals in the P3AT/P3AT blends at the macroscale, but also changed their cocrystalline and phase‐separated structures, and crystal orientations at the micro/nanoscale. The formation mechanism of large‐area anisotropically aligned crystals in these blends was first discussed. It is reported that rubbing at increased temperatures could melt the side chains of P3ATs [18]. Thus, it weakened the entanglement of P3AT/P3AT blended chains and greatly improved the film plasticity during the rubbing process. Simultaneously, some oriented chains were produced as seeds and guided the subsequent growth of crystal domains. Therefore, the P3AT/P3AT blended films with greatly enhanced plasticity were readily aligned along the rubbing direction to form large‐area anisotropically aligned crystals.

Subsequently, we discuss the cocrystallization and phase separation in the P3AT/P3AT blends rendered by rubbing. Both P3HT/P3OT and P3HT/P3DDT blends displayed phase‐separated structures in the as‐cast state. After rubbing, P3HT/P3OT blend became cocrystals at 60°C and remained cocrystals at 120°C. Differently, some P3HT cocrystallized with P3DDT in the P3HT/P3DDT blend after rubbing at 60°C, and other P3HT remained sole crystals simultaneously. After rubbing at 120°C, P3HT and P3DDT changed back to phase‐separated structures. All these results indicate the cocrystallization ability is P3HT/P3OT > P3HT/P3DDT. Different cocrystallization abilities could be analyzed from their crystal coherence length (reflecting the crystal domain size) along the (100) direction (L ₁₀₀) (see the detailed calculation in Supporting Information, Table S2). The P3HT and P3OT exhibited close L ₁₀₀ of 14.5 and 13.6 nm in the as‐cast P3HT/P3OT blend, respectively. The P3HT and P3DDT showed L ₁₀₀ of 11.2 and 5.2 nm in the as‐cast P3HT/P3DDT blend, respectively, with a larger ΔL ₁₀₀ gap than that of P3HT/P3OT blend. It implied a better compatibility between P3HT and P3OT in their blend. Therefore, P3HT/P3OT blend had a stronger cocrystallization ability than P3HT/P3DDT blend.

Next, the evolution of crystal orientation in the P3AT/P3AT blends was discussed. Both P3HT/P3OT and P3HT/P3DDT blends formed mixed edge‐on and face‐on crystals after rubbing at 60°C, but reverted back to edge‐on crystals after rubbing at 120°C. To scrutinize why the blends behaved like that, the control samples of three single homopolymer films, P3HT, P3OT, and P3DDT were rubbed under the same conditions. We can see the P3HT homopolymer changed from edge‐on orientation before rubbing to mixed edge‐on and face‐on orientation after rubbing at 60°C, then resumed back to the edge‐on orientation after rubbing at 120°C (Figure S12a). In contrast, P3OT and P3DDT remained major edge‐on orientation when rubbed at both 60°C and 120°C (Figure S12b,c). It indicated the P3HT dominated the crystal orientation transition in the P3HT/P3OT and P3HT/P3DDT blends. There is a competition between rubbing‐induced face‐on orientation and heating‐induced edge‐on orientation for P3HT. At 60°C, the shear force rendered by rubbing aligned the polymer backbone parallel to the substrate and π–π stacking perpendicular to the substrate, producing the face‐on orientation. At 120°C, the thermal annealing facilitated the formation of more thermodynamically stable edge‐on orientation. Therefore, both P3HT/P3OT and P3HT/P3DDT blends adopted mixed face‐on and edge‐on orientations after rubbing at 60°C, while recovered to the edge‐on orientation after rubbing at 120°C.

2.5. Raman and Optical Properties of P3AT/P3AT Blends After Rubbing

After thoroughly elucidating the crystalline structures and morphologies in these P3HT/P3OT and P3HT/P3DDT blends, the Raman and optical properties were characterized by Polarized Raman spectroscopy and polarized UV–vis spectroscopy (Figure 5 and Figures S13 and S14). Taking the P3HT/P3OT blended films as an example, Raman measurement was performed with the polarization vector parallel (θ = 0°) or perpendicular (θ = 90°) to the rubbing direction. In both directions, the Raman peaks were observed at 1445 and 1380 cm⁻¹, which were attributed to the stretching vibration of the C═C and C─C bonds on the thiophene ring, respectively (Figure 5a) [31]. Because these peaks are highly sensitive to the delocalization of π‐electrons on the P3AT backbones, the degree of alignment can be quantified by the dichroic ratio (DR_Raman = I _0°/I _90°) of the peak intensity. Unlike the DR_Raman values (∼ 1) of their blended films in the as‐cast state before rubbing, the average DR_Raman of P3HT/P3OT and P3HT/P3DDT blended films after rubbing at 120°C greatly increased to 5.15 and 5.62, respectively (Figure 5b). This is because rubbing greatly improved the orientation of the P3AT backbone, leading to much higher DR_Raman values than their as‐cast films.

FIGURE 5.

(a) Polarized Raman and (c) polarized UV−vis spectra of P3HT/P3OT film after rubbing at 120°C. In (a), the angle (0° or 90°) indicates the polarization of the incident light is parallel or perpendicular to the rubbing direction, respectively. In (c), the angle (θ: 0°–90°) is between the polarizer axis and the rubbing direction. (b, d) The corresponding (b) DR_Raman and (d) DR_UV of P3HT/P3OT and P3HT/P3DDT blended films before and after rubbing at 120°C.

Polarized UV–vis spectroscopy was further used to analyze the optical properties of P3HT/P3OT and P3HT/P3DDT blended films after rubbing at 120°C. The P3HT/P3OT blended film showed three characteristic absorption peaks around 510, 550, and 600 nm (Figure 5c). The main absorption peak at 510 nm belongs to the intra‐chain π‐π* transition of polythiophenes, and the two shoulder peaks near 550 and 600 nm correspond to the inter‐chain π‐π interaction [32]. The degree of alignment can be quantified by the DR_UV of absorbance (A) when the light beam was parallel to the rubbing direction (θ = 0°) to that in the perpendicular direction (θ = 90°), i.e., DR_UV = A _0°/A _90°. Before rubbing, the DR_UV values of these two blended films were all around 1, proving the isotropically aligned crystals. After rubbing at 120°C, their DR_UV values increased to 2.56 and 2.86 for P3HT/P3OT and P3HT/P3DDT blended films, respectively (Figure 5d). Clearly, the increased DR_UV values after rubbing indicated the presence of large‐scale anisotropically aligned crystals in these blends. Interestingly, the DR_Raman values were nearly twice of DR_UV. A possible reason is Raman spectroscopy measures the change of the surface structures through reflection, while UV–vis spectroscopy measures the change of the surface and bulk structures through transmission. Since rubbing is a process more acting on the polymer surface, the DR values from UV–vis are smaller than those from Raman spectroscopy.

3. Conclusions

In summary, we unraveled the transition between phase separation and cocrystallization of P3HT/P3OT and P3HT/P3DDT blends induced by a robust rubbing strategy, as well as their changes in crystal orientations (i.e., edge‐on and face‐on) and large‐scale anisotropically aligned crystalline films. Specifically, upon rubbing at increased temperatures, P3HT/P3OT blended films changed from a phase‐separated structure in the as‐cast state to their cocrystalline structures after rubbing at 60°C and 120°C. Different from P3HT/P3OT blend, the P3HT/P3DDT blend changed from phase‐separated structure in the as‐cast state to a partial cocrystalline structure after rubbing at 60°C, then back to phase‐separated structure after rubbing at 120°C, indicating a weaker cocrystallization ability than that of P3HT/P3OT blend. Simultaneously, as a competition existed between the rubbing‐induced face‐on orientation and the heating‐induced edge‐on orientation in P3HT, their crystal orientation transitioned from the edge‐on in the as‐cast state to the mixed edge‐on and face‐on after rubbing at 60°C and then back to the edge‐on orientation after rubbing at 120°C. At a larger scale, the rubbing rendered these two blended films into anisotropically aligned crystals. These changes in multilevel crystalline structures and associated kinetics are revealed via a suite of techniques (i.e., GIWAXS, CPOM, AFM, Raman, and UV−vis spectroscopy). Clearly, this study demonstrates the crystalline structural evolution in conjugated polymer blends during rubbing, providing a deeper fundamental understanding on the interplay between different conjugated components and their response to external fields.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: marc70314‐sup‐0001‐SuppMat.pdf.

Data Availability Statement

The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.