Face-On and Edge-On Orientation Transition and Self-Epitaxial

Article pubs.acs.org/Macromolecules

Face-On and Edge-On Orientation Transition and Self-Epitaxial Crystallization of All-Conjugated Diblock Copolymer Hua Yang, Rui Zhang, Lei Wang, Jidong Zhang, Xinhong Yu, Jiangang Liu, Rubo Xing, Yanhou Geng, and Yanchun Han* State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, China Dongguan Institute of Neutron Science, No. 1 Zhongziyuan Road, Dalang, Dongguan 523803, China S Supporting Information *

ABSTRACT: The orientation transition and self-epitaxial crystallization of all-conjugated diblock copolymers poly(pphenylene)-block-(3-hexylthiophene) (PPP-b-P3HT) were systematically investigated by in situ temperature-resolved two-dimensional grazing incidence X-ray diffraction (2D GIXD) in step-by-step heating and cooling process. B39T18 was selected; the results of 2D GIXD showed that the PPP block crystal adopted a face-on orientation while the crystallization of P3HT block was hindered in as-casted films. Three different molecular orientations transition were obtained in self-epitaxial crystallization circles. First, P3HT crystallize with edge-on during the heating process and induced the PPP blocks crystallized with edge-on during the cooling process. Then, the as-casted film was heated in the melting temperature region of PPP blocks and isothermally crystallized. The partial melting of PPP blocks promoted the P3HT blocks crystallize in a face-on due to the steric limitation effect; PPP blocks crystallized with a face-on via the self-epitaxy during cooling. Furthermore, the face-on transformed to thermodynamically stable edge-on in the melt annealing process.

1. INTRODUCTION Because of the anisotropy of transport properties, tuning a proper molecular orientation is crucial to optimize the final performance of different electronic devices based on conjugated polymers.1−8 As to field-effect transistors, the favored molecular orientation is edge-on, in which the directions of π−π stacking and backbones beneficial for carrier transport between electrodes are both in-plane.9−11 As to photovoltaic devices, the face-on molecular orientation is preferred for a better carrier transport in the vertical direction of active layers.11−14 In addition to field-effect transistors and photovoltaic devices, the molecular orientation of conjugated polymers also influences the photovoltaic performance and thermoelectric behavior of the corresponding devices based on conjugated polymers.15−17 In our previous work, the molecular orientation can be controlled to transform from face-on to edge-on in PPP-bP3HT block copolymer films by melting annealing. However, this transition is irreversible because the edge-on orientation is a more stable state in thermodynamics.18−20 Therefore, the reverse transition from edge-on to face-on seems not to be favored. However, it is important to demonstrate this disfavorable molecular transition in order to understand the self-assembly behavior of conjugated polymers and develop higher performance of photovoltaic devices. Along with thermal annealing as a common method to tune molecular orientations, epitaxy crystallization is another © 2015 American Chemical Society

effective way of controlling in-plane anisotropy and molecular orientations.21−23 Because of the close lattice parameters between polymers and the template crystals, the polymer crystal could grow along the predominant lattice direction of templates. A preparation of aligned P3HT films can be gotten by epitaxy crystallization induced by crystalline solvent like TCB. Brinkmann et al. reported that the molecules displayed a face-on orientation in the spherulite films due to the epitaxy effect of TCB crystals by blending TCB into solvent of P3HT solutions.23 However, the tuning of alignment or orientation of molecules by epitaxy is often achieved through adding a heterogeneous substance like TCB or using specific patterning substrates. To realize a spontaneous epitaxy without heterogeneity, in other words, to tune the molecular orientation by “self-epitaxy” is preferred, which prevents the operation of removing templates like TCB or the potential disadvantages of heterogeneity. Based on this idea, crystalline diblock copolymers are good candidates. Because of the covalent linking of blocks, an aligned crystallization finished ahead may first act as an epitaxy template to induce the alignment of crystals of the other block if they have close lattice periods. Therefore, by considering the different dominant molecular orientations of Received: August 31, 2015 Revised: September 24, 2015 Published: October 9, 2015 7557

DOI: 10.1021/acs.macromol.5b01804 Macromolecules 2015, 48, 7557−7566

Article

Macromolecules

IPXRD, the scanning speed is 5 s per step with 0.05° step size (2θ). The measurements were obtained in a scanning interval of 2θ between 3° and 30°. Two-dimensional GIXRD images were conducted at the BL1W1A beamline of Beijing Synchrotron Radiation Facility (BSRF) (λ = 1.54 Å) and BL14B1 beamlineof Shanghai Synchrotron Radiation Facility (SSRF) (λ = 1.24 Å). The incidence angle is 0.16°, and the exposure time is 60 s. In situ temperature-resolved synchrotron GIXD experiments were performed at BL1W1A beamline of BSRF. A TA Instruments Q2000 differential scanning calorimeter (DSC) was used to probe the thermal transitions of the polymers at a heating/ cooling rate of 10/−10 °C/min under a nitrogen flow. The thermal behaviors of these diblock copolymers are characterized by DSC. The detailed data of melting and crystallization temperatures for all these copolymers and homopolymers are summarized in Table 1.

each blocks, we choose the block polymer of PPP-b-P3HT studied previously to investigate the “self-epitaxy” behavior. The 2D GIXD with area detector provides invaluable information with regard to molecular packing, crystallite size, and crystallite orientation owing to shape and intensity distribution of diffraction peaks in reciprocal (qxy−qz) space.2,3,5,24−26 In this work, the thermodynamic state of the PPP-b-P3HT block copolymer films is tuned by thermal annealing. 2D GIXD measurements were performed based on synchrotron source with high brightness, thus ensuring high-resolution diffraction events along with the information in qz (out-of-plane) and qxy (in-plane) directions recorded on a 2D area detector. By designing schemes of heating and melting crystallization, the melting subsequence and molten degree of different blocks are controlled. Through a systematic in situ GIXD characterization under a series of varying temperatures, we observed the evolution of molecular orientation of both blocks with their precedence of crystallization. According to the results, we proved the existence of self-epitaxy effect in block copolymers and proposed the corresponding mechanism.

Table 1. DSC Characteristics of Poly(p-phenylene)-block-(3hexylthiophene) (PPP-b-P3HT) Diblock Copolymers and Homopolymers

2. EXPERIMENTAL SECTION 2.1. Materials. The conjugated diblock copolymers poly[(pphenylene)-block-(3-hexylthiophene)]s (PPP-b-P3HT) with PPP/ P3HT molar ratio of 81:19, 68:32, and 48:52 were synthesized by the Grignard metathesis (GRIM) polymerization method according to our previous report.27 In the diblock copolymer PPP-b-P3HT, the molecular weights of different blocks were well-controlled through the feed ratio of the monomers and the initiator. The degrees of polymerization of PPP and P3HT blocks estimated from the H NMR spectrum were 39/9, 39/18, and 41/44, respectively. 2.2. Sample Preparation. The diblock copolymers were dissolved in a nonselective solvent, chlorobenzene (ClB) (purchased from Sigma-Aldrich Co.), with a concentration of 2−5 mg/mL. The solutions are heated up to 80 °C for a complete dissolution. After cooling to room temperature, the solutions were placed in a dark and vibrationless environment for over 24 h. The thick films of the homopolymers (P3HT and PPP homopolymers blends were taken for comparison) and diblock PPP-b-P3HT were prepared by drop-casting the corresponding solutions in chlorobenzene (20 μL) with concentrations of 2 mg/mL placed onto precleaned silicon wafers with dimensions of 1.5 cm × 1.5 cm. The samples were placed in the closed vessels at room temperature for the slow evaporation of the solvent. Prior to drop-casting, the wafers were obtained by (i) sonication in acetone for 30 min, (ii) sonication in ethanol for 30 min, (iii) cleaning with a 70/30 v/v solution of 98% H2SO4/30% H2O2 (piranha solution) between 90 and 110 °C for 30 min, and (iv) sonication in deionized water for 30 min and thoroughly rinsed. The substrates were subsequently dried in a flow of nitrogen. 2.3. Characterization. Grazing incidence X-ray diffraction (GIXD), in-plane X-ray diffraction (IPXRD), and two-dimensional GIXRD were performed to characterize the morphologies and the crystalline structures of the films. Differential scanning calorimetry (DSC) was performed to characterize the thermal properties of the homopolymers and the diblock copolymers. The grazing incidence X-ray diffraction (GIXRD) profiles were obtained by using a Bruker D8 Discover reflector with an X-ray generation power of 40 kV tube voltages and 40 mA tube current, using Cu Kα radiation (λ = 1.54 Å). The in-plane X-ray diffraction (IPXRD) profiles were obtained by using a Rigaku SmartLab X-ray diffractometer with an X-ray generation power of 40 kV tube voltage and 30 mA tube current. In situ temperature-resolved GIXD experiments were performed on a Rigaku SmartLab X-ray diffractometer equipped with the Anton Pear DHS 900 hot stage and the heating/cooling rate of 10/−10 °C/min under vacuum conditions. The diffraction was recorded in the 2θ−χ mode. In both GIXRD and

polymer

Tm(PPP) [°C]

B33 B39T9 B39T18 B41T44 T36

88.4 90.2 91.4 92.0

Tm(P3HT) [°C]

231.5 222.6

Tc(PPP) [°C] 36.9 51.3 74.0 76.4

Tc(P3HT) [°C]

149.5 189.6 191.4

3. RESULTS AND DISCUSSION 3.1. Results. In the following, we would choose B39T18, B39T9, and B41T44 to investigate whether the PPP blocks also have an epitaxy effect on crystallization of P3HT blocks in order to demonstrate a mutual transition between edge-on and face-on orientations. Compared with blends of polyphenylene and polythiophene without any covalent connection between them, the covalent bond between the two blocks in copolymers could lead to the interference of block crystallization by each other, in other words, a limitation effect. In our samples, we observed self-epitaxy crystallization induced by limitation effect only in B39T18, while such a phenomenon was not found in B41T44, B80T40, and homopolymers of polyphenylene and polythiophene. 3.1.1. PPP Epitaxially Grown as the Seed on P3HT. In Table 1, the melting point and crystallization temperature of PPP homopolymers are fixed to be 88.4 and 36.9 °C, respectively, and the melting point and crystallization temperature of P3HT are fixed to be 222.6 and 191.4 °C, respectively. In these diblock copolymers, the melting point and crystallization temperature of PPP blocks increase, while the melting point and crystallization temperature of P3HT blocks decrease compared to homopolymers. In DSC results, two melting peaks of B39T18 were observed at 92 and 188 °C during heating, corresponding to the melting point of PPP and P3HT block (Tm(PPP) = 92 °C; Tm(P3HT) = 188 °C), respectively. During cooling, the crystallization peak of PPP and P3HT blocks were observed at 75 and 175 °C, respectively (Tc(PPP) = 75 °C; Tc(P3HT) = 150 °C). Based on these results, the melting temperature of polymer is within a region not a fixed value, and three different results could be achieved by controlling annealing at 78−97 °C. In our previous work, it was found that the PPP and P3HT could crystallize in the orthorhombic system with unit cell: a = 21.20 Å, b = 3.78 Å, and c = 4.24 Å; a = 16.44 Å, b = 3.80 Å, and c = 7.80 Å, respectively.18,19 Moreover, the PPP block 7558


Article

Macromolecules

Figure 1. Temperature dependence of 2D GIXD patterns for B39T18 during the stepwise heating and cooling process: (a−d) heating stage; (d−g) cooling stage. The insets illustrate schematic diagrams for face-on orientations of PPP blocks in the pristine films and P3HT blocks. The insets in (d) and (e) represent the P3HT adopt edge-on orientations and the PPP blocks meltdown. The inset in (g) reveals the P3HT and PPP adopt edge-on orientations through thermal annealing.

Å, the (100) peak of P3HT blocks emerged, which represents the stacking of alkyl chain of P3HT blocks. These results imply that the edge-on nucleation of P3HT blocks occurred during their crystallization in the heating process as the melting of PPP blocks went on. When heating up to 120 °C (Figure 1d), the intensity of (100)T in the vertical direction at qz = 0.40 Å−1 increased and the in horizon direction at qxy = 0.40 Å−1 decreased. It meant that a large proportion of P3HT blocks adopted edge-on orientation. As the cooling process went on, we observed that only a peak along the vertical axis at qz = 0.40 Å−1 at 100 and 70 °C in Figures 1e and 1f. When the temperature was reduced to 30 °C as shown in Figure 1g, another diffraction peak emerged at qz = 0.30 Å−1, indicating a d-spacing of 21.20 Å. This peak could be associated with the diffraction peak of (100) B , suggesting the subsequent crystallization of PPP blocks with edge-on orientation. These results implied that the PPP blocks crystallized with an edge-on orientation induced by the edge-on P3HT crystals grown on P3HT as the seed via self-epitaxial crystallization during the cooling stage. The temperature-resolved one-dimensional out-of-plane WAXD profiles (Figure 2) provide a more intuitive approach to analyze the variation of (100) diffraction position and intensity belonging to side-chain stacking. As shown in Figure 2, a recognizable diffraction peak appeared at a 2θ angle of ∼21.1° in the pristine film, corresponding to a spacing distance

crystal adopts a face-on orientation while the P3HT adopts edge-on orientation in the pristine drop-casting film. To investigate the crystallization behavior of B39T18, we employed in situ temperature-resolved GIXD measurements. All samples were followed by stepwise heating and cooling at the rate of 10/−10 °C/min under vacuum conditions. In Figure 1, during the stepwise heating and cooling process for B39T18, the horizontal and vertical axis indicate the magnitude of scattering vector q in the xy plane and z directions, respectively. The major diffraction is confirmed at the horizon direction at qxy = 0.30 Å−1 corresponding to a d-spacing of 21.20 Å in pristine film as shown in Figure 1a, which was the diffraction peak of (100) indicating the packing of hexyloxy side chains of PPP blocks. It is more important that the diffraction peak displays distinct orientation, i.e., face-on orientation with the π−π stacking direction of P3HT backbones perpendicular to the substrate. The PPP block crystal adopted a face-on orientation while the crystallization of P3HT block was hindered in pristine film. When the temperature was decreased to 70 °C, the observed diffraction pattern (Figure 1b) has a close resemblance with the pristine film since the temperature is lower than the melting point of PPP blocks. After a further heating to 100 °C (Figure 1c), the intensity of diffraction at the horizon direction is surveyed at qxy = 0.30 Å−1, corresponding to the (100) of PPP blocks is disappeared. However, in the vertical direction at qz = 0.40 Å−1 indicating a d-spacing of 16.4 7559


Article

Macromolecules

occurrence of edge-on nucleation. As the temperature increased to 100 and 120 °C, the intensity of (100)T was increased. When the temperature increases to 100 and 70 °C, the intensity of (100)T increased further. Finally, the (100)B appeared as the temperature reduced to temperature lower than crystallization temperature of PPP, which indicated that PPP blocks also crystallized with edge-on orientation via the self-epitaxial crystallization from edge-on nuclears. It is noteworthy that the diffraction peak of (100)T appeared at a 2θ angle of ∼5.35°, indicating a spacing distance of 16.5 Å, which displayed a bit decrease of distance result from cooling effect. In general, the crystallization behaviors of PPP and P3HT blocks influenced each other with molecular packing more regular through thermal annealing. 3.1.2. P3HT Grown on PPP via Confined Epitaxial Crystallization. The meltdown process of polymers could be indicated as a melting range in DSC curves. Hence, when the pristine film was heated in the melting temperature region of PPP blocks, the partly molten PPP blocks promoted the extension of P3HT blocks, leading P3HT blocks to crystallize in a face-on orientation based on the face-on orientation of PPP blocks via the steric limitation effect. As shown in Figure 3, during the stepwise heating and cooling process for B39T18, we control the temperature of thermal annealing in the melting temperature region of PPP blocks for isothermal crystallization unlike the situation in Figure 1. It was first confirmed the PPP

Figure 2. Temperature dependence of 1D out-of-plane GIXD profiles for B39T18 during the stepwise heating and cooling process.

of 4.24 Å. This peak can be assigned to the reflection from crystallographic (001) plane of PPP crystals (denoted as(001)B), which coincides with the PPP adopting face-on orientation as shown in Figure 1a. When the temperature increased to 100 °C, a recognizable diffraction peak appeared at a 2θ angle of ∼5.2° corresponding to a spacing distance of 16.9 Å. This peak can be assigned to the reflection from the crystallographic (100) plane of P3HT crystals (denoted as (100)T), indicating that P3HT blocks crystallized with the

Figure 3. Temperature dependence of 2D GIXD patterns for B39T18 during the stepwise heating and cooling process: (a−e) heating stage; (e−i) cooling stage. 7560


Article

Macromolecules blocks adopted face-on orientation and the P3HT blocks were amorphous in pristine film as shown in Figure 3a, where major diffraction corresponded to (100) planes of PPP blocks along the horizon direction (in-plane) at qxy = 0.30 Å−1. When heating up to 80 °C (Figure 3c), we found that the intensity of diffraction at qxy = 0.30 Å−1 in the horizon direction, indicating the (100) plane of PPP blocks was receded. Meanwhile, a faint diffraction around qxy = 0.40 Å−1 emerged. Furthermore, when heating up to 90 °C and resting for 5 min (Figure 3d), the intensity of (100)B at qxy = 0.30 Å−1 in the horizon direction was decreased, revealing primary meltdown of fractional PPP with low molecular weight in the melting region. However, the intensity of (100)T at qxy = 0.40 Å−1 was increased. These results indicated that the face-on nucleation of P3HT, which was hindered in the pristine films, occurred, and the crystallization of P3HT blocks took place in the heating process as the meltdown of PPP blocks went on. It is speculated that the procedure is induced by steric hindrance, in concrete terms, when the PPP partial melting some interspace should be released for service of P3HT stretch. However, a large proportion of PPP adopted the face-on orientation. Hence, the P3HT only stretched in a confined space and consequently underwent the self-epitaxial crystallization by the unmolten PPP molecular, as a result of face-on molecular orientation of P3HT. After a further heating to 120 °C to surpass the melting range, all the PPP are molten as shown in Figure 3e. It was observed that (100)B diffraction at qxy = 0.30 Å−1 disappeared. However, the (100)T diffraction peak at qxy = 0.40 Å−1 indicated an unchanged face-on orientation. As the cooling process went on, the (100) diffraction peak of PPP block at qxy = 0.40 Å−1 was surveyed at 100 and 70 °C (Figure 3f,g). When the temperature decreased to 70 °C as shown in Figure 3h, another peak corresponding to the diffraction of (100)B emerged at qxy = 0.30 Å−1, suggesting that the subsequent crystallization of PPP blocks adopted a face-on orientation. These results imply that the face-on orientation of PPP blocks in crystals were induced by the face-on P3HT crystals grown on P3HT as the seed via self-epitaxial crystallization during cooling. It was found that the crystallization of P3HT block occurred during heating, while the diffraction of PPP blocks could be detected after their melting down. The molecular orientation of PPP blocks was face-on in the copolymers with longer PPP blocks, which was different from the edge-on orientation of P3HT blocks. Therefore, the face-on orientation of PPP blocks would prevent P3HT blocks from crystallizing, leading to the amorphous P3HT blocks. However, the hindrance to crystallization of P3HT blocks by face-on PPP blocks is removed immediately after PPP block crystals were molten. It was possible to observe the face-on and edge-on configuration on the substrate surface of derived from the film morphology. However, the film morphology studies by AFM characterization (Figure S1) and UV−vis (Figure S2) are mainly caused by crystallization ability rather than molecular orientation in this work. The in situ temperature-resolved one-dimensional in-plane XRD profiles provided a more quantitative and intuitive approach to investigate the crystallization behavior. All the samples were followed by the stepwise heating and cooling process at the rate of 5/−5 °C/min for 5 min. Figure 4a showed the evolution of (100) diffraction for PPP block and P3HT blocks in the 2θ range from 2° to 10° in the in-plane XRD profiles of B39T18 during the stepwise heating and

Figure 4. Temperature dependence of in-plane XRD profiles for B39T18 during the stepwise heating and cooling process: (a) the (100) peaks of PPP and P3HT blocks; (b) the (001) peaks of PPP and P3HT blocks dependence on temperature.

cooling process. A recognizable (100)B diffraction appeared at a 2θ angle of ∼4.2° in the pristine film, indicating that PPP blocks crystallized with a face-on orientation. When the temperature was elevated to 80 °C, the intensity of the (100)B peak decreased as the temperature increased. When the temperature increased to 85 °C, another diffraction peak emerged at a 2θ of ∼5.2°, indicating a spacing distance of 16.9 Å. This peak could be associated with the primary reflection from the (100)T plane of P3HT crystals (denoted as (100)T), which suggested the subsequent crystallization of P3HT blocks. The intensity of the (100)T peak also increased as the temperature increased (from 85 to 92 °C). When the temperature reached up to 95 °C, the (100)B peak disappeared. During the stepwise cooling process from 120 to 70 °C, only the (100)T peak existed until the temperature decreases to the crystallization temperature of PPP blocks. The result showed that when the temperature increased from room temperature to the melting region of PPP blocks (78 to ∼97 °C), P3HT blocks started to crystallize and their crystallinity increased with the temperature rising and reached a plateau at 95 °C, while PPP blocks began to crystallize subsequently during cooling to 50 °C, followed by a rapid increase of crystallinity. It was noteworthy that the PPP and P3HT blocks both adopted face-on orientation in their crystals. 7561


Article

Macromolecules

Figure 5. Temperature dependence of 2D GIXD patterns for B39T18 during the stepwise heating and cooling process: (a−e) heating stage; (e−i) cooling stage.

again, which is different from the orientation transition from face-on to edge-on of PPP blocks when annealing films above Tm(P3HT). Thus, it was proved that during the face-on transition of P3HT blocks induced by PPP blocks not only the side chains but also the backbones adopted a face-on orientation. Based on these results, it was concluded that the side chains were molten and amorphous after heating the film to the melting point, and they reorganized and induced the reassembly of backbones upon cooling. Meanwhile, the backbones were also influenced and reorganized in the cooling procedure. 3.1.3. Face-On to Edge-On Orientation Transformation. In order to clarify the thermodynamically favorable state, the in situ temperature-resolved 2D-GIXD were carried on again. The variation of the in-plane (001) diffraction reflecting period along backbone direction was investigated. As shown in Figures 5a and 3i, the orientations of PPP and P3HT blocks were faceon. When the temperature increased to 100 °C, the (100)B peak disappeareed. However, the (100)T was present until heating stepwise to 200 °C (Figure 5d). When the temperature increased to 230 °C, the (100)T peak disappeared as well. During the cooling process from 200 to 100 °C only the (100)T ring existed, and then the (100)B ring existed at 30 °C. It was noteworthy that the diffraction of P3HT did not display a distinct anisotropy peak but a ring with a higher intensity along the vertical direction. We speculated that the crystals lose the nucleus of crystallization during the cooling proces when the

Figure 4a shows the thermometric change tendency of (100) peaks associated with the stacking of alkyl side chains of B39T18. However, whether the backbones of PPP and P3HT crystallized remained unknown. In order to clarify whether only the side chain or both the side chains and backbones adopted the face-on orientation during the orientation transition of P3HT blocks induced by self-epitaxy effect, a further investigation into B39T18 within a 2θ region from 18° to 28° of the in-plane XRD profiles was made during the stepwise heating and cooling process. In pristine films, only (001) peaks of PPP blocks were observed. As shown in Figure 4b, a recognizable diffraction peak appeared at a 2θ angle of ∼21.1° in the pristine film to indicate a spacing distance of 4.24 Å, which proved that the backbones of PPP adopted face-on orientation. Considering that the intensity of (001) peaks was proportional to the crystallinity of PPP and P3HT blocks, the gradual decrease of the (001)B peak intensity during heating suggested the melting of PPP blocks with annealing temperature exceeding their melting points. Meanwhile, the (001) diffraction belonging to P3HT blocks appeared and became more intense with the PPP blocks melting, which suggested that the melting PPP blocks provided free volume for T blocks to extend and stack orderly. When elevating annealing temperature far above Tm(PPP), the (001) diffraction belonging to PPP blocks totally disappeared, and only (001) diffractions belonging to P3HT blocks remained. During the cooling down of Tm(PPP), (001)B diffraction peaks appeared 7562


Article

Macromolecules

Figure 6. Three different self-epitaxial crystallization circles by controlling the heating process. Blue indicates P3HT blocks while red indicates PPP blocks.

PPP and P3HT blocks melted at 230 °C, resulting in the isotropic diffraction rings. As mentioned above, we investigated three different selfepitaxial crystallization circles by controlling the heating process, as shown in Figure 6. In different temperature processes, PPP-b-P3HT underwent different pathways to yield edge-on and face-on orientations. First, when the ascasted film was heated to 120 °C, P3HT started crystallizing with edge-on orientation. During cooling, PPP blocks also crystallized with an orientation of edge-on induced by the formed edge-on P3HT crystals. Second, when the pristine film was heated to 80 °C and underwent isothermal crystallization, the partial melting of PPP blocks promoted the extension of P3HT blocks to result in P3HT blocks crystallizing in a face-on orientation. The face-on orientation of P3HT was stable even when PPP blocks were melted and totally heated to 120 °C. During cooling, PPP blocks crystallized with face-on orientation under the self-epitaxy of P3HT blocks. Thus, the PPP and P3HT blocks crystals both adopted face-on orientation. Third, the films of the second process by heating above 230 °C, the PPP and P3HT blocks were melted. During cooling, P3HT blocks with the higher melting point crystallized with a more stable edge-on orientation again during cooling, which acted as self-epitaxy template to induce an edge-on orientation of PPP blocks. 3.2. Discussion. 3.2.1. Necessary Condition of SelfEpitaxial Crystallization. On the basis of the results mentioned above, we proposed that some necessary conditions must be satisfied before demonstrating the self-epitaxial crystallization: (i) the miss ratio of two blocks in self-epitaxial crystallization process; (ii) the PPP block is longer than P3HT block, the crystallization of PPP blocks dominate in the pristine films and hinder the crystallization of P3HT blocks; however, the P3HT blocks could recrystallize during thermal annealing; (iii) the covalent binding with two blocks. First, during self-epitaxial crystallization of diblock copolymer, the block finishing aligned crystallization at first could act as an epitaxy template to induce the aligned crystallization of the other block if they had close lattice periods. The PPP and

P3HT could crystallize in the orthorhombic system with unit cell: a = 21.20 Å, b = 3.78 Å, c = 4.24 Å and a = 16.44 Å, b = 3.80 Å, c = 7.80 Å, respectively.18 The miss ratio of face-to-edge epitaxial growth between PPP and P3HT blocks arose from the disparity between the (001) of PPP and the (001) of P3HT. Δ=

d(001)B − d(001)T d(001)B

This value was 0.07, which was less than 0.15 and satisfied this necessary condition of epitaxial crystallization. Second, B41T44 and B39T9 were chosen to carry on the same annealing scheme for comparison to clarify whether block ratio was an influential factor of self-epitaxy crystallization. For B41T44, the crystallization of P3HT block occurred during heating, and the diffraction of PPP blocks could be detected after their melting. Thus, it was confirmed that the P3HT blocks could recrystallize during thermal annealing and acted as epitaxy crystal nuclei to induce crystallization of PPP blocks. Instead, the P3HT blocks were too short to crystallize sufficiently even when PPP blocks as a hindrance crystallizing were molten in B39T9. Therefore, self-epitaxy crystallization was not observed in B39T9. The results of in situ characterization are presented in Figure 7. However, when the lengths of PPP and P3HT were close such as B41T44, the edge-on orientation of the P3HT blocks and face-on orientation of the PPP blocks coexisted as shown in Figure 8a. For diblock copolymer with both crystalline blocks, the one with a higher crystallization temperature (Tc) crystallized first and then exerted an effect on crystallization of the other one, which could determine the final film morphology. During the heating process, after the PPP blocks melted the P3HT blocks had to crystallize grown on the nuclear of itself with an edge-on orientation. Third, the blend of homopolymers, B40 and T18, was also chosen to carry on the same annealing scheme to investigate the orientation variation with annealing process (Figure 9). It was observed that P3HT blocks could not crystallize on PPP block nuclei by epitaxy due to no covalent connection between B40 and T18. It suggested that the covalent bond between PPP 7563


Article

Macromolecules

Figure 9. Temperature dependence of 2D GIXD patterns for blends of B40 and T18 during the stepwise heating and cooling process: (a−e) heating stage; (e−i) cooling stage.


temperature was elevated above T m (PPP) but below Tm(P3HT), the crystals of P3HT blocks with a face-on orientation were not changed in spite of PPP block melting. As the films were cooled, the face-on P3HT block crystals induced a same orientation of PPP blocks as the epitaxy nuclei. At last, the face-on orientations of both PPP and P3HT blocks were formed. In another annealing scheme, we controlled the annealing temperature within the melting region of PPP blocks (78−97 °C) and made the portion with a smaller MW melt first. The melting process provided free volume for P3HT blocks to extend and stack orderly. However, most PPP blocks still displayed face-on orientation, preventing the torsion of thiophene rings and limit P3HT blocks to adopt a same face-on orientation. Then by elevating annealing temperature above Tm(P3HT), both PPP and P3HT blocks melted with disappearance of both face-on and edge-on orientation. There were no epitaxy nuclei in this stage. During cooling, the P3HT blocks crystallize first due to their higher Tc. However, the original anisotropic orientation in films became isotropic due to the lack of epitaxy nuclei, that is, the coexistence of face-on and edge-on orientations. We obtained some different process of self-epitaxial crystallization: when annealing above melting point of P3HT blocks, they started crystallizing with edge-on orientation of chains. During cooling, the formation of edge-on P3HT crystals induced a orientation transition of PPP blocks from face-on to edge-on. Meanwhile, by controlling annealing temperature in melting region of PPP blocks, the partial melting of them promoted the extension of P3HT blocks and made P3HT blocks crystallize in a face-on orientation on aggregates of PPP blocks due to the steric limitation effect. The face-on orientation of P3HT was stable even when PPP blocks melted totally, which in turn induced the face-on orientation of PPP block crystals to grow under the epitaxy of P3HT blocks during cooling. In comparison, P3HT blocks with the higher melting point crystallized with a more stable edge-on orientation again and acted as epitaxy template of PPP blocks during cooling by annealing above melting points of the two face-on blocks because crystal nuclear for self-epitaxy was destroyed at a high


and P3HT blocks be important to the self-epitaxy crystallization in block copolymers. For B39T19, it satisfied the three necessary conditions for self-epitaxial crystallization. We chose the temperatures in the melting region of PPP blocks to anneal pristine copolymer films with face-on PPP blocks and amorphous P3HT blocks for 3 min. During this process, the portion of PPP blocks with smaller molecular weights melted first, while the portion with higher MW stayed unchanged. The free volume formed by partly melting of PPP blocks provided void for P3HT blocks to extend the chains and stack orderly. However, the motion of P3HT blocks was still hindered due to most PPP blocks displaying face-on orientation. It led to the same face-on orientation of P3HT blocks as PPP blocks. When the annealing 7564


Macromolecules

■

temperature. We annealed the film with both face-on blocks above Tm(P3HT) to melt all epitaxy nuclei and eliminate all face-on and edge-on orientation. During cooling, although P3HT blocks crystallized first due to their higher Tc, their orientation was destroyed. Fortunately, most P3HT blocks crystallizing with a preferred edge-on orientation could induce B blocks to adopt the same orientation. With the progression of cooling, face-on nuclei were hardly observed and the prevailing orientation in the final film was edge-on. As mentioned above, the P3HT transformed from amorphous state to crystalline state through annealing, and the covalent bond was a necessary condition of self-epitaxial crystallization. The block length and the temperature mutually affect the self-epitaxial crystallization of all-conjugated diblock copolymer.

AUTHOR INFORMATION

Corresponding Author

*Tel 86-431-85262175, Fax 86-431-85262126, e-mail ychan@ ciac.ac.cn (Y.H.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21334006) and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant XDB12020300). We also thank Beijing Synchrotron Radiation Facility (BSRF) 1W1A and Shanghai Synchrotron Radiation Facility (SSRF) BL14U1 for 2D GIXD measurements.

■

4. CONCLUSION In this work, we investigated the molecular orientation and tuned self-epitaxial crystallization of B39T18 by controlling the heating process. Three different self-epitaxial crystallization circles were conducted in order to see the orientation change of the two blocks. First, when the pristine film was heated to 120 °C, above the melting temperature of PPP blocks, P3HT started crystallizing with edge-on orientation in the heating process when PPP blocks were molten since the edge-on orientation was a more stable state for P3HT blocks. During cooling, PPP blocks also crystallized with an orientation of edge-on induced by the formed edge-on P3HT crystals previously via self-epitaxial crystallization. Second, when the pristine film was heated in the melting temperature region (78−97 °C) of PPP blocks and underwent isothermal crystallization for 30 min, the partial melting of PPP blocks promoted the extension of P3HT blocks to result in P3HT blocks crystallizing in a face-on orientation due to the steric limitation effect exerted by the face-on PPP blocks. The face-on orientation of P3HT was stable even when PPP blocks were melted totally in the heating process. During cooling, PPP blocks crystallized with face-on orientation under the selfepitaxy of P3HT blocks. Thus, the PPP and P3HT blocks crystals were both adopted face-on orientation after this heating−cooling circle. Furthermore, by heating above the melting points of the two face-on blocks (230 °C), the crystal nucleus of self-epitaxy for the PPP and P3HT blocks were absent. However, P3HT blocks with the higher melting point crystallized with a more stable edge-on orientation again during cooling, which acted as self-epitaxy template to induce an edgeon orientation of PPP blocks. We also observed that the selfepitaxy crystallization induced by limitation effect only in B39T18, while no such phenomenon was found in B39T9, B41T44, and homopolymers blends of PPP and P3HT. This result was attributed to the satisfaction of the two necessary requirements for self-epitaxial crystallization; one was the covalent bonding between blocks, and the other was P3HT transition from amorphous to crystalline through annealing.

■

Article

REFERENCES

(1) Chang, J. F.; Sun, B. Q.; Breiby, D. W.; Nielsen, M. M.; Solling, T. I.; Giles, M.; McCulloch, I.; Sirringhaus, H. Chem. Mater. 2004, 16 (23), 4772−4776. (2) Chabinyc, M. L.; Toney, M. F.; Kline, R. J.; McCulloch, I.; Heeney, M. J. Am. Chem. Soc. 2007, 129 (11), 3226−3237. (3) Rivnay, J.; Steyrleuthner, R.; Jimison, L. H.; Casadei, A.; Chen, Z. H.; Toney, M. F.; Facchetti, A.; Neher, D.; Salleo, A. Macromolecules 2011, 44 (13), 5246−5255. (4) Yang, H. C.; Shin, T. J.; Yang, L.; Cho, K.; Ryu, C. Y.; Bao, Z. N. Adv. Funct. Mater. 2005, 15 (4), 671−676. (5) Yang, H. H.; LeFevre, S. W.; Ryu, C. Y.; Bao, Z. N. Appl. Phys. Lett. 2007, 90 (17), 172116. (6) Kline, R. J.; Mcgehee, M. D.; Toney, M. F. Nat. Mater. 2006, 5 (3), 222−228. (7) Kim, D. H.; Park, Y. D.; Jang, Y. S.; Yang, H. C.; Kim, Y. H.; Han, J. I.; Moon, D. G.; Park, S. J.; Chang, T. Y.; Chang, C. W.; Joo, M. K.; Ryu, C. Y.; Cho, K. W. Adv. Funct. Mater. 2005, 15 (1), 77−82. (8) Kim, D. H.; Jang, Y.; Park, Y. D.; Cho, K. Langmuir 2005, 21 (8), 3203−3206. (9) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.; Bredas, J. L.; Logdlund, M.; Salaneck, W. R. Nature 1999, 397 (6715), 121− 128. (10) 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. Nature 1999, 401 (6754), 685−688. (11) Kline, R. J.; McGehee, M. D.; Kadnikova, E. N.; Liu, J. S.; Frechet, J. M. J.; Toney, M. F. Macromolecules 2005, 38 (8), 3312− 3319. (12) Gomez, E. D.; Barteau, K. P.; Wang, H.; Toney, M. F.; Loo, Y. L. Chem. Commun. 2011, 47 (1), 436−438. (13) Verploegen, E.; Mondal, R.; Bettinger, C. J.; Sok, S.; Toney, M. F.; Bao, Z. A. Adv. Funct. Mater. 2010, 20 (20), 3519−3529. (14) Muller, C.; Aghamohammadi, M.; Himmelberger, S.; Sonar, P.; Garriga, M.; Salleo, A.; Campoy-Quiles, M. Adv. Funct. Mater. 2013, 23 (19), 2368−2377. (15) He, M.; Han, W.; Ge, J.; Yu, W. J.; Yang, Y. L.; Qiu, F.; Lin, Z. Q. Nanoscale 2011, 3 (8), 3159−3163. (16) He, M.; Han, W.; Ge, J.; Yang, Y. L.; Qiu, F.; Lin, Z. Q. Energy Environ. Sci. 2011, 4 (8), 2894−2902. (17) He, M.; Ge, J.; Lin, Z. Q.; Feng, X. H.; Wang, X. W.; Lu, H. B.; Yang, Y. L.; Qiu, F. Energy Environ. Sci. 2012, 5 (8), 8351−8358. (18) Yang, H.; Wang, L.; Zhang, J. D.; Yu, X. H.; Geng, Y. H.; Han, Y. C. Macromol. Chem. Phys. 2014, 215 (5), 405−411. (19) Yang, H.; Wang, L.; Yu, X.; Zhang, J.; Geng, Y.; Han, Y. Chem. Asian J. 2014, 9 (8), 2239−2248. (20) Yu, X. H.; Yang, H.; Wu, S. P.; Geng, Y. H.; Han, Y. C. Macromolecules 2012, 45 (1), 266−274. (21) Brinkmann, M. Macromolecules 2007, 40 (21), 7532−7541.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01804. Experimental section; Figures S1 and S2 (PDF) 7565


Article

Macromolecules (22) Brinkmann, M.; Chandezon, F.; Pansu, R. B.; Julien-Rabant, C. Adv. Funct. Mater. 2009, 19 (17), 2759−2766. (23) Brinkmann, M.; Rannou, P. Adv. Funct. Mater. 2007, 17 (1), 101−108. (24) Rivnay, J.; Toney, M. F.; Zheng, Y.; Kauvar, I. V.; Chen, Z. H.; Wagner, V.; Facchetti, A.; Salleo, A. Adv. Mater. 2010, 22 (39), 4359− 4363. (25) Cho, E.; Risko, C.; Kim, D.; Gysel, R.; Miller, N. C.; Breiby, D. W.; McGehee, M. D.; Toney, M. F.; Kline, R. J.; Bredas, J. L. J. Am. Chem. Soc. 2012, 134 (14), 6177−6190. (26) Himmelberger, S.; Dacuna, J.; Rivnay, J.; Jimison, L. H.; McCarthy-Ward, T.; Heeney, M.; McCulloch, I.; Toney, M. F.; Salleo, A. Adv. Funct. Mater. 2013, 23 (16), 2091−2098. (27) Wu, S. P.; Bu, L. J.; Huang, L.; Yu, X. H.; Han, Y. C.; Geng, Y. H.; Wang, F. S. Polymer 2009, 50 (26), 6245−6251.

7566


Face-On and Edge-On Orientation Transition and Self-Epitaxial

Recommend Documents