Microphase Separation and Crystallization of All-Conjugated

Article pubs.acs.org/Macromolecules

Microphase Separation and Crystallization of All-Conjugated Phenylene−Thiophene Diblock Copolymers Xinhong Yu, Hua Yang, Shupeng Wu, 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, People’s Republic of China S Supporting Information *

ABSTRACT: Microphase separation and crystallization in thin films of all-conjugated diblock copolymers poly(2,5-dihexyloxyp-phenylene)-block-(3-hexylthiophene) (PPP-b-P3HT) with various main chain lengths were investigated under solvent vapor annealing or thermal annealing. It is shown that crystallization and microphase separation coexisted during thin film forming process due to the incompatibility and crystallization of the two conjugated blocks. When the B34T66 copolymer film was annealed in chlorobenzene vapor or at high temperature, crystallization broke out the microphase-separated structure. On the other hand, upon annealing of the films of B62T38 and B75T25 with higher PPP ratio, crystallization was confined in the microphase-separated domains. The copolymer composition plays a key role in determining the crystallization and the final morphology of the thin films.

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induced by π−π interactions and the unique ordered morphologies compared with those of rod−coil block copolymers.18,19 Several all-conjugated thiophene-based rod− rod block copolymers have been synthesized because of their excellent charge-transporting mobility and high regioregular structure.20−31 Hashimoto and co-workers synthesized allconjugated diblock copolymer of P3HT-block-poly[3-(2ethylhexyl)thiophene] (P3EHT) and discovered that the less crystalline P3EHT segment could promote the self-organization of P3HT domains in film.23 Fully conjugated block polythiophenes in which each block contains a unique alkyl side chain have been synthesized.24,25 Qiu and co-workers confirmed that diblock copolythiophenes with alkyl side-chain length different by two carbon atoms cocrystallized into a uniform crystal and, when the side chain lengths were different by more than two carbon atoms, preferred to microphaseseparate into two independently crystallized domains of each block.25 Side chain mainly interferes with intermolecular interactions; Seferos and co-workers synthesized selenophene−thiophene block copolymers with blocks of distinct

INTRODUCTION Conjugated polymers have been attracting much attention as new electronic and optoelectronic substances mainly used for active semiconducting layers in low-cost electronic devices.1−4 The orientation and nanostructures of semiconducting polymers in solid state play an important role in determining the device performance.5−7 To precisely control the organization and the nanostructure in the thin films of rod-like conjugated semiconductor polymer, a fascinating route is to covalently link with coil-like polymers.8−15 Block copolymers containing polymeric semiconducting blocks can self-organize into well-defined microphase-separated structures driven by factors such as immiscibility or crystallizability difference between the blocks. There have been several reports on semiconducting polymer-based block copolymers containing a conjugated block and a nonconjugated block such as polystyrene (PS), polyisoprene (PI), poly(methyl acrylate) (PMMA),12,16 etc. The addition of such “coil-like segments” has an effect on the crystallinity and leads to new morphological behavior. However, the coil segments are generally insulating and could dilute the concentration of the semiconducting blocks in the films and further limit the opotoelectronic properties.17 All-conjugated block copolymers of the rod−rod type have become a focus of interest due to the strong self-assembly © 2011 American Chemical Society

Received: May 5, 2011 Revised: November 30, 2011 Published: December 22, 2011 266

dx.doi.org/10.1021/ma201024z | Macromolecules 2012, 45, 266−274

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controlled through the feed ratio of the monomers and the initiator, according to our previous report. The number-average molecular weights (Mn) of the studied PPP-b-P3HT estimated from GPC were around 28 400, 29 700, 32 400, and 41 100, with polydispersity index of 1.46, 1.34, 1.30, and 1.36. The degree of polymerization of PPP and P3HT segments estimated from the 1H NMR spectrum were 27/140, 41/80, 69/43, and 138/45, respectively. Chlorobenzene (ClB) was purchased from Beijing Chemical Reagent Co. Sample Preparation. The diblock copolymers were dissolved in chlorobenzene (a nonselective solvent) with concentration of 5 mg/mL. The solutions were heated up to 80 °C for complete dissolution. After cooling to room temperature, the solutions were standing in dark and vibration-free environment over 24 h at room temperature (the aging process has a negligible effect on the film morphologies) to obtain homogeneous samples. The films of diblock copolymers were prepared by spin-coating solutions onto precleaned silicon wafers at 1500 rpm for 30 s using a commercial spin-coater (KW-4A, chemat Technology Inc.). Prior to spin-coating, the wafers were cleaned with a 70/30 v/v solution of 98% H2SO4/30% H2O2 at 80 °C for 30 min, then thoroughly rinsed with deionized water, and finally blown dry in nitrogen. For the solvent vapor treatment, the spin-coated samples were exposed to saturated chlorobenzene vapor in closed vessels at room temperature (20 °C) for different periods. Then, the samples were removed from the vessels quickly for fast drying. Prior to the heat treatment, the spin-coated thin films were vacuumdried to remove the residual solvents. The films were first melt at 280 °C (a temperature higher than the melting points of PPP and P3HT blocks) for 1 h and then cooled to room temperature slowly. Characterization. Transmission electron microscopy (TEM), selected area electron diffraction (SAED), high-resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and two-dimensional grazing incident X-ray diffraction (2D GIXRD) techniques were applied to characterize the morphology and structure of diblock copolymer thin films. AFM characterization was performed in tapping mode, using a SPA300HV with a SPI3800 N controller (Seiko Instruments Inc., Japan). A silicon microcantilever with spring constant 2 N/m and resonance frequency ∼70 kHz, (Olympus Co., Japan) with an etched conical tip was used for scanning. TEM images and SAED patterns were obtained with a JEM-1011 transmission electron microscope (JEOL Inc., Japan) operated at 100 kV accelerating voltage. Samples for TEM were prepared by floating thin film on a carbon-coated copper grid. The samples were dried at room temperature for 24 h before TEM experiments. HRTEM was carried out on a FEI Tecnai G2 microscope operated at an accelerating voltage of 200 kV. XRD profiles were obtained by using a Bruker D8 Discover Reflector with X-ray generation power of 40 kV tube voltage and 40 mA tube current. The measurements were obtained in a scanning interval of 2θ between 3° and 30°. To increase XRD peak intensity for investigating the crystallinity and orientation that prevail throughout the film, we employed an incident angle (α = 0.2°) slightly above the critical angle (αc = 0.18°). DSC curves were performed using a Perkin-Elmer DSC7 at a heating/cooling rate of 10/−10 °C min−1 under a nitrogen flow. 2D GIXRD was measured at Beijing Synchrotron Radiation Facility on beamline 1W1A with the incident energy of 8 keV (λ = 0.154 nm). The thin films were aligned using a Huber 5-circle diffractormeter and point detector, and then their 2D GIXRD patterns were recorded by a Mar345 area detector.

heterocycles and discovered that phase separation as well as the optical properties can be controlled by the heterocycle in the polymer chain.26 In contrast to the microphase separation of coil−coil block copolymers, most previous reports on all-conjugated diblock copolymers mainly focused on the crystallization instead of microphase separation of them. Crystallization of one block was a major drive to the nanowire formation. Additional contributions from the rigidity and the interchain overlapping of the conjugated orbits complicated the self-assembling behavior.18,19 Microphase separation and crystallization are two kinds of phase transitions in polymer systems.32−34 The final phase structure depends on the competition between microphase separation and crystallization.32 With respect to new all-conjugated diblock copolymers, the superstructure and growth have yet to be investigated systematically. Although nanowires composed of microphase-separated crystal domains are always obtained in conjugated diblock copolymers, the crystalline structure and growth mechanism remain unrevealed and need extensive investigation. In this article, we take a family of fire-new crystalline− crystalline all-conjugated diblock copolymers, poly(2,5-dihexyloxy-p-phenylene)-block-(3-hexylthiophene) (PPP-b-P3HT), as the research objects and systematically explore their crystallization behavior and nanoscale morphology with different main chain block ratios. We studied the effects of block ratio and annealing process on the transition between microphase separation and crystallization of the diblock copolymer thin films. Through the modulation of the block composition and annealing process, the competition between the microphase separation and crystallization was tuned, leading to observations of confined crystallization and breakout crystallization.

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EXPERIMENTAL SECTION

Materials. Conjugated diblock copolymers PPP-b-P3HT with PPP:P3HT molar ratios of 16:84, 34:66, 62:38, and 75:25 were synthesized by Grignard metathesis (GRIM) polymerization method according to our previous report.35 The molecular weights and thermal behavior of these diblock copolymers are listed in Tables 1 and 2. The molecular weights of different blocks in PPP-b-P3HT were well-

Table 1. Summary of Composition and Molecular Weight of PPP-b-P3HT Diblock Copolymersa polymer

m

n

m/n (%)

Mn

PDI

B16T84 B34T66 B62T38 B75T25

27 41 69 138

140 80 43 45

16:84 34:66 62:38 75:25

28 400 29 700 32 400 41 100

1.46 1.34 1.30 1.36

a

m, n: the estimated degree of polymerization of PPP (B) and P3HT (T) block in the synthesized diblock copolymers; Mn: number-average molecular weight; PDI: polydispersity index.

Table 2. Summary of DSC Measurements of the Series of Diblock Copolymersa

a

polymer

Tm(P3HT) [°C]

B16T84 B34T66 B62T38 B75T25

237.7 239.0 231.0 219.3

Tm(PPP) [°C]

Tc(P3HT) [°C]

Tc(PPP) [°C]

88.1 96.6 99.2

201.4 224.8 185.7 172.8

81.2 84.4

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RESULTS AND DISCUSSION The thermodynamic incompatibility between the blocks makes the films prone to phase separate into domains of each block. Meanwhile, the conjugated polymer with a regular end-to-end arrangement of side chains allows for efficient π−π stacking of the conjugated backbone to form crystalline supramolecular

Tm: melting point; Tc: crystallization temperature. 267


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Figure 1. (a−d) TEM images (bar length: 500 nm), (a′−d′) AFM topographic images, and (a″−d″) SAED patterns of PPP-b-P3HT films with different block ratios: (a, a′, a″) B75T25, (b, b′, b″) B62T38, (c, c′, c″) B34T66, (d, d′, d″) B16T84. Thin films were prepared by spin-coating from 5 mg/mL chlorobenzene solutions. (e) XRD patterns of PPP-b-P3HT films with different block ratios.

structure.36 The final phase structure in crystalline diblock conjugated copolymers depends on the competition between these two processes. 3.1. Competition of Crystallization and Microphase Separation in the Thin Film Forming Process. Figure 1

shows the morphologies of PPP-b-P3HT films (B75T25, B62T38, B34T66, B16T84) spin-coated from 5 mg/mL chlorobenzene solutions on Si substrate. TEM images (Figure 1a−c) show disordered spinodal-like morphologies in the as-cast B75T25, B62T38, and B34T66 films, respectively. The TEM contrast 268


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the film morphology by the modulation of the extent of crystallization and microphase separation. Here we choose solvent vapor annealing and thermal annealing to promote the crystallization of both blocks. When the as-prepared B34T66 film with a relatively high P3HT content was annealed in chlorobenzene vapor or at high temperature, crystallization broke out the microphase-separated structure, yielding a transition from microphase-separated structure into crystalline structure.32 The evolution of microdomain structure in the B34T66 thin films was investigated by varying the annealing time in chlorobenzene vapor. Exposed to chlorobenzene vapor, the spinodal-like morphology disappeared gradually. With annealing time increasing to 5 h (Figure 2a,a′), fibrillar aggregates were generated on the film surface. The amount of the fibrillar aggregates increased with the extended annealing time (12 days) and covered the whole film eventually (Figure 2b,b′). In the thermal annealing process, the spin-coated diblock copolymer thin films were heated above the melt point of two blocks and slowly cooled down to room temperature. Typical AFM images of B34T66 thin films after thermal annealing show network-like fibrillar crystals with lengths on the order of micrometers (Figure 2c). The TEM images confirmed the morphology observed in AFM images (Figure 2c′). That is to say, upon thermal annealing, the spinodal-like microphaseseparated morphology disappears, and crystals dominate the film structure. The crystallinity and molecule packing in the diblock copolymer thin films after different annealing processes were further examined by SAED and XRD spectra shown in Figure 2. The SAED patterns (in the insets of Figure 2b,c′) show two diffraction rings corresponding to P3HT and PPP block, respectively. So in such fibrils, the crystals of P3HT and PPP coexist. Compared with the as-prepared thin film, the diffraction peak in XRD spectra (Figure 2d) shifts to higher angle after solvent vapor treatment. As for thin films of diblock copolymer after annealing at 280 °C, the fibers of B34T66 show diffraction peaks that are characteristic of PPP and P3HT blocks. A prominent peak appears at 2θ ≈ 5.4°, corresponding to an d-spacing of 1.61 nm of the well-organized lamellar structure of P3HT block. A shoulder peak at 2θ ≈ 4.37°corresponds to the PPP block with a d-spacing of 2.02 nm. Such characteristic crystallographic structures of the block copolymer fibers is confirmed by the SAED patterns shown in the inset of Figure 2c′. The SAED patterns show a clear outer diffraction ring with a d-spacing of 0.38−0.39 nm, corresponding to the P3HT block and a inner diffraction ring with a d-spacing of 0.43−0.44 nm from the diffraction of PPP. 3.3. Confined Crystallization in Block Copolymers B62T38 and B75T25 with a Higher Block Ratio of PPP. Confined crystallization occurred in the films of B62T38 and B75T25 with higher PPP block ratios when the films are annealed in chlorobenzene vapor or at high temperature, as shown in Figure 3. When B62T38 and B75T25 thin films were annealed in chlorobenzene vapor, the morphology of the thin films changed from netlike to string of beads (Figure 3a,b). Meanwhile, the spinodal-like morphology can be obtained regardless of the treating time (Figure 3a′,b′). Figure 3c,c′,d,d′ shows the morphologies of B62T38 and B75T25 thin films after thermal annealing. Large fibers with several micrometers in length can be resolved in AFM images (Figure 3c,d). With further observation at higher magnification by TEM, each fiber is composed of several nanoscale ribbons (Figure 3c′,d′).

originates from the electron density of projected objects. Here, the sulfur atoms in P3HT should provide more electron scattering and thus appear darker. The sizes of the bright and dark domains in the TEM images can be approximately scaled with the degree of polymerization of both blocks in the diblock copolymers. B75T25 showed bright domains with a width of about 50 nm, while B62T38 and B34T66 showed narrower bright domains. Meanwhile, B75T25 and B62T38 showed dark domains with similar widths of around 10 nm, while B34T66 showed broader dark domains. B75T25 and B62T38 show similar widths of dark domains, probably due to the similar degree of polymerization of P3HT block in both copolymers. This difference in images also suggests that the bright and dark domains may correspond to the PPP and P3HT blocks, respectively. The AFM images of the prepared block copolymer films could provide further details of the surface structures. AFM images give microphase-separated structures consistent with the TEM images. As for the PPP-b-P3HT with the block ratio of 16:84, fiber structures dominate the whole film instead of the spinodal-like morphology observed in other systems (Figure 1d,d′). The crystal structures of the films were investigated by SAED (Figure 1a″−d″). For B62T38 and B34T66, the SAED patterns show two blur diffraction rings. The outer rings in these SAED patterns are indexed as the diffractions from crystallographic (020) planes, corresponding to a π−π stacking distance of 0.38−0.39 nm, which is in good agreement with the previously reported value of 0.38 nm in films of poly(alkylthiophene) homopolymers.37 The inner ring is due to the diffraction of PPP, with a d-spacing of 0.43−0.44 nm, which is comparable to that in films of poly(p-phenylene) homopolymers (not shown here). On the other hand, SAED patterns for B75T25 and B16T84 give an intensive ring and a diffusive ring. For B75T25, the intensive ring is caused by the diffraction from crystallographic plane associated with PPP, and the diffusive ring is due to the diffraction of P3HT. For B16T84, the intensive ring and the diffusive ring are due to the diffraction of P3HT and PPP, respectively. XRD was also used to investigate the crystal structures of the thin films (Figure 1e). The B75T25 shows a diffraction peak at 2θ ≈ 4.37°, which is comparable with the diffraction of PPP homopolymer (shown in Figure S1). With the increase of P3HT block, the XRD spectra of the films of B62T38 and B34T66 show much broader nonsymmetric diffraction peaks, which are caused by the overlap of the diffraction peaks of PPP and P3HT. It is consistent with the SAED profiles, which show two diffraction rings. We used 2D GIXRD measurements with higher beam energy to resolve the diffractions of the spincoated films before and after thermal annealing. The results showed excellent consistency to the XRD data (shown in Figures S2 and S3). As for B16T84, the peak at 2θ ≈ 5.4° corresponds to an interlayer d-spacing of 1.61 nm, and the other two diffraction peaks at around 10.8° and 16.2° represent second- and third-order diffractions of the lamella. The values are close to those reported for P3HT thin films and suggest the crystalline P3HT lamellae orient parallel to the substrate. No diffraction peaks associated with PPP are observed, which indicates that its diffraction peak may be too weak and overlapped by P3HT diffractions. 3.2. Breakout Crystallization in Block Copolymers B34T66 with a Higher Block Ratio of P3HT. The competition between crystallization and microphase separation determined the final film morphology. Therefore, we can tune 269


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Figure 2. AFM topographic and phase images of thin B34T66 films with a higher block ratio of P3HT exposed to chlorobenzene vapor for different times: (a, a′) 5 h, (b, b′) 12 days (inset: SAED pattern), (c) AFM, and (c′) TEM images of thin B34T66 films annealed at 280 °C for 1 h (bar length: 500 nm; inset: SAED pattern). (d) XRD patterns of B34T66 thin films treated with different conditions. 270


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Figure 3. AFM and TEM images of thin films of block copolymers upon different annealing process: (a, a′) B62T38 and (b, b′) B75T25 exposed to chlorobenzene vapor for 12 days, (c, c′) B62T38 and (d, d′) B75T25 annealed at 280 °C for 1 h. (a′−c′) Bar length: 500 nm; (d′) bar length: 1 μm. 271


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Figure 4. XRD patterns of (a) B62T38 and (b) B75T25 films treated with different conditions.

molecular chains to form crystals through π−π stacking. For block copolymers with longer P3HT block, the P3HT chains can gradually diffuse to the neighbor chains and allow for efficient π−π stacking of P3HT conjugated backbones to form crystalline supramolecular structures.38 Because of the large amount of P3HT chains, crystals can develop into connected fibers, destroying the spinodal-like morphologies. In block copolymers with longer PPP block, the diffusion of P3HT chains is obstructed by the majority PPP block. Thus, the P3HT crystallization is confined in the microphase-separated domains. In the case of thermal annealing process, the microphaseseparated morphology prevails in the thin film at the molten state. Upon slow cooling down to the crystallization temperature of P3HT, the PPP blocks remain a coiled conformation due to its low melting point. The molten PPP blocks are mobile. Therefore, during the slow-cooling process, the P3HT blocks tend to form a thermodynamically favorable crystal structure consisting of edge-on side chains and parallel π−π stacking planes of P3HT with respect to the substrate. For block copolymers with longer P3HT block, the majority P3HT chains can crystallize and break out the preformed microphase separation structure. When the temperature further reduced to the crystallization temperature of PPP block, the PPP block also form crystals. The crystallinity of the PPP block is restricted by the crystallization of P3HT block due to limited mobility of the P3HT blocks frozen in the lattice.22,23 It therefore results in a larger ratio of the intensities of the two diffraction peaks in XRD patterns (Figure 2) from the P3HT- and PPP-rich domains than the block ratio. This result is further confirmed by the missing of the recrystallization peak of PPP block in the cooling process. In the case of block copolymers with longer PPP block, the preferential P3HT crystallization is confined in the microphase-separated domains. In turn, the majority block of PPP self-organize into crystals under the restriction of the crystallization of P3HT block. The crystallization ability of the P3HT plays a key role in determining the final morphology of the thin films. We further investigated the nanometer scale morphologies in the nanoribbons of B62T38 and B75T25 with HRTEM. As shown in Figure 5, each nanoribbon composed of one main line and lots of short fibers perpendicular at both sides of the main line. More details were available from dark-field scanning transmission electron microscopy (STEM) measurements where bright features represent domains with a high electron scattering ability and are most likely due to the P3HT-rich phases. Topographic elemental mapping demonstrates that the sulfur is rich in the main line regions and decays rapidly in the

We speculate that thermal annealing yielded phase-separated nanoribbons, where the PPP and P3HT blocks crystallized within microphase-separated domains. The crystal structures of the thin films were also checked with SAED (inset in Figure 3a′−d′).The inner diffraction ring with a d-spacing of 0.43−0.44 nm corresponding to the PPP block became clearer in B62T38 and B75T25 films after solvent vapor treatment. Meanwhile, two clear diffraction rings with a d-spacing of 0.38−0.39 nm and 0.43−0.44 nm from the diffraction of P3HT and PPP are obtained after thermal annealing. XRD measurements were performed on the thin films of diblock copolymers with higher PPP content after different annealing processes (Figure 4). Compared with the as-prepared films, the diffraction peak shifted to lower angle after treated with nonselective solvent vapor. Upon slow cooling from the melt, the PPP-b-P3HT copolymers clearly showed two distinct crystalline domains with relative intensities of X-ray diffraction peaks from the PPP- and P3HT-rich domains approximately scaling with the block ratio. A prominent peak corresponding to the PPP block was observed with a d-spacing of 2.02 nm, while a shoulder at 2θ ≈ 5.4° is characteristic of P3HT block. These results show that the crystals are composed of two different crystalline domains formed by segregated PPP and P3HT blocks. The results are consistent with the thermal behaviors of block copolymers that two endothermic peaks and two exothermic peaks were observed during the heating and cooling process. 3.4. Theoretical Consideration of the Transition between Crystallization and Microphase Separation. For PPP-b-P3HT block copolymer system, crystallization and microphase separation coexist during the thin film forming process. Here we choose chlorobenzene as the nonselective solvent; the slow solvent evaporation speed during spin-coating facilitates the growth of crystalline domains in thin films. So a weak laterally microphase-separated structure due to incompatibility between PPP and P3HT and the crystallization of blocks is observed. In block copolymer B16T84 with a higher block ratio of P3HT, the crystallization of P3HT blocks dominates the surface morphology of the thin film, possibly because the priority crystallization of P3HT blocks breaks the microphase separation. These results also lead to a hypothesis that the crystallization ability of P3HT blocks is higher than PPP blocks probably because of the steric hindrance of more alkyl side chains in PPP. Similar behavior has been observed in other diblock copolymer systems.23 The diblock copolymer film absorbs chlorobenzene during annealing in chlorobenzene vapor; thus, the mobility of block copolymer molecules is enhanced, which assists the conjugated 272


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Figure 5. (a, b) HRTEM images of B62T38 and B75T25 after annealing at 280 °C for 1 h. (c) Dark-field STEM image of B75T25 thin films. In (c), electron-dense regions appear brighter. (d, d′) STEM image and elemental mapping of sulfur content as a function of position.

and the Ministry of Science and Technology of China (2009CB623604). A portion of this work is based on the data obtained at 1W1A, BSRF. The authors gratefully acknowledge the assistance of scientists of Diffuse X-ray Scattering Station during the experiments.

surrounding area. This indicates the distribution of P3HT chains in such self-assembled structures concentrated in the main line area relatively. Such a structure further confirms the overwhelming of phase separation over crystallization.

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CONCLUSION In conclusion, we investigated the crystallization and microphase separation of crystalline−crystalline conjugated diblock copolymers with different main chains. There exists a competition between the crystallization and microphase separation in crystalline−crystalline conjugated block copolymers. We further tune the thin film morphology through the modulation of the competition between crystallization and microphase separation. It is the crystallization ability of the P3HT determines the final morphology of the thin film. Break-out crystallization and confined crystallization have been observed in block copolymers with a higher and lower block ratio of P3HT, respectively.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S3. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *Tel 86-431-85262175; Fax 86-431-85262126; e-mail ychan@ ciac.jl.cn.

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ACKNOWLEDGMENTS This work was subsidized by the National Natural Science Foundation of China (20923003, 21004064, and 20834005) 273


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