Article pubs.acs.org/Macromolecules
Improved Hierarchical Ordering in Supramolecules via Symmetrically Bifunctionalized Organic Semiconductor Keun Hyung Lee,†,¶ Peter Bai,† Benjamin J. Rancatore,‡ Bo He,§,∥ Yi Liu,§,∥ and Ting Xu*,†,‡,§ †
Department of Materials Science and Engineering and ‡Department of Chemistry, University of California, Berkeley, Berkeley, California 94720, United States § Materials Sciences Division and ∥The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ¶ Department of Chemical Engineering, Inha University, Incheon 402-751, Republic of Korea S Supporting Information *
ABSTRACT: Block copolymer (BCP)-based supramolecular systems provide a versatile approach to manipulate functional structures spanning several nanometers to macroscopic length scales. Most studies to date focused on supramolecules containing asymmetrically end-functionalized small molecules, and it remains challenging to obtain molecular control over small molecule ordering within the BCP microdomain. Here we designed symmetrically end-functionalized bis-phenol quarterthiophene (BP4T) small molecules and systematically investigated how the end-group chemistry of the small molecules affects the supramolecular assembly process and the resulting morphology. Bifunctionalized small molecules can bridge two adjacent polymer blocks and lead to macroscopically aligned hierarchical assemblies at much higher degree of ordering than previously observed for asymmetrically functionalized small molecule analogues. The supramolecular morphology is very sensitive to the stoichiometry between the BP4T and polymer repeat unit because of the specific molecular organization within BCP microdomain. Furthermore, similar thermoresponsiveness of supramolecule, i.e., ∼40% change in the supramolecular periodicity during the heating and cooling cycles, can be obtained at BP4T loading stoichiometry of 0.5, much smaller than that of asymmetrically functionalized small molecule. These results clearly demonstrate that supramolecular assemblies can be readily manipulated by engineering the small molecule chemistry. Present studies provide basic design principles and an effective route to fabricate well-defined hierarchical assemblies for functional and stimuli-responsive nanomaterials.
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INTRODUCTION Block copolymer (BCP)-based supramolecules are generated by noncovalent association of low mass molecules to one block of BCPs through hydrogen bonding, metal coordination, electrostatic interaction, and π−π conjugation.1−10 The supramolecular approach affords versatile opportunities to combine desirable functionalities and properties of the small molecules into solution processable BCPs while avoiding additional synthesis. The hierarchical morphologies and molecular packing of the supramolecules are tunable by varying BCP, small molecule, the ratio between the constituents, and the assembly conditions. These properties of the supramolecules make them versatile candidates to design functional nanomaterials for a number of applications. For example, microscopic assemblies and their phase transitions enable switching of electrical, optical, and mechanical properties upon applying external stimuli.11−15 Most studies on supramolecular systems were conducted by using weak-interacting alkyl molecules such as 3-pentadecylphenol (PDP) attaching to poly(4-vinylpyridine).11−13,16−25 The supramolecule’s phase behavior including order−order transitions (OOTs) and order−disorder transitions (ODTs), assembly kinetics, and the hierarchical morphologies have been © XXXX American Chemical Society
systematically investigated. Other functional molecules, e.g. liquid crystals and organic semiconductors, have also been incorporated within the polymer scaffolds to form ordered nanostructures.1,3,4,26−29 However, previous studies mostly dealt with monofunctionalized hydrogen bond donor molecules. Incorporation of organic molecules with plural binding sites can change the enthalpic and entropic interactions between compartments, the kinetic pathway, and ultimately the supramolecular morphology. While potential cross-linking polymer chains by the multifunctional small molecule may present more complications, the multivalent functionalities also present opportunities to enhance structural ordering and to design and fabricate novel functional polymer nanocomposites. Thus, there is need to understand the effects of chemical structure and stoichiometry of multifunctionalized small molecules on the assemblies in BCP-based supramolecules at different length scales. Here, we report the systematic studies of BCP-based supramolecules consisting of bifunctional oligothiophene Received: February 13, 2016 Revised: March 20, 2016
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DOI: 10.1021/acs.macromol.6b00317 Macromolecules XXXX, XXX, XXX−XXX
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Figure 1. (a) Chemical structures of a PS-b-P4VP copolymer and a BP4T small molecule. DSC thermograms for BP4T and PS-b-P4VP(BP4T)r supramolecules with different BP4T contents at second heating (b) and cooling (c) cycles. The coefficient r represents the molar stoichiometry of BP4T to 4VP. TEM grids. The grids were exposed to iodine vapor that selectively stains P4VP(BP4T)r for 20 min to increase the contrast between PS and P4VP(BP4T)r domains. TEM images were obtained using a FEI Tecnai 12 transmission electron microscope. Small-Angle X-ray Scattering (SAXS) and Wide-Angle X-ray Scattering (WAXS). SAXS and WAXS experiments were performed at the beamline 7.3.3 in the Advanced Light Source (ALS), Lawrence Berkeley National Laboratory. The wavelength of the X-rays was 1.24 Å (10 keV). 2D scattering patterns were collected on a Pilatus 1M detector and then circularly averaged to produce 1D profiles. All samples were sealed in standard DSC pans. For variable temperature experiments, samples were thermally equilibrated for 10 min at each temperature on a homemade heating stage before X-ray illumination.
small molecules and a diblock copolymer host, polystyreneblock-poly(4-vinylpyridine) (PS-b-P4VP). A linear bis-phenol quarterthiophene (BP4T) is used, and the phenol group in each end enables BP4T to form two hydrogen bonds with P4VP chains. The resultant BCP-based supramolecules form lamellarin-lamellar nanostructures similarly to that of the monofunctionalized small molecule. The ordering of the P4VP(BP4T) comb blocks is significantly enhanced presumably due to the bridging of BP4T molecules across adjacent P4VP chains. There is significant BCP period change (∼40%) upon heating and cooling cycles. However, these hierarchically structured assemblies can only be obtained within a fairly narrow range of BP4T/4VP ratio. This study demonstrates the first example of supramolecular assemblies containing multivalent small molecules and provides an effective route to better control the hierarchical ordering in a supramolecular system with useful electronic, optical, and thermoresponsive properties.
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RESULTS AND DISCUSSION The small molecule 3,3′-([2,2′:5′,2″:5″,2‴-quaterthiophene]5,5‴-diylbis(decane-10,1-diyl))diphenol (BP4T) was designed based on previous studies using monofunctionalized quarterthiophene, called “4T”.22 BP4T was synthesized by crosscoupling of 2,2′-bithiophene precursors, and detailed procedures are shown in the Supporting Information.30 Both ends of the BP4T small molecule were designed to hydrogen bond with PS-b-P4VP to investigate how the number of functionalities on the filler molecule affects the coassembly of supramolecules. Chemical structure of resulting supramolecule, PS-b-P4VP(BP4T), is shown in Figure 1a. Differential scanning calorimetry (DSC) was first conducted on pure BP4T and PS-b-P4VP(BP4T)r supramolecules to study the molecular ordering of the small molecular filler and the supramolecule as a function of BP4T/4VP stoichiometry (Figure 1b,c). The subscript r that represents the molar ratio between BP4T and 4VP was varied from 0.25 to 1. For pristine BP4T, two endothermic peaks at 146 and 166 °C and exothermic peaks at 129 and 163 °C are observed in the heating and cooling curves, respectively. These peaks in the thermograms remain in the same position for consecutive heating and cooling cycles. These melting and crystallization peaks in DSC are consistent with the in situ SAXS result for pristine BP4T (see Supporting Information Figure S1). The scattering peak at q = 0.138 Å−1 shows a crystalline BP4T small molecule that has a periodicity of 4.6 nm at temperatures below 140 °C. Such crystals melt at 146 °C and form the second
EXPERIMENTAL SECTION
Materials. PS(40 000 Da)-b-P4VP(5600 Da) (PDI = 1.1) was purchased from Polymer Source, Inc. Tetrahydrofuran was purchased from Sigma-Aldrich and filtered through basic alumina prior to use. Synthetic procedures for BP4T small molecule is described in detail in the Supporting Information. Sample Preparation. PS-b-P4VP and BP4T were separately dissolved in THF first, then mixed, and stirred overnight for hydrogen bond formation between 4VPs and BP4Ts. The mole ratio (r) of BP4T to 4VP unit was varied from 0.25, 0.5, 0.75, and 1. The solution was transferred in a Teflon beaker, and the solvent was allowed to evaporate slowly. Casted films were then placed in a vacuum oven at 30 °C for 12 h to remove the residual solvent. The bulk PS-bP4VP(BP4T)r films were then collected from the beakers and annealed at various temperatures in a vacuum oven using hermetic DSC pans. Differential Scanning Calorimetry (DSC). The BP4T and PS-bP4VP(BP4T)r films were sealed in hermetic Al pans, and the measurements were performed on a TA-Q200, while heating and cooling the samples from 30 to 200 °C and then to 30 °C for three cycles at a ramp rate of 10 °C/min. Transmission Electron Microscopy (TEM). The bulk samples were embedded in an epoxy resin and then cured at 60 °C overnight. Thin slices of the samples were microtomed by RMC MT-X Ultramicrotome (Boeckler Instruments) and transferred on copper B
DOI: 10.1021/acs.macromol.6b00317 Macromolecules XXXX, XXX, XXX−XXX
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Figure 2. (a) TEM image of PS-b-P4VP(BP4T)0.5 supramolecule and the image analysis for BCP (b, e) and P4VP(BP4T) comb block (c, f) on the yellow rectangular boxes for a PS-b-P4VP(BP4T)0.5 supramolecule annealed at 190 °C. (d) Schematic illustration of coassembly of BP4T and PS-bP4VP.
Figure 3. TEM images of PS-b-P4VP(BP4T)r supramolecules with different BP4T contents (r = 0.25, 0.75, and 1). The samples sealed in aluminum DSC pans were annealed at 190 °C for 5 h.
crystalline structure with 4.0 nm in period (q = 0.158 Å−1) as depicted in the SAXS profile at 150 °C. These BP4T crystals then melt to isotropic liquid at 166 °C, which corresponds to the complete disappearance of the BP4T scattering peak. With decreasing the temperatures from a melt state, DSC and SAXS data exhibit crystallization and scattering peaks accordingly. For a PS-b-P4VP(BP4T)0.25 composite, very broad peaks at ∼170 °C in both heating and cooling processes were collected. This implies that BP4T crystals with considerable size distribution are randomly sequestered in the P4VP domains. Upon increasing the small molecule ratio to 0.5, in this case a ratio between hydrogen bond acceptor and donor is 1; original peaks corresponding to the small molecule crystallization and fusion completely disappear, and two new peaks are observed at 180 °C/185 °C and 179 °C/182 °C in heating and cooling cycles, respectively. Increased crystallization and melting points are probably due to the homogeneously distributed nanoscopic BP4Ts confined inside the polymer microdomains.27 When we further increase the r to 0.75 and 1, both melting peaks of pure BP4T and higher Tm for assembled BP4T are observable in DSC traces. This suggests that the system cannot uptake additional small molecules once the 4VP sites are fully coupled with phenolic hydrogens, and the extra amount of small
molecules is most likely expelled and phase-separated from the supramolecular assembly. The DSC data suggest PS-b-P4VP(BP4T)0.5 is expected to be a coassembly of BCP and BP4T without phase separation, and thereby further investigation was performed. Incorporation of the BP4T small molecule into the P4VP microdomain was studied by the sequential DSC scans (see Figure S2). In the first heating curve, there were two melting peaks at 146 and 181 °C that coincide with the phase-separated small molecules and the coassembled PS-b-P4VP(BP4T) in the as-cast composite, respectively. In the second and third heating cycles, however, the BP4T crystallization peak at low temperature completely vanished while intensity of the comb block melting peaks were enhanced indicating all the phase-separated BP4Ts have diffused to P4VP subdomains and hydrogen bonded with 4VP units to form supramolecular composites. Crystallization peaks in the cooling curves remained in the same position because the composites were completely melted in the first heating cycle. The morphologies of the PS-b-P4VP(BP4T)0.5 post thermal treatment were studied by transmission electron microscopy (TEM) conducted on microtomed thin sections of the supramolecule. The sample was annealed at 190 °C, which is C
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Macromolecules higher than the melting temperature of the composites, for 5 h and then slowly cooled down to room temperature over several hours. To enhance the contrast of TEM images, iodine vapor was used to stain the samples. Morphologically, PS-bP4VP(BP4T)0.5 exhibits well-defined lamellar-within-lamellar structure post thermal treatment (Figure 2a−c). The grain size of lamellar morphology is over tens of micrometers without any additional treatment to bias their macroscopic alignment. The schematic drawing of the supramolecular assembly is illustrated in Figure 2d. BCP lamellar structure with ∼56 nm in period is formed by symmetric PS coil and P4VP(BP4T) comb blocks (Figure 2e). Considering the fact that the contour length of the P4VP block used in this study is ∼27 nm, P4VP chains are almost fully extended by incorporating BP4T fillers into the domains. Within the comb block, the second 4.7 nm lamellae, which are comparable to the length of a stretched BP4T molecule, are observable implying that one layer of bifuctional BP4T molecules holds two neighboring P4VP chains (Figure 2f). No evidence of small molecule phase separation was detected, which suggests that all the BP4T small molecules are involved in the hydrogen bonding with 4VPs. When the stoichiometry between function groups (phenol and pyridine) is mismatched, well-ordered lamellae-withinlamellae structure cannot be obtained. When the BP4T to 4VP ratio is 0.25, one can observe both P4VP(BP4T) cylinders embedded in PS matrix and lamellar morphology due to the inhomogeneous distribution of BP4T small molecule in the supramolecule (Figure 3a). When the amount of BP4T is larger than 0.5, e.g. r = 0.75 and 1, the TEM images show wellaligned BCP lamellae as shown in Figure 3b,c. However, phaseseparated small molecules were also visible, which is consistent with the DSC results in Figure 1. More phase-separated BP4Ts were observed with higher r = 1 because more unbound filler molecules existed in the sample. These results suggest that BP4T-based supramolecules tend to expel extra small molecules out of the system once all the 4VP binding sites have participated in the hydrogen bonding. This behavior is different from previously reported monofunctional small molecule-based supramolecules that allow loose unbound filler molecules in the system.27 We attribute this to the bridging of BP4T molecules across adjacent P4VP chains. The coassembly process and the structural evolution of the supramolecule PS-b-P4VP(BP4T)0.5 was traced by using in situ SAXS while heating from 30 to 190 °C at a heating rate of 20 °C/min and then cooling back to 30 °C (Figure 4). Before the SAXS data acquisition the sample was thermally equilibrated for 10 min at each temperature. The supramolecule was solvent casted, and the trace solvent was removed by drying in a vacuum oven at room temperature. When annealed at temperatures near and above the melting temperature of the BP4T small molecule, PS-b-P4VP(BP4T)0.5 supramolecule forms well-ordered hierarchical assemblies with two distinct periods that correspond to BCP lamellae and P4VP(BP4T) comb domains.17,20 Upon heating the PS-b-P4VP(BP4T)0.5 from 30 to 190 °C, phase-separated small molecules are incorporated into the P4VP blocks, and a hierarchically ordered supramolecule is created. At temperatures below 110 °C, one can identify a diffraction peak at q = 0.137 Å−1 and the second-order peak at q = 0.264 Å−1 which corresponds to the small molecule periodicity of 4.6 nm. This periodicity is exactly the same as that of pristine BP4T (see Figure S1). Upon heating the sample to 120 °C, a scattering peak at around q = 0.0111 Å−1
Figure 4. In situ SAXS profiles of PS-b-P4VP(BP4T)0.5 supramolecules. The supramolecules were heated from 30 to 190 °C and cooled down to 30 °C. The red SAXS plots correspond to SAXS profiles during the heating process, and the blue plots are for the cooling cycle. Temperatures for each SAXS profile from the bottom are 30, 70, 110, 120, 130, 140, 150, 160, 170, 180, 190, 180, 170, 160, 140, 120, 100, and 30 °C, respectively.
corresponding to BCP periodicity of 56 nm evolves, and a diffraction peak from a small molecule shifts to a smaller value, q = 0.133 Å−1 (∼4.7 nm in period). This supports the incorporation of BP4T molecules into the P4VP domains by hydrogen bonding and the formation of ordered lamellae in the P4VP(BP4T) comb blocks. As we increase the temperature from 120 °C, BCP periodicity gradually decreases and becomes 54 nm at 170 °C. When further increasing the temperature to 180 °C, the BCP periodicity shrinks to 37 nm (q = 0.0167 Å−1), and the intensity of small molecule packing peaks decrease due to the melting of the P4VP(BP4T) comb block.27 Such behavior is consistent with the DSC result shown in Figure 1. At 190 °C, the BCP scattering peaks with 37 nm in period at low q regime becomes pronounced in the melt state, but the peaks corresponding to the comb block ordering of 4.7 nm are completely disappeared. Such results show that BP4T molecules no longer form ordered structures in the comb domains at high temperature. Dynamic phase behavior of the BP4Ts in the PS-b-P4VP polymer is qualitatively similar to the monofunctional 4T molecules in the same polymer host.27 However, the onset temperature for the BP4T incorporation into the polymer microdomain (∼120 °C) is 50 °C higher than that of the monofunctional small molecule such as PDP (∼70 °C).27 This suggests that the hydrogen bonding between the bifunctional BP4T fillers is stable up to ∼120 °C and is much more robust than the monofunctional counterpart.20 Upon cooling the PS-b-P4VP(BP4T)0.5 supramolecule from 190 to 30 °C, P4VP(BP4T) comb block crystallizes and the BCP periodicity expands back to 54 nm. A substantial increase in BCP periodicity upon cooling from its melt state is mainly attributed to the P4VP chain stretching induced by the crystallization of BP4T small molecules. PS domains also vary to compensate the cross-sectional mismatch upon comb domain deformation.27 It is noteworthy that the BCP thirdorder scattering peak at q = 0.033 Å−1 increases in the cooldown process while the second- and fourth-order peaks are not observable, indicating almost symmetric PS coil and P4VP(BP4T) comb domains in size, which is supported by the TEM images. On the other hand, small molecule crystallization peak at q = 0.13 Å−1, ∼4.7 nm in size, was invariant to the D
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Figure 5. TEM images of PS-b-P4VP(BP4T)0.5 supramolecules annealed at selected temperatures. For (b), (c), and (d), composite films were annealed at 130, 155, and 170 °C, respectively, for 20 h to provide sufficient time for assembly. For (e) and (f) (with higher magnification), a supramolecular film was treated at 190 °C for 5 h. All thermal treatments were done on supramolecular composites sealed in aluminum DSC pans.
treatment temperature is further increased to 190 °C, T > Tm of the composite, macroscopically aligned lamellae on the order of tens of micrometers in grain size were created. PS coil and P4VP(BP4T) comb domains were clearly developed and formed parallel lamellae. Transmission SAXS results on PS-b-P4VP(BP4T)0.5 composites thermally annealed at various temperatures also support the BCP assembly and comb block ordering (see Figure S3a,b). The supramolecule forms well-ordered hierarchical structures with BCP lamellae (54 nm) and comb lamellae (4.7 nm) when treated near and above the melting temperature of the small molecule BP4T. WAXS profiles in Figure S3c demonstrate the incorporation of BP4T molecules and their molecular ordering within the supramolecular assemblies. For pure BP4T, scattering peaks at around 1.5 and 2.0 Å−1 imply herringbone packing of BP4T like other oligothiophenes.32 As we anneal the composites at high temperatures, the scattering peaks merge and become broader, indicating small BP4T crystals get sequestered well within the P4VP(BP4T) domains.27
temperature change. These results demonstrate a simple route to generate thermoresponsive hierarchical supramolecules using simple thermal treatment.12 It is noteworthy that BP4T can generate much more effective periodicity changes in the composites than monofunctional small molecule such as PDP and 4T by using a considerably low amount of the BP4T molecule.16,27 Structural evolution of the PS-b-P4VP(BP4T)0.5 supramolecules can also be identified by the TEM data from thermally annealed samples at selected temperatures (Figure 5). For as-cast films, macrophase separation of BP4Ts and the mixtures of hexagonally packed P4VP(BP4T) cylinders and randomly oriented small lamellae are clearly observable in Figure 5a. Planar BP4T molecules tend to crystallize and phaseseparate when the solvent evaporates, and therefore only a part of the BP4Ts is incorporated in the BCP microdomains at first.31 As the annealing temperature is increased to 130 °C, which is lower than the melting temperature but higher than the glass transition temperature of the block polymer, P4VP(BP4T) cylinders were no longer observable and shortrange-ordered lamellar morphology was seen in Figure 5b, implying increased amount of BP4Ts are incorporated in BCPs. This explains that bifunctional BP4Ts can diffuse through PS domains and form hydrogen bonds with 4VP units in the block polymer at temperatures below Tm of the small molecule.27 However, phase-separated small molecules still exist suggesting the rate of small molecule incorporation is considerably slow. By thermally annealing the samples at T > Tm of the small molecule (ca. 155 and 170 °C), supramolecular lamellae on the order of few hundred nanometers in grain size were developed quite well without phase separation of the small molecules. In these conditions, BP4T molecules have enough mobility to diffuse into the P4VP domains and form well-defined supramolecular assemblies. Ordering of the BCP lamellae is slightly better for the sample annealed at 170 °C, but no qualitative difference was found between the two samples. As
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CONCLUSION
Coassembly of symmetrically end-functionalized BP4T fillers with PS-b-P4VP BCPs is systematically investigated as functions of small molecule content and annealing temperature. The BP4T small molecule used in this study is designed to possess two phenol groups in both ends that act as hydrogen bonding donors in the assembly process. When the ratio r between BP4T and 4VP is 0.5 and thermally treated above the melting temperature of the composite, the supramolecule creates macroscopically aligned hierarchical morphology with two distinct periodicities of 54 and 4.7 nm for BCP and comb domain, respectively. Crystallization of BP4T from a melt state leads to a very large BCP periodicity increase (from 37 to 54 nm) similar to a previously reported monofunctionalized small molecule. When the r is smaller than 0.5, the composite shows E
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randomly distributed cylinders and short lamellae. If r is greater than 0.5, the supramolecules exhibit long-range ordered lamellae. However, phase-separated BP4T molecules are also seen in these samples because the system does not incorporate more filler molecules into the supramolecular assemblies when the 4VP sites are already fully saturated with hydrogen bonds with other BP4Ts. Morphological observation with different BP4T ratio and the width of the comb lamellae suggest that bifunctional BP4T molecules are incorporated into the P4VPs by bridging two P4VP chains. Our results broaden the fundamental understanding about the supramolecular approach when multifunctional molecules are used as a filler material. Results presented here show that bifunctionalized small molecules can be used to improve ordering in supramolecular structures over several length scales at higher precision.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00317.
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Experimental details (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (T.X.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the U.S. Department of Energy under Contract DE-AC02-05CH11231 through the “Organic/ inorganic Nanocomposite Program”. Part of the work was performed at the Molecular Foundry and the Advanced Light Source, both of which were supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract DE-AC02-05CH11231.
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DOI: 10.1021/acs.macromol.6b00317 Macromolecules XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.macromol.6b00317 Macromolecules XXXX, XXX, XXX−XXX
