Article pubs.acs.org/Macromolecules
Ultrahigh Molecular Weight Linear Block Copolymers: Rapid Access by Reversible-Deactivation Radical Polymerization and SelfAssembly into Large Domain Nanostructures Jose Kenneth D. Mapas,† Tim Thomay,‡ Alexander N. Cartwright,‡ Jan Ilavsky,§ and Javid Rzayev*,† †
Department of Chemistry, University at Buffalo, The State University of New York, Buffalo, New York 14260-3000, United States Department of Electrical Engineering, University at Buffalo, The State University of New York, Buffalo, New York 14260-1900, United States § Advanced Photon Source Division, Argonne National Laboratory, Argonne, Illinois 60439, United States ‡
S Supporting Information *
ABSTRACT: Block copolymer (BCP) derived periodic nanostructures with domain sizes larger than 150 nm present a versatile platform for the fabrication of photonic materials. So far, the access to such materials has been limited to highly synthetically involved protocols. Herein, we report a simple, “user-friendly” method for the preparation of ultrahigh molecular weight linear poly(solketal methacrylate-b-styrene) block copolymers by a combination of Cu-wire-mediated ATRP and RAFT polymerizations. The synthesized copolymers with molecular weights up to 1.6 million g/mol and moderate dispersities readily assemble into highly ordered cylindrical or lamella microstructures with domain sizes as large as 292 nm, as determined by ultrasmall-angle X-ray scattering and scanning electron microscopy analyses. Solvent cast films of the synthesized block copolymers exhibit stop bands in the visible spectrum correlated to their domain spacings. The described method opens new avenues for facilitated fabrication and the advancement of fundamental understanding of BCPderived photonic nanomaterials for a variety of applications.
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INTRODUCTION Block copolymers (BCP) are an important class of soft materials that feature two or more chemically distinct polymer blocks covalently linked together. Thermodynamic incompatibility between the blocks drives microphase separation in melt state, producing periodic nanomaterials whose morphology and domain sizes are dictated by the copolymer composition and molecular weight.1−4 Owing to the tunable physical and chemical characteristics of block copolymers and easily accessible domain sizes of 5 × 105 g/mol), is extremely limited due to synthetic challenges associated with the preparation of very long linear block copolymers, and kinetic limitations in the self-assembly of highly entangled UHMW block copolymer melts. So far, arduous anionic polymerizations have afforded the exclusive synthetic pathway to UHMW linear block copolymer based materials,10,17,18 while kinetic limitations have been addressed by utilizing BCPs with a brush-like molecular architecture, exhibiting low density of entanglements.12,19,20 In both cases, © XXXX American Chemical Society
laborious synthetic protocols are required, which limits the availability of large domain size nanomaterials to a broader scientific community. Herein, we disclose a simple and “userfriendly” method for the preparation of ultrahigh molecular weight linear block copolymers that readily assemble into highly desirable periodic nanostructures with large domain sizes (>150 nm) and photonic properties (Figure 1). Reversible-deactivation radical polymerization (RDRP) has been used to produce well-defined polymers having linear, branched, comb, star, and network architectures.21−23 The precise control over molecular dimensions and architecture stems from the dynamic equilibrium between active and
Figure 1. Fabrication of large domain spacing photonic nanomaterials from UHMW BCPs prepared by radical polymerization. Received: April 25, 2016
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DOI: 10.1021/acs.macromol.6b00863 Macromolecules XXXX, XXX, XXX−XXX
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Tetradetector Array system equipped with two PLgel PolyPore columns (Polymer Laboratories, Varian Inc.). The detector unit contained refractive index, UV, viscosity, low (7°), and right angle light scattering modules. Measurements were carried out in THF as the mobile phase at 30 °C. The system was calibrated with 10 polystyrene standards having molecular weights ranging from 1.2 × 106 to 500 g/ mol. Refractive index increments (dn/dc) for PMMA, PSM, and acetylated PHEMA were measured to be 0.089, 0.067, and 0.071 mL/g in THF (T = 30 °C; λ = 630 nm), respectively, and were used to determine the absolute molecular weights of the homopolymers. Scanning electron microscopy (SEM) images were obtained by a Carl Zeiss AURIGA instrument using secondary electron detector at an accelerating voltage of 3.0 kV. Prior to SEM analysis, fractured polymer samples were coated with a 1−2 nm gold layer. Optical measurements were obtained from an Ocean Optics spectrometer with a thermal light source (Euromex). Transmission measurements were done on samples sandwiched between glass microscope slides that were mounted on a copper mask. The samples were scanned from 190 to 850 nm with an integration time of 1s. Sample transmission data were normalized against the transmission data through a copper mask. Ultrasmall-angle X-ray Scattering (USAXS) and pinhole SAXS measurements were performed at the Advanced Photon Source (APS) beamline 9ID-C at the Argonne National Laboratory. USAXS and pinhole SAXS data were sequentially acquired and was merged into a single data set using the Irena SAS package.37,38 SM Polymerization. Solketal methacrylate (1 mL, 5.13 mmol), Me6TREN (0.68 μL, 2.54 μmol), Cu0 wire (9 pieces), and DMSO (0.48 mL) were added to a reaction flask. Subsequently, a solution of cumyl dithiobenzoate in DMSO (0.0815 M) was added to the flask (31.5 μL, 2.56 μmol). The mixture was then degassed by three cycles of freeze−pump−thaw, and placed in an oil bath at 100 °C. After a predetermined time, the flask was cooled to room temperature, and an aliquot of the solution was taken for percent conversion analysis by 1H NMR. The contents of the flask were diluted with dichloromethane and passed through a neutral alumina column, then precipitated in methanol (twice). The polymer was dried overnight under vacuum. MMA Polymerization. Methyl methacrylate (1 mL, 9.42 mmol), Me6TREN (1.3 μL, 4.86 μmol), Cu0 wire (9 pieces), and DMSO (0.44 mL) were added to a reaction flask. Subsequently, a solution of cumyl dithiobenzoate in DMSO (0.0816 M) was added to the flask (58.3 μL, 4.75 μmol). The mixture was then degassed by three cycles of freeze− pump−thaw, and placed in an oil bath at 100 °C. After a predetermined time, the flask was cooled to room temperature, and an aliquot of the solution was taken for percent conversion analysis by 1 H NMR. The contents of the flask were diluted with dichloromethane and passed through a neutral alumina column, then precipitated in methanol (twice). The polymer was dried overnight under vacuum. HEMA Polymerization. 2-Hydroxyethyl methacrylate (1 mL, 8.24 mmol), a solution of Me6TREN in DMSO (0.0799 M) (1.3 μL, 4.86 μmol), Cu0 wire (9 pieces), and DMSO (2 mL) were added to a reaction flask. Subsequently, a solution of cumyl dithiobenzoate in DMSO (0.0767 M) was added to the flask (53.2 μL, 4.08 μmol). The mixture was then degassed by three cycles of freeze−pump−thaw, and placed in an oil bath at 100 °C. After a predetermined time, the flask was cooled to room temperature, and an aliquot of the solution was taken for percent conversion analysis by 1H NMR. The contents of the flask were diluted with methanol and passed through a neutral alumina column then precipitated in diethyl ether (twice). The polymer was dried overnight under vacuum. PHEMA was acetylated for SEC analysis in THF. PHEMA (20 mg) was dissolved in 0.50 mL of pyridine. Acetic anhydride (0.1 mL) was added dropwise to the solution, and the mixture was stirred at room temperature for 12 h. After the reaction, the mixture was diluted with dichloromethane, then precipitated in methanol (twice). The polymer was dried overnight under vacuum to yield a white solid. Example of Synthesis of PSM−PS (SK-2). PSM homopolymer (Mn = 401 900 g/mol, 0.096 g, 0.24 μmol) and AIBN (0.02 μmol from 7 mM stock solution in styrene) were dissolved in styrene (1.06 mL, 9.25 mmol) in a reaction flask equipped with a stir bar. This mixture was allowed to stir until the solids were completely dissolved. The
dormant polymer chains, achieved either by reversible deactivation, such as the case in atom transfer radical polymerization (ATRP), or by reversible transfer, which occurs in reversible addition−fragmentation chain-transfer (RAFT) polymerization. Compared to anionic polymerizations, RDRPs are more tolerant to functional groups, applicable to a broader range of monomers, and require less stringent conditions. Recently, there has been progress in circumventing inherent limitations of RDRP and developing protocols to achieve homopolymers with ultrahigh molecular weights. Various UHMW poly(meth)acrylates have been synthesized by Cumediated processes24−30 and by high-pressure RAFT polymerization.31 Despite an increasing number of procedures for the synthesis of UHMW (meth)acrylic homopolymers, the access to UHMW linear block copolymers by RDRP methods has been very limited, highlighting the difficulty of reinitiating the second block off a very long polymer chain. Matyjaszewski et al. was the first group to report the synthesis of UHMW poly(methyl methacrylate-b-butyl methacrylate)27 and poly(methyl methacrylate-b-methyl acrylate)26 by ATRP processes, while Davis et al. recently disclosed the synthesis of a highly compositionally asymmetric and polystyrene-rich amphiphilic block copolymer by emulsion RAFT polymerization. 32 However, RDRP synthesis of UHMW linear block copolymers that phase separate into ordered periodic nanostructures has not been reported to date. In this paper, we describe a robust RDRP protocol for the synthesis of UHMW polymethacrylates and their block copolymers with styrene (Scheme 1). The utilized method is Scheme 1. Synthesis of UHMW Block Copolymers by RDRP
halide-free, does not require any sensitive catalysts/reagents to start the process and relies on a combination of Cu-mediated RDRP and RAFT polymerization. We also demonstrate that upon simple solvent casting, these copolymers readily selfassemble into photonic nanomaterials with domain sizes as large as 292 nm, which we believe to be the largest reported for a pure linear BCP.
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EXPERIMENTAL SECTION
Materials. Solvents and reagents were purchased from commercial sources and used directly without purification unless noted otherwise. Azobis(isobutyronitrile) (AIBN) was recrystallized from methanol. DMSO was vacuum distilled and stored over 4 Å molecular sieves. Styrene (S), methyl methacrylate (MMA), and solketal methacrylate (SM) were passed through basic alumina column prior to polymeriz a ti o n to r e m o v e a n y i nh i b i to r s . S M 3 3 a n d t r i s ( 2 dimethylaminoethyl)amine (Me6TREN)34 were prepared according to literature procedures. 2-hydroxyethyl methacrylate (HEMA)35 and Cu0 wire36 (20 gauge, length = 5 mm) were purified using literature procedures. Measurements. All 1H NMR spectra were recorded on a Varian INOVA-500 (500 MHz) spectrometer by using CDCl3, d6-DMSO, or CD2Cl2 as solvent. Size exclusion chromatography (SEC) analyses were performed using Viscotek’s GPC Max and TDA 302 B
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Macromolecules mixture was then bubbled with N2 for 15 min, and placed in an oil bath at 65 °C. After 24 h, the flask was cooled to room temperature and the contents were diluted with dichloromethane and precipitated in hexanes (twice). The resulting polymer was suspended in boiling acetonitrile to remove residual PSM homopolymer. The polymer was then dried overnight under vacuum to yield a powdery solid (94 mg). SEC (polystyrene calibration): Mn = 296 kg/mol, Đ = 1.63. 1H NMR: n(PS) = 2,880.
catalyst in the presence of Cu wire and anisole as the solvent, in particular for generating UHMW polymethacrylates.27,42 Initiating radicals are generated from the CTA in the presence of a Cu(I) catalyst; and owing to rapid chain transfer facilitated by CTAs, polymers with low dispersities are produced even in the absence of deactivating Cu(II) species.27 Zhu et al. have demonstrated that methyl methacrylate can also be polymerized in a controlled fashion in the presence of only a RAFT CTA dithioester and Cu(0)/N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA) catalyst in DMSO.41 The polymerization was relatively slow, and the synthesis of UHMW polymers was not attempted. Generally, even in the presence of Cu(0), methacrylates require long reaction times (24−120 h) to produce polymers with ultrahigh molecular weights.27,29,30 In this work, we utilized the capacity of Cu(0)-Me6TREN catalyst system to enhance polymerization rates,43 and the control provided by CDB via the RAFT process, to develop a polymerization protocol that produced high molecular weight polymethacrylates in a rapid but controlled manner. Solketal methacrylate was chosen due to its fast polymerization kinetics and a latent diol functionality, which will be exploited in future publications. While SM provided the best results, we successfully applied similar conditions to achieve controlled polymerization of other methacrylates, such as MMA and HEMA (Supporting Information). UHMW block copolymers were prepared by taking advantage of the dithioester end-groups on the PSM to promote RAFT polymerization of styrene (Scheme 1), notorious for its low kp values. We found that using high monomer-to-CTA ratio and stopping the reaction at low conversions (∼10%) afforded the desired copolymers. Thus, a polystyrene (PS) block with Mn = 420 kg/mol can be installed in 24 h by a simple RAFT polymerization in the presence of PSM macro-CTA. The polymerization was twice as fast as a similar RAFT polymerization of styrene conducted in the presence of a small molecule CTA (5.9% conversion after 24 h). We attribute this behavior to a decrease in the rate of diffusion-limited termination processes when macro-CTA is used, which aids the formation of UHMW polystyrene block copolymers. The SEC traces of the block copolymers appeared at lower retention times, compared to the macro-CTA agent (Figure 2d), and exhibited moderate increase in dispersity values (Đ = 1.4−1.6) (Table 1). Relatively high dispersity of the PS block is expected here as a result of extremely low concentrations of the dithiobenzoate end groups, which will make it harder for the addition reaction of the RAFT process to compete with monomer propagation, resulting in broadening of
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RESULTS AND DISCUSSION Controlled polymerization of solketal methacrylate (SM) was conducted using a Cu(0)-mediated RDRP procedure, where cumyl dithiobenzoate (CDB) served as the initiator, copper wire as the catalyst precursor, and Me6TREN as the ligand (Scheme 1). The polymerization followed a first-order behavior and produced PSM polymers with low dispersities (Đ), featuring linear evolution of polymer molecular weight with monomer conversion (Figure 2). The obtained Mn values were
Figure 2. Polymerization of solketal methacrylate ([SM]: [CDB]: [Me6TREN] = 2,000:1:1). (A) First-order kinetic plot, (B) evolution of molecular weight and dispersity with conversion, (C) SEC analysis of PSM homopolymers, and (D) SEC characterization of PSM−PS block copolymer.
consistently higher than theoretically predicted ones, likely due to low initiation efficiency. The reaction was rapid, reaching 60% conversion (Mn = 402 kg/mol, Đ = 1.27) in 1 h, after which it abruptly stopped, possibly due to high viscosity of the reaction medium. Under more dilute conditions (1 M), higher conversions (78%) could be achieved, but polymer dispersity increased significantly (1.90). We also conducted control experiments in the absence of CDB, Cu wire or Me6-TREN. In each case, less than 5% conversion was obtained after 90 min, indicating that all three components were necessary for the successful outcome of the polymerization. Cu(0) has the ability to activate radical initiators in the presence of a ligand,39−41 and has been reported to facilitate the synthesis of UHMW polymers. For example, Percec et al. have been able to achieve ultrafast polymerization of methyl acrylate in the presence of Cu(0)/Me6-TREN catalyst with an alkyl bromide initiator and DMSO as the solvent.28 Matyjaszewski et al. have recently demonstrated the ability of RAFT chain transfer agents (CTAs) to function as pseudohalide initiators for ATRP with a Cu(I)/tris(2-pyridylmethyl)amine (TPMA)
Table 1. Structural and Morphological Characteristics of PSM−PS Block Copolymers polymer
f PSa
SK-1 SK-2 SK-3 SK-4 SK-5 SK-6
0.46 0.42 0.62 0.64 0.58 0.72
Mn,total (g/mol)b
Đc
d (nm)d
morph.e
× × × × × ×
1.49 1.63 1.39 1.63 1.60 1.39
209 178 235 222 257 292
C C/L L L L L
9.7 7.0 1.4 1.1 9.7 1.6
105 105 106 106 105 106
a
Volume fraction of PS. bCalculated from a combination of SEC-LS and 1H NMR analyses. cDetermined by SEC in THF using PS calibration. dPrincipal domain spacing from USAXS. eDetermined by USAXS and SEM (L, lamella, C, cylinders). C
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Figure 3. USAXS and SEM analyses of block copolymers (A) SK-1 and (B) SK-2 with cylindrical morphology and (C) SK-3 and (D) SK-4 with lamella morphology.
the molecular weight distribution.44 After polymerization, the product was washed in boiling acetonitrile (selective solvent for PSM) to remove unreacted PSM chains and in cyclohexane (selective solvent for PS) to remove any polystyrene homopolymer byproduct. These treatments did not produce significant changes in the molecular weight distribution of the copolymers (Supporting Information). The length of the PS block was calculated from NMR spectra by comparing the signal integral areas of the aromatic PS peak at 6.3−7.3 ppm to that of the PSM peak at 4.3 ppm (Supporting Information). Using these methodology, a series of block copolymers with varying molecular weights and polystyrene volume fractions were obtained (Table 1). One must note that these copolymers do not boast low dispersity values. Recent studies have shown that high chain length dispersity in block copolymers, while having an impact of the phase diagram, does not preclude the formation of well-ordered morphologies with uniform microdomain sizes.45−51 It does, however, have a generally favorable impact on polymer rheological properties and processing.52−55 Differential scanning calorimetry analysis of the block copolymer powders revealed two distinct glass transitions (Supporting Information), corresponding to the phase separated PSM and PS domains. Free-standing films of the block copolymers were prepared by solvent casting from oxylene or toluene. Ultrasmall angle X-ray scattering (USAXS) analysis of the copolymer films38 showed a strong primary scattering peak and a number of higher order Bragg reflections, suggesting the formation of highly ordered periodic nanostructures despite chain length dispersity. Domain sizes were calculated from USAXS data to be in the range of 180−290 nm (Table 1). Scanning electron microscopy (SEM) analysis in conjunction with USAXS data allowed for unambiguous identification of the lamella and cylindrical morphologies (Table 1 and Supporting Information). As shown in Figure 3d, copolymer SK-4 featured 5 higher order reflections in the USAXS pattern, indicating the formation of an ordered lamella morphology with 222 nm domain spacing. Copolymer SK-1, on the other hand, exhibited a hexagonally packed cylindrical morphology with domain spacing of 209 nm, as characterized by SEM and USAXS (Figure 3a).
Morphological characterization of the synthesized block copolymers revealed a shift in the phase boundaries consistent with the disperse nature of the PS block. For example, asymmetric copolymer SK-6 ( f PS = 0.72) formed lamella morphology (expected for monodisperse BCPs with fA = 0.4− 0.6),3 while nearly symmetric copolymer SK-1 (f PS = 0.46) exhibited cylindrical morphology. These results suggest that the interfaces curve toward PS domains (containing chains with high chain length dispersity), as has been observed for other disperse block copolymers.49,56 Additionally, we speculate that BCP chain length dispersity aids in the formation of large domain spacing nanomaterials in two ways: by lattice spacing dilation,46 which results in domain sizes larger than what is expected from a monodisperse BCP with similar composition, and by improved kinetics due to the presence of shorter chains. Photonic crystals are materials having periodic dielectric structures that introduce an optical band gap, which can manipulate and control the propagation of light. In particular, if the periodic structures have an optical thickness of a quarter of the wavelength it is possible to construct a highly reflective mirror. Self-assembled linear block copolymers have been shown to exhibit photonic band gaps in the short visible wavelength range,11,15 often with help of additives (homopolymer or solvent) to swell the microdomains.18,57,58 The copolymer films produced in this work appeared colored to a naked eye without the need for any additives or manipulations. As evidenced from the preliminary optical characterization (Figure 4), the transmission spectra of the PSM−PS films featured a highly reflective spectral band (stop band), whose wavelength increased with increasing domain spacing obtained from USAXS, showing a good correlation between the materials microstructure and its optical properties.
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CONCLUSIONS In summary, we developed a simple RDRP-based protocol for the preparation of UHMW linear block copolymers. Cu-wiremediated process in the presence of Me6-TREN and cumyl dithiobenzoate provided rapid access to high molecular weight poly(solketal methacrylate). RAFT polymerization of styrene initiated from dithiobenzoate end-groups of PSM allowed for the formation of PSM−PS block copolymers with molecular D
DOI: 10.1021/acs.macromol.6b00863 Macromolecules XXXX, XXX, XXX−XXX
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Figure 4. Transmittance spectra of UHMW PSM−PS block copolymer thin films, and optical images illustrating reflected (top row) and transmitted (bottom row) colors of the prepared films.
weights up to 1.6 million g/mol. Despite chain length dispersity, the synthesized copolymers readily assembled into highly ordered morphologies with uniform microdomain sizes as high as 292 nm. Lamella and cylindrical morphologies were observed by USAXS and SEM analyses at polymer compositions skewed toward high polystyrene content compared to monodisperse block copolymers, consistent with the presence of a disperse polystyrene block. Ordered block copolymer films exhibited photonic properties with stop bands in the visible spectrum. The access to BCP-based large domain spacing nanomaterials through a “user-friendly” synthetic protocol is poised to advance their research, applications, and broader impact.
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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.6b00863. Experimental details; NMR, SEC, DSC, USAXS and SEM characterization of the synthesized copolymers (PDF)
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AUTHOR INFORMATION
Corresponding Author
*(J.R.) E-mail: jrzayev@buffalo.edu. Notes
The authors declare no competing financial interests.
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ACKNOWLEDGMENTS We acknowledge financial support from the National Science Foundation (DMR-1409467) and the University at Buffalo. The use of the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-06CH11357.
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REFERENCES
(1) Bates, F. S. Polymer-Polymer Phase-Behavior. Science 1991, 251, 898−905. E
DOI: 10.1021/acs.macromol.6b00863 Macromolecules XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.macromol.6b00863 Macromolecules XXXX, XXX, XXX−XXX
