Article pubs.acs.org/Macromolecules
Microphase-Separated Thiol−Ene Conetworks from Telechelic Macromonomers with Asymmetric Molecular Weights Kelly R. McLeod† and Gregory N. Tew*,†,‡,§ †
Department of Polymer Science and Engineering, ‡Department of Veterinary and Animal Sciences, and §Molecular and Cellular Biology Program, University of Massachusetts Amherst, Amherst, Massachusetts 01003, United States S Supporting Information *
ABSTRACT: Phase-separated networks made from different macromonomers with disparate properties provide a platform where the molecular weight between cross-links (Mc) can be varied leading to tunable mechanical and conductive properties. To form networks with bimodal Mc’s, two volume fraction series, A and B, were synthesized from telechelic polystyrene (PS) and poly(ethylene oxide) (PEO) of varying molecular weights (MW), using thiol− norbornene chemistry to tune Mc. Series A consisted of 4K PEO and 12K PS, and series B consisted of 12K PEO and 5K PS. Phase separation in the network was confirmed by DSC where two distinct glass transition temperatures were observed and by SAXS where broad, weakly ordered scattering was observed. The networks were further characterized to probe how bimodal Mc affects the mechanical and conductive properties of phase-separated networks. The two series demonstrated that the asymmetric MW studied herein had little effect on mechanical and lithium conductive properties while changes in these properties were primarily influenced by the volume fraction of PEO.
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solvent quality,12−15 or during phase separation, which limits the degree of phase separation that can occur, kinetically trapping the phase separation in nonequilibrium states.16−18 Cross-linking also increases the solvent resistance and mechanical properties of the system, generally advantageous properties for many applications. When designing cross-linked polymer networks, the molecular weight between cross-links, Mc, is a critical parameter, influencing both the modulus and the morphology.11,19 Traditionally formed through chain-growth polymerizations, crosslinked networks are synthesized by using both difunctional and monofunctional monomers, where the difunctional monomer forms cross-links as the reaction progresses. While the ratio of difunctional to monofunctional monomers can be tuned to affect the average Mc, the resulting Mc’s are highly polydisperse, leading to heterogeneous cross-link densities and subpar material properties.20 Precisely controlled networks with a monomodal Mc can be obtained by utilizing efficient end-linking of polymers with multifunctional cross-linkers.12−15,21−28 The Mc is thus controlled by tailoring the molecular weight (MW) of the constituent polymers. To form reproducible, end-linked networks, highly efficient cross-linking chemistry, such as activated ester or thiol−ene chemistry, must be used.27,28 Networks synthesized from highly efficient end-linking chemistry can have either a monomodal Mc, where all macromonomers have the same MW, or multimodal Mc’s,
INTRODUCTION Many applications demand polymeric materials fulfill multiple requirements to be of practical use. Rather than synthesize a new polymer with the appropriate level of each property, multiple polymers can be combined to fulfill the needs of the application. One example of this approach that has received considerable attention recently are amphiphilic polymeric conetworks (APCNs). In these systems, it is typical to covalently link polymers with disparate properties, combining hydrophilic and hydrophobic blocks into one bulk system.1−6 An application area is polymeric membranes used for ion transport since they need to incorporate both efficient transport and mechanical stability. One commonly studied system includes the hydrophilic, ionconducting polymer poly(ethylene oxide) (PEO) and the hydrophobic, mechanically reinforcing polymer polystyrene (PS).1,7−9 Combinations of hydrophobic and hydrophilic polymers phase separate, and the bulk performance becomes highly dependent on the morphology of the phase separation, which is determined by both the volume fraction of each phase and the interaction strength, χ, of the two phases.10,11 If the hydrophilic phase exists as a sphere inside the insulating hydrophobic block, the bulk material has little ability to transport; a phase that is continuous throughout the system is needed. Even when a targeted morphology is achieved, the morphology can change as χ is affected by temperature or exposure to solvents such as water vapor.11 By cross-linking a phase-separated network, the free migration of phases is hindered, limiting the changes in morphology. These networks can be cross-linked before phase separation, which is then driven by decreasing temperature or © XXXX American Chemical Society
Received: August 3, 2017 Revised: September 21, 2017
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DOI: 10.1021/acs.macromol.7b01681 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
NMR and GPC characterization data can be found in Figures S1−S5 and Table S1. Network Synthesis. For the network solution, the macromonomer concentration was held at 100 mg polymer/mL DMF. The other solution components were added in the following ratio: 1.1:1 mol norbornene:thiol cross-linker; 1:10 mol LiTFSI:EO repeat unit; and 5:1 mol polymer:photoinitiator. DMF was added to bring the solution concentration to 100 mg/mL. The solution was transferred into rectangular molds made of microscope slides clamped on either side of a 0.81 mm thick Teflon spacer to form rectangular samples and irradiated with 365 nm light from a Blak-Ray 100 W B-110 AP/R lamp for 1 h. Alternatively, solutions were transferred into molds made from 6 mL disposable syringes and irradiated using the same conditions to form cylindrical samples. Networks were dried, first under a nitrogen stream and then under vacuum at 60 °C overnight, to ensure removal of residual solvent. Characterization. 1H NMR of the macromonomers was acquired using a Bruker 500 MHz Ascend fitted with a cryo-probe. Gel permeation chromatography of macromonomers was performed using an Agilent 1260 series system with both refractive index and ultraviolet detectors, a 5 μm guard column, two 5 μm analytical Mixed-C columns, and a 5 μm analytical Mixed-D column; all columns are PL gel from Agilent. All columns were connected in series and incubated at 40 °C with THF as the eluent at a flow rate of 1.0 mL/min. PEO macromonomers were compared to PMMA standards, while PS macromonomers were compared to PS standards. Differential scanning calorimetry data were taken using a TA Instruments Q200 DSC under nitrogen flow of 50 mL/min. Network samples of 3−5 mg were hermetically sealed in aluminum pans. Samples were heated to 130 °C at a rate of 10 °C/min, cooled to −60 °C at 5 °C/min, and heated again to 130 °C at 10 °C/min. The final heating curve was used for evaluation. For SAXS, samples of approximately 1 mm thickness were mounted in the sample chamber using Kapton tape. SAXS patterns were obtained from an Osmic MaxFlux Cu Kα X-ray source with a wavelength of 1.54 Å and a 2D gas-filled wire array detector (both Molecular Metrology, Inc.) at a distance of 1.476 m from the sample. Domain spacings were calculated from the principal scattering maxima (q*) calculated using the Scherrer equation, d = 2π/q. For DMA, rectangular samples of approximately 25 mm by 7 mm by 1 mm were cut and used in a TA Instruments Q800 DMA equipped with a film tension clamp. The multifrequency strain method was used with a preload force of 1 mN, stretching to 0.1% strain at 1 Hz, and heated from −60 to 130 °C at a rate of 3 °C/min after 500 s equilibration at the lowest temperature. For electrical impedance spectroscopy, circular samples of approximately 4 mm in diameter and 0.3−0.5 mm in thickness were cut. Aluminum SEM mounts were sputter-coated using a Cressington 108 sputter coater with a 10 nm layer of gold, and the samples were placed on top, surrounded by a Teflon spacer with a 4 mm in diameter disc cut out of the middle to make room for the sample. The spacer and sample were sandwiched between two mounts and loaded into a custom system that multiplexes the impedance analyzer to positions inside a Cascade TEK TVO-2 vacuum oven. Samples were heated under vacuum to 100 °C and held for 4 h to ensure all solvent and moisture were removed and allowed to cool to room temperature. Impedance spectra was recorded for each sample every 30 min in the frequency range of 10 MHz−0.1 Hz. The bulk resistance to ion conduction, R, was calculated by fitting a constant function to the first plateau of the impedance magnitude occurring at high frequencies. Conductivity was then calculated from the known sample area, A = 0.072 cm2, and the thickness of the spacer tape, d = 0.029 cm, as σ = d/AR.
where the macromonomers have different MW’s. It has been shown that bimodal networks with minimal defects and the same average Mc as a corresponding monomodal network showed similar modulus and domain spacing, but with improved toughness and ultimate stress at break.21,22 Despite these improvements, research into bimodal networks has been limited, focusing almost exclusively on the morphology and mechanical behavior of single-component polymer networks.21−26,29,30 Cakmak et al. reported an amphiphilic conetwork with a bimodal distribution in PDMS cross-linker length, but these PDMS crosslinkers were reacted with PDMS chains randomly grafted to the dimethylacrylamide backbone, which precludes precise control over all Mc’s.31−33 To the best of our knowledge, no study has been conducted on a two-component, phase-separated network with a bimodal distribution of Mc’s, even though such a network would have two handles to manipulate network properties. The phase separation can be tuned by the volume fraction of each phase, while the Mc, set by macromonomer MW, affects modulus and domain spacing.11,19 Additionally, some properties are known to be dependent on the macromonomer MW, such as the lithium conductivity increasing with increasing PEO MW.15,34,35 Previous research in the Tew group has focused on synthesizing phase-separated networks from telechelic macromonomers using the thiol−norbornene reaction.12,13 This chemistry was used to prepare poly(ethylene oxide) (PEO) and polystyrene (PS) networks doped with lithium bis(trifluoromethane)sulfonimide (LiTFSI). These networks exhibited high lithium conductivity and mechanical properties over a wide range of compositions and increased access to the bicontinuous phase, highlighting the potential of these multicomponent phase-separated networks.14,15 Herein, the saltdoped PEO−PS system was used to investigate the effect of bimodal Mc’s on phase separation. The thiol−norbornene platform produces well-defined networks where the chemistry can be tailored to control both the Mc’s and the volume fraction of each component. Two volume fraction series of networks were synthesized and characterized for their phase separation and morphology. The connectivity of the PEO phase was probed by measuring the lithium ion conductivity while the PS phase was probed by measuring the storage modulus.
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EXPERIMENTAL SECTION
Materials. Dihydroxyl-terminated PEO, dihydroxyl-terminated PS, 5-exo-norbornene-2-carboxylic acid (99% exo), triphenylphosphine, diisopropyl azodicarboxylate (DIAD), pentaerythritol tetrakis(3mercaptopropionate), 2-hydroxy-4′-(1-hydroxyethoxy)-2-methylpropiophenone (photoinitiator), bis(trifluoromethane)sulfonimide, tetrahydrofuran (THF), dichloromethane (DCM), methanol, dimethylformamide (DMF), and diethyl ether were purchased from Alfa Aesar, Sigma-Aldrich, Acros Organics, Polymer Source, Ciba, or Fisher. DCM was distilled under nitrogen from calcium hydride, THF was distilled under nitrogen from sodium/benzophenone, and all other reactants were used without further purification. Macromonomer Functionalization. The macromonomers were functionalized with norbornene end groups using the Mitsunobu reaction as previously reported by our group.12−15 In brief, the macromonomer was dried overnight in a vacuum oven at 60 °C and dissolved under nitrogen in a septum-sealed flask with solvent (DCM for PEO, THF for PS), magnetic stir bar, exo-5-norbornenecarboxylic acid, and triphenylphosphine. The flask temperature was lowered to 0 °C using an ice bath. Diisopropyl azodicarboxylate was diluted in dry solvent under nitrogen and added dropwise to the reaction flask, and the reaction was stirred overnight. After the reaction was complete, the macromonomers were purified using precipitation into diethyl ether (for PEO) or methanol (for PS) and vacuum filtration. All subsequent
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RESULTS AND DISCUSSION Macromonomer Functionalization. In this study, networks were formed from two telechelic macromonomersPEO, a soft lithium conducting polymer, and PS, a rigid mechanically reinforcing polymerand a small molecule tetrafunctional cross-linker. Telechelic macromonomers were used to ensure control over the molecular weights between cross-links of the network since the Mc is equivalent to the MW of the B
DOI: 10.1021/acs.macromol.7b01681 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 1. Schematic depiction of the network formation from telechelic macromonomers. The cartoon represents a network from series B.
macromonomers. The telechelic macromonomers were synthesized using the Mitsunobu reaction, converting hydroxyl end groups to norbornene as previously reported12−15 on commercially available PEO and PS. The PEO molecular weights studied were 4 kg/mol (4K) and 12 kg/mol (12K), and PS molecular weights were 5 kg/mol (5K) and 12K. The Mitsunobu reaction resulted in functionalization of greater than 90% as determined by 1H NMR (Figures S1−S4). Network Synthesis. To cross-link the networks, the telechelic macromonomers were mixed in a solution of dimethylformamide, photoinitiator, tetrafunctional thiol crosslinker, and LiTFSI, as depicted in Figure 1. LiTFSI was added as a source of lithium ions for conduction and to prevent the crystallization of PEO.36 Adding the LiTFSI before cross-linking ensures the networks all contain the same ratio of 1 mol LiTFSI:10 mol EO repeat units, a ratio commonly used in the PS−PEO literature of lithium-conducting polymer electrolytes.35,37−39 The solution was then irradiated with UV light to cross-link the macromonomers after which the solvent was removed to drive phase separation. While the presence of the LiTFSI salt does not allow for the quantification of the sol fraction, previous research with this network system has indicated that the sol fractions are low (
