Melt Processable Poly(acrylonitrile)-based Precursors for Carbon Fiber Production and Advanced Polymeric Membranes for Gas Separation and Water Electrolysis Applications
Miller Jr, Gregory Charles
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An effort concerned with the feasibility of achieving melt-processable polyacrylonitrile copolymer system precursors for producing high modulus carbon fibers is detailed. High molecular weight poly(acrylonitrile-ran-methyl acrylate) (PAN-MA) copolymer with high acrylonitrile content were mixed with various water containing binary melting point modifiers to produce systems that formed stable melts at temperatures below the temperature corresponding to the onset of PAN-MA crosslinking. The structure of the copolymer was found to be 96.5 ± 0.13 mole % acrylonitrile and 4.40 ± 0.13 mole % methyl acrylate by 1H-NMR with an Mw ]= 238 kDa and dispersity of 1.9 determined by size exclusion chromatography. A reduction in the Tm of the copolymer of 200 C was established for a copolymer/melting point modifier system containing copolymer mixed with water and acetonitrile with the following composition: PAN-MA/ACN/H2O 55/25/20 wt:wt:wt. This corresponds to the greatest reduction in a PAN-based copolymer melting temperature yet reported. From isothermal DSC and pressurized capillary rheometry experiments it was found that the stability of the resulting melts shows a strong temperature dependence, but does not show a strong dependence on shear rate. Copolymer mixtures with H2O and acetonitrile or H2O and adiponitrile were found to be suitable for melt-extrusion at 170 C with viscosities ranging from 1800-2000 Pa*s with stabilities greater than 1 hour. The modification of membranes to improve gas separation properties is of considerable interest. Crosslinking is one route to modify membranes, but the resulting effects on thin membranes have yet to be investigated to understand the impact of such modification at thicknesses that are relevant to industrial membranes. In this study, the influences of UV irradiation and physical aging on O2 and N2 gas permeation properties of thin (~ 150 nm) glassy poly(arylene ether ketone) (PAEK) films at 35 C and 2 atm were investigated. Thin PAEK films prepared from tetramethyl bisphenol A and 4,4'-difluorobenzophenone were UV irradiated on both sides in air or N2 at wavelengths of 254 nm or 365 nm. This induced crosslinking and, in some cases, photooxidation. Gas permeability decreased and O2/N2 selectivity increased as UV irradiation and aging time were increased. At 254 nm, samples irradiated in air had lower permeability coefficients and higher selectivities than samples irradiated in N2, and this was ascribed to additional decreases in free volume due to photooxidation in air-irradiated samples. Additionally, air-irradiated samples at 254 nm exhibited less physical aging than non-crosslinked and N2-irradiated samples at 254 nm, possibly due to interactions among photooxidative polar products that may restrict polymer chain mobility, thereby lowering the aging rate. The influence of water vapor on physical aging of air-irradiated samples was examined. Finally, irradiation at 254 nm leads to more extensive crosslinking and/or photooxidation than irradiation at 365 nm, possibly due to greater UV absorption by the polymer and the higher probability of radical formation at the lower wavelength. Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) is utilized for gas separation membranes. It has a relatively high free volume with high gas permeabilities but suffers from low selectivities. PPO polymers with Mn's from 2000-22,000 g/mole were synthesized and blended with a poly(arylene ether ketone) derived from bisphenol A and difluorobenzophenone (BPA-PAEK). DSC showed that the blends with all but the lowest molecular weight PPO had two Tgs, thus suggesting that two phases were present. The ketone carbon and benzylic methyl groups on the BPA-PAEK and the PPO polymers crosslinked upon exposure to UV light. The gel fractions after UV exposure were high and the tensile properties were similar to the PPO control polymer that is currently used as a gas separation membrane. The crosslinked blends had improved gas selectivities over their linear counterparts. The 90/10 wt/wt 22k PPO/BPA PAEK crosslinked blends gained the most O2/N2 selectivity and maintained a high permeability. Two series of high molecular weight disulfonated poly(arylene ether sulfone) random copolymers were synthesized as proton exchange membranes for high temperature water electrolyzers. These copolymers differed based on the position of the ether bonds on the aromatic rings. One series was comprised of fully para-substituted hydroquinone comonomer and the other series incorporated 25 mole % of a meta-substituted comonomer, resorcinol, and 75 mole % hydroquinone. The influence of the substitution position on water uptake and electrochemical properties of the membranes were investigated and compared to the state-of-the-art membrane, Nafion. Mechanical properties of the membranes were measured for the first time in fully hydrated conditions at room and elevated temperatures. While submerged in water, these hydrocarbon-based copolymers had moduli an order of magnitude higher than Nafion membrane. Selected copolymers of each series showed dramatically increased proton conductivity at elevated temperature and fully hydrated conditions while their H2 gas permeabilities were well controlled over a wide range of temperatures. These improved properties were attributed to the high glass transition temperature of poly(arylene ether sulfone)s.
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