The effect of casting solvent on the sorption and diffusion of water vapor in poly(γ‐methyl L‐glutamate)

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Abstract
Water vapor permeability of poly(γ‐methyl L‐glutamate) (PMLG) membranes prepared by using dichloroethane, trifluoroacetic acid, and formic acid as solvents was studied. The membranes prepared by casting dichloroethane and trifluoroacetic acid solutions of the polymer, designated as PMLG–DCE and PMLG–TFA, respectivley, had α‐helical structures according to infrared absorption spectra, while the membranes prepared by allowing the PMLG–TFA membranes to swell in formic acid, designated as PMLG–FA, had mainly a β‐sheet structure. The amounts of water sorbed by PMLG–DCE, PMLG–TFA, and PMLG–FA increased in that order. The isotherms of PMLG–TFA and PMLG–FA were sigmoidal‐shaped isotherms, and the heat of sorption for PMLG–TFA and PMLG–FA was larger than that for PMLG–DCE, which suggested the presence of the sorption sites. The diffusion coefficients of water for PMLG–DCE increased and then decreased with increasing concentration. On the other hand, the diffusion coefficients for PMLG–TFA and PMLG–FA increased with concentration. The activation energies of diffusion for PMLG–DCE, PMLG–TFA, and PMLG–FA increased in that order. These results were discussed in connection with the molecular conformations of poly(γ‐methyl L‐glutamate) in the membranes. From these results, it is assumed that the molecular chains in the PMLG–TFA membranes are mainly in α‐helical and partly random‐coil conformations.