Effect of the β relaxation process on the transport and sorption of small gas molecules in polyesters and copolyesters

The purpose of this project was to determine the effect of the magnitude and position of the β relaxation process on the transport of small gas molecules through selected amorphous polyesters and copolyesters based primarily on poly(ethylene terephthalate) [PET] and poly(1,4-cyclohexylenedimethylene terephthalate) [PCDT]. The polymer samples were thin films extruded on a small Brabender extruder and showed no orientation when placed under crossed polarizers. Using the Rheovibron DDV-II-C Viscoelastometer and Mocon Oxtran 100 and Permatran C permeability instruments, the magnitude and location of the low temperature molecular motions have been related to oxygen and carbon dioxide permeability through the bulk polymer. Coefficients for diffusion of oxygen were determined by the half-time method and permeabilities from steady state measurements. Solubilities were obtained from the ratios of these coefficients. Certain aromatic dicarboxylic acid modifications of the base polyester systems were found to restrict the molecular motions that occur in the β relaxation region and decrease the diffusion of oxygen and carbon dioxide through these materials. The dicarboxylic acids investigated were isophthalic, 2,6-naphthalenedicarboxylic and monochloroterephthalic acids. Modifications with the 2,6-naphthalenedicarboxylic acid produced a new transition just below T_g which appears to be due to the motions of naphthalenedicarboxylate units within the polymer backbone. Partial and complete replacement of ethylene glycol linkage with the 1,4-cyclohexanedimethanol group increased the molecular motions in the β relaxation region and the diffusion of oxygen through and solubility in the bulk polymer. The molecular motions in the β relaxation region were found to affect the oxygen permeability through a change in the slope of the Arrhenius plot of log P versus 1⁄T. The energy of activation for oxygen permeation above the discontinuity was greater than below and found to be essentially the same for all polyesters investigated. Therefore, it has been shown that selected dicarboxylic acid modifications of base polyesters, PET and PCDT, which restrict the molecular motions that occur in the β relaxation region will in turn decrease the diffusion of oxygen and carbon dioxide through these polyesters. Thus, the magnitude of the β relaxation and transport of small gas molecules, such as oxygen and carbon dioxide, in polyesters can be controlled through chain modifications.