Nonequilibrium Molecular Characteristics of Glassy Polymers

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Introduction: The transient structure and property changes that occur in glassy polymers during chemical and physical aging and/or annealing below the glass-transition temperature Tg are of great current research interest. This time-dependent behavior, also referred to as volume or enthalpy relaxation (or recovery), originates from the nonequilibrium nature of the glassy state. From an application point of view, it is apparent that this phenomenon is the controlling factor in predicting the "initial" behavior as well as the practical performance of glassy or partially glassy polymeric materials during their useful lifetimes. Research Summary: In this research, the PIs probe the effects of controlled (and accelerated) chemical and physical aging and/or rejuvenation on achievable barrier property levels in a carefully selected glassy polymer of major commercial significance (polyethylene terephthalate). This will be accomplished by conditioning the polymer over a range of temperatures below Tg with small molecule penetrants (at high chemical potentials) which are interactive with small segment regions. Most importantly, the microstructural changes which specifically control the behavior of transport parameters in sorption and diffusion will be directly measured and identified for the first time, via monitoring of the kinetics of the isomerization reaction which governs the distribution of polymer segmental conformers. The research will be accomplished through combined experiments utilizing gas transmission and Fourier Transform I.R. Spectroscopy, together with enthalpy relaxation studies. Significance: The results of this work will contribute strongly to the development of a comprehensive molecular theory which may lead to the design of a variety of new and/or improved processes for the production of glassy polymers with superior properties in packaging applications, as protective coatings, and for use as permselective membranes in energy-efficient separations of gaseous mixtures, to cite only a few examples. Also, knowledge of both transport and mechanical property variations (which can be interrelated through free volume and its distribution) will be acquired in this research. It is anticipated that novel polymer microstructural states with significantly enhanced property levels and application lifetimes will be discovered.