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Alternatives to the Narayanaswamy expression (N) for nonlinearity, r0 = A exp[xAh*/RT + (1 -x)Ah*/RT{], were derived from the Adam-Gibbs (AG) theory and fitted to experimental data on five polymers. Two AG-derived expressions were evaluated: r0 = A exp[B/RT In (Tf/T2)] ("AGL") and r0 = A exp[D/RT(l ~T2/Tf}] ("AGV"). The N and two AG expressions gave comparably good fits for most thermal histories, AGV giving somewhat better fits at the longest annealing times. Reported variations in N parameters with thermal history were shown to be qualitatively consistent with AG predictions. The N parameter, x, was shown to be a direct measure of T{jT2 (T/ = glassy state value of Tf); the N activation energy, Ah*, was found to vary inversely with the AG parameters B and D. Correlations of B and D with T{/ T2 were observed and shown to be consistent with T{ approaching T2 as the AG primary activation energy decreased to zero.The Kohlrausch-Williams-Watt parameter, ß, also decreased with decreasing T{/T2, suggesting increased cooperativity as T2 is approached. Variations in AG parameters, obtained directly for polymer glasses and indirectly from published N parameters for nonpolymeric glasses, were consistent with generally observed variatons in non-Arrhenius behavior above Tg. It was concluded that nonlinear behavior near and below Tg is determined by the same factors that influence equilibrium behavior above Tg.

Physical aging refers to structural relaxation of the glassy state toward the metastable equilibrium amorphous state, and it is accompanied by changes in almost all physical properties. These changes, which must be taken into account in the design, manufacture, and use of glassy polymer materials and devices, present a daunting challenge to theorists.

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