Comparison of enthalpy relaxation between two different molecular masses of a bisphenol-A polycarbonate |
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Authors: | Pearl Lee-Sullivan M. Bettle |
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Affiliation: | (1) Department of Mechanical Engineering, University of Waterloo, 200 University Avenue West, Waterloo ON, Canada N2L 3G1;(2) Department of Mechanical Engineering, University of New Brunswick, P.O. Box 4400, Fredericton NB, Canada E3B 5A3 |
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Abstract: | Summary The present work is an extension of an earlier study that compared the stress relaxation between two molecular masses of a bisphenol-A polycarbonate due to thermal aging. The enthalpy relaxation of the same materials has been characterized. First, by measuring the change in enthalpy loss (ΔHa) and fictive temperature (Tf) as a function of aging temperature (Ta) ranging from -25 to 120°C, using differential scanning calorimetry. For the limited aging time of 120 h, ΔHa and Tf changes were only appreciable for (Tg -70 K)<Ta<Tg . While the influence of molecular mass was somewhat discernible, enthalpy measurements were not as sensitive as stress relaxation tests in differentiating molecular mass effects. In a second investigation, the kinetics of enthalpy relaxation upon isothermal aging at 130°C was evaluated using the peak shift method and found to be comparable to literature values. The plot of ΔHa as a function of log (aging time) showed two distinct regions: a brief non-linear portion (less than 1 h aging) which is followed by a linear relationship as typically reported in the literature. In contrast to the linear region, the non-linear relaxation behaviour of the poorly aged state does not appear to be dependent on molecular mass. |
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Keywords: | fictive temperature physical aging annealing enthalpic relaxation heat-aging |
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