A comparative study is made of the surface and bulk crystallinities of poly(ethylene terephthalate) films subjected to the following treatments: a) annealed at different temperatures in dry nitrogen, b) annealed in inert atmosphere and then boiled in water, and c) heated in water at various temperatures up to 100°C and also in an autoclave at temperatures above 100°C. It is observed that the surface crystallinity is higher than the bulk crystallinity for shorter heating times and lower heating temperatures. However, the inequality between the surface and bulk crystallinities is reduced as the temperature or the time of heat treatment is increased.
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Padhye et al. (1974) studied this question.
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