Specific heat of synthetic high polymers. VII. Polyethylene terephthalate

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Abstract
From specific heat measurements on drawn, undrawn, and annealed polyethylene terephthalate over the temperature range – 10 to 280° it has been possible to observe the magnitude of: (a) the glass transition, (b) recrystallization effects, and (c) the change of crystallinity with temperature. The change in the function cp/T at Tg is shown to be roughly proportional to the fraction of amorphous polymer present. Drawing the fiber reduces Δcp/T, but increases the rate of recrystallization so that, on the unannealed drawn samples, recrystallization occurs over a range of temperatures such that the glass transition cannot be observed. Good agreement is obtained between the crystallinity estimated from enthalpies and that calculated from density data except in the case of the samples that recrystallized during the calorimetric measurements. The estimated heat of fusion is in agreement with that calculated from the statistical thermodynamic theory of the effect of copolymers on the melting point. The influence of the surface free energy of the crystallites on their melting point. The influence of the surface free energy of the crystallites on their melting range is discussed.