Calorimetric studies of drawn polyethylene terephthalate

Dynamic differential calorimetry and density measurements have been made on drawn polyethylene terephthalate. The crystallinity and heats of fusion depend on the type of molecular rearrangements produced by the extent of draw and the temperature of draw. At low temperatures (near the glass transition) the molecules relieve strain by uncoiling with little sliding or alignment occurring. At temperatures above 100°C the molecules glide past one another to relieve strain with little uncoiling Strain‐induced crystals are formed in the samples drawn more than 160% at temperatures near the glass transition; at high strain, both chain alignment and uncoiling occur and produce crystallization at these temperatures. At higher temperatures and high strain, the uncoiling is too small to allow crystallization. Annealing a drawn sample under strain resulted in an increase in uncoiling which is proportional to the strain.