Differential Scanning Calorimetry of Stressed Polymers

Stresses which have been frozen into a polymer by cooling to below its glass transition temperature (Tg) are relieved when the polymer is reheated above its Tg. Polymers which have been elongated will contract above Tg. This sudden contraction just above Tg is detected as a sharp drop in the differential scanning calorimetric curve. This is the reverse of the well known superheating endotherm accompanying the expansion of annealed polymers. The decrease in the differential scanning calorimetric curve immediately after the Tg inflection can be quantitatively related to the residual strain in poly(vinyl chloride) and polycarbonate.