Recovery of deformed polymers. III. Thermal properties

Measurements of heat capacity as a function of temperature are used to evaluate the changes in heat content of three deformed polymers during isothermal annealing treatments. The polymers investigated are the same as those used in previous studies of length recovery, viz., amorphous polycarbonate (PC) and semicrystalline polyethylene (PE), and polypropylene (PP). Cold-drawn PC is characterized by a smaller heat content than the undrawn material, while drawn PE and PP have larger heat contents than their undrawn forms. In all cases, the differences between the original and drawn states are of the order of a few cal gm^-1. These differences are observed to anneal out completely in the case of PC heat treated in the vicinity of its glass transition, while only incomplete recovery is observed in the case of PE and PP. Activation-energy spectra characterizing the recovery process are derived from the variation of heat content with annealing time. The spectra are centered about 22.6 kcal (g atm)^-1 for PC, about 24.1 kcal(g atm)^-1 for PE, and about 19.2 kcal(g atm)^-1 for PP. The spectra characterizing enthalpy recovery are similar to those derived previously from retraction kinetics, and indicate the similarity of relaxation processes involved in the two recovery phenomena.