CALORIMETRIC STUDY OF FIRST ORDER TRANSITIONS IN POLY(VINYL CHLORIDE)

A calorimetric study of first order transitions in the specific heat of PVC is reported. The results show that various types of commercial PVC exhibit a melting range which starts at about 150 °C and extends into the thermal decomposition region. A heat of fusion of 750 cal/mole was calculated from the melting energy and the change in density upon melting. A similar melting range starting at 180 °C was found in PVC polymerized at -30 °C. Once melted, this primary cristallinity does not reappear after annealing. It can be restored by dissolution and subsequent careful precipitation. Annealing above the glass transition produces secondary crystallinity which melts just above the annealing temperature and has a heat of fusion of only about 200 cal/mole. The secondary crystallinity is not formed unless the primary crystallinity has at least partly been eliminated by prior heating. Secondary crystallinity was also found in plasticized PVC. For compounds containing 30 parts of DOP per 100 parts of PVC, storage at room temperature means annealing above the glass transition. Consequently, secondary crystallinity is formed during storage at room temperature, which accounts for the stiffening of plasticized compounds in the course of time. © 1969 Dr. Dietrich Steinkopff Verlag.