Thermal transitions of ethylene-vinyl chloride copolymers

Abstract

Differential scanning calorimetry (d.s.c.) measurements were performed on a series of ethylene-vinyl chloride (E–V) copolymers for the purpose of studying the dependence of their thermal transitions upon their microstructure. The method of preparation, via reductive dechlorination of poly(vinyl chloride) with tributyltin hydride, resulted in a series of E–V copolymers differing only in comonomer content, sequence distribution and stereoregularity of adjacent V units. Chain length distribution and branching frequencies were identical for each member of the series. Extrapolation of glass transition temperatures, T g, measured for our E–V copolymers to pure polyethylene (PE) predicted a T g = −85°C ± 10°C for amorphous PE. E–V copolymers with greater than 60 mol% E units exhibited melting endotherms characterized by melting temperatures from 20°C to 128°C and degrees of crystallinity from 12 to 63%. Observed melting temperatures were plotted against the composition of the E–V copolymers and compared to Flory's equation for melting point depression of random copolymers containing one crystallizable and one non-crystallizable monomer unit. The melting point depressions observed for our E–V copolymers were in agreement with Flory's theory, if the CH 2CH 2 moiety is considered to be the crystallizable unit and theCHmoiety is assumed to prevent the CH 2CH 2 units attached on either side from being incorporated into the crystal. This implies that among all possible comonomer triad sequences only the EEE triad may crystallize. Therefore only those E–V copolymers with average lengths of consecutive E units greater than 2 exhibit crystallinity.