WIDE-ANGLE X-RAY-DIFFRACTION STUDY OF THE PHASE-BEHAVIOR OF VINYL ALCOHOL ETHYLENE COPOLYMERS

The influence of the composition and thermal history on the type of la ttice developed has been studied by wide-angle X-ray diffraction measu rements in three vinyl alcohol-ethylene, VAE, copolymers with differen t compositions, which were crystallized from the melt at various cooli ng rates. The results are compared with those of the corresponding hom opolymers, poly(vinyl alcohol) and low-density polyethylene. Both the composition and the cooling rate have been found to determine the crys talline structure of these VAE copolymers. Thus, for the samples slowl y crystallized from the melt, a monoclinic lattice is obtained for cop olymers VAE71 and VAE68 (with compositions of 71 and 68 mol % VA, resp ectively) while an orthorhombic lattice has been found for copolymer V AE56 (56 mol % VA). The effect of the cooling rate is very important, since copolymers VAE71 and VAE68 lead to the orthorhombic form when th e samples are quenched from the melt. Copolymer VAE56 has been found t o crystallize in an orthorhombic lattice under all the crystallization conditions here studied. The polymorphism exhibited by these copolyme rs is not the usual case for two different unit cells with well-define d lattice parameters: VAE71 and VAE68 show, by the influence of the co oling rate, a continuous change of some of the lattice constants. For instance, the angle beta of the monoclinic lattice decreases smoothly from an ''equilibrium'' value of about 92.2 degrees in the slowly crys tallized samples to a final value in the quenched samples of 90 degree s (orthorhombic unit cell). The degree of crystallinity determined fro m the X-ray diffraction patterns has been found to be in the quenched specimens considerably smaller than that in the slowly crystallized sa mples, standing in contrast to the constancy of the enthalpy of meltin g. Moreover, it seems that there are no differences in the melting tem peratures between the two crystalline modifications.