Crystallization of ethylene/vinylacetate random copolymers

Abstract

Abstract It may be anticipated that uncrystallizable units, if introduced randomely into a crystallizable polymer chain, might strongly prohibit the chain folding and enhance intermolecular crystallization, which will give rise to the fringed micelle structure. In anticipation of this, crystallization and resultant texture were studied for random copolymers of ethylene (E) and vinyl acetate (VAc) as functions of composition and crystallization temperature. Crystallization was carried out isothermally from solution as well as from melt. DTA melting behavior of the melt‐crystallized samples revealed that two types of crystals with different melting temperatures were formed during the isothermal crystallization of the copolymers. Since the height of the peak appearing at the higher temperature T 1 decreased and that at the lower temperature T 2 increased with the increasing VAc content and crystallization temperature, the former peak may be assigned to the chain folded crystals and the latter to the bundle‐like crystals. In other words, the transition from the chain folding crystallization to the bundle‐like crystallization did not occur sharply at a given composition as expected but the two types of crystallization coexist in a rather wide range of compositions, depending on the crystallization temperature. It became also apparent that molecular fractionation occurred during the isothermal crystallization with respect to the composition of the copolymer. Electron microscopy could not confirm the existence of the bundle‐like crystals, probably due to the fact that the individual bundle‐like crystal was not large enough to be distinguished from much larger chain‐folded lamellae with more definite shapes. On the other hand, solution grown crystals gave a single melting peak and a single X‐ray long spacing, suggesting only the chain folding crystallization occurred in the whole range of compositions studied. Molecular fractionation was again evident and the material left uncrystallized in the solution increased with the VAc content. These results may imply that the intermolecular bundle‐like crystallization depends strongly on the probability of finding other chain molecules around a given molecule. The X‐ray crystallinity as well as the heat of fusion measured suggest that the thickness of the amorphous surface layer (loose loops) of the solution grown crystals increases with the VAc content.