EFFECT OF VINYLACETATE CONTENT ON CRYSTALLINITY AND 2ND-ORDER TRANSITIONS IN ETHYLENE-VINYLACETATE COPOLYMERS

Thermal characteristics of ethylene-vinylacetate (EVA) copolymers havi ng vinylacetate contents ranging from 5 to 40 w/w % are studied by dif ferential scanning calorimetry. It is first shown that EVA copolymers having a vinylacetate content lower than 30 w/w % obey the Flory and B urfield theories of copolymer crystallisation. The minimum sequence le ngth of CH2 ethylenic entities required to participate in a crystallin e lamella is also deduced. One can conclude that EVA copolymers repres ent cases of ''total exclusion'' of the noncrystallizable comonomer. M oreover, it is observed that when the vinylacetate content is increase d, the relative quantity of polyethylene amorphous phase increases and the degree of crystallinity decreases; whereas the beta transition te mperature of a characteristic-oriented amorphous phase is kept constan t. A phase model of ethylene-vinylacetate copolymers, based on an enri chment process of the interlamellar amorphous phase by polyethylene se gments originating from the crystalline phase, at increasing vinylacet ate content, is proposed. (C) 1997 John Wiley & Sons, Inc.