Influence of structural and topological constraints on the crystallizationand melting behavior of polymers. 1. Ethylene/1-octene copolymers

Studies of the crystallization, melting, and morphology of random ethylene/ 1-octene copolymers by a combination of differential scanning calorimetry a nd atomic force microscopy are presented. Two different crystallization mec hanisms prevalent in separate temperature ranges are inferred from the effe ct of cooling rate on the temperature dependence of crystallinity, from the reversibility of crystallization/melting phenomena at the lowest temperatu res, and from the temperature dependence of kinetic parameters describing i sothermal crystallization and melting. Morphological studies of these copol ymers demonstrate the coexistence of two distinct crystalline superstructur es (i.e., lamellae and fringed-micellar or chain cluster structures) which we tentatively associate with the two crystallization mechanisms. The multi ple melting behavior of these copolymers is associated with the existence o f separate morphological entities and is not explained by a mechanism of me lting-recrystallization-remelting. Finally, the upward shift of the melting endotherm of secondary crystals (i.e.,these formed by the low temperature mechanism) with longer crystallization times is explained by a decrease in the molar conformational entropy of the remaining amorphous fraction as a r esult of secondary crystallization.