Influence of structural and topological constraints on the crystallization and 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 and atomic force microscopy are presented. Two different crystallization mechanisms prevalent in separate temperature ranges are inferred from the effect of cooling rate on the temperature dependence of crystallinity, from the reversibility of crystallization/melting phenomena at the lowest temperatures, and from the temperature dependence of kinetic parameters describing isothermal crystallization and melting. Morphological studies of these copolymers demonstrate the coexistence of two distinct crystalline superstructures (i.e., lamellae and fringed-micellar or chain cluster structures) which we tentatively associate with the two crystallization mechanisms. The multiple melting behavior of these copolymers is associated with the existence of separate morphological entities and is not explained by a mechanism of melting-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 result of secondary crystallization.