Macromolecules 1999, 32, 6221-6235
Influence of Structural and Topological Constraints on the Crystallization
and Melting Behavior of Polymers.
1. Ethylene/1-Octene Copolymers?
A. Alizadeh, L. Richardson, J. Xu, S. McCartney, and H. Marand*
Departments of Chemistry and Materials Science and Engineering, Virginia
Polytechnic Institute and State University, Blacksburg, Virginia 24061-0212
Y. W. Cheung and S. Chum
Polyolefin Research, The Dow Chemical Company, Freeport, Texas 77541
Received April 29, 1999; Revised Manuscript Received June 23, 1999
ABSTRACT: 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.