Influence of Structural and Topological Constraints on the Crystallization and Melting Behavior of Polymers. 1. Ethylene/1-Octene Copolymers

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.

Phase Transitions of Bulk Statistical Copolymers Studied by Dynamic Monte Carlo Simulations

“…This demonstrates that annealing disturbs both heating and cooling crystallinity curves only locally. This observation is consistent with recent DSC measurements on ethylene-1-octene copolymers 39,41 and reveals a kind of master curve irrespective of local disturbances caused by the scanning rate or the isothermal annealing. It seems logical to conclude that, on cooling, long sequences crystallize earlier forming large crystallites that remelt only at higher temperatures.…”

“…In experiments, the large-scale aggregation of crystallites can be observed to change from spherulites to dispersed granular domains. 3,38,39,55 Clearly, the sizes of crystallites are limited by the monomer sequence lengths. Long monomer sequences can form well-developed lamellar crystallites with chain folding.…”

Review and critical analysis of the morphology of random ionomers across many length scales

“…From a critical perspective, there does not seem to be any piece or pieces of evidence that could not be explained either by water absorption and/or slow crystallization of ethylene segments. Further, this same peak at $508C that grows with room temperature annealing occurs in both unneutralized acid copolymers [112] as well as other ethylene copolymers, [113][114][115], although the effect is not nearly as dramatic. The best way to prove that such a transition exists is to choose a system that cannot crystallize, and rigorously exclude water to eliminate these two possible explanations.…”

Evolution of a Metallocenic sPP with Time: Changes in Crystalline Content and Enthalpic Relaxation

“…Additionally, a small endotherm is observed at 40 8C. This feature might arise from the annealing process related to maintenance of the sample at room temperature, as observed in other olefinic materials, [47,51,52] where the annealing peak appears at temperature around 15-308 higher than the annealing temperature (room temperature in most cases). Alternatively, considering the complicated polymorphic behavior of sPP aforementioned, this endotherm could also be associated with a transformation (or simply melting) from a polymorph different from the major crystalline structure that melts at 94 8C.…”