Crystallization, Melting Behavior, Physical Properties, and Physical Aging of Ethylene/1-Octene Copolymers
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The time dependence of the physical properties of ethylene/1-octene (EO)-copolymers after primary crystallization is investigated by calorimetry, density, and creep measurements. The temporal evolution of the multiple melting of EO-copolymers is monitored by differential scanning calorimetry. The low temperature endotherm displays an evolution similar to that observed for the enthalpy recovery in glasses after physical aging. Using this analogy, a calorimetry-aging rate is defined, which quantifies the change in the low endotherm temperature with time. Similarly a density-aging rate is defined from the evolution of density with time. A non-classical creep behavior is observed for short aging times, consistent with crystallization-induced shrinkage. The change in crystallinity during aging leads to a change in the shape of the relaxation spectrum. Hence, analysis of creep data cannot be carried out using Struik's superposition method. For both short and long aging times, the creep rate exhibits a dependence on copolymer composition similar to those associated with the calorimetry- and the density-aging rates, suggesting a common origin for the evolution of the low endotherm, the creep behavior and the bulk density. The calorimetry, density, and creep data are reexamined based on the following assumptions: First, a single population of small crystals is formed during crystallization at low temperature; Second, these small crystals increase in stability under isothermal conditions, easily melt and recrystallize during heating and serve as efficient thermo-reversible cross-links to increase the conformational constraints in the residual amorphous fraction. These assumptions appear to be consistent with all observations made to date.
- Masters Theses