(21d) Melting and Crystallization Temperatures of Ethylene-Octene Block Copolymers in Compressed Carbon Dioxide and Nitrogen

Melting and crystallization temperatures of poly(ethylene-co-octene) and a rheologically modified form of the polymer have been evaluated in both carbon dioxide and nitrogen over a pressure range up to 200 bar. The measurements have been made using high-pressure torsional braid analysis.

The results show that the unmodified polymer shows a depression of the melting and crystallization temperatures in both carbon dioxide and nitrogen. However, at pressures higher than about 150 bar, the trend is observed to reverse, and the transition temperatures tend to increase from their lowered values. In the modified form of the polymer, while these transition temperatures are still initially lowered in carbon dioxide, they display an increase in nitrogen at all pressures.

The differences are discussed in terms of changes in the relative rigidity, damping, and relative importance of the diluent versus hydrostatic effects on the thermal transitions of polymers in compressed fluids like carbon dioxide or nitrogen.