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 lightly crosslinked form of the polymer have been measured in both carbon dioxide and nitrogen over a pressure range up to 200 bar. The measurements have been made by high-pressure torsional braid analysis. Swelling with and sorption of CO₂ and N₂ in these polymers were also determined at 200 bar as a function of temperature using a magnetic suspension balance. The results show that both polymers display a depression of both the melting and crystallization temperatures in carbon dioxide. However, at pressures above 100 bar, a tendency for these transition temperatures to increase is observed. In compressed nitrogen, uncrosslinked polymer shows a small decrease in T _m and T _c again with the trend showing an increase at pressures above 100 bar. In contrast, the crosslinked polymer displays an increase in T _m and T _c at all nitrogen pressures. The differences in the T _m and T _c of these polymers in CO₂ and N₂ 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 these compressed fluids.