Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)

First Advisor

Maciej Radosz


Phase equilibria of polyethylene and ethylene-copolymers in sub- and supercritical solutions are investigated in this work. Fluid-liquid and fluid-solid phase transitions data are measured for tetracontane, a prototype of linear polyethylene, in sub- and supercritical propane, poly(ethylene-co -hexene-1) in suband supercritical. propane and ethylene + 1-hexene mixture, and poly(ethylene-co-octene-1) in sub- and supercritical propane and ethylene. The experiments are carried out in a batch optical cell equipped with a transmitted-light probe. The experimental data are correlated and predicted using two equations of state derived from of the Statistical Associating Fluid Theory (SAFT). In the case of tetracontane + propane, increasing the tetracontane concentration and increasing pressure increase the fluid-solid transition temperature. Also, increasing the long-chain-n-alkane molecular weight increases the fluid-liquid transition pressure. The SAFT predictions are in a reasonable agreement with the experimental data for both fluid-liquid and fluid-solid transitions. For poly(ethylene-co-hexene-1), both poly(ethylene- co-hexene-1) concentration and molecular weight have a relatively weak effect on the phase behavior. The fluid-liquid pressure and fluid-solid temperature decrease as branch density increases. The fluid-liquid pressure depends strongly on the solvent type. When the ethylene concentration increases in poly(ethylene-co-hexene-1)+ethylene+hexene-1 mixtures, the fluid-liquid pressure increases, whereas the fluid-solid temperature decreases. For poly(ethylene-co-octene-1)+propane, the poly(ethylene- co-octene-1) concentration has a weak effect on both fluid-liquid and fluid-solid transitions in the range of 0.01 to 0.1 polymer fraction. Increasing the poly(ethylene-co-octene-1) molecular weight increases both fluid-liquid and fluid-solid transitions, whereas increasing the poly(ethylene-co-octene-1) branch density decreases both transitions in poly(ethylene-co-octene-1)+propane system. Only fluid-liquid, cloud-point pressure, is measured for poly(ethylene- co-octene-1)+ethylene. Furthermore, the fluid-liquid pressure strongly depends on molecular weight and short-chain branch density, but weakly on polymer concentration in the range from 0.05 to 0.15 polymer fraction. Poly(ethylene- co-octene-1) solutions in ethylene exhibit upper- and lower-critical-solution-temperature behavior only. Once again, SAFT calculations are in reasonable agreement with the experimental data.