Phase behaviors for the supercritical ethylene + 1-hexene + hexane + polyethylene systems at elevated temperatures and pressures

Phase behaviors of binary, ternary and quaternary polymer solution systems of ethylene, 1-hexene, hexane and polyethylene (PE) were investigated. Three types of nearly monodispersed polyethylene (PE15k:   M¯w=15.4×103 g mol−1;   M¯w/M¯n=1.17,   PE17k:   M¯w=17.2×103 g mol−1;   M¯w/M¯n=1.12,   PE108k:   M¯w=108×103 g mol−1;   M¯w/M¯n=1.32) were used. Measurements were carried out with a synthetic-type apparatus at temperatures as high as 473 K, using PE weight fractions ranging from 0.019 to 0.123 and ethylene weight fractions ranging from 0 to 0.102. As for the 1-hexene + hexane + PE and the ethylene + 1-hexene + hexane + PE systems, the ratios of the weight fractions of 1-hexene to hexane were kept constant at 1.0. Lower critical solution temperature (LCST)-type phase behavior was observed for each system measured. The liquid–liquid (LL) phase boundaries of the 1-hexene + PE and the 1-hexene + hexane + PE systems were slightly higher than those of the hexane + PE systems depending on 1-hexene concentration. On the other hand, the phase boundaries of the ethylene + 1-hexene + PE and the ethylene + 1-hexene + hexane + PE systems were quite similar to those of the ethylene + hexane + PE system.The LL phase boundaries were correlated and predicted by the Sanchez–Lacombe equation of state (S–L EOS) using binary interaction parameters. The characteristic parameters of the S–L EOS for 1-hexene were determined by correlating the vapor pressure of 1-hexene and the LL phase boundaries of the 1-hexene + PE system. The correlated results approximately reproduced the experimental results, although a slight deviation was observed in the region where the concentration of polymer is high. Moreover, the predicted LL boundaries for the 1-hexene + hexane + PE, the ethylene + 1-hexene + PE and the ethylene + 1-hexene + hexane + PE systems approximated the experimental results.