Phase equilibria and modelling of polymer systems

Abstract

Polymer solutions can form homogenous mixtures or they can undergo phase separation. With linear low density polyethylene (LLDPE) production using solution polymerization process, better understanding of phase equilibrium and phase separation is important to design such desired process.

The goal of this work is to determine liquid–liquid phase boundaries during the phase separation. For binary and multicomponent systems, liquid-liquid phase boundaries are determined by measuring liquid-liquid phase separation points also known as cloud points. These cloud point measurements are conducted at a temperature range of 130 ℃ to 250 ℃ and pressures up to 100 bar for systems in 15-29 mass percentage (m-%) polymer concentration. For binary systems consisting of polyethylene and n-hexane, as well as for multicomponent systems containing polyethylene, ethylene monomer, 1-butane or 1-octene comonomer, butane or Iso-octane, and a hexane solvent, cloud point values were measured for six different types of polymers. And these experiments were done in a pressure and temperature controlled variable volume cell.

Lower critical solution temperature (LCST) behavior was observed in the investigated temperature range. With increasing polymer concentration (increasing solubility of polymers), cloud points are observed at lower pressure values. On the other hand, with an increase in size and density difference between the polymer and solvent, a decrease in polymer solubility is observed. Hence this causes the cloud point values shift to higher pressures.