Liquid–liquid phase separation in a polyethylene blend monitored by crystallization kinetics and crystal-decorated phase morphologies

Abstract

A series of polyethylene (PE) blends consisting of a linear high density polyethylene (HDPE) and a linear low density polyethylene (LLDPE) with an octane-chain branch density of 120/1000 carbon was prepared at different concentrations. The two components of this set of blends possessed isorefractive indices, thus, making it difficult to detect their liquid–liquid phase separation via scattering techniques. Above the experimentally observed melting temperature of HDPE, Tm=133°C, this series of blends can be considered to be in the liquid state. The LLDPE crystallization temperature was below 50°C; therefore, above 80°C and below the melting temperature of HDPE, a series of crystalline–amorphous PE blends could be created. A specifically designed two-step isothermal experimental procedure was utilized to monitor the liquid–liquid phase separation of this set of blends. The first step was to quench the system from temperatures of known miscibility and isothermally anneal them at a temperature higher than the equilibrium melting temperature of the HDPE for the purpose of allowing the phase morphology to develop from liquid–liquid phase separation. The second step was to quench the system to a temperature at which the HDPE could rapidly crystallize. The time for developing 50% of the total crystallinity (t1/2) was used to monitor the crystallization kinetics. Because phase separation results in HDPE-rich domains where the crystallization rates are increased, this technique provided an experimental measure to identify the binodal curve of the liquid–liquid phase separation for the system indicated by faster t1/2. The annealing temperature in the first step that exhibits an onset of the decrease in t1/2 is the temperature of the binodal point for that blend composition. In addition, the HDPE-rich domains crystallized to form spherulites which decorate the phase-separated morphology. Therefore, the crystal dispersion indicates whether the phase separation followed a nucleation-and-growth process or a spinodal decomposition process. These crystal-decorated morphologies enabled the spinodal curve to be experimentally determined for the first time in this set of blends.