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

A series of polyethylene (PE) blends consisting of a linear high density polyethylene (HDPE) and a linear low density polyethylene (LLDPE) with a hexane-chain branch density of 120/1000 was prepared at different concentrations. This set of blends possessed isorefractive indices and thus, was difficult to detect their liquid-liquid phase separation via scattering techniques. The LLDPE only crystallized below 50°C; therefore, above 80°C and below the melting temperature of HDPE, we had a series of crystalline-amorphous PE blends. Above the melting temperature of HDPE, this series of blends were in liquid state. 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 quench the system from temperatures of known miscibility and isothermally anneal the blend at a temperature higher than the equilibrium melting temperature of the HDPE for the purpose of allowing the phase morphology to develop via a liquid-liquid phase separation. The second step was to quench the system to a temperature at which the HDPE could rapidly crystallize. Monitoring the overall crystallization kinetics via the time for developing 50% of the crytallinity (t1/2) provided an experimental approach to identify the liquid-liquid phase separation binodal curve of the system because phase separation results in HDPE-rich domains exhibiting increased crystallization rates and thus shortening the t1/2. The annealing temperature in the first step at which the t1/2 of the system exhibits an onset of the decrease is the temperature of the binodal point for that blend composition. In addition, the HDPE-rich domains crystallized to formed spherulites which decorate the liquid-liquid 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 in this set of blends with isorefractive indice.