The effect of phase separation on crystallization of polyethylene/poly(ethylene-alt-propylene) blend

The influence of liquid–liquid phase separation (LLPS) on crystallization kinetics in off-critical and near-critical blends of polyethylene/poly(ethylene-alt-propylene) (PE/PEP) is studied to explore the origin of the two crystallization peaks in differential scanning calorimetry with the same long period. The PE/PEP blend systems show upper critical solution temperature (UCST) type of phase diagram and the crystallization temperature of PE component is much lower than the UCST, LLPS occurs first. The crystal nuclei first form at the interface region of two phases and then it is necessary for the PE chains to diffuse there to continue the crystallization process from both PE-rich and PEP-rich regions. As a result, the crystallization kinetics pathway is dynamically influenced by LLPS period of time, quench depth and initial composition because they control inter-diffusion coefficient in both phases. In the off-critical blend, the LLPS is extremely slow. The area ratio of the two DSC crystallization peaks doesn't fit with the ratio of coexistence compositions of PE in PE-rich and PEP-rich phases until LLPS has reached its equilibrium before crystallization. The crystallization of PE dominates the final morphology. Small and randomly dispersed crystals could be observed because the inter-diffusion of PE chains in PEP-rich phase is greatly hindered and they could not contribute to the formation of more perfect crystals. In the near-critical blend, LLPS kinetics is comparatively much faster, so the area of the two DCS crystallization peaks agrees with the coexistence compositions of PE. LLPS is the key factor to determine the final morphology.