Miscibility and Co-Continuous Morphology of Polypropylene-Polyethylene Blends

Abstract

Polyethylene has been used to modify the physical and mechanical behavior of PP by forming mechanical blends. Four varieties of PE, namely low density polyethylene (LDPE), high density polyethylene (HDPE), linear low density polyethylene (LLDPE), and very low density polyethylene (VLDPE) are in common use in physical blends with iPP or PP copolymers. PP and PE blends are compatible but only partially miscible. PP and LDPE or HDPE are generally immiscible in the liquid state. Most polyolefins are immiscible in the liquid state since small differences in the shape of the molecules overcome the weak intermolecular forces. A further immisciblity arises as the polyolefins crystallize into different crystal domains. If a blend of PP and LLDPE is cooled from a miscible melt, it may separate into two phases. If a blend of PP and PE is cooled and remains miscible, it can be held at a temperature such that the PP crystallizes from the molten blend. After a time, long enough for the PP to completely crystallize, the blend is further cooled and the remaining PE will crystallize. This phenomenon results in PP forming a continuous phase in the PE, leading to a co-continuous morphology, even when PP is the minor phase. This gives a unique phase structure to the blend. Crystallization on continuous cooling produces a blend of finely divided immiscible crystals of polypropylene dispersed in LLDPE. Polypropylene has been found to be miscible in LLDPE, although in some LLDPE it is partially miscible with some crystal growth from solution and others from droplets.