REGULARLY PHASE-SEPARATED STRUCTURE IN AN INJECTION-MOLDED BLEND OF ISOTACTIC POLYPROPYLENE AND HIGH-DENSITY POLYETHYLENE

High-density polyethylene (HDPE) is known to be immiscible with isotac tic polypropylene (PP) in the quiescent state. In an injection-molded blend of PP/HDPE (= 60/40 wt. ratio), a regularly phase separated stru cture with periodic distance of about 0.15 mu m was found. The regular structure could not be formed by a simple melt mixing of immiscible p olymer pair but only by the spinodal decomposition (under zero shear r ate in mold) from a single-phase mixture attained (by UCST depression or LCST elevation) in high sheer fields in an injection machine. The s hear-dependent mixing and demixing mechanism was supported by transmis sion electron microscopy: the volume ratio of PP-rich region and HDPE- rich region being different from the charge ratio and the HDPE crystal lamellae developed in the PP-rich region. The mechanism was also supp orted by microscopic observation of the structure development (charact eristic to the late stage of spinodal decomposition) during isothermal annealing above the melting point of PP; i.e. bicontinuous structure with nice regularity developed and grew up self-similarly. (C) 1998 El sevier Science Ltd. All rights reserved.