Effect of molecular weight distribution on the structure and properties of polypropylene cast film and stretched microporous membrane

Abstract Two isotactic polypropylene (iPP) resins with approximate weight-average molecular weight ( Mw ) and different molecular weight distribution ( MWD ) were chosen to prepare cast films by the melt-stretching method. The lamellar morphology and mechanical properties of the initial precursor films and stretched microporous membranes were characterized. It was found that PP1 resin with a broader MWD exhibits a higher relaxation time of around 5.5 s, a larger entanglement density of 24.7 mol/m 3 and fast crystalline ability. The lamellar lateral size, orientation degree and entanglement density of the corresponding cast film are significantly increasing, resulting in higher elastic recovery and more apparent strain-hardening behavior. The PP resin with broader MWD also benefits the pore formation, resulting in a larger pore size and better air permeability of the stretched microporous membrane.