Maleic anhydride functionalization of polypropylene with suppressed molecular weight reduction via solid-state shear pulverization

Abstract

Polypropylene (PP) is sometimes functionalized with polar molecules like maleic anhydride (MA) to improve interfacial adhesion and to allow for reactive compatibilization. The conventional method of synthesizing PP grafted with MA (PP-g-MA) is by post-polymerization reactive extrusion at high temperature (180–220 °C). Under these conditions, the extent of β-scission (a radical chemistry that results in cleavage of C–C backbone bonds) is significant; thus, the product of functionalization by reactive extrusion suffers dramatic molecular weight (MW) reduction and degradation of properties. We present a novel method of synthesizing PP-g-MA while strongly suppressing MW reduction using solid-state shear pulverization (SSSP). By taking advantage of the relatively low temperature conditions associated with SSSP and with the use of azobisisobutyronitrile as radical initiator, significant MA grafting levels are achieved while strongly suppressing β-scission. For a PP-g-MA sample with 0.5 wt% grafted MA, characterization of number-average MW (Mn) before and after functionalization reveals that the frequency of chain scission events per repeat unit is reduced by more than 90% when synthesis is done by SSSP as opposed to reactive extrusion. Consequently, relative to the neat PP from which it was made, the PP-g-MA sample (with 0.5 wt% grafted MA) synthesized via SSSP exhibits only 8 and 25–32% reductions in Mn and weight-average MW (Mw), respectively; this is greatly improved over the 51 and 71% reductions in Mn and Mw reported in the literature for PP-g-MA (with 0.5 wt% grafted MA) synthesized by reactive extrusion.