Reactive processing of polyethylene: effect of peroxide-induced graft copolymerization of some acrylic monomers on polymer structure melt rheology and relaxation behavior

Abstract

Melt processing of polyethylene (PE) at 170°C in a shear mixer in the presence of selected doses of dicumyl peroxide (DCP) and an acrylic monomer (acrylic acid (AA), ethyl acrylate (EA) and butyl acrylate (BA)) resorted in synthesis of acrylic grafted PE via radical graft copolymerization. The graft copolymers of PE, made free from the respective acrylic homopolymers by appropriate solvent extraction, showed development of higher shear stress ( τ) compared to the control (ungrafted) PE and simply DCP-treated PE when sheared at different rates ( D) at different temperatures (160–190°C). Melts of the graft copolymers of PE and DCP-treated PE, much like the melt of the control PE showed pseudoplastic flow behavior and all of them assumed a more pronounced pseudoplastic character over a range of relatively high rates of shear and at a relatively low melt temperature. The rupture shear parameters, measure of thixotropic hysteresis and relaxation behaviors of the differently modified PE and the control PE have been evaluated and compared. Observed effects and odd trends have been understood and interpreted considering (a) establishment of crosslinkages in the PE chains due to peroxide action that become much restricted in the presence of an acrylic monomer used for graft copolymerization, (b) uncertain and uncontrolled degree of chain entanglements resulting from possible uneven distributions of the peroxide and monomer in the PE melt during the reactive processing, (c) enhancement in molecular weight on grafting and (d) disappearance of residual crystalline orientations in the PE melt above 170°C.