Influence of Novel Zwitterionic Grafts on the Properties of Polypropylene, Ethylene-Propylene Rubber and their Blends
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Polyampholyte derivatives of ethylene-propylene rubber (EPR-g-PA) and polypropylene (PP-g-PA) were prepared through the reaction a tertiary amino alcohol, 2- [2-(dimethylamino)ethoxy]ethanol (DMAEE), with maleated ethylene-propylene rubber (EPR-g-MAn) and maleated polypropylene (PP-g-MAn) respectively, forming zwitterionic grafts. The resulting zwitterionomers contained grafted ammonium and carboxylate functionality. Conversion of EPR-g-MAn to EPR-g-PA substantially increased viscoelasticity and tensile modulus due to the presence of ionic associations between the polymer chains. EPR-g-PA showed similar viscoelastic properties to an EPR lightly cross-linked using dicumyl peroxide but with greater strain hardening and tensile elongation. Relative to its precursor material, EPR-g-MAn, EPR-g-PA exhibited equivalent adhesion to metals and improved adhesion to other ionomers, which was believed to relate to the presence of ionic associations at the substrate interface. The in-situ reaction of EPR-g-MAn with DMAEE during blending with PP produced a blend with superior impact properties compared to the thermoplastic vulcanizate (TPV) blend of peroxide cross-linked EPR and PP due to the elimination of peroxide induced degradation of the PP phase. Conversion of PP-g-MAn to PP-g-PA had a negligible effect on its viscoelastic and mechanical properties. This was believed to relate to its low graft content, containing only 0.13 mol% grafted subunits. Despite its low graft content PP-g-PA was capable of forming strong associations with EPR-g-PA. Immiscible blends of EPR-g-PA and PP-g-PA exhibited improvements in droplet dispersion, tensile elongation and impact strength relative to their precursor blends of EPR-g-MAn and PP-g-MAn. Similarly, the addition of 7.5 wt% PP-g-PA to blends of 25/75 EPR-g-PA/PP was found to substantially improve droplet dispersion, impact strength and tensile elongation. These results suggest the preferential localization of PP-g-PA at the interface and the interaction of zwitterionic functionality across the phase interface, improving the compatibility between PP and EPR-g-PA. This conclusion was further supported by the frequency dependent viscoelasticity of blends containing both PP-g-PA and EPR-g-PA which was found to deviate substantially from conventional immiscible blend behavior, exhibiting properties consistent with reactively compatibilized blends.