Functional Nitroxyls in Polypropylene Based Thermoplastic Vulcanizates
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Peroxide vulcanization of polypropylene (PP) based thermoplastic vulcanizates (TPVs) is hindered by PP’s susceptibility to radical degradation. PP degradation occurs through chain scission of tertiary alkyl macroradicals, reducing molecular weight, and negatively effecting TPV blend morphology. Nitroxyl chemistry using acryloyloxy-2,2,6,6-tetramethylpiperidine-N-oxyl (AOTEMPO) was exploited to control the dynamics and rheological outcomes of peroxide polyolefin modifications. PP melt viscosity was partially retained by introducing a competition between macromonomer oligomerization and β-scission. Further improvements were observed when synergizing the polymer-bound acrylate functionality of AOTEMPO, with trimethylolpropane triacrylate (TMPTA) a multifunctional coagent. Resulting TPVs contained smaller, more dispersed crosslinked elastomer particles when compared to peroxide-only formulations, owing to the retention of PP molecular weight. A follow up investigation into the effects of AOTEMPO on TPV physical property performance was completed for low elastomer content blends (0-20wt%). AOTEMPO formulations provided improved particle dispersion compared to peroxide only formulations, owing to the retention of melt viscosity. These improved particle dispersions led to better impact performance for AOTEMPO formulations over peroxide only formulations. Additionally, AOTEMPO formulations provided improved tensile properties compared to peroxide only formulations, owing to the branched architecture formed during polyolefin modification.