Effect of ionic content on ballistic self-healing in EMAA copolymers and ionomers

A specific class of poly(ethylene-co-methacrylic acid) copolymers and ionomers (EMAA) possesses a unique ability to instantly self-heal following ballistic projectile puncture. The present study has comprehensively explored the relationship between ionic content, morphology and the self-healing property response to high energy impact, using a range of EMAA copolymers which vary from non-ionic to highly neutralized. DSC, DMA, FTIR and rheological methods were used to understand the thermal, rheological, chemical and microstructural responses which can occur during temperature variations experienced during high energy impact. Ballistic puncture testing probed the self-healing response over a wide range of temperatures, from −50 °C to 140 °C, allowing the production of a self-healing “phase-style diagram” identifying regions of self-healing success or failure as a function of ionic content and impact temperature. Moderate ionic content proved the most beneficial to healing for tests below the order–disorder transition (Ti = 40 °C); while above and into the melt (Tm = 90 °C) healing improved with increasing ionic content. Finally, a previously established, instrumented non-ballistic puncture method was used to record the tensile forces and displacements during a simulated puncture experiment allowing delineation of elastic vs. elastomeric (flow) properties as a function of ionic content. Overall, the relative mobility and strength of the associative regions whether arising from hydrogen bonding in the case of the non-ionic species, or ionic association in the case of the neutralized ionomers, was demonstrated to control the self-healing performance when subjected to high energy impact.