Self-Healing of Thermoplastic Poly(Ethylene-co-Methacrylic Acid) Copolymers Following Projectile Puncture
Kalista, Jr., Stephen James
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Poly(ethylene-co-methacrylic acid) (EMAA) ionomer polymers carry great potential for use in a wide variety of unique applications due to their property of “self-healing” following projectile impact. Following puncture, certain films based on these materials are observed to “heal”, with the penetration opening recovering to an air-tight condition. Specifically, four polymers of this class were examined, including DuPont™ Surlyn® 8920, Surlyn® 8940, Nucrel® 925, and Nucrel® 960. Though these differ in their amount of ionic content, all expressed a certain degree of self-healing. Thin films were prepared by a compression molding process and punctured at temperatures ranging from room up to that of the melt using a pellet gun. Samples were then assessed for self-healing. A quantitative post-puncture burst-test method examined the strength or quality of the healed site in the four examples. A comparison of this data provided an understanding of the importance of ionic content and the mechanism of puncture healing. Additional damage modes were also examined to determine other cases where healing occurs and the requirements necessary to elicit the healing response. In addition, interesting composite materials consisting of carbon nanotube filled ionomers were fabricated by a melt-mixing process which produced potentially self-healing composites with superior mechanical properties. By comparing peel testing, projectile testing, the quantitative healed strength, and other characteristics, it was determined that healing is not a function of the ionic content of the materials involved. Further, healing was determined to occur due to a synergy of thermomechanical properties facilitated by the addition of the methacrylic acid groups to the polymer backbone.
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