Results are reported from an ongoing experimental investigation of the effects of thermo-oxidative aging on the mechanical behavior of an epoxy shape memory polymer (SMP). Chemo-rheological degradation due to macromolecular scission and cross-linking is one of the main factors contributing to the chemical aging of thermo-responsive SMPs. This aging may manifest as residual strain or irreversible material property changes, which can affect the performance and limit the useful life of a SMP. A relatively new epoxy SMP based on the diglycidyl ether of bisphenol A is synthesized, and specimens are tested under uni-axial tension using a dynamic mechanical analyzer. Fundamental viscoelastic behavior and thermal expansion coefficients are first characterized, showing a glass transition near 60 °C. Shape memory cycle experiments are performed at shape fixing temperatures of 80, 125, 150 and 175 °C, and the effect of fixing time at each temperature is examined upon subsequent strain recovery at 80 °C. Performance parameters such as recovery ratio, speed of recovery and residual strain are quantified as a function of shape fixing time and temperature. No effect of chemical aging was seen at a fixing temperature of 80 °C, although the recovery ratio decreases initially with increasing fixing time and stabilizes near 92 %. Only minor effects of chemical aging are seen in the mechanical responses for fixing temperatures of 125 and 150 °C, but specimens exhibit progressively more noticeable color changes that indicate oxidation. Significant effects are observed at the highest fixing temperature of 175 °C, where chemical aging at longer fixing times results in a reduction in recovery rate across the rubber-glass transition temperature, progressively larger residual strains, lack of complete strain recovery at 80 °C, and higher temperatures to achieve 90 % strain recovery.
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The Influence of Thermo-Oxidative Degradation on the Behavior of Epoxy Shape Memory Polymers
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Dasharathi, K, & Shaw, JA. "The Influence of Thermo-Oxidative Degradation on the Behavior of Epoxy Shape Memory Polymers." Proceedings of the ASME 2014 Conference on Smart Materials, Adaptive Structures and Intelligent Systems. Volume 2: Mechanics and Behavior of Active Materials; Integrated System Design and Implementation; Bioinspired Smart Materials and Systems; Energy Harvesting. Newport, Rhode Island, USA. September 8–10, 2014. V002T02A005. ASME. https://doi.org/10.1115/SMASIS2014-7478
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