Wednesday, 13 October 2004 - 4:20 PM

This presentation is part of : Mechanics and Thermodynamics of Polymers

A One-dimensional Constitutive Model for theThermomechanical Behavior of Shape Memory Polymers

Yiping Liu1, Ken Gall2, Martin L Dunn, and Alan R Greenberg. (1) University of Colorado, Campus box 0427, Boulder, CO 80309-0427, (2) University of Colorado at Boulder, Boulder, CO 80309

Shape memory polymers (SMPs) have the capacity to hold the pre-deformed temporary shape at a lower temperature and recover the original shape upon reheating. In this study, the thermomechanics of the shape storage and recovery behavior of a thermoset SMP is systematically investigated in tension and compression under various strain and stress constraining conditions. During cooling, the strain constraint is chosen for shape fixation while the stress evolution is monitored. During heating, three cases of constraint are selected for strain/stress recovery: unconstrained strain recovery, stress recovery at full strain constraint and stress recovery at reduced strain constraint. Due to the high stiffness at temperatures well below Tg, the evolution of the stress under strain constraint is highly affected by the thermal expansion of the polymer at the low temperature. Based on the experimental results, a one-dimensional constitutive model is developed in a continuum approach. The model is motivated by the macroscale mechanics of the SMP network. The entropy and internal energy levels as functions of temperature are quantified. The foundation of the model is that the entropy change is gradually frozen during cooling and released during reheating as recovery strain or stress. The fraction of the entropy storage is a function of temperature, which can be obtained by curve fitting of the free strain recovery curve. This model can well predict the trends of the stress evolution during shape storage and the stress recovery with/without low temperature unloading. The future improvement of the model is discussed.

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