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Abstract Amorphous shape-memory polymers (SMPs) can be programmed to deform by heating above the glass transition temperature (Tg) to a temporary shape, which can then be fixed after cooled down below Tg. In the stress-free state, the shape of polymers can be recovered through increasing the temperature to above Tg. However, some SMPs lose the shape-fixity ability and recover from a temporary shape to a permanent shape in the environment with high relative humility, which is named as the moisture-driven shape-memory effect (SME). On the one hand, this effect is detrimental to the application of thermally-activated SME. On the other hand, this phenomenon can be harnessed to achieve shape recovery in the ambient condition, which can be potentially applied in various areas, since no external heat is needed to activate the shape recovery. In this paper, we develop a chemo-thermo-mechanical model to simulate the moisture-driven shape-memory behaviors of amorphous polymers. The model adopts the concept of free volume to describe the glass transition behaviors. As temperature decreases, the free volume also decreases, resulting in an increase in viscosity and a transition from the rubbery state to the glassy state. The diffusion of moisture into the polymer matrix increases the free volume and decreases the viscosity, and eventually causes the shape recovery. Fick’s law is used to simulate the diffusion of moisture in the polymer matrix. The coupled multi-field model is further implemented into the finite element analysis. The results show that the theory can qualitatively capture the influences of relative humility and recovery temperature on the shape recovery performance, represented as the increases in recovery rate and final recovery ratio with increasing the relative humility and recovery temperature. The model also reveals that the diffusion rate of water molecules significantly affects the recovery behaviors. The model is further demonstrated to be capable of describing the moisture-driven shape-memory effects involving complex finite deformation conditions.
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Received: 11 November 2019
Published: 12 June 2020
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2020, Vol. 41 
