水蒸汽驱动高聚物形状记忆效应的力学模拟

Modeling the Moisture-driven Shape-memory Effect in Polymers

部分形状记忆高聚物在相对湿度较高的环境中会从其临时形状恢复到永久形状,这种效应被称之为水蒸汽驱动形状记忆效应.由于不需要升高温度就可实现形状恢复,水蒸汽驱动的形状记忆效应在多个领域都有着潜在的应用价值.本文拟建立一个热-力-化学多场耦合的理论模型来模拟非晶态高聚物的水蒸汽驱动形状记忆行为.该理论模型采用自由体积的概念来模拟玻璃态转变,采用Fick定律来模拟水蒸汽在高聚物基体中的扩散行为.相关有限元模拟结果表明,该模型能定性地描述文献中观察到的恢复温度、相对湿度以及溶剂分子扩散速度对形状恢复行为的影响,也能模拟复杂变形条件下水蒸汽驱动的形状记忆效应.

Amorphous shape-memory polymers(SMPs)can be deformed to a temporary shape above the glass transition temperature(T_g),which can then be fixed after cooled down below T_g.In the stress-free state,the shape of polymers can be recovered through increasing the temperature to above T_g.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.