Modelling Constrained Recovery of UV-Curable Shape Memory Polymer toward 4D Printing

Ultraviolet (UV) curable shape memory polymers (SMPs) are a family of smart materials that have been widely used in four-dimensional (4D) printing due to their fine compatibility to stereolithography based additive manufacturing and superior performance in creating programmable shapeshifting. Currently, the deployment of 4D printed shape memory structure is idealized as a free recovery process, which, however, frequently leads to overestimation of shape recovery ratio in practice due to the existence of external loading or constraint. Herein, we treat the UV crosslinked SMP as a thermoviscoelastic solid and derive analytical solutions to predict the maximum recovery stress under fully constrained shape recovery and shape recovery ratio under partially constrained shape recovery. Effects of training parameters including programming strain, programming temperature, fixing temperature, storage time, recovery temperature and constraining stress have been investigated through parametric studies. Good agreement has been achieved among theoretical prediction, experimental investigation and numerical simulation. Hopefully, our model can provide a theoretical tool that quantitatively guides the design of 4D printed shape memory structures toward practical applications.