Stress-free two-way shape memory property and microstructure evolution of single-phase polymer networks

摘要

In this work, single-phase polymer networks with excellent stress-free two-way shape memory effect (2W-SME) were successfully fabricated based on poly(ethylene-co-vinyl acetate) (EVA) and dicumyl peroxide. The microstructural evolution during actuation was repeatable and reversible, as demonstrated by the in situ wide-angle X-ray scattering. During multiple heating and cooling cycles, the (110) and (200) reflections of EVA appeared with increasing intensity, and the orientation degree was high at the low temperature point (10℃), suggesting that the oriented crystallites are present throughout the entire sample and hold extended conformation of the polymer strands in the amorphous phase. When heating to a high temperature (T_(high) = 70℃), partial melting of crystallites leads to the contraction of the material, and the intensity of the (110) and (200) reflections and the orientation became smaller, indicating that the residual oriented crystallites could still provide an anisotropic skeleton for the oriented recrystallization of the amorphous molecular chain. Moreover, the reversible actuation strain showed a maximum value of 15.6 at a T_(high) of 72℃ and was present even at a T_(high) of 80℃ when the polymer had a very small crystalline fraction (2.2). Considering the remarkable stress-free 2W-SME, EVA may have potential applications in smart grippers, actuators, and sensors.