Two-Way Reversible Shape Memory Properties of Benzoyl Peroxide Crosslinked Poly(ethylene-co-vinyl acetate) under Different Stress Conditions

Two-way (reversible) chemically crosslinked semicrystalline shape memory polymers are synthesized using poly(ethylene-co-vinyl acetate) (PEVA) with benzoyl peroxide (BPO). The two-way shape memory effect (2W-SME) is achieved under both constant stress and stress-free conditions. It is found that the stress-free 2W-SME can be achieved by the relationship between the initial prestretching strain (R-prestretch) and recovery strain (R-rec). Under the same prestretching stress, the stress-free two-way shape memory behavior can be controlled by variation of R-rec using a different setting temperature (T-set) in the recovery process. More importantly, the driving force and recovery force, as one of the key indicators for two-way shape memory materials, are investigated, and they significantly change depending on the BPO content. The sample with high BPO content shows excellent high-temperature creep resistant performance. A highly crosslinked structure can suppress viscous flow and provides sufficient force to allow the sample to recover its initial shape after crystal melting. Therefore, the PEVA/BPO samples are able to contract during heating. The presence of an oriented crystal structure with high applied stress that causes sample elongation during cooling is also investigated. These findings for PEVA/BPO two-way shape memory polymers will contribute to their applications as soft actuators in various fields.