Research And Development On Two-way Shape Memory Polymer And Its Composite

Shape memory polymers(SMPs)are stimulus-sensitive smart materials that can memorize and recover permanent shapes under external stimuli,such as heat,light,electric or magnetic field,pH,and specific solvent.This property of SMPs is the basis for their extensive applications in biomedical devices,aerospace structures,and novel sensors.Conventional one-way SMPs(1W-SMPs)can be readily deformed into a temporary shape at elevated temperatures.The obtained temporary shape can be fixed by reducing the temperature.Upon reheating,1W-SMPs undergo a reversible phase transition that triggers shape recovery.But,a re-programming process is required to recur this shape memory behavior.Thus,this one-way shape memory behavior is irreversible.By contrast,two-way SMPs(2W-SMPs)can change reversibly between two temporary shapes through cyclic heating and cooling.Obviously,research on 2W-SMPis important to increase the applications of SMPs.In this study,we prepared a set of chemically crosslinked ethylene vinyl acetate copolymer(EVA)samples by using dicumyl peroxide(DCP)as thermal initiator.All samples showed a good one-way shape memory property;in particular,the property of the crosslinked EVA with 4.0wt% DCP was considered near-ideal.A high cross-link density was needed to achieve the two-way shape memory effect.The 2W-SMP with 4.0wt% DCP,with the highest crosslink density,showed the optimal shape recovery property but general reversible strain.The crosslinked EVA with 3.0wt% DCP showed the highest reversible strain because of balanced crosslinking and free chains.Furthermore,an electrical-active 2W-SMP nanocomposite incorporated with vapor-grown carbon nanofibers(VGCFs)was fabricated and investigated.At a content of VGCF above 15.0wt%,electrical resistivity decreased to below 20.4 Ω?cm.The composite with 15.0wt% VGCF exhibited good electrical-activetwo-way shape memory effect,because of the high electrical conductivity and good dispersion of VGCNF in the matrix.The last,a novel temperature controlled switch was made by tailoring,coating of the conductive adhesive,and programming the 2W-SM EVA.This temperature controlled switch could unfold at 52.7 ± 2.8 °C and refold at 27.3±1.4 °C,caused circuit opening and closing respectively.This switch was excepted to be used as high-temperature protection circuit devices.These findings may contribute to design an appropriate shape memory composites and shape memory protocol based on application-specific requirements.