Two-way reversible shape memory effects in a free-standing polymer composite

Abstract

Shape memory polymers (SMPs) have attracted significant research efforts due to theirease in manufacturing and highly tailorable thermomechanical properties. SMPs can betemporarily programmed and fixed in a nonequilibrium shape and are capable of recoveringthe original undeformed shape upon exposure to a stimulus, the most common beingtemperature. Most SMPs exhibit a one-way shape memory (1W-SM) effect since oneprogramming step can only yield one shape memory cycle; an additional shape memorycycle requires an extra programming step. Recently, a novel SMP that demonstrates both1W-SM and two-way shape memory (2W-SM) effects was demonstrated by one of theauthors (Mather). However, to achieve two-way actuation this SMP relies ona constant externally applied load. In this paper, an SMP composite where apre-stretched 2W-SMP is embedded in an elastomeric matrix is developed. Thiscomposite demonstrates 2W-SM effects in response to changes in temperaturewithout the requirement of a constant external load. A transversal actuation of ∼ 10% of actuator length is achieved. Cyclic tests show that the transversal actuation stabilizesafter an initial training cycle and shows no significant decreases after four cycles. A simpleanalytic model considering the programming stress and actuator dimensions is presentedand shown to agree well with the transverse displacement of the actuator. The model alsopredicts that larger actuation can be achieved when larger pre-stretch of 2W-SMP is used.The scheme used for this polymer composite can promote the design of new shape memorycomposites at micro- and nano-length scales to meet different application requirements.