Preparation and Characterization of Shape Memory Elastomeric Composites

Shape memory polymers (SMPs) are a class of smart poly-meric materials that have the ability to " memorize " a permanent shape, be manipulated to retain or " fix " a temporary shape, and later recover to its original (permanent) shape upon a stimulus such as heat, electricity, or irradiation. 1-3 A large number of SMPs have been developed and utilized for actua-tors, deployable medical devices, smart adhesives, and sensors, among others. However, very few of the existing SMPs are soft and elastomeric at the application temperature, although the demand for soft actuators is evident. 4 Three existing examples we are aware of include a main-chain liquid crystalline elasto-mer (LCE), 5 EPDM ionomers incorporating a range of crystal-lizable fatty acid salts, 6 and hydrogels with crystallizable alkyl side chains. 7 Each case involved some custom synthesis that presents inherent challenges to utilization at large scale. Here we report the development of a unique shape memory elasto-meric composite (SMEC) using a new and broadly applicable approach, which involves an interpenetrating combination of a crystallizable thermoplastic microfiber network (functioning as the " switch phase " for shape memory) with an elastomeric matrix. Our SMEC is composed of two commercially available polymers ;a silicone rubber (Sylgard 184 from Dow Corning; hereafter " Sylgard ") and poly(ε-caprolactone) (PCL; M w = 65 000 g/mol from Aldrich);and fabricated via a two-step process shown in Scheme 1. PCL was first electrospun from a 15 wt % chloroform/DMF (volume ratio=8:2) solution (voltage=15 kV, flow rate= 1 mL/h). The resulting nonwoven fiber mat (thickness = 0.5 mm) was then immersed in a two-part mixture of Sylgard 184 (mixing ratio = 10:1) and vacuum (30 in.Hg) was applied for 20 min to ensure complete infiltration of Sylgard 184 into the PCL fiber mat. After carefully removing the extra Sylgard 184 resin on the surface with a spatula, the infiltrated Sylgard/PCL was cured at room temperature for >48 h. The Sylgard/PCL composites fabricated showed an average PCL weight fraction (measured gravimetrically) of 25.6% (or a volume fraction of 23.6%, calculated using the densities of PCL (1.145 g/cm 3) and Sylgard (1.03 g/cm 3)) with a small standard deviation of 0.5% (sample size = 5), indicating good reproducibility of this method. The morphology of the Sylgard/PCL composite was studied by scanning electron microscopy (SEM). The surfaces of the as spun PCL fiber mat (fiber diameter = 1.93 (0.60 μm; see analysis …