Shape Memory Characteristics of Injection Molded Poly(lactic acid) Multiscale Hybrid Composites

In this study, we showed that hybrid reinforcement-a combination of nanoparticles and fibers-can provide more effective reinforcement for increasing the recovery stress of a shape memory polymer (SMP) than using either filler individually. We mixed carbon fibers (CF) and carbon nanotubes (CNT) into a poly(lactic acid) (PLA) matrix on a twin-screw extruder and injection molded specimen from the hybrid composite. Subsequently, some of the specimens were subjected to crystallizing heat treatment, while others were kept as molded to study the effects of crystallinity as well. We investigated the properties of the specimens with scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and mechanical and thermomechanical tests. We found that the CF helped disperse the CNT properly, allowing them to reinforce more effectively. The CF increased the recovery stress of the samples significantly while decreasing the precision of the recovery due to the rigid nature of the reinforcement. Dispersed CNT could further increase the recovery stress without impairing precision because dispersed CNT formed a deformable reinforcing structure that did not increase elongation at break or plastic strain.