Designing a polymer blend nanocomposite with triple shape memory effects

In this work, a polymer blend nanocomposite containing two widespread natural biopolymers (poly (L-lactic acid) and polyvinyl acetate) and nanohydroxyapatite (nHA) was manufactured. Triple-shape memory behavior was observed when 10 wt% of nHA was incorporated to the polymer blends. In contrast, adding lower content of nHA (1 and 5 wt% of nHA) did not bring about this behavior but only led to an increase in glass transition temperature (T_g). The triple-shape memory behavior was attributed to the phase separation in the polymer nanocomposites, which was confirmed by Atomic Force Microscope (AFM) and Dynamic Mechanical Analysis (DMA). The thermal responsive dual- and triple-shape memory behavior was thoroughly examined. According to the DMA results, the addition of nHA improved the thermal stability of nanocomposites, which in turn led to an increase in storage modulus. Subsequently, the results of the shape memory test indicated that the interaction between the matrix and the nanofiller could improve the shape memory properties of the nanocomposites. However, further increasing the nanofiller content (e.g., to 15 wt%) led to formation of aggregation in the polymers and consequently degraded shape recovery ratio. Further research also demonstrated that increasing thermomechanical cycles had a significant effect on shape memory behavior. In other words, there is a synergistic effect of adding nanofiller and increasing thermomechanical cycles on improving the shape memory properties.