Influence of carbon nanotubes and polypyrrole on the thermal, mechanical and electroactive shape-memory properties of polyurethane nanocomposites

Electroactive shape-memory composites obtained by shape-memory polyurethane block copolymer (PU) and three kinds of fillers of multi-walled carbon nanotubes (MWNTs), polypyrrole (PPy) and PPy-coated MWNTs were investigated. In addition, a series of composites with different MWNTs and different PPy compositions were prepared, and the relationship among the thermal, mechanical, morphology, and electroactive shape memory of the composites was investigated. Differential scanning calorimetric results suggested that the addition of MWNTs into the polyurethane increased the crystallinity, as confirmed by X-ray diffraction; however, the presence of PPy decreased it. A scanning electron microscopy observation indicated that a homogeneous dispersion of MWNTs throughout the PU matrix and a strong interfacial adhesion between MWNTs and the matrix were achieved in the PU nanocomposites. A significant improvement in the mechanical properties of the MWNT-PU composite (2.5%) was obtained: a 200% increase in modulus and a 37% increase in the tensile strength over pure PU. On the other hand, the tensile strength and modulus decreased with the addition of PPy into the composites. An addition of combined MWNTs and PPy as a coating for the CNT contributed to an enhancement in the conductivity of the PU-MWNTs composites. The electrical conductivity was the highest for the MWNT-PU composites coated with PPy on the film surface. The conductivity of 0.098 S/cm in this composite was sufficient to show electroactive shape recovery by heating above a transition temperature of 40–48 °C, due to the melting of the polycaprolactone soft segment domain. A good shape recovery in the 90–95% range could be obtained during a shape recovery test when a voltage of 25 V was applied.