Modification of tensile mechanical and dielectric properties in shape memory polyurethane-carbon black 0-3 composites

Shape memory polymers are gaining more and more consideration, particularly when their recovery properties couple with bio-compatibility. Much attention is paid to the possibility of inducing thermomechanical actuation in such materials by means of techniques alternative to conventional heating. Within this frame, a commercial polyurethane based shape memory polymer was modified by the addition of sub-micrometric carbon black (CB up to a 30% weight fractions). The resulting 0-3 composites were studied and compared with the original polymer. The neat polymer was first characterised by differential scanning calorimetry in order to determine its activation temperature. The optimal dispersion of the filler within the polymeric matrix was verified by scanning electron microscopy on a fracture surface. Strain-stress tensile characteristics of the same polymer and its composites were measured by multiple thermal-mechanical cycles under different loading and recovery times, allowing a comparison of both the recovery capability and strain fixity; also direct ultimate strain measurements were performed on each formulation. Recovery extent worsened by the addition of the carbon black, showing a decreasing monotonic trend with the filler content, and it also was markedly influenced by the loading-recovery time. Complex dielectric permittivity spectra (10^3÷10^8 Hz) of the various composites were measured and compared with those of the neat polymer. Apart from the 30%wt sample the spectra were almost flat, showing a slight dispersive behaviour toward higher frequencies probably due to a secondary relaxation process of the polymeric matrix; a monotonic growth of the dielectric constant was also observed at all frequencies with the increase of the filler content. The dielectric spectrum of the 30%wt composite was completely different from the others, showing a strong dispersive behaviour even at low frequencies. This suggests the onset of a microscopic dissipative mechanism, perhaps originating from a Maxwell-Wagner polarisation at CB grains/polymer matrix interfaces or from a charge carriers relaxation process. However, the conductivity percolation threshold was not reached in the examined formulations.