Nanoscale Investigations of alpha- and gamma-Crystal Phases in PVDF-Based Nanocomposites

The impact of carbon nanotube (CNT) incorporation into semicrystalline poly(vinylidene fluoride), PVDF, was investigated at both the macro and nanoscales. A special effort was devoted to probe the local morphology and the mechanical, ferroelectric, piezoelectric, and electrical conductivity response by means of atomic force microscopy. Incorporation of CNTs mainly induces the development of the polar gamma-phase, and as a consequence, the coexistence of the gamma-phase with the most stable nonpolar alpha-phase is observed. A maximum gamma-phase content is reached at 0.7 wt % CNT loading. The spherulitic morphology of the PVDF alpha-phase is assessed, in conjunction with the lack of any ferroelectric response, while the presence of the polar gamma-phase is confirmed, owing to dear piezoresponse signals. Local piezoelectric measurements on gamma-phase domains yield a maximum effective coefficient vertical bar d(33)vertical bar approximate to 13 pm/V, thus underlining the potential for applications of such functional PVDF-based nanocomposites in advanced piezoelectric devices. An increase in macroscopic conductivity with CNT content is observed, with a percolation threshold achieved for a composition close to 0.7 wt %. Nanoscale investigation of the electrical conductivity confirms the presence of some infinite CNT cluster homogeneously distributed over the surface. The macroscopic viscoelastic behavior of the composite reflects the reinforcing effect of CNTs, while the nanomechanical characterization yields a local contact modulus of the gamma-phase domains larger than that of its alpha-phase counterpart, in agreement with the fact that the CNTs act as gamma-phase promoters and subsequently reinforce the gamma-domains.