Nanoscale investigations of α- and γ-crystal phases in PVDF based nanocomposites.

The impact of carbon nanotubes (CNTs) incorporation into semi-crystalline poly(vinylidene fluoride), PVDF, was investigated at both macro- and nanoscales. A special effort was devoted to probe the local morphology, and mechanical, ferroelectric, piezoelectric and electrical conductivity response by means of atomic force microscopy tools. Incorporation of CNTs mainly induces the development of the polar γ-phase and as a consequence the coexistence of the γ-phase with the most stable non-polar α-phase is observed. A maximum γ-phase content is reached at 0.7 wt% CNT loading. The spherulitic morphology of the PVDF α-phase is assessed, in conjunction with the lack of any ferroelectric response, while the presence of the polar γ-phase is confirmed, owing to clear piezoresponse signals. Local piezoelectric measurements on γ-phase domains yield a maximum effective coefficient │d33│ ~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 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 γ-phase domains larger than that of its α-phase counterpart, in agreement with the fact that the CNTs act as γ-phase promoters and subsequently reinforce the γ-domains.