Nanoscale Investigations of α- and γ-Crystal Phases in PVDF-Based Nanocomposites

Abstract

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 γ-phase, and as a consequence, the coexistence of the γ-phase with the most stable nonpolar 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 | d| ≈ 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 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.

Light-induced reversible phase transition in polyvinylidene fluoride-based nanocomposites

“…It is also known that the conductivity increases beyond the percolation threshold of the material, which is reflected in the decrease in dielectric constant and increase in dielectric loss. In the case of CNTs, the conductivity increases rapidly after 0.4 wt% loading in PVDF in general [39]. This may also suggest that the change in local conductivity can also affect the phase transformation behavior.…”

“…Since we employ a continuous laser source in Raman measurements, it is likely that there is some amount of heat that is generated within the material. Further, it can be expected that the localized photocurrents and pyroelectric currents generated due to laser heating combined with a decrease in electrical resistivity caused by the presence of CNTs [39][40][41]. The high content of β phase (non-centrosymmetric phase) in the PVDF-CNTs composite produces pyroelectricity induced charges due to a temporary voltage generated at the instant when it is excited with due to the temperature induced by the laser locally [42,43].…”

Multiscale understanding of electric polarization in poly(vinylidene fluoride)-based ferroelectric polymers

“…The g-phase is usually obtained via solvent crystallization, 13 or via re-crystallization at temperatures slightly above melting point 14 or via the addition of fillers such as carbon-based materials. 15 To achieve better dielectric and ferroelectric properties, the fundamental physics of crystalline polymorphism, dipole rotation and macroscopic ordering of molecules still need to be clarified. The anisotropy of the polarizability of PVDF-based materials at low frequencies (Hz to MHz) has been well studied, [16][17][18][19][20] where films with in-plane distribution of polymer chains (c-axis) show relatively high dielectric constant (11-14 at 100 Hz) and low coercive field (30-70 kV mm À1 measured at 10 Hz), compared to films with randomly distributed polymer chains (dielectric constant: 7; coercive field: 260 kV mm À1 ), since the CH 2 -CF 2 dipoles can respond easily to an applied external electric field.…”

Hybrid lead-free polymer-based nanocomposites with improved piezoelectric response for biomedical energy-harvesting applications: A review

“…A spin-coated PVDF film containing 0.7 wt% CNTs (8.5 nm in diameter and 1.5 µm in length) was shown to consist of the polar electroactive γ-phase mixed with the non-polar α-phase. The γphase domains yielded a d 33 value of 13 pm/V [104]. Composite CNT-filled PVDF films prepared by the spray coating method exhibited an approximately 50-fold increase in the output power as the CNT concentration was increased from 0.015 to 0.075 wt% [105].…”