Doping Effect of Poly(vinylidene fluoride) on Carbon Nanofibers Deduced by Thermoelectric Analysis of Their Melt Mixed Films

The effect of temperature on the electrical conductivity (σ) and Seebeck coefficient (S) of n-type vapor grown carbon nanofibers (CNFs) and poly(vinylidene fluoride) (PVDF) melt-mixed with 15 wt% of those CNFs is analyzed. At 40 °C, the CNFs show stable n-type character (S=−4.8 µV·K⁻¹) with an σ of ca.165 S·m⁻¹, while the PVDF/CNF composite film shows an σ of ca. 9 S·m⁻¹ and near-zero S (S=−0.5 µV·K⁻¹). This experimental reduction in S is studied by the density functional tight binding (DFTB) method revealing a contact electron transfer from the CNFs to the PVDF in the interface. Moreover, in the temperature range from 40 °C to 100 °C, the σ(T) of the CNFs and PVDF/CNF film, successfully described by the 3D variable range hopping (VRH) model, is explained as consequence of a thermally activated backscattering mechanism. On the contrary, the S(T) from 40 °C to 100 °C of the PVDF/CNF film, which satisfactorily matches the model proposed for some multi-walled carbon nanotube (MWCNT) doped mats; however, it does not follow the increase in S(T) found for CNFs. All these findings are presented with the aim of discerning the role of these n-type vapor grown carbon nanofibers on the σ and S of their melt-mixed polymer composites.