Carbon Materials Filling Modified Structure And Properties Of Poly (vinylidene Fluoride Compound System

Poly(vinylidene fluoride)(PVDF) has been widely used in many fields in recent years because of its excellent performance. In comparison with the common plastics, however, the potential applications of PVDF are not fully developed. In order to further extend its applications, it is necessary to improve its properties or to endow it with special functions by hybridization to meet strict standards of some engineering fields. Among those commonly used fillers, carbonous materials such as carbon fiber (CF), carbon nanofiber (CNF) and carbon nanotube (CNT) play an important role in the fabrication of PVDF based composites because of their excellent mechanical strength and electrical conductivity as well as their unique aspect ratio structure. Hitherto much work has been reported on the carbonous materials filled PVDF composites, but they are still worthy of further study because the relations between their hierarchical structures and final properties have not yet been fully established, which are essential for the structure design and performance control of PVDF based composites.In this work, therefore, the carbonous fillers with different structures and scales were used to compound with PVDF to prepare PVDF based composites for structure-property relation study. Three systems, including PVDF/CF, PVDF/CNF and PVDF/CNT composites, were prepared by melt mixing. The mechanical properties, crystallization, viscoelasticity, electrical conductivity and dielectric behaviors of those composites were then explored deeply. The effects of the short-range and long-range structures of carbonous fillers on the macroscopic properties of the composites were further described using mechanical models, kinetic models and viscoelastic theory, aiming at establishing the relations between hierarchical structure of the fillers and properties, and finally, providing useful information on the structure design and property control of the PVDF based composites.(1) For the PVDF/CF composites:as the CF loadings achieve up to28wt%, the tensile strength, Young’s modulus and bending strength of the composites increase by about179.56%,107.8%and139.83%, respectively, compared with those of the neat PVDF. CF has evident reinforcement effect on the PVDF and the Krenchel and Nielsen models can be well used to describe the CF concentration effect, especially at the lower loading levels. The short-range aspect ratio structure and the long-range orientation one of the CF are the two most important structural parameters to final properties of the composites. The effective aspect ratio and orientation degree of the CF in the PVDF matrix can be well evaluated by the Halpin-Tsai and the Krenchel-COX models, respectively, which agree well with the experimental observations. In addition, the CF shows better reinforcement effect after surface treatment due to reduced interfacial tension and increased interfacial area between two phases.(2) For the PVDF/CNF composites:the presence of CNF improves both the tensile strength and the impact strength. As the loadings of CNF achieve up to3wt%and above, however, the mechanical strength of the composites decreases due to poor dispersion of CNF. The electrical property improvements of the composites by the CNF are more significantly than those by the CF:the conductivity and the permittivity of the PVDF/CNF composites increases by about9orders and2orders, respectively, compared with those of the neat PVDF. Moreover, the presence of CNF has an evident heterogeneous nucleating effect on the crystallization of PVDF, promoting formation of the nuclei and facilitating crystallization rate of PVDF as a result. But the crystal form of PVDF is not changed with addition of CNF.(3) For the PVDF/CNT composites:the presence of CNT enhances the pseudoplastic flow behavior, accompanied by the increased flow activation energy. However, the linear flow region is not sensitive to the temperature whether driven by shear rate or by strain. In the oscillatory shear flow, the solid-like response is attributed to the percolation of CNT. But the formation of percolated CNT network is temperature dependent, and the percolation threshold values reduce with increase of temperature, hence the time-temperature superposition principle is only suitable for the composites before percolation. The two-phase viscoelastic model can be well used to describe the linear responses of composites and to obtain structural parameters related to the short-range and long-range structures of CNT in the PVDF matrix. Additionally, the presence of CNT also has large influence on the mechanical properties and crystallization behavior of the PVDF, which is similar with those by the CF or CNF.