Conductive Network and beta Polymorph Content Evolution Caused by Thermal Treatment in Carbon Nanotubes-BaTiO3 Hybrids Reinforced Polyvinylidene Fluoride Composites

摘要

A good dispersion of carbon nanotube (CNT) in polyvinylidene fluoride (PVDF) is realized by using CNT and BaTiO3 (BT) hybrids (H-CNT-BT) with a special core shell structure. Thus, a high dielectric performance is achieved for the composite (H-CNT-BT/PVDF). Carried by BT, CNT is easy to connect with each other and thus more interface area may be created which helps to achieve an extremely low percolation threshold (f(c)). Moreover, the dielectric permittivity of the composite near fc is increased more than three times after thermal treatment while dielectric loss remains at a low level. In order to study more comprehensively about the influence of thermal treatment, in situ synchrotron X-ray is used to detect recrystalline behavior of PVDF. Results of wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) show that after thermal treatment, the content of beta polymorph has increased nearly double times at the interface of CNT-PVDF, and the thickness of amorphous layers (La) in PVDF's long periods (LP) has shrunk around 10 A. Increased beta polymorph at the interface of CNT-PVDF may form an ideal structure with the grading dielectric permittivities from the center to the border which decreases the contrast between CNT and PVDF. Meanwhile, the evolution of CNT's network possibly occurs in the procedure of La shrinkage, where the strong interfacial polarization may be aroused. Besides, an increase in the thickness of crystalline lamella may also arouse more orientational polarization and improve dielectric properties at high frequency. Combining with BT's buffer role for blocking possible leakage current during the percolative behavior, the dielectric loss of composite can remain at a very low level even after thermal treatment. In addition, experimental results show that prolonging annealing duration or increasing annealing cycles favors stabilization of CNT's dynamic percolation, which reduces the sensitivity of CNT's network in the composite and further improves dielectric properties. After thermal treatment, the dielectric permittivity reachs 1172, but dielectric loss remains at 0.55 at 100 Hz. To our best knowledge, this high dielectric performance is really rare, only found in recent reports.