We discussed the effect of Polycarbonate (PC) type and Vapor-Grown Carbon Fiber (VGCF) size on the mechanical properties of PC/VGCF composites prepared by melt compounding using a twin screw extruder. Four kinds of PC which have different melt volume flow rate, and two kinds of VGCF which have different aspect ratio (VGCF-S which is the aspect ratio of 100, and VGCF-H which is the aspect ratio of 40.) were used in this study. In addition, the several VGCF contents were set up in each PC/VGCF composite. The Young's modulus of PC/VGCF composite overall increases with VGCF content in each case. However, the enhancement of Young's modulus in PC with low and high melt viscosity is little. Tensile strength, which is maximum stress during tensile testing, of PC/VGCF composite increases a little with VGCF content except for the case using PC with high melt viscosity. Though VGCF-S is the smaller surface area and higher aspect ratio as compared with VGCF-H, the effect of each VGCF on the mechanical properties of PC is almost same. The dispersion, aggromeration and breaking of VGCF could be interpreted from the investigation of the morphology, thermal properties and Halpin-Tsai model, which predicts the Young's modulus of the polymer/fiber composites. We concluded that the VGCF could be easily broken in high viscosity matrix.

The effect of polycarbonate (PC) matrix viscosity on the thermal conductivity of a PC/vapor-grown carbon fiber (VGCF) composite was investigated in this study in terms of the rheological properties of the PC/VGCF using two types of VGCF. Two types of VGCF, which have different aspect ratios (VGCF-h has an aspect ratio of 40, whereas VGCF-s has an aspect ratio of 100), were added to two types of PC with different viscosities. The storage modulus (G¢) and loss modulus (G 00 ) of the PC slightly increased and thermal conductivity gradually increased with the content of VGCF-h. By adding VGCF-s to low-viscosity PC, rheological properties originating in the network structure were observed. Thermal conductivity of low-viscosity PC drastically increased with the content of VGCF-s. By analyzing the length of VGCF in each composite, we found that the length of VGCF decreased with mixing. It was also easy to shorten VGCF in high-viscosity PC. We clarified that the thermal conductivity of PC/VGCF could be controlled with the viscosity of the polymer matrix because the spread of the network structure of VGCF and/or the breaking of VGCF depended on the viscosity of the polymer matrix.