Vapor Grown Carbon Fiber (VGCF) is, due to its extraordinary electrical properties, a most promising filler for polymer composites, which are mostly prepared through melt mixing. However, little is known about the effects of melt mixing on the length of VGCF and the electrical conductivity of composites. Here, we have measured the length of VGCF and studied its relationship with the electrical conductivity in VGCF/polymer composites by varying melt mixing time.
We used a batch type twin-screw kneader, to make composites of polycarbonate (PC) and VGCF at 260°C under 450rpm of screw speed. When the mixing time became longer, the surface resistivity increased. SEM pictures of the cross sections under various mixing times were almost identical. Longer mixing times also made the complex modules G* decrease in measurements of dynamic viscoelasticity. The length measurement of VGCF showed that the average length of VGCF as received was 5.94μm. When melt mixing times increased from 7 to 600 seconds, the VGCF length decreased from 5.76 to 3.96μm. It is believed that the initial clumps of VGCF are unexpectedly easily dispersed by a very small shear strain. At larger strains, the VGCF lengths reduce causing the increase in surface reisistivity.

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