Polymer beads in microtubule-filled composites: Effects on length distributions and permittivities

The effects of adding poly(methylmethacrylate) beads to tubule-filled composites were examined in terms of changes in tubule distribution and dielectric properties of the composites. The beads had an average diameter of 51+/-29 mum, whereas the copper-coated tubules, which were shaped like hollow cylinders and derived from a phospholipid template, had an average length of 30+/-22 mum and an average diameter of 1 mum. Incorporating a small amount of beads increased the permittivity value of the composite, but adding more beads resulted in a dramatic decrease in the permittivity value. In fact, a permittivity maximum appeared at around 5 vol % beads for various tubule loadings. The results could be explained in terms of volume exclusion and decreased tubule length effects. At low bead loadings, the volume exclusion effect dominated as the tubules were pushed closer together, resulting in a higher permittivity value. At higher bead loadings, the tubules were broken into shorter pieces and this became the dominant effect. Consequently, the shorter tubules formed fewer conducting pathways and the permittivity declined in value. In addition, the De Loor model for composites containing a dielectric matrix and conductors as fillers correctly predicted a permittivity maximum and provided the best fit to the permittivity data. (C) 2004 American Institute of Physics.