Quantitative evaluation of the dispersion ability of different preparation methods and DC electrical conductivity of vapor grown carbon nanofiber/epoxy composites

The aim of this work is to quantitatively analyze the dispersion ability of different methods for the preparation of vapor grown carbon nanofiber - epoxy composites. Four different dispersion methods were used, differing in stress level intensity: blender mixing, capillary rheometry mixing, 3 roll milling and planetary centrifuge mixing. Furthermore, the relationship between dispersion and DC conductivity of the composites was evaluated. For the dispersion analysis, four nanofiber concentrations ranging from 0.1 to 3.0 wt.% were prepared for each method, while the DC measurements were performed for eight concentrations, ranging from 0.0 to 4.0 wt.%. The dispersion was analyzed by transmitted light optical microscopy and greyscale analysis, following a methodology previously established. The results show that as the VGCNF content increases the dispersion level decreases, as indicated by the increase of the variance of the corresponding greyscale histograms. The 3 roll-mill method produces the samples with the highest dispersion levels, whilst the samples from the remaining methods show large VGCNF agglomerates. The dispersion was also estimated calculated along the length of the samples, indicating a symmetric variation of dispersion from the center. The dispersion method also strongly influences the overall composite electrical response. No relationship was found between the electrical conductivity and the greyscale analysis achieved by the different methods. Thus, this method for the quantification of dispersion works well for lengthscales around 0.1 μm, but this is above the relevant scale that determines the electrical response.Foundation for Science and Technology, Lisbon, for financial support through the 3 degrees Quadro Comunitariode Apoio, the POCTI and FEDER programmes, projects PTDC/CTM/69316/2006, PTDC/CTM-NAN/112574/2009 and NANO/NMed-SD/0156/2007, and grant SFRH/BD/41191/2007 (PC). Joint Luso American Foundation (FLAD) - NSF U.S. Research Networks Program research grant (FH and DK). We also thank Albermarle for the hardener, Hexion Specialty Chemicals for the epoxy resin, and Applied Sciences for providing their facilities. The authors also thank the COST actions MP1003 'European Scientific Network for Artificial Muscles' (ESNAM) and MP0902 "Composites of inorganic Nanotubes and Polymers (COlNAPO)" for their support. PC thanks A.J. Paleo and J. Silva for interesting discussions