Quantitative evaluation of the dispersion achievable using 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 achievable using 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 along the length of the samples was also estimated, indicating 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 length scales around 0.1 μm, but this is above the relevant scale that determines the electrical response.