Thermal analysis of epoxy-based nanocomposites: Have solvent effects been overlooked?

Abstract

Analysis of the available published data calls into question some of the reported effects of silver, graphene, and boron nitride nanoparticles on the cure behavior and glass transition temperature (Tg) of epoxy-based conductive nanocomposites. The usefulness of most studies is limited, as the final Tg value is only reported for a single composition. More useful studies provide a comparison of Tg across compositions and/or to the Tg of the neat matrix material. However, the main focus is on nanoparticle effect, while the presence of residual solvent is generally overlooked. Although the data are too sparse for firm conclusions, reductions in Tg seem to correlate with a known residual solvent or mild degas conditions (low temperature, unagitated), while increases are more likely when no solvent is used or after rigorous degassing. Using solvent control groups to differentiate solvent effects from filler effects, we conducted our own differential scanning calorimetry experiments. It was found that silver microparticles have no statistically significant effect on Tg, but appeared to when solvent content was not accounted for. The erroneous apparent effect is very similar to reported effects for nanoparticles in the literature. If a solvent must be used, we recommend that the residual solvent content be quantified and provide the Tg of a control sample with a representative solvent content without nanoparticles. Analysis of the present literature also suggests that (i) nanoparticle surface chemistry has a significant effect and (ii) poor dispersion/aggregation results in a reduction in Tg.