Polymers and related polymer-based networks commonly have low thermal conductivity in the range of 0.1-0.2 W m-1 K-1, which is a limiting factor for their usage in the course of continuously increasing miniaturization and heat generation in electronic applications. Basically, two strategies can be applied in order to increase the transport of phonons in polymers: (i) the embedment of thermally conductive inorganic materials, yielding composite materials, and (ii) the involvement of aromatic units enabling microscopic anisotropy by pi-pi-stacking.
In this study, the thermal conductivity of resins based on Bisphenol A diglycidyl ether BADGE and 1,2,7,8-diepoxyoctane DEO was compared. DEO can be derived from pseudopelletierine, which is contained in the bark of the pomegranate tree. DEO-based epoxy resins, hence, potentially are a natural and sustainable alternative to BADGE. The epoxy compounds were cured with isophorone diamine IPDA and o-dianisidine DAN. Notably, isophorone diamine is derived from isophorone, which naturally occurs in cranberries. The formulations were produced without filler and with 5 wt.-% of SiO2 nanoparticles.
Significantly enhanced thermal conductivity in the range of 0.4 W m-1 K-1 occurs only in DEO-based polymer networks that were cured with DAN (and do not contain SiO2 fillers). This observation is argued to originate from pi-pi-stacking of the aromatic units of DAN enabled by the higher flexibility of the aliphatic carbon chain of DEO compared to that of BADGE. This assumption is further supported by the facts that significantly improved thermal conductivity occurs only above the glass-transition temperature (with higher flexibility of the polymer segments) and that nanoparticles appear to disrupt the pi-pi-stacking of the aromatic groups.In summary, it can be argued that the bisphenol-free epoxy/amine resin (with an epoxy compound derivable from natural resources) shows favorably higher thermal conductivity in comparison to the petrol-based bisphenol-based epoxy/amine resins.
