Enhanced thermal conductivity of photopolymerizable rubbery and glassy thiol-ene composites filled with hexagonal boron nitride

In this study, a facile and speedy method of making highly thermally conductive polymer composites containing hexagonal boron nitride (h-BN) was demonstrated. Rubbery and glassy at room temperature thiol-ene based composites were fabricated by mechanical mixing of unmodified h-BN microparticles with different liquid thiol and ene monomers followed by UV-curing of the cast films. Thermal diffusivity of the composites was directly measured by a Light Flash Analyzer (LFA) while thermal conductivity calculated from thermal diffusivity, density, and thermal capacity at constant pressure. Scanning electron microscopy (SEM) was used to investigate the morphology of the composites, in particular, the particle orientation and spatial arrangement. Both rubbery and glassy thiol-ene based composites exhibited very high levels of thermal conductivity for the composites prepared via mechanical mixing. For instance, the thermal conductivity of the rubbery and glassy composites with 40 wt% of h-BN were 1.38 W/m·K and 0.74 W/m·K as compared to 0.19 W/m·K and 0.11 W/m·K for the unfilled networks, respectively. The thermal conductivity versus h-BN content experimental data were fit to the Nielsen model which showed a good agreement. But the only fitting parameter of the model, which is related to the particle aspect ratio, turned out to be greater than the calculated one based on the real dimensions of h-BN filler. The SEM data have shed light on this behavior. The h-BN phase was uniformly dispersed and mainly made of tiny aggregates in which the stacked platelets demonstrated one way sliding, like steps in a staircase, thus forming quasiparticles with an increased aspect ratio. The study also revealed the effect of h-BN particle size and processing conditions such as adding a small amount of solvent used to reduce viscosity of the thiol/ene/h-BN mixture prior to photopolymerization on the thermal conductivity of the composites.