Electron-beam curing of a novel liquid crystal thermoset resin

In this paper we report on the processing and properties of a novel, “dual-curing” liquid crystal (LC) resin cured by electron-beam irradiation. The LC monomer contains both acrylate and acetylene reactive groups. Neat resin samples were e-beam cured in either the LC phase at 65 °C or the isotropic phase at 90 °C. The experimental variables included the total e-beam radiation dose (150 or 250 kiloGray {kGy or J/g}) and thermal post-cure cycle (none or 190 °C/1 h). Cured polymer specimens were characterized by dynamic mechanical analysis (DMA) and thermo-mechanical analysis (TMA). The results demonstrated that e-beam exposure alone at 150 or 250 kGy was not sufficient to fully cure the polymer, although it was sufficient to lock-in the isotropic or LC morphology. Differences in morphology had a significant impact on the DMA and TMA results for non-post-cured specimens, where LC samples demonstrated more robust mechanical properties at elevated temperatures compared to isotropic specimens. After a thermal post-cure cycle the DMA storage modulus differences between the LC and isotropic samples were not as great. However, large differences in thermal expansion coefficients remained. The data suggest the possibility that the acrylate bonds react during e-beam exposure, followed by conversion of the acetylene bonds during the thermal post-cure. Also, e-beam curing in the LC phase likely will allow for tighter molecular packing and more efficient polymerization resulting in higher crosslink densities.