Delayed Thiol-epoxy photopolymerization: A general and effective strategy to prepare thick composites

Abstract

Photoinduced thiol-epoxy click polymerization possesses both the characteristics and advantages of photopolymerization and click reactions. However, the photopolymerization of pigmented or highly filled thiol-epoxy thick composites still remains a great challenge due to the light screening effect derived from the competitive absorption, reflection, and scattering of the pigments or functional fillers. In this article, we present a simple and versatile strategy to prepare thick composites via delayed thiol-epoxy photopolymerization. The irradiation of a small area with a light-emitting diode (LED) point light source at room temperature leads to the decomposition of a photobase generator and the released active basic species can uniformly disperse throughout the whole system, including unirradiated areas, via mechanical stirring. No polymerization was observed at room temperature and therefore the liquid formulations can be further processed with molds of arbitrary size and desired shapes. It is only by increasing the temperature that base-catalyzed thiol-epoxy polymerization occurs and controllable preparation of thick thiol-epoxy materials can be achieved. The formed networks display excellent uniformity in different radii and depths with comparable functionality conversions, similar Tg values, and thermal decomposition temperatures. The presented strategy can be applied to prepare thick composites with glass fibers possessing improved mechanical properties and dark composites containing 2 wt % carbon nanotubes.