Propagating waves of network formation induced by light

The exposure of a photopolymerizable liquid (e.g., multifunctional thiol-ene) to ultraviolet radiation often leads to a propagating wavefront of network formation that invades the unpolymerized material from the illuminated surface of the photosensitive material. We theoretically describe this light-driven frontal photo-polymerization (FPP) process, which is the basis of many commercially important fabrication methods, in terms of an order parameter ϕ(x,t) characterizing the extent of monomer-to-polymer conversion, the temporally and spatially evolving optical attenuation μ(x,t) of the medium, and the height h(t) of the resulting solidified material. The non-trivial aspects of this frontal polymerization process derive from the coupling of μ(x,t) and the growing non-uniform network ϕ(x,t) and we consider limiting situations in which the optical attenuation increases ('photodarkening') or decreases ('photobleaching') in time to illustrate the general nature of FPP front propagation and the essential variables on which it depends. Since FPP fabrication of complex three-dimensional structures containing components having different material characteristics would greatly extend the practical utility of this method, we explore the influence of nanoparticle (silica, titania, and multi-wall carbon nanotube) additives on FPP front propagation. We also characterize the influence of temperature on the kinetics of FPP since this factor can often be controlled in practice. The experiments elucidate basic physical aspects of FPP and are well described by our model with a sensible variation of relevant physical parameters (optical attenuation and chemical rate constant) governing frontal growth. Our results are of interest both from the standpoints of complex structure fabrication and for understanding the fundamental nature of the FPP process.