Abstract
Frontal photopolymerization (FPP) is a versatile directional solidification process that can be used to rapidly fabricate polymer network materials by selectively exposing a photosensitive monomer bath to light. A characteristic feature of FPP is that the monomer-to-polymer conversion profiles take on the form of traveling waves that propagate into the unpolymerized bulk from the illuminated surface. Practical implementations of FPP require detailed knowledge about the conversion profile and speed of these traveling waves. The purpose of this theoretical study is to (i) determine the conditions under which FPP occurs and (ii) explore how optical attenuation and mass transport can be used to finely tune the conversion profile and propagation kinetics. Our findings quantify the strong optical attenuation and slow mass transport relative to the rate of polymerization required for FPP. The shape of the traveling wave is primarily controlled by the magnitude of the optical attenuation coefficients of the neat and polymerized material. Unexpectedly, we find that mass diffusion can increase the net extent of polymerization and accelerate the growth of the solid network. The theoretical predictions are found to be in excellent agreement with experimental data acquired for representative systems.
Highlights
Photopolymerization is a common solidification process that is used to create cross-linked polymer networks by subjecting a photosensitive monomer-rich bath to light, typically ultraviolet radiation [Fig. 1(a)]
The results from this study indicate that photopolymerization will be frontal provided that mass transport is sufficiently slow, as measured relative to the rate of the photopolymerization reaction, and the optical attenuation is strong
The optical attenuation coefficients play a primary role in selecting the width of the interfacial layer, with the width decreasing as these coefficients increase
Summary
Photopolymerization is a common solidification process that is used to create cross-linked polymer networks by subjecting a photosensitive monomer-rich bath to light, typically ultraviolet (uv) radiation [Fig. 1(a)]. At one end of the spectrum are accurate physicochemical models accounting for each of the reaction steps (minimally photoinitiation, propagation, and termination) [19,20,21,22], nonuniform distributions of polymer chain length [23,24], generation and diffusion of thermal energy [15], mass transport [25], and intricate optical effects [26,27] Such models, often suffer from an excessive number of parameters, some of which cannot be measured experimentally, and they offer limited insight and practical use.
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