Coarse‐grained models for frontal photopolymerization with evolving conversion profile

Abstract

We introduce a series of ‘minimal’ models to describe a common light-driven directional solidification process, known as frontal photopolymerization (FPP), focusing on experimental observables: the solidification kinetics, light attenuation and spatiotemporal monomer-to-polymer conversion. Specifically, we focus on FPP propagation that yields conversion profiles that are not invariant with time, and which cannot be simply described by the presence of mass or heat diffusion. The models are assessed against experimental data for the photopolymerization of a model trimethacrylate system. We find that the simplest model, comprising a single equation of motion for the conversion fraction ϕ and a generalized Beer–Lambert law, can only describe the experimental data by assuming an unphysical variation in optical absorption. Introducing a ϕ-dependent reaction constant Keff is found to require a time dependence, regardless of the functional form in ϕ. We conclude by introducing a ‘minimal’ chemical model, which is based on a simple three-step reaction scheme involving the spatiotemporal evolution of the photoinitiator fraction, relative fraction of radicals and monomer conversion fraction, that is able to capture the experimental data with a small number of parameters and under reasonable FPP assumptions. Our framework provides important predictive ability for ubiquitous solidification and patterning processes, including three-dimensional printing, via photopolymerization

Frontal free‐radical photopolymerization of thick samples: Applications to LED‐induced fiber‐reinforced polymers

“…29 This effect, known as frontal photopolymerization (FPP) or photofrontal polymerization, 30 creates a front of polymerization that propagates within the sample during irradiation. 28,31 Different authors have proposed some models to account for the frontal polymerization 29,[32][33][34][35] and it has been concluded that if the photoinitiator bleaches, the front propagates at a constant velocity in the sample. 36 This has been used to generate gradient optical materials 37,38 or microfluidic devices.…”

Monomer diffusion into static and evolving polymer networks during frontal photopolymerisation

“…A novel model of FPP, incorporating mass transport and the mechanical response of the polymer network, was found to describe all experimental data obtained using a commercial thiol-ene formulation (NOA81). By accounting for heat generation and transport 36 or multiple reaction steps, 72 the applicability of the model can be extended to a range of noncommercial thiol-ene and acrylate systems, as previously demonstrated. 27 Significantly, the model yields an estimate of the time at which swelling is relevant, and is found to agree with experiments.…”

“…where K is an effective rate coefficient for the conversion process. A more precise description of K in terms of detailed chemical reactions can be found in Purnama et al 72 The intensity of light is modelled using a Beer-Lambert law…”