2024 AIChE Annual Meeting
Novel Modeling of Highly Crosslinked Thiol-Ene Photopolymerization Reactions to Accurately Predict Diffusion Kinetics
A growing interest in holographic polymers requires researchers to have greater control of the polymerization process to generate improved holographic properties (increase refractive index and decrease haze). As such, a strong focus has been aimed towards improving models for holographic polymerizations. One promising approach uses thiol-ene step growth photopolymerization (also known as thiol-ene “click” chemistry) to create holographic polymers. The reaction kinetics of this “click” reaction are well understood and are traditionally controlled with temperature, concentration, light exposure, and light intensity. However, viscosity increases beyond the gel point and radical termination kinetics prevent the polymerization from reaching conversions greater than 80%, even at high temperatures. Here, experimental results are used to develop novel models for the polymerization process in an effort to predict the conditions necessary to achieve higher conversion and superior control over local conversion for improved image clarity. Previous models have effectively predicted reaction kinetics, but fail to predict diffusion kinetics due to high crosslink density and viscosity. Improvements have been made to model accuracy for highly crosslinked polymers by obtaining viscosity and reaction data from differential scanning calorimetry (DSC) and photo-DSC for high molecular weight monomers with high functionality (3 or more functional groups). Pentaerythritol triallyl ether (APE) was used as a novel high molecular weight monomer containing three alkene functional groups with many degrees of rotational freedom. It is predicted that rotational freedom will reduce glass transition temperature leading to higher conversions when reacted with PETMP. The APE PETMP polymerization was characterized by generating an empirical correlation curve between glass transition temperature and conversion. This information was used to accurately model reaction rate and conversion using reaction kinetics and free volume theory. Further investigation into “click” chemistry will focus on improving models that expand our understanding of the fundamental process of diffusion in highly crosslinked polymerizations. This has applications to other highly crosslinked step growth polymerizations (e.g. polyester and polyurethane) and will allow for greater control of local conversion; which is important in the continued development of polymer printing processes (e.g. StereoLithogrAphy) and holographic polymers for use in augmented reality, security, and information storage.