Hybrid Free-Radical/Cationic Phase-Separated UV-Curable System: Impact of Photoinitiator Content and Monomer Fraction on Surface Morphologies and Gloss Appearance

Abstract

Simultaneous photopolymerization of radical and cationic systems is one strategy to generate polymer network architectures named interpenetrating polymer networks (IPNs). In these hybrid systems, phase separation and final polymer morphology are ultimately governed by thermodynamic incompatibility and polymerization kinetics. This behavior is quite complex, as numerous factors can affect polymerization kinetics including monomer/oligomer viscosity and structure, light intensity, photoinitiator content and absorbance, cross-linking, vitrification, etc. In this work, the impact of photoinitiator concentration and monomer fraction on surface morphologies in a hybrid radical/cationic phase-separated system was examined. Wrinkles formed on the surface of photopolymerized films depend on the polymerization rate and acrylate/epoxy ratio. This phenomenon is partially explained by the rapid polymerization rate associated with the development of an epoxy matrix and a smaller acrylate domain. The size and shape of the wrinkles can be controlled by varying formulation parameters (mainly, composition) and photoinitiator content. It was possible to create surface roughness and consequently decrease the gloss by controlling the polymerization kinetics and phase-separated morphology. This study demonstrates that the morphology, polymerization kinetics, and film properties (e.g., gloss, transparency) can be manipulated with the ratio of the acrylate/epoxy mixture and the photoinitiator content.

Effect of Copolymer on the Wrinkle Structure Formation and Gloss of a Phase-Separated Ternary Free-Radical/Cationic Hybrid System for the Application of Self-Matting Coatings

“…In our previous study [35], we demonstrated the immiscibility of CE and PBDUA prior to photopolymerization, which was dependent on the ratio of these two species. Based on these findings, a third monomer was included (HDDMA) in the mixtures studied here, to ensure compatibility.…”

“…Specifically, thermodynamics dictates the degree of incompatibility between monomers and monomer-polymer solutions and, thus, the driving force for phase segregation. According to our previous study, CE and PBUDA show incompatibility in their monomer liquid states, depending on their ratio, which leads to the formation of a heterogeneous and totally phase-separated network [35].…”

“…The concentration of the radical and cationic photoinitiators (PIs) was 1 wt.% and 3 wt.%, respectively. This PI ratio was chosen based on our previous study, which showed that at a high concentration of cationic PI, the epoxide part formed very rough surfaces, while the radical PI had little impact on the final degree of conversion [35]. The reference system was modified by incorporating 1, 2, 3, 5, 7, and 10 wt.% PBGMA, while maintaining the CE/PBUDA/HDDMA ratio.…”

“…In the present work, the miscibility of a ternary mixture comprised of a polybutadiene urethane diacrylate (PBUDA), a cycloaliphatic diepoxyde (CE), and hexanediol dimethacrylate (HDDMA) was studied. In fact, we previously demonstrated the immiscibility of CE and PBDUA prior to photopolymerization, which was dependent on the ratio of these two species [35]. Here, HDDMA, which had complete miscibility with both CE and PBUDA, was added to ensure compatibility.…”

Photoinitiator Selection and Concentration in Photopolymer Formulations towards Large-Format Additive Manufacturing

“…As the reaction progresses, the viscosity of the solution increases, and radicals can become occluded in the growing polymer network. This radical trapping is a prevalent termination step for glassy photopolymers such as Bis-EMA, which can become vitrified before all monomer is consumed [36][37][38][39][40].…”