Controlling network morphology in hybrid radical/cationic photopolymerized systems

Abstract

Controlling phase separation during radical/cationic hybrid photopolymerization may provide a method for manipulating polymer morphology and material properties of photocured networks. However, regulating phase separation during the highly dynamic cross-linking process remains challenging. In this study, we combined a cationic oxetane/epoxide comonomer system with a methacrylate resin to examine the effects of the cationic comonomer ratio on reaction kinetics, network morphology, and bulk material properties. At high loadings of oxetane, the dissimilar reaction rates between the rapid free-radical and relatively slow cationic photopolymerization led to highly phase separated structures. Conversely, small additions of epoxide significantly accelerated the cationic polymerization enabling control over the rate difference between hybrid photopolymerizations. Ultimately, modifications to the comonomer ratio enabled substantial control over kinetics and polymer structure. When the dual photopolymerizations occurred on similar time scales, greater network interpenetration was exhibited, whereas increased phase separation was induced with more sequential polymerizations. This ability to control polymer structure through comonomer content permits a broad range of tailorable mechanical properties for photocured films. Notably, although photopolymer tensile strength and elongation can be adapted, tensile toughness is maintained across a wide array of phase separated morphologies and is significantly improved relative to the neat methacrylate network. Finally, these systems were incorporated in 3D constructs using stereolithography, demonstrating that decreased phase separation size-scale enables greater impact strength of 3D printed objects. The results of this work show that internally regulating cationic polymerization rate using comonomer composition provides control over polymer structure and material properties in radical/cationic photopolymer systems.