Thermally promoted cationic photopolymerization of ring-opening monomers for Hot Lithography

Abstract

Cationic photopolymerization of ring-opening monomers offers unmatched possibilities for the production of polymer materials. Besides the unique features of the light-induced cationic curing reaction, such as oxygen insensitivity and highest reaction conversions, cationically ring-opening monomers generate outstanding mechanical, chemical and electrical properties with lowest amounts of volumetric shrinkage upon cure. Out of the great variety of applicable monomers, epoxy resins represent by far the most important substance class for the production of polymer materials in a great number of industrial applications. Due to these benefits, the direct 3D printing of materials by cationic photopolymerization is a desired goal of lithography-based additive manufacturing technologies (L-AMTs), however, suffers from strongly reduced reaction rates in comparison to predominant free-radical photopolymerization of acrylates and methacrylates. With the introduction of Hot Lithography as an innovative L-AMT operating at temperatures up to 140 °C, the use of thermally promoted cationic photopolymerization for the direct 3D printing of ring-opening monomers is challenged. To facilitate the implementation of cationic UV-curing in Hot Lithography, this thesis aims for the investigation of cationic photopolymerization at elevated temperatures. Firstly, a photoinitiation system for the application in unmediated 3D printing of epoxy monomers under significant thermal promotion was established. Furthermore, this cationic photoinitiation systems was applied on the photopolymerization of much-noticed 2-oxazoline monomers, which have not been investigated for the production of structural materials by UV-curing before. After extensive studies of the cationic photopolymerization reaction of synthesized 2-oxazoline model compounds, the highly promising thermomechanical properties of poly(2-oxazoline) photopolymers were evaluated. Finally, the novel system was used for cationic 3D printing of poly(2-oxazoline) structures with highest precision and detailed resolution in Hot Lithography.