(233f) Photocurable Prepolymers with Distinct Structure and Functionality

In recent years, photopolymerization has become increasingly important based on its energy efficiency and lack of volatile organic components. With its inherently fast processing, photopolymerization is used in many applications ranging from adhesives and coatings to 3D manufacturing.  UV curable formulations regularly employ macromolecular prepolymer/oligomer species to help govern the bulk properties of the final films. However, the photopolymerization reaction generates a large degree of polymer network heterogeneity due to localized reaction gradients inside the film, and increased internal stresses within the system leading to premature failure and broad glass transition. In our work, nitroxide-mediated polymerization (NMP) was used to synthesize novel prepolymer molecules with unique architectures and placement of epoxy and (meth)acrylic functionalities. NMP accomplishes this via a reversible termination reaction, which allows simple batch feeds to generate the desired architectures without affecting the photopolymerization process. Through the use of these architectured photocurable prepolymers, we have shown the ability to modify the glass transition temperature of UV curable thermosets of identical chemical composition.  This is achieved by manipulating the reactive group placement which affects how the predominantly homopolymer molecules incorporate into the resulting photocured network.  By concentrating the reactive epoxy/ (meth)acrylate moieties at the ends of the architectured prepolymers, two domains can be created within the photocured thermoset. One domain results from the unreactive prepolymer backbone while the other is due to a high degree of crosslinking with the reactive diluents. The use of these architectured prepolymers also drives the morphology of the resulting thin films largely due to the formation of multiple domains. The formation of multiple domains allows multiple glass transitions to be generated within a single material. By generating domains with high and low T_g’s, through the selection of prepolymer and reactive diluent, the resulting materials display both glassy and rubbery behavior resulting in architectured prepolymer formulations having increased the overall toughness of the materials as well as enhanced creep resistance.