Photopolymerization of Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) 'click'-based networks for new-age materials
Date
2018
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
The copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) reaction is one of the most powerful tools in the ‘click’ chemistry toolbox and is immensely useful for an array of materials research avenues and applications. My research explores novel photo-CuAAC based networks as a viable alternative to currently used methacrylate-based dental composites to overcome numerous deleterious attributes of the latter, such as incomplete polymerization, degradation susceptible ester bonds, and brittle failure. The photoinitiated CuAAC i.e. photo-CuAAC is triggered with a Norrish Type II photoinitiating system (photosensitizer camphorquinone and an accelerator tertiary amine) which is sensitive to blue-light. I demonstrate that this blue light initiating system prevents azide decomposition that occurs under UV irradiation. The rapidly photopolymerized CuAAC networks exhibit polymerization after irradiation is ceased. The resultant materials possess narrow glass transition temperatures (Tg) and are glassy (>2 GPa) at room temperature. Surprisingly, these transparent photo-CuAAC networks exhibit extraordinarily high toughness uncommon in glassy thermosets. Furthermore, photo-CuAAC composites filled with silica particles exhibit ≈30% higher functional group conversion and comparable modulus to current methacrylate-based dental composites. I further simplify the Type II photoinitiating system to a one-component system by eliminating the need of leachable accelerating amines. The optimized alkyne monomer structure acts as a radical generating species, that yields rapid polymerization rates even at low concentrations and provides a handle to tune the Tg of network. ☐ Photopolymerized methacrylate networks formed via chain polymerization possesses results in a brittle, topologically heterogeneous network structure. In contrast, step-polymerized CuAAC networks are tough and topologically homogenous. I demonstrate a unique approach to integrate the CuAAC and methacrylate polymerizations in a one-pot blue-light activated scheme to form a first-of-its-kind interpenetrating polymeric network (IPN). Although simultaneously triggered under ambient conditions, the network formation proceeds sequentially owing to copper-induced methacrylate inhibition. The resulting glassy IPN material exhibits significantly enhanced toughness as compared with pure methacrylate network. This new IPN process is proposed to eliminate the requirement of additives commonly used in methacrylate-based plastics manufacturing to mitigate brittle fracture. The entangled CuAAC network in the IPN acts as a non-leachable additive enhancing its toughness, also making it a greener synthetic approach for many applications that include dental implants and coatings. Additionally, the entangled methacrylate chains lower the water uptake in bulk IPN materials. Finally, the material exhibits unique shape-memory attribute, where the material can be manipulated in glassy state (below Tg) and regain its original shape upon heating. In sum, IPN photopolymerization enables precise fabrication of tough, glassy materials with complex shapes, with potential implementation in numerous applications, from ion-conductive membranes to 3D printed shape memory thermosets.
Description
Keywords
Pure sciences, Applied sciences, Click chemistry, Crosslinked network, CuAAC polymerization, Dental material, Interpenetrating polymeric network, Photopolymerization