Researcher(s)
- Matthew Poshusta, Chemical Engineering, University of Delaware
Faculty Mentor(s)
- Chris Kloxin, Chemical and Biomolecular Engineering, University of Delaware
- Jovan Tatar, Materials Science and Engineering, University of Delaware
Abstract
The addition of catechol groups into polymer networks has been theorized to greatly improve their adhesive properties. The experimental design tested thiol-yne and thiol-epoxy reactions, with and without the addition of propargyl dihydroxybenzoate (catechol). The thiol-yne reaction was photopolymerized and was triggered immediately or latently. However, the thiol-epoxy reaction was base catalyzed by 5 wt% triethylamine. The aim of this study was to determine reaction conditions that attain the greatest overall thiol conversion without sacrificing the glass transition temperature in catechol-containing polymer networks. Fourier transform infrared (FTIR) spectroscopy was used to measure conversion data, while dynamic mechanical analysis (DMA) was performed to measure glass transition temperatures.Initial thiol-yne photopolymerizations with no catechol or propargyl benzoate (catechol control) resulted in a thiol conversion of approximately 70%. Epoxy groups were then stoichiometrically added to the system to react with the remaining 30% of thiol groups. The resulting ternary system was catalyzed by UV light and a basic environment. Adding epoxy groups into the thiol-yne photopolymerized system allowed the thiol to reach nearly complete conversion (i.e., 95%) before either propargyl benzoate or catechol were added. The system had a glass transition temperature of approximately 58oC without the propargyl benzoate or catechol. However, the thiol groups only reached 84% conversion when the thiol-epoxy reaction was allowed for 90 minutes before exposure to UV light to polymerize the thiol-yne monomers. With the addition of propargyl benzoate, the system reached 98% thiol conversion when cured instantly. The glass transition temperature decreased to 42oC with the addition of propargyl benzoate, likely due to a lower crosslink density driven by the lower functionality of the singular alkyne group compared to the trialkyne monomer. When catechols were introduced to the dual-cure system the thiol and alkyne conversions dropped significantly. The thiol reached a 60% conversion while the alkyne reached 42%. However, the epoxy groups continued until completion (100% conversion). When this system was subjected to a 90-minute latent cure, the thiol reached 50% conversion following the same pattern as observed previously. The lower conversion of the catechol-containing polymer networks may be attributed to the radical capturing effects of the catechols. This effect may hinder the overall conversion of the thiol-yne reaction while allowing the thiol-epoxy reaction to continue. In the future, using alternate forms of catalysis and avoiding exposure to UV light may increase the overall cure of the system.