A. Saha, M. Finale, S. Chowdhury
New Mexico Institute of Mining and Technology,
Keywords: refractory, plasmonic, titanium nitride, thermosets, 3-D printing, Diels alders
Summary:Additive manufacturing of thermosets is attractive due to their potential for structural applications, especially for automotive and aerospace parts. Irreversible crosslinking and slow curing rate of thermosets like epoxy, however, make layer-by-layer printing difficult. We have developed a reversible thermosets polymer incorporating thermo-reversible covalent adaptable network (Diels–Alder (DA) reactions) into it and mixing that with refractory plasmonic titanium nitride (TiN) nanomaterials.1 TiN nanoparticles can efficiently, and rapidly convert visible light into localized heating to trigger the polymerization and depolymerization reaction. Additionally, it provides an opportunity for targeted repair of defects on a 3-D printed epoxy structure and smoothing the rough surface of the finished products. The nanoparticle loading, light intensity, and heat dissipation were studied to determine the optimal conditions for enhancing DA reactions. We could reach a temperature as high as 1500C and starts melting within 10 seconds with a 0.78 W/cm2halogen lamp light source for the epoxy sample with 0.5 wt% titanium nitride nanoparticles loading. The sample without the nanoparticles could only reach 750C when exposed to a similar intensity halogen lamp. Our in-situ FTIR study of reversible epoxy/TiN composites under light suggested that broad-spectrum white light can significantly enhance the polymerization and depolymerization reactions. Interestingly, the reaction rate with light is significantly faster than the reaction driven by heat even though the average bulk temperature of the samples is similar. Plasmonic TiN nanostructures concentrate incident light in nanometer-sized volumes to generate nanoscale heating which causes the local temperature around nanoparticles hotter than the bulk temperature measured. This can lead to enhanced chemical reactions at the near field of these plasmonic nanoparticles ultimately showing improved reaction rates in the bulk scale. However, we also found that increasing temperature higher than 1400C can cause irreversible cross-linking due to homo-polymerization of maleimide, one of the reactants for DA reactions, which can adversely affect the reversibility of DA epoxy.2 References: 1. Mojtabai, K. D.; Lindholm, S. J.; McReynolds, B. T.; Penners, N.; McCoy, J. D.; Chowdhury, S.; Lee, Y., ACS Applied Polymer Materials 2022, 4 (4), 2703-2711. 2. McReynolds, B. T.; Mojtabai, K. D.; Penners, N.; Kim, G.; Lindholm, S.; Lee, Y.; McCoy, J. D.; Chowdhury, S., Polymers 2023, 15 (5), 1106.