Early Cure Analysis to Inform Direct Ink Writing of HTPB Polyurethane: Insights from Spectroscopy, Rheology, and Molecular Simulations

This work analyzed the curing process of hydroxyl-terminated polybutadiene (HTPB) polyurethane networks in a direct ink writing (DIW)-inspired investigation using spectroscopy, rheology, and molecular simulations. Because the bulk performance of DIW-produced specimen is driven by the extrudate surface chemistry and bulk viscoelastic properties, coupled rheology and Fourier-transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy were used to sample critical physics in strategic locations within the material (i.e., at the surface and bulk) simultaneously. Data obtained from FTIR-ATR spectroscopy and rheology yielded similar trends with complementary details at early cure stages (i.e., before gel point), which are critical to the DIW process. Infrared spectroscopy provided valuable information for monitoring surface network conversion, while rheology gave precise information on when the material transitioned to a solid phase via gelation. Molecular simulations provided a predicted ideal curing process to compare relationships between isocyanate ratios and the impact of additional plasticizers on the curing process. The results showed that while adding plasticizer led to changes in both rheological and spectroscopic results, almost no changes were detected in the simulations. Overall, this study emphasizes the importance of better understanding the location-specific reaction process of HTPB polyurethane network formation for high-fidelity DIW implementation through coupled rheology and spectroscopy.