Functionalization of Titania Nanoparticles with Carboxyl Groups for Improved Mechanical Performance of Hydroxyl Terminated Polybutadiene (HTPB) Composites

Solid fuel ramjets (SFRJs) offer an increased density-specific impulse compared to their liquid variants due to the higher density of the polymeric fuels (e.g., hydroxyl-terminated polybutadiene or HTPB) and particulate additives used. These additives influence the burn rate and mechanical properties of the HTPB matrix. Functionalizing the surface chemistry of these particles can affect the dispersion and modify the surface interaction with the HTPB matrix. This study investigated the effects of using phosphonoundecanoic acid (PA) to functionalize the surface with COOH-terminated ligands on TiO2 nanoparticles. The COOH ligands in PA are intended to cross-link with HTPB and the isophorone diisocyanate (IPDI) curative. The PA concentration affected the zeta potential of TiO2 particles where the 2 mM and untreated surfaces had the most negative zeta potential. The zeta potential changed from −26 mV for untreated particles to −10 mV for untreated TiO2 due to the removal of adsorbed moieties. All TiO2 nanoparticles were incorporated into HTPB at 60 wt % solids loading and cured with IPDI at 1.28, 1.78, and 2.37 NCO/OH ratios. Nanoindentation and tensile measurements on the HTPB–TiO2 composites observed a mechanical property trend that followed that observed from zeta potential measurements, indicating the importance of dispersion on the effects of mechanical properties particularly for the lower NCO/OH ratios. The degree of functionalization enabled the modulus of the 1.28 NCO/OH sample to be adjusted from 0.70 to 4.82 MPa and the ductility to change by a factor of 3, suggesting the ability to optimize mechanical performance with functionalization degree.