Functionalization\nof Titania Nanoparticles with Carboxyl\nGroups for Improved Mechanical Performance of Hydroxyl Terminated\nPolybutadiene (HTPB) Composites

Solid fuel ramjets\n(SFRJs) offer an increased density-specific\nimpulse compared to their liquid variants due to the higher density\nof the polymeric fuels (e.g., hydroxyl-terminated polybutadiene or\nHTPB) and particulate additives used. These additives influence the\nburn rate and mechanical properties of the HTPB matrix. Functionalizing\nthe surface chemistry of these particles can affect the dispersion\nand modify the surface interaction with the HTPB matrix. This study\ninvestigated the effects of using phosphonoundecanoic acid (PA) to\nfunctionalize the surface with COOH-terminated ligands on TiO<sub>2</sub> nanoparticles. The COOH ligands in PA are intended to cross-link\nwith HTPB and the isophorone diisocyanate (IPDI) curative. The PA\nconcentration affected the zeta potential of TiO<sub>2</sub> particles\nwhere the 2 mM and untreated surfaces had the most negative zeta potential.\nThe zeta potential changed from −26 mV for untreated particles\nto −10 mV for untreated TiO<sub>2</sub> due to the removal\nof adsorbed moieties. All TiO<sub>2</sub> nanoparticles were incorporated\ninto HTPB at 60 wt % solids loading and cured with IPDI at 1.28, 1.78,\nand 2.37 NCO/OH ratios. Nanoindentation and tensile measurements on\nthe HTPB–TiO<sub>2</sub> composites observed a mechanical property\ntrend that followed that observed from zeta potential measurements,\nindicating the importance of dispersion on the effects of mechanical\nproperties particularly for the lower NCO/OH ratios. The degree of\nfunctionalization enabled the modulus of the 1.28 NCO/OH sample to\nbe adjusted from 0.70 to 4.82 MPa and the ductility to change by a\nfactor of 3, suggesting the ability to optimize mechanical performance\nwith functionalization degree.