Moisture-Curable Poly(hydroxyurethanes) Composites Reinforced with Silica Nanofillers

Despite safety advantages over conventional polyurethanes, one of the biggest drawbacks of polyhydroxyurethanes (PHUs) are their comparatively weak mechanical properties derived from their limited molecular weights via poly(addition). Improvement of mechanical properties of a neat, moisture-curable PHU, synthesized from typical diamine and dicarbonate precursors, was attained by blending both fumed silica and amino-functionalized silica, independently. Varying filler loadings, and both amine-terminated and cyclic carbonate-terminated PHU prepolymers (synthesized in bulk using telechelic amino-terminated poly(propylene glycol), number average molecular weight M_n~2000 g mol^-1) and diglycerol dicarbonate, resulted in a much-improved formulation in terms of hardness, modulus, and tensile properties. The amine-terminated prepolymer cross-linked with 3-glycidyloxypropyltrimethoxysilane (GLYMO) and 3.5 wt% of fumed silica had the highest hardness, 2.01 MPa, and the highest modulus, 5.07 MPa, according to nanoindentation tests, and we obtained E of 0.0280 MPa, σ_u of 0.118 MPa, and ε_u of 13.2%, all of which were unobtainable for the benchmark PHU as it was mechanically too weak to perform the tests. All samples were successfully cross-linked, and we found that those reinforced with amino-functionalized silica were weaker than their fumed silica counterparts due to the formation of aggregates, which were confirmed using SEM and TEM imaging.