Tuning Silica SurfaceProperties for Enhanced Performancein Si–UHMWPE Battery Separators

By chemically tuning the surface of precipitated silica, we propose an approach to vary the hydrophilicity and elucidate its impact on the state of dispersion of silica aggregates in hydrophobic materials. Precipitated silica underwent reversible chemical modification, which transformed its surface from a hydrophilic surface to a hydrophobic surface in order to promote interactions with hydrophobic environments, e.g., suspension of silica in hydrophobic solvents and dispersion in nanocomposites. Hence, tunable hydrophobic molecules, i.e., 3-mercaptopropyltrimethoxysilane (MPTMS), were grafted onto the surface of silica. In the first step, the properties of the surface of silica were adapted to enhance the dispersion of particles in a hydrophobic medium (e.g., processing hydrophobic polymers filled with silica). Afterward, the obtained modified silica underwent chemical tuning to recover part of its initial hydrophilicity, which is desired for some applications like battery separators. Thereby, the grafted molecules onto the surface of the silica were oxidized to decrease the hydrophobicity of the grafted functions. For each surface treatment of silica particles, solid-state NMR analyses were used to confirm qualitatively the presence of the grafted molecules onto the surface of silica, and TGA analysis and conductance measurements were used to quantify the grafted molecules. Water sorption isotherms were also used to characterize the hydrophilic change of silica. Finally, the obtained silicas were used in formulations of UHMWPE (ultrahigh molecular weight polyethylene)–silica battery separators that were characterized by scanning electron microscopy (SEM), small-angle neutron scattering (SANS), porosity measurements, and electrical resistance measurements. The silica grafted and then oxidized presented the best dispersion in the UHMWPE while presenting the high hydrophilicity needed for the low resistivity of the membrane.