(672a) Spectrokinetic Approach Towards Understanding Effect of H2o on Surface Treatment of Silica Filler

The mechanical strength of silica filler-reinforced polydimethylsiloxane is controlled by silylating filler surfaces with trimethyl siloxane (TMS) groups by reaction with hexamethyldisilazane (HMDZ). The molecular mechanisms of HMDZ silylation under practical conditions remain unclear, especially in the presence of HMDZ, water (H₂O), and intermediates formed in situ.¹ Here, we investigate these questions using spectrokinetic analysis of HMDZ silylation of fumed silica, which utilizes calibrated absorption coefficients together and time-resolved spectra to obtain surface coverages as functions of time at each set of reaction conditions. Subsequently, measurements of initial rates report on the influence of H₂O on rates, the total coverage of TMS groups, and the mechanism of the reaction.

Uncalcined and calcined silica samples, the latter consists primarily of isolated silanol groups, give similar initial silylation rates, indicating that HMDZ preferentially reacts with isolated silanol species. Initial rates increase in proportion to the HMDZ partial pressure (P_HMDZ) in the absence of H₂O but depend sublinearly on P_HMDZ in the presence of water (0.06 – 0.23 kPa HMDZ, 2.34 kPa H₂O, 303 K), and decrease by an order of magnitude with increasing P_H2O (0.06- 2.34 kPa H₂O, 0.12 kPa HMDZ) over uncalcined silica. These comparisons indicate HMDZ not only reacts with isolated silanol sites but also hydrolyzes to form trimethylsilanol (TMSOH) in situ.² Infrared spectra confirm the presence of TMSOH, and experiments show both HMDZ and TMSOH act as silylating reagents albeit with different rates. Uncovering the effect of H₂O in HMDZ silylation kinetics provides guidance for the rational design of silica filler treatment conditions.