Studies of the simulation of silane coupling agent structures on particulate fillers; the pH effect

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Abstract
The molecular structure of γ‐methacryloxypropyltrimethoxysilane (γ‐MPS) deposited on the surfaces of particulate mineral fillers was modeled by adjusting the pH of an aqueous alcoholic solution of the silane in absence of a substrate. Hydrolyzates obtained from the solution were studied by gel permeation chromatography (GPC) and Fourier transform infrared spectroscopy (FT‐IR) and found to be polymethacryloxypropylsilsesquioxanes whose configuration and molecular‐weight distribution were dependent on the solution pH. In acidic environments (below pH 4) the polycondensation mechanism appeared to proceed in a selective manner to form polycyclic blocks which resemble a beaded chain following condensation between blocks. Under more neutral and basic conditions (at or above pH 4) the polycondensation mechanism, resulted in polymethacryloxypropylsilsesquioxanes with the more equilibrated double chain ladder configuration. The molecular weight distribution of these structures was dependent on the stability of the silanol or the silanolate ion. Extremely high‐molecular‐weight and polydisperse polymethacryloxypropylsilsesquioxanes were generated at near neutral pH due to the formation of the double chain ladder configuration and the high silanol instability in this pH range. The two different polymethacryloxypropylsilsesquioxane configurations were also observed on γ‐MPS modified metal oxide substrates. The structures help explain some of the previously observed trends with respect to γ‐MPS adsorption on particulate mineral fillers. In strongly basic environments the methacrylate function of γ‐MPS partially hydrolyzes to form sodium carboxylate, and carbon dioxide is absorbed from the air and reacts to form sodium carbonate.

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