Poly(Dimethylsiloxane) Elastomers from Aqueous Emulsions: I. Preparations, and Characterization of the Curing and Aging Processes

Abstract

Abstract Poly(dimethylsiloxane) (PDMS) elastomers were prepared from aqueous emulsions, in combination with one of several carefully chosen alkoxy silanes as a crosslinking agent, and either stannous-2-ethylhexanoate or dioctylitin dilaurate as catalyst. Conditions were generally chosen so as to generate silica/silicate phases within the elastomeric particles or at their surfaces, for improvements in mechanical properties of films to be cast from the emulsions. A series of such films was obtained from emulsions consisting of various combinations of these ingredients, and was then characterized, primarily by measurements of soluble fractions, and by extents of equilibrium swelling (as inverse measures of degrees of crosslinking). Of primary interest were experimental results clarifying the mechanisms for the PDMS crosslinking and for the subsequent “aging” of these materials. For all the systems, increase in concentration of crosslinking agent was found to decrease the fraction of soluble polymer present and to decrease the extent of swelling. The extent to which this occurred, however, varied significantly with the nature of the alkoxy silane crosslinking agent, its concentration, and the nature of the catalyst. The observed differences were used to propose mechanisms with regard to the role of the silanes in the formation of silica-like phases, as well as for the endlinking of the PDMS chains. The formation of this inorganic phase was found to have a large effect on the extraction and swelling results, particularly through its barrier effects on the surfaces of the emulsion particles. Aging effects were found to depend not only on the nature of the reactants and their concentrations in the preparation of the emulsions, but also on whether the aging had been carried out in the dry state or in the wet state. Two major effects were decreases in soluble polymer fraction and extent of swelling with time. These changes were associated with increases in degree of crosslinking, but opposing changes can arise from bond cleavages or reorganizations. It appeared that dry-state aging effects are greatest for systems with a low concentration of the precipitated inorganic phase, with the effects of aging being suppressed by the presence of the inorganic, filler-like phases.