Poly(dimethylsiloxane) elastomers from aqueous emulsions: I. Preparations,and characterization of the curing and aging processes

Poly(dimethylsiloxane) (PDMS) elastomers were prepared from aqueous emulsio ns, in combination with one of several carefully chosen alkoxy silanes as a crosslinking agent, and either stannous-2-ethylhexanoate or dioctylitin di laurate as catalyst. Conditions were generally chosen so as to generate sil ica/silicate phases within the elastomeric particles or at their surfaces, for improvements in mechanical properties of films to be cast from the emul sions. A series of such films was obtained from emulsions consisting of var ious combinations of these ingredients, and was then characterized, primari ly by measurements of soluble fractions, and by extents of equilibrium swel ling (as inverse measures of degrees of crosslinking). Of primary interest were experimental results clarifying the mechanisms for the PDMS crosslinki ng and for the subsequent "aging" of these materials. For all the systems, increase in concentration of crosslinking agent was fo und to decrease the fraction of soluble polymer present and to decrease the extent of swelling. The extent to which this occurred, however, varied sig nificantly with the nature of the alkoxy silane crosslinking agent, its con centration, and the nature of the catalyst, The observed differences were u sed to propose mechanisms with regard to the role of the silanes in the for mation of silica-like phases, as well as for the endlinking of the PDMS cha ins. 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 w hether 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 reo rganizations. It appeared that dry-state aging effects are greatest for sys tems 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.