Clay nanolayer reinforcement of a silicone elastomer

A synthetic fluorohectorite clay in which the exchange cations have been replaced by hexadecyltrimethylammonium ions, abbreviated C16FH, has been shown to readily intercalate linear poly(dimethylsiloxane) (PDMS) molecules containing terminal hydroxyl groups. The extent of gallery swelling increased with increasing PDMS molecular weight over the range 400-4200. Little or no intercalation was observed for PDMS molecules terminated by methyl groups, indicating that terminal silanol interactions with the gallery surfaces are an important part of the gallery swelling mechanism. These interfacial interactions may also account for the unusual correlation between the extent of gallery swelling and the molecular weight of the intercalated linear polymer. Cross-linking reactions between PDMS-4200 and tetraethyl orthosilicate in the presence of the C16FH organoclay afforded elastomeric nanocomposites in which the clay nanolayers were exfoliated. The nanolayer-reinforced polymer exhibited substantially improved tensile properties and resistance to swelling by an organic solvent in comparison to the pristine polymer. Also, nanolayer reinforcement greatly reduced the structural damage caused by the internal strain induced upon allowing the solvent to evaporate from the swollen polymer network. Although synthetic fluorohectorite has one of the highest nanolayer aspect ratios among smectite clays, relatively small reductions in oxygen permeability were observed for the nanocomposites. A more or less random orientation of the clay nanolayers in the polymer matrix, as indicated from TEM images of thin sectioned samples, was responsible for the lack of an effective permeant barrier.