The transport of octamethylcyclotetrasiloxane (D-4) and polydimethylsiloxane (PDMS) in lightly cross-linked silicone rubber

The transport of octamethylcyclotetrasiloxane (D-4), one of the major const ituents of silicone fluids and rubbers, and low viscosity polydimethylsilox ane oil into a silica filled cross-linked silicone elastomeric rubber was m easured as a function of temperature, cross-link density of the rubber, and concentration of the D-4 in methanol solution. A small amount of material, approximately 3 wt%,, is extracted from the rubber with hexane. The extrac tion process has a large effect upon D-4 solubility in the rubber, increasi ng from approximately 160 to 180 wt% after extraction. The heats of solutio n for both penetrants into the rubber are essentially zero and the activati on energies for diffusion are small, approximately 8 and 15 kJ mol(-1) for D-4 and PDMS, respectively. The diffusion process is Fickian and the diffus ion coefficient of D-4 into silicone/silica rubbers is essentially independ ent of concentration over the concentration investigated, i.e. from 1 to 10 0 vol% D-4 in methanol. The permeability, i.e. the product of the diffusion coefficient and the solubility, decreases rapidly for D-4 concentrations l ess than 50 vol% (0.1 mol fraction). This suggests that the permeation of D -4 out of any encapsulation device, such as a silicone breast implant, is l inearly dependent upon the concentration of D-4 in the prosthesis. Swelling is isotropic and was measured by dimensional changes in rectangular sample s and correlates well with the volume of D-4 sorbed.