Swelling of medical grade silicones in liquids and calculation of their cross-link densities

Abstract 

Four medical grade silicones were swollen, until they reached equilibrium (i.e. constant mass) in eight liquids at 25°C. The greatest swelling was obtained with n-heptane but the volume fraction, ϕ, of the silicones in their swollen state was not significantly different (p<0.05) in this liquid than in cyclohexane. For each grade of silicone, ϕ was plotted against δ_l, the liquid solubility parameter, for each liquid in which it was swollen. A second-order polynomial was plotted through the results; the minimum in this polynomial provided a value for the polymer solubility parameter, δ_p. The Flory polymer–liquid interaction parameter, χ, was calculated for the four best liquids, using Hildebrand's solubility parameter theory. An alternative method for calculating χ, directly from swelling measurements, was shown to produce physically unreasonable results. The cross-link density, υ, was calculated, from ϕ and χ, for each grade of silicone, using the Flory–Rehner equation. Since the values of two parameters involved in Hildebrand's theory cannot be determined reliably and because the Flory–Rehner equation is an approximation, absolute values of υ cannot be obtained. However, the relative values of υ obtained were higher for the harder grades then for the softer grades and similarly, the grades with the higher Young's modulus had higher υ values.

Keywords: Cross-link density, Flory–Rehner equation, Hildebrand's solubility parameter theory, Silicones, Swelling of polymers in liquids