Cross-Linking of Methyl Silicone Rubbers. Part II. Analysis of Extractables from Samples Cross-Linked under Various Conditions

Abstract

Abstract During the cross-linking under themal conditions of two pre-polymers, a variety of compounds may be formed in addition to the desired cross-linked network, i.e., branched chain, large ring, and linear chain of twice pre-polymer molecular weight. In order to evaluate these processes with as little chemical complication as possible, a series of two component methyl silicone pre-polymers (Sylgard 184, 186, and 188) have been cross-linked with vinyl silicone pre-polymer at various rations of reactive groups and concentrations in silicone oil. These rubbers were evaluated by thermomechanical analysis (TMA) and then swollen with n-hexane. The cross-link density was evaluated from TMA and hexane swelling. The extracts were studied by gel permeation chromatography. As the “catalyst” concentration (active hydrogen containing prepolymer) was increased from 5 to 60%, the percent and average molecular weight of the hexane-soluble fraction was found to decrease for the 186 and 188 samples. No higher molecular weight fraction was extractable from the 184 sample. For all samples the modulus and cross-link density increased until 10 to 30% catalyst was present. Above this concentration these parameters dropped off gradually. Evidence is offered to support the view that polymerization in dilute soulution promotes cross-linking. The molecular weight changes noted for the extractables from the 186 and 188 sample suggests that branched chain formation predominates as a side reaction product. Little original pre-polymer is extractable in an unreacted state. Differential scanning calorimetry was employed to follow the thermodynamics of the reaction.