Influence of cross-linker concentration on the cross-linking of PDMS and the network structures formed

The cross-linking of linear di-vinyl-terminated poly(dimethylsiloxanes) (PDMS) with tetrakis(dimethylsiloxane) was studied in the presence of different concentrations of the cross-linker (H/V = ratio of Si-H groups of the cross-linker and C=C bonds). The consumption of the Si-H and C=C bonds was monitored simultaneously by in situ Confocal Raman Microscopy (CRM) and ATR-FTIR spectroscopy. When formulations with H/V >= 1.0 are cross-linked at low temperature (25 degrees C) in air and atmospheric humidity conditions, hydrosilylation and secondary reactions occur simultaneously at early stages of the reaction. For H/V = 1.0 the C=C bonds are also consumed by side reactions. Films cross-linked from formulations with different H/V ratios were studied by NMR imaging, swelling/extraction experiments and SEM. Films cross-linked with H/V = 1.0 showed a slower magnetization decay due to the presence of a large percentage of extractable material not connected to the cross-linked network. After extraction, all the films show faster relaxation behavior, explained by the presence of two types of chemical cross-links as well as one type of physical cross-links. These cross-links result from the occurrence of hydrosilylation and secondary reactions and counterbalance each other at different H/V ratios. (C) 2009 Elsevier Ltd. All rights reserved.