Spectroscopic Characterization of Low Molecular Weight Fluids From Silicone Elastomers

The migration of low molecular weight (LMW) silicone fluid from the bulk to the surface of polydimethylsiloxane (PDMS) elastomers results in hydrophobic recovery after thermal or UV damage. The identification and analysis of these silicone fluids is an important step in understanding the process which ultimately affects the service lifetime of the elastomer and may provide one approach to assessing residual service lifetime in applications such as high voltage insulation. LMW silicone fluids were extracted from the bulk of four different silicone elastomers used as the weather sheds on high voltage insulators and differences were observed both in the total amount of fluid and the amounts obtained on successive extractions. MALDI-MS analysis of the extracted fluids showed significant differences in the proportions of cyclic and linear species. The extracted fluids were also analyzed by FTIR and Raman spectroscopy to determine trimethyl silyl end-group concentration using a calibrated series of PDMS standards containing known proportions of linear and cyclic species. Comparison of FTIR and Raman analysis with the MALDI-MS showed a major limitation in vibrational spectroscopic analysis of cyclic/linear content occurred when the linear species terminated in dimethylhydroxyl rather than trimethyl silyl groups.