Formation of skin-core structure in filled room-temperature vulcanized polydimethylsiloxane after long exposure to moderate temperatures

This study highlights the formation of a skin-core effect inside a filled silicone rubber exposed to air for durations up to 600 days at temperatures between 30 °C and 70 °C. Coupling mechanical and physico-chemical characterisations reveal that the volatile components, such as crosslinking by-products and oligomers, evaporate from the elastomeric network during the exposure, resulting in the shrinkage of the sample. This phenomenon gives rise to the formation of a composition gradient from the sample's surface towards its core. All the samples become more brittle with ageing, but the tensile modulus increase only slightly at the longest durations and highest temperatures. More fillers are found on the outer surface while the infrared spectra in the core of the sample remain unaffected by neither the temperature nor the duration. The nanoindentation highlights gradients of elastic modulus on the cross-section and provides an estimation of the skin thickness. By wrapping the samples in aluminum foil, the evaporation of volatile components is prevented. The measured properties remain constant with the duration of thermal exposure even at the highest temperature. • Thermal ageing of filled RTV PDMS ranging from 30 °C to 70 °C from 10 to 600 days. • Characterisation by tensile tests, nanoindentation, solvent swelling and ATR-FTIR. • Evaporation of cross-linking by-products and oligomers leads to embrittlement and slight stiffening • Coupling ATR-FTIR and nanoindentation revealing a skin-core effect.