Optical response to stress in pyrene labelled polydimethylsiloxane elastomers: Monitoring strain in 1D and 2D

Pyrene-labelled polydimethylsiloxane (Py-PDMS) elastomers were synthesised by the condensation reaction of [3-(pyren-1-ylmethoxy)propyl]triethoxysilane (Py-TEOS) with a mixture of α,ω-dihydroxyPDMS (Mn = 15,000 or 26,000 or 45,000) (PDMS) and tetraethoxysilane (TEOS) and were cured using a Sn(II) catalyst. The resulting mechanoresponsive fluorescent Py-PDMS elastomers were studied by differential scanning calorimetry and dynamic mechanical analysis. No significant changes were observed in the thermal or mechanical properties of the elastomers containing Py when compared to an otherwise identical sample not containing Py. Unlabelled PDMS elastomers were also synthesised and painted with Py-TEOS to compare the mechanical effect of Py incorporated either internally or on the surface of the PDMS elastomers. All of the Py containing elastomers were demonstrated to be fluorescent showing both monomer and excimer emissions under suitable excitation in a photoluminescent spectrometer. The elastomers were placed in a photoluminescence spectrometer and subjected to cycles of extension and relaxation (strain = 0–20%) whilst changes in the emission spectra were monitored. The resulting spectra of the mechanoresponsive Py-PDMS elastomers emission peaks displayed clear and reproducible changes in fluorescence intensity upon stretching and relaxation. The intensity of the monomer and excimer emission peaks was observed to increase with elongation of the sample and decrease upon relaxation. Furthermore, the ratio of the intensities of the excimer:monomer peaks (IE:IM) decreased with elongation and increased with relaxation. The fluorescent response of the Py-PDMS elastomers (MW 26,000) showed no appreciable hysteresis, suggesting that fluorescent labelling of elastomers is a valid approach for the non-invasive in situ monitoring of stress and strain in such materials. Furthermore, resulting fluorescent optical microscopy images taken of the elastomer samples whilst under extension and relaxation cycles were used to reveal a 2D map of fluorescent intensity clearly showing intensity changes with elongation of the elastomers.