Effect of precursor macromonomer molecular weight on poly(dimethylsiloxane) film morphology and nitroaromatic vapor sorption

The capability to detect nitro-based explosives from their vapor is often limited by their low vapor pressure. One approach for overcoming this limitation is to use a solid-state pre-concentrator. The sorption and desorption of nitroaromatic vapors by poly(dimethylsiloxane)-based (PDMS) films fabricated from three different molecular weights of hydroxy-terminated poly(dimethylsiloxane) (HO-PDMS) macromonomers has been investigated. It was found that independent of macromonomer molecular weight, all the PDMS films were able to sorb nitro-based explosive analyte. However, for PDMS films of similar thickness, those formed from the lowest molecular weight macromonomer sorbed the least analyte and had the poorest retention capability. Atomic Force Microscopy (AFM) suggested that at least a proportion of the analyte was adsorbed onto the surface of the PDMS film formed from the low molecular weight macromonomer. PDMS films from the higher molecular weight macromonomers sorb more analyte with the vapor diffusing into the bulk of the film. PDMS films formed from the 750 cSt macromonomer were found to have the best analyte sorption and retention properties. The best pre-concentrator film was determined to increase the available analyte for vapor detection by up to 2 orders of magnitude.