Swelling and plasticization of PDMS and PTMSP in methanol and dimethyl carbonate vapors and liquids: Volume, mechanical properties, Raman spectra

The swelling and plasticization of polydimethylsiloxane (PDMS), a rubbery polymeric network, and poly [(trimethylsilyl)propyne] (PTMSP), a glassy polymer, in methanol and dimethyl carbonate (DMC) vapors (a) and liquids (b) was studied using visible-light microscopy (ad a), dynamic mechanical analysis (DMA, ad a) and confocal Raman microscopy (ad b). The properties of swollen PDMS were found to be as expected: Volume swelling followed volume additivity and swelling in mixed vapors was predictable from single component swelling data, elastic moduli followed the scaling law and Raman spectra of pure and swollen PDMS were indistinguishable. On the contrary, swollen PTMSP showed pronouncedly lower volume changes than those expected from volume additivity and non-predictable swelling in mixed vapors, preferential saturation of the nonequilibrium packing defects of the polymer, an initial plateau for the storage modulus and pronounced spectral changes when swollen with the liquids. The initially rigid physical structure of PTMSP was found to be relaxed when the concentration of the volatile compound in the polymer exceeded the capacity of the first adsorption layer, which is one of the parameters of the Guggenheim, Anderson, de Boer (GAB) model of multilayer adsorption. Hence, the general mechanism for sorption of volatile compounds in glassy polymers is likely adsorption and gradual physical relaxation.