Sorption and diffusion of organic solvents through photo-crosslinked SBS block copolymers

Styrene-butadiene-styrene (SBS) block copolymer exposed to UV radiation in the presence of unimolecular and bimolecular photoinitiators yields a crosslinked polymer. When 2,2-dimethoxy-2-phenylacetophenone is used as photoinitiator the photocrosslinking takes place fundamentally through the addition of the benzoyl and/or dimethoxyphenyl radicals to the double bonds (cis, trans and vinyl) of the SBS block copolymer. The reactivity of the radicals towards the three double bonds is as follows: vinyl > cis > trans. Diffusion and sorption of chloroform and toluene through SBS block copolymer have been studied in the temperature interval of 25–60°C by conventional weight gain experiments. The diffusion results have been analyzed in terms of the simple Fickian model. Diffusion of toluene and chloroform in crosslinked SBS block copolymer was found to deviate slightly from the normal Fickian character which was attributed to the diphasic morphology of the SBS copolymer. The approach to a normal Fickian behaviour as the temperature and crosslinking increase has been explained considering the decrease in the degree of completion of the microphase separation with the increase of both the crosslinking and temperature. Temperature was found to activate the diffusion process, which was supported by higher intrinsic diffusion coefficient values and slopes values at higher temperatures. Entropy and enthalpy of sorption were estimated by using the Van’t Hoff relationship. Crosslinked SBS block copolymer–solvent interaction parameter χ were computed using the Flory–Rehner equation. The χ parameter (0.40 and 0.41 for chloroform and toluene, respectively) indicated strong polymer–solvent interaction. The crosslinking densities of the SBS block copolymer irradiated were relatively low (M̄c values go from 23,000 for an irradiation time of 30 min until 5000 for an irradiation time of 120 min).