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 cross linked polymer. When 2,2-dimethoxy-2-phenylacetophenone is used as photoini tiator the photocrosslinking takes place fundamentally through the addition of the benzoyl and/or dimethoxyphenyl radicals to the double bonds (cis, t rans and vinyl) of the SBS block copolymer. The reactivity of the radicals towards the three double bonds is as follows: vinyl > cis > trans. Diffusio n and sorption of chloroform and toluene through SBS block copolymer have b een studied in the temperature interval of 25-60 degrees 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 crossli nked SBS block copolymer was found to deviate slightly from the normal Fick ian character which was attributed to the diphasic morphology of the SBS co polymer. The approach to a normal Fickian behaviour as the temperature and crosslinking increase has been explained considering the decrease in the de gree of completion of the microphase separation with the increase of both t he crosslinking and temperature. Temperature was found to activate the diff usion process, which was supported by higher intrinsic diffusion coefficien t values and slopes values at higher temperatures. Entropy and enthalpy of sorption were estimated by using the Van't Hoff relationship. Crosslinked S BS block copolymer-solvent interaction parameter (chi) were computed using the Flory-Rehner equation. The ii parameter (0.40 and 0.41 for chloroform a nd toluene, respectively) indicated strong polymer-solvent interaction. The crosslinking densities of the SBS block copolymer irradiated were relative ly low ((M) over bar(c) Values go from 23,000 for an irradiation time of 30 min until 5000 for an irradiation time of 120 min). (C) 2000 Elsevier Scie nce Ltd. All rights reserved.