Relating mass transport properties to viscoelastic response in micro-phase separated Styrene block copolymers

The correlation between linear viscoelasticity and mass transport properties in styrene block copolymers has been investigated and discussed in terms of segmental dynamics of the soft block. The samples were phase separated polystyrene-block-polybutadiene-block-polystyrene [SBS], polystyrene-block-polyisoprene-block-polystyrene [SIS] and polystyrene-block-poly(ethylene butylene)-block-polystyrene [SEBS]. All samples have a styrene content of approximately 30%. Linear viscoelastic characterization was carried out performing small angle oscillatory shear experiments in a wide range of temperatures between the soft block Tg and room temperature. The latter experimental data were analyzed by Time-Temperature Superposition. The relaxation spectrum has been determined and the monomeric friction factor has been extracted. This value has been correlated with results from diffusion of a small foreign molecule (cyclohexane) in the block copolymers. Solubility and diffusivity of cyclohexane in films samples of the investigated copolymers as function of concentration at 60ºC have been measured by means of sequences of differential vapour sorption experiments run in a pressure decay apparatus. The effects of both molecular architecture (linear and star-branched for SBS) and microstructure of the elastomer mid-segment (polybutadiene vs poly(ethylene butylene) and polyisoprene) were considered. Solubility results were interpreted in terms of contributions from rubbery and glassy phases and pertinent results were used to derive the mobility coefficient from measured data of fickian diffusivity. Comparison between mobility coefficients in the binary system as function of solute concentration revealed that friction factor for solute molecules in SEBS and SIS is significantly higher than in SBS.