Effect of Casting Solvent on the Mechanical Properties of a Styrene-Butadiene-Styrene Block Copolymer

Abstract

Abstract The tensile mechanical properties of a number of well characterized solution-cast styrene—butadiene-styrene block copolymer systems are reported within the strain range of 0<ε≤5.0. It was found from the results of repeated stretching experiments (up to 100%) at room temperature that polymers cast from solvents for polystyrene or from solvents for both polystyrene and polybutadiene showed lower stresses on each cycle whereas the stress levels for the polybutadiene solvent-cast sample remained essentially constant. The polystyrene solvent-cast and polystyrene/polybutadiene solvent-cast samples exhibited a yield at ε≈0.3. No “yield” was apparent with the polybutadiene solvent-cast sample. The explanation for these behaviors lies in the material of the continuous phase. When the continuous phase is polystyrene there will be long-time recovery after the yield and consequently the system will be left under residual stress so that lower strains are observed on subsequent straining. When polybutadiene is the continuous phase (a phase inversion) no such yielding is apparent, the form of the stress-strain curve is that of an elastomeric material, and hence the repeated stress-strain curves are practically superposable. It was concluded that the low strain region is governed by the material of the continuous phase. From the viscoelastic measurements it was found that at 1.0≤ε≤6.0 the values of the modulus-temperature dependencies were comparable to that given by polystyrene and that the linearity and the slopes of plots of the logaT as a function of temperature were indicative of a glassy and, moreover, a polystyrene response. It was concluded that the strain region 2.0≤ε≤5.0 is governed by the polystyrene content, whether it be in the continuous phase or not. These results, together with certain proposed model structure, are discussed with the available literature.