Influence of block molecular weight on the properties of styrene-ethylenebutylene-styrene block copolymers

Abstract

Styrene-ethylene butylene-styrene (S-EB-S) block copolymers with similar polystyrene contents and varying molecular weights (S-EB-S-1, molecular weight: 8833-41223-8833; S-EB-S-2, molecular weight: 15844-70534-15844; S-EB-S-3, molecular weight: 26133-111067-26133) were used in this study. The domain size of the polystyrene phase marginally increases with an increase in polystyrene segmental weight as observed by atomic force microscopy. Dynamic mechanical measurements of these polymers were carried out over a wide range of temperatures and frequencies. These polymers exhibited three peaks: α, β, and γ in the tan δ-temperature curve. With increase in the molecular weight of the S-EB-S polymers, the α-transition temperature shifted to higher values, while the β- and γ-transitions remained unaltered. Also, the elastic modulus and storage modulus decreased with increase in the molecular weight. The rheological behavior of the various S-EB-S polymers was studied using a Monsanto Processability Tester. These systems exhibited pseudoplastic flow behavior. The shear viscosity of these S-EB-S polymers decreased with an increase in the molecular weight from S-EB-S-1 to S-EB-S-3 polymers because of the wall slip and plug flow. The activation energy of the melt flow process was found to vary between 4 and 0.6 kcal/mol in the range of shear rates studied. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1621–1628, 2000