Solution and bulk flow properties of linear and branched styrene–butadiene rubbers

The solution and bulk flow properties of various styrene‐butadiene rubbers have been investigated. Bulk flow properties were measured with the Instron capillary rheometer, while the Weissenberg rheogoniometer was used to measure to steady‐state and oscillatory flow properties of 15% wt solutions in decalin. A comparison of the rheological information obtained from the solution and bulk behavior with GPC and intrinsic viscosity data makes it possible to distinguish between different forms of branching and between the effects of molecular weight distribution and branching. Branching decreases the hydrodynamic volume of a polymer coil relative to its molecular weight, so that branched polymers have lower solution viscosities at the same molecular weight than linear polymers. This effect is more pronounced for short‐chain and dendritic forms of branching than for star‐shaped long‐chain branching. At higher concentrations and especially for bulk polymers, however, star‐shaped long‐chain branching will give rise to much higher viscosities, which is hardly, if at all, found for short‐chain or dendritic forms of branching. Star‐shaped branching greatly affects the shear sensitivity of the bulk viscosity, whereas it has relatively little effect on polymer fluid elasticity either in bulk or in solution, this elasticity being mostly determined by the width of the molecular weight distribution Mz/Mw.