Sedimentation analysis of styrene–butadiene copolymer rubber

Abstract

AbstractMethyl n‐propyl ketone (MNPK) at 21°C. and methyl isobutyl ketone (MIBK) at 46°C. are found to be theta conditions for a styrene–butadiene copolymer rubber commercially designated as SBR‐1500. Data for specific refractive index increment and density indicate that these solvents are appropriate for ultracentrifugation of this polymer by the method of schlieren optics. Sedimentation measurements are made on one whole polymer and six fractions in MNPK at 21°C. It is demonstrated that in this theta solvent the distribution of sedimentation coefficient s (at the limit of zero concentration) of a given SBR‐1500 can be determined, with a good accuracy, from a single sedimentation run at one nonzero concentration below 0.4 g./dl. A relation between s and molecular weight M necessary to convert this distribution of s to the distribution of M is derived by making use of the theory of Flory which correlates s limiting viscosity number, and M. It is believed that the theta conditions and the procedure of analysis presented in this paper are readily utilized for heterogeneity studies on this particular type of synthetic rubber SBR‐1500.