Streaming birefringence behavior of dextrans produced by leuconostoc mesenteroides

The results of a large number of streaming birefringence studies on dextran fractions, both native and degraded, produced by Leuconostoc mesenteroides are reported. The following solvents have been examined, all in admixture with varying proportions of glycerol (to enhance the solvent viscosity): water (with and without prior autoclaving), aqueous sodium hydroxide, formamide, and ethylenediamine. The results are surprisingly independent of the solvent used. Ethylenediamine–glycerol (equal parts by weight) was adopted for further investigation because of the high solvent viscosity. This solvent was found to yield a small birefringence with extinction angle near zero degrees, which, however, contributed no appreciable error except at polymer concentrations below 0.3%. Results were found to be substantially independent of polymer concentration and of temperature. Characteristically, the dependence of extinction angle on gradient was found to be much less than is to be expected for solutions of homogeneous, rigid molecules. It is concluded that this cannot be due to heterogeneity, at least in the case of certain well‐fractionated preparation, and must be attributed to flexibility of the polymer molecule. It is shown that extrapolation of the apparent rotary diffusion constants to zero gradient can be carried out in most cases with some confidence. The resulting values, which should characterize the rotary diffusion properties of the undistorted polymers, are shown to be quite reasonable in terms of the known intrinsic viscosity and molecular weight of the fractions. The reduced birefringence of the various fractions is shown to be rather rigorously proportional to their intrinsic viscosity. Further, the extinction angles are found to approach a lower limit, at high gradient, which is related also to the intrinsic viscosity through the empirical equation X lim = 24/[η] 1/4 .