TEMPO-Mediated Oxidation of Pullulan and Influence of Ionic Strength and Linear Charge Density on the Dimensions of the Obtained Polyelectrolyte Chains

Abstract

The applied pH during the TEMPO-mediated (2,2,6,6-tetramethylpiperidine-1-oxyl) oxidation of the polysaccharide pullulan has a substantial influence on the molar mass distribution of the products. The least degradation was found in the pH range 9.2−9.7 and especially at higher pH, significant degradation occurs due to β-elimination. Four samples were prepared at pH 9.4 with different degrees of oxidation and the influence of the charge parameter ξ on the rms radius of gyration at various ionic strengths (Cs) was determined with size exclusion chromatography (SEC) using a multiangle laser light scattering (MALLS) detection system. Experimental values were compared to values obtained with the wormlike chain model for polyelectrolytes in combination with excluded-volume theory. The ionic strength−linear charge density dependence of the rms radius of gyration is quite accurately described, whereas the prediction of its absolute value is more problematic for lack of a quantitative description of solvent quality. However, very reasonable values for the rms radii of gyration are obtained using eq 15, originally derived as a scaling relation. Furthermore, it is shown that there is no discrepancy between the apparent persistence length, proportional to ξ/Cs1/2, and the real persistence length, proportional to ξ2/Cs, when excluded-volume effects are accounted for.