TEMPO-mediated oxidation of pullulan and influence of ionic strength and linear charge density on the dimensions of the obtained polyelectrolyte chains

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 beta-elimination. Four samples were prepared at pH 9.4 with different degrees of oxidation and the influence of the charge parameter xi on the rms radius of gyration at various ionic strengths (C-s) 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 xi/C-s(1/2), and the real persistence length, proportional to xi(2)/C-s, when excluded-volume effects are accounted for.