Synthesis, characterization and solution behaviour of oxidized pullulan

Abstract

Various amounts of carboxyl groups were introduced at C-6 of the non-ionic, water soluble polysaccharide, i.e. pullulan, by applying the well-established TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl), sodium hypochlorite/sodium bromide oxidation protocol, varying the reaction time. The oxidized products, more water-soluble than pullulan, were further characterized by FT-IR, 1H NMR and 13C NMR techniques, in order to assess the degree of oxidation. The absolute molecular weight measurements performed using a multiangle laser light scattering molecular weight analyzer, reveals a sharp drop of the molecular weight of the samples oxidized for longer reaction times. The second virial coefficients (A2), increased from unoxidized pullulan to the oxidized samples. Dynamic light scattering (DLS) measurements provide zeta potentials and hydrodynamic radius for all studied samples. The viscosity of the initial and oxidized pullulan dilute aqueous solutions was studied in detail. All oxidized samples except the highest oxidized pullulan sample (OxPu8) showed strong polyelectrolyte behaviour, whereas this effect is less pronounced for OxPu8 due to the high degradation of the chains. The intrinsic viscosity and the interaction parameter were determined at 25°C as a function of solvent ionic strength according to Wolf approach. The dependence of these parameters on the salt concentration follows Boltzmann sigmoid model.