The effect of riboflavin-photoinduced degradation of alginate matrices on the diffusion of poly(oxyethylene) probes in the polymer network

The effect of irradiation, in the wavelength range of 310–800 nm, on the tracer diffusion of poly(oxyethylene) (POE) of different molecular weights embedded in various alginate matrices with the photosensitizer riboflavin (RF) present, has been investigated with the aid of pulsed field gradient spin-echo NMR (PGSE-NMR). Both alginate solutions of different polymer concentration were studied as well as corresponding acid gels of alginate produced by introduction of different amount of glucono-δ-lactone (GDL). In 2 wt.% alginate solutions, the values of the tracer diffusion coefficient suggest a strong obstruction effect as the probe molecular weight increases. Faster probe diffusion was observed for the irradiated samples, which indicates a photochemical scission of the polymer chains and the formation of a fragmented polymer network that facilitates the migration of the tracer chains. A semidilute alginate/RF solution was transformed into a gel by adding sufficient amount of GDL. GDL lowers the pH of the solution under the pKa of alginate, favouring intermolecular associations and the evolution of a less homogeneous network with more open structure. Therefore, the POE chains were shown to diffuse faster in the acid gel matrix than in the corresponding more homogeneous alginate solutions. The photochemically induced cleavage again promoted faster migration of the probe chains in the irradiated samples. The probe diffusion of the eight-arm star-shaped POE sample revealed an augmented obstruction effect with increasing alginate concentration and higher values of the diffusion coefficient were found in gels. The evolution of a tighter network inhibits the diffusion of the probe molecules. At lower alginate concentrations the values of the tracer diffusion coefficients are higher for the irradiated samples than for the non-exposed systems.