A physico-chemical study on poly(vinyl alcohol)/Poly(acrylic acid)/water in sol and gel phase

The effect of poly(acrylic acid) (PAA) and/or of its sodium salt (PANa) on the physico-chemical properties of both poly(vinyl alcohol) (PVA) aqueous solutions and on the corresponding hydrogels formed through the freeze/thaw method has been investigated. These gels could be suitable for delivery of small molecules in transdermal applications. Mutual diffusion, conductivity and density measurements have been used to characterize the sol phase, while small angle neutron scattering has been employed for sol and gel systems. Investigations performed have revealed that PAA/PVA polymers are well soluble in water, at least up to 20% wt of the total amount of solutes. Gel formation through freeze/thaw method has been observed for PAA/PVA systems only if the concentration of the former polymer is smaller than 4% wt. When the PAA amount is increased, a two phase system (a PVA-rich gel phase and a PAA-rich aqueous phase) is formed. Spontaneous gel formation has been observed at room temperature when the total concentration of PAA and PANa is ~2.5% wt and their ratio is 1.5. The study of diffusivity and conductivity of ternary liquid systems allows to have an insight of the driving force implied in the gelation process. SANS reveals that gels can be described, on a microscopic scale length, in terms of a network composed by the presence of small crystallites, highly connected by swollen amorphous tie chains. In conclusion, the presence of different domains in the gel phase as highlighted by SANS measurements, suggests to study these systems even in the presence of suitable hydrophobic domains, to have a multi-domain system useful for the releasing of both hydrophilic and hydrophobic molecules, with particular regard for transdermal applications