Some equilibrium and kinetic aspects of water sorption in poly(ortho ester) s

Abstract

Poly(ortho ester)s are a class of erodible polymers suitable for the construction of drug delivery systems. Since the release of bioactive substances from these polymers depends on the hydrolytic cleavage of the backbone, the isothermal sorption and transport of water in various poly(ortho ester)s have been investigated. At 25°C and relative vapor pressure equal to unity, the amount of water sorbed at saturation ranges from 0.3% to 0.75% w/w. Application of a modified Henry's law to the data yields reasonable correlation. The diffusion coefficient of water in poly(ortho ester)s appears to be independent of film thickness, molecular weight and water activity. At experimental temperatures above the glass transition temperature of the polymers, a marked increase in water diffusivity was observed. The activation energies of the transport process are within the range usually found for synthetic polymers. These equilibrium and kinetic studies suggest that membranes made of copolymers tend to have a more open structure than single diol poly(ortho ester)s.