Studies on the sorption of moisture by polymers. I. Effect of crystallinity

Abstract

Abstract The relation between the sorption of moisture by polymers and their crystalline structure is reviewed, and it is concluded that, if a dry polymer can form well‐defined crystallites, then these crystallites will usually be inaccessible to water vapor. The ratio of the moisture sorption of a polymer to that of its completely amorphous form will thus be a measure of the noncrystalline fraction. Methods of estimating the sorption of the amorphous form of a polymer that is normally partially crystalline are then discussed. In one method, the polymer is prepared in various degrees of crystallinity, and the relation between sorption and crystallinity is extrapolated to zero crystallinity. A variant of this method involves the measurement of the accessibility to some simple reagent that reacts only in the noncrystalline regions, e.g. , D 2 O; the relation between sorption and accessibility is extrapolated to 100% accessibility. The second main technique is based on the fact that conversion of a partially crystalline homopolymer into a random copolymer frequently destroys the crystallinity over a wide range of compositions, within which there is a linear relation between sorption and composition. Extrapolation of this linear relation enables the sorption of the completely amorphous homopolymer to be found. These methods have been applied to several polymers, e.g. , cellulose, polyvinyl alcohol, wool. For cellulose, in particular, very good agreement is found between many different methods, and the sorption isotherm of amorphous cellulose has been calculated.