Crystal Structure of Polyvinyl Alcohol

Abstract

IT has often been said (by me¹ as well as by others) that the molecular repeat distance (from X-ray diffraction photographs) of polyvinyl alcohol—2·52 A.²—indicates not only that the chain has a simple plane zigzag configuration, but also that all the hydroxyl groups (in the section of a molecule in one crystallite) lie on the same side of the zigzag plane. This is not necessarily true ; so far as X-ray diffraction is concerned, the observed repeat distance is equally compatible with a molecular structure in which hydroxyl groups are randomly placed in left-and right-hand positions. This possibility was not considered until recently ; it was assumed that the high degree of crystallinity of polyvinyl alcohol implied regularity of molecular structure. Recently, however, it has been found³ that interpolymers of ethylene and vinyl alcohol are crystalline ; thus hydroxyl groups may replace hydrogen atoms at random on a carbon chain without destroying crystallinity ; the high crystallinity of polyvinyl alcohol cannot, therefore, be regarded as evidence against the idea of stereochemical irregularity in this molecule.