Effect of OH Segmental Length on the Iodine Complex Formation of Ethylene–Vinyl Alcohol Random Copolymers

Abstract

Poly(vinyl alcohol) (PVA) is known to form the crystalline complexes with iodine ions, in which the linear I3– ions form the columnar arrays and are connected to the neighboring PVA chains of planar-zigzag conformation through the charge transfer between OH groups and I3– ions. In order to clarify the effective OH segmental length necessary for the formation of such an iodine complex along the chain axis, the complex formation behavior has been investigated systematically for a series of ethylene–vinyl alcohol (EVOH) random copolymers on the basis of the experimental data of 2D X-ray diffraction, visible-ultraviolet (UV–Vis) absorption spectra, Raman spectra, and infrared spectra. By comparing the observed X-ray diffraction patterns, the EVOH copolymers with VOH content near 100 mol % were found to take the relatively regular complex II structure, while the EVOH copolymers of 70–50 VOH mol % were found to take the structure of complex I, in which the PVA zigzag chains are packed together randomly with and without coupling with iodine ions. In this way, an increase of VOH monomer content in the copolymer generates the longer VOH unit sequences, which can couple with I3– ions to form the crystalline complex with higher regularity. The infrared bands of OH and CO stretching modes were found to shift by iodine complexation, indicating that the OH···O hydrogen bonds between the neighboring PVA chains are broken in the complex formation process, and the interactions between I3– ions and OH groups, which may be assumed as a kind of intermolecular hydrogen bonds, become stronger with an increase of VOH content in the EVOH copolymers as well as the increase in the concentration of iodine solution.