CRYSTAL-STRUCTURE OF POLY(ETHYLENIMINE)-ACETIC ACID COMPLEX

Linear poly(ethylenimine) was found to form a highly crystalline complex with acetic acid with a molar ratio of 1:1, (CH2CH2NH·CH3COOH)n, when the polymer, irrespective of whether it was in the state of its anhydrate or hydrates, was exposed to acetic acid vapour. The crystal structure of the complex was determined by X-ray diffraction. The crystal system is monoclinic with cell constants a = 5.76 A ̊* * 1 A ̊ = 10-1 nm, b = 6.20 A ̊, c(chain axis) = 7.39 A ̊ and γ = 95° (c-unique), and the space group is P21/m. The unit cell contains two monomeric units (one chain) and two acetic acid molecules. The polymer chains are essentially planar zigzag, the fibre repeat comprising two monomeric units. Planar acetic acid molecules are stacked horizontally along the polymer chain axis at the levels of the two polymer nitrogen atoms. Hydrogen bonds of the types NH ... O and OH ... N link the polymer chain and the acetic acid molecule alternately to form an infinite chain structure at the two levels; the scheme of the hydrogen bondings is similar to that in acetic acid crystals. The melting temperature of the complex is ≈ 135°C, which is higher than those of the polymer hydrates by about 55°C. © 1990.