The role of hydrogen bonding in rigid-rod polymers: the crystal structure of a polybenzobisimidazole model compound

Microstructural information about the nature of the polymer-water interacti on in rigid-rod polybenzimidazole polymer fibers (PBI) is derived from the X-ray crystallographic analysis of 1,7-dihydro-2,6-diphenylbenzo-[1,2-d;4,5 -d']diimidazole tetrahydrate, C20H14N4.4H(2)O. MW = 382.20 amu, a model com pound for poly(p-phenylene-benzobisimidazole) (PBDI). The model compound cr ystallizes in a monoclinic crystal system, space group P2(1)/c, with n = 9. 008(2) Angstrom, b = 24.967(7) Angstrom, c = 9.870(5) Angstrom, beta = 119. 82(3)degrees, and Z = 4. Molecules pack in a herringbone fashion, intersper sed with a network of solvent water molecules. Hydrogen-bonded water molecu les bridge molecules of the model compound that are related by an inversion center. Each water molecule acts both as a hydrogen bond donor and as a hy drogen bond acceptor. A plausible model for the packing of chains in heat-t reated fibers elf PBDI would involve polymer chains extending in the direct ion of elongation, in the manner found for polybenzothiazoles (PBZT) and po lybenzoxazoles (PBO), but with a network of hydrogen-bonded water molecules providing strong lateral interactions between polymer molecules. A compari son is made with the structure of poly-{2,6-diimidazo[4,5-b:4',5'-e]pyridin ylene-1,4(2,5-dihydroxy) phenylene} (PIPD or 'M5' fiber) since the enhanced compressive behavior of PIPD is attributed to a hydrogen-bonding network b etween polymer chains. (C) 2000 Published by Elsevier Science Ltd.