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

Abstract

Microstructural information about the nature of the polymer–water interaction 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, C20H14N4·4H2O, MW=382.20amu, a model compound for poly(p-phenylene-benzobisimidazole) (PBDI). The model compound crystallizes in a monoclinic crystal system, space group P21/c, with a=9.008(2)Å,b=24.967(7)Å,c=9.870(5)Å,β=119.82(3)°, and Z=4. Molecules pack in a herringbone fashion, interspersed with a network of solvent water molecules. Hydrogen-bonded water molecules 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 hydrogen bond acceptor. A plausible model for the packing of chains in heat-treated fibers of PBDI would involve polymer chains extending in the direction of elongation, in the manner found for polybenzothiazoles (PBZT) and polybenzoxazoles (PBO), but with a network of hydrogen-bonded water molecules providing strong lateral interactions between polymer molecules. A comparison is made with the structure of poly-{2,6-diimidazo[4,5-b:4′,5′-e]pyridinylene-1,4(2,5-dihydroxy) phenylene} (PIPD or ‘M5’ fiber) since the enhanced compressive behavior of PIPD is attributed to a hydrogen-bonding network between polymer chains.