Site-specific reversible scission of hydrogen bonds in polymers: an investigation of polyamides and their Lewis acid-base complexes by infrared spectroscopy

Lewis acid--base complexation in conjunction with Fourier transform infrared spectroscopy was used to study the scission and re-formation of N--H...O=C hydrogen bonds in seven aliphatic polyamides and one aromatic polyamide. The characteristic shifts of the N--H stretch frequency, the amide I mode, and the amide II mode in the vibrational spectra of several polyamides on GaCl sub 3 complexation were examined at room temperature (25 deg C). Complete scission of the intermolecular H bonds in polyamides was achieved by the 1:1 Lewis acid complexation of the basic oxygen sites of the carbonyl groups in the polymers, thus "liberating" the N--H groups from H bonding. Facile decomplexation of the polyamide--Lewis acid complexes resulted in the reformation of the H bonds and, consequently, the characteristic solid-state structure and properties of the polyamides. Thus, the H bonds in polyamides function like a molecular zipper: Lewis acid--base complexation "unzips" and decomplexation "zips" the macromolecular chains. These results have important implications for the understanding of H bonding in polymers and its effects on structure and properties. The results also suggest a possible application of the approach of reversible H bond scission to ultradrawing of polymers with H bonding to highly oriented materials with high strength and high modulus. Spectra. 16 ref.--AA