BENDING OF POLYAMIDE RIGID RODLIKE SEGMENTS

Linear rigid rodlike polyamide macromolecules tend to orient parallel in the bulk and concentrated solution. They then respond to applied stress as an ensemble and not individually. Conversely, when isotropic gels of rigid rodlike polyamide networks are deformed, then the rodlike segments between the stiff branchpoints respond individually and bend in order to relieve the applied stress. The deformations may be external in origin, as in the case of an applied mechanical stress, or internal in origin, as in the case of network collapse due to loss of solvent from the gel. Activation energy data from the literature indicate that the preferred mechanism for chain bending in the para-aromatic polyamides is by an interconversion of anti and syn configurations, facilitated by 180° rotation around the ring-to-carbonyl bond. Each such interconversion contributes about 20 deg to the change in chain direction. Solid-state 13C CP/ MAS NMR spectra of the strained polyamide networks show an extra resonance downfield from the usual position of the amide carbonyl in the unstrained state. Similarly, IR spectra of strained networks show amide I peaks at frequencies higher than for the unstrained networks. These extra peaks were shown by NMR and IR not to be associated with the branchpoint residues or the nitro substitution on some of the aromatic rings. No extra peaks or shifts in peak position were observed in the spectra of the linear rigid rodlike polyamide analogues. Data generated by us and collected from the NMR and IR literature about model compounds show shifts in peak positions with strain in the same direction and of similar magnitude to the ones observed by us in the strained rigid polyamide networks. © 1990, American Chemical Society. All rights reserved.