Infrared Study of the Hydrogen Bonding Association in Polyamides Plasticized by Benzenesulfonamides. Part I: Self-Association in Amide and Sulfonamide Systems; Part II: Amide—Sulfonamide Interaction

Abstract

The molecular associations of N-( n-hexyl)hexanamide (PA-12MC) and N-( n-butyl)benzenesulfonamide (BBSA), taken as model compounds for polydodecamide (PA-12) and benzenesulfonamide plasticizers, respectively, were examined by Fourier transform infrared spectroscopy. In solution, the amide is distributed between a self-associated form, involving intermolecular hydrogen bonding, and isolated species. Dissociation is favored in an electron donor solvent due to hydrogen bonding between the amide and the solvent. Comparatively, BBSA has much less tendency to dissociate. Molecular modeling suggests that BBSA dimer associations exist in the condensed state thanks to intermolecular hydrogen bonds, while gas-phase infrared spectroscopy supports a stabilizing intramolecular interaction between the sulfonamide proton and the sulfonyl lone pairs for isolated molecules. Mixtures of the amide model compound with BBSA show the creation of a strong S-N-H … O=C hydrogen bond between the sulfonamide proton and the amide's carbonyl lone pairs. The amide N-H groups liberated from the former amide-amide interaction find themselves involved in a weaker C-N-H … O=S hydrogen bond (“free N-H”) with the plasticizer's sulfonyl lone pairs, the concentration of these bonds being maximum at mid-composition. For polyamide/BSAs mixtures, the accessibility of the amide and sulfonamide groups can restrict these associations. Mixtures of AAPA, an aliphatic amorphous polyamide, with a plasticizer bearing a branched alkyl chain, generate a low free N-H concentration and cause phase separation to occur, which confirms the steric sensibility of their interaction. A bifunctional benzenesulfonamide plasticizer appears to be less efficient than BBSA and leads to an increased dispersion in hydrogen bond distribution, both of which could be ascribed to the bulk of this molecule. Incorporation of BBSA in semicrystalline PA-12 leads to a behavior identical to that of AAPA/BBSA mixtures.