A polarised μ-FTIR study on a model system for nylon 6 6: implications for the nylon Brill structure

Polarised transmission FTIR microscopy studies (μ-FTIR) have been performed on a monodisperse 3-amide oligomer. The oligomer is a model compound for nylon 6 6; it has essentially the same room temperature crystal structure, and it undergoes the same high temperature transition, the Brill transition, prior to melting. However, the oligoamide forms extended chain, rather than chain-folded, crystals, and so crystals are produced that are essentially 100% crystalline, and of ∼μm–mm size. Consequently, this material is ideally suited for polarised μ-FTIR single crystal studies. The thermal polarised FTIR behaviour of this material provides definitive proof that the Brill transition does not involve major rearrangement of hydrogen bonds, since the strong parallel polarisation of both the NH stretch and amide I bands are retained right up to melting. Quantitative infrared dichroism measurements indicate that a maximum of 5° rotation of the N–H bonds about the extended chain axis occurs prior to melting. These results strongly suggests that the equivalent Brill transition in nylon 6 6 also proceeds without significant hydrogen bond rearrangement. In addition we have investigated the behaviour of designated ‘Brill’, ‘crystalline’, ‘amorphous’ and ‘fold’ bands that are present in our spectra.