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

Polarised transmission FTIR microscopy studies (mu -FTIR) have been perform ed 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 transitio n, prior to melting. However, the oligoamide forms extended chain, rather t han chain-folded, crystals, and so crystals are produced that are essential ly 100% crystalline, and of similar to mum-mm size. Consequently, this mate rial is ideally suited for polarised mu -FTIR single crystal studies. The t hermal 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 am ide I bands are retained right up to melting. Quantitative infrared dichroi sm measurements indicate that a maximum of 5 degrees rotation of the N-H bo nds about the extended chain axis occurs prior to melting. These results st rongly suggests that the equivalent Brill transition in nylon 6 6 also proc eeds without significant hydrogen bond rearrangement. In addition we have i nvestigated the behaviour of designated 'Brill', 'crystalline',,amorphous' and 'fold' bands that are present in our spectra. (C) 2001 Published by Els evier Science Ltd.