Phase behaviour and orientational order of a main-chain nematic polyester: a combined SANS and NMR study

A main-chain semiflexible copolyester, containing 4,4′-dihydroxy-α,α′-dimethylbenzalazine as a mesogenic unit was studied by SANS and solid state NMR. SANS scattering patterns from mixtures of labelled and unlabelled LCP indicated the presence of an additional coherent signal superimposed on the single chain scattering function. The scattering from the pure polymers (hydrogenous, partially deuterated and fully deuterated LCP) was investigated to assess the origin of this extra signal. The results suggest that the disclination structure associated with a domain structure, typical of liquid crystalline systems, may be observable by SANS. In addition NMR experiments excluded the possibility of the existence of a biphasic region over a wide temperature range. NMR spectra evidenced a pure nematic phase for measurements performed on heating above the solid to nematic phase transition. The orientational order of the mesogenic units was investigated by proton and deuterium NMR. The order parameters were found to be very high: at the nematic to solid transition Szz was estimated to be 0.965 and it was approaching 0.67 at the nematic to isotropic phase transition.