The influence of monomer structures on the liquid crystalline order of aramide polymers: An NMR analysis

Natural abundance NMR methods were employed to analyze the liquid crystalline behavior of poly(p-phenylene-2,6-naphthylamide) and poly(p-phenylene-4,4'-biphenylamide), two members of the aramide polymer family. These macromolecules were dissolved in absolute sulfuric acid and the extent of order in their liquid crystal phases was evaluated with the aid of solid phase C-13 tensor data and by total simulations of their lyotropic NMR line shapes as a function of temperature and concentration. These measurements revealed that as reported recently for other phenyl-based aramides, the nematic order of polymers in these lyotropic phases is essentially independent of temperature while slightly dependent on concentration. When considered in unison with these previous C-13 NMR analyzes this study also suggests that nematic order in aramides can be controlled by the choice of the monomeric chemical structures, increasing along the series naphthyl less than or similar to phenyl < biphenyl. Although the origin of this trend is not apparent when polymers are considered in their preferred all-anti/ all-trans backbone conformations, its nature can be rationalized in terms of macromolecular semiflexibility arguments involving sync<-->anti rearrangements of consecutive amide groups. These rearrangements impart worm-like displacements to otherwise rigid macromolecules that help understand the observed trend in order parameters, while simultaneously explaining the relative disorder exhibited by these aramide solutions in comparison with rigid rod theoretical predictions.