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 crystalli ne behavior of poly(p-phenylene-2,6-naphthylamide) and poly(p-phenylene-4,4 '-biphenylamide), two members of the aramide polymer family. These macromol ecules 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 th at as reported recently for other phenyl-based aramides, the nematic order of polymers in these lyotropic phases is essentially independent of tempera ture while slightly dependent on concentration. When considered in unison w ith 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 p henyl < biphenyl. Although the origin of this trend is not apparent when po lymers are considered in their preferred all-anti/ all-trans backbone confo rmations, its nature can be rationalized in terms of macromolecular semifle xibility arguments involving sync<-->anti rearrangements of consecutive ami de groups. These rearrangements impart worm-like displacements to otherwise rigid macromolecules that help understand the observed trend in order para meters, while simultaneously explaining the relative disorder exhibited by these aramide solutions in comparison with rigid rod theoretical prediction s.