All-aromatic, charged, rodlike polyamides possess high-aspect-ratios that enable the formation of a lyotropic, liquid crystalline (LC) phase. Poly(3,3’-dihydroxybenzidine terephthalamide) (DHTA) is a polyaramid that contains two weakly acidic phenoxy groups per repeat unit. In 1.5 M NaOH/H₂O, these groups are converted into charged phenoxide ions, which enable DHTA to exist as a water-soluble polyelectrolyte. Driven by excluded volume interactions (Onsager-type LC), DHTA polyelectrolytes form a LC phase. The LC phase behavior was investigated by varying the DHTA molecular weight and by introducing a nonlinear 1,3-substituted phenylene regioisomer to the DHTA backbone. The onset of the nematic phase was inversely related to the polymer M_n, occurring at 13.0 wt.% for M_n = 2.7 kg/mol and at 6.5 wt.% for M_n = 18 kg/mol in 1.5 M NaOH. In comparison to a non-aggregated molecular LC, poly(p-phenylene terephthalamide) (PPTA) (e.g., Kevlar^® or Twaron^®), these concentrations indicate supramolecular self-assemblies are formed. These self-assemblies were confirmed by the presence of high-aspect-ratio fibrillar structures from TEM imaging, which bear a strong resemblance to peptide nanofiber structures. Furthermore, in dilute concentrations, DHTA exhibits the Tyndall effect, indicating that DHTA exists as a colloid in 1.5 M NaOH. Incorporation of nonlinear moieties greater than 10 mol% into the DHTA backbone disrupts the formation of the LC phase, highlighting the importance of chain geometry for LC phase formation. Our results strongly suggest that weak polyelectrolytes based on rigid, rodlike polyamides form lyotropic, nematic LC phases due to their propensity to form supramolecular fibrillar structures.

Maruti Hegde

The University of North Carolina at Chapel Hill College of Arts and Sciences

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