A synthetic strategy towards sulfonated all-aromatic polyamides and poly(benzoxazole-co-amides)

Herein, we describe the synthesis and properties of a liquid crystal all-aromatic polyamide with tunable sulfonated content that can be converted into its poly(benzoxazole-co-amide) analog using a simple thermal treatment step. The parent all-aromatic polyamide, poly(3,3′-dihydroxybenzidine terephthalamide) (DHTA), is modified by incorporating a sulfonated comonomer, 2,5-diaminobenzenesulfonic acid (DABS) (i.e., 5, 10, 25, and 50 mol% DABS), to yield sulfonated, random copolyamides (DHTA-y-DABS). The ortho-positioning of the hydroxyl group relative to the amide bond allows DHTA-y-DABS to undergo a cyclodehydration reaction from 200 to 450 °C, forming a sulfonated poly(benzoxazole-co-amide) while preserving the sulfonate functionalities. Swelling experiments show that the DHTA-y-DABS-Na+ films exhibit water uptake values ranging from 9 to 41 wt%. PBO-y-DABS-Na+ films swell between 4 and 15 wt% in water. Dry films exhibit excellent mechanical properties. Young's moduli are between 8 and 10 GPa, yield strength values range from 117 to 215 MPa and elongation at break values range from 3 to 10%.