Thermal transition behaviors, solubility, and mechanical properties of wholly aromatic para-, meta-poly(ether-amide)s: effect on numbers of para-aryl ether linkages

In order to make clear how the numbers of para-aryl ether linkages affect the solubility, thermal transition behavior and mechanical properties of para-(p-PEAs), meta-poly(ether-amide)s (m-PEAs), a series of para-substituted aryl ether diamine monomers with 2, 3, 4 or 5 ether linkages were synthesized, and then reacted with terephthalic acid and isophthalic acid via the Yamazaki–Higashi phosphorylation method to prepare wholly aromatic PEAs. The PEAs with intrinsic viscosity values in the range of 0.81–1.16 dL g−1 were obtained and characterized by FT-IR, NMR, and elemental analysis. Although the Tgs and Tms of PEAs decrease with the increasing numbers of aryl ether linkages, their Tgs and Tms still are above 180 °C and 320 °C, respectively. p-PEAs have a high crystallinity making it difficult to dissolve them in common organic solvents even when 5 aryl ether linkages are introduced. Conversely, except for m-PEA5 with a lower polarity, the other m-PEAs show good solubility due to their low crystallinity. Besides, the decomposition temperatures at 5% weight loss of all PEAs exceed 430 °C and their residual yields at 700 °C are above 50% in N2, illustrating that they have excellent thermal stability. It is found that by casting m-PEAs DMAc solution in a dish, the transparent films with high Young's modulus (2.6–3.5 GPa) can be successfully obtained.