Effects of Para‐ or Meta‐Aromatic Amide Units on the Microstructure, Thermal, Mechanical, Rheological, and Dielectric Properties of Thermotropic Liquid Crystalline Poly(ester amide)s

Abstract

The impact of para‐ and meta‐aromatic amide repeat units on the microstructure, thermal behavior, mechanical properties, rheological characteristics, and dielectric properties of thermotropic liquid crystalline poly(ester amide)s (TPEAs) is reported. For this purpose, two distinct series of TPEAs are synthesized using 75.0 mol% of 4‐hydroxybenzoic acid (HBA) and 25.0–17.5 mol% of 6‐hydroxy‐2‐naphtoic acid (HNA) with 0.0–7.5 mol% terephthalic acid/para‐acetylaminophenol (TPA/PAP) or isophthalic acid/para‐acetylaminophenol (IPA/PAP) are synthesized via bulk polycondensation reaction. Infrared and electron microscopic data reveal the existence of intermolecular hydrogen bonds and associated fibrillar structures of TPEAs with TPA/PAP‐ or IPA/PAP‐based aromatic amide units. Accordingly, the glass transition temperatures, dynamic storage moduli, and melt viscosities of TPEAs increased with the increment of the aromatic amide units, although they are higher for TPEAs with IPA/PAP‐derived units, compared to TPEAs with TPA/PAP‐derived units. The melting temperatures of TPEAs increase with the para‐aromatic amide unit owing to the intermolecular hydrogen bonding and the rigid linearity, whereas the melting points decrease with the meta‐aromatic amide unit due to the structural nonlinearity. Interestingly, the dielectric constants and losses of TPEAs decrease with the aromatic amide unit content, which is even dominant for TPEAs with IPA/PAP‐based repeat units. TPEAs can be thus utilized as super‐engineering plastics, industrial fibers, printed circuit board materials, etc., requiring enhanced mechanical and thermal transition properties but lower dielectric constants and losses.