Improved thermal, mechanical, and dielectric properties of thermotropic liquid crystalline poly(ester amide)s containing lactam-derived segmental units

We prepared two different series of thermotropic liquid crystalline poly(ester amide)s (TPEAs) via bulk polycondensation reactions of 4-hydroxybenzoic acid (HBA, 75.0 mol %) and 6-hydroxy-2-naphthoic acid (HNA, 25.0–17.5 mol %) with ε-caprolactam (CL, 0.0–7.5 mol %) or γ-butyrolactam (BL, 0.0–7.5 mol %). Our investigation aimed to explore the impact of lactam-derived aliphatic amide units’ length and content on the liquid crystalline feature, microstructure, thermal transition, thermal stability, mechanical/relaxational/rheological behavior, and dielectric characteristics of TPEAs. The molecular structure and fibrillar formation of TPEAs containing aliphatic amide units were confirmed through FT-IR and SEM analysis. With increasing CL or BL-derived units from 1.0 mol % to 5.0 mol %, we observed an increase in crystallinity, glass transition temperature, melting temperature, complex melt viscosity, and storage modulus of TPEAs, attributed to the enhanced intermolecular hydrogen bonding interactions. However, at 7.5 mol % CL or BL-derived unit content, these properties decreased due to increased chain irregularity as well as lowered crystallinity. Notably, the dielectric constants (3.31–3.04 at 2 MHz) of TPEAs decreased with the increase of lactam-derived aliphatic amide units owing to the restricted chain mobility by intermolecular hydrogen bonding interactions. We believe that the TPEAs synthesized in this study hold significant potential for applications in technical plastics, superfibers, and printed circuit board materials.