Liquid crystalline poly(ester imide)s (LCPEIs) were synthesized by solution polymerization from 4-hydroxybenzoic acid (4-HBA), 6-hydroxy-2-naphthoic acid (HNA) and N-(3-carboxyphenyl)-4-hydroxyphthalimide (3-CHP), with the capping groups of benzocyclobutene (BCB)-containing compounds (BCB-HP for phenolic hydroxyl group and BCB-CP for aromatic carboxylic acid). Subsequent cross-linking of the BCB capping groups upon hot pressing afforded the cured LCPEI films. Optimal properties of these films were achieved by adjusting the capping BCB-HP/BCB-CP contents.These LCPEIs showed favorable thermal properties with a relatively high glass transition temperature (Tg, 137–167 °C) and low melting temperature (Tm, 186–194 °C). With the increase in BCB capping content, the tensile modulus, tensile strength, and coefficient of thermal expansion (CTE) exhibited a non-linear tendency of first decreasing and then increasing. LCPEI-3.0 (3 mol% BCB) showed optimal performance: a relatively low CTE (20 × 10−6 K−1), a relatively high storage modulus (2.55 GPa), a moderate tensile modulus (2.65 GPa), a relatively low dielectric constant (Dk = 3.17) with low dielectric loss (Df = 0.0034) at 10 GHz, and excellent hydrophobicity (water contact angle = 133°). This improvement embodies an effective strategy to combine advantages of polyester, polyimide, and benzocyclobutene to achieve favorable and excellent comprehensive properties for convenient processability and practical application prospects.
Pan et al. (Sat,) studied this question.