Structure and properties of fully aromatic thermotropic liquid‐crystalline copolyesters containing m ‐hydroxybenzoic acid units

A series of fully aromatic copolyesters based on p ‐acetoxybenzoic acid ( p ‐ABA), hydroquinone diacetate (HQDA), terephthalic acid (TPA), and m ‐acetoxybenzoic acid ( m ‐ABA) were prepared by a modified melt‐polycondensation reaction. The copolyesters were prepared in two series in which series I is based on a two monomer system ( p ‐ABA/ m ‐ABA) and series II is based on a three monomer system (HQDA/TPA/ m ‐ABA). The effect of different linear structures and kink moiety on the properties of the copolyesters were investigated systematically. The copolyesters were characterized by differential scanning calorimetry, thermogravimetric analysis, 1 H‐NMR, polarized optical microscopy, X‐ray diffraction, and intrinsic viscosity measurements. It was found that the copolyesters exhibited nematic liquid‐crystalline phases when the content of linear ( p ‐ABA or HQDA/TPA) units was over 67 mol %. The melting or flow‐transition temperatures were decreased with an increase of the content of kink m ‐ABA units. The copolyesters that contained p ‐ABA as linear units had a significantly higher degree of crystallinity compared to the copolyesters that contain HQDA/TPA as the linear units, implying a higher blockiness in the polymer chains as a result of the higher reactivity of the p ‐ABA units in the p ‐ABA/ m ‐ABA system. All the copolyesters had poor solubility except the copolyesters containing at least 40 mol % of m ‐ABA, which were soluble in a dichloromethane/trifloroacetic acid (70:30 v/v) mixture. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3260–3269, 2003