749 Research Article Received: 29 May 2009 Revised: 7 September 2009 Accepted: 8 September 2009 Published online in Wiley Interscience: 11 February 2010 (www.interscience.wiley.com) DOI 10.1002/pi.2779 Thermal and morphological properties of thermotropic liquid-crystalline copolyesters containing poly(ethylene terephthalate), 4-hydroxyphenylacetic acid and main-chain rigid aromatic units Volkan Babacan, a Serpil Aksoy, a Zekeriya Yerlikaya b* and Haydar Altinok c Abstract Thermotropic liquid-crystalline polymers (TLCPs) have aroused considerable interest due to their attractive properties as high- performance materials. Significant research attention has been devoted to investigating the relationship among monomer structures, syntheses and end-use properties of TLCPs. The study reported here concerns the preparation, characterization and melt spinning of novel copolyesters containing two different flexible units together with two different aromatic units in the polymer chains. A range of copolyesters based on p-hydroxybenzoic acid (p-HBA), m-hydroxybenzoic acid, p-hydroxyphenylacetic acid and poly(ethylene terephthalate) were synthesized. The liquid crystallinity, thermal properties and degrees of crystallinity of these copolyesters were investigated using hot-stage polarized light microscopy, differential scanning calorimetry, thermogravimetry and wide-angle X-ray diffraction. Copolyester fibres were characterized using scanning electron microscopy. The copolyesters were melt-processable, thermally stable and could be processed above their melting temperatures without degradation. The degree of crystal structure was found to depend upon the content of p-HBA. The fibres prepared showed that polymer chains had a well-developed fibrillar structure. Novel TLCPs containing flexible units in the main chain were synthesized and characterized. Copolyesters containing p-HBA units ranging from 55 to 70 mol% exhibited phase-separated liquid-crystalline morphology, appropriate melting temperatures and high thermal stability for melt processing. c  2010 Society of Chemical Industry Keywords: liquid-crystalline polymers; PET; thermal properties; degree of crystallinity; fibre INTRODUCTION Thermotropic liquid-crystalline polymers (TLCPs) have aroused considerable interest due to their attractive properties as high- performance engineering materials. In the last three decades, significant research attention has been devoted to investigating the relationship among monomer structures, syntheses and end- use properties of TLCPs. 1–11 The anisotropic nature of these systems leads to a high degree of molecular orientation during processing in the melted state. The ordered phase can be achieved by different routes, depending on whether the polymer chains are rigid or flexible. The ability to form anisotropic and ordered phases is exploited, and because of the long relaxation time anisotropy is maintained during solidification. For flexible semi-rigid chains the ordered phase is mechanically induced with some assistance from the flow field. 7,9,12 – 27 Liquid-crystalline behaviour and transition temperatures de- pend on the copolymerization of ester groups, use of rod-like arylene moieties of different length, use of kink units and in- corporation of flexible aliphatic spacers. Systems based on a rigid rod–flexible spacer concept have the virtue of yielding thermally processable polymers that can be manipulated by the nature and mole fractions of the flexible spacers. For this purpose, a wide range of polymers containing rigid rods and flexible spacers of different lengths, containing one or more –CH 2 –aliphatic groups in the backbone have been prepared and characterized. 15,16,26 – 29 However, no systematic investiga- tion of the thermal and liquid-crystalline properties has been hitherto reported in the open literature on copolyesters contain- ing m-hydroxybenzoic acid (m-HBA) and p-hydroxyphenylacetic acid (p-HPAA) monomers in equal molar ratios, p-hydroxybenzoic acid (p-HBA) and poly(ethylene terephthalate) (PET) of different compositions. In one of our previous studies, we prepared by a modified melt polycondensation reaction 14 a series of totally aromatic ∗ Correspondence to: Zekeriya Yerlikaya, Department of Science Education, Faculty of Education, Kastamonu University, 37200 Kastamonu, Turkey. E-mail: y.zekeriya@gmail.com a Department of Chemistry, Gazi University, Teknikokullar, 06500 Ankara, Turkey b Department of Science Education, Faculty of Education, Kastamonu University, 37200 Kastamonu, Turkey c Kırıkkale University, Department of Chemistry, 71450 Yahs ¸ihan Kirikkale, Turkey Polym Int 2010; 59: 749–755 www.soci.org c  2010 Society of Chemical Industry