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The Influence of Thermal Annealing on Microstructure and Mechanical Properties in High Performance Liquid Crystal Copolyesters Adriana Reyes-Mayer 1,2, Amaury Constant 3, Angel Romo-Uribe1,* and Michael Jaffe 4 1 Laboratorio de Nanopolimeros y Coloides, Instituto de Ciencias Fisicas, Universidad Nacional Autonoma de Mexico, Cuernavaca, Mor. 62210, MEXICO. 2 Facultad de Ciencias, Universidad Autonoma del Estado de México, Instituto Literario #100, Toluca, Edo de Mexico, C.P. 50000, MEXICO. 3 Faculte des Sciences d’Orsay, Universite Paris Sud XI, Orsay, Cedex, FRANCE. 4 New Jersey Institute of Technology, Newark NJ, U.S.A. * Contact author: [email protected] ABSTRACT In this research we have focused on the influence of thermal treatment for periods of time on the thermal and mechanical properties of extruded films of a series of high-performance thermotropic liquid crystal polymers (LCPs). The dependence of microstructure, thermal and mechanical properties on the extent of thermal treatment is investigated. Especially synthesized wholly aromatic LCPs based on hydroxybenzoic acid (B), hydroxynaphthoic acid (N), terephthalic acid (TA) and biphenol (BP) are kindly supplied by Hoechst Celanese Research Corp in the form of 50 Pm thick extruded films. Thus, the influence of monomer composition is also studied in order to contrast the influence of molecular conformation. Thermal treatments are carried out at temperatures close to the solid-to-nematic transition (Ts→n) for up to several hours under dry air conditions. The results show a profound influence of thermal annealing on morphology and mechanical modulus when annealing is carried out c.a. 40ºC below Ts→n, where solid-to-nematic transition and Young’s modulus are significantly increased. INTRODUCTION Thermotropic liquid crystalline polymers (LCPs) have received considerable attention since the first report by scientists at Eastman Co. [1]. Since then the most successful LCPs, from the commercial point of view, are those developed by the Celanese Co. and commercialized by Ticona. The commercial LCPs display remarkable physical properties including tolerance for high temperatures, resistance to chemicals and acids, and high tensile strength [2,3]. These properties make them ideal for applications into the electronics, medical, dental, automotive and aerospace industries [4,6]. Thermotropic LCPs are wholly aromatics, i.e., there are not flexible spacers in the backbone (for instance, methylene CH2 groups). Manipulating the orientation of the anisotropic macromolecules optimizes mechanical properties of LCP fibers [2]. Molecular alignment and microstructure are controlled through processing conditions during fiber spinning [7] and heat treatment [8,10]. At the molecular level the degree of straightness in the molecular architecture of the polymers studied here (figure 1) depends upon the amount of 2,6hydroxynaphthoic acid (N), which upsets the straight conformation of the chain. The double aromatic ring in the N monomer acts as a parallel offset thereby disturbing th