Fracture behavior of heat treated liquid crystalline polymers
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Fracture behavior of heat treated liquid crystalline polymers A Reyes-Mayer 1, B Alvarado-Tenorio 1, A Romo-Uribe1*, O Flores1, B Campillo1 and M Jaffe 2 1 Laboratorio de Nanopolimeros y Coloides, Instituto de Ciencias Fisicas, Universidad Nacional Autonoma de Mexico, Cuernavaca, Mor. 62210, MEXICO. 2 New Jersey Institute of Technology, Newark NJ, U.S.A. * To whom correspondence should be addressed: [email protected] ABSTRACT Thermotropic polymers are thermally treated in air at temperatures Ta, where T =Ta Tsn=40°C, and Tsn is the solid-to-nematic transition. Samples are extruded thin films of a series of thermotropic random copolyesters termed B-N, COTBP and RD1000. The thermal treatment produces a second endotherm without changing Tsn for B-N and RD1000. However, for COTBP Tsn is significantly increased. Regardless of the complex thermal behavior exhibited by the thermotropes, the thermal treatment produces a significant increase in Young's modulus, more than 30% for B-N and over 100% for COTBP. The increase in mechanical modulus is correlated with a thermally-induced fiber-like morphology. INTRODUCTION Thermotropic polymers exhibit high mechanical properties and excellent chemical and thermal resistance, making them suitable for high performance applications in automotive, electronic and aerospace industries [1-2]. Many thermotropic liquid crystalline polymers (LCPs) have been synthesized, but their mechanical properties are often poor at elevated temperatures. For example, copolymers of HBA and HNA show a 75% drop in tensile modulus on heating from room temperature to 150°C [3]. Several groups have investigated dynamic mechanical and dielectric properties of LCPs. The influence of monomer concentration, type of monomers, and the effect of temperature treatment on the mechanical relaxations of thermotropic LCPs have also been studied [4-6]. Moreover, the influence of degree of molecular orientation on mechanical properties has also been investigated [5]. The tensile properties of thermotropic LCPs can be improved by heat treatment. The optimal annealing temperature has been reported to be between 10 and 30°C below the melting point [7]. In a previous report on a series of thermotropic random copolyesters we have shown that thermal treatment indeed produces an increase in tensile modulus [8]. In this work we further investigate the influence of thermal treatment on microstructure using small-angle light scattering (SALS), X-ray scattering and scanning electron microscopy (SEM). EXPERIMENTAL PROCEDURE Materials. Extruded thin films of three thermotropic liquid crystalline copolyesters have been kindly supplied by the former Hoechst Celanese Research Corp. (Summit, NJ, USA). These are termed B-N, COTBP and RD1000. B-N is a copolyester based on random units of ~73 mol % B and ~27 mol % N. The chemical formula is shown in Figure 1a. This material is marketed by Ticona under the tradename of Vectra A®. COTBP (Figure 1b) is a random copolyester made
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of 66 mol% 1,4-hydroxybenzoic acid (B), 5 mol%2,6-hy
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