The elastic modulus of poly(ethylene terephthalate-co-p-oxybenzoate)

Abstract

The experimental approach to improve the mechanical properties of polymers proceeds essentially along two different lines: the modification of the processing of conventional polymers in order to obtain extended chain structures with better orientation, and the design of stiff, "rod-like" molecules exhibiting nematic liquid crystalline behaviour in the melt, which can be easily oriented in the solid state. High orientation and excellent mechanical properties can be achieved by solid-state or solution processing of flexible polymers which do not show mesophases [1-3]. More recently, thermotropic liquid crystalline polymers have attracted attention as a consequence of their good mechanical properties and easy processability [4, 5]. In fact the orientation of the liquid crystalline domains, which leads to a superior degree of orientation of the macromolecules along the fibre axis has been related to the possibility of producing materials with excellent levels of mechanical anisotropy. Different types of thermotropic nematogenics have been investigated [6]. Among these, the class of polymers which has received the highest attention is the copolyester, produced by Tennessee Eastman Company, initially synthesized by Jackson and Kuhfuss in 1976 [7]. The polymer is a polyethylene terephthalate modified by p-hydroxybenzoic acid (PET/PHB 60). Several papers have been published on the rheological [4] and on structural characterization [8, 9] of this new material. Most effort has been concentrated on the evaluation of the mechanical properties with the purpose to relate them to the processing of the polymer [4, 5]. Acierno et al. [4] found that the higher elastic moduli were reached when the fibres were spun at a lower temperature. They inferred that an important contribution to the high modulus would arise from crystallites of the hydroxybenzoic acid sequences. For the material extruded at a higher temperature (260 ° C) they measured a relatively high tensile modulus (about 20 GPa) which increases slightly with the draw ratio. More recently Tealdi et al. [10] reported higher elastic moduli for the polymer processed at higher temperatures than those found by Acierno et al. However, no indication of the exact value of the orientation of the macromolecules is provided in either paper. Sugiyama et al. [11] calculated the function of orientation of Hermans ( f ) using wide-angle X-ray patterns for fibres processed in different conditions of temperature and drawing. They found thatfdecreases with temperature and shear rate, but is practically independent of the spinline drawdown ratio. They inferred that almost all the orientation appears to be developed in the capillary. Muramatsu and Krigbaum [5] related the orientation, measured by X-ray, with the rheological behaviour and mechanical properties of the fibres. However, they do not provide any form of the function of orientation. In this letter we refer to the mechanical properties of fibres of PET/PHB 60 spun at 260 °C. The high modulus found is related to the high level of orientation, obtained from the large values of the order parameters, calculated from the X-ray diffraction patterns. Spinning of the nematic phase was performed using a Ceast rheometer. The piston-type extruder was operated at constant velocity, with a die having a 1 mm diameter. Fibres were spun at a single temperature, 260 ° C, using a capillary with LID = 10 and varying the span draw ratio. The extrusion rate, V0, was 45 cm min and the output flow rate, Q, was 0.35cm3min ~. About 5g polymer was used for each extrusion. In order to avoid moisture, the batches were previously dried overnight in a vacuum oven at 100 ° C. The spun fibres were collected with a take-up machine placed at a distance 0.3 m from the bottom of the capillary. The Vf/Vo ratios take-up velocity, Vr, was varied to obtain between 10 and 300. The true Vr/V0 ratios were evaluated as cross-sectional area variation and determined by measuring the decrease in the diameter of the fibres, using a microscope. Wide-angle X-ray diffraction patterns were recorded with a Siemens diffractometer using a flat film camera with CuKe radiation. Because of the small dimensions of the fibres, the exposure time ranged between 8 and 24h. Samples were fractured in liquid nitrogen and the fracture surface examined using a Cambridge Stereoscan 100 scanning electron microscope. The real component of the complex modulus, E', was determined using a Dynamic-mechanical spectrometer Dynastat system. One of the most important features of the Dynastat is the rise time control circuit that allows step-functions in load or displacement to be imposed on the sample without overshoot, but with rise time of an underdumped servo. Dynamic mechanical experiments were performed at room temperature and at a frequency of 1 Hz.