Detection of nematic-isotropic transition of a main-chain thermotropic liquid crystalline poly(aryl ether ketone) by dynamic rheological technique

Abstract

Thermotropic liquid crystalline polymers (TLCPs) have attracted much research attention in the past two decades due to the attracting properties, especially melt processing characteristics [1]. Most of the main-chain TLCPs synthesized up to date were polysters with flexible spacers [2–4]. The commonly used techniques in investigating phase behavior and to identifying the mesophase textures of LCPs include differential scanning calorimetry (DSC), wide angle X-ray diffraction (XRD) as well as polarized optical microscopic (POM) [5, 6]. The application of POM in TLCP characterization is sometimes found to be problematic because of the high viscosity of polymeric materials. Rheological response is very sensitive to morphological variation of polymers and in turn can be utilized to investigating phase transition of TLCPs [7, 8]. Rheological studies on lyotropic liquid crystalline polymers have afforded fruitful achievements [9, 10]. Comparatively dear rheological results from TLCPs were available. The reason lies in the fact that most TLCPs which have accessible mesophase-to-isotropic phase transition temperatures and mesophase temperature region broad enough that is suitable for rheological studies have not been synthesized in large quantity and at the same time most commercial available TLCPs have inaccessible mesophase-to-isotropic transition temperatures because they will decompose before reaching the clearing point. Poly(aryl ether ketone)s are a very important class of special engineering plastics which have been widely used in applications because of their excellent comprehensive properties [11]. The major drawbacks relative to processing are the high melting temperature and the high melt viscosity. Bennett and Farris first reported in 1994 thermotropic liquid crystalline poly(aryl ether ketone) copolymers based on 4,4′biphenol, bulk substituted hydroquinone and 1,4difluorobenzophene [12]. Afterwards several series of thermotropic liquid crystalline poly(aryl ether ketone) copolymers have been prepared [13, 14]. The realization of thermotropic liquid crystalline properties in poly(aryl ether ketone)s is showing promise in solving the above-mentioned problem in processing because they have been shown to have melt viscosity significantly lower that the isotropic polymers [15]. Here in the present work we describe a dynamic rheological detection of the mesophase to isotropic transition of a novel main-chain thermotropic liquid crystalline poly(aryl ether ketone) which was synthesized via an aromatic nucleophilic substitution reaction of 4,4′-biphenol, tert-butylhydroquinone with 1,4-bis(4-fluorobenzoyl)benzene. The details leading to the copolymer synthesis was described in one of our previous papers [16]. The copolymer containing 30 mol% biphenol moiety with a 19F NMR-based number-averaged molecular weight of 11 000 was employed in rehological measurements. The polymer has a nominal melting temperature of 335 ◦C and a nematic to isotropic phase transition temperature of 402 ◦C as determined by DSC. This copolymer is high temperature resistant. Thermogravimetric analysis demonstrated that it had a decomposition temperature above 500 ◦C at a 5% weight loss in nitrogen. This polymer is referred to as BB3 throughout this paper, where the number 3 stands for the 30 mol% tert-butylhydroquinone moiety in the copolymer. The ARES (Advanced Rheological Expansion System, Rheometrics, USA) was used for rheological measurements. The 25 mm-parallel platen geometry was adopted. The sample was subjected to dynamic oscillatory stressing (small-amplitude oscillatory shear, SAOS) in which the range of frequencies covered was from 0.1 to 100 rad/s. Before applying a frequency sweep, strain sweep at each fixed frequency was performed to determine the range of linear viscoelastic response (LVR) of the sample. The strain amplitude was varied from 2% to 20%, over which LVR holds. This strain amplitude also ensured to achieve sufficient levels of transducer torque during temperature increasing period for all the experiments. The temperature precision of the rheometer was satisfactorily controlled to be within ±1 ◦C. Data acquisition was completed by using a computer that was interfaced with the rheometer. The unique feature of polymer BB3 is the presence of both the crystal-to-liquid crystal and liquid crystal-toisotropic state transitions on its DSC thermogram. The