LOW AND HIGH SHEAR RATE RHEOLOGY OF INJECTION MOULDING GRADE LIQUID CRYSTAL POLYMERS

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1 LOW AND HIGH SHEAR RATE RHEOLOGY OF INJECTION MOULDING GRADE LIQUID CRYSTAL POLYMERS Ahmed Rahman, Rahul K Gupta*, Sati N. Bhattacharya, Shishir Ray 1 and Franco Costa 1 Rheology and Materials Processing Centre, School of Civil, Environmental and Chemical Engineering, RMIT University, Melbourne 3001,Vic, Australia 1 Autodesk Pty Ltd Colchester Rd Kilsyth 3137, Vic, Australia Corresponding Author s rahul.gupta@rmit.edu.au ABSTRACT The rheology of liquid crystalline polymers (LCPs) is a topic of practical importance because of their unique properties. It has a particular kind of structural orientation at one or two dimensional level. Whenever a LCP is subjected to a shear field, there is a molecular reorientation process taking place resulting in the formation of a polydomain structure of randomly oriented nematic micro domains. Based on the micro structural evolution the shear behaviour, in most cases LCP does not show a plateau region or a zero shear viscosity. It has been reported that accurate measurements of the shear rheological properties of (LCPs) are critical to allow appropriate prediction, as it flows under pressure in a typical injection moulding machine. Rheological properties of four thermotropic LCPs were investigated from low to high shear rate regions. Advanced Rheometric Expansion System (ARES) and capillary rheometer were used for low shear rate and high shear rate respectively. The complex viscosities as well as shear viscosities of the four LCPs show a typical shear-thinning behaviour. The abnormal temperature dependence of the viscosities can be explained by the nematic-isotropic transition, in which the rod-like molecules lost part of their orientation ordering and became partially isotropic with increase in temperature. INTRODUCTION Liquid Crystal Polymers (LCPs) are materials of great importance due to their lightweight, superior strength and stiffness(acierno and Collyer, 1997). A lot of research work has been carried out for its exceptional characteristics over the last two decades. It is already established that the flow behaviour of Liquid Crystal Polymers exhibits peculiar characteristics which are not shown in convention polymers. One of the attractive features of liquid crystalline polymers (LCPs) is their potential ability to easily become oriented in a flow process, a property which can be exploited to fabricate articles with exceptional properties(leonov 2008, Marrucci and Guido, 1995). Because of this orientation ability, the rigid rodlike molecules of LCPs show unusual rheological properties. The two major aspects for abnormal behaviours of LCPs are disclinations (rotation of molecules from the direction of flow to the perpendicular of flow) and tumbling nature of molecules under shear flow. It was found that the director, i.e., the average direction of the rods, exhibits tumbling, kayaking and wagging, flow-aligning, or log-rolling types of motion depending on the applied shear rates. The theoretical investigations revealed

2 that all peculiar behaviors of LCPs are to be attributed to the periodic oscillation of the director(hashimi and Takeshi 2007). Shear dependency of viscosity By increasing the shear rate, from very small values to very large ones, the viscosity first decreases (region I), then levels off (region II), then again decreases (region III) (Onogi and Asada, 1980). Figure 1: Schematic diagram of three zone shear dependency of viscosity (Muir and Porter 1989). Whenever a nematic LCP is subjected to a shear field, there is a molecular reorientation process taking place resulting in the formation of a polydomain structure of randomly oriented nematic micro domains which finally lead to behaviour like any other isotropic normal polymer. The alignment of the molecular domains now results in a different kind of texture evolution with the material subjected to shear flows. LCPs depict a typical tumbling behaviour under shear flow (the direction of molecular orientation rotates continuously in a shearing flow) resulting in a different micro structural and texture evolution(kle'man, 1989). In region I, LCPs exhibit shear thinning behaviour with low shear rate, i.e., the viscosity decreases with increasing shear rate. But the conventional polymers do not display the region I behaviour, i.e., their viscosity always approaches a constant value as the shear rate tends to zero. At region I, the texture of nematic phase consists of many small domains, in which the local orientations of the director vary from one domain to another. Under this circumstance, each small domain may act as if it is a discrete particle suspended in a very low viscosity fluid (nematic solvent)(chen 2008). Thus, at extremely low shear rate, the polydomain hardly deforms. The structural evolution of such a defect suspension would generate the shear thinning behavior at low shear rates(marrucci and Maffettone, 1993).Then the viscosity is almost constant with shear rate (Newtonian behavior), which is referred to the region II. With the increase of shear rate, the effect of coalescence also increased. For this reason, the number of small domains also reduced. At region III (very high shear rate zone), the molecules of LCPS start to move with flow aligning directions, which leads to strong shear thinning behaviour(muir and Porter 1989). Temperature dependency of viscosity LCPs are showing anomalous behaviour of viscosity with respect to temperature. Thermoplastic LCPs exhibit low viscosity at nematic phase (tumbling the rigid molecules along with the director and molecules are positionally disordered but orientationaly ordered). In spite of having a fairly rigid chain structure, LCPs can be easily processed in nematic phase. At this phase, viscosity decreases with increase in

3 temperature and low energy is required for processing nematic LCPs. It is observed that the shear viscosities are higher in the isotropic phase, despite its higher temperature, than in the anisotropic phase (Kiss and Porter 1980, Kiss and Porter, 1978). Further increase in temperature, viscosity also increases due to biphasic transition. By thermal activation, part of the rod-like molecules become a more viscous isotropic phase and the increase of temperature gives rise to increasing portion of isotropic phase at the expense of anisotropic phase (Hsies et al., 1999, Gao et al., 1996, Fan et al., 2003) Negative first normal stress (N 1 ) It is observed that the first normal stress difference (between the shear and the gradient directions) is positive at low shear rates, and at very high ones. It becomes negative in an intermediate range of shear rate values, roughly located at the beginning of region III (Marrucci and Guido, 1995). The negative normal stress demonstrated in LCPs is due to its tumbling behaviour. The director of rod like molecules keeps rotating along with the shear field indefinitely. After a reasonable large shear rate, the ellipsoid shape of domain gets deformed here too. At this stage, the rotation of molecule tends to rotate perpendicular to the flow of axis. The difference with respect to the equilibrium ellipsoid now is in the opposite direction than in the cases previously considered; there follows that N l is negative in these conditions. The value of N l becomes positive again at high shear rate. This will occur when the strength of the flow has become so large that tumbling is suppressed altogether, and the molecules increasingly align in the shear direction. In other words, at some critical value of the shear rate, a transition takes place between tumbling and the flow-aligning behaviour (Marrucci and Guido, 1995). This paper presents shear rheological study of a series of four LCPs, filled as well as unfilled at low as well as high shear rate. EXPERIMENTAL MATERIALS AND TECHNIQUES Materials: Four aromatic nematic thermotropic LCPs were selected in the form of pellets. The generic names of these polymers are as follows: 1. LCP Polymer A unfilled 2. LCP Polymer A with 35% glass fibre. 3. LCP Polymer B unfilled 4. LCP Polymer B with 30% glass fibre. Preparation of samples: These LCP materials were dried at 120 C for eight hours to achieve target moistures content of less than 0.02 %. These dried materials were then compression molded at the processing temperature window as specified by the manufacturer. Shear Rheology: The 2 mm thick compression moulded samples were cut into 25 mm diameter discs. Low shear rheological properties were studied using the Advanced Rheometrics Expansion System (ARES), a constant strain rate rheometer in both dynamic and steady shear modes with parallel plate geometry. The tests were conducted using 25 mm diameter plates at temperatures between 320 C and 360 C for the various LCP

4 materials. All measurements were performed using a force transducer with a range of 0.2 to 200 g-cm torque. Prior to any test, the zero-gap between the parallel plates was calibrated at the required temperature. For each test, the sample was placed between preheated fixtures for 10 minutes to reach thermal equilibrium, and then the gap was set. Dynamic frequency sweeps were conducted to determine the microstructure and dynamics of the materials. These tests were conducted at low strain amplitudes within the linear viscoelastic region and at frequencies ranging from 0.01 rad/s to 100 rad/s. The high shear rate rheology test was conducted at Autodesk research facilities. CEAST double piston-driven capillary rheometer measurements (using circular dies) have been carried out for high shear rate data. The high shear rate was conducted from 10 s -1 to 6000 s -1. Two different types of temperature range were used for two polymers. The tests for LCP polymer A were conducted from 340 C to 360 C with 10 C increments and for LCP polymer B from 320 C to 340 C with 10 C increments. RESULTS AND DISCUSSION Figures 2 to 7 present the magnitude of the measured complex viscosity and shear viscosities for the four LCPs against various frequencies and shear rates at different temperatures. The viscosity-frequency curves exhibit typical shear thinning behaviour. These curves for various temperatures are parallel. From these viscosity-frequency curves, no constant viscosity plateau was observed as reported in the literature. LCPs have the potential to easily become oriented (by tumbling of rod like molecules) in a flow process. This easy orientation ability is related to the fact that molecular orientation is a collective or cooperative property in a liquid crystalline nematic phase so that the applied field is not counteracted by thermal motion(fan et al., 2003, Marrucci and Guido, 1995). At very high shear rate, random orientation occurs, which leads to the behaviour of normal polymer. At this stage, the tumbling is suppressed altogether by the strength of flow, and the molecules increasingly align in the shear direction. 1.00E E+05 LCPA 350 C LCP A 350 C LCP A 360 C LCP A 360c C o m p ex visco sity (P a.s) Frequency (rad/s) Figure 2: Complex viscosities of filled and unfilled LCP A as a function of frequency

5 1.00E+06 Complex viscosity (Pa.s) 1.00E+05 LCP B C LCP 330 C LCP B 320 C LCP B 330 C 1.00E Frequency (rad/s) Figure 3: Complex viscosities of filled and unfilled LCP B as a function of frequency LCP A 340C LCP A 350 C Figure 4: Shear viscosity at high shear rate for unfilled LCP polymer A LCP B 320 LCP B Figure 5: Shear viscosity at high shear rate for unfilled LCP polymer B

6 LCP A 340C LCP A 350C LCP A 360C 1.00E+05 Figure 6: Shear viscosity at high shear rate for filled LCP polymer A LCP B 340C LCP 330C LCP 320C Figure 7: Shear viscosity at high shear rate for filled LCP polymer B It is observed from Figures 4 that the viscosity decreases with temperature increase for high shear rates. It is typical for most of the polymers. However, at low frequency, viscosity increases as temperature increases (Figure 2) for LCP A. This abnormal dependency of viscosities at higher temperature at low shear rate can be expressed by the biphasic nature of LCPs, which is responsible for thermal activation. Due to this, parts of molecules become more viscous isotropic phase. Thus an increase of temperature leads to an increase in viscosity due to transition from anisotropic to the isotropic.(fan et al., 2003).

7 CONCLUSION: Due to the temperature effect, the complex viscosities as well as shear viscosities give rise to different responses. In our case, all the four LCPs show a typical shear thinning behaviour. The abnormal temperature dependence i.e. increases in the viscosity as temperature increases can be due to the nematic-isotropic transition. There is a molecular reorientation process taking place for thermotropic LCPs under shear. This process is responsible for the formation of a polydomain structure, which finally leads to the behaviour like that of any other isotropic polymer. The magnitude of viscosities for filled LCPs was found to be higher than that of the unfilled LCPs. REFERENCES Acierno, D. & Collyer, A. A. (1997) Rheology and Processing of Liquid Crystal Polymers, London, Chapman & Hall. Chen, H. (2008) Simulations of shearing rheology of thermotropic liquid crystal polymers. The University of Akron. Fan, Y., Shaocong, D. & Tanner, I. R. (2003) Rheological properties of some thermotropic liquid crystalline polymers. Korea-Australia Rheology Journal, 15, Gao, P., Lu, X. H. & Chai, C. K. (1996) Rheology of low nematic transition temperature thermotropic liquid crystalline copolyester HBA/HQ/SA. Polym. Eng. Sci, 36, Hashimi, S. A. R. & Takeshi, K. (2007) Shear Rate Dependence of Viscosity and First Normal Stress Difference of LCP/PET Blends at Solid and Molten States of LCP. Journal of Applied Polymer Science, 104, Hsies, T. T., Tiu, C., Simon, G. P. & Wu, R. W. (1999) Rheology and miscibility of thermotropic liquid crystalline polymer blends,. J. Non-Newt. Fluid Mech., 86, Kiss, G. & Porter, R. S. (1978) Rheology of concentrated solution of poly(γbenzylglutamate). Journal of Polymer Science, Polymer Symposia, 65, Kiss, G. & Porter, R. S. (1980) Rheology of concentrated solution of helical polypeptides. Journal of Polymer Science, Polymer Physics Edition, 18, Kle'man, M. (1989) Defects in Liquid Crystal. Rep. Prog. Phys, 52, Leonov, A. I. (2008) Algebraic Theory of Linear Viscoelastic Nematodynamics. Math Phys Anal Geom, 11, Marrucci, G. & Guido, S. (1995) Shear Flow Rheology of Liquid Crystalline Polymers. Int. J. Polyer Analysis & Characterization, 1, Marrucci, G. & Maffettone, P. L. (1993) Liquid Crystalline Polymers, New York, Pergamon. Muir, M. C. & Porter, R. S. (1989) Processing rheology of liquid crystal polymers: a Review. Mol. Cryst. Liq. Cryst., 169, Onogi, S. & Asada, T. (1980) Rheology, New York, Plenum Press.