Rheology of thermotropic liquid crystal polymers for injection moulding.
Creators: name: Rahman, Ahmed
Publisher: name: RMIT University
Year: 2024

description: Thermotropic Liquid Crystal Polymers (TLCPs) are advanced high temperature processing polymers with unique physical properties. They contain rigid rod like molecules, which exhibit structural orientation during the flow process at one or two dimensional levels. This structural orientation makes the rheological properties of TLCPs often extraordinary. The rheological properties of TLCPs in injection moulding are still not well understood.<br>In this study, the rheological and morphological property of four thermotropic filled and unfilled TLCPs were investigated from low to high shear rate regions. The effect of incorporation of fillers and their orientation with shear was also investigated. Thermal analyses were conducted to identify the phase transition behaviour. In addition, wide angle x-ray diffraction (WAXD) was conducted to find out the crystallization peak. PVT experiments were carried out to find out the abnormal volume change/shrinkage. Low shear rheology was conducted by the ARES rheometry and high shear rheology was conducted by the CEAST capillary rheometry.<br>The FTIR experiment reveals the presence of major constituents (i.e. cyno and phenyl mesogen groups, CH aromatic deformation, CH2 asymmetric alkyl chain and Amide (I) in TLCPs. Aspect ratio of fibers as well as orientation of the rod like molecules and the fillers with application of shear were determined by the ESEM study. DSC measurements specified a sharp peak for the melting point of all four samples. However, small and broader peaks were observed for nematic isotropic (TN-I) transition temperature.<br>VIII<br>The rheological behaviour of TLCPs is highly dependent on thermal and deformation histories compared to conventional polymers. The initial identical conditions were carefully maintained to get reproducible rheological data. The dynamic rheological measurements were used to determine the molecular weight distribution (MWD). The nematic-isotropic phase transition temperature was also confirmed by dynamic rheological measurements. From this study, it was determined that the nematic-isotropic phase transition occurred only at low shear rate region with the increase of temperature. In addition, hysteresis was observed for the dynamic moduli data in the low shear rate region for forward and reverse direction measurements, which indicated time dependent rheology and shear induced crystallization of LCPs. WAXD measurements confirmed the shear induced crystallization in LCPs. The architecture and morphology of LCP molecules varied with temperature and shear rate, which makes LCP thermo-rheologically not simple.<br>The negative first normal stress difference (N1) is one of the most important rheological characteristics observed for these LCPs, which is very uncommon for thermotropic LCPs. In this study, it was revealed that filled LCPs (aspect ratio of filler greater than 100) exhibited the negative first normal stress difference, N1 in the low shear rate region. Moreover, in the high shear rate region (shear rate &gt; 2000 s -1) the viscosity of filled LCPs showed more shear thinning behaviour compared to the unfilled LCPs due to the rheological hybrid effects (a phenomenon in which the melt viscosity of a ternary polymer blend decreases with increasing filler loading, influenced by the minor polymer phase in the blend) (Ding et al., 2006).<br>Very limited research has been conducted for the rheological modelling of liquid crystal polymers at high shear rate. None of the available rheological models for LCPs has been used to<br>IX<br>model the flow in any injection moulding process. The current lack of suitable models prevents the progress in processing of LCPs.<br>The complicated rheological properties and characteristic anisotropic properties of TLCPs has been investigated by simulating the recently developed Leonov’s viscoelastic constitutive equations. The simulated viscosities have been compared with experimentally measured viscosity data, indicating a good fit in the shear rate range of 10 to 2000 s-1. For unfilled TLCPs, good comparison has been observed between the simulated values of N1 and positive experimental data. The rectangular hyperbolic model also fitted well with the positive first normal stress data of LCPs. For filled TLCPs the model does not predict negative values of N1 even though the experimentally determined values are negative. The modelling of this behaviour remains a challenge to the rheologist.