Experimental understanding of the viscosity reduction ability of TLCPs with different PEs

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1 Korea-Australia Rheology Journal, Vol.26, No.3, pp (August 2014) DOI: /s Experimental understanding of the viscosity reduction ability of TLCPs with different PEs Youhong Tang 1, *, Min Zuo 2 and Ping Gao 3, 1 Centre for NanoScale Science and Technology, School of Computer Science, Engineering and Mathematics, Flinders University, South Australia 5043, Australia 2 Department of Polymer Science and Engineering, Zhejiang University, Hangzhou , China 3 Department of Chemical and Biomolecular Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China (Received December 16, 2013; final revision April 7, 2014; accepted April 20, 2014) In this study, two thermotropic liquid crystalline polyesters (TLCPs) synthesized by polycondensation of p- hydroxybenzoic acid /hydroquinone/ poly dicarboxylic acid were used as viscosity reduction agents for polyethylene (PE). The TLCPs had different thermal, rheological and other physical properties that were quantitatively characterized. The two TLCPs were blended with high density PE (HDPE) and high molecular mass PE (HMMPE) by simple twin screw extrusion under the same weight ratio of 1.0 wt% and were each rheologically characterized at 190 o C. The TLCPs acted as processing modifiers for the PEs and the bulk viscosity of the blends decreased dramatically. However, the viscosity reduction ability was not identical: one TLCP had obviously higher viscosity reduction ability on the HDPE, with a maximum viscosity reduction ratio of 68.1%, whereas the other TLCP had higher viscosity reduction ability on the HMMPE, with a maximum viscosity reduction ratio of 98.7%. Proposed explanations for these differences are evaluated. Keywords: liquid crystalline polymer, flexible spacer, viscosity reduction, polymer melt, rheological properties 1. Introduction *Corresponding author: youhong.tang@flinders.edu.au Corresponding author: kepgao@ust.hk Polymer composites containing small amounts of a thermotropic liquid crystalline copolyester (TLCP) in a matrix of thermoplastic have attracted technical interest in recent years for two main reasons. Firstly, by the use of TLCP to enhance the mechanical properties of the matrix polymer through in situ formation of fibrous TLCP dispersion during processing, it may be possible to develop self-reinforced composites that exploit the outstanding tensile properties of fibres made from LCPs (Pawlikowshki et al., 1991; Chen et al., 2009). Secondly, it is known that TLCP can act as a flow modifier, resulting in a substantial reduction in pressure drop during melt extrusion (Whitehouse et al., 1997; Chan et al., 1999, 2001; Chen and Gao 2005a, 2005b; Ruggiero and Acierno, 2007; Tang et al., 2010a, 2010b; Wu et al., 2013). For the latter, our previous studies have shown that TLCPs containing p-hydroxybenzoic acid /hydroquinone /sebacic acid (HBA/HQ/SA) are effective processing aids for high density polyethylene (HDPE) (Whitehouse et al., 1997; Chan et al., 1999) and high molecular mass polyethylene (HMMPE) (Chan et al., 2001; Chen and Gao 2005a, 2005b). Whitehouse et al. (1997) used a TLCP, denoted TLCP(1), as a processing aid for high density polyethylene (HDPE). At a temperature of 185 o C, when TLCP(1) was in the nematic regime, the processing window for HDPE was increased tenfold from a maximum shear rate of 100 1/s for pure HDPE to /s for a 2 wt% TLCP(1)/HDPE blend. There were large viscosity reductions of between 85% and 90% compared with the pure HDPE when the blended material was used at a wall shear stress value of approximately Pa. Chan et al. (2001) used the same TLCP(1) blended with the same HDPE at low concentrations of 2 wt% and less. Viscosity reduction of ~93% was observed at 185 o C when the TLCP was fully nematic and ~89% at 220 o C when the TLCP was a nematic-isotropic biphase. Wall slip was shown to contribute negligibly to the viscosity and the mechanism was elucidated that the TLCP droplet firstly deformed into long fibrils during entry flow, followed by chain alignment of the nematic TLCP molecules within the nematic TLCP droplets. Such chain alignment forces the neighbouring PE molecules to align and disentangle, leading to reduced bulk viscosity. Chan et al. (1999) also reported a TLCP, denoted TLCP(3), as a processing aid in the extrusion of high molecular mass polyethylene. This TLCP(3) was predominately nematic phase at 190 o C and predominantly isotropic at 230 o C. TLCP(3) was an effective processing aid for HMMPE, particularly at 190 o C, with viscosity reductions in excess of 90% with a 1 wt% TLCP(3)/HMMPE blend. Shear-induced interactions between HMMPE melt and TLCP(3) were investigated using large amplitude oscillatory shear and capillary shear 2014 The Korean Society of Rheology and Springer 303

2 Youhong Tang, Min Zuo and Ping Gao Fig. 2. Molecular structure of TLCPs. Fig. 1. Schematic diagram illustrating the flow regimes characterized by alternative flow patterns A and B as a function of maximum fluid velocity developed from the centre region. Region B: a homogeneous highly viscous melt where the maximum fluid velocity is below the critical velocity. Region A: the melt in the centre region flows at velocities above the critical velocity for chain disengagement (Chen and Gao, 2005b). by polarized optical microscopy (POM) and transmission electron microscopy (TEM). A strong interfacial diffusion was found between the oriented TLCP(3) filament and the HMMPE matrix with an interfacial thickness up to 30 nm. This was attributed to the flexible spacer effects within the SA units on the main chain TLCP (Chan and Gao 2005a). A binary flow pattern model was proposed to simulate the rheological responses of the TLCP(3)/HMMPE blend. In this model, when the centreline velocity in capillary exceeds a critical value, the corresponding elongation rate is high enough to stretch the PE chains neighbouring the elongated TLCPs from the random coil configuration to the extended chain configuration. With a further increase in shear rate, the PE chains near the elongated TLCP became disentangled and caused the bulk viscosity reduction, as shown in Fig. 1. With no adjustable parameters, the model successfully predicted both the onset and the completion of transition to the extended chain flow regime (Chen and Gao, 2005b). Our previous experimental observations (Chen and Gao, 2005a, 2005b) suggested that only elongated TLCP particles can induce molecular order in the neighbouring PE phase and hence increase the rate of PE crystallization. The alignment-induced interfacial interactions were attributed to the flexible spacers of the SA segments within the dispersed TLCP domains. The chain alignment within both components at the interface assisted better molecular contact between SA segments and neighbouring PE molecules and hence enhanced interfacial compatibility. In the current study, two TLCPs were synthesized by polycondensation with the same molar ratio of monomers of HBA, HQ and poly dicarboxylic acid (PDA), but having different -(CH 2 )- lengths. Nuclear magnetic resonance (NMR) and X-ray photoelectron spectroscopy (XPS) were used to characterize the length of the -(CH 2 )- in the TLCPs. 1.0 wt% of TLCPs were blended with HDPE and HMMPE respectively and the blends were rheologically characterized at 190 o C by capillary rheometry (Göttfert Rheograph, 2003A, Germany). TLCPs with different -(CH 2 )- lengths displayed different viscosity reduction ability on HMMPE and HDPE. 2. Experimental 2.1. Materials and sample preparation The HMMPE, Marlex HXM TR571, with a melt flow index (MFI) of 2.5 g/10 min (ASTM D1238, 190ºC/21.6 kg) and the HDPE, HMM6060, with a MFI of 6.5 g/10 min (ASTM D1238, 190ºC/21.6 kg) were kindly supplied by Chevron Phillips Marlex, USA. The TLCPs used here, TLCP(B) and TLCP(G), were copolymers containing HBA, HQ and PDA. They were synthesized and kindly supplied by B. P. Chemicals Ltd., UK. As they are proprietary materials, exact details of their chemical composition cannot be revealed. The chemical structure can be represented as in Fig. 2. For the PE blends, dried 1 wt% TLCP in powder form and PE in pellet form were mechanically pre-mixed at room temperature until macroscopically homogeneous. The mixture was then twice extruded using a Dr. Collin twin screw extruder (Dr. Collin GMBH, Germany) at 190 o C at different speeds (75 rad/s and 300 rad/s respectively). The extrudate was palletized and kept dry overnight inside an oven to remove moisture generated during the process Characterizations A high temperature chromatograph Waters 150 o C (Waters, Milford, USA) operating at temperatures up to 150 o C was used. The pump of the Waters system was bypassed using an Agilent G1311A quarternary pump (Agilent, Waldbronn, Germany). The operating temperature was 150 o C. Trichlorobenzene (HPLC grade) was used as the mobile phase. The 1 H-NMR spectra were measured at 45 o C on a NMR spectrometer (Bruker ARX 300, Germany) using chloroform-d as the solvent, and the chemical shifts were reported on the δ scale using tetramethylsilane (TMS) as the internal reference. The phase transition temperatures of the TLCPs were determined via differential scanning calorimetry (DSC) (PYRIS diamond DSC, PerkinElmer Instruments, USA), using indium as the calibration standard, with heating or cooling rate of 10.0 o C/min under nitrogen atmosphere. The surface compositions of the TLCPs were measured by 304 Korea-Australia Rheology J., Vol. 26, No. 3 (2014)

3 Experimental understanding of the viscosity reduction ability of TLCPs with different PEs XPS (PerkinElmer Surface Science Analysis System model PHI 5600) equipped with a monochromatic Al Kα X-ray source. Low-energy flooding electrons were used to neutralize the charges built up on the samples, and the binding energy (BE) scale was adjusted to have the adventitious carbon peak at ev. Multiplex scans of carbon C1s, oxygen O1s and silicon Si2p were acquired at the constant pass energy of 23.5 ev. The mesophase structures of the liquid crystalline phases of the TLCPs were investigated by POM using an Olympus microscope BX 50 with a Cambridge shear system CSS450 connected to a hot stage. The most outstanding feature of this setup was that it permitted investigation of texture changes at different temperatures and under varying shear rates. Mesophase structure images were obtained at different temperatures after pre-shearing the samples at a low shear rate, i.e /s for more than 3600 seconds at 185 o C, to remove any shear history and anchored defects. Controlled strain rheological measurements were performed using an Advanced Rheometric Expansion System (ARES) (TA instruments, USA) with a 200 g-cm transducer within the resolution limit of 0.02 g-cm. For all tests reported here, 50 mm parallel plate fixtures were used. All measurements were performed at 185 o C in nitrogen atmosphere, where TLCP had been shown to exhibit stable rheological properties under the nematic phase. Care was taken to ensure a controlled thermomechanical history as follows: the rheometer was heated to the testing temperature and allowed to reach equilibrium. Fresh samples, dried in a vacuum oven for 2 days at 120 o C, were loaded in the preheated rheometer, heated up to 185 o C then held for 10 min. Decreased gap to the testing gap of 1.0 mm and kept isotherm for 30 min to reach thermal and deformation equilibrium before measurements were started. The experiments were repeated no fewer than three times to check reproducibility. In each case, a fresh sample was used. The rheological behaviours of the blends were characterized by a capillary rheometer (CR) (Göttfert Rheograph 2003A, Germany) at 190 o C. Here, the controlled piston speed mode was used with the round hole capillary dies (nominal L/D ratio equal to 30/1 and die entrance angle 180 ). The real die diameters used here were recalibrated before use (calibrated die diameters D = mm for the nominal D = 1.0 mm die). For each measurement at a certain apparent shear rate, pressure drop has been monitored online. Measurements have been conducted when pressure drop became steady state with normal waiting time about s. 3. Results and Discussions Table 1. Molecular weight of TLCPs from high temperature GPC. M n M w M w /M n TLCP(B) TLCP(G) Fig. 3. (Color online) TLCP(B) (left) and TLCP(G) (right) dissolved in CDCl Flexible spacer length determination in TLCPs From material information provided by B. P. Chemicals Ltd., UK, both TLCPs were synthesized in the same condition with monomers of HBA/HQ/PDA under different reaction batches. Due to the different lengths of -(CH 2 )- in the PDA and the properties of random polymerization, the two TLCPs had similar chemical structures but differed in thermal, rheological and other physical behaviours. High temperature GPC results for TLCP(B) and TLCP(G) are presented in Table 1 with number average molecular weight (M n ), weight average molecular weight (M w ) and polydispersity index (PDI = M w /M n ). From the table, the two TLCPs are quite different in molecular weight and molecular weight distribution, due to the different batches of random polymerization and lengths of the flexible spacer -(CH 2 )-. This indicates that TLCP(B) had a lower molecular weight and lower distribution of individual molecular masses in the batch of polymers than those of TLCP(G). In previous studies (Whitehouse et al., 1997; Chan et al., 1999, 2001; Chen and Gao 2005a, 2005b), it was presumed that a strong interaction existed between the TLCP and the PE. Comparing the chemical structure of the polymer chains, the interaction was believed to be mainly due to the flexible spacer -(CH 2 )- which was common to both TLCP and PE chains. Efforts were made to validate the hypothesis of such interactions in our previous reports. In the current study, we focus on the effects of the flexible spacer length on the final viscosity reduction ability of TLCP/PE blends. With this aim, the length of the flexible spacer -(CH 2 ) m - in TLCPs was quantitatively analysed. Experimentally, TLCPs were dissolved in CDCl 3 for 1 H NMR tests. Fig. 3 shows the TLCP/CDCl 3 solutions with the same concentration. It is evident that the two TLCPs have different solubility in CDCl 3, with a transparent solution for TLCP(G) and a translucent solution for TLCP(B) Korea-Australia Rheology J., Vol. 26, No. 3 (2014) 305

4 Youhong Tang, Min Zuo and Ping Gao Table 2. Summary of 1 H NMR normalized data for each segment. PDA HBA HQ H in α-c H in β-c H in other C TLCP(B) TLCP(G) Table 3. Calculation of x, y, z and m in the two TLCPs from 1 H NMR data. Ratio TLCP(B) TLCP(G) HBA/HQ 4x/4y 1.36/ /2.3 HBA/PDA (α-c) 4x/4z 1.36/ /2.6 HBA/PDA (other-c) 4x/((2 m 8)*z) 1.26/ /8.1 x+y+z=1 x y z m Table 4. XPS data for TLCPs. C1s O1s Si2p [C1s/O1s] exp [C1s/O1s] theo TLCP(B) % 27.87% TLCP(G) % 23.00% Fig. 4. (Color online) 1 H NMR data for TLCP(B) and TLCP(G) at 45 o C in d-chloroform. at room temperature. Fig. 4 presents the experimental results from 1 H NMR for TLCPs at 45 o C for the purpose of achieving better solubility. In the figure, the peak intensity (with the centre at chemical shift of 8.25 ppm) of 1 H in HBA was normalized as 1 and the intensities of the other peaks were normalized accordingly. The corresponding 1 H positions and relative intensities in TLCPs are marked in the figures. Clearly, two of the peaks present in the TLCP(G) spectrum are absent in the TLCP(B), namely the peak at chemical shift of 1.90 ppm with relative intensity 1.05 and the peak at chemical shift of 7.55 pm with relative intensity Table 2 summarizes the relative intensity of the 1 H NMR spectrum with different segments for the TLCPs. From the table, it is obvious that the two kinds of TLCP have similar ratio of HBA, HQ and α-c and β-c in PDA segments but differ in other C in PDA segments. The TLCPs used here had the same molar ratios of HBA, HQ and PDA but different m in -(CH 2 ) m -. The mean m in each TLCP was calculated based on the above 1 H NMR data, and the results are presented in Table 3. Table 3 shows that TLCP(B) had the mean -(CH 2 )- length of m = 6.6 whereas for TLCP(G) it was m = 10. Table 4 presents the XPS results for the two TLCPs. The ratios of C1s/O1s from XPS experimental results and from the molecular structure by using m = 6.6 for TLCP(B) and 10 for TLCP(G) and listed in the table. From Table 4, the theoretical results and experimental data are in good agreement 3.2. Thermal properties of TLCPs The DSC thermograph for the second heating of the TLCPs is shown in Fig. 5. For TLCP(G), the material begins to soften at ~120ºC, going from solid to semi-solid. Two reasonably well-defined melting peaks are located at 145.3ºC and 165.2ºC, respectively, both of which correspond to the crystal to nematic transition. As the segmental distribution is severe in this TLCP, this transition peak splits into a shoulder and a peak. The mesophase structure at temperatures of interest to this study is identified as nematic. Under the microscope the material appears to have a predominantly nematic texture up to a temperature of 220ºC. At 216.9ºC, a broad nematic-to-isotropic transition melting peak, or clearing point, is 306 Korea-Australia Rheology J., Vol. 26, No. 3 (2014)

5 Experimental understanding of the viscosity reduction ability of TLCPs with different PEs Fig. 5. (Color online) Second DSC heating curves of TLCPs. Table 5. Crystalline-to-nematic transition (T C-N ) and nematic-toisotropic (T N-I ) temperatures of TLCP(B) and TLCP(G). Sample T C-N T N-I TLCP(B) TLCP(G) recorded by DSC. The broad and asymmetric transition peaks indicate that the TLCP is characterized by a broad composition distribution which is attributed to the sequential distribution of the HBA and PDA-HQ segments (Pazzagli et al., 2000). This gives a visually biphasic structure in the temperature range ~ ºC. This is in contrast with the TLCP(B), in which there are two reasonably well-defined melting peaks at the temperatures of 116.8ºC and 135.5ºC, as shown in Fig. 5. The nematic to isotropic transition melting peak is located at o C. Table 5 shows the transition temperature for the TLCPs. The superiority of these TLCPs lies in their low crystalline-to-nematic transition temperatures. This enables them to map isotropic polymers with low processing temperatures of ~ ºC, e.g. PE. This is one of the most important criteria for the use of any TLCP as a viscosity reducing agent (Cogswell et al., 1983, 1984a, 1984b). The PDA, especially the flexible chains of -(CH 2 )-, acts as an effective spacer to lower the melting temperature of HBA and HQ rigid rod molecules (Noël, 1992). Confirmation of the above transition temperatures was provided by the POM experiments, and images at different temperatures are shown in Figs Fig. 6 shows the TLCP(B) texture at 185ºC, after being first sheared at a shear rate of 0.5 1/s and then annealed for 2 hr (Fig. 6c). The equilibrium texture in Fig. 6c shows two pairs of dark brushes originating from two point defects. These brushes are the characteristic texture of Schlieren, which also Fig. 6. (Color online) POM images of TLCP(B) at different temperatures (a) 125 o C; (b) 150 o C; (c) 185 o C; (d) 230 o C. belongs to the nematic category. Two brushes represent the half-strength defect which is commonly observed in long chain TLCP as the elastic anisotropy is high compared to the low molecular LCP (Kléman, 1991). The textures in Figs. 6a-6d were obtained by first heating the sample up to 185ºC to melt all the crystals and remove the wall defect texture described above. Then, the TLCP was cooled at a cooling rate of 10ºC 1/min to 150ºC (Fig. 6b) and 125 o C (Fig. 6a). At each temperature, the image was taken after the sample has already annealed for at least 1 hr to attain the equilibrium stage. At 150 o C (Fig. 6b), the material is still molten and retains the nematic structures but some crystalline structures can be seen, which may due to the molecules with high melting temperature. At 125 o C (Fig. 6a), only crystalline structure of TLCP(B) is obtained. At a temperature of 230ºC (Fig. 6d), an isotropic phase is clearly present alongside the nematic phase. There are two phases present, namely the nematic phase as the dispersed phase and the isotropic phase as the continuous phase. Each nematic region contains defect lines, is highly birefringent and contains domains of anisotropy. In contrast, the isotropic phase allows no light through under cross-polarizers. After shearing at 0.1 1/s for ~1 hr, the TLCP is found to be in a more relaxed state than before shearing, as evidenced by the presence of fewer line defects inside the nematic regions and these regions taking on a spherical form. Extinction bands are still present within the nematic regions. Fig. 7 shows the textures of TLCP(G) at the above mentioned temperatures with the same thermal history as that of TLCP(B). It has a similar nematic phase at 185 o C (Fig. 7c), crystalline structures at 125 o C (Fig. 7a) and nematic and isotropic biphase at 230 o C (Fig. 7d). However, it still retains the crystalline structure at 150 o C (Fig. 7b). From the above, it Korea-Australia Rheology J., Vol. 26, No. 3 (2014) 307

6 Youhong Tang, Min Zuo and Ping Gao Fig. 7. (Color online) POM images of TLCP(G) at different temperatures (a) 125 o C; (b) 150 o C; (c) 185 o C; (d) 230 o C. is clearly shown that the textures displayed in Figs. 6 and 7 are in good agreement with the transition temperatures obtained in the DSC results in Fig Rheological properties of TLCPs The transitions from linear to nonlinear viscoelastic behaviour, as manifested in the dynamic strain sweep experiments for the TLCPs, are shown in Fig. 8a. The TLCP(G) shows an increase more than three times greater in complex viscosity Eta * compared with the TLCP(B) in the linear viscoelastic region where the strain amplitude is less than the critical value. In the nematic phase, the textures of TLCP are very stable, giving a very large linear viscoelastic response region. The observed large extent of linear viscoelastic behaviour in the nematic phase can be compared with observations made on Vectra B950. Here, Eta * exhibits the expected shear-thinning behaviour, with the critical strain amplitude for the transition about 20.0% for both TLCPs. As well, the strain amplitude dependence of Eta * in the shear-thinning regime in the TLCP(G) is higher than that of the TLCP(B). Fig. 8b further shows that the viscosities of the TLCPs were less than 1 Pa.s when the shear rate was greater than 5.0 1/s, and the TLCP(B) had much lower viscosity than the TLCP(G). These values are exceptionally low, even among the TLCPs. Some studies (Whitehouse et al., 1997; Chan et al., 1999, 2001; Chen and Gao 2005a, 2005b) have shown that a small amount of a similar type of as-received TLCP effectively reduced the viscosity of HDPE or HMMPE at a processing temperature close to the nematic transition region of the TLCP. If these TLCPs are blended with HDPE or HMMPE, this low viscosity will produce an extremely low viscosity ratio, which is an important factor influencing the rheological properties of a blend system. Fig. 8. (Color online) (a) Dynamic strain sweep and (b) steady shear behaviour of TLCP(B) and TLCP(G) at 185 o C Viscosity reduction ability Fig. 9a shows the apparent shear viscosity of HDPE, 1.0 wt% TLCP(B)/HDPE and 1.0 wt% TLCP(G)/HDPE as the function of shear stress at wall at 190 o C with L/D = 30 and die diameter equal to 1.0 mm. From the graph, it is evident that a dramatic viscosity reduction occurred in the two blends, with a maximum processing window increasing from /s (HDPE) to /s (TLCP(B)/ HDPE) and /s (TLCP(G)/HDPE) respectively. Based on an equivalent wall stress of Pa, the viscosity reductions for the different blends were: TLCP(B)/ HDPE had 68.1% viscosity reduction and TLCP(G)/ HDPE had 51.1% viscosity reduction, compared with HDPE. TLCP(B) had a slightly higher viscosity reduction capability in HDPE but the TLCP(B)/HDPE blend had a slightly narrower processing window than the TLCP(G)/ HDPE blend. Fig. 9b shows the apparent shear viscosity of HMMPE, 1.0 wt% TLCP(B)/HMMPE and 1.0 wt% TLCP(G)/HMMPE as the function of shear stress at wall at 190 o C with L/D = 30 and die diameter equal to 1.0 mm. 308 Korea-Australia Rheology J., Vol. 26, No. 3 (2014)

7 Experimental understanding of the viscosity reduction ability of TLCPs with different PEs Table 6. Rheological behaviours of PE and its TLCP blends at 190 o C. Max. Viscosity Increase Viscosity process reduction ratio (%) (Pa.s) rate (1/s) ratio (%) HDPE wt% TLCP(B)/HDPE wt% TLCP(G)/HDPE HMMPE wt% TLCP(B)/HMMPE wt% TLCP(G)/HMMPE Note: viscosity values are from Pa.s for HDPE and its blends, for HMMPE and its blends. Fig. 9. (Color online) Viscosity reduction ability of TLCP(B) and TLCP(G) for HDPE and HMMPE at 190 o C. The TLCP(B)/HMMPE blend had similar viscosity to that of HMMPE, because yielding had not occurred, while the viscosity of the TLCP(G)/HMMPE blend had 98.7% reduction compared to HMMPE (at an equivalent wall stress of Pa). The yielding stress for the TLCP(G)/HMMPE, where the apparent shear viscosity decreased with the constant shear stress at wall (Chen and Gao, 2005b), was about of Pa. Meanwhile the processing window of the TLCP(B)/HMMPE blend had not broadened but the TLCP(G)/HMMPE blend processing window enlarged almost 7 times from /s (HMMPE) to /s (TLCP(G)/HMMPE blend). Table 6 provides the detailed rheological behaviours of the TLCPs in PE matrices. The TLCP(G) had a much greater viscosity reduction efficiency to HMMPE and the blend had a much wider processing window than the TLCP(B) and its HMMPE blend. The HDPE and HMMPE were industrially obtained through the polymerization of ethylene in the presence of Ziegler Natta catalysts. Both have a low degree of branching and thus strong intermolecular forces and tensile strength. As reported previously, the mechanism of these kinds of TLCP as viscosity reduction agents is the elongated TLCP particles which induce molecular order in the neighbouring PE phase and hence disentangle PE molecular chains near the aligned TLCPs to dramatically reduce the bulk viscosity of the blends. The alignmentinduced strong interfacial interactions are attributed to the flexible spacer -(CH 2 )- within the dispersed TLCP domains. The chain alignment within both components at the interface may generate better molecular contact between the flexible spacer -(CH 2 )- and the neighbouring PE molecules, hence enhancing interfacial compatibility. With enhanced interfacial compatibility, PE molecular chains in the neighbouring elongated TLCP are easily disentangled and thereby cause the blend s low critical transition shear rate. The most important aspect is that the critical transition shear rate should fall into the process window of pure PE to disentangle PE molecular chains before the PE becomes unprocessable. Enhanced interfacial compatibility between flexible spacers -(CH 2 )- and the neighbouring PE molecules lower the transition shear rate and promote viscosity reduction. In this study using TLCP(B) and TLCP(G), the longer flexible spacer length of -(CH 2 )- in the TLCP(G) with higher PDI enhanced the interfacial compatibility with PE, especially with HMMPE, increasing the extent of the processing window as well as reducing the bulk viscosity of the blends. 4. Conclusions Two kinds of thermotropic liquid crystalline copolyesters (TLCP(B) and TLCP(G)) with similar chemical structures but differences in the length of -(CH 2 )- in monomer of poly dicarboxylic acid were used as viscosity reduction agents for different molecular weight polyethylenes (HDPE and Korea-Australia Rheology J., Vol. 26, No. 3 (2014) 309

8 Youhong Tang, Min Zuo and Ping Gao HMMPE). The molecular differences between the two TLCPs were characterized quantitatively by NMR and XPS and were attributed to the different lengths of flexible spacer -(CH 2 )- in the molecular structures. Compared to the TLCP(G), the TLCP(B) had lower molecular weight, PDI, thermal transition temperature and viscosity. The viscosity reduction ability of the TLCPs was characterized at 190 o C, where both were in their nematic phase structure. The TLCPs had comparable viscosity reduction ability for HDPE but the TLCP(G) had much greater viscosity reduction ability than the TLCP(B) for HMMPE. Due to the longer flexible spacer, TLCP(G) had much better interfacial compatibility with high molecular weight PE chains, which caused the PE molecular chains neighbouring the elongated TLCP to disentangle at the critical shear rate lower than the HMMPE unprocessable shear rate, thereby generating the highly efficient viscosity reduction. However, the critical transition shear rate for the TLCP(B)/HMMPE blend was higher than the HMMPE unprocessable shear rate due to the weaker interfacial compatibility and the viscosity reduction ability could not be utilized before an unstable process occurred in the HMMPE. Acknowledgements The authors gratefully acknowledge the Research Grant Council of Hong Kong (Grant number HKUST6256/02) for financial support. Y Tang is grateful for the research support of a Discovery Early Career Research Award (DECRA) from the Australian Research Council. References Chan, C.K., C. Whitehouse, and P. Gao, 1999, The effect of TLCP melt structure on the bulk viscosity of high molecular mass polyethylene, Polym. Eng. Sci. 39, Chan, C.K., C. Whitehouse, P. Gao, and C.K. Chai, 2001, Flow induced chain alignment and disentanglement as the viscosity reduction mechanism within TLCP/HDPE blends, Polymer 42, Chan, C.K. and P. Gao, 2005a, Shear-induced interactions in blends of HMMPE containing a small amount of thermotropic copolyester HBA/HQ/SA, Polymer 46, Chan C.K. and P. Gao, 2005b, A phenomenological model for the viscosity reductions in blends of HMMPE containing a small quantity of thermotropic liquid crystalline copolyester HBA/HQ/SA, Polymer 46, Chen, L., H.Z. Huang, Y.Z. Wang, J. Jow, and K. Su, 2009, Transesterification-controlled compatibility and microfibrillation in PC-ABS composites reinforced by phosphorus-containing thermotropic liquid crystalline polyester, Polymer 50, Cogswell, F.N., B.P. Griffin, and J.B. Rose, 1983, Compositions of melt-processable polymers having improved processability, U. S. Patents Cogswell, F.N., B.P. Griffin, and J.B. Rose, 1984a, Compositions of melt-processable polymers having improved processability, U. S. Patents Cogswell, F.N., B.P. Griffin, and J.B. Rose, 1984b, Compositions of melt-processable polymers having improved processability, U. S. Patents Kléman, M., 1991, Defects and textures in liquid-crystalline polymers, In: Liquid crystallinity in polymers: principles and fundamental properties, Ciferri, A. ed., VCH, Weinheim, pp Noël, C., 1992, Characterization of mesophases, In: Liquid crystal polymers: from structure to applications, Collyer, A.A. ed., Elsevier Applied Science, London, pp Pawlikowshki, G.T., D. Dutta, and R.A. Weiss, 1991, Molecular composites and self-reinforced liquid crystalline polymer blends, Annu. Rev. Mater. Sci. 21, Pazzagli, F., M. Paci, P. Magagnini, U. Pedretti, C. Corno, G. Bertolini, and C.A. Veracini, 2000, Effect of polymerization conditions on the microstructure of a liquid crystalline copolyester, J. Appl. Polym. Sci. 77, Ruggiero, V. and D. Acierno, 2007, Effects of the addition of small amounts of thermotropic liquid crystalline polymer on the processing characteristics of polyphenylene oxide-polyamide alloys, Adv. Polym. Technol. 26, Tang, Y.H., P. Gao, L. Ye, and C.B. Zhao, 2010a, Experimental measurement and numerical simulation of viscosity reduction effects in HMMPE containing a small amount of exfoliated organoclay-modified TLCP composite, Polymer 51, Tang, Y.H., P. Gao, L. Ye, and C.B. Zhao, 2010b, Organoclay/ thermotropic liquid crystalline polymer nanocomposites. Part III. effects of fully exfoliated organoclay on morphology, thermal and rheological properties, J. Polym. Sci. Part B: Polym. Phys. 48, Whitehouse, C., X.H. Lu, P. Gao, and C.K. Chai, 1997, The viscosity reducing effects of very low concentrations of a thermotropic copolyester in a matrix of HDPE, Polym. Eng. Sci. 37, Wu, T., P.Q. Liu, X. Wang, L.X. Zeng, G.D. Ye, and J.J. Xu, 2013, Effects of novel thermotropic liquid crystalline polyester with aryl-ether linkages on the processability and properties of poly(ether ether ketone)s fibers, J. Appl. Polym. Sci. 128, Korea-Australia Rheology J., Vol. 26, No. 3 (2014)