THESIS
1996
xi, 176 leaves : ill., photos. (some col.) ; 30 cm
Abstract
This work aims to improve the processability of high molecular mass polyethylene (HMMPE) using a small quantity of thermotropic liquid crystalline polymer (TLCP) (1-5 wt%) as a processing aid. The HMMPE has weight average molecular weight over 300,000 kg/kmol, which offers exceptionally good mechanical properties, however, it is considered as unprocessable using conventional techniques due to its high viscosity. The liquid crystalline polymer (LCP) used is a terpolymer of hydroxybenzoic acid, hydroquinone and sebacic acid (HBA/HQ/SA), developed by British Petroleum Ltd., which has melt transition temperatures matching the melt processing temperature of polyethylene....[
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This work aims to improve the processability of high molecular mass polyethylene (HMMPE) using a small quantity of thermotropic liquid crystalline polymer (TLCP) (1-5 wt%) as a processing aid. The HMMPE has weight average molecular weight over 300,000 kg/kmol, which offers exceptionally good mechanical properties, however, it is considered as unprocessable using conventional techniques due to its high viscosity. The liquid crystalline polymer (LCP) used is a terpolymer of hydroxybenzoic acid, hydroquinone and sebacic acid (HBA/HQ/SA), developed by British Petroleum Ltd., which has melt transition temperatures matching the melt processing temperature of polyethylene.
Firstly, a significant improvement in melt processibility in terms of both bulk viscosity and melt flow stability has been achieved at very low LCP loading. For examples, a bulk viscosity reduction of up to 75% was achieved by adding only 1 wt% LCP. The level of viscosity reduction was nearly constant at all shear rates. In addition, no melt flow instability or pressure oscillation during flow was observed for shear rates up to 1000 s
-l, whereas the pure HMMPE showed melt fracture and pressure oscillation at shear rates as low as 15 s
-l at 175°C.
Secondly, the observed processability enhancement is LCP structure dependent. Maximum viscosity reduction was achieved when the LCP melt was in either the crystalline/nematic biphase or in the nematic single phase.
Thirdly, morphological studies using both scanning electron microscopy and high temperature optical microscopy show that very weak phase interactions exist between the two blend components. High temperature optical microscopy shows that the dispersed LCP droplets exist as spherical shape in the quiescent state. These spherical droplets can be elongated under steady shear. The droplet elongation increases with shear rates and is highly sensitive to LCP melt structure. The crystalline/nematic LCP droplets deform most readily and the nematic/isotropic LCP droplets were the most difficult ones to deform, while the single nematic phase LCP droplet deformation lies in between. The observed droplet deformation is also consistent with the interfacial tension values obtained using Taylor's theory within different LCP phases. Maximum interfacial tension was obtained when the LCP becomes more isotropic and minimum was observed in the crystalline/nematic biphase.
Finally, the mechanism of viscosity reduction was studied by evaluating the effect of wall slip on the flow behavior of the blend and the pure HMMPE. It was found that surface slip exists in both the blend and the homopolymer. After slip correction, using Mooney's technique, there still exists a significant viscosity difference between the pure HMMPE and the blend. For example, a 1bulk viscosity difference of 54-67% was observed at shear rates of l0-50 s
-1 for the 2 wt% blend extruded at 195°C after slip contribution has been removed. There is also a trend for further increase in viscosity difference between the true viscosity of pure HMMPE and the blend at high shear rates. We can then conclude that the interfacial slip dominates the observed viscosity reduction. This conclusion is also supported by our droplet deformation dynamics studied using the optical shearing apparatus.
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