Linking the Rheological Behavior to the Processing of Thermotropic Liquid Crystalline Polymers in the Super-cooled State
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Thermotropic liquid crystalline polymers (TLCPs) have attracted great interest because of the combination of their promising properties, which includes high stiffness and strength, excellent processability, and outstanding chemical resistance. TLCPs exhibit inherently low viscosity relative to many other conventional thermoplastics. The low melt viscosity is detrimental to processes requiring high melt strength, such as extrusion blow molding, film blowing, thermoforming and multilayer coextrusion. Our laboratory has developed a unique method to increase the viscosity of TLCPs by first raising the temperature above the melting point (Tm) to exclude all solid crystalline structure, and then lowering the temperature below Tm to super cool the materials. Additionally, the super-cooling behavior of TLCPs allows them to be blended with other thermoplastics possessing lower processing temperatures. The initial focus of this dissertation is to investigate the processing temperature of a representative TLCP in the super-cooled state, using the methods of small amplitude oscillatory shear (SAOS), the startup of shear flow and differential scanning calorimetry (DSC). The TLCP used in this work is synthesized from 4-hydroxybenzoic acid (HBA), terephthalic acid (TA), hydroquinone (HQ) and hydroquinone derivatives (HQ-derivatives). The TLCP of HBA/TA/HQ/HQ-derivatives has a melting point, Tm, of around 280 oC. Once melted, the TLCP can be cooled 30 oC below the Tm while still maintaining its processability. As the TLCP was cooled to 250 oC, a one order magnitude increase in viscosity was obtained at a shear rate of 0.1 s- 1. Additionally, super cooling the TLCP did not significantly affect the relaxation of shear stress after preshearing. However, the recovery of the transient shear stress in the interrupted shear measurements was suppressed to a great extent in the super-cooled state. The second part of this work is concerned with the extrusion blow molding of polymeric blends containing the TLCP of HBA/TA/HQ/HQ-derivatives and high density polyethylene (HDPE), using a single screw extruder. The blends were processed at a temperature of 260 oC which is 20 oC below Tm of the TLCP such that the thermal degradation of HDPE was minimized. Bottles were successfully produced from the blends containing 10, 20 and 50 wt% TLCP. The TLCP/HDPE blend bottles exhibited an enhanced modulus relative to pure HDPE. However, the improvement in tensile strength was marginal. At 10 and 20 wt% TLCP contents, the TLCP phase existed as platelets, which aligned along the machine direction. A co-continuous morphology was observed for the blend containing 50 wt% TLCP. The preliminary effectiveness of maleic anhydride grafted HDPE (MA-g-HDPE) as a compatibilizer for the TLCP/HDPE system was also studied. The injection molded ternary TLCP/HDPE/MA-g-HDPE blends demonstrated superior mechanical properties over the binary TLCP/HDPE blends, especially in tensile strength. Consequently, it is promising to apply the ternary blends of TLCP/HDPE/MA-g-HDPE in the blow molding process for improved mechanical properties. Finally, this work tends to determine how the isothermal crystallization behavior of a TLCP can be adjusted by blending it with another TLCP of lower melting point. One TLCP (Tm~350 oC) used is a copolyester of HBA/TA/HQ/HQ-derivatives with high HBA content. The other TLCP (Tm~280 oC) is a copolyesteramide of 60 mol% hydroxynaphthoic acid, 20 mol% terephthalic acid and 20 mol% 4-aminophenol. The TLCP/TLCP blends and neat TLCPs were first melted well above their melting points, then cooled to the predetermined temperatures below the melting temperatures at 10 oC/min to monitor the isothermal crystallization. As the content of the low melting TLCP increased in the blends, the temperature at which isothermal crystallization occurred decreased. Comparing with neat TLCPs, the blend of 75% low melting TLCP crystallized at a lower temperature than the pure matrices, and the blend remained as a stable super-cooled fluid in the temperature range from 220 to 280 oC. Under isothermal conditions, differential scanning calorimetry (DSC) was not capable of reliably detecting the the low energy released in the initial stage of crystallization. In contrast, small amplitude oscillatory shear (SAOS) was more sensitive to detecting isothermal crystallization than DSC.
- Doctoral Dissertations