热致性液晶共聚酯的拉伸流动行为

EXTENSIONAL FLOW BEHAVIOR OF A THERMOTROPIC LIQUID CRYSTAL COPOLYESTER

采用入口收缩流动的实验方法研究了改性PET/ 80PHB液晶共聚酯LCP80的拉伸流动行为 ,考察了拉伸速率、温度等对其拉伸粘度、Trouton比的影响 .实验结果表明 ,LCP80的入口压降值很大 ,其中由拉伸引起的入口压降是主要的 .在该文实验条件下LCP80均表现出拉伸稀化现象 ,并且Trouton比值都远大于 3 .根据流动中液晶织态结构的变化解释了实验现象 ,并对入口收缩流动的实验数据处理方法作了改进 ,比Beery的方法更为合理 ,也具有更广的适用性 .

The extensional flow behavior of a modified PET/PHB type thermotropic liquid crystal copolyester (LCP80) was studied by using the contraction flow method with a capillary rheometer.The investigation focused on the effects of the extensional strain rate and temperature on the extensional viscosity and Trouton ratio through experimental measurements.The nematic texture variation and molecular orientation during extensional flow at various temperatures were also observed.Experimental results have shown that the entrance pressure losses,which decreased with temperature and increased with the flow rate,were particularly large.The entrance pressure loss reached about 30% of the total pressure drop even at L/D=50,which was significantly larger than that of conventional polymers.It meant that the end correction in the capillary rheometer could not be omitted even when a long capillary is used.In the entrance pressure loss the extensional component was dominant and the shear component was less than 4% in all cases.This was attributed to the orientation development due to extensional flow as the polymer flew through the converging entry section,which was confirmed by the macroscopic ordered texture development after extension.At extensional strain rates from 1 to 100 s -1 ,LCP80 exhibited extension thinning,this phenomenon was more obvious at temperatures above 300℃ due to the increased orientation ability of the rigid molecule chains.The extensional viscosity and the shear viscosity both could be empirically described by the power law model at various temperatures,but the model parameters were quite different.Besides,the Trouton ratios,decreasing with the temperature and extensional rate,were always much greater than the Newtonian values,indicating a high elasticity of the LCP80 melt.The above rheological properties were explained according to the texture observation.