聚合物熔体微尺度剪切黏度测量方法与黏度模型

研究微尺度效应下聚合物熔体黏度时,发现不同入口修正方法获得的剪切黏度随特征尺寸变化的规律不同,这对于聚合物微成型理论和技术尤为重要。采用直径分别为1 000μm、500μm、350μm的毛细管口模,在相同试验条件下分别用零口模法和Bagley法两种入口修正方法,研究高密度聚乙烯(High density polyethylene,HDPE)、短链支化的聚丙烯(Polypropylene,PP)、聚甲基丙烯酸甲脂(Polymethylmethacrylate,PMMA)和聚苯乙烯(Polystyrene,PS)四种材料的剪切黏度变化规律。结果发现,两种方法获得的PMMA和PS黏度随口模直径变化的规律相反,指出传统入口修正方法在测量微尺度黏度时存在局限。基于入口收敛流动特征,提出一种考虑微尺度效应下压力影响的测量方法,并用该方法给出四种材料剪切黏度随口模直径变化的真实规律。试验剪切速度范围内,四种材料剪切黏度均随口模直径的减小而减小,平均变化幅度为9.9%～38.3%,并从分子结构角度揭示四种材料黏度变化程度不同的机理。基于黏度变化规律,采用唯象性方法建立适用于宏—微观尺度下的黏度模型。试验结果表明该模型的理论预测结果与试验结果平均误差小于3.7%,验证了模型的正确性和有效性。

Micro-scale Shear Viscosity Testing Approach and Viscosity Model of Polymer Melts

When shear viscosity of polymer melts under micro-scale effect is studied,different shear viscosity variations are obtained by using various entrance correction methods,which is of significant importance for micro forming theory and technology.Shear viscosities of four polymers,including high density polyethylene(HDPE),polypropylene(PP),polymethylmethacrylate(PMMA) and polystyrene(PS),are investigated under the same experimental condition by both orifice correction and Bagley correction.Capillary dies with diameters of 1 000 μm,500 μm and 350 μm are used.Experimental results show that the shear viscosity variations with die diameter alteration of PMMA and PS obtained by the two methods are opposite,which indicates that there are limitations when traditional entrance correction method is utilized in the measurement of micro-scale viscosity.Thus an improved testing approach is developed considering the pressure effect under micro-scale,based on the characteristics of the entrance converging flow.And the authentic variations of the shear viscosity with different die diameters are presented for the four polymers.The results show that the shear viscosities decrease with the decrease of die diameter for all the four materials in the test shear rate range,and the change amplitude is from 9.9% to 38.3%.The mechanism of different viscosity variation degree of the four materials is revealed from the perspective of molecular structures.Based on the phenomenological theory,a viscosity model,suitable for both macro and micro scales,is established.Good agreement between the model predictions and experimental data is obtained with an average error less than 3.7%,confirming the validity and efficiency of the proposed model.