Stress-optical measurements at a flow stagnation point in confined geometries such as the cross-slot provide an elegant way to perform extensional testing for polymer melts. This technique is especially useful for samples which have a steady-state that cannot be reached (easily) in standard elongational rheometry, for example, highly branched polymers which show a non-homogeneous deformation that occurs in stretching experiments for Hencky strains above 4. In contrast to filament stretching, the cross-slot provides one point at which steady-state extensional flow may be sustained indefinitely. In this study, a Cambridge multi-pass rheometer [ Coventry, K. D., and M. R. Mackley, J. Rheol. 52, 401–415 (2008) ] is used to generate planar elongational flow in a cross-slot geometry for different polyethylene melts. The experimental results are compared to finite element flow simulations using the multi-mode Pompom constitutive equations. The steady-state elongational viscosity at the stagnation point is computed from the flow-induced stress birefringence and the strain-rate determined from numerical calculations of the flow field. We apply this technique to a range of different branched high- and low-density polyethylene melts. This demonstrates both the effectiveness of this technique and shows how the stress distribution in a complex flow depends on molecular structure. Cross slot extensional rheometry therefore provides a very promising technique for parameterizing molecular constitutive equations for LCB melts.
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机译:在狭缝等狭窄几何形状的流动停滞点进行应力光学测量,为进行聚合物熔体拉伸测试提供了一种绝佳的方法。对于在标准伸长流变仪中无法(轻松)达到稳态的样品,例如对于在高于4的Hencky应变的拉伸实验中表现出不均匀变形的高支化聚合物,该技术尤其有用。与长丝拉伸相反,十字槽提供了一个可以无限期维持稳态拉伸流的点。在这项研究中,使用了剑桥多程流变仪[Coventry,K. D.和M. R. Mackley,J. Rheol。 52,401–415(2008)]用于针对不同的聚乙烯熔体在交叉槽几何形状中产生平面伸长流。将实验结果与使用多模式Pompom本构方程的有限元流模拟进行了比较。根据流动引起的应力双折射和由流场的数值计算确定的应变率,计算出驻点处的稳态伸长粘度。我们将此技术应用于各种不同的支链高密度和低密度聚乙烯熔体。这既证明了该技术的有效性,又表明了复杂流中的应力分布如何取决于分子结构。因此,缝隙延伸流变法为参数化LCB熔体的分子本构方程提供了非常有前途的技术。
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