流变技术在高分子表征中的应用:如何正确地进行剪切流变测试

Toward Correct Measurements of Shear Rheometry

流变学是高分子加工和应用的重要基础,流变学表征对于深入理解高分子流动行为非常重要,获取的流变参数可用于指导高分子加工.本文首先总结了剪切流变测试中的基本假设:(1)设置的应变施加在样品上,(2)应力来源于样品自身的响应和(3)施加的流场为纯粹的剪切流场;之后具体阐述了这些假设失效的情形和所导致的常见的实验错误;最后,通过结合一些实验实例具体说明如何培养良好的测试习惯和获得可靠的测试结果.

Rheology is an important tool for characterizing polymeric materials.The knowledge of polymer dynamics obtained from rheological measurements can guide the processing of polymeric materials.Nevertheless,it is a challenging task to conduct precise measurements on shear rheometers.First,we summarize the basic assumptions of rheological measurements:(1)the applied strain is on the sample,(2)the stress reflects sample’s own stress response,and(3)the flow field is purely the shear flow.Second,we highlight the conditions under which the above assumptions become invalid,which leads to incorrect measurements.For example,the first assumption is violated when compliance of the equipment or the wall-slip becomes non-negligible,the second assumption is violated when either inertia of the equipment,that of the sample,or the interfacial tension contributes non-negligibly to the overall torque,and the third assumption is violated when there is the secondary flow,shear-banding,or edge fracture.We show some examples of the incorrect measurements owing to these violations of the assumptions.Finally,we stress the importance of good experimental practice in rheological experiments.