Lee Johnny Ching-Wei, Hong Yu-Tong, Weigandt Katie M, Kelley Elizabeth G, Kong Hyunjoon, Rogers Simon A
Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801.
Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899.
J Rheol (N Y N Y). 2019 Nov;63(6). doi: 10.1122/1.5111358.
Rheological measurements in which the applied stress or strain is oscillated are widely used to interrogate viscoelastic properties due to the independent control over the time scale and length scale afforded by changes in amplitude and frequency. Taking a nontraditional approach, we treat stress-controlled oscillatory tests as creep tests with transiently varying stress and apply an analysis typically used for steady creep and recovery experiments. Defining zero strain as the state prior to external shearing, it is shown that strain responses to small-amplitude oscillatory stressing are naturally shifted from the starting point by an amount proportional to the phase of the applied stress. The phenomenology is experimentally observed with entangled polymerlike micelles and polyethylene oxide solutions. A theory of strain shifting in the steady alternating state is provided based on recovery rheology, where differences between total strain and recoverable strains are acknowledged. User-controlled variables, such as the amplitude of the stress, the angular frequency, and the phase of the stress, as well a lone material parameter, the zero-shear viscosity, are shown to dictate the amount of shifting. A rapid and efficient approach of determining the zero-shear viscosity is, therefore, presented. We investigate the microstructural evolution via small-angle neutron scattering when strain shifting appears. The microscopic orientation is shown to correlate to the recoverable strain independent of the shifting. Additional measurements are carried out on collagen, pluronic-hyaluronic acid, alginate gels, and polystyrene melts to show the generic nature of the strain shift phenomenon. In addition, we demonstrate that the strain-shift knowledge can be applied to determine the horizontal shift factor in time-temperature superposition, free of any numerical fitting procedures.
施加应力或应变呈振荡形式的流变学测量方法被广泛用于研究粘弹性特性，这是因为通过改变振幅和频率能够独立控制时间尺度和长度尺度。我们采用一种非传统方法，将应力控制的振荡测试视为应力瞬变的蠕变测试，并应用通常用于稳态蠕变和恢复实验的分析方法。将零应变定义为外部剪切之前的状态，结果表明，对应于小振幅振荡应力的应变响应会自然地从起始点偏移，偏移量与所施加应力的相位成正比。这种现象在缠结的聚合物状胶束和聚环氧乙烷溶液中通过实验观察到。基于恢复流变学提供了一种稳态交变状态下应变偏移的理论，其中考虑了总应变和可恢复应变之间的差异。结果表明，用户控制的变量，如应力振幅、角频率和应力相位，以及唯一的材料参数零剪切粘度，决定了偏移量。因此，提出了一种快速有效的确定零剪切粘度的方法。当出现应变偏移时，我们通过小角中子散射研究微观结构演变。结果表明，微观取向与可恢复应变相关，与偏移无关。我们还对胶原蛋白、普朗尼克 - 透明质酸、藻酸盐凝胶和聚苯乙烯熔体进行了额外测量，以展示应变偏移现象的普遍性。此外，我们证明，应变偏移知识可用于确定时间 - 温度叠加中的水平偏移因子，无需任何数值拟合程序。
