Department of Chemistry, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397, Japan.
Langmuir. 2011 Jun 21;27(12):7400-9. doi: 10.1021/la104826s. Epub 2011 May 16.
In a previous paper, we reported for the first time the lamellar-to-onion transition with increasing temperature at around 67 °C under a constant shear rate (0.3-10 s(-1)) in a nonionic surfactant C(16)E(7)/water system. In this study, the first temperature-shear rate diagram has been constructed in a wider range of shear rate (0.05-30 s(-1)) than in our previous study based on the temperature dependence of the shear stress at constant shear rate. The results suggest that the critical temperature above which the transition begins does not depend on the shear rate very much, although it takes a very shallow minimum. Then we have performed simultaneous measurements of small-angle X-ray scattering/shear stress (rheo-SAXS) with a stepwise increase in temperature of 0.1 K per 15 min at a constant shear rate of 3 s(-1) near the transition temperature. When the temperature exceeds 67 °C, just before the increase in the shear stress, the intensity of the Bragg peak for the velocity gradient direction (approximately proportional to the number of lamellae with their normal along this direction) is suddenly increased. As the temperature increases by 0.2 K, the shear stress begins to increase. At the same time, the peak intensity in the velocity gradient direction rapidly decreases and instead the intensity in the neutral direction increases. As the temperature increases further, the intensities in both the neutral and gradient directions decrease whereas the intensity in the flow direction increases, corresponding to the formation of onions. We have also performed rheo-SAXS experiments with a stepwise increase in shear rate at 72 °C. The sequence of the change in the intensity in each direction is almost the same in the temperature scan experiments at constant shear rate, suggesting that the transition mechanisms along these two paths are similar. The abrupt enhancement of the lamellar orientation with the layer normal along the velocity gradient direction just before the transition is the first finding and strongly supports the coherent buckling mechanism in the lamellar-to-onion transition proposed by Zilman and Granek (Zilman, A. G.; Granek, R. Eur. Phys. J. B 1999, 11, 593).
在之前的一篇论文中，我们首次报道了在恒定剪切速率（0.3-10 s(-1))下，非离子表面活性剂 C(16)E(7)/水体系中温度升高至约 67°C 时的层状-洋葱状转变。在本研究中，我们构建了第一张更宽剪切速率范围（0.05-30 s(-1))的温度-剪切速率图，该范围比我们之前基于恒定剪切速率下剪切应力随温度的变化关系的研究范围更广。结果表明，开始发生转变的临界温度对剪切速率的依赖性不大，尽管它呈现出非常浅的最小值。然后，我们在接近转变温度时，以 0.1 K/15 min 的升温速率，在恒定剪切速率为 3 s(-1)下，同时进行小角 X 射线散射/剪切应力（流变 SAXS）的分步测量。当温度超过 67°C 时，即在剪切应力增加之前，速度梯度方向的布拉格峰强度（大致与沿该方向的层片数量成正比）突然增加。随着温度升高 0.2 K，剪切应力开始增加。与此同时，速度梯度方向的峰强度迅速降低，而中性方向的强度增加。随着温度的进一步升高，中性和梯度方向的强度都降低，而流动方向的强度增加，对应于洋葱的形成。我们还在 72°C 时进行了逐步增加剪切速率的流变 SAXS 实验。在恒定剪切速率的温度扫描实验中，各方向强度变化的顺序几乎相同，这表明这两种途径的转变机制相似。在转变之前，层片取向突然增强，其层法线沿速度梯度方向，这是第一个发现，强烈支持了 Zilman 和 Granek 提出的层状-洋葱状转变中的相干屈曲机制（Zilman, A. G.; Granek, R. Eur. Phys. J. B 1999, 11, 593）。
