Direct observation of a polymer chain in the vicinity of critical strain rate of coil-stretch transition

众所周知，溶液中的柔性聚合物链在临界应变率ε_c下表现为线圈-拉伸跃迁（CST），作为对拉长流场的响应。通过测量流动诱导双折射作为应变速率的函数来确定ε_c。CST本身是单个分子的事件，而流体双折射平均在溶液中的分子上。已知单个分子在拉伸流场作用下的变形过程取决于每个分子在临界应变率附近的构象和取向。宏观上确定的ε_c值是否真的反映了这种分子特征，即单个分子变形的系综平均值是否真的对应于ε_c，这是未知的。我们将宏观的ε_c值与荧光显微观察到的分子延伸过程测定的值进行比较，以期弄清这一尚未解决的问题。利用光刻法在硅片上制造微交叉槽，在显微镜下产生了细长流场。在一台四辊轧机上进行了宏观观察。荧光显微镜测定的ε_c是流动双折射法测定值的三分之一。这种临界应变率的差异归因于宏观流动双折射法中检测信号的特殊平均程序。

It is well known that a flexible polymer chain in a solution manifests a coil-stretch transition (CST) at a critical strain rate ε_c as a response to an elongational flow field. ε_c has been determined by measuring flow induced birefringence as a function of strain rate. CST itself is an event of individual molecules, while flow birefringence averages over molecules in the solution. Deformation process of individual molecules by the elongational flow field has been known to depend on the conformation and orientation of each molecule in the vicinity of critical strain rate. It is unknown if macroscopically determined ε_c value actually reflects such molecular feature, that is, if an ensemble average of individual molecular deformation actually corresponds to ε_c. We aimed to make it clear this still unsolved question by comparing the macroscopic ε_c value with that determined by the molecular extension process observed using fluorescent micrograph. Elongational flow field under the microscope was generated by a micro-cross-slot fabricated by a photo etching of silicon wafer. Macroscopic observation was performed on a four roller mill. ε_c determined by fluorescent micrograph was one third of that determined by flow birefringence method. This difference in critical strain rate was attributed to a particular averaging procedure of detecting signals in the macroscopic flow birefringence method.