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

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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.