  The viscoelastic behavior of liquid polymers changes depending on the
microstructure of the polymer chains. To investigate the unsteady structural
changes induced by polymer chain morphology under stress, we develop a simple
rheo-optical technique combining a high-speed polarization camera and
dripping-onto-substrate capillary breakup extensional rheometry (CaBER-DoS),
which uniaxially extends and aligns the polymer along the stress direction.
Using this technique, the viscoelastic behavior and birefringence field of an
extending liquid polymer are simultaneously measured. Unlike the conditions
that occur under shear-stress loading with a rotational rheometer, this
technique provides a linear relation between birefringence and extensional
stress. The measurements reflect the structural change in a flexible polymer (a
mixed solution of PEO and CNCs) in the temporal evolution of the birefringence
and orientation angle, which is a measure of the orientation state and
coil-stretch transition of a polymer chain. Remarkably, within the
elasto-capillary regime, the measured birefringence remains constant with a
constant orientation state, while the Weissenberg number gradually increases,
which provides experimental evidence of the coil-stretch transition of the
polymer chain under constant extensional stress loading.
