We developed a high-sensitive apparatus for oscillatory flow birefringence measurement in a co-cylindrical geometry. The apparatus was comprised of a conventional rheometer, an Argon ion laser, and a lock-in amplifier. The laser was irradiated to samples under sinusoidal strains, and transmitted light intensities were analyzed using the lock-in amplifier to evaluate flow birefringence. TTL signals, which were transformed from sinusoidal strains using a comparator, were used as reference signals for lock-on detection at low frequencies. First, phase shifts between the true strain and TTL reference signal generated by the comparator were calibrated. Next, reliability of the apparatus was assessed using a wormlike micellar solution, whose flow birefringence behavior are well known. Finally, sensitivity of the birefringence measurement was checked using a low birefringent cellulose nanofiber (CNF) dispersion. Flow birefringence behavior of the CNF dispersion became measurable thanks to a long optical pass length of the geometry. The apparatus will be a strong tool to reveal dynamics of low birefringent solutions.
Dynamic viscoelasticity and dynamic birefringence of a microgel system were investigated around the liquid–solid transition concentration to clarify the molecular origin of the viscoelastic response of the microgel system. The complex modulus showed viscoelastic liquid-like behavior at concentrations, c, below a threshold c jamming for random close packing of the microgels, whereas viscoelastic solid-like behavior at c > c jamming. The imaginary part of the complex strain-optical coefficient changed its sign with increasing angular frequency ω in a liquid-like regime, suggesting that the stress and birefringence involved three relaxation mechanisms. Utilizing the stress-optical rule, SOR, for each relaxation process, the complex shear modulus at c < c jamming was separated into the orientational stress of network strands in temporary contacted microgels, deformation of particle, and the redistribution process, from high frequencies in this order. On the other hand, in a solid-like regime at c > c jamming, the ordinary SOR held well with a single stress-optical coefficient, C, implying that only one stress origin is dominant, which can be attributed to the orientational stress of densely packed microgels in permanent contact. The strain-optical coefficients evaluated using the Onuki–Doi theory reproduced the measurements qualitatively and supported these assignments.
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