Rheo-Optical Study on Dispersion Systems of Cellulose Nanofibers, Microgels, and Their Mixture

概要

In this thesis, the stress-optical relationship of the viscoelasticity in dispersion systems of TEMPO-oxidized cellulose nanofiber (CNF), the microgel particle (MGP), and their mixture was studied.

In Chapter Ⅱ, the viscoelastic response of CNF dispersions in the entangled regime was investigated. Width of the cellulose nanofiber was firstly determined as 3.46 ±0.2 nm by using UV-VIS-NIR absorption spectroscopy. The critical concentrations for onset of semi-dilute and concentrated regime were evaluated as c1=0.033 kg/m3 and c2=6.56 kg/m3 respectively with the obtained width. The contribution of orientation mode and bending mode were analyzed by rheo-optical measurements, indicating that the birefringence was reduced from the orientational birefringence, probably due to the non-affine deformation of the system resulting from the slippage occurring between CNFs In spite of these non-idealness of deformation, the stress-optical relaxation seemed to hold well and the stress-optical coefficient for the bending mode was successfully determined. Cbend-2.5*10-7 Pa-1, which was approximately 30 times smaller than the stress-optical coefficient for the orientation mode, Cor-7.8*10-6 Pa-1, as shown in Fig.1.

In Chapter Ⅲ , the highly-sensitive apparatus (shown in Fig.2) for oscillatory flow birefringence measurements in a co-cylindrical geometry was developed. A conventional rheometer is equipped with an Argon ion laser, optical train, and a lock-in amplifier for birefringence measurements. The laser was irradiated to sinusoidally oscillating samples in an outer cylinder of the geometry, and amplitudes and phase angle differences of the transmitted light through samples were analyzed using the lock-in amplifier. The phase shift between the Transistor-Transistor Logic signals (nominal strains) and sinusoidal signals (true strains) was calibrated with a half shaded glassy fixture, manifesting phase shit depended on strains as well as frequencies. The reliability of the apparatus was further examined with the CTAB / NaSal solution whose rheo-optical behavior has been clearly researched. It was expected that the dynamic flow birefringence measurement using the high-sensitive apparatus can provide new insights on dynamics of low birefringent polymeric materials which used to be difficult.

In Chapter Ⅳ, the validity of stress optical rule (SOR) for soft deformable colloids was examined. I measured the dynamic birefringence and linear viscoelasticity of MGP dispersions of 0.8 kg/m3 ≤ c ≤ 40 kg/m3. For microgels dispersions in regime of c ≲ cglass, where cglass = 2.16 kg/m3 was the mass concentration corresponding to the maximum of random close packing volume fraction 0.64, the imaginary part of the complex strain optical coefficient, K*, changed its sign with angular frequency while the real part exhibited negative values within whole frequency window. The dynamic modulus G* could be described with three relaxation mechanisms, which were assigned to the reorientation of polymer segments of internal microgel, the form birefringence, and the Brownian stress. This assignment could be supported by the extension of the Doi-Onuki theory to strain-induced birefringence of MGP dispersions. However, in the concentration regime of c ≳ cglass, the SOR held well with one stress-optical coefficient, implying there was only one relaxation mechanism, which could be attributed to the segment orientation of network strands in gel particles. Thus, it was concluded that the rheo-optical research provided a practicable method for the research of microgel particles on a microscopic level.