Various novel mathematical approaches to analyze the tensile properties and molecular structure in polymeric solids are introduced. The failure behavior such as yielding and necking is treated as a phase modification resulting from an elemental catastrophe phenomenon. The ductile fracture behavior under tension is characterized by a stochastic Ito-process where the probability density functions of fracture time, strength, and toughness followed Gaussian statistics. In addition, a grath-theoretical approach is applied to give the radius of gyration and dynamics of polymer molecules which is a key parameter in morphology formation being directly related to mechanical failure and fracture.
Viscosity is a fundamental parameter in fluid dynamics and the only physical property for determining the relationship between stress and strain, in other words, force and deformation. In addition, it may be a measure of strength of molecular motions and their interactions. As for the commercially available viscometers or rheometers, they still have problems about usable range of the minimum torque, condition control of measurement environment, time-consuming work of exchanging sample, and so on. Furthermore, it remains difficulty to clear which mode of molecular motion represents the measured data. Considering the slow progress of technique for solving these problems, I have developed many innovative systems to accurately measure and/or directly observe the rheological properties. In this paper, I introduce the newly developed systems, and show the remarkable results.
We investigated the elongational flows of the weakly entangled linear polymer melt using a coarse- grained molecular dynamics simulation. We extended the uniform extensional flow (UEF) method developed by Nicholson and Rutledge (D. A.Nicholson and G. C. Rutledge, J. Chem. Phys., 145 244903 (2016)) for application to Langevin dynamics. We succeeded in observing the elongational viscosity of the weakly entangled linear polymer melt from the equilibrium state to the steady state using the extended UEF method, whereas the conventional rectangular parallelepiped shape technique for extensional flows has failed to do so for over 20 years.
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.
Deformation characteristics of micro spherical hydrogels in single state have been examined in recovery processes from elongated states. The micro gel particles with core-shell internal structure and without membrane on its surface were dyed and tested. Deformability of two kinds of the gel particles with different cross-link density in the shell layer were evaluated. The size of these particles with low degree of cross-linking were reduced by the dyeing process. Strain recovery process of the micro gel particles were obtained with a high speed camera and approximated exponential recovery curves in particle height were fitted. Moreover, a characteristic time is defined with the formula. The characteristic times of the micro gel particles with low degree of cross-linking in the shell layer were relatively shorter than those of particles with high degree. It is considered that the structure of the shell layer would be changed by dyeing processes. However, it is revealed that the characteristic time calculated in strain recovery processes are useful to feature the deformability of the micro gels.
Poly(vinyl alcohol) (PVA) hydrogels were prepared by the repeated freezing and thawing cycles, and creep behavior of the gels was examined. In the gel preparation, the salt K2CO3 was introduced to obtain the clear gel structure. By the creep we also prepared the condensed PVA gels with unidirectional structure. The mechanical anisotropy was observed for the unidirectional gel.