Variation of complex shear modulus of glassy poly(methyl methacrylate) (PMMA) was monitored during uniaxial stretching to investigate the frequency dispersion of strain-induced nonlinear relaxation. With increasing strain εn, the storage modulus G' decreased to a steady value appearing at post-yield range of strain accompanied by a marked increase of the loss modulus G", indicating that glassy structures changed into more unstable ones due to stretching. The variation of the moduli was more remarkable when the timescale of observation, i.e. frequency ƒ of dynamic measurement, was closer to that of deformation, i.e. strain rate εn. Increment of G" as a function of εn was identical independently of εn when observed at a fixed condition of α = εn/ƒ. This observation indicates that β relaxation is not altered by imposition of large strain and that the frequency dispersion of the nonlinear stress relaxation is determined by relative distance from the timescale of deformation and the amount of imposed strain. Relationships between ΔG', decrement of G', and εn were not superposable at a fixed α, because of negative εn dependence of ΔG' in the post-yield regime. This presumably shows that strain aging, structural relaxation of the strain-induced unstable glassy structures under finite stress, occurs during deformation.
Chitosan (CS) and poly(ethylene oxide) (PEO) are miscible with each other, because of the hydrogen bonding between CS and PEO that becomes stoichiometric at the PEO content of wPEO = 20 wt%. This study analyzed the previously measured dielectric data of this stoichiometric CS/PEO blend being doped with two kinds of salt, potassium sulfate (K2SO4) and calcium sulfate (CaSO4). Specifically, the casting solvent, acetic acid, was intentionally left in the blend to an amount almost equimolar to K and Ca, so that the doped blend contained two types of cations, H+ and K+ or Ca2+. The target of this study was to examine a synergetic effect of H+ and the metal cation, if any, on the dielectric behavior and ionic conduction. Comparison of the complex dielectric permittivity ε* of the K2SO4-doped and neat (undoped) blends suggested that K+ largely increased ε* by governing the electrode polarization at low frequencies (ω) and the hopping conduction at high ω. In contrast, doping of CaSO4 considerably decreased ε* of the blend at low ω, even though the total concentration of the cationic species (H+ and Ca2+) was larger for the doped blend. This unusual, anti-synergetic effect was discussed in relation to constraint for the segmental motion of CS due to Ca2+ and the resulting suppression of segmental-motion-aided fast hopping and slow diffusion of H+.
The objective of this work is to study the peristaltic motion of an incompressible Giesekus fluid in a circular cylindrical tube. The problem is modeled in a fixed frame of reference and then transformed into a frame that moves with the wave speed. The most widely taken assumptions of long wavelength and low Reynolds number are applied in the wave frame. Both exact and approximate solutions of governing equation for stream function are obtained at each cross-section by solving nonlinear algebraic equations. The comparison of the two solutions is presented graphically. The exact solution is then used to analyze the effects of parameters of interest on velocity profile, pressure rise per wavelength and trapping phenomenon. The results disclose that the magnitude of velocity increases in the middle-most region of the tube whereas it decreases in the vicinity of wall with increasing the Giesekus model parameters. It is also observed that the size and circulation of the trapped bolus decrease with increasing the Giesekus model parameters. Moreover, much greater mixing is realized in a plane channel than in a circular tube.
Flow-induced orientational changes in a 0.5 wt % Xanthan gum solution in planar channels with an abrupt expansion were examined by measurement of the flow-induced birefringence and velocity fields. Three kinds of 1:4 abrupt expansion channels with different cross-sectional aspect ratios (of 1, 2, and 5) in the upstream region were tested in the experiments. A similar channel with a different size but the same aspect ratio of 1 was tested for comparison. Remarkable differences were found in the development of flow-induced orientation near the centerline after the abrupt expansion, depending on the aspect ratio. In the cases of the aspect ratios of 1 and 2, the polymer molecules were temporally aligned perpendicular to the flow direction due to negative elongational flows generated after the abrupt expansion. In contrast, similar phenomena were not observed with an aspect ratio of 5. From these results, the typical flow-induced orientational changes in polymers in the planar channels just after the abrupt expansion were found to be substantially affected by the cross-sectional aspect ratio in the upstream region.