Steady shear flows of liquid n-butane were simulated at T=150K and T=300K by an isothermal nonequilibrium molecular dynamics. In the model molecule of n-butane, the bond lengths are fixed by SHAKE algorithm, and the bond angle and dihedral angle are controlled by the corresponding potentials respectively. The liquids exhibit shear-thinning in viscosity at high shear rates and the shear-thinning becomes more remarkable with lowering the temperature. The shear flow also affects the molecular conformation; at the high shear rates, the distribution of the bond angle is more concentrated around θ0 where the bond angle bending potential is minimum, and the trans configuration becomes increasingly populated. It is suggested that the non-Newtonian viscosity is caused by both the flow-induced orientation and the change of molecular conformation with shear rate. Comparison of our results with those obtained by Morriss, Davies, and Evans [J. Chem. Phys. 94, 7420 (1991)] indicates that the shear-thickening observed in the latter work is caused by the increase in the gauche population at high shear rates.
Flow visualizations and velocity measurements for entry flows of shear-thinning aqueous solutions of polyacrylamide through three kinds of multi-hole contractions have been carried out using the tracer method and LDV technique. In these experiments effects of the number of contractions, distance between contractions and rheology of solutions on the flow structures have been studied. For the multi-hole contractions square arrays of 9 holes and 25 holes were used. In the steady flow regime dimensionless lip vortex length is almost the same for all multi-hole contractions. Swirling-like vortices are also observed for both three-by-three and five-by-five contractions with dimensionless pitch 4, while two-dimensional lip vortices are observed for the three-by-three contraction with dimensionless pitch 8. It is found from LDV measurements that periodic oscillations of the main flow occur at the onset of unsteady flow for all multi-hole contractions and the frequency of the oscillation increases as the Weissenberg number increases. For multi-hole contractions, flow interaction through each contraction is very important factor which decides both steady and unsteady flow structures.
The physical aging behavior of amorphous poly (ethylene terephthalate) (PET) has been studied as a function of aging time and temperature. Volume recovery measurements are performed for samples quenched from an equilibrium temperature of 80°C to aging temperatures between 20°C and 75°C. The isothermal volume curves are plotted in the form of normalized fractional free volume (=(V-V∞)/V∞, where V∞ is the equibilium value of specific volume V) versus logarithmic aging time. All the data can be superimposed by appropriate horizontal and vertical shifts of the individual curves. The horizontal shift factors show the WLF type of temperature dependence, while the vertical ones correspond to the volume change due to thermal expansion. The specific volume V∞conf of the fully relaxed glass estimated from the reduced master curve is different from that calculated from linear extrapolation of the equilibrium specific volume V∞ in the liqiud state to temperature below the glass transition temperature (Tg). The equilibrium Tg of the amorphous PET is determined to be 57°C from the estimated value of V∞conf versus temperature curve.
Brownian dynamics (BD) simulations of the steady and transient rheological behaviors of FENE dumbbell model for dilute polymer solutions in shear flow are reported. The simulations include examination of the effects of distributions of maximum allowable extension (FENE parameter b) and of relaxation time parameter α. Shear-thinning behavior of shear viscosity and the first normal stress coefficient, overshoot phenomenon in a start-up shear flow and stress relaxation for cessation of shear flow are also confirmed for distribution of b or α, which can be often observed in polymer solutions. Shear-thinning and overshoot behaviors are due to the non-linearity of FENE spring. Furthermore, it is found that the distribution of b has little influence on shear flow properties while the distribution of α can affect both the steady and transient shear flow properties under the distribution condition used in the simulations. In conclusion, BD simulation is a good tool to easily predict the effect of the distribution of maximum extension and relaxation time of dumbbells on the rheological behavior, however it can not calculate precisely the rheological properties in low shear rate region of λ0γ<0.1. This situation remains as a future task.
The effect of physical aging on stress and birefringence relaxation behaviors in polycarbonate (PC) glass was investigated. The birefringence relaxed monotonically at the glass transition-to-rubbery plateau region for the non-aged PC, suggesting that chain orientation relaxes in a single relaxation process in this region. On the other hand, stepwise relaxation behavior was observed in birefringence for the aged PC. The birefringence relaxation curves at various temperatures for the aged PC could not be superimposed by horizontal shift. The complicated relaxation behavior could be caused not only by the physical aging but also by the rejuvenation of enthalpy during the relaxation measurement.
The purpose of this investigation is to evaluate the dynamic viscoelastic properties of artificial denture teeth using a viscoelastic spectrometer. Specimens were porcelain and resin teeth mounted in denture base. The measurement of viscoelastic frequency spectrum based on the fast fourier transform of displacement to applied random forces was analyzed using spectrum analyzer. The materials was tested under two dynamic conditions. One was a static load of 10.0N and a dynamic peak load of 3.0N. Second was a static load of 20.0N and a dynamic peak load of 5.0N. The dynamic stiffness of the porcelain specimens measured under each condition was significantly (P<.001) higher than that of the resin specimens. The phase lag of the porcelain specimens measured under each condition was significantly (P<.001) lower than that of the resin specimens. The results of this study revealed that the resin teeth had greater toughness and higher shock-absorbing ability than the porcelain teeth. It was indicated that the porcelain teeth were more brittle material than the resin teeth.
Dynamic viscoelasticity of ethylene-co-methacrylic acid copolymer (EMAA) and its ionomers (EMAA-Zn and EMAA-Na) has been measured in the melt state. The time-temperature superposition principle is applicable to data of angular frequency dependences of storage modulus (G') and loss modulus (G") for EMAA very well from 140°C to 200°C. On the other hand, data for ionomers give two separated master curves with different temperature dependences of shift factor, one from the data obtained in the range of 140°C-170°C and the other in the range of 180°C-200°C. Even at 200°C, the moduli G' and G" are still much higher than those of EMAA. These results indicate that a structure change in EMAA ionomers occurs between 170°C and 180°C. The free volume fraction (fr) and the thermal expansion coefficient (αf) in the range of 180°C-200°C are larger than those in the range of 140°C-170°C for EMAA ionomers. This implies that in higher temperature range, some hydrocarbon chains depart from cross-links of ion aggregates and move much more freely.
Stress Relaxation behavior under large step shear strain γ for ethylene-co-methacrylic acid copolymer (EMAA) and its ionomers (EMAA-Zn and EMAA-Na) was studied in the melt state. EMAA ionomers show higher relaxation modulus G(t, γ) and slower relaxation rate than EMAA. At 140°C the strain dependence of G (t, γ) for EMAA ionomers is very strong. The corresponding damping function h(γ) decreases with γ in two steps around γ=0.6 and γ=4. This suggests that the ion aggregates become small in two steps. However, at 200°C h(γ) monotonically decreases with γ, indicating that the ion aggregates are already small and become smaller monotonically with increasing γ. Compared with EMAA-Na, EMAA-Zn has higher stress relaxation modulus because of strong interactions due to cooperation between the carboxyl acid group and Zn2+.