Kiyohito Koyama

Brownian simulations of a network of reptating primitive chains

A new model for Brownian dynamics simulations of entangled polymeric liquids is proposed here. Chains are coarse grained at the level of segments between consecutive entanglements; hence, the system is in fact a network of primitive chains. The model incorporates not only the “individual” mechanisms of reptation and tube length fluctuation, but also collective contributions arising from the 3D network structure of the entangled system, such as constraint release. Chain coupling is achieved by fulfilling force balance on the entanglement nodes. The Langevin equation for the nodes contains both the tension in the chain segments emanating from the node and an osmotic force arising from density fluctuations. Entanglements are modeled as slip links, each connecting two chain strands. The motion of monomers through slip links, which ultimately generates reptation as well as tube length fluctuations, is also described by a suitable Langevin equation. Creation and release of entanglements is controlled by the num...

High-melt-strength polypropylene with electron beam irradiation in the presence of polyfunctional monomers

High-melt-strength polypropylene (PP) was achieved with irradiation by an electron beam generated from an accelerator in the presence of polyfunctional monomers (PFM). Among 16 PFMs, the relatively shorter molecular chain bifunctional monomers such as 1,4-butanediol diacrylate (BDDA) and 1,6-hexanediol diacrylate (HDDA) were the most effective for enhancing the melt strength of PP. The concentration and dose of the HDDA to obtain the high melt strength PP in irradiation under nitrogen gas atmosphere were 1.5 mmol/100 g PP and 1 kGy, respectively. DSC measurement and dynamic mechanical analysis showed that the thermal behavior of the high-melt-strength PP was different from that of the original PP. Crystallinity and crystallization temperature during cooling after heating were lower and higher in high melt strength PP than original PP, respectively. Elongational viscosity at 180°C of the high-melt-strength PP showed a remarkable increase at a certain elongational time with constant strain rate, demonstrating the typical property of high-melt-strength samples. This implies that a few higher molecular chains of PP, formed by intermolecular combination of its chain by HDDA in irradiation, give higher melt strength to induce entanglement of molecular chains.

Nonlinear dielectric constant and ferroelectric‐to‐paraelectric phase transition in copolymers of vinylidene fluoride and trifluoroethylene

Second‐ and third‐order dielectric constant e2 and e3 of three copolymers of vinylidene fluoride and trifluoroethylene were measured over a temperature range from 20 °C to higher than the ferroelectric‐to‐paraelectric phase transition temperature Tc . The thermodynamical theory predicts that e2 is negative below Tc  and vanishes at Tc , whereas, for the second‐order transition, e3 is positive below Tc but turns to negative at Tc and, for the first‐order transition, e3 does not change its sign below and above Tc . According to this criterion, it is concluded that the transition is of the second order for a copolymer with vinylidene fluoride fraction of 52 mole %, but of the first order for 65 and 73 mole % copolymers. This conclusion can explain the difference in slope of e2 versus remanent polarization among the copolymers.

Effect of chain structure on the melt rheology of modified polypropylene

The effect of molecular structure of polypropylene (PP) on the melt rheological properties were investigated for electron irradiated polymer and di-2-ethylhexyl peroxy dicarbonate (EHPC)-treated polymer. The modifications were examined in terms of the rheological behaviors, molecular weight distribution, and the degree of branching. The high melt strength PP was obtained by irradiating with 50 and 80 kGy and adding EHPC. The modified PPs showed the strain hardening in the uniaxial elongational viscosity, though the linear elongational viscosity was lower than that of the unmodified PP. Low angle laser light-scattering measurements of the modified PPs showed the interesting results; high irradiation doses such as 50 and 80 kGy caused higher molecular weight chains branching. Nevertheless, the long branching chains were not detected for the EHPC modified PP, which also showed the strain hardening in uniaxial elongational flow. In this article, the relation between chain structure and rheological properties is discussed.

Mixing performance experiments in impeller stirred tanks subjected to unsteady rotational speeds

Two unsteady stirring approaches, co-reverse periodic rotation and time-periodic fluctuation of rotational speed, are adopted to enhance global mixing in a stirred tank with high viscosity materials. The relations of complete decolorization time with Re and with mean energy dissipation per unit volumetric weight are used as quantitative methods to compare the mixing performance under an unsteady stirring with that under a steady one. The experimental results show that mixing time can be significantly reduced in stirred tanks when unsteady stirring approaches are used. However, it is also found experimentally that for the method of co-reverse periodic rotation, only when the Re is larger than a critical value, can significant enhancements in mixing be obtained. In contrast to this, for the case of time-periodic RPM fluctuations, such a critical value is not as easy to define. The higher the frequency of periodic co-reverse rotation and the larger the amplitude of time-periodic fluctuation, the shorter is the mixing time. In both cases, after Re becomes larger than a certain value, further increases in Re yields relatively small returns. The experimental results are presented and discussed with the help of photographs taken during the mixing process.

Fourier-transform rheology under medium amplitude oscillatory shear for linear and branched polymer melts

Nonlinear response of linear and branched polymers has been investigated under medium strain amplitude oscillatory shear (strain amplitude range from 10% to 100%) with Fourier-transform rheology. A power law relationship was found between the relative third intensity (I3∕I1), which is an indicator of nonlinearity, and the strain amplitude at low and medium strain amplitudes. On a log-log plot, the intercept and slope of I3∕I1 were investigated at different excitation frequencies and temperatures. Simulation results with three different constitutive equations [Giesekus, exponential Phan-Thien Tanner (E-PTT), pom-pom model] were also compared. Experimental results show that the intercept was affected by the excitation frequency and temperature, and the slope of I3∕I1 for linear polymer remained constant regardless of molecular weight, molecular weight distribution, and excitation frequency in accordance with the predictions of the constitutive equations (Giesekus and E-PTT). It should be noted that the slop...

Elongational viscosity at a constant elongational strain rate of polypropylene melt

Abstract The measurement of elongational viscosity has been carried out with an improved elongational rheometer, sensitive to small elongational forces and measuring simultaneously the diameter of elongating samples. Force distribution was evaluated for various conditions. The method of reduced variables, successful in linear viscoelastic properties, can be applied to the temperature dependence of elongational viscosity only at small elongational strain rate. For large elongational strain rates, the elongational viscosity was found to be non-linear and a new method characterizing non-linearity was proposed. The non-linearity parameter in elongational viscosity was independent of elongational strain rate and almost independent of temperature. Non-linearity in elongational viscosity is discussed in terms of a structural change in the polymer melt.

Flow-induced whisker orientation and viscosity for molten composite systems in a uniaxial elongational flow field

Abstract The elongational viscosity of polystyrenepotassium titanate whisker composite melt was estimated by using a Meissner-type uniaxial elongational rheometer. The time dependence of flow-induced whisker orientation in a uniaxial elongational flow field was analysed in terms of the average polar angle with respect to the flow direction by wide-angle X-ray diffraction, in order to study the influence of whisker orientation on the elongational viscosity of composite melts. The whiskers prevented the strain-hardening phenomenon in elongational flow for polystyrene. The elongational viscosity was almost independent of the orientational change of the whiskers. The orientation of the whiskers was dependent on strain only. To understand this phenomenon, whisker orientation was discussed in comparison with Jeffery theory. The tendencies of the average polar angle were similar to the tendency of Jeffery theory in a uniaxial elongational flow field.

`Positive' and `negative' electro-rheological effect of liquid blends

The electrorheological (ER) effect of urethane-modified polypropylene glycol (UPPG)/dimethylsiloxane (DMS) blends was investigated by rheological measurement and observation of the domain structure under shear flow. It was found that: (i) the cause of an ER effect is in close association with the domain composition under no electric field; the ER effect appears when UPPG forms droplets, while when it forms a continuous phase, the ER effect does not appear; (ii) UPPG droplets elongate and bridge between the electrodes when an electric field is applied and the bridges remain even under the shear; and (iii) the ER effect is positive in the case that the viscosity of UPPG is larger than that of DMS and negative in the case that the viscosity of UPPG is smaller than that of DMS. The cause of both positive and negative ER effects for UPPG/DMS blends is the connection of UPPG between electrodes under the electric field.

Influence of glass beads on the elongational viscosity of polyethylene with anomalous strain rate dependence of the strain-hardening

Abstract The elongational viscosity of HDPE and the same polymer filled with glass beads was measured. The strain-hardening of neat HDPE showed anomalous strain rate dependence; the behaviour became weaker as the strain rate increased. The strain-hardening of the HDPE filled with glass beads weakened with increasing bead contents. Suppression of the strain-hardening by the beads is stronger at high strain rate, where the strain-hardening of neat HDPE is weak. Especially, the strain-hardening did not appear if the bead contents and the strain rate are sufficiently high.