Yuichi Masubuchi

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...

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.

Entanglement molecular weight and frequency response of sliplink networks

Brownian dynamics simulations of the linear viscoelastic response of entangled polymers have been performed, and compared quantitatively with some existing solution data at a fixed concentration and variable molecular weight. The model is a three-dimensional network where the nodes are sliplinks connecting chains in pair. The simulations make use of Langevin equations both for the node motion in space, and for the one-dimensional monomer sliding through sliplinks. Comparison with data is very satisfactory, but the molecular weight between entanglements that emerges from the model is unconventionally small.

`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.

Crystallization kinetics of polypropylene under high pressure and steady shear flow

Abstract We investigate the simultaneous effect of pressure P and shear flow on the crystallization process of polypropylene using a modified pressure–volume–temperature measurement system that has been developed in our laboratory. We focus on the inverse of half-conversion time 1/ t 1/2 that is defined by the inverse of the time when the relative crystallinity of system reaches to one half. It is found that the pressure dependence of the quantity 1/ t 1/2 can be expressed by a shifted degree of super-cooling Δ T ( P ) by pressure. We also find that 1/t 1/2 (T,P, γ ) under both pressure and shear flow can be described by a sum of 1/t 1/2 (T,P, γ =0) under no shear and a linearly increasing term with shear rate γ .

Spontaneous self-assembly process for threadlike micelles

More than 100 micros dissipative particle dynamics simulations were carried out to investigate the spontaneous formation process of threadlike micelles from the random configuration for surfactant molecules. Stable spherical micelles were formed during the earlier stage. These spherical micelles fused to each other and grew into rodlike and threadlike micelles during the later stage. The length and radius of a micelle were estimated by tracing the backbone positions and the distance between the head group particles and the backbone of the micelles, respectively. The ratio of the largest to the smallest principal moments of inertia for each micelle was calculated as the micelle shape.

Rheological Properties of Native Silk Fibroins from Domestic and Wild Silkworms, and Flow Analysis in Each Spinneret by a Finite Element Method

Silkworms can produce strong and tough fibers at room temperature and from an aqueous solution. Therefore, it seems useful to study the mechanism of fiber formation by silkworms for development of synthetic polymers with excellent mechanical properties. The rheological behaviors of native silk dopes stored in the silk glands of Bombyx mori and Samia cynthia ricini were clarified, and flow simulations of the dopes in each spinneret were performed with a Finite Element Method. Dynamic viscoelastic measurements revealed that silk fibroin stored in silk glands forms a transient network at room temperature, and that the molecular weight for the network node corresponds to the molecular weight of a heterodimer of H-chain and L-chain (B. mori) and a homodimer of H-chains (S. c. ricini), respectively. Also, each dope exhibited zero-shear viscosity and then shear thinning like polymer melts. In addition, shear thickening due to flow-induced crystallization was observed. The critical shear rate for crystallization of B. mori dopes was smaller than that of S. c. ricini dopes. From the flow simulation, it is suggested that domestic and wild silkworms are able to crystallize the dopes in the stiff plate region by controlling shear rate using the same magnitude of extrusion pressure despite differences in rheological properties.

Multi-chain slip-spring model for entangled polymer dynamics

It has been established that entangled polymer dynamics can be reasonably described by single chain models such as tube and slip-link models. Although the entanglement effect is a result of hard-core interaction between chains, linkage between the single chain models and the real multi-chain system has not been established yet. In this study, we propose a multi-chain slip-spring model where bead-spring chains are dispersed in space and connected by slip-springs inspired by the single chain slip-spring model [A. E. Likhtman, Macromolecules 38, 6128 (2005)]. In this model the entanglement effect is replaced by the slip-springs, not by the hard-core interaction between beads so that this model is located in the niche between conventional multi-chain simulations and single chain models. The set of state variables are the position of beads and the connectivity (indices) of the slip-springs between beads. The dynamics of the system is described by the time evolution equation and stochastic transition dynamics for these variables. We propose a simple model which is based on the well-defined total free-energy and detailed balance condition. The free energy in our model contains a repulsive interaction between beads, which compensate the attractive interaction artificially generated by the slip-springs. The explicit expression of linear relaxation modulus is also derived by the linear response theory. We also propose a possible numerical scheme to perform simulations. Simulations reproduced expected bead number dependence in transitional regime between Rouse and entangled dynamics for the chain structure, the central bead diffusion, and the linear relaxation modulus.

Development of shear flow thermal rheometer for direct measurement of crystallization fraction of polymer melts under shear deformation

Recently viscoelastic measurement has received considerable attention as a complementary method to estimate the crystallization dynamics under shear. However, because of the complex relationship between the crystallization fraction and dynamic mechanical results, it has been difficult to obtain accurate crystallization fraction under shear. To conduct the direct determination of crystallization fraction under shear, the shear flow thermal rheometer (SFTR) having differential thermal analysis (DTA) has been successfully developed using a rotational shear rheometer. Two thermocouples to measure the reference and the sample temperatures were installed at each optimal position as DTA equipment. Two sheets of polyetheretherketone (PEEK) were put on two aluminum plates to prevent heat transfer between the sample and the aluminum plates for accurate measurement of DTA. The influences of using PEEK, instead of an aluminum plate and the thermocouple on rheology results were confirmed to be negligible. In the SFTR, shear strain was given from the lower plate. Both stress and differential temperature responses were simultaneously detected by the upper plate and thermocouples, respectively. The advantage of the SFTR over conventional rheometers was that the crystallinity fraction was able to be directly determined from DTA. In non-isothermal and isothermal preliminary experiments of iso-polypropylene, it was demonstrated that the SFTR provided two kinds of data by the simultaneous measurement of DTA and viscosity. The crystallinity fraction under shear was directly estimated from DTA in the SFTR.

Thermal analysis of shear induced crystallization by the shear flow thermal rheometer: isothermal crystallization of polypropylene

Abstract In situ differential thermal analysis during steady shear induced crystallization of polypropylene were performed by the shear flow thermal rheometer. Time development of crystallinity was obtained and analyzed by the Avrami equation; Xc(t)=1−exp{−k(t−t0)n}, where Xc is relative crystallinity, k the kinetic constant, t0 the induction time and n the Avrami exponent. With an increase of shear rate, k decreases slightly and t0 decreases drastically. The increase of n was also observed but the magnitude of the change was insignificant. The observed value of n suggested the spherulite growth and inhomogeneous nucleation of the crystal even under shear. Against the crystallization temperature, the parameters behaved similar to those under quiescent conditions except that t0 increases extremely with the increase in t0.