Models for entangled polymer dynamics can be conveniently thought of as belonging to two major categories - multichain models, such as the Kremer-Grest model simulated using molecular dynamics and single chain models, such as the tube and sliplink models. The single chain models typically assume that each polymer chain moves essentially independently of the other chains in the system. Hence they also assume that the cross-correlations between the different chains make negligible contribution to the various physical quantities (for e.g., the time-dependent relaxation modulus). However, Cao and Likhtman [Phys Rev Lett 104, 207801 (2010)] using molecular dynamics simulations have shown that there exist significant cross-correlation contributions to the time-dependent orientational relaxation function. In this study, the effect of cross-correlation was investigated using the primitive chain network model, a multi-chain sliplink model. Orientational relaxation functions for both the subchain vectors between entanglements and also for the end-to-end vector of whole chain were evaluated. In the case of the subchain relaxation function (and which corresponds to the time-dependent relaxation modulus by the stress-optical rule), the cross-correlation contribution to the total relaxation function appears to progressively increase with time. Further, the ratio of the cross-correlation function to the total relaxation function reaches a maximum value of approximately 40 % in the terminal region. Besides indicating that the cross-correlation contributions to the total relaxation function are non-negligible, this also demonstrates that the relaxation spectrum itself is modified if the cross-correlation contribution is neglected from the total relaxation. Specifically, the cross-correlation contribution affects the relaxation spectrum in the time range where the effect of the constraint release process is expected to be important. On the other hand, the results indicate that effect of cross-correlations on the shape of the relaxation curve is not significant at long times around the terminal region and that the autocorrelation and the total correlation functions can be superimposed by rescaling the unit of modulus. In contrast, the contribution from cross-correlation to the end-to-end relaxation (that corresponds to the dielectric relaxation of type-A polymers) is less significant than that for the subchain relaxation. In fact, no significant difference was found in the prediction of dielectric relaxation between the end-to-end relaxation functions with and without the contribution of the cross-correlations.
We recently developed a new dispersant which can decrease the viscosity of concentrated polyethylene (PE) particle dispersions significantly. This dispersant is a block copolymer constructed with side chain crystalline monomers and solvent compatible monomers (Side Chain Crystalline Block Copolymer : SCCBC). Also the viscosity of this dispersion shows reversible temperature dependence that the viscosity increases with increasing temperature up to almost the original value of the dispersion without the dispersant. This phenomenon is named as “Thermal Rheology” and the fluid is named as “Thermal Rheological (TR) Fluid”. In the previous report, we found this TR Fluid effect depends on the randomness of the copolymer and solvent species qualitatively. In this investigation we researched about copolymer molecular composition and weight dependence on the TR Fluid effects. We also researched about deformation mode dependence. From this research, the TR Fluid effects strongly depend on the molecular weight, especially on the molecular weight of side chain crystalline composition. Also the TR Fluid effects are significant in low shear rate region and dynamic deformation mode.
The non-linear stress relaxation behavior of scarcely entangled polymer chains was investigated with primitive chain network simulations in the transitional regime of molecular weight for the type-A and type-B damping. After checking the consistency of simulation results with experimental data for linear viscoelasticity, stress relaxation was examined under large step shear deformations. As reported in experiments, the time-strain separability was observed in the examined molecular weight regime so that damping function was obtained. The simulated damping function increased (the nonlinearity became less significant) with decreasing chain length, which was in accord to experimental data for polystyrene solutions. Then, the mechanism of molecular weight dependence of the damping function was analyzed on the basis of kinetics of the sliplink network. For the short chains showing the type-B behavior, the equilibration of sliplink network is faster than that for longer chain due to high concentration of chain end that enhances recreation of the sliplinks. This fast equilibration of sliplink network smears the inhomogeneity of tension along the backbone and reduces the nonlinearity of damping function. The transition from the type-A to type-B behavior mainly reflects the difference of equilibration rate of entanglement network structure.
The viscoelastic behavior of polymerized ionic liquids made up of poly(1-vinylimidazole-ran-1-ethyl-3-vinylimidazolium bis(trifluoromethane)sulfonylimide) (P(VI/C2TFSI)) with varying degrees of quarternization (χq) was investigated to understand the relationship between macro physical properties and the charge density of polymer chains. The time-temperature superposition principle was found to be valid for P(VI/C2TFSI) irrespective of χq value and yielded a universal master curve. The terminal response times decreased with increasing χq due to the dilution of polymer chains caused by the increasing TFSI- concentration. Master curves of P(VI/C2TFSI) with high χq showed broad shoulders in the glass-to-rubber transition region, which reflected either rotational motion of TFSI- or sub-Rouse motion of the polymer chains. The fragility index decreased with increasing χq, indicating that the polymerized ionic liquids were less fragile in comparison with the ionomers or electrically neutral polymers. By observing the change in the fragility index, the transition between ionomers and polymerized ionic liquids was determined to be χq = 0.10 ∼ 0.25.
We study the shear-induced lamellar/onion transformation of the amphiphilic triblock copolymer-embedded surfactant lamellar phase. Increase in the mole fraction of polymer XP and the degrees of polymerization of hydrophobic PO chain NPO allows to increase the critical shear stress. The critical shear stress σC can be scaled by the effective increment of the bending modulus, which is originally predicted for the hydrophilic polymer-grafted membranes, Δκ ∼ XPN1.2. NPO dependence of the onion size is attributed to the disturbed compression modulus of the membrane via the embedded hydrophobic chain. All of these influences on the onion formation will be attributed to the configuration entropy of the embedded hydrophobic polymers in the bilayer.