Tadashi Inoue

Shear small-angle light scattering studies of shear-induced concentration fluctuations and steady state viscoelastic properties

We aimed at elucidating the influence of shear-induced structures (shear-enhanced concentration fluctuations and/or shear-induced phase separation), as observed by rheo-optical methods with small-angle light scattering under shear flow (shear-SALS) and shear-microscopy, on viscoelastic properties in semidilute polystyrene (PS) solutions of 6.0 wt % concentration using dioctyl phthalate (DOP) as a Theta solvent and tricresyl phosphate (TCP) as a good solvent. In order to quantify the effects of the shear-induced structures, we conducted a numerical analysis of rheological properties in a homogeneous solution based on the constitutive equation developed by Kaye-Bernstein, Kearsley, and Zapas (K-BKZ). In the low-to-intermediate shear rate gamma region between tau(w) (-1) and tau(e) (-1), where tau(w) and tau(e) are, respectively, terminal relaxation time and the relaxation time for chain stretching, the steady state rheological properties, such as shear stress sigma and the first normal stress difference N(1), for the PS/DOP and PS/TCP solutions are found to be almost same and also well predicted by the K-BKZ equation, in spite of the fact that there is a significant difference in the shear-induced structures as observed by shear-SALS and shear-microscopy. This implies that the contribution of the concentration fluctuations built up by shear flow to the rheological properties seems very small in this gamma region. On the other hand, once gamma exceeds tau(e) (-1), sigma and N(1) for both PS/DOP and PS/TCP start to deviate from the predicted values. Moreover, when gamma further increases and becomes higher than gamma(a,DOP) (sufficiently higher than tau(e) (-1)), above which rheological and scattering anomalies are observed for PS/DOP, sigma and N(1) for PS/DOP and PS/TCP are significantly larger than those predicted by K-BKZ. Particularly, a steep increase of sigma and N(1) for PS/DOP above gamma(a,DOP) is attributed to an excess free energy stored in the system via the deformation of interface of well-defined domains, which are aligned into the stringlike structure developed parallel to the flow axis, and stretching of the chains connecting the domains in the stringlike structures. Thus, we advocate that the effect of shear-induced structures should be well considered on the behavior of sigma and N(1) at the high gamma region above tau(e) (-1) in semidilute polymer solutions.

Dynamical coupling between stress and concentration fluctuations in a dynamically asymmetric polymer mixture, investigated by time-resolved small-angle neutron scattering combined with linear mechanical measurements

By time-resolved small-angle neutron scattering (SANS) monitoring of two kinetic phenomena of (i) an early stage of spinodal decomposition and (ii) relaxation of shear-induced phase separation after shear cessation, we quantitatively investigated a growth or relaxation rate R(q) of the concentration fluctuations in a polymer mixture of polystyrene (PS)/poly (vinyl methyl ether) (PVME), where q is the magnitude of scattering vector. R(q) thus determined is largely influenced according to a scheme of dynamical coupling between stress and diffusion. The coupling effect gives rich varieties depending on two crossovers of (i) from viscous to gel-like limits and (ii) from dynamic symmetry to asymmetry, which are tuned by temperature change in the intermediate temperature region between largely different glass transition temperatures, Tg, for each constituent. With respect to q-behaviors of R(q), we observed as follows; (i) in a conventional (dynamically symmetric) viscous limit (T ≥ 110 °C), R(q) shows q2, (ii) in a dynamically asymmetric viscous limit (T = 100–80 °C) R(q) becomes q-independent, (iii) shallow in a dynamically asymmetric gel-limit (T = 80–60 °C), R(q) shows q2 for low q and q-independent for high q, and (iv) deep in a gel-like dynamically asymmetric limit (50–40 °C), R(q) splits into fast and slow modes (showing q2 and q-independent, respectively). In order to explain the rich variety found in R(q), we employed a time-dependent Gintzburg-Landau equation (TDGL) modified to consider the stress and diffusion coupling effect. Two mechanical relaxation times of terminal reptation and Rouse modes were determined by linear viscoelasticity measurements to successfully reproduce the rich variety in R(q) according to the modified TDGL.

Dynamic birefringence and non-linear rheology of diblock copolymer micellar solutions

Dynamic viscoelasticity, dynamic birefringence, and non-linear rheology of styrene–ethylenepropylene diblock copolymer, SEP, in isotridecyl isononanoate solution were examined in order to clarify the rheology–structure relationship for block copolymer micellar solutions. SAXS experiments at room temperature indicated formation of the BCC super-lattice of the micelles, composed of S chain core and EP chain corona. The complex strain-optical coefficient, being the complex ratio of birefringence to strain in dynamic measurements, changed its sign with angular frequency, ω, and exhibited a very complicated ω dependence, indicating that the stress relaxed through several relaxation mechanisms. Assuming the stress-optical rule for each mechanism, the complex shear modulus was separated into three components. The three components could be assigned to the reorientation of corona chains, reorientation of core chains, and deformation of the lattice. Steady shear viscosity, η, showed strong thinning behavior. Shear rate dependence of η and the first normal stress difference were described well with the BKZ constitutive equation composed of the above three mechanisms having different damping parameters.

High frequency viscoelastic measurements using optical tweezers on wormlike micelles of nonionic and cationic surfactants in aqueous solutions

Linear viscoelasticity of two kinds of wormlike micelles in aqueous solutions, one is nonionic surfactants and the other one is cationic surfactant with organic salt, was measured over a wide frequency range with Brownian motion tracking microrheology (BMTR) using optical tweezers and a conventional rheometer. In BMTR measurements, the Brownian motion of a small particle embodied in the sample is traced and the complex modulus is calculated from the trajectory. The wideband linear viscoelastic spectra thus obtained for each of wormlike micelles were classified into the following three relaxation types as already known. Type A is similar to the spectrum of the nonentangle polymer solutions, type B similar to that of the entangle polymer systems, and type C has a single Maxwell relaxation at low frequencies. In the high-frequency region, spectra of all types showed a common power-law relaxation, which reflects the reorientation of the viscoelastic segment of the wormlike micelles, indicating that the dynamics of wormlike micelles is identical with that of the ordinary polymeric systems. For type A solutions, the molar mass of wormlike micelles was estimated by fitting the beads-spring models to the viscoelastic spectra. For type B solutions, the molar mass was estimated by using the universality of entangled system. For the case of nonionic micelles, thus determined molar mass is in good agreement with the reported result with the light scattering measurement.

Component Dynamics in Polystyrene/4‐Pentyl‐4′‐Cyanobiphenyl Blend

We studied the dielectric and viscoelastic relaxation behaviors of polystyrene (PS)/4‐pentyl‐4′‐cyanobiphenyl (5CB) miscible blend, and examined the relationship between segmental motion of PS and rotational motion of 5CB in the blend. Viscoelastic measurements could detect the global motion of PS‐chain, and the longest relaxation time τL of PS was determined. On the other hand, 5CB motion was detected by dielectric measurement, because of much larger electric dipole of cyano group in 5CB than that of PS. The dielectric spectra of the blends were bimodal, indicating the existence of two distinct dynamical modes related to the 5CB motion. The relaxation times of these two modes are named τslow and τfast. Temperature dependences of τslow and τL were found to be almost the same, and τslow was close to the segmental relaxation time τseg of PS estimated from the τL values. This means that these two motions, PS‐segmental motion and slow mode of 5CB are cooperative. On the other hand, temperature dependence of τ...

Cu/Zn-superoxide dismutase forms fibrillar hydrogels in a pH-dependent manner via a water-rich extended intermediate state

Under certain conditions, amyloid-like fibrils can develop into three-dimensional networks and form hydrogels by a self-assembly process. When Cu/Zn superoxide dismutase (SOD1), an anti-oxidative enzyme, undergoes misfolding, fibrillar aggregates are formed, which are a hallmark of a certain form of familial amyotrophic lateral sclerosis (ALS). However, the issue of whether SOD1 fibrils can be assembled into hydrogels remains to be tested. Here, we show that the SOD1 polypeptides undergo hydrogelation accompanied by the formation of thioflavin T-positive fibrils at pH 3.0 and 4.0, but not at pH 5.0 where precipitates are formed. The results of viscoelastic analyses indicate that the properties of SOD1 hydrogels (2%) were similar to and slightly more fragile than a 0.25% agarose gel. In addition, monitoring by a quartz crystal microbalance with admittance analysis showed that the denaturing of immobilized SOD1 on a sensor under the hydrogelation conditions at pH 3.0 and 4.0 resulted in an increase in the effective acoustic thickness from ~3.3 nm (a folded rigid form) to ~50 and ~100 nm (an extended water-rich state), respectively. In contrast, when SOD1 was denatured under the same conditions at pH 5.0, a compact water-poor state with an effective acoustic thickness of ~10 nm was formed. The addition of physiological concentrations of NaCl to the pH 4.0 sample induced a further extension of the SOD1 with larger amounts of water molecules (with an effective acoustic thickness of ~200 nm) but suppressed hydrogel formation. These results suggest that different denatured intermediate states of the protein before self-assembly play a major role in determining the characteristics of the resulting aggregates and that a conformational change to a suitable level of extended water-rich intermediate state before and/or during intermolecular assembling is required for fibrillation and hydrogelation in the case of globular proteins.

Relationship between global and segmental dynamics of poly(butylene oxide) studied by broadband dielectric spectroscopy

Relationship between segmental relaxation and normal-mode relaxation has been studied for molten poly(butylene oxide)s having various molecular weights by broadband dielectric spectroscopy over a wide temperature (T) range. We found that T dependence of the segmental relaxation time, τs, was weaker than the normal mode time, τn, at high T(>250 K ∼ Tg + 50 K), and the τn/τs ratio systematically decreased with increasing temperature. This high temperature complexity, whose mechanism has not been discussed in detail so far, was quantitatively explained by assuming the two step processes: local conformation change of polymers (elementary process) occurs first, and then the motion of a segment unit (second process) occurs. It was also found that the elementary process was strongly correlated with the experimentally observed β-relaxation.