We examined thermal and rheological behaviors for miscible polymer blends, poly(2-vinyl pyridine) / poly(4-vinyl phenol) (2VPy/VPh), in which intermolecular hydrogen bonds play an important role. The molecular weights of the 2VPy and VPh components are higher and lower than the critical sizes of the entanglement, respectively. In the mixture of these two polymers, only the 2VPy chains entangle each other, and the low molecular weight VPh is expected to act as a diluent. We found that the time-temperature superposition principle approximately held. The Williams-Landel-Ferry equation with a single parameter set (C1 and C2) could represent their shift factors by setting the reference temperatures to be Tg + 64 ºC. From the comparison of the composite curves, we found that the intermolecular hydrogen bonds did not affect the entanglement densities but made the zero shear viscosities larger and the relaxation times longer.
Acrylamide copolymer-based gel substrates with different viscoelasticity were employed to evaluate the viscoelasticity effect on the direct relation among cancer stemness, cellular motility and mesenchymal properties with induction of epithelial-mesenchymal transition (EMT) of human breast adenocarcinoma (MCF-7) cells in both normoxia and hypoxia. Cellular migration speed (S) of MCF-7 cells was significantly upregulated with decreasing in coefficient of damping (tanδ). The softer gel substrate produced a large amount of surface molecule of cancer stem cells (CSC) marker CD44. In contrast, for the stem cell biomarker CD133 expression, their tanδ-dependent manner was not contributed by EMT phenomenon and was an independent from acquisition of the EMT. The substrate damping as potential physical parameter emerged the important linkage to cellular motility, cancer stemness, and EMT induction.
Ianniruberto and Marrucci [Macromolecules, 46, 267-275, 2013] developed a theory whereby entangled branched polymers behave like linear ones in fast elongational flows. In order to test such prediction, Huang et al [Macromolecules, 49, 6694-6699, 2016] performed elongational measurements on star polymer melts, indeed revealing that, in fast flows, the elongational viscosity is insensitive to the molecular structure, provided the molecular weight of the “backbone” (spanning the largest end-to-end distance of the molecule) is the same. Inspired by these studies, we here report on results obtained with multi-chain sliplink simulations for symmetric and asymmetric star polymer melts, as well as calculations of the Rouse time of the examined branched structures (not previously determined by Ianniruberto and Marrucci). The simulations semi-quantitatively reproduce the experimental data if the Kuhn-segment orientation-induced reduction of friction (SORF) is accounted for. The observed insensitivity of the nonlinear elongational viscosity to the molecular structure for the same span molar mass may be due to several factors. In the symmetric case, the calculated Rouse time of the star marginally differs from that of the linear molecule, so that possible differences in the observed stress fall within the experimental uncertainty. Secondly, it is possible that flow-induced formation of hooked star pairs (or even larger aggregates) makes the effective Rouse time of the aggregate even closer to that of the linear polymer because the friction center moves towards the branchpoint of the star molecule. In the asymmetric case, it is shown that the stress contributed by the short arms is negligible with respect to that of the long ones. However, such stress reduction is balanced by a dilution effect whereby the unstretched arms reduce SORF as they decrease the Kuhn-segment order parameter of the system. As a result of that dilution, the stress contributed by the backbone is larger. The two effects compensate one another, so that the overall stress is virtually the same of the other architectures.
Linear viscoelastic behavior of cellulose nano-fiber (CNF) suspension dispersed in concentrated aqueous sucrose solution at the concentration of 60 wt% was investigated. Because the strong inter-particle interaction due to many hydrogen bonds formed between hydroxy groups on the surface of dispersed CNF particles is quite effectively reduced by using the concentrated aqueous sucrose solution as a dispersion medium, the viscoelastic behavior observed in the suspension clearly demonstrated a flow region showing fundamental viscoelastic parameters such as the zero-shear viscosity, steady state compliance, and the average relaxation time as functions of the concentration of CNF particles. A theoretical model for monodisperse rigid rod particle suspensions proposed by Doi and Edwards was slightly modified and expanded to rigid rodlike particle suspensions with a relatively broad particle length distribution, and was successfully applied to the obtained viscoelastic behavior of the CNF suspension.
For moderately entangled high-cis polyisoprene (molecular weight = 30.5 × 103) head-modified with an associative metal-carboxylate (salt) group, PI30-COOM with M = Li, Na, and K, linear viscoelastic and dielectric measurements were conducted to examine an effect(s) of the head-to-head association on the chain dynamics. The PI30-COOM chains had type-A dipoles so that their large-scale dynamics was reflected in both viscoelastic and dielectric data at low angular frequencies. The salt groups associate and dissociate (without ionization) with a rate that changes with the temperature T, as known for ionomers having non-polar backbones. Correspondingly, PI30-COOM exhibited failure of the time-temperature superposition for both viscoelastic and dielectric data. This failure was characterized through comparison with non-associative reference homo-PI, the PI30 unimer (a precursor of PI30-COOM), (PI30)2 dimer, and (PI30)6 star-type hexamer. It turned out that the viscoelastic data of PI30-COOLi at low and intermediate T (−20 º and 20 ºC), respectively, were close to those of the star-hexamer and dimer data in the iso-frictional state, and a further increase of T resulted in deviation from the dimer data toward the unimer data. This “crossover” was observed also for PI30-COONa and PI30-COOK but at lower T, which possibly reflected a barrier for the dissociation of the COOM groups lowering in the order of COOLi > COONa > COOK. The dielectric data of PI30-COOM showed a qualitatively similar crossover but at higher T compared to the viscoelastic crossover. This difference between the viscoelastic and dielectric behavior was discussed in relation to the dynamic tube dilation mechanism and also to the motional coupling (conformational transfer) among the PI30-COOM chains coexisting in different association forms.
We discussed the relationship between rheological behavior and macroscopic shape of aggregates in shear flow of TEMPO oxidized nanocellulose (TOCN) suspension. The viscosity curve of TOCN suspensions is characterized by two shear thinning regions, separated by an intermediate plateau. The first normal stress difference (N1) measured at the same time showed a tendency to increase sharply in the negative direction in the vicinity of the plateau region of viscosity, and thereafter to increase in the positive direction. Small-angle Light Scattering (SALS) measurements reveal that these changes in rheological behavior correspond to the behavior of TOCN aggregates extending from an isotropic sphere to anisotropic ellipse. That is, in the low shear rate region, TOCN is dispersed as aggregates having a three-dimensional network structure in which fibers are entangled with each other. As the shear rate increases, the aggregates flow and the viscosity decreases. When a certain critical shear rate is reached, the aggregates try to deform into an ellipse to reduce flow resistance. At that time, transient shear stress is generated in aggregates, which causes a viscous plateau region. In addition, the size of aggregates in the direction perpendicular to the flow tends to decrease as they grow in the direction of the flow, resulting in a sudden negative normal stress. As the shear rate increases further, aggregates grow into an ellipse, the degree of anisotropy increases, and the complete thinning behavior begins again.
A viscoelastic constitutive model for floc-forming suspensions developed in a previous study couples a population balance equation for the floc aggregation-breakage and a White-Metzner-type viscoelastic model. In the White-Metzner model, the viscosity and the relaxation time were respectively represented by a Krieger-Dougherty model and a power-law model, which depend on the effective volume fraction of flocs. The relation between the effective floc radius and the length of a monomer, which is the minimum unit of a fiber, was described using the mass to radius fractal dimension df. The present study considered the effect of df on the rheological properties of the proposed model and analyzed its shear property by simulating startup shear flows. The steady shear viscosity is larger for suspensions of smaller df and shows a shear-thinning property, which appears more strongly with deceasing df. The first normal stress coefficient shows fractal-dimension dependence and is larger for smaller df. These phenomena are relevant to a characteristic of the present model whereby the effective volume fraction is larger for suspensions of flocs of smaller df. Furthermore, analyses of the transient behavior of shear rheology revealed that the change in the floc size distribution proceeded in a much shorter time as compared to the relaxation time λ of the White-Metzner model, and hence the growth of macroscopic properties, such as shear viscosity and the first normal stress coefficient, was mainly dominated by the steady-state value of λ, although it depends on temporal change in the floc size distribution of flocs.
We investigated the relationship between rheological properties and tiger-striped flow marks in injection moldings of polypropylene alloys prepared under a wide range of molding conditions, taking into consideration the visibility of flow marks. The three polypropylene alloys used in this study were composed of ethylene propylene block copolymer, ethylene butene rubber, and talc. By using a reduced shear rate in which the shear rate of the flow front in the mold was normalized by the relaxation time of the polypropylene alloys, it was possible to perform flow mark evaluations that considered the influences of molding conditions such as injection speed, molding temperature, and mold temperature. In all three polypropylene alloys, the flow mark evaluations improved as the reduced shear rate increased in the high reduced shear rate region. On the other hand, the flow marks were generated in the low reduced shear rate region, but they gradually became less noticeable and the flow mark evaluations improved as the reduced shear rate decreased from about 50. Design guidelines for polypropylene alloys recommend shortening their relaxation time in order to eliminate flow marks in the low reduced shear region. These material design guidelines are markedly different from those for eliminating flow marks in the high reduced shear region.
The linear and nonlinear viscoelastic properties of five polybutadienes (PBs: Y1, Y2, V, Co1, and Co2) synthesized using three different catalysts and two processes were measured to investigate their molecular structures, with a focus on long-branched structures. Concentrated PB solutions (10 %, 30 %, 50 %) were prepared with liquid paraffin as the solvent. The storage modulus G′ and loss modulus G″ of the PB solutions and PB melts were measured. The relationships between G′c−2 and G″c−2 corrected by concentration (c) are strongly dependent on the presence or absence of long branches in PBs. At G″c−2 = 20,000 Pa, the value of G′c−2 for long-branched PBs increases with PB concentration, and also increases for more complex long-branched structures. The values of G′c−2 for Y2 and V were not dependent on concentration. The results showed that Y2 and V have linear molecular structures and that Y1, Co1, and Co2 have long-branched structures, with the order of complexity being Y1 > Co1 > Co2. From the damping functions evaluated by large-deformation stress relaxation measurements of the 30 % PB solutions, the order of long-branched structural complexity was Y1 > Co1 > Co2 > V ≈ Y2. This order was essentially the same as that based on the linear viscoelasticity of the PB samples. The rheological method in this study provides structural information about long-branched PBs.
A series of six linear low-density polyethylene (LLDPE)/low-density polyethylene (LDPE) blends were prepared by melt blending of different combinations of three LLDPEs made using a metallocene catalyst, one LLDPE made using a Ziegler-Natta catalyst, and two tubular-type LDPEs. The linear viscoelasticity of the blends was then measured. The cross-term complex shear modulus G12* from a viscoelastic constitutive law was calculated using the viscoelasticity of each blend and its components. The miscibility of the LLDPE/LDPE blends was evaluated by comparing G12* when varying the composition of each blend system. If G12* of blends with different compositions matched over a wide angular frequency range, the miscibility of the blend system is considered to be good. On the other hand, miscibility of the blend system is thought to be poor if G12* depended on the composition.