Index of content:
Volume 53, Issue 5, September 2009
53(2009); http://dx.doi.org/10.1122/1.3193720View Description Hide Description
Non-aqueous layered silicate suspensions exhibit a complex rheological behavior due to a multiple length scale structure, which is sensitive to flow and flow history. In the present work, the nature of flow-induced non-equilibrium and metastable structures in non-aqueous layered silicate suspensions based on natural and organo-modified sodium montmorillonites was examined using rheometry and confocal laser scanning microscopy(CLSM). The scaling behavior of their linear and non-linear viscoelastic properties was investigated. Based on fractal scaling theories, the scaling laws of the solid-like properties were ascribed to the presence of space-filling percolating networks consisting of clusters with a mass-fractal dimensionality, . CLSM allowed us to detect the formation of aggregates under flow and to characterize their microscopic length scale. The shear-rate dependency of the microstructurecharacteristic length scale was attributed to a reversible shear-induced aggregation process. Upon cessation of flow, the observed thixotropic behavior of these suspensions was inferred from the CLSM observations to stem from local rearrangements at the nano-scale.
Obtaining reliable transient rheological data on concentrated short fiber suspensions using a rotational rheometer53(2009); http://dx.doi.org/10.1122/1.3177348View Description Hide Description
The conventional method for obtaining transient rheological data on short glass fiber-filled polymeric fluids is to use the parallel disk (PP) geometry in a rotational rheometer. Using the PP geometry large transient stress overshoot behavior was observed during the startup of flow measurements on a 30 wt % short glass fiber-filled polybutylene terephthalate. A contributing factor to this behavior is believed to be induced fiber collisions caused by the inhomogeneous velocity field (radial varying velocity gradient). A novel approach was taken in which a “donut” shaped sample was used in a cone-and-plate device (CP-D) to maintain a sufficient gap to fiber length ratio. The magnitude of the first normal stress difference was reduced by 70%, and the time to reach steady state was reduced by 100 strain units. The Lipscomb model coupled with the Folgar–Tucker model for the evolution of fiber orientation was fit to the stress growth behavior measured using both the PP geometry and CP-D resulting in different parameters. In addition, the fitted model parameters were found to depend on the initial fiber orientation. It is believed that the CP-D allows for an accurate determination of the stress growth behavior and eventually will allow one to obtain unambiguous model parameters.
Concentration dependence of the longest relaxation times of dilute and semi-dilute polymer solutions53(2009); http://dx.doi.org/10.1122/1.3160734View Description Hide Description
The longest relaxation times of polymer solutions of semi-flexible T4 DNA and flexible 18 M molar mass polyacrylamide (PAAm) in dilute and semi-dilute concentration range are studied by the polymer extension relaxation of stretched single DNA molecules and by the stress relaxation of PAAm solutionsmeasurements. For both polymer solutions, the longest relaxation time normalized by the value at infinite dilution with the same solventviscosity increases with increasing concentration. In the dilute regime, the longest relaxation time increases just slightly with increasing concentration as as well as the empirical relation of up to with , where is the overlap concentration, in accord with the theory and previous experiments. For the semi-dilute solutions, the scaling of with concentration shows two different exponents in two concentration regions, corresponding to the unentangled and entangled regimes. The exponents are consistent with those expected by the theory of dynamical scaling for semi-dilute polymer solutions. The crossover concentration from the unentangled to entangled regime is found to be in a good solvent, in accord with other experiments based on the relaxation of a single molecule, the diffusion coefficient, and the solutionviscosity measurements. We also test the universality of the concentration dependence of the longest relaxation time for both flexible and semi-flexible polymers in both good and solvents in the dilute and semi-dilute regimes by analyzing literature data sets.
53(2009); http://dx.doi.org/10.1122/1.3190170View Description Hide Description
We study the influence of micellar kinetics on the rheological behavior of worm-like micelles composed of tri-block copolymers of ethylene oxide and propylene oxide in an aqueous solution containing KCl and ethanol. The kinetics of the micelles are adjusted by changing the ethanol concentration, according to a previous study in which the lifetime of the micelles was shown to decrease exponentially with increasing ethanol concentration. At higher ethanol concentrations (15 vol % EtOH), the worm-like micelles behave like Maxwell fluids at low frequencies, but have an upturn at higher frequencies, probably due to Rouse or breathing relaxation modes. At low ethanol concentrations (5 and 8 vol % EtOH) where the lifetime of the micelles is long, the rheological behavior is clearly non-Maxwellian, revealing a spectrum of relaxation times. The slow, block copolymer dependent growth of the micelles leads to scaling of viscosity with surfactant concentration, which varies with time. In this slow breaking regime, stirring of the solutions causes an increase of the viscosity, which slowly decreases once stirring is stopped. This apparent increase of the viscosity may be induced by the linking of ring-like micelles or by the formation of clusters of worm-like micelles (non-equilibrium structures), which disassemble when stirring is stopped.
Rheological, morphological and structural properties of PE/PA/nanoclay ternary blends: Effect of clay weight fraction53(2009); http://dx.doi.org/10.1122/1.3153551View Description Hide Description
The effect of an organically modified layered silicate on the rheological, morphological, and structural properties of immiscible polyethylene/polyamide (PE/PA) blends was investigated. The blends have been prepared for PA weight fractions ranging from 10 to 90% and at clay weight fractions from 1 to 6%. Scanning electron microscopy and transmission electron microscopy have been used to study the morphology and the structure of the blends. The dispersed phase size was shown to decrease with increasing clay content up to 2% and tends to stabilize at higher fractions. For PE matrix blends, clay particles were shown to be essentially located at the interface of the two polymers, forming an interphase whose thickness grows with clay fraction. For PA matrix blends with 2% of clay, the interphase thickness is stabilized at 11 nm; further clay addition leads to dispersion of clay within PA. Oscillatory and steady shear measurements have shown that PE matrix ternary blends behaved like polymer blends and underlined the contribution of an interphase at high clay fractions. For sufficiently filled PA matrix blends, a yield behavior was observed. The behavior of PA matrix ternary blends, dominated by the organoclay dispersed in PA, is similar to that of nanocomposites.
Interface slippage study between polyamide 12 and ethylene butene copolymer melt in capillary extrusion53(2009); http://dx.doi.org/10.1122/1.3198245View Description Hide Description
Extrusion of a polyamide 12 (PA12) material through a capillary die coated with an ethylene butene copolymer (EBM) was studied. The EBM coated die significantly increased the flow rates of the PA12 melt compared to a clean die at the same extrusion pressure. Introducing a maleic anhydride grafted ethylene-octene copolymer (EOM-g-MAH) into the EBM suppressed the effect. This behavior seems only explained by significant interface slippage between PA12 and EBM melts, which could be eliminated by introducing covalent chemical bonds across the interface. A mathematical analysis was carried out to calculate the interface slippage. The shear stress where slippage began to occur was around 0.045 MPa and the slippage velocity was around 15 mm/s at 0.1 MPa. Adding EOM-g-MAH could largely decrease the interfacial tension between EBM and PA12, thus largely decrease the interface slippage.
53(2009); http://dx.doi.org/10.1122/1.3191781View Description Hide Description
We measure the stress relaxation of linear comb polymer solutions, after a large amplitude step shear strain. We apply the time-temperature superposition principle in order to construct stress relaxation master curves that span many orders of magnitude in time and cover the entire comb relaxation from early branch retraction to backbone reptation. We find evidence of distinct relaxation processes and dynamic tube dilation that can be attributed to the architectural features of the polymer.
Injection of a viscoplastic material inside a tube or between two parallel disks: Conditions for wall detachment of the advancing front53(2009); http://dx.doi.org/10.1122/1.3191779View Description Hide Description
The injection of a viscoplasticmaterial, driven by a constant pressure drop, inside a pipe or between two parallel coaxial disks under creeping flow conditions is examined. The transient nature of both flow arrangements requires solving a time-dependent problem and fully accounting for the advancing liquid/air interface. Materialviscoplasticity is described by the Papanastasiou constitutive equation. A quasi-elliptic grid generation scheme is employed for the construction of the mesh, combined with local mesh refinement near the material front and, periodically, full mesh reconstruction. All equations are solved using the mixed finite element/Galerkin formulation coupled with the implicit Euler method. For a viscoplastic fluid, the flow field changes qualitatively from that of a Newtonian fluid because the material gets detached from the walls. For small Bingham numbers, the contact line moves in the flow direction, so that initially the flow resembles that of a Newtonian fluid, but even in that case detachment eventually occurs. The distance covered by the contact line, before detachment takes place, decreases as the Bingham number increases. For large enough Bingham numbers, the fluid may even detach from the wall without advancing appreciably. In pipe flow, when detachment occurs, unyielded material arises at the front and the flow changes into one under constant flow rate with pressure distribution that does not vary with time. In the flow between disks, it remains decelerating and the material keeps rearranging at its front because of the increased cross section through which it advances. The wall detachment we predict has been observed experimentally by Bates and Bridgwater [Chem. Eng. Sci. 55, 3003–3012 (2000)] in radial flow of pastes between two disks.
53(2009); http://dx.doi.org/10.1122/1.3160733View Description Hide Description
We consider the response of entangled four-arm polybutadiene star solutions to steady shear and to startup of steady shear in the nonlinear shear-rate regime. Data are reported both for the shear stress, measured in a cone and plate geometry using a temperature controlled ARES rheometer, and for birefringencemeasured in a Couette device using two-color birefringence. These data are then compared with predictions from the Mead–Larson–Doi (MLD) and Graham, Likhtman, McLeish and Milner (GLaMM) models for linear chains, but with the reptation mechanism turned off as an “ad hoc” means of accounting for the effect of the immobile branch point in these systems. The results for both models are reasonable. However, with the Milner–McLeish model for chain length fluctuations included, the MLD model gives better results at the lowest shear rates where the deep retractions of the arms are a significant contributor to chain relaxation. On the other hand, the local implementation of convective constraint release (CCR) in the GLaMM model gives better predictions for higher shear rates between the inverse reptation and inverse Rouse times, where the CCR mechanism largely obviates any contribution of the deep arm retractions to the relaxation process.
53(2009); http://dx.doi.org/10.1122/1.3177005View Description Hide Description
Tropoelastin, the native monomeric form of elastin, and elastin-like polypeptides undergo a process of temperature-induced phase separation (coacervation) resulting in self-organization of a polymeric network that can subsequently be cross-linked into an elastomeric polymer. In this study, the rheological properties of a recombinantly produced polypeptide mimicking the sequences and domain arrangements of tropoelastin were investigated to understand post-coacervation changes in structure and assembly. This polypeptide, designated EP20-24-24, consisted of three hydrophobic domains of native human tropoelastin flanking two cross-linking domains. Temperature-induced formation of coacervate droplets resulted in a small increase in viscosity. A higher temperature produced a second temperature-dependent transition to a gel state characterized by a much higher viscosity, strong shear thinning, and a high ratio of storage to loss moduli. This gel state was fully reversible if the temperature was immediately lowered. However, incubation above for 60 min resulted in a further transformation of the network, limiting thermal reversibility of the gelation process. These results demonstrate that for elastin-like polypeptides mimicking the sequence and domain arrangements of tropoelastin, temperature-induced coacervation and transition of the coacervate to a gel-like state are distinguishable events with separate transition temperatures. Moreover, gels formed by these elastin-like polypeptides undergo a process of maturation, reducing the reversibility of the gel state.
Real-time depth sectioning: Isolating the effect of stress on structure development in pressure-driven flow53(2009); http://dx.doi.org/10.1122/1.3164970View Description Hide Description
Transient structure development at a specific distance from the channel wall in a pressure-driven flow is obtained from a set of real-time measurements that integrate contributions throughout the thickness of a rectangular channel. This “depth sectioning method” retains the advantages of pressure-driven flow while revealing flow-induced structures as a function of stress. The method is illustrated by applying it to isothermal shear-induced crystallization of an isotactic polypropylene using both synchrotron x-ray scattering and optical retardance. Real-time, depth-resolved information about the development of oriented precursors reveals features that cannot be extracted from ex-situ observation of the final morphology and that are obscured in the depth-averaged in-situ measurements. For example, at and at the highest shear stress examined (65 kPa), oriented thread-like nuclei formed rapidly, saturated within the first 7 s of flow, developed significant crystalline overgrowth during flow and did not relax after cessation of shear. At lower stresses, threads formed later and increased at a slower rate. The depth sectioning method can be applied to the flow-induced structure development in diverse complex fluids, including block copolymers, colloidal systems, and liquid-crystalline polymers.
53(2009); http://dx.doi.org/10.1122/1.3193713View Description Hide Description
The present work investigates nonlinear behavior in large amplitude oscillatory shear (LAOS) of four different polymericmaterials using simultaneous conventional rheometric measurements and particle-tracking velocimetric observations. In contrast to most studies in the literature that treat nonlinearity in LAOS in steady state, we emphasize by the present four examples that nonlinearity in LAOS often arise in complex fluids due to time-dependent rearrangement of their microstructures in response to LAOS. Consequently, no correlation is obvious between strain dependence of the steady-state stress response and the time-dependent characteristics of the steady-state response. For instance, a highly viscoelastic material made of nano-sized polybutadiene particles exhibits homogeneous deformation and an approximate sinusoidal wave despite strong strain softening. In a second example, a well-entangled polybutadiene solution becomes inhomogeneous over time, and the corresponding nonlinearity (i.e., strain softening) took a finite time to develop to its fullest. In the example of wall slip of a monodisperse entangled polyisoprene melt, contrary to the literature claim that even harmonics would emerge, we show that the stress response only involves odd harmonics in the absence of any edge fracture. Last, a polydisperse poly(dimethyl siloxane) melt experiences homogeneous LAOS without displaying significant higher harmonics in the absence of any edge failure. In contrast, the Fourier transform analysis shows that meniscus failure is responsible for the emergence of higher harmonics including some even ones.