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Journal of RheologyVolume 48, Issue 3 (May-Jun 2004) |
Contents
Poly(D-lactic acid) as a rheological modifier of poly(L-lactic acid): Shear and biaxial extensional flow behaviorHideki Yamanea,c), Kosuke Sasaia), Masayuki Takanoa), and Masaoki Takahashib) a) Graduate School of Science and Technology b) Department of Polymer Science and Engineering AbstractThe effect of addition of a small amount of poly(D-lactic acid) (PDLA) on the melt rheology of poly(L-lactic acid) (PLLA) was investigated. PDLA added to PLLA produces a stereocomplex which stays unmelted even above Tm of PLLA. Because of the imbalanced contents of component polymers, the stereocomplex does not grow into a large crystallite but rather acts as a crosslinking point of PLLA chains resulting in the apparent introduction of the long chain branching as well as in the apparent increase in molecular weight. Such changes in the molecular structure give rise to the change in the melt rheology depending on the content and the molecular weight of PDLA. The addition of low Mw PDLA significantly affects the shear rheology of PLLA melts while high Mw PDLA does not give such a significant effect. However the addition of PDLA with high and low Mw's gives a strong strain hardening character to PLLA melt even at a very low PDLA content. c) Corresponding author. Email: hyamane@ipc.kit.ac.jp Relating rheology and molecular structure of model branched polystyrene melts by molecular stress function theoryM. H. Wagnera), J. Hepperleb), and H. Münstedtb) a) Polymertechnik/Polymerphysik b) Institute of
Polymer Materials AbstractA quantitative analysis of the experimentally observed strain-hardening characteristics of comb-shaped polystyrene melts with different numbers and lengths of grafted side chains is made using the Molecular Stress Function (MSF) model. This is based on a specific strain energy function, which assumes that the backbone of the grafted molecule is stretched by deformation, while the side chains are compressed. It is demonstrated that the experimentally observed slope of the elongational viscosity after inception of strain-hardening depends on the ratio b of total molar mass to backbone molar mass as predicted by the model. The steady-state (plateau) value of the elongational viscosity depends on the maximum relative stretch, f2MAX, which can be supported by chain segments. It is found that f2MAX increases with the molar mass fraction of side chains, and at the same molar mass fraction, more short side chains lead to higher values of f2MAX than a few long side chains. Surprisingly, even side chains which are shorter than the entanglement length are found to contribute to strain-hardening. Using the same nonlinear parameters as in elongational flow plus a parameter taking into account the additional constraint release in rotational flows, the shear damping function data of the grafted polystyrene melts are modeled quantitatively by the MSF theory. Sphere migration in oscillatory Couette flow of a viscoelastic fluidBradley M. Lormand and Ronald J. Phillips AbstractObservations are presented of the migration of spherical particles in large-amplitude, oscillatory flows between concentric cylinders. The ratio of the gap between the cylinders to the cylinder radii was 1:20, yielding an approximately linear shear flow in the absence of any particles. The particles had diameters d that were comparable to the gap width, in the range 0.3mm < d < 0.6mm, and were suspended in a 13% solution of polyisobutylene in tetradecane. Particle motion was observed by using horizontal and vertical CCD cameras that were interfaced with a computer. The results show that, in a linear velocity field, in low frequency and large-amplitude oscillations, spherical particles near solid boundaries generally migrate toward the nearest boundary, in keeping with previous simulation results for steady flows. However, the migration velocity depends strongly on the sphere diameter, frequency and amplitude of the oscillation. For example, at small amplitudes large “dead zones” are apparent, in which the sphere does not migrate at all, and these zones are not centered in the gap. In addition, at frequencies greater than 1 Hz, the particles do not migrate all the way to the outer wall, but stop short by a frequency-dependent distance. When the particles do migrate, the migration velocity is determined by a single dimensionless parameter that is derived here by a scaling analysis. Rupture of entangled polymeric liquids in elongational flow with dissipationa)Yogesh M. Joshi and Morton M. Dennb) AbstractThe scaling theory for rupture of entangled polymeric liquids, which is based on a critical recoverable strain [Joshi and Denn, J. Rheology 47, 291-298 (2003)], is extended to include the viscoelastic regime, where the total strain at rupture is a decreasing function of the elongation rate. Rupture can be predicted in both the viscoelastic and rubbery regimes using the scaling theory together with a constitutive equation for the stress in the entangled liquid. The scaling theory with the Wagner model captures the essential features of the available experimental data, and the quantitative agreement is reasonable. a)
The editorial process for this manuscript was carried out by Editorial
Board member Ronald Larson. A rheological study of the phase transition in thermoplastic polyurethanes: Critical gel behaviour and microstructure developmentSilvia Cossara), Dario Nichettia), and Nino Grizzutib) a) Chiorino SpA, Via Sant'Agata, 9 - 13900 Biella, Italy b) Dipartimento di Ingegneria Chimica AbstractThe phase transition behaviour of a commercial thermoplastic polyurethane (TPU) is investigated by rheological techniques. Oscillatory shear flow experiments are performed during both the isothermal annealing following a rapid cooling from the melt state, and the subsequent heating ramp (re-melting). Rheological measurements reveal that the micro-phase separation between hard and soft segments and the concurrent crystallization of the hard phase domains produce a sol-to-gel type of transition. We show that the microstructure of TPUs at the critical gel point is strictly related to the applied thermal history. In particular, the critical gel properties during the cooling down transition are found to be quantitatively different from those measured during the reverse, heating up process. When coupled to Differential Scanning Calorimetry (DSC) measurements under the same thermal history, the rheological technique is shown to be a useful tool to determine the mechanisms of microstructure evolution. Rheology and reptation of linear polymers: Ultra high molecular weight chain dynamics in the meltJ. F. Vegaa), S. Rastogi, G. W. M. Petersb),
and H. E. H. Meijer AbstractThe melt rheology of ultra high molecular weight polymeric materials characterized by a narrow molecular weight distribution has been analyzed. Ultra high molecular weight polyethylene obtained from a metallocene catalyst shows a well-developed “plateau” modulus in a range of angular frequency of more than three decades. The characteristic value of the “plateau” modulus (~2 MPa) is in close agreement with those reported for a model high molecular weight monodisperse polyethylene. From this value one can determine a characteristic molecular weight between entanglements of 1200 g-mol-1. The molecular weight dependency of different, experimentally based relaxation times obtained from the linear viscoelastic response exhibits an exponent power law close to 3.0 for these materials. This seems to contradict the 3.4 dependence observed in the usual molecular weight range, which is based on the chain contour length fluctuation approach, but is in agreement with the latest reptation-based models. These models predict a crossover from the 3.4 to a 3.0 exponent for very long chains as used here at a constant critical value of the molecular weight Mr close to 100Mc, (200Me when using the well accepted relationship Mc = 2Me). This predicted crossover is independent of the polymer's chemical composition. However, combining results from our experiments with results from literature shows that the experimental values of Mr extent from 15Mc for polystyrene (PS), 25Mc for polyisobutilene (PIB), 100 Mc for polybutadyene (PBd) to 220Mc for polyethylene (PE). These results are not predicted by molecular models and demand for new theoretical considerations of chain dynamics, in which the chemical structure is, most probably, a key factor that should be taken into account. It should be noticed that the influence of the molecular weight distribution on the differences observed is not understood. Unfortunately, it is very difficult to obtain monodisperse samples of ultra high molecular weight polyethylene and, therefore, the use of the samples studied here the best choice possible up to now to test and revisit basic and novel aspects of the rheology of polyolefin's. a) J.F. Vega is a
visiting post-doc from Consejo Superior de Investigaciones Cientificas,
Instituto de Estructura de la Materia, Serrano 113bis, E-28006, Madrid,
Spain. Experimental characterization of extrudate swell behavior of linear polybutadieneZhiyong Zhu and Shi-Qing Wang AbstractThis paper describes an experimental study of the time-dependent extrudate swell behavior of a series of linear polybutadienes as a function of molecular weight (MW) and molecular weight distribution (MWD). The “initial” extrudate swell, i.e., the extrudate swell measured near the die exit, is universally found to be around 1.10, independent of MW, MWD and the extrusion rate. Like the initial extrudate swell, the extrudate surface velocity at the die exit, Vs, when normalized by the average capillary flow velocity, Vp, is also a relatively universal function of the distance X from the exit plane. It increases from 0.3 to 1 over a normalized distance from X/D = 0 to X/D = 0.035 regardless of MW, MWD and the extrusion rate. In the presence of exit slip, which can be produced by coating the exit wall with a fluoropolymer, Vs is nearly equal to Vp. The extrudate swell at a given distance from the exit is shown to be independent of the molecular weight for monodisperse samples. A small tail in the MWD at the high end produces considerably larger extrudate swell. Although the extrudate swell increases with increasing applied stress for a given sample, a series of polymer solutions of different concentrations show that the real controlling variable is the applied stress normalized by the material's elastic plateau modulus. The extrudate swell is also found to depend systematically on the die diameter at all stages of its growth. Moreover, the extrudate swell is greater when it is allowed to grow in media (other than the air) such as oil and water. These two last effects can be understood in terms of the free-surface-to-volume ratio of the extrudate: the larger this ratio is more rapidly the residual molecular deformation in the extrudate can relax. A constitutive equation describing the nonlinear elastic response of aqueous foams and concentrated emulsionsR. Höhler, S. Cohen-Addad, and V. Labiausse AbstractWe present a constitutive equation describing the nonlinear elastic response of aqueous foams and concentrated emulsions, undergoing affine deformations. We derive it from the expression of the interfacial energy of strained aqueous foams or concentrated emulsions, using the formalism of large deformation continuum mechanics. By linearizing the strain energy potential, we obtain a constitutive equation of the Mooney-Rivlin form. The predicted nonlinear elastic response is compared to previous models and numerical simulations. Furthermore, we discuss the approximations used, in particular regarding the non-affine character of the deformations in real foams and concentrated emulsions. Computer simulation of the film blowing process incorporating crystallization and viscoelesticityIyad A. Muslet and Musa R. Kamala) AbstractA comprehensive two-dimensional simulation of the film blowing process is developed, based on a mathematical model that incorporates the Phan-Thien and Tanner (PTT) and the Neo-Hookean constitutive equations with crystallization effects. The PTT constitutive equation is employed in the liquid-like region, while the Neo-Hookean constitutive equation is employed in the solid-like region. The orientation-induced crystallization is accounted for by incorporating the Nakamura non-isothermal equation along with Ziabicki's equation. The proposed model provides predictions of the bubble shape and dimensions, the position of the freeze-line, and the evolution of temperature, crystallinity, stresses and deformation. The predictions of the model show good agreement with analytical and computational solutions for some limiting cases and with experimental results reported by various workers. a) Author to whom correspondence should be addressed. Electronic mail: musa.kamal@mcgiil.ca Extensional rheometer for creep flow at high tensile stress: Part I. Description and validationM. Stadlbauera), H. Janeschitz-Kriegl, M. Lipp,
G. Eder, and R. Forstner AbstractAn extensional rheometer for creep experiments at relatively high tensile stresses is presented. Elongational viscosities and creep recoveries as functions of time can be measured. The new device is called Windbix Extensional Rheometer. “Windbix” is a neologism reminding of the Dutch word for air-gun (“Wind Buks”) with reference to the device's pneumatic mechanism to create the tensile force. The rheometer was validated with a LDPE identical to IUPAC A. Results are presented and partly compared with published data. The new filament stretching rheometer uses a pneumatic (or “expansion”) tube for creating a pre-adjustable tensile stress up to 0.3 MPa, which is constant during extension of the sample. The time needed for the adjustment of this stress is less than 0.02 s. In principle, this fast mechanism enables rheological measurements at high strain rates. As a prerequisite the usual rigid end-plates are replaced by flexible textile mats, pliably following the contraction of the top of the sample during elongation. In this way, influences of necking are effectively reduced. Isothermal extension takes place in a thermostated glass tube flushed with inert gas. In principle samples of suitable polymers can be extended from an initial length of 10 mm up to a final length of nearly 1 m. This corresponds to a total Hencky strain up to 4.6. Originally, this machine was designed for a quantitative evaluation of the influence of extensional flow on the crystallization kinetics of semi-crystalline polymers, where a rapid short term extension is indispensable (see second paper). a) Author to whom all correspondence should be addressed. Present occupation: Borealis GmbH, St.-Peter-Str. 25, A-4021 Linz, Austria. New extensional rheometer for creep flow at high tensile stress: Part II. Flow induced nucleation for the crystallization of iPPM. Stadlbauera), H. Janeschitz-Kriegl, G. Eder
and E. Ratajski AbstractA novel extensional rheometer for creep experiments has been presented in part I. The device can be used also for an investigation of the influence of short term elongational flow on the structure development in crystalline polymers. It enables the application of defined portions of mechanical work to the polymer sample in its state of undercooled melt. For the purpose short-term elongational creep is used. With increasing mechanical work the number density of nuclei in iPP increases by decades. Results, as previously obtained with shear flow, are introduced for comparison. Prospects of a theoretical description are discussed. a) Author to whom all correspondence should be addressed. Present occupation: Borealis GmbH, St.-Peter-Str. 25, A-4021 Linz, Austria. Rheological study of transient polymer networks crosslinked by two-component associative groups: Inversion of the gel skeletal structureT. Indei and F. Tanaka AbstractOn the basis of transient network theory, we study linear viscoelasticity of physical gels formed by polymers carrying two different species of associative groups. As the simplest but important case, this paper treats linear chains carrying two A functional groups at their both ends and one B group at the center of the chain. It turns out that the complex modulus is described by the two-mode Maxwell model when conditions bA >> bB or bA << bB holds, where bi (i = A, B) is the detaching rate of an i associative group from the junction. In the latter case, two rubbery plateaux appear in the storage modulus, and structural transformations from a network of A backbone to that of connected B micelles are expected to occur at a characteristic frequency w » bA + bB. Phase deformation under shear in an immiscible polymer blend : Influence of strong permanent elastic propertiesY. Deyrail and P. Cassagnau AbstractThe influence of a strong permanent elasticity on droplet deformation is studied. Deformation under shear of a crosslinked dispersed copolymer of ethylene and vinyl acetate (EVA) in a poly(dimethylsiloxane) (PDMS) matrix was investigated using a hot optical shear device. In the first part of this work, deformation of reactive EVA droplets at different extents of crosslinking under isothermal temperature was studied in order to better understand the system. As expected, droplet deformation decreases with increasing crosslinking level. However, even when the droplets are crosslinked beyond the gel point, they continue to deform. Following this, the control of dispersed phase morphology under shear flow using the extent of crosslinking of the dispersed phase was also studied. Fixed fibrillar morphology can be obtained by adjusting the shear rate so that crosslinking reaction occurs before the break-up of the EVA fibers. In the same way, fixed nodular morphology can be generated if fiber relaxation occurs before the crosslinking level becomes too high. Depending on the shear rate needed to generate fibers, the final nodular morphology obtained can be coarse or fine. |
Please e-mail suggestions and comments to albertco@umche.maine.edu. Updated 20 May 2004 |