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Dynamic rheology of the immiscible blends of liquid crystalline polymers and flexible chain polymers
Immiscible blends containing liquid crystalline polymers (LCP) as dispersed phases show different dynamic rheological properties than those composed of flexible polymers. The widely used Palierne’s model was shown by many authors to be insufficient to describe the frequency dependence of dynamic modulus of such blends. A new model was presented to describe the dynamic rheology of the immiscible blend containing LCP as a dispersed phase. The flexible chain polymer matrix was assumed to be a linear viscoelastic material under small amplitude oscillatory shear flow, and the LCP was assumed to be an Ericksen’s transversely isotropic fluid. The Rapini-Papoular equation of anisotropic interfacial energy was used to account for the effect of nematic orientation on the interfacial tension. It was found that the orientation of the director and the anchoring energy greatly influenced the storage modulus at the “shoulder” regime. The overall dynamic modulus of the blend can be well described by the model with suitable choice of the orientation of the director and anchoring energy of LCP. 相似文献
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The effect of nanometer sized silica particles (R16 nm) on the flow-induced morphology of immiscible polymer blends is studied. Polydimethylsiloxane (PDMS) and polyisobutylene (PIB) are chosen as model components. A stable droplet/matrix microstructure is obtained for blends of 30% PIB in 70% PDMS or vice versa. Rheological measurements are used to show that the silica particles alter the sensitivity of the of dispersed phase/matrix microstructure to shear flow. Coalescence is suppressed or at least slowed down on a practical time scale, especially when PDMS is the matrix phase. The effect of mixing conditions, pre-shear rate and particle concentration on the blend morphology are studied. Cryo-SEM is used to observe the accumulation of the particles at the interface. Blends stabilized by solid particles could provide an interesting alternative to blends compatibilized by block-copolymers.This paper was presented at the first Annual European Rheology Conference (AERC) held in Guimarães, Portugal, September 11-13, 2003. 相似文献
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We examined the effect of interfacially active particles on the morphology and rheology of droplet/matrix blends of two immiscible homopolymers. Experiments were conducted on polybutadiene/polydimethylsiloxane (10/90) blend and the inverse system. The effects of fumed silica nanoparticles, at low particle loadings (0.1–2.0 wt%), were examined by direct flow visualization and by rheology. Fumed silica nanoparticles were found to significantly affect the morphology of polymer blends, inducing droplet cluster structure and decreasing the droplet size, regardless of which phase wets the particles preferentially. This is surprising in light of much past research that shows that particles are capable of bridging and thus induce droplet cluster structure in droplet/matrix systems only when they are preferentially wetted by the continuous phase. Therefore, there should exist other possible mechanisms responsible for these droplet cluster structures except for the bridging mechanism. We proposed a particle-flocculating mechanism based on the fact that fumed silica particles readily flocculate due to their high aspect ratio, fractal-like shape, or interparticle attractions. Optical microscopy also reveals that the clustering structure becomes more extensive, and the droplet sizes in the clusters become smaller when the particle loading is increased. Rheologically, the chief effect of particles is to change the flow behavior from a liquid-like rheology to gel-like behavior. This gel-like behavior can be attributed to droplet clustering. Moreover, it should be emphasized that such gel-like behavior can be seen in the blends regardless of which phase wets the particles preferentially, suggesting that, once again, bridging is not the only cause of droplet clustering. 相似文献
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《ournal of non Newtonian Fluid Mechanics》1998,77(3):191-199
Spatial inhomogeneities of blends are taken into account by letting the free energy depend on gradients of the quantities chosen to characterize the interface. The governing equations as well as new stresses that arise due to the inhomogeneity are derived from a requirement of compatibility with thermodynamics. 相似文献
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Pickering emulsions are emulsions whose drops are stabilized against coalescence by particles adsorbed at their interface.
Recent research on oil/water/particle systems shows that particles can sometimes adsorb at two oil/water interfaces. Such “bridging particles” can glue together drops of oil in water or vice versa. We hypothesize that
the same effect should apply in immiscible polymer blends with droplet-matrix morphologies, viz., added particles should glue
together drops and give rise to particle-bridged drop clusters. We test this hypothesis in PIB-in-PDMS blends [PIB, poly(isobutylene);
PDMS, poly(dimethylsiloxane)] with fumed silica particles. Direct visualization shows that the particles can indeed induce
clustering of the drops, and the blends appear to show gel-like behavior. Such gel-like behavior is confirmed by dynamic oscillatory
experiments. However, we are unable to conclusively attribute the gel-like behavior to droplet clustering: Association of
the fumed silica particles in the bulk, which itself causes gel-like behavior, confounds the results and prevents clear analysis
of the gluing effect of the particles. We conclude that PIB/PDMS/fumed silica is not a good model system, for studying particle-containing
polymer blends. We instead propose that spherical monodisperse silica particles can offer a far more convenient model system,
and provide direct visual evidence of gluing of PIB drops in a PDMS matrix. 相似文献
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We propose a family of new models making a direct link between flow and structure for immiscible mixtures of viscoelastic
fluids undergoing high deformation flows. The morphology is treated both at local (Doi-Ohta-type) and more macroscopic (droplet-like)
scales. The governing equations, that include expressions for the extra stress tensor, agree with the conservation laws and
with the observed compatibility with thermodynamics. In the particular case in which only one of the two characterizations
of the morphology is used, extended versions of the Doi-Ohta and the Maffettone and Minale models are obtained. The extension
consists of involving explicitly the free energy in the governing equations and completing the expressions for the extra stress
tensor.
Received: 24 January 2001 Accepted: 24 April 2001 相似文献
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Rheological behavior of a PS/PE model viscoelastic immiscible blend compatibilized by two types of interfacial modifiers
was investigated. Dynamic, steady shear, and transient experiments were performed to probe the effect of the interfacial modification
on the rheological behavior of the blend. While the effect was relatively small in dynamic and steady shear experiments, significant
signature of the presence of the copolymer was observed in transient experiments after start up of shear flow.
The magnitude of the departure from Doi-Otha theory (worked out for non-compatibilized blends) was evaluated.
Received: 6 March 2000 Accepted: 15 June 2000 相似文献
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Strain recovery after the cessation of shear was studied in model immiscible blends composed of polyisobutylene drops (10–30%
by weight) in a polydimethylsiloxane matrix. Blends of viscosity ratio (viscosity of the drops relative to the matrix viscosity)
ranging from 0.3 to 1.7 were studied. Most of the strain recovery was attributable to interfacial tension, and could be well-described
by just two parameters: the ultimate recovery and a single retardation time. Both these parameters were found to increase
with the capillary number of the drops prior to cessation of shear. For blends that had reached steady shear conditions, the
ultimate recovery decreased with increasing viscosity ratio, whereas the retardation time increased with increasing viscosity
ratio. The retardation time was well-predicted, but the ultimate recovery was over-predicted by a linear viscoelastic model
developed previously by Vinckier et al. (Rheol Acta 38:65–72, 1999). 相似文献
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We consider the effects of 2.7-μm-diameter hydrophobic silica particles added to droplet–matrix blends of polyethylene oxide
(PEO) and polyisobutylene (PIB). The particles adsorb on the surface of the PEO drops but protrude considerably into the PIB
phase. Hence, it is possible for a single particle to adsorb onto two PEO drops simultaneously. Such particles are called
“bridging” particles, and they the glue drops into noncoalescing clusters. Flow visualization studies show that shearing the
sample promotes bridging-induced clustering of drops and that the structure of the clusters depends on the shear rate. Rheologically,
the most significant consequence of bridging-induced drop clustering appears to be a plateau in G′ at low frequencies characteristic of gel-like behavior. The gel-like behavior develops fully after shearing the sample,
and the kinetics of gel formation are faster with increasing shear stress or increasing drop volume fraction. The gel-like
behavior suggests that the bridging-induced drop clusters form a weak network. Apart from particle bridging, optical microscopy
also reveals that particles can organize into a hexagonal lattice on the drops’ surfaces, a phenomenon that has only been
noted in aqueous systems previously. Finally, rheology and flow visualization both suggest that particles promote coalescence
of drops. This is surprising in light of much past research that shows that particles that are preferentially wetted by the
continuous phase generally hinder coalescence in droplet–matrix systems. 相似文献
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Particles have been shown to adsorb at the interface between immiscible homopolymer melts and to affect the morphology of
blends of those homopolymers. We examined the effect of such interfacially active particles on the morphology of droplet/matrix
blends of model immiscible homopolymers. Experiments were conducted on blends of polydimethylsiloxane and 1,4-polyisoprene
blended in either a 20:80 or 80:20 weight ratio. The effects of three different particle types, fluoropolymer particles, iron
particles, and iron oxyhydroxide particles, all at a loading of 0.5 vol.%, were examined by rheology and by direct flow visualization.
Particles were found to significantly affect the strain recovery behavior of polymer blends, increasing or decreasing the
ultimate recovery, slowing down or accelerating the recovery kinetics, and changing the dependence of these parameters on
the applied stress prior to cessation of shear. These rheological observations were found to correlate reasonably well with
particle-induced changes in drop size. The particles can both increase as well as decrease the drop size, depending on the
particle type, as well as on which phase is continuous. The cases in which particles cause a decrease in drop size are analogous
to the particle stabilization of “Pickering emulsions” well-known from the literature on oil/water systems. We hypothesize
that cases in which particles increase drop size are analogous to the “bridging–dewetting” mechanism known in the aqueous
foam literature. 相似文献
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Most methods of measuring the interfacial tension between two immiscible polymers are based on the analysis of the shape
that a drop of one polymer immersed in the other one exhibits under the action of flow or gravity. In such a situation, the
small, yet nonzero mutual solubility between the two polymers acts toward mass transfer between the drop and the surrounding
fluid. In this work, diffusion effects on the interfacial tension of the pair polyisobutylene/polydimethylsiloxane have been
investigated by drop deformation under shear flow. When the drop was made of polyisobutylene, drop size decreased with time
due to diffusion. Drop shrinkage was associated with a significant increase in interfacial tension, until an apparent plateau
value was reached. The effect was attributed to a selective migration of molecular weights, which would act to enrich the
drop with higher molar mass material. To support such an interpretation, drop viscosity was evaluated by drop shape analysis
and it was actually found to increase with time. In some cases, the ratio between drop and continuous phase viscosity became
higher than the critical value for drop breakup in shear flow. Upon inverting the phases (i.e., when the drop was made of
polydimethylsiloxane), no significant transient effects were observed. In the light of these results, the problem of what
are the correct values of interfacial tension and viscosity ratio for a polymer blend of a certain composition will also be
discussed.
Received: 25 January 1999 Accepted: 24 May 1999 相似文献
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Stress relaxation in immiscible blends is studied for a well defined shear history, i.e. after prolonged steady state shearing. Model systems are used that consist of quasi-Newtonian liquid polymers. Hence the relaxation is dominated by changes in the morphology of the interface. Both shear stress and the first normal stress are considered. The measurements cover the entire concentration range. For dilute blends the interfacial contribution to the stress relaxation compares well with model predictions. Deviations occur when the matrix phase is slightly elastic. In that case the similarity between the relaxation of shear and normal stresses is also lost. The latter is attributed to a wider drop size distribution.Increasing the concentration of the disperse phase results in a complex evolution of the characteristic relaxation times. The normal stresses relax systematically slower than the shear stresses and the concentration curve includes two maxima. Even for equiviscous components the concentration curves are not symmetrical. It is concluded that even a slight degree of elasticity in the matrix phase drastically affects the morphology and the interfacial relaxation of such blends. 相似文献
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The effect of added compatibilizer on the strain recovery of model immiscible blends after cessation of shear was studied. Blends were composed of polyisobutylene drops (up to 30% by weight) in a polydimethylsiloxane matrix, with viscosity ratio (viscosity of the drops relative to the matrix viscosity) ranging from 0.3 to 1.7. Up to 1% by weight of a PIB-PDMS diblock copolymer was added as compatibilizer. The ultimate recovery recorded after reaching steady-shear conditions increased significantly due to added compatibilizer. Furthermore, the compatibilizer also slowed down the kinetics of the recovery; however, unlike uncompatibilized blends, the recovery could no longer be captured by a single retardation time. The largest increase in ultimate recovery due to compatibilizer occurred at the lowest viscosity ratio. In contrast, the greatest slowing down of the recovery due to compatibilizer occurred at the highest viscosity ratio. The rheological data by themselves are insufficient to reach a definitive conclusion about the mechanism of compatibilizer action. The results are consistent with the effects of flow-induced gradients in compatibilizer concentration. An alternative constitutive modeling approach that captures compatibilizer effects in terms of an interfacial dilational elasticity can reproduce the recovery curves qualitatively, but some predictions of the model contradict experimental results. 相似文献
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Immiscible polymer blends are known to display an unusual elastic recovery after stress release. Recoil after steady-state
shearing is well understood and obeys specific scaling relations. Releasing the stress before the steady-state morphology
has been reached results in a more complex elastic recovery, including very large final values. This behaviour is investigated
systematically. Model blends are used, consisting of nearly inelastic components; hence the measured recoil can be attributed
totally to contributions from the interface. The instantaneous structure at the onset of the recoil can vary greatly in transient
experiments, ranging from slightly deformed droplets to highly elongated filaments. The effects of this initial structure
on the ultimate recoil and time scale of the recovery are studied. The morphological changes during recovery are considered
as well. It is demonstrated that they can be computed from the normal stresses during stress relaxation with comparable initial
morphologies. This indicates that the same morphological changes occur during stress relaxation and constrained recoil. A
scaling relation for the recoil curves has been derived from the Doi-Ohta theory, which is confirmed by the experiments.
Received: 9 December 1998 Accepted: 5 April 1999 相似文献
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Mosto Bousmina 《Rheologica Acta》1999,38(3):251-254
When interfacial tension is increased from zero to infinity, the storage dynamic modulus predicted by Palierne's model varies
in a nearly Gaussian fashion with almost equal asymptotic values at the limits of low and high interfacial tension. We report
a simple physical discussion of such an effect.
Received: 13 January 1999 Accepted: 22 March 1999 相似文献
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Carol B. Gell Ramanan Krishnamoorti Eugene Kim Dr. William W. Graessley Lewis J. Fetters 《Rheologica Acta》1997,36(3):217-228
Linear viscoelasticity and tracer diffusion were investigated as functions of temperature, component molecular weight and blend composition for entangled, single-phase blends of nearly monodisperse poly(ethylene-alt-propylene) (PEP) and head-to-head polypropylene (HHPP). Both components are non-polar and, despite evidence for slight differences of component glass temperatures in their blends, the viscoelastic data obey time-temperature superposition rather well. The properties of the blends were compared at constant T-T
g
(blend) with predictions of the tube-model theories. The composition dependence of viscosity agrees best with the double-reptation prediction, as had been found earlier for molecular weight blends. The variation in plateau modulus with composition is consistent with reptation, but the changes are too small to provide a definitive test. The tracer diffusion coefficients, D
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PEP and D
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HHPP are nearly independent of composition, consistent with the reptation prediction and in sharp contrast with tracer diffusion for blends with specific associations. Results for the recoverable compliance depart from this pattern, varying differently and much less strongly with composition than the predictions of either single or double reptation. It thus seems that microstructural blends may behave in significantly more complex ways than molecular weight blends even for components with only weak dispersive interactions and rather modest differences in glass temperature and plateau modulus.Dedicated to Prof. John D. Ferry on the occasion of his 85th birthday. 相似文献