Electrorheological properties of PDMS/carbon black suspensions under shear flow

Suspensions of polydimethylsiloxane (PDMS) containing low amounts (1 wt.% or less) of a highly conducting carbon black (CB) filler are rendered conductive and exhibit electrorheological (ER) responses under shear flow when exposed to an externally applied AC electric field. The presence of columnar structures, consisting of CB particles aligned in the direction of the electric field is evidenced through optical microscopy experiments. The appearance of yielding behavior and positive ER response, manifested by an increase in the viscosity of the suspensions, depend strongly on the filler loading, strength of the electric field, magnitude of the shear field, and viscosity of the medium. The responses are stronger at low filler loadings, below the percolation threshold, and at very low shear rates, where the microstructure of the dispersed phase remains intact. At higher shear rates, corresponding to Mason numbers (Mn) above 1, the structure is disrupted and thus does not contribute to the observed shear stress. The rheological characterization is accompanied with admittance measurements, to demonstrate that the induced polarization forces between particles lead to the formation of electrically conductive structures within the polymer matrix. A critical comparison with the qualitative predictions based on the theory of induced dipole–dipole interactions shows that the theory is valid for these dilute systems.     

Melt shear rheology of carbon nanofiber/polystyrene composites

The rheological behavior and morphology of carbon nanofiber/polystyrene (CNF/PS) composites in their melt phase have been characterized both through experimental measurements and modeling. Composites prepared in the two different processes of solvent casting and melt blending are contrasted; melt-blended and solvent-cast composites were each prepared with CNF loadings of 2, 5, and 10 wt%. A morphological study revealed that the melt blending process results in composites with shorter CNFs than in the solvent-cast composites, due to damage caused by the higher stresses the CNFs encounter in melt blending, and that both processes retain the diameter of the as-received CNFs. The addition of carbon nanofiber to the polystyrene through either melt blending or solvent casting increases the linear viscoelastic moduli, G′ and G″, and steady-state viscosity, η, in the melt phase monotonically with CNF concentration, more so in solvent cast composites with their longer CNFs. The melt phase of solvent-cast composites with higher CNF concentrations exhibit a plateau of the elastic modulus, G′, at low frequencies, an apparent yield stress, and large first normal stress difference, N ₁, at low strain rates, which can be attributed to contact-based network nanostructure formed by the long CNFs. A nanostructurally-based model for CNF/PS composites in their melt phase is presented which considers the composite system as rigid rods in a viscoelastic fluid matrix. Except for two coupling parameters, all material constants in the model for the composite systems are deduced from morphological and shear flow measurements of its separate nanofiber and polymer melt constituents of the composite. These two coupling parameters are polymer–fiber interaction parameter, σ, and interfiber interaction parameter, C _I. Through comparison with our experimental measurements of the composite systems, we deduce that σ is effectively 1 (corresponding to no polymer–fiber interaction) for all CNF/PS nanocomposites studied. The dependence of CNF orientation on strain rate which we observe in our experiments is captured in the model by considering the interfiber interaction parameter, C _I, as a function of strain rate. Applied to shear flows, the model predicts the melt-phase, steady-state viscosities, and normal stress differences of the CNF/PS composites as functions of shear rate, polymer matrix properties, fiber length, and mass concentration consistent with our experimental measurements.     

Impact of mixing procedure on phase morphology and fracture mechanical properties of carbon black-filled NR/SBR blends

Based on a viscoelastic model, the filler distribution and the amount of interphase of carbon black-filled blends of natural rubber (NR) with styrene-butadiene rubber (SBR) are evaluated. Hereby, the total dissipated energy \(G''\) during dynamical straining is decomposed into the contributions of the different polymer phases and the interphase. For the NR/SBR blends, we find a higher filling of the SBR phase and the interphase and a lower filling of the NR phase. The filler distribution itself depends not only on the affinity of the polymer to the filler but also on the mixing procedure. This is investigated by studying NR/SBR blends prepared by two different mixing procedures. In the standard mixing procedure, the polymers are mixed first, and then, the filler is added. In the batch mixing procedure, the filler is previously mixed in the NR only and then blended with SBR. Batch mixing is resulting in an increase in the filling of the interphase due to filler transfer from NR to SBR. The results for the filler distribution are compared to fatigue crack propagation rates under pulsed excitation. The crack propagation is accelerated when substituting NR with SBR. The batched samples show higher crack propagation rates at higher tearing energies due to a worse dispersion of the carbon black and/or higher filler loading of the interphase.     

Rheological behavior of a semi-crystalline polymer during isothermal crystallization

The theological behavior of a molten semi-crystalline polymer, namely, a high density polyethylene (HDPE), was investigated during isothermal crystallization from the melt, using dynamic oscillatory experiments at 1 tad/s in a parallel plates rheometer. The theological results were compared with those obtained from differential scanning calorimetry in the same conditions. During the crystallization, the molten and crystallizing polymer provides a useful model for filled polymers, the crystalline phase being the filler and the liquid phase being the matrix. In most cases, the filler can be considered to be spherical shaped (spherulites). Owing to the amorphous phase linking liquid and crystallites, the adhesion between matrix and filler in this system is perfect. The filler content increases continuously during the crystallization. This model might be used to test laws relating the theological parameters to the volume fraction of filler. Problems related to the rheometry for such systems are discussed and the key parameters insuring reproducibility and accuracy in the measurements are pointed out. The relative sensitivity of the various theological parameters (storage and loss moduli, loss angle) to structural changes of the liquid has been out forward. Some preliminary equations relating the variation of these parameters to the volume fraction of filler, through the use of simple fractal exponents have been derived and discussed in comparison with laws provided by various authors.     

Vapor pressure and void size effects on failure of a constrained ductile film

To achieve certain properties, semiconductor adhesives and molding compounds are made by blending filler particles with polymer matrix. Moisture collects at filler particle/polymer matrix interfaces and within voids of the composite. At reflow temperatures, the moisture vaporizes. The rapidly expanding vapor creates high internal pressure on pre-existing voids and particle/matrix interfaces. The simultaneous action of thermal stresses and internal vapor pressure drives both pre-existing and newly nucleated voids to grow and coalesce causing material failure. Particularly susceptible are polymeric films and adhesives joining elastic substrates, e.g. Ag filled epoxy. Several competing failure mechanisms are studied including: near-tip void growth and coalescence with the crack; extensive void growth and formation of an extended damaged zone emanating from the crack; and rapid void growth at highly stressed sites at large distances ahead of the crack, leading to multiple damaged zones. This competition is driven by the interplay between stress elevation induced by constrained plastic flow and stress relaxation due to vapor pressure assisted void growth.A model problem of a ductile film bonded between two elastic substrates, with a centerline crack, is studied. The computational study employs a Gurson porous material model incorporating vapor pressure effects. The formation of multiple damaged zones is favored when the film contains small voids or dilute second-phase particle distribution. The presence of large voids or high vapor pressure favor the growth of a self-similar damage zone emanating from the crack. High vapor pressure accelerates film cracking that can cause device failures.     

Co-continuity interval in immiscible polymer blends by dynamic mechanical spectroscopy in the molten and solid state

The work focuses on the detection of the co-continuity window in immiscible polymer blends. The purpose of the paper is to describe how rheological techniques can help to evaluate the composition range of the co-continuous morphology through the study of a particular system: PEO/PVDF-HFP. First, the blends were characterized by selective dissolution experiments and SEM observations. Then the ability of dynamic mechanical spectroscopy to detect the co-continuity was investigated in the melt and in the solid state. The evolution of the storage modulus of molten blends with their composition at a constant low frequency gives information about the co-continuity interval, especially as far as the onset of the continuity of the PEO phase is concerned. Then the immiscibility of the polymers and the continuity of PVDF-HFP as a function of blend composition have been highlighted by means of dynamic mechanical spectrometry below the melting point of PVDF-HFP. Comparison with results from classical methods shows fair agreement.This paper was presented at the first Annual European Rheology Conference (AERC) held in Guimarães, Portugal, September 11-13, 2003.     

Rheological properties of carbon black-filled blends of liquid-crystalline copolyester with thermoplastic polysulfone

Rheological properties of a two-phase polymeric blend containing LCCPE of poly(ethylene terephthalate) and p-hydroxybenzoic acid and thermoplastic polysulfone with varying concentrations of polymeric components and particulate filler have been studied. The theological behavior of such blends at different temperatures is governed by variation of the degree of ordering of LC-CPE macromolecules associated with the phase transition in the CPE at 260°C. Experimental results are discussed on the basis of concepts of compatibility of polymeric components in the melt or, if the system is incompatible, of the degree of interphase interaction between the components, as well as the impact of the filler and of the shear straining conditions on structurization in the system and compatibility. The filler exerts a compatibilizing effect on blend components, while the shear stress encourages the phase separation in the system. An extremal variation of viscosity of the LC-CPE/carbon black, silica and talk blends with the filler concentration on both at the flow in a uniform shear stress field and at the capillary flow has been found. Normalization of the filler concentration with respect to its specific surface yields a unified concentration dependence of the relative viscosity of LC-CPE filled with solid particles of various natures and specific surfaces.     

Viscoelastic plastic rheological model for particle filled polymer melts

A viscoelastic plastic model for suspension of small particles in polymer melts has been developed. In this model, the total stress is assumed to be the sum of stress in the polymer matrix and the filler network. A nonlinear viscoelastic model along with a yield criterion were used to represent the stresses in the polymer matrix and the filler network, respectively. The yield function is defined in terms of differential equations with an internal parameter. The internal parameter models the evolution of structure changes during floc rupture and restoration. The theoretical results were obtained for steady and oscillatory shear flow and compared with experimental data for particle filled thermoplastic melt. The experimental data included the steady state shear strress over a wide range of shear rates, the transient stress in a start up shear flow, stress relaxation after cessation of a steady state shear flow, the step shear and the oscillatory shear flow at various amplitudes.     

Rheological behavior of filled propylene/ethylene copolymers

A new family of propylene-ethylene copolymers developed by The Dow Chemical Company has narrow molecular weight distributions for industrial polymers and broad chemical composition distributions. This molecular architecture makes possible high filler loadings while maintaining good processability. To provide a fundamental understanding of the lower than expected viscosity, a study of the shear rheological behavior of two series of composites was performed. The composites consist of stearate-coated calcium carbonate particles suspended in a propylene-ethylene copolymer. The matrix in one series was a new copolymer, and in the other series, it was a traditional metallocene copolymer. For both systems, the viscosity increases dramatically with increasing filler loading; however, the viscosity is lower in the case of the composites of the new copolymer. The stearate coating on the calcium carbonate particles decreases the adhesion of the polymer to the filler surface, allowing particle-matrix interfacial slip. A high temperature atomic force microscopy study has indicated the existence of ethylene-enriched zones in the matrix immediately surrounding the particles in the new copolymer. We hypothesize that these high ethylene content chains around the particle surface enhance particle-matrix interfacial slip resulting in the lower composite viscosity.     

Effect of poly(ethylene oxide) on the rheological behavior of silica suspensions

The steady-shear viscosity, dynamic viscoelasticity, and sedimentation behavior were measured for silica suspensions dispersed in aqueous solutions of poly(ethylene oxide) (PEO). For suspensions prepared with polymer solutions in which the transient network is developed by entanglements, the viscosity at a given shear rate decreases, shows a minimum, and then increases with increasing particle concentration. Because the suspensions are sterically stabilized under the conditions where the particle surfaces are fully covered with by a thick layer of adsorbed polymer, the viscosity decrease can be attributed to the reduction of network density in solution. But under the low coverage conditions, the particles are flocculated by bridging and this leads to a viscosity increase with shear-thinning profiles. The polymer chains with high molecular weights form flexible bridges between particles. The stress-dependent curve of storage modulus measured by a stress amplitude sweep shows an increase prior to a drastic drop due to structural breakdown. The increase in elastic responses may arise from the restoring forces of extended bridges with high deformability. The effect of PEO on the rheological behavior of silica suspensions can be explained by a combination of concentration reduction of polymer in solution and flocculation by bridging.     

环境水氧对纳米氮化铝/聚四氟乙烯复合材料摩擦学性能的影响

采用超声分散法制备出纳米氮化铝/聚四氟乙烯(AlN/PTFE)复合材料,使用线性往复摩擦磨损试验机在大气和干燥氩气中对比测试了该复合材料摩擦学性能. 结果表明:大气环境下,纳米氮化铝质量分数为5%时可以将聚四氟乙烯磨损率降低4个数量级[1×10?7 mm3/(N·m)]. 而在同样摩擦测试条件的干燥氩气环境中,使用纳米氮化铝只能将聚四氟乙烯磨损率降低2个数量级[1×10?5 mm3/(N·m)]. 利用三维轮廓仪、扫描电镜、红外光谱仪和光电子能谱仪对金属对偶表面形成转移膜的形貌和化学成分进行分析. 研究发现:大气环境的摩擦过程中,聚四氟乙烯与环境水氧发生摩擦化学反应生成了富含羧酸盐的转移膜,显著提高了复合材料耐磨性能;干燥氩气中,水氧的缺失使复合材料无法在摩擦中生成富含羧酸盐的转移膜,影响材料耐磨性能的进一步提高.      

Dynamic and transient rheological properties of glass filled polymer melts

The dynamic and transient rheological properties of a low density polyethylene melt and a plasticized polyvinylchloride melt filled with glass beads were measured at 200 °C and 180 °C respectively in a modified Weissenberg Rheogoniometer R-17. Its main modification consisted of the use of a piezoelectric transducer instead of the conventional torsion bar, and of the interfacing of a microcomputer Apple II plus to the Rheogoniometer for data acquisition and analysis. The glass beads were pretreated with silane and titanate coupling agents to observe the effect of the chemical modification of the polymer filler interface on these properties. It was observed that both the dynamic viscosity and the storage modulus increased with the weight fraction, but this last parameter did appreciably affect the stress growth and stress relaxation curves of the polymeric matrices at low shear rates. The effect of coupling agents on these properties was varied.     

Rheology of immiscible blends with particle-induced drop clusters

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.     

A unified model for polystyrene–nanorod and polystyrene–nanoplatelet melt composites

We present a unified constitutive model capable of predicting the steady shear rheology of polystyrene (PS)–nanoparticle melt composites, where particles can be rods, platelets, or any geometry in between, as validated against experimental measurements. The composite model incorporates the rheological properties of the polymer matrix, the aspect ratio and characteristic length scale of the nanoparticles, the orientation of the nanoparticles, hydrodynamic particle–particle interactions, the interaction between the nanoparticles and the polymer, and flow conditions of melt processing. We demonstrate that our constitutive model predicts both the steady rheology of PS–carbon nanofiber composites and the steady rheology of PS–nanoclay composites. Along with presenting the model and validating it against experimental measurements, we evaluate three different closure approximations, an important constitutive assumption in a kinetic theory model, for both polymer–nanoparticle systems. Both composite systems are most accurately modeled with a quadratic closure approximation.     

Analysis of the influence of polymer viscosity on the dispersion of magnesium hydroxide in a polyolefin matrix

The dispersion of Mg(OH)₂ agglomerates at low concentration in a polymer melt was investigated in a transparent counter-rotating shear cell. The influence of the viscosity of the matrix, the initial agglomerate size and the infiltration of the matrix was evaluated. Mg(OH)₂ agglomerates have a low cohesion and a fractal structure. Two dispersive mechanisms already mentioned in the literature were identified: erosion and rupture. Critical conditions for rupture were measured and particle size analysis was performed in order to determine the kinetics of erosion. The infiltration of the matrix, which depends on the viscosity, was found to play a key role on dispersion mechanisms. In contrast to previous works, infiltration is more important with the high viscosity matrix. In infiltrated matrix, rupture was found to occur firstly through plastic deformation of the infiltrated agglomerate, and then the agglomerates split into small fragments. In the low viscosity matrix, fragments produced either by rupture or erosion are small aggregates.     

Rheological determination of polymer-filler affinity

An experimental method to quantify the affinity of polymers for certain fillers is presented. In this method, the relative viscosity of filled and unfilled polymer systems as determined by capillary rheometry is taken as a measure of the thickness of the adsorbed layer of polymeric material on the filler surface and hence of the affinity of the polymer for the filler. A styrene-butadienestyrene block copolymer is thus found to have a higher affinity for talc than do polypropylene and an ethylene-propylene-diene terpolymer.     

Design of new HDPE/silica nanocomposite and its enhanced melt strength

Linear polymers are restricted to use in processes that involve severe extensional deformation, such as fiber spinning, film blowing, and thermoforming. To extend their applicability, the extensional properties of polymer melts should be enhanced such that strain hardening, which is defined as an increase in extensional viscosity under a large strain that deviates from the linear viscoelastic curve, is pronounced. In this study, a novel preparation method of linear polymer/inorganic nanocomposites was proposed with a main focus on enhanced melt strength. The design of molecular structure consists of three components—linear polymer, compatibilizer, and surface-modified particles. High-density polyethylene was used as a linear polymer while polyethylene grafted with maleic anhydride was used as a compatibilizer. Silica particles were synthesized and modified on their surfaces by 3-aminopropyltriethoxysilane. The strain hardening behavior of the surface-modified silica composites was pronounced. However, such a result was not observed for the composites of the same composition with pure-silica. In addition, the dispersion of the modified silica was much better than that of pure-silica.     

磨损增强型超疏水材料的制备及性能研究

通过在有机硅烷中掺杂微纳米级尺度的SiO2颗粒,利用硅烷的水解和聚合成功地制备了1种具有优异耐磨性能和稳定性的超疏水材料.通过该方法所制备的超疏水材料不仅具有良好的耐磨性,而且其超疏水性能可在一定条件下通过表面的磨损得到增强或恢复.该超疏水材料在较苛刻的环境下仍能保持良好的化学稳定性.扫描电镜分析表明贯穿于整个材料且构成材料厚度的微纳米聚合物复合层是赋予该超疏水材料耐磨损性能的主要原因.     

Effect of combined polymers on the loss of efficiency caused by mechanical degradation in drag reducing flows through straight tubes

The mechanical molecular degradation in drag reducing flows is a huge problem in the effort to improve the efficiency of drag reducers, which is clearly increased when a combination of materials is used. Here, we analyze mixtures of three kinds of water-soluble polymers: Poly (ethylene oxide) (PEO), Polyacrylamide (PAM), and Xanthan Gum (XG). Two kinds of mixtures are tested: (a) PAM and XG; (b) PEO and XG. The synergy between the polymers is clearly noticeable. The values of the drag reduction (DR) obtained by the polymer–polymer combination was larger than that observed for a single polymer in a solution with the same total concentration of the mixture. Our tests are conducted in straight tubes where the total pressure is fixed. The values of DR are computed step-by-step, as the total amount of solution pass through the system. In doing so, we carefully took into account the loss of efficiency caused by the turbulent flow in the straight tubes. It is quite clear that the degradation of the flexible polymers (PEO and PAM) is delayed in the mixtures. In other words, besides the increase in the DR, the flexible polymers are more resistant when in the presence of the rigid one (XG). Such observation is the main conclusion of this work.     

Stress-induced interfacial failure in filled polymer melts

We study a dynamic interfacial slip phenomenon in filled polymer melts. When a poly(dimethyl siloxane) melt of weight average molecular weight M _w = 93 700 is mixed with glass beads of diameter up to 45 m, the sample shows some evidence of breakdown of interfacial adhesion between the glass beads and the PDMS matrix at a level of oscillatory stress under which the pure PDMS melt exhibits no decay. The decay of viscoelastic properties with time is essentially independent of the amplitude of shear strain as long as the magnitude of the oscillating stress is sufficiently high. It is suggested that much higher local stress than the apparent applied stress may be generated between the narrowly spaced beads. The interfacial slip was observable because it was measured against a natural length scale d in the filled polymer melts which is the filler size or the inter-filler distance and is much smaller than the dimensions of the flow cell. The decay under high stress and healing of interfacial adhesion upon a large reduction in stress may be related to the de-bonding and reformation of hydrogen bonds between the PDMS chains and the glass bead surfaces.