Moshe Gottlieb

The viscosity-volume fraction relation for suspensions of rod-like particles by falling-ball rheometry

The relative viscosities of suspensions of randomly oriented rods in a Newtonian fluid were measured using falling-ball rheometry. The rods were monodisperse and sufficiently large to render colloidal and Brownian forces negligible. Steel and brass ball bearings were dropped along the centreline of cylindrical columns containing the suspensions. The terminal velocities of the falling balls were measured and used to determine the average viscosities of the suspensions. The suspensions behaved as Newtonian fluids in that they were characterized by a constant viscosity. They exhibited a linear relative viscosity-volume fraction relationship for volume fractions less than 0.125, and, for volume fractions between 0.125 and 0.2315, the specific viscosity increased with the cube of the volume fraction. The relative viscosity was found to be independent of falling-ball size for a ratio of falling ball to fibre length greater than 0.3. It was found to be independent of the diameter of the containing cylindrical column for a ratio of column diameter to fibre length greater than 3.2. The value determined for the intrinsic viscosity is in good agreement with theoretical predictions for suspensions of randomly oriented rods.

Surface-tension-driven breakup of viscoelastic liquid threads

A linearized stability analysis is carried out for the breakup of small-diameter liquid filaments of dilute polymer solutions into droplets. Oldroyds 8-constant model expressed in a corotational reference frame is used as the rheological equation of state. The crucial idea in this theory is the recognition that the liquid may be subject to an unrelaxed axial tension due to its prior history. If the tension is zero, the present analysis predicts that jets of shear-thinning liquids are less stable than comparable jets of Newtonian liquids; this is in agreement with previous analyses. However, when the axial tension is not zero, and provided the stress relaxation time constant is sufficiently large, the new theory predicts that the axial elastic tension can be a significant stabilizing influence. With reasonable values for the tension and stress relaxation time the theory explains the great stability observed for jets of some shear- thinning, dilute polymer solutions. The theory explains why drops produced from jets of such liquids are larger than drops from corresponding Newtonian liquids. The theory also appears capable of explaining the sudden appearance of irregularly spaced bulges on jets after long distances of t,ravel with little amplification of disturbances.

Experimental tests of entanglement models of rubber elasticity: 1. Uniaxial extension-compression

Abstract The predictions of several entanglement models of rubber elasticity for the uniaxial stress-strain response of crosslinked polymer networks are examined. It is found that the Gaylord tube model and the Flory constrained junction fluctuation model both agree well with the experimental data.

Optimization of a flame-retarded polypropylene composite

Abstract Composite materials demand constant improvements in mechanical and flame retardant (FR) properties. The goal of this project is to study the effect of additives on these properties in polypropylene composites containing glass fibers, pentabromobenzyl acrylate (PBBMA) as a primary FR and magnesium hydroxide as a secondary FR. Optimal composition is reached by means of statistical design of experiments (DOE) rather than by “trial and error” approach. The DOE approach allows minimization of the number of experiments, investigation of the influence of each additive and the mutual interactions between additives. It also allows here, prediction of optimal sample properties better than 8% from experimental values. The optimal composition exhibits improved mechanical and FR properties. Both FRs reduce the impact strength while enhancing flame retardancy. Glass fibers increase the modulus, but have only a moderate effect on the impact strength due to poor adhesion with PP. The interpretation of the effect of glass fibers on the flammability is inconclusive.

Effect of interfaces on the crystallization behavior of PDMS

The reversible thermal behavior of a non-entangled semicrystalline polymer, poly(dimethylsiloxane), PDMS, was investigated in the presence of sub-micron particles. Filled polymer systems of this type are characterized by a large surface-to-volume ratio but lack the external confinement that is typical for a thin film geometry. Differential-scanning calorimetry (DSC) measurements indicate that the presence of the nanometric solid additives enhances the crystallization rate as compared to native PDMS melts. Different types of additives and surface interactions resulted in a similar effect, suggesting that the origin of the enhanced crystallinity is non-specific. The effect is attributed to entropic interactions in the boundary layer.

Dilute solution viscosity of red microalga exopolysaccharide

The red microalga Porphyridium sp (P. sp) is encapsulated in a sulphated polysaccharide. The external part of this capsule dissolves in the growth medium. This extracellular polysaccharide is a heteropolyelectrolyte with molecular weight of ∼ 6 × 106 Da. The effect of solvent, counterion and pH on chain flexibility and structural features in dilute solution of the exopolysaccharide was investigated by intrinsic viscometry. From the dependence of the intrinsic viscosity [η] on ionic strength, it was estimated that the stiffness of P. sp polysaccharide chains is in the same range as that of xanthan and DNA. The effect of the counterion on [η] is found to be specific and dependent on the type and valence of the counterion. The polyelectrolyte behaviour of the polymer is confirmed by the decrease of [η] with the addition of salt without any observable order-disorder conformational transition in aqueous salt solutions in the commonly used range of ionic strength (0.01–1.0). At considerably lower ionic strength (<0.01) there is an indication of a transition in the P. sp polyion conformation, most likely reflecting a contraction of the polymer chain from a highly stretched to a stiff, wormlike chain. It is hypothesized from the overall dilute solution features that the P. sp biopolymer chain molecules adopt stiff ordered conformation in solution.

Rheological behavior of a suspension of randomly oriented rods

Falling‐ball rheometry is used to study a suspension (volume fraction solids=0.05) of large, neutrally buoyant rods (length=3.175 cm, diameter=0.154 cm) in a viscous Newtonian liquid. We find that all sizes of balls studied (0.65 cm ≤ diameter ≤1.90 cm) experience the same continuum that is characterized by a single viscosity which we have measured to be 2.41 times greater than that of the pure suspending liquid. The viscosity increase due to the presence of the rods is over 12 times greater than that observed for a suspension of uniform spheres having the same volume fraction. The results are free of any flow induced clustering and orientation effects and are in reasonable agreement with theoretical predictions for randomly oriented rods.

The steady shear viscosity of filled polymeric liquids described by a linear superposition of two relaxation mechanisms

Filled polymeric liquids often exhibit apparent yielding and shear thinning in steady shear flow. Yielding results from non-hydrodynamic particle—particle interactions, while shear thinning results from the non-Newtonian behavior of the polymer melt. A simple equation, based on the linear superposition of two relaxation mechanisms, is proposed to describe the viscosity of filled polymer melts over a wide range of shear rates and filler volume fraction.The viscosity is written as the sum of two generalized Newtonian liquid models. The resulting equation can describe a wide range of shear-thinning viscosity curves, and a hierarchy of equations is obtained by simplifying the general case. Some of the parameters in the equation can be related to the properties of the unfilled liquid and the solid volume fraction. One adjustable parameter, a yield stress, is necessary to describe the viscosity at low rates where non-hydrodynamic particle—particle interaction dominate. At high shear rates, where particle—particle interactions are dominated by interparticle hydrodynamics, no adjustable parameters are necessary. A single equation describes both the high and low shear rate regimes. Predictions of the equation closely fit published viscosity data of filled polymer melts.

Amphiphilic poly(ethylene oxide)/poly(dimethylsiloxane)polymers:: 1. Synthesis and characterization of cross-linked hydrogels

Three types of poly(ethylene oxide) (PEO) networks cross-linked by silicone-based compounds have been formed. One type of network prepared from allyl-substituted poly(ethylene glycol) (PEG), is stable whereas the other two are degradable. The degree of swelling of the resulting hydrogels depends strongly on the PEO/silicone ratio and on temperature. From the weight fraction of soluble material in the networks, the extent of cross-linking reaction has been estimated. The molecular weight between cross-links has been calculated independently from the weight fraction of soluble material in the networks and from swelling data.

Rheological investigation of single-walled carbon nanotubes – induced structural ordering in CTAB solutions

The rheological behavior of aqueous dispersions of single-walled carbon nanotubes (SWNTs) in solutions of the cationic surfactant cetyl trimethyl ammonium bromide (CTAB) was investigated. The steady shear viscosity as a function of the applied shear rate was monitored in different concentrations of surfactant which correspond to different mesophases. We found that the presence of SWNTs had a dramatic effect on the behavior of the combined system not observed with other additives: a significant increase in the low shear-rate viscosity of SWNT dispersions, and shear thinning replacing Newtonian behavior were observed for CTAB concentrations below the onset of the surfactant hexagonal phase. As CTAB concentration increases the rheological behavior of the SWNT-CTAB system and the native CTAB solutions become more alike. We suggest that the origin of the observed phenomena is the good size-match between SWNTs and elongated CTAB micelles. Thus dispersed SWNTs may induce the formation of size-matched elongated CTAB micelles that further orient under the action of external shear. A similar effect was not observed in dispersions of multi-walled carbon nanotubes or carbon black particles, suggesting that the cooperative behavior is not invoked when significant size-mismatch exists between the surfactant micelles and the dispersed additives.