Extensional Viscosity Measurements Research Articles

Measurement and control of extensional viscosity in barrier coating dispersions

This study aimed to understand the effect of various rheological additives on the extensional viscosity of barrier coating dispersions, as well as to understand the role extensional viscosity plays in stabilizing a liquid curtain. The apparent extensional viscosity was measured using two devices that create accelerating flows: a capillary viscometer and an orifice rheometer. Additives tested include several polyvinyl alcohols, a high molecular weight polyethylene oxide, and carboxymethylcellulose. Extensional viscosity plays a significant role in stabilizing a liquid curtain, as it slows down hole expansion and prevents impurities and disturbances from causing holes in the first place. Some of the additives could substantially increase the extensional viscosity of the dispersions without increasing the shear viscosity outside the typical range of processability for a curtain coater. Some of the additives exhibited coil-stretch transition, meaning they start increasing extensional viscosity above a certain extension rate. Polymers with low chain lengths exhibited finite extensibility, which indicates the polymer chain has fully extended and cannot provide further extensional viscosity, even though the extending force is increased. Polymeric additives with stiff or branched chains significantly raised shear viscosity without increasing extensional viscosity. Both methods could reliably measure extensional viscosity in curtain coating barrier dispersions.

Extensional rheometry of mobile fluids. Part II: Comparison between the uniaxial, planar, and biaxial extensional rheology of dilute polymer solutions using numerically optimized stagnation point microfluidic devices

Part I of this paper [Haward et al., J. Rheol. 67, 995–1009 (2023)] presents a three-dimensional microfluidic device (the optimized uniaxial and biaxial extensional rheometer, OUBER) for generating near-homogeneous uniaxial and biaxial elongational flows. Here, in Part II, the OUBER device is employed to examine the uniaxial and biaxial extensional rheology of model dilute polymer solutions, compared with measurements made under planar extension in the optimized-shape cross-slot extensional rheometer [OSCER, Haward et al. Phys. Rev. Lett. 109, 128301 (2012)]. In each case, micro-particle image velocimetry is used to measure the extension rate as a function of the imposed flow conditions, and excess pressure drop measurements enable estimation of the tensile stress difference generated in the fluid via a new analysis based on the macroscopic power balance for flow through each device. Based on this analysis, for the most dilute polymer sample tested, which is “ultradilute”, the extensional viscosity is well described by Peterlin’s finitely extensible nonlinear elastic dumbbell model. In this limit, the biaxial extensional viscosity at high Weissenberg numbers (Wi) is half that of the uniaxial and planar extensional viscosities. At higher polymer concentrations, although the fluids remain dilute, the experimental measurements deviate from the model predictions, which is attributed to the onset of intermolecular interactions as the polymer chains unravel in the extensional flows. Of practical significance (and fundamental interest), elastic instability occurs at a significantly lower Wi in uniaxial extensional flow than in either biaxial or planar extensional flow, thereby limiting the utility of this flow type for extensional viscosity measurement.

Plantago Ovata Husk: An Investigation of Raw Aqueous Extracts. Osmotic, Hydrodynamic and Complex Rheological Characterisation

The aim of the study was to characterize raw aqueous extracts from Plantago ovata husk in terms of molecular chain mass, osmotic, hydrodynamic, and rheological properties. The raw extracts used in this study have not been yet investigated in the indicated research area. Determination of the molecular weight of the chains present in the extract was performed by gel permeation chromatography (GPC). Osmotic properties were characterized using membrane osmometry. Rheological properties were investigated via classical rotational rheology with normal force measurements, as well as less common but equally important measurements of extensional viscosity. Two types of chains with an average molecular mass of 200 and 1780 kDa were found. The values of the first virial coefficient (B2) indicate the predominance of biopolymer-biopolymer interactions. The hydrodynamic radius established at 25 and 30 °C was 74 and 67 nm, respectively, and lower than at 40 °C (>600 nm). The first critical concentration was determined: c*=0.11 g·dL-1. The dominance of negative normal force values resulting from the formation of a pseudo-gel structure of the heteroxylates was demonstrated. Extensional viscosity measurement results revealed that the studied extracts cannot be treated as simple shear-thinning fluids, as indicated by shear flow, but should be considered as viscoelastic fluids.

Modification of polylactide by reactive blending with polyhydroxybutyrate oligomers formed by thermal recycling through E1cB-elimination pathway

This study aimed to thermally recycle polyhydroxybutyrate (PHB), a microbial polyester with low thermal stability and limited re-processability, into crotonate-ended oligomers through the E1cB-elimination pathway. The resulting products were used as a functional additive to improve the mechanical and rheological properties of polylactide (PLA), which has intrinsic brittleness and low melt stability. The PLA was modified through a reactive blending process in an internal mixer. The quantitative 1H NMR analysis confirmed the crotonate functionality of the oligomers and their consumption during the reactive blending process. Tensile testing revealed that reactive blending 20 wt% PHB oligomers with PLA improve tensile toughness 24 times over pure PLA while keeping the elastic modulus above 1 GPa. Oscillatory-shear rheometry confirmed the improved shear-thinning behavior of reactive blends, while extensional viscosity measurements revealed a significant strain hardening behavior. The increase in melt stability was attributed to the formation of high molecular weight structures as a result of possible chain extension and branching reactions. DSC traces showed that branching and grafting points acted as nucleation sites, accelerating PLA melt crystallization. It was also found that increasing the PLA crystallinity above 44% gives rise to crystallization-induced phase separation, despite initial miscibility.

Effect of viscoelasticity on the foaming behaviour of long-chain branched polypropylene with different branching degrees analysed by using bubble-growth modelling

Long-chain branched polypropylene (LCB-PP) was generated by a melt grafting reaction with trimethylolpropane triacrylate (TMPTA) as the grafting monomer and zinc dimethyldithiocarbamate (ZDMC) or styrene (St) as the co-monomers. High-temperature size exclusion chromatography (HT-SEC) was used to characterize the molecular mass, molecular mass distribution and branching degree of the modified PP. Subsequently, linear and nonlinear viscoelasticity data were obtained by a small amplitude oscillatory shear (SAOS) test and uniaxial extensional viscosity measurement. The pressure balanced bubble-growth (PBB) model was used to calculate the critical coalescence radius (CCR) curves for PP to quantitatively correlate the viscoelasticity and foamability and evaluate the foamability of PP combined with batch foaming. It is shown that with increasing branching degree, the foam shows a better resistance to bubble coalescence, resulting in a more regular bubble structure and a higher expansion ratio. Moreover, viscosity should be controlled over an appropriate range for realising good foamability; otherwise, excessive viscosity can suppress bubble growth and limit the expansion ratio. MPPZ4 modified with 4 wt% TMPTA and 0.4 wt% ZDMC showed the best foamability. A combination of the PBB model and batch foaming experiment is used to quantitatively explain the contribution of LCB to foaming.

Instrumental characteristics from extensional rheology and tribology of polysaccharide solutions.

Instrumental characteristics from extensional rheology and tribology for aqueous xanthan gum (XG) and locust bean gum (LBG) solutions were studied in the presence or absence of simulated saliva. Extensional viscosity was calculated from the filament shrinkage behavior using a capillary breakup extensional rheometer, whereas the friction coefficient was measured using a set-up consisting of polydimethylsiloxane substrate and a glass ball bearing on a rotational rheometer. Increase in extensional viscosity was detected immediately after initiation of extensional flow, particularly XG, and also immediately before the filament rupture, particularly LBG. Extensional viscosity tended to decrease with increased addition of simulated saliva for XG, while to increase for LBG. In both cases, effect of cations in the saliva was greater than that of mucin. From the shape of the Stribeck curve (i.e., dependence of the friction coefficient on the entrainment speed) and comparison of the friction coefficient itself, lubricity of XG was greater than that of LBG. Simulated saliva added decreased the friction coefficient for each polysaccharide through functions of cations rather than mucin. Extensional viscosity and tribological measurements revealed mechanical properties of polysaccharide solutions which cannot be determined or quantified by shear viscosity alone.

Development of an online rheometer for simultaneous measurement of shear and extensional viscosity during the polymer extrusion process

Abstract The demand for valid and reliable online process surveillance control of modern production plants in extrusion and compounding has considerably increased in recent years. We present an online rheometer for simultaneously measuring the extensional and shear viscosity of polymer melts and filled compounds during processing. The unique die design features a hyperbolic slit contraction that enables constant extensional rates along the die. Further, pressure transducers can be incorporated directly into the two slit sections, which prevents material from accumulating in pressure holes. The rheometer was implemented to a compounding line containing a Leistritz ZSE27 MAXX co-rotating twin-screw extruder and strand pelletizing. We demonstrated the applicability for condition monitoring based on several examples. The rheological behavior of a polypropylene random copolymer is characterized during twelve successive extrusion cycles. We compared our results to oscillatory shear measurements with a plate-plate rheometer. Decrease of extensional viscosity were less severe than the decrease of shear viscosity during multiple extrusion runs. Blending a high and a low viscous polypropylene produced similar behavior. The Trouton ratio which is based on both the extensional and shear viscosity was introduced as an additional quality parameter. For distinguishing between different PP/PS compounds or PP glass fiber compounds, the Trouton ratio is a valid and beneficial parameter. Additionally, different methods for evaluating the melt flow rate (MFR) from shear viscosity data were compared.

Gellan gum: A new member in the dysphagia thickener family

In this study, gellan gum (GG) (0.075–0.3 wt%) is proposed as a new dysphagia thickener and compared against commercial starch-based thickeners (modified starch with or without gums, 5 wt%) and xanthan gum (XG, 0.5–1.0 wt%) using apparent viscosity, oral tribology using polydimethylsiloxane (PDMS) ball-on-disc set up and ζ-potential measurements. The measurements were conducted in presence of artificial saliva containing mucin with or without α-amylase at 37 °C. Viscosity results suggested that the commercial starch-based thickeners behaved like water in orally relevant shear, largely associated with the hydrolysis of modified starch by α-amylase, whereas, XG and GG showed no responsiveness to α-amylase. In the case of oral tribology, artificial saliva containing mucin adsorbed to the PDMS surfaces reducing friction as compared to water. The increase in boundary friction coefficients in commercial starch-based thickeners was likely associated with α-amylase-induced hydrolysis, increasing the PDMS-PDMS asperity contacts. Interestingly, the tribological behaviour of XG and GG was dictated mainly by viscous lubrication. However, in simulated oral conditions, the increase in friction coefficients in presence of XG and GG was influenced by depletion of artificial saliva from the PDMS surfaces due to electrostatic interaction between the gums and mucin. A combination of rheological and tribological techniques in orally relevant conditions appears as a reliable approach to understand the potential of GG (0.3 wt%) to act as a dysphagia thickener that offers similar mechanical properties as XG (1.0 wt%) at a lower concentration. Extensional viscosity measurement of GG is needed to understand its applications in dysphagia management.

The application of an extrusion modular slit head of a special construction in the in-line extensional viscosity measurements of polymers

The article has presented the measuring capabilities of the author's modular slit head for the assessment of extensional viscosity (ηE). The measurements were carried out using an in-line extruder rheometer. The test material was low-density polyethylene (LDPE). The assessment of the extensional rate (ε˙) and extensional viscosity was made based on the analysis of the entrance pressure loss (Δpe), as proposed by Cogswell, the so-called convergent channel method. The unique construction of the head and the flanged measuring module connection system enabled the characteristic of the polyethylene to be determined in the conditions of extensional flow in a convergent and divergent slit channels.

The evaluation of extensional viscosity of highly filled polyolefins composites films with calcium carbonate

This article reports the results of the extensional viscosity ( ) of polyolefins composites films. The test material was composed of ternary MDPE/iPP/CaCO3 composites with a calcium carbonate content of 48–72% by mass. The concentrations of the composites and reference materials that were PE‐MD/iPP mixtures are summarized in Table 1. For all materials the viscosity curves (rotational rheometer) and the change in mass flow rate were determined. Two types of investigated films were produced: cast films and blow films. The extensional viscosity of films (thickness ≈ 80 μm) was performed on a SER‐2 Universal Testing Platform. The influence of the addition of calcium carbonate, extrusion techniques and the direction of sample cut (MD and TD) to change the extensional viscosity were discussed. The extensional viscosity measurements ( ) were taken at a temperature of 140°C and for a strain rate (Hencky strain rate) of = 0.1, 0.2 and 0.5 s−1. POLYM. ENG. SCI., 59:E155–E163, 2019. © 2018 Society of Plastics Engineers

Unraveling the Bimodality of Polypropylene Film Grades Using Rheological Shear and Elongational Measurements: Inconsistent Results of Gel Permeation Chromatography

As the main parameter to describe the molecular structure of a linear polymer, molecular weight distribution of five different polypropylene grades was evaluated through measurements of gel permeation chromatography and rheology experiments. From their dynamic shear data, the relaxation spectra, h(τ), were calculated from which molecular weight distribution was estimated using molecular viscoelastic theories and then compared with the gel permeation chromatography results. Generally good agreement, yet with narrower molecular weight distribution rheology curves was found. Exception was observed for one grade for which rheological data predicted bimodal distribution curve comprising a small shoulder of high molecular weight, not seen in the corresponding gel permeation chromatography curve. Additionally, a higher value of the generalized mixing parameter (i.e. β) was found for this bimodal grade which was attributed to an increase in the number of entanglements per chain and better network connectivity. Furthermore, measurements of the transient extensional viscosity also revealed enhanced strain hardening and melt strength for this particular grade. The results showed that small amount of high molecular weight fraction of linear chains could drastically alter the processing behavior of polypropylene film grades.

Passive non-linear microrheology for determining extensional viscosity

Extensional viscosity is a key property of complex fluids that greatly influences the non-equilibrium behavior and processing of polymer solutions, melts, and colloidal suspensions. In this work, we use microfluidics to determine steady extensional viscosity for polymer solutions by directly observing particle migration in planar extensional flow. Tracer particles are suspended in semi-dilute solutions of DNA and polyethylene oxide, and a Stokes trap is used to confine single particles in extensional flows of polymer solutions in a cross-slot device. Particles are observed to migrate in the direction transverse to flow due to normal stresses, and particle migration is tracked and quantified using a piezo-nanopositioning stage during the microfluidic flow experiment. Particle migration trajectories are then analyzed using a second-order fluid model that accurately predicts that migration arises due to normal stress differences. Using this analytical framework, extensional viscosities can be determined from particle migration experiments, and the results are in reasonable agreement with bulk rheological measurements of extensional viscosity based on a dripping-onto-substrate method. Overall, this work demonstrates that non-equilibrium properties of complex fluids can be determined by passive yet non-linear microrheology.

Extensional flow of carboxymethylcellulose sodium salt measured on the opposed-nozzle device

ABSTRACTThe study presents results of the shear viscosity and apparent extensional viscosity measurements performed for semidilute solutions of carboxymethylcellulose sodium salt with two different average molecular weights and three different degrees of substitution, in distilled water and with the addition of NaCl. An increase in the weight average molecular weight of Na-CMC triggers a rapid increase in the apparent extensional viscosity and the Trouton ratio. Another finding was that the values of the apparent extensional viscosity and the Trouton ratio rose with a decrease in the degree of substitution, which can be attributed to the presence of interchain hydrogen bonds. The addition of NaCl to the Na-CMC solutions induces a reduction in the apparent extensional viscosity and the Trouton ratio, which can be linked to a change in the configuration of the polyelectrolyte chain.

Fiber-spun polypropylene/polyethylene terephthalate microfibrillar composites with enhanced tensile and rheological properties and foaming ability

Modification of the mechanical and rheological properties of polypropylene (PP) is accomplished through the in-situ generation of polyethylene terephthalate (PET) fibrils via fiber spinning of PP/PET (95/5 wt%). This modification increases the tensile strength of PP fibers by up to 46% and elongation at break by up to seven-folds. The change in the tensile properties is governed by the draw ratio. Uniaxial extensional viscosity measurements show a strain-hardening behavior in the fiber-spun PP/PET, not observed in the neat PP or in the melt-blended PP/PET. The oscillatory shear behavior in the linear viscoelastic region is studied to understand the percolation properties of PET fibrils in PP. When the PET domains are fibrillated, the storage (G′) and loss (G″) moduli increase and their slopes decrease at low frequencies compared to neat PP or melt-blended PP/PET, indicating that the fibril network responds elastically over long timescales. The wide-angle X-ray scattering (WAXS) data shows the presence of γ-polymorph crystals of PP in both PP and the fiber-spun PP/PET. Foam extrusion is used as a model polymer process to study the effect of the PET fibrils on the processability of PP. Results reveal that the presence of PET fibrils in PP yields foams that exhibit up to two orders of magnitude higher cell densities and up to a five-fold increase in the expansion ratio relative to the neat PP. Enhancing the foaming ability of polymer blends by fibrillating the dispersed phase using fiber spinning is technologically promising.

A reactive extrusion process with the aid of ultrasound for preparing cross-linked polypropylene

Introducing the cross-linked structure in polypropylene (PP) was highly desired to meet the requirement of high melt strength, which was a key for the application of PP in thermoforming, blow molding and foaming where elongation flows dominated. In this work, the power ultrasound was introduced into the reactive extrusion at the exit die of an extruder for preparing cross-linked PP. The main target was to maintain the highly cross-linked structure during reactive extrusion process in case of low peroxide concentration. The results of the dynamic rheological properties, melt flow index, and extensional viscosity measurement showed that the content of PP gel and the melt strength of PP increased obviously with the employment of a 300 W ultrasound while the contents of functional monomers and peroxide were constant. Fourier transform-infrared spectroscopy analysis, differential scanning calorimeter characterization, and gel permeation chromatograph measurement proved that the increases of content of PP gel and melt strength of PP were caused by that the ultrasonic waves could induce the chain scission and recombination reaction when the PP melts flowed through the exit die assembled with a ultrasonic probe. POLYM. ENG. SCI., 2016. © 2016 Society of Plastics Engineers

Measurements and modeling of temperature-strain rate dependent uniaxial and planar extensional viscosities for branched LDPE polymer melt

Abstract In this work, novel rectangle and circular orifice (zero-length) dies have been utilized for temperature-strain rate dependent planar and uniaxial extensional viscosity measurements for the LDPE polymer melt by using standard twin bore capillary rheometer and Cogswell model and the capability of five different constitutive equations (novel generalized Newtonian model, original Yao model, extended Yao model, modified White-Metzner model, modified Leonov model) to describe the measured experimental data has been tested. It has been shown that chain branching causes the strain hardening occurrence in both uniaxial and planar extensional viscosities and its maximum is shifted to the higher strain rates if the temperature is increased. The level of uniaxial extensional strain hardening for the branched LDPE sample has been found to be higher in comparison with the planar extensional viscosity within wide range of temperatures.

Rheology of natural and imitation mozzarella cheese at conditions relevant to pizza baking

The rheology of mozzarella and imitation cheese was studied at 60 °C, with small amplitude oscillatory shear (SAOS), shear and extensional flow measurement at low strain rates in the range 0.01–1 s−1. These conditions were chosen to replicate those experienced by the cheese during pizza baking and consumption. The extensional viscosity measurements were carried out by means of hyperbolic contraction flow, an alternative method to traditional extensional measurements. The extensional viscosity measured by hyperbolic contraction flow was related to the cheese elasticity and consequently to its stretchability, a major quality characteristic when this is consumed on pizza pies. The rheology of the two cheese materials could be explained by the structural observations made by confocal laser scanning microscopy (CLSM).

Relationship of Extensional Viscosity and Liquid Crystalline Transition to Length Distribution in Carbon Nanotube Solutions

We demonstrate that the length of carbon nanotubes (CNTs) can be determined simply and accurately from extensional viscosity measurements of semidilute CNT solutions. The method is based on measuring the extensional viscosity of CNT solutions in chlorosulfonic acid with a customized capillary thinning rheometer and determining CNT aspect ratio from the theoretical relation between extensional viscosity and aspect ratio in semidilute solutions of rigid rods. We measure CNT diameter d by transmission electron microscopy (TEM) and arrive at CNT length L. By studying samples grown by different methods, we show that the method works well for CNT lengths ranging from 0.4 to at least 20 μm, a wider range than for previous techniques. Moreover, we measure the isotropic-to-nematic transition concentration (i.e., isotropic cloud point) φiso of CNT solutions and show that this transition follows Onsager-like scaling φiso ∼ d/L. We characterize the length distributions of CNT samples by combining the measurements of ...

Correlation between in-line measurements of tomato ketchup shear viscosity and extensional viscosity

The viscosity and shear thinning behavior are essential characteristics of tomato ketchup. A real-time monitoring of those characteristics during processing is important to obtain a good quality of the final product and to reduce production waste. This work investigates the measurement of rheological in-line flow properties of tomato ketchup, using a real-time technique that combines ultrasound velocity profiling (UVP) and pressure difference (PD) assessment. In-line data were compared to those obtained off-line using a rotational viscometer. There was a poor correlation with the Bostwick measurement, whereas the flow curves calculated from flow velocimetry data were very similar to those measured off-line. The extensional viscosity of ketchup was determined through the measurement of Hyperbolic Contraction Flow; the curve followed a trend similar to that for the shear viscosity over the deformation rate investigated.

Generation of inkjet droplet of suspension in polymer solution

In the present study the generation of inkjet drops of suspensions of spherical particles in polyvinylpyrrolidone (PVP) solution in 1-heptanol is investigated experimentally. The particle size was 2 µm and the particle volume fraction was in the range of 0–0.18. The molecular weight of PVP was 1,300 kg/mol and the concentration was 0–4,000 ppm. The diffusive wave spectroscopy measurement shows that the PVP solution has a weak elasticity. The extensional viscosity measurement reveals that both the polymer solution and the suspension show strain hardening behavior. The jetting experiments show that the drop velocity of the suspension is larger than that of the polymer solution without particles. The difference in drop velocity is ascribed to the combined effect of the extensional viscosity and shear viscosity. The drop size of the suspension is larger than the drop size of the polymer solution without particles at the same driving voltage even though the viscosity of the suspension is larger. This difference is ascribed to the change in the free surface shape near the nozzle exit due to the viscosity difference and the increased inertial term due to the density increase by the addition of particles. Because the surface tensions of the fluids tested here are the same, the effect of surface tension force cannot be examined even though the force analysis shows that the surface tension force is larger than the other forces such as extensional and convective forces.