Creep recovery behavior of metallocene linear low-density polyethylenes

The rheological behavior of two metallocene linear low-density polyethylenes (mLLDPE) is investigated in shear creep recovery measurements using a magnetic bearing torsional creep apparatus of high accuracy. The two mLLDPE used are homogeneous with respect to the comonomer distribution. The most interesting feature of the two mLLDPE is that their molecular mass distributions are alike. Therefore, as one of the mLLDPE contains long-chain branches, the influence of long-chain branching on the elastic properties of polyethylene melts could be investigated. It was found that long-chain branches increase the elasticity of the melt characterized by the steady-state recoverable compliance. The long-chain branched mLLDPE has a flow activation energy of 45 kJ/mol which is distinctly higher than that of the other mLLDPE. The shear thinning behavior is much more pronounced for the long-chain branched mLLDPE. A discrepancy between the weight average molecular mass M _w calculated from size exclusion chromatography measurements by the universal calibration method and the zero shear viscosities of the two mLLDPE was observed. These observations are discussed with reference to the molecular architecture of the long-chain branched mLLDPE. The rheological properties of the long-chain branched mLLDPE are compared with those of a classical long-chain branched LDPE. It is surprisingly found that the rheological behavior is very much the same for these two products although their molecular mass distributions and presumedly the branching structures differ largely. Received: 15 February 1999 Accepted: 10 June 1999     

Influence of long-chain branches in polyethylenes on linear viscoelastic flow properties in shear

 This contribution presents a survey on the influence of long-chain branching on the linear viscoelastic properties zero shear-rate viscosity and steady-state recoverable compliance of polyethylene melts. The materials chosen are linear and slightly long-chain branched metallocene-catalyzed polyethylenes of narrow molecular mass distribution as well as linear and highly long-chain branched polyethylenes of broad molecular mass distribution. The linear viscoelastic flow properties are determined in shear creep and recovery experiments by means of a magnetic bearing torsional creep apparatus. The analysis of the molecular structure of the polyethylenes is performed by a coupled size exclusion chromatography and multi-angle laser light scattering device. Polyethylenes with a slight degree of long-chain branching exhibit a surprisingly high zero shear-rate viscosity in comparison to linear polyethylenes whereas the highly branched polyethylenes have a much lower viscosity compared to linear samples. Slightly branched polyethylenes have got a higher steady-state compliance in comparison to linear products of similar polydispersity, whereas the highly branched polyethylenes of broad molecular mass distribution exhibit a surprisingly low elasticity in comparison to linear polyethylenes of broad molecular mass distribution. In addition sparse levels of long-chain branching cause a different time dependence in comparison to linear polyethylenes. The experimental findings are interpreted by comparison with rheological results from literature on model branched polymers of different molecular topography and chemical composition. Received: 12 July 2001 Accepted: 30 October 2001     

Influence of thermal history on rheological properties of various bitumen

This paper focuses on the influence of thermal history on the rheological properties of unmodified and polymer modified bitumen (PMB), measured at elevated service temperatures, and contributes to the development of test methods for measuring binder properties, which can be used as indicators for asphalt rutting. It was found that the storing and preparation conditions prior to the rheological measurement can have a large influence, especially in the range of long loading times or low frequencies. For elastomer modification, the homogenization and sample pouring temperature and the corresponding change in microstructure, as revealed by fluorescence microscopy, have a large impact on the rheological measurements. For binders with semi-crystalline modifiers, the storage conditions between sample preparation and testing have the largest impact on the rheological behaviour. This can be related to variations in crystallinity, as shown by calorimetry. The main conclusion from this study is that sample preparation and handling is extremely important for the rheological properties of PMBs. Reproducibility can only be achieved when these conditions are controlled more accurately, especially in binder specification tests for rutting susceptibility.     

Influence of molecular weight on some rheological properties of plasticized polyvinylchloride

Conclusions From an overall analysis of the above results it appears that the rheological properties of plasticized PVC are governed, below about 200 °C, not only by the usual parameters (composition, temperature, molecular weight) but also by the crystallinity of the polymer. Recent estimations of the crystallinity degree in commercial PVC samples vary from 3% (45) to about 10% (46), the actual value depending on the thermal history of the sample.Apparently even small values of the crystallinity degree can affect rather strongly the rheological behaviour of plasticized PVC in the rubbery and flow regions.For the fractions here investigated, the fractional precipitation procedure used to carry out the separation (1) leads probably to a fractionation based not only on molecular weight but also on stereoregularity (47), the result being that the fractions precipitated firstly are more polydisperse and more crystallizable. In dilute solution, they give easily aggregates (1), stable up to relatively high temperatures (2).The dynamic-mechanical data discussed above (fig. 1) as well as the anomalies presented by the rheological results (fig. 7 to 12) can be therefore explained on the basis of a model in which supermolecular structures linked together by crystalline bonds are present in the samples.The marked dependence of PVC mechanical properties on the annealing temperature described recently byAndrews andKazama (48) seems to give considerable support to such a model.Presented at the Jahrestagung der Deutschen Rheologen, Berlin-Dahlem, 20.–22. Mai 1968.The authors are indebted to Mr.Sangiovanni and Mr.Zinelli for most of the experimental results.     

Determining polymer molecular weight distributions from rheological properties using the dual-constraint model

Although multiple models now exist for predicting the linear viscoelasticity of a polydisperse linear polymer from its molecular weight distribution (MWD) and for inverting this process by predicting the MWD from the linear rheology, such inverse predictions do not yet exist for long-chain branched polymers. Here, we develop and test a method of inverting the dual-constraint model (Pattamaprom et al., Rheol Acta 39:517–531, 2000; Pattamaprom and Larson, Macromolecules 34:5229–5237, 2001), a model that is able to predict the linear rheology of polydisperse linear and star-branched polymers. As a first step, we apply this method only to polydisperse linear polymers, by comparing the inverse predictions of the dual-constraint model to experimental GPC traces. We show that these predictions are usually at least as good, or better than, the inverse predictions obtained from the Doi–Edwards double-reptation model (Tsenoglou, ACS Polym Prepr 28:185–186, 1987; des Cloizeaux, J Europhys Lett 5:437–442, 1988; Mead, J Rheol 38:1797–1827, 1994), which we take as a “benchmark”—an acceptable invertible model for polydisperse linear polymers. By changing the predefined type of molecular weight distribution from log normal, which has two fitting parameters, to GEX, which has three parameters, the predictions of the dual-constraint model are slightly improved. These results suggest that models that are complex enough to predict branched polymer rheology can be inverted, at least for linear polymers, to obtain molecular weight distribution. Further work will be required to invert such models to allow prediction of the molecular weight distribution of branched polymers.     

Influence of dispersion procedure on rheological properties of aqueous solutions of high molecular weight PEO

The linear and nonlinear viscoelastic behaviors of poly(ethylene oxide) (PEO) in aqueous media have been investigated as a function of concentration and molecular weight. A particular interest has been paid to study the effect of turbulent flow under stirring, inducing both shear and elongational stresses, on the rheological behavior of the polymer solutions. The comparison of intrinsic viscosity and viscoelastic properties between shaken and stirred PEO solutions is discussed at the molecular scale in terms of chain scission and aggregation. Results point out that the effect of the mechanical history on the rheological response of PEO solutions depends also on the concentration regime and molecular weight. Indeed, the influence of the dispersion procedure vanishes by decreasing both the concentration and the molecular weight.     

Correlation between rheological properties of polyolefin melts and solutions and their molecular structure

Conclusion In this work a method has been described to determine the weight average molecular weight and the branching of polyolefins and this by means of very simple physical measurements. The branching index can very easily be separated in a small and long chain branching index. The knowledge of the molecular weight and this branching index permits us to derive a generalised masterrheogram for all kinds of polyolefins which also explains the degree of non-Newtonian behaviour.Paper read at the Annual Meeting of the German Rheologists, Berlin-Dahlem, May 20–21, 1968.     

Correlation between rheological behaviour in uniaxial elongation and film blowing properties of various polyethylenes

Six various low density polyethylenes and one blend were rheologically characterized in elongation. Their different strain-hardening behaviour could qualitatively be related to their molecular structure. All the materials were blown to films on laboratory equipment under various conditions. The take-up force and the film homogeneity were determined quantitatively, the bubble stability was visually assessed. The bubble stability increased with growing take-up force. The take-up force was found to be the stronger the higher the elongational viscosity was. The homogeneity of film thickness is not related to the bubble stability but to the occurrence of strain hardening in the uniaxial elongational experiment at high Hencky strains. Measurements of the uniaxial elongational behaviour of polyethylene melts are a valuable and promising way to assist the development and optimisation of film blowing materials.

Rheological behaviour and molecular structure of long-chain branched semifluorinated thermoplastics

The long-chain branched thermoplastic tetrafluoroethylene–hexafluoropropylene–vinylidenefluoride terpolymers (LCB THV) investigated in this paper are new polymers with a unique combination of properties like a high stability against aging or weathering and a very good chemical resistance. But not much is known about the rheological behaviour of the LCB THV, yet. In this paper, non-linear rheological properties like shear thinning and strain hardening are studied. Two different types of the THV with different contents of comonomers and, therefore, different melting points are examined. The THV with the higher melting point is insoluble. The other with the lower melting temperature is soluble and, therefore, was characterised by size exclusion chromatography coupled with light scattering with respect to its molecular structure. The results of the rheological measurements show a pronounced shear-thinning and strain-hardening behaviour for the long-chain branched materials. Both properties are of great importance for processing operations governed by shear and elongational flows.

Influence of surfactant addition on the rheological properties of aqueous Welan matrices

In the present work the effects produced by the presence of two different surfactants (Abil B 8842 and Triton N 101) on the rheological properties of aqueous welan matrices are studied, both in steady and in oscillatory shear conditions. Welan is an acidic microbial polysaccharide having high thermal, pH, and salt stability. At sufficiently low concentrations it forms aqueous weak gel matrices which can be profitably used to regulate the rheological properties of disperse systems and improve their stability. Different systems are examined, having the same polysaccharide concentration (0.25 wt%) and different surfactant concentrations (up to 40 wt%, far beyond the range of practical interest for emulsion preparation). All the systems exhibit marked shear-thinning properties which can be described quite satisfactorily by the Cross equation. The concentration dependence of the zero-shear-rate viscosity as well as the mechanical spectra confirm that, in the concentration range considered, the aqueous welan systems are typically weakly structured fluids. The influence of both surfactants is examined in detail by comparing the behavior of the different classes of systems. Both surfactants reduce the polymer contribution at low shear, whereas an opposite action is exerted at high concentration and shear. These contrasting effects are ascribed to the different structural features of the polymer matrix under low stresses and high shear conditions, respectively. Received: 6 February 2000 Accepted: 1 November 2000     

Influence of the dynamic rheological properties of polypropene on its calendering ability

Summary We tried to correlate the rheological properties in dynamic mode with the calendering ability of polypropene. Complex dynamic viscosity experiments have been correlated with the calendering processing using the relaxation times distribution spectra. These spectra allow to classify the materials according to their ability to calendering. Associating the bank to a long times range and the nip to a short times one, it is possible to give a rheological explanation of defects such asV-shaped defects or sharkskin.With 1 figure and 1 table     

Recoverable strains and retardation times of monodisperse suspensions of silicon dioxide spheres in poly(dimethylsiloxane)

Steady-state viscosities η, steady-state recoverable strains γ _rs and characteristic retardation time τ _1/2 were measured for suspensions of monodisperse silicon dioxide (SiO₂) spheres in poly(dimethylsiloxane) (PDMS) with various volume fractions Φ of the suspended spheres at various creep stresses σ ₀. Two different regions are found in plots of η/η _m vs γ _rs, where η/η _m denotes the relative viscosity of the suspensions. In one region, η/η _m is proportional to γ _rs, while γ _rs is independent of η/η _m in the other region. In both regions, τ _1/2 is the functions of the shear strain rate in the steady-state of creep test independently of Φ. The origin of the elasticity is related to the ‘maximally distorted’ cages recovered owing to the repulsive interaction between the SiO₂ spheres and recovery of the cages in the shear-induced clusters of the suspended spheres.     

The effect of liquid crystal structure on the rheological properties of nitrocellulose-dimethylacetamide solutions

Solutions of Pyro grade nitrocellulose (NC) in dimethyl acetamide (DMA), containing between 42.5% and 60% NC (w/w), have been studied by differential scanning calorimetry and polarised light microscopy. The results showed that NC forms a lyotropic liquid crystal structure in DMA. A transition from the liquid crystal phase to an isotropic phase occurred over the temperature range 27 °C to 67 °C, and the enthalpy of transition increased with NC concentration. Rheological properties were determined using an extrusion rheometer with a slit die. The solutions were shown to have a yield stress for flow which increased with increasing NC concentration. The solutions were also found to be thixotropic.     

The rheological properties of solder and solar pastes and the effect on stencil printing

Solar and solder pastes are widely used in the electronics industry. Solder paste is the principal joining medium in the assembly of surface mount components, whilst solar paste is used in the manufacture of semiconductor solar cells in the photo-voltaic industry. The stencil printing of both solder and solar pastes is a very important and critical stage in the assembly process. With miniaturisation of components, this is likely to continue. The challenge in stencil printing at such dimensions is in achieving repeatable deposition of both solar and solder pastes from print to print. To meet this challenge requires an understanding of the flow behaviour of both solar and solder pastes. The rheological properties of solar and solder pastes have been evaluated through three different types of experiments. Existing models were applied to compare their rheological behaviour under these schemes. One striking difference was that solar paste showed a higher viscosity than solder paste. Both solar and solder pastes were found to be non-Newtonian materials, showing a decrease in viscosity with increasing shear rates. In this paper we investigate the rheological properties of both solder and solar paste under steady shear and creep-recovery tests. Received: 12 October 1999 Accepted: 11 July 2000     

Influence of rheological properties of bitumen emulsions on the performance of surface dressing systems

Three bitumen emulsions, used in road surface dressing construction, one conventional and two polymer modified, have been tested in a CarriMed rheometer to determine their rheological properties G ^* and . These properties were determined over a temperature range and curing time conditions that resemble those experienced in-situ upon laying. A tensile loading test called the Pull-Off Test was developed to determine the adhesive strength of surface dressing systems and was used to study the influence of temperature and curing time on the strength development of dressing systems containing the three emulsions in question. To establish reliability criteria for the developed test, its parameters were correlated with the G ^* values of the emulsions evaluated under similar temperature and curing time conditions.     

Influence of molecular weight and molecular structure of polystyrenes on turbulent drag reduction

Summary An investigation was carried out on drag reduction of diluted solutions on nine linear polystyrenes of different molecular weight in toluene in the turbulent region (from 5,000 to 40,000 Reynolds numbers). The universal curve established byVirk et al. is only confirmed for very low concentration depending on the molecular weight and on the Reynolds number. If the maximum drag reduction varies with the molecular weight according to previous results, the intrinsic concentration shows a dependence with the cube of the molecular weight. This type of dependence explains the observed effects of the polydispersity and degradation. Moreover, no critical molecular weight below which no drag reduction occurs is observed. The study of PS samples with non-linear structure shows the total ineffectiveness of these structures giving support to the suggestion that an elongation of the polymer in the turbulent flow is responsible for the drag reduction. From the comparison between Polyox and PS properties it is concluded that both polymers have a comparable effectiveness and degradability when compared to the same DP or to the same [].

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Zusammenfassung Untersucht wurde die Verminderung des Reibungswiderstandes (VRW) verdünnter Lösungen von neun linearen Polystyrolen mit verschiedenem Molekulargewicht in Toluol im turbulenten Bereich bei Reynoldsschen Zahlen zwischen 5000 und 40000. Die vonVirk und Mitarbeitern festgestellte universelle Kurve wird nur für sehr verdünnte Lösungen, abhängig von der Reynoldsschen Zahl und dem Molekulargewicht, bestätigt. Wie schon in früheren Untersuchungen gefunden wurde, nimmt der Maximale VRW-Effekt mit dem Molekulargewicht zu, und die intrinsic-Konzentration zeigt eine Abhängigkeit mit der dritten Potenz des Molekulargewichts. Diese Abhängigkeit erklärt die beobachteten Effekte der Polydispersität und der Degradation. Weiterhin wird kein kritisches Molekulargewicht, unterhalb dessen kein VRW-Effekt auftritt, beobachtet. Die Untersuchung von nichtlinearen Polystyrolen beweist die völlige Unwirksamkeit dieser Strukturen, was den Schluß nahelegt, daß die Verlängerung des Polymers in der turbulenten Strömung für die Verringerung des Reibungswiderstandes verantwortlich ist. Durch einen Vergleich der Eigenschaften von Polyäthylenoxid und Polystyrol wird gefunden, daß beide Polymere eine vergleichbare Wirksamkeit und Degradation zeigen, wenn man Proben von gleichem Polymerisationsgrad oder gleichem Staudinger-Index [] in Beziehung setzt.

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With 9 figures and 2 tables     

In-line measurement of rheological properties of polymer melts

Shear viscosity (), first normal stress difference (N ₁), and extensional viscosity ( _E ) of polymer melts measured under processing conditions are important in process modeling, quality control, and process control. A slit rheometer that could simultaneously measure , N ₁, and the planar extensional viscosity ( _p ) was designed and tested by attaching it in-line to a laboratory model single-screw extruder. A tube (circular cross-section) rheometer to measure and the uniaxial extensional viscosity ( _u ) simultaneously was also designed and tested. Two commercial grades of LDPE (low density polyethylene) with melt index values of 6 and 12 were used as test materials for the study. Exit and hole pressure methods were used to estimate N ₁, and the entrance pressure drop method using the analyses of Cogswell, Binding, and Gibson (the last analysis used with the axisymmetric case only) was used to estimate _E .The hole pressure method was considered better than the exit pressure method to estimate N ₁ (due to the greater susceptibility of the latter to experimental errors). From the hole pressure method N ₁ was obtained from 100 kPa to 500 kPa over a range of shear rates from 40 s^–1 to 700 s^–1. Among the analyses used to estimate the extensional viscosity, Cogswell's is recommended due to its simpler equations without loss of much information compared to the other analyses. The range of extension rates achieved was 1 to 30 s^–1. The combination of the hole pressure and entrance pressure drop methods in a slit rheometer is a feasible design for a process rheometer, allowing the simultaneous measurement of the shear viscosity, first normal stress difference and planar extensional viscosity under processing conditions. Similarly, combining the entrance pressure drop measurements with a tube rheometer is also feasible and convenient.