Rheological Properties Of Polymer Melts Research Articles

Two-relaxation-time lattice Boltzmann model of the velocity profiles and volumetric flow rate of generalized Newtonian fluids in a single-screw extruder

Single-screw extruders and injection molding machines are essential equipment in polymer processing. It is of great importance for the optimization of operating parameters and the design of extrusion screw to predict the throughput of an extruder and the metering time of an injection molding machine according to the geometric parameters of the screw, operating parameters, and the rheological behavior of materials. Most polymer melts exhibit non-Newtonian behavior. The lattice Boltzmann method has many advantages in simulating the flow of non-Newtonian fluids. Herein, the dimensionless velocity profiles and dimensionless volumetric flow rate of generalized Newtonian fluids in a screw channel have been studied using the two-relaxation-time lattice Boltzmann method (TRT–LBM). The numerical results of power-law fluids are in good agreement with the analytical solutions, which verifies the validity of TRT–LBM. Through research, the change rule of the dimensionless volumetric flow rate of Bingham fluids with dimensionless pressure gradient has been obtained. It was found that the rheological properties of polymer melts and the dimensionless pressure gradient significantly affect the dimensionless velocity profiles and dimensionless volumetric flow rate. The dimensionless volume flow rate has some unexpected changes with the increase of the dimensionless pressure gradient. This study can provide theoretical guidance for the optimization of operating parameters and the design of extrusion screws.

Molecular configuration evolution model and simulation for polymer melts using a non-equilibrium irreversible thermodynamics method

Almost all flows of polymers are accompanied by evolution of the molecular configuration, which has a great influence on material properties. However, the existing molecular configuration evolution models are mainly limited to the configuration evolution of a single molecular chain in a dilute solution ignoring intermolecular forces; it is impossible to predict the evolution of the molecular configuration of polymer melt with a complex entanglement network. Thus, we propose a non-isothermal compressible viscoelastic molecular configuration evolution model for polymer melt using non-equilibrium irreversible thermodynamic methods. The proposed model can more accurately describe the rheological properties of polymer melts compared with the commonly used XPP model, especially at high shear rates. The predicted molecular configuration in rotary Couette flow agrees well with the measured dielectric anisotropy results. Finally, the present model and a simulation method are successfully applied to predict the evolution of polymer molecular configuration and birefringence in three-dimensional injection compression molding, and the predicted birefringence is consistent with the measured results. A typical skin-core structure, resulting from the competition of the flow field and the molecular Brownian thermal motion in the injection molding, is investigated.

Scaling Characteristics of Rotational Dynamics and Rheology of Linear Polymer Melts in Shear Flow

Rheological properties of polymer melts under weak flow reflect the orientational anisotropy of the chains, which is established experimentally as the validity of the stress-optical rule (SOR). In ...

Influence of hydrostatic pressure on rheological properties of polymer melts—A review

Rheological properties of polymer melts are significantly dependent on molecular structure and external parameters. While numerous experimental data of the dependence of the rheological behavior on stress, shear rate, and temperature are available in the literature, much less is known on the effect of hydrostatic pressure. This article reviews the knowledge about the pressure dependence of rheological properties of polymer melts. The different experimental devices for measurements under pressure are described, and the methods of determining the pressure coefficients of viscosity are critically analyzed. Very few investigations of the pressure dependence of viscoelastic properties are addressed. A comparison of the pressure coefficients of commercial polymers obtained by various authors from viscosity measurements with different methods shows the deficits in getting exact data. It can be said, however, that the pressure coefficients increase in the order of high density polyethylene, linear low-density polyethylene, low-density polyethylene, polypropylene, and polystyrene. For amorphous polymers and, particularly, for polystyrene, it is shown how the pressure coefficients decrease with growing temperature. For polyolefins, this dependency is less significant. The free volume concept is discussed with respect to an interpretation of the experimental findings. Results of using the hole fraction theory for describing the pressure dependence of viscosity are presented and critically assessed.

ε-Caprolactone as a medium for improving dispersability of graphene oxide in polyamide based composites

We described preparation of composites of graphene oxide with copolymers containing ε-caprolactam and ε-caprolactone structural units. Graphene oxide was prepared using modified Hummers method and characterized by elemental analysis and X-ray photoelectron spectroscopy measurements, interlayer distance was evaluated from X-ray diffraction. Two types of copolymers - polyesteramides - with molar ratio of ε-caprolactam to ε-caprolactone units in polymerization feed 80/20 and 90/10 were prepared by polymerization casting method. Graphene oxide content in both types of polyesteramides was in range from 0.1 to 1wt.%. Thermogravimetric analysis and differential scanning calorimetry were utilized to study thermal stability and crystallization behaviour of materials. Prepared composites were characterized by tensile modulus and impact strength. Rheological properties of polymer melts and viscometry of diluted polymer solutions revealed the physical character of polymer-filler interactions.

Parameter Free Prediction of Rheological Properties of Homopolymer Melts by Dynamic Monte Carlo Simulation

A biased Monte Carlo method is developed to simulate shear flow of linear homopolymers between neutral hard walls. Simulations are conducted on a face- centered cubic (FCC) lattice at full occupancy (φ = 1) using a modified version of Pakula's cooperative motion (COMOTION) algorithm. As chain lengths are increased under quiescent conditions, a clear crossover in chain dynamics from Rouse behavior to reptation-like behavior is revealed in characteristic correlation functions. Shear flow is simulated through the use of a biasing technique that favors segmental movement in the direction of flow; this combination of cooperative motion with flow is referred to as the COMOFLO algorithm. Using this technique, rheological properties are calculated as a function of shear rate and chain length. The viscosity and the first and second normal stress coefficients are calculated and found to be consistent with well-established experimental facts. The zero-shear viscosity scales linearly with low molecular weights (η0 ∼ N 1.0 ) and scales to the 3.5 power at higher molecular weights (η0 ∼ N 3.5 ). This crossover in viscosity scaling occurs at the same chain lengths (N ∼ 150) as the transition in chain dynamics. Because the simulations are conducted in the athermal limit, there are no parameter inputs. The known observable linear viscoelastic rheological properties of polymer melts are thus predicted in an a priori fashion.

Experimental Study of Rheological Characteristics of Polymer Melts in the Micro-Die

Compared with in the conventional scales, the rheological properties of polymer melts have changed in the micro-scales. Based on the principle of capillary flow and modified with the method of Bagley or zero die micro-die, the rheological characteristics of polypropylene (PP) were investigated using the HAAKE capillary rheometer under the micro-die and conventional die. The results show that: the viscosity of polymer melt modified with the method of bagley or zero die micro-die in the different L/D ratio of micro-die is the same and the shear viscosity of PP decreases with the increase of shear rate; Compared with the viscosity in the conventional scales, the measured viscosity of PP in the micro-scales was reduced by 46% in 0.25mm diameter die, 44% in the 0.20mm diameter die and 88% in the 0.15mm diameter die at the same shear rate; a layer of fixed polymer melt was found in the inwall of the micro-die through the calculation of wall slip rate.

Shear viscosity measurements of polymer melts using injection molding machine with adjustable slit die

Rheological properties of polymer melts were studied using a slit die attached to an injection molding machine. The die has three exchangeable inserts to set the slit height to 0.75, 1.0, or 1.5 mm. The modular design eases the die cleaning, and measurements with variable slit heights enable the detection of possible wall slip. Two grades of polypropylene and two grades of polystyrene were studied using the slit die and the results were compared to those obtained using a capillary rheometer and a rotational rheometer in steady and dynamic modes. The examination of the selected polymers with different slits gave essentially the same results at each slit height, verifying the absence of wall slip. The agreement with the conventional off-line rheometer results was very good for all polymers studied. This suggests that a simple slit die construction can be used as an inexpensive rheometer to measure rheological properties of polymer melts for processing purposes. The thermo-mechanical history of the investigated melt in the in-line slit die experiments resembles that in actual melt processing and, therefore, determination of rheological properties under true processing conditions is possible.

Rheological Properties of Polymers: Structure and Morphology of Molten Polymer Blends

The article reviews a brief literature on the rheological properties of polymer melts and blends. Experimental results on polymer blends are summarized. Technically, vital types of multi-phase polymers such as compounds and blends are discussed. The importance of the rheological properties of polymer mixtures in the development of the phase structure is discussed. And the importance of considering the stress and/or strain history of a material sample in a rheological investigation is discussed. Finally, the outlook on the past, present and future developments in the field of polymer rheology are given. The review concludes with a brief discussion on the opportunities and challenges in the field of polymer blends and blend rheology.

Polymer melt dynamics: Microscopic roots of fractional viscoelasticity

The rheological properties of polymer melts and other complex macromolecular fluids are often successfully modeled by phenomenological constitutive equations containing fractional differential operators. We suggest a molecular basis for such fractional equations in terms of the generalized Langevin equation (GLE) that underlies the renormalized Rouse model developed by Schweizer [J. Chem. Phys. 91, 5802 (1989)]. The GLE describes the dynamics of the segments of a tagged chain under the action of random forces originating in the fast fluctuations of the surrounding polymer matrix. By representing these random forces as fractional Gaussian noise, and transforming the GLE into an equivalent diffusion equation for the density of the tagged chain segments, we obtain an analytical expression for the dynamic shear relaxation modulus G(t) , which we then show decays as a power law in time. This power-law relaxation is the root of fractional viscoelastic behavior.

Tension thickening, molecular shape, and flow birefringence of an H-shaped polymer melt in steady shear and planar extension

Despite recent advances in the design of extensional rheometers optimized for strain and stress controlled operation in steady, dynamic, and transient modes, obtaining reliable steady-state elongational data for macromolecular systems is still a formidable task, limiting today's approach to trial-and-error efforts rather than based on a deep understanding of the deformation processes occurring under elongation. Guided, in particular, by the need to understand the special rheology of branched polymers, we studied a model, unentangled H-shaped polyethylene melt using nonequilibrium molecular dynamics simulations based on a recently developed rigorous statistical mechanics algorithm. The melt has been simulated under steady shear and steady planar extension, over a wide range of deformation rates. In shear, the steady-state shear viscosity is observed to decrease monotonically as the shear rate increases; furthermore, the degree of shear thinning of the viscosity and of the first- and second-normal stress coefficients is observed to be similar to that of a linear analog of the same total chain length. By contrast, in planar extension, the primary steady-state elongational viscosity eta(1) is observed to exhibit a tension-thickening behavior as the elongation rate epsilon increases, which we analyze here in terms of (a) perturbations in the instantaneous intrinsic chain shape and (b) differences in the stress distribution along chain contour. The maximum in the plot of eta(1) with epsilon occurs when the arm-stretching mode becomes active and is followed by a rather abrupt tension-thinning behavior. In contrast, the second elongational viscosity eta(2) shows only a tension-thinning behavior. As an interesting point, the simulations predict the same value for the stress optical coefficient in the two flows, revealing an important rheo-optical characteristic. In agreement with experimental indications on significantly longer systems, our results confirm the importance of chain branching on the unique rheological properties of polymer melts in extension.

Flow of a magnetic fluid between eccentric rotating disks

We discuss the flow of a magnetic fluid between two parallel disks rotating about non-coincident axes, normal to the disks, but with same angular velocity. This device is known as an orthogonal rheometer and has been used to measure the rheological properties of polymer melts and viscoelastic fluids. For a certain range of effective relaxation times, we obtain an exact solution of this three-dimensional problem in a magnetic fluid and determine the forces exerted by the fluid on one of the rotating disks.

Forme de la distribution des masses molaires pour supprimer le défaut de peau de requin

In this Note, we focused on the sharkskin defect, classically observed in linear polymer extrusion. The aim of this work is to link the sharkskin onset to the rheological properties of polymer melt and more particularly to the shape of the molecular weight distribution. Using the model of Brochard–de Gennes, we show that it is possible to eliminate the sharkskin if M n ≈ 10 M e . To cite this article: A. Allal et al., C. R. Physique 8 (2007).

Dewetting of thin polymer films

We study the dewetting of thin polymer films deposited on slippery substrate. Recent experiments on these systems have revealed many unexpected features. We develop here a model that takes into account the rheological properties of polymer melts, focussing on two dewetting geometries (the receding of a straight edge, and the opening of a hole). We show that the friction law associated with the slippage between the film and the substrate has a direct influence on the dewetting dynamic. In addition, we demonstrate that residual stresses, which can be stored in the films due to their viscoelasticity, are a source of destabilization for polymer films, and accelerate the dewetting process.

From atomistic simulations to slip-link models of entangled polymer melts: Hierarchical strategies for the prediction of rheological properties

Chain entanglements are thought to govern the rheology of long-chain polymer melts and the large-deformation properties of solid polymers. Although they have been invoked in tube and slip-link theories of polymer dynamics, their precise definition and their direct experimental observation have been elusive. Recently, algorithms enabling the full equilibration of long-chain polymer melt models on all length scales opened up the way to the detection and identification of entanglements by computer simulation, using topological reduction of configurations or dynamical analysis of simulation trajectories. After a reference to the tube model and to coarse-graining strategies for simulating polymers, this brief review discusses slip-link models that have been developed for the mesoscopic analysis and simulation of rheological properties of polymer melts. It then presents algorithms for the reduction of model melt configurations sampled in the course of atomistic or coarse-grained molecular simulations to networks of entanglements comparable to those invoked in slip-link models. Estimates of the molar mass between entanglements and of the Kuhn length of primitive paths obtained through topological analysis and through dynamical analyses of simulation trajectories are compared against experimental evidence. Statistical descriptors are then developed for the entanglement network structure, as obtained by simulation. Entanglement analysis is shown to be an important missing link in quantitatively relating the detailed chemical constitution of polymers to their rheological and large-scale deformation properties through a hierarchy of atomistic, coarse-grained, and mesoscopic slip-link model simulations.

A test case for predicting the rheological properties of polymeric liquids: the multiple coupled Maxwell modes model

In this work, the behavior of the multiple coupled Maxwell modes (MCMM) model is examined with regard to the rheological properties of polymer melts in diverse flow fields, including (i) transient, (ii) steady-state shear flow, (iii) small-amplitude oscillatory shear flow, and (iv) transient uniaxial elongational flow. This model is specialized to the case of pair-wise coupling, i.e., each mode interacts with only one other mode. Consequently, each pair of modes acts independently of the others. For a typical polymer melt of industrial interest, a simple optimization technique is developed for determining the number of independent mode pairs, as well as precise values for the corresponding parameters. The goal of this ongoing study is to fit the parameters of various rheological models using a limited number of simple, standard experiments, and then to see whether or not the models can predict data taken from more complicated experiments. In this paper, only the first step is taken in this direction: we examine for one particularly promising rheological model, the MCMM model (but herein restricted to pair-wise coupling), whether or not it can achieve this goal. This restricted model is chosen because it mimics the effect of pair-wise coupling between relaxation modes that is prevalent in current rheological models. Using this model as a test case allows the optimization technique and analytical methodology for achieving the overall goal to be developed. The outcome of this study with regard to developing the methodology was successful, but the particular model chosen, written in terms of coupled Maxwell modes with pair-wise coupling, is not general enough to predict well typical polymer melt rheological behavior.

高分子系のメソ領域シミュレータの開発

We have developed an integrated simulation system for soft materials named OCTA. OCTA is developed for the meso-scale simulation, which is important to study the material properties such as rheological properties. OCTA consists of four simulation programs (COGNAC, PASTA, SUSHI, MUFFIN) and a simulation platform (GOURMET), and is designed for collaborative operations of multi programs. Various application studies using OCTA are conducted, e.g. rheological properties of polymer melt, structure and dynamics of polymer melt confined between nano-scale gap, interfacial strength of polymer blends, phase separation of polymer blend under shear flow, and the brief results are introduced in this paper.

Polymer crystallization in complex conditions. Towards more realistic modelling of industrial processes

Crystallization plays an important role in processing many industrial polymers. Crystallinity controls mechanical and sorptional properties of solid materials and strongly affects rheological properties of polymer melts and solutions. Therefore, realistic modeling of technological processes involving crystallizable polymers (melt spinning, film blowing, injection molding, etc.) requires that crystallization in the course of processing is taken into account. In contrast to idealized laboratory conditions usually applied in academic studies, crystallization in processing conditions covers wide range of rapidly changing temperatures and pressures. Simple models of non-isothermal, and stress-induced crystallization kinetics are discussed. Consideration of crystallization in the computer model of melt spinning reveals new effects, absent in earlier models, in which crystallization was ignored.

Elongational rheology of polymer melts and solutions

Elongational rheological properties of polymer melts and solutions may be measured using nonlubricated flow characteristics through a semihyperbolic converging die. The effects of body forces related to developing orientation in the fluid during converging extensional flow are so strong that the shearing contribution become negligible in comparison, eliminating the need for lubrication to achieve an essentially pure elongational flow. The effective elongational viscosities of polypropylene melts and lyocell solutions correlated with shear-flow determinations were used to estimate the enthalpy and entropy changes as function of processing conditions. The flow of lyocell solutions through a converging die had, as a result, not only phase separation and cellulose crystallization, but also microfibers formation and high orientation. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 69: 2357–2367, 1998

Evaluation of rheological properties of non‐newtonian fluids in internal mixers: An alternative method based on the power law model

AbstractA new method was developed to evaluate rheological properties of polymer melts such as shear stress, shear rate, apparent viscosity and other rheological parameters in internal mixers. It is based on the classical power law model where the power law index n is directly evaluated from a set of data containing speed S, torque M, and the consistency index m is indirectly determined by an empirical relation. The method is based on a model with only one geometrical parameter (α), which involves a chamber radius R2 and an equivalent radius Re. It is assumed that the measuring head consists of two adjacent sets of coaxial cylinders. This method has advantages over previously reported models that use two parameters and do not propose a straightforward method to evaluate m. The pseudoplastic nature of the polymer melt decreases as the rate of loss of structural points, i.e., molecular entanglements and network junctions, increases, which produces greater mobility of the melt. A relationship between α and C(n) is found, which is simpler than other models previously reported. These results further demonstrate the feasibility of evaluating rheological properties of polymer melts in internal mixers.