Properties Of Polymer Melts Research Articles

On‐line measurement of the PVT properties of polymer melts using a gear pump

AbstractThis paper presents a newly developed experimental apparatus, based on an extrusion system, which can economically and accurately measure the pressure–volume–temperature (PVT) properties of polymeric fluids. The density or specific volume of a polymer melt is determined by measuring the mass and volume flow rates of the melt. A positive displacement gear pump mounted on an extruder is used to measure the volume flow rate of the melt. In order to reduce the leakage across the gear pump, the difference between the upstream and downstream pressures is minimized by using a variable resistance die attached to the downstream outlet of the gear pump. The positive displacement volume of a gear pump was determined in calibration experiments with water and oil with the aid of a syringe pump. Two critical sets of experiments were carried out to measure the specific volumes of polystyrene and polypropylene as case examples. The measured results were compatible with the known PVT data, confirming the validity of the system. © 2004 Wiley Periodicals, Inc. Adv Polym Techn 23: 316–327, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.20020

Dissipative particle dynamics simulations of polymer melts. I. Building potential of mean force for polyethylene and cis-polybutadiene.

We present the derivation of coarse-grained force fields for two types of polymers, polyethylene (PE), and cis-polybutadiene (cis-PB), using the concept of potential of mean force. Coarse-grained force fields were obtained from microscopic simulations for several coarse-graining levels, i.e., different number of monomers lambda per mesoscopic unit called "bead." These force fields are then used in dissipative particle dynamics (DPD) simulations to study structural and dynamical properties of polymer melts of PE and cis-PB. The radial distribution functions g(R), the end-to-end distance R0, the end-to-end vector relaxation time tau, and the chain center of mass self-diffusion D(CM), are computed for different chain lengths at different coarse-graining factor lambda. Scaling laws typical of the Rouse regime are obtained for both polymers for chain lengths ranging from 6 to 50 beads. It is found that the end-to-end distance R0 obtained from DPD simulations agree well with values obtained from both microscopic simulations and experiments. The dependence of the friction coefficient used in DPD simulations versus the coarse-graining level is discussed in view of the overall scaling of the dynamical properties.

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.

Application of the ISM EoS for polymer melts

A method for predicting an analytical equation of state for polymer melts from surface tension and liquid state density at the freezing temperature ( γ f, ρ f), as scaling constants, is presented. B 2( T) follows a promising corresponding-states principle. Calculation of α( T) and b( T), the two other temperature-dependent constants of the equation of state, are made possible by scaling. As a result, γ f and ρ f are sufficient for the calculation of thermophysical properties of polymer melts. We applied the procedure to predict specific volumes of polyethylene glycol (PEG), polypropylene glycol (PPG), polypropylene (PP) and polyvinylchloride (PVC) at compressed state with temperature range from 298.15 to 423.15 K and pressures up to 200 MPa. The experimental specific volumes were correlated satisfactorily with our procedure and the results are within 3%.

Simulation of Melt Spinning of Nylon-6 Using the Finite Element Method

The properties of polymer melts are different from those of monomeric materials because polymer materials have crystalline and amorphous regions. In addition, polymer melts exhibit elasticity as well as viscosity. Therefore, the viscoelasticity, crystallinity and temperature of polymer melts must be considered for an accurate simulation of the melt spinning process for crystalline polymers. In this study, we simulated the melt spinning process of Nylon-6 by using the streamline-upwinding finite element method. The non-isothermal Phan-Thien Tanner model proposed by Sugeng and Phan-Thien was employed as a constitutive equation. We investigated the distribution of temperature and crystallinity on the free surface of the filament, and the filament diameter.The calculations were nearly independent of gravity. The primary normal stress increases and the filament diameter decreases with an increase of take-up speed. The filament diameter obtained by the non-isothermal flow simulation is smaller than that for the isothermal case, and the filament diameter depends on the crystallinity. The filament shape for a purely viscous fluid is the same as that for a viscoelastic one.

Harmonic analysis in rheological property measurement

Sudden change in the linear stress-strain relationship has long been known to produce interesting discontinuities in the flow properties of polymer melts, concentrated particle suspensions, and gels as the shear rate amplitude is increased. The non-linear effect produces stress harmonics, which contain information about the material properties that is otherwise difficult to obtain. This note describes a generalized procedure for extracting yield stress material functions from the discrete Fourier transform of the stress signal.

Lattice polymers with structured monomers: A Monte Carlo study of thermodynamic properties of melts and solutions

The influence of monomer structure on the thermodynamic properties of polymer melts and solutions is explored in Monte Carlo computations. The internal energy, the specific heat, and the chemical potential are determined for united atom lattice models of polyethylene, polypropylene, polyethylethylene, and polyisobutylene, all with M=60 united atom groups. Computations are performed for a range of melt volume fractions 0<φ⩽0.7 and effective temperatures 3.3⩽T*⩽∞. The results reveal fine differences between the internal energies of the four different melts (solutions) and a pronounced influence of monomer structure on the specific heat and the chemical potential. The computed thermodynamic quantities are confronted with the analytical results of the lattice cluster theory (LCT). It is shown that for most thermodynamic states the LCT correctly predicts the qualitative effect of monomer structure on the thermodynamic quantities. However, large errors observed in the LCT values for the specific heat, and a qualitative departure between simulation and theory for the difference between the internal energies of polyethylethylene and polyethylene, stress the need both for improvements in the analytical solution of the LCT and for further tests of its accuracy.

In-line dielectric monitoring during extrusion of filled polymers

Real-time monitoring of the dielectric properties of polymer melts and filled polymer melts has been carried out during extrusion. The measurements are obtained using a dielectric cell that is placed directly in line with the extruder machine. The dielectric cell consists of interdigitating electrodes that are deposited on the inside of a ceramic ring that is electrically insulated and temperature controlled to the set point of extrusion. As the processed resin passes through the ring, its permittivity and conductivity are measured. The spatial sensitivity of the cell was determined experimentally and was biased to the resin flowing near the electrodes. Using the spatial sensitivity function, we examined the time profile of the transition from one composition to another during extrusion. We demonstrate the operation of the cell during the processing of polystyrene filled with aluminum oxide and calcium carbonate and of polyethylene-ethyl vinyl acetate copolymer filled with montmorillonite clay.

Viscoelastic Properties of Polybutadiene in the Glassy Regime from Molecular Dynamic Simulations

Introduction A thorough understanding of the dynamic-mechanical properties of polymer melts and glasses is key to many, if not most, applications of polymers as well as to the processing of polymers. It is well-known that amorphous and semicrystalline polymers exhibit viscoelastic response to an applied stress or strain. For a small constant shear strain γ the shear stress is proportional to the strain and is given by the stress relaxation equation1

Sources of error and other difficulties in extensional rheometry revisited: commenting and complementing a recent paper by T. Schweizer

In a recent paper by T. Schweizer (Schweizer 2000) a large collection of experimental difficulties associated with the measurement of uniaxial extensional properties of polymer melts in the Rheometrics RME extensional rheometer is described. The work covers topics such as sample preparation for different types of polymers (sensitive or not to moisture) supplied in different shapes (pellets or powder), the necessary corrections to the tensile force, and the ever-present problem of determining the true strain rates of the experiments. The aim of the present paper is to complement and expand the work of Schweizer by pointing out other experimental problems that are the cause of errors in extensional rheometry of polymer melts. The present analysis, however, is not exclusively dedicated to the RME, unlike that of Schweizer, being directed instead to a general class of apparatus that work according to the principle of stretching a constant length sample between pairs of counter-rotating rollers; for example, all the data shown was obtained with our own extensional rheometer (Maia et al. 1999). This work will focus on the importance of the correct choice of the supporting media used for sample heating and support, the importance of end-effects, and the influence that the griping surfaces can have in such measurements.

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.

Simulations of Filled Polymers on Multiple Length Scales

ABSTRACTWe present simulation results of the effect of nanoscopic and micron-sized fillers on the structure, dynamics and mechanical properties of polymer melts and blends. At the smallest length scales, we use molecular dynamics simulations to study the effect of a single nano-filler on the structure and dynamics of the surrounding melt. We find a tendency for polymer chains to be elongated and flattened near the filler surface. Additionally, the simulations show that the dynamics of the polymers can be dramatically altered by the choice of polymer-filler interactions. We use time-dependent Ginzburg-Landau simulations to model the mesoscale phase-separation of an ultra-thin blend film in the presence of an immobilized filler particle. These simulations show the influence of filler particles on the mesoscale blend structure when one component of the blend preferentially wets the filler. Finally, we present some preliminary finite element calculations used to predict the effect of mesoscale structure on macroscopic ultrathin film mechanical properties.

The role of intermolecular interactions in the viscoelastic properties of polymer melts

AbstractThe exact microscopic expression for the stress tensor in polymer liquids contains a tensor product of the the segment position vector with the total, intra‐ plus inter‐chain, force acting on the segment. On the other hand, the widely accepted theory of viscoelasticity of polymer melts1) is based on the assumption, that contributions from interchain interactions to the viscosity of polymer melts is negligible relative to the effectively intrachain entropic interactions. Starting from the exact Green‐Kubo formula for the viscosity, the Rouse dynamic correlation functions, and Newton's second law, we show that the intrachain assumption is inadequate. Rather, the intrachain and interchain forces acting on polymer segments cancel each other largely. The intrachain contribution therefore cannot be dominant as anticipated in the usual treatment1), or, in other words, the interchain contribution cannot be ignored. The main contribution to viscoelastic properties of polymer melts can only arise from a part of the total stress tensor as already suggested by M. Fixman based on a different argument2). It is concluded that the viscosity is of a purely interchain nature, and is determined by the tensor product of the vector connecting the centers‐of‐mass of neighboring macromolecules on the one hand, and the total force by which macromolecules interact, on the other, just in the case of simple liquids.

A novel capillary rheometer for measurement of flow properties of polymer melts

This paper introduces a novel type of capillary rheometer, with respect to its design and operation, for investigating flow properties of a polymer melt. The constructed capillary rheometer is fitted into a standard tensile testing machine such that the mode of extrusion could be altered. The equipment features the possibility of moving either the piston or the barrel, the latter being novel in design. In addition, this work presents a novel pressure sensor for determining the pressure drop along the die in the capillary rheometer. It is found that the constructed capillary rheometer, coupled with the novel pressure sensor, is a useful device to monitor the flow properties of the polymer melt. The preliminary results show that the flow properties are dependent not only on the test condition used, but also on the mode of operation.

Alternative off-lattice model with continuous backbone mass for polymers

We present an alternative model for polymers with flexible one-dimensional rod chains as backbone. This model is able to simulate polymers consisting of large monomer units that cannot be described by a bead spring model with satisfying results. In spite of using large coarse grained units no bond crossing can occur because of a continuous excluded volume along the backbone. Our model provides an effective way to describe anisotropic monomers with respect to their mass and their nonbonded interactions. The rods interact with confocal force fields. The geometric shape of the chemically realistic monomers is conserved by using ellipsoids which represents the interaction volume as building units for the coarse grained model of the monomers. Static and dynamic properties of linear polymer melts are investigated using a parametrization for the bisphenol-A-polycarbonate. We give evidence that our model is able to reproduce the expected scaling behavior of static and dynamic properties of polymer melts. Moreover, we are able to study the reptation behavior in a computationally efficient way.

The pressure dependence of the shear and elongational properties of polymer melts

A capillary rheometer has been modified, by the addition of a second chamber and valve arrangement below the main die, in order to measure the pressure drops associated with the capillary and entry flows of a number of polymer melts as a function of pressure. The five polymer melts investigated are high- and low-density polyethylene, polypropylene, polymethyl-methacrylate and polystyrene, each of which is tested at three temperatures within the normal processing range, at apparent shear rates between 50 and 2500 s −1 and at mean pressures ranging from atmospheric up to 70 MPa. The capillary pressure drop data are used to obtain shear viscosity functions using conventional capillary rheometry expressions, whilst extensional viscosities are estimated from orifice pressure drop data via the Cogswell–Binding analysis. Both the shear and extensional viscosity curves for all of the polymers are seen to exhibit an exponential pressure dependence that can be characterised by pressure coefficients that are found to be independent of temperature. Trouton ratios for the polymers can be specified by an expression with separable strain rate and pressure dependence terms, the latter of which is again exponential. The pressure coefficients of the Trouton ratio terms then orders the pressure dependence: PS>PMMA>PP>HDPE>LDPE. Our major conclusion is that the Trouton ratio for some of the polymer melts can be a strong function of the pressure, indicating that the variation of extensional properties with pressure can be greater than that of the shear properties.

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

Pressure Drop of the Polymer Melt Flow in a Pipe

Rheological properties of polymer melts are often described with a linear model, the two-parameter power-law model in industrial applications, while the real polymer melts behave more or less nonlinearly, particularly in the low shear rate regions. In a lot of real flows, such as the simple pipe flow, there is even a zero shear rate region. Therefore, the adequacy of this linear approximation has to be examined. In this paper, the polymer melt flow in a pipe is studied using the nonlinear three-parameter Powell model. Numerical results are presented in several graphs, which can be used in engineering calculations. A comparison with the test data has been given. The restrictions of using the power-law model and the impact in handling the experimental data are discussed.

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.