Transient Viscoelastic Properties Research Articles

Poly(phenylene sulfide)/low‐melting‐point metal composites. I. Transient viscoelastic properties and crystallization kinetics

AbstractPoly(phenylene sulfide)/low‐melting‐point metal composites (PPSMs) with various loading levels were prepared by melt compounding. The nonisothermal crystallization behavior and transient viscoelastic properties were characterized by the DSC, POM, DMA, and parallel‐plate rheometer. The results reveal that the low‐melting‐point metal (LMPM) particles show nice dispersion at relative low content levels (< 30 wt %). The PPSMs composites present dual characteristics of both the filled polymer composite and polymer blend system in their transient viscoelastic behaviors, which results in occurrence of the stress overshoots with long relaxation time and nonzero residual stress especially at high shear levels. During the crystallization process, the presence of those deformable LMPM droplets facilitates the crystallization kinetics of PPS because of their flow‐promoting action. On the other hand, the LMPM has no heterogeneous nucleating effect and, only plays the role of inert filler, which results in the degradation of the crystal structure of PPS. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 677–690, 2008

Viscoelastic properties of polyarylene ether nitriles/thermotropic liquid crystalline polymer blend

Polyarylene ether nitriles (PEN)/thermotropic liquid crystalline polymer (TLCP) blend was prepared via melt mixing. The immiscible phase morphologies, linear and nonlinear, as well as transient viscoelastic properties of the blend were studied using SEM, rheometer, and DMA. The linear dynamic viscoelastic behavior of the blend shows temperature dependence due to further evolution of the immiscible morphology and, as a result, the principle of time-temperature superposition (TTS) is invalid. In the steady shear flow, the discrete TLCP phase is difficult to be broken up because of the high viscosity ratio of the blend systems, while is easy to be coarsened and followed by elongation, and finally, to form fibrous morphology at high TLCP content and high shear level. During this morphological evolution process, the transient stress response presents step increase and nonzero residual relaxation behavior, leading to increase of the dynamic viscoelastic responses after steady preshear. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

The effect of transient viscoelastic properties on interfacial instabilities in superposed pressure driven channel flows

In a recent study, Ganpule and Khomami (submitted to J. Non-Newtonian Fluid Mech.) have shown that in order to accurately describe the experimentally observed interfacial instability phenomenon in superposed channel flow of viscoelastic fluids, a constitutive equation that can accurately depict not only the steady viscometric properties of the experimental test fluids, but also their transient viscoelastic properties must be used in the analysis. In the present study, the effect of differences in transient viscoelastic properties which can arise either due to the differences in the predictive capabilities of various constitutive models or from the presence of multiple modes of relaxation on the interfacial instabilities of the superposed pressure driven channel flows has been investigated. Specifically, a linear stability analysis is performed using nonlinear constitutive equations which predict identical steady viscometric properties but different transient viscoelastic properties. It is shown that different nonlinear constitutive equations give rise to the same mechanism of interfacial instability, but the boundaries of the neutral stability contours and the magnitudes of the growth/decay rates, especially at intermediate and shortwaves, are shifted due to the overshoots in the transient viscoelastic responses predicted by the constitutive equations. In addition, the effect of the presence of multiple modes of relaxation on interfacial stability is studied using single and multiple mode upper convected Maxwell (UCM) fluids and it is shown that pronounced differences in the intermediate and shortwave linear stability predictions arise due to the fact that the increase in the number of modes gives rise to additional fast as well as slow relaxation modes and the presence of these additional relaxation modes gives rise to differences in the transient viscoelastic response of the fluids in the absence of any overshoots. The effect of fluid inertia on the interfacial stability of viscoelastic liquids is examined and it is shown that at longwaves, inertia has a pronounced effect on the stability of the interface, whereas at shortwaves, elastic and viscous effects dominate. Furthermore, the mechanism of viscoelastic interfacial instabilities is studied by a careful examination of disturbance eigenfunctions as well as performing a disturbance energy analysis. The results indicate that the mechanism of viscoelastic interfacial instabilities can be described in terms of interaction of mechanisms of purely viscous and purely elastic instabilities. However, since more than one mechanism for the instability is at work, the disturbance energy analysis can not clearly distinguish between them due to the fact that the eigenfunctions used in the energy analysis contain the information regarding both viscous and elastic effects. Hence, the mechanism of the instability must be determined by a careful examination of disturbance eigenfunctions.

An investigation of interfacial instabilities in the superposed channel flow of viscoelastic fluids

Interfacial instabilities of superposed pressure driven channel flow of viscoelastic fluids are investigated theoretically using linear stability analysis. Nonlinear constitutive equations which accurately depict the steady as well as transient viscoelastic properties of typical polymeric melts and solutions with various degrees of flexibility and accuracy are used to assess the constitutive complexity required to accurately describe the stability characteristics of this class of flows by comparing the results of the stability analysis with the experimental results of Wilson and Khomami (J. Non. Newtonian Fluid Mech. 41 (1992) 255; J. Rheol. 37 (1993) 315) and Khomami and Ranjbaran (Rheol. Acta 36 (1997) 1–22). It is shown that the multimode Giesekus model, which can accurately describe the steady as well transient behavior of the polymeric test fluids used in the experiments, can quantitatively describe the interfacial instability phenomenon in terms of the neutral stability contour as well as the growth/decay rate behavior. A rigorous energy analysis based on a newly developed disturbance-energy equation for viscoelastic flows is performed to investigate the mechanism of the purely viscous and purely elastic interfacial instabilities in pressure and drag driven channel flows. The mechanism of shortwave purely viscous instability is found to be due to the viscosity mismatch and the subsequent perturbation vorticity mismatch at the interface (i.e. interfacial friction), whereas the mechanism of the longwave purely viscous instability is found to be due to the bulk Reynolds stresses. The mechanism of purely elastic instability is found to be due to the coupling between the perturbation velocity and the jump in normal stresses across the interface at longwaves as well as shortwaves. An examination of perturbation velocity field reveals that for purely elastic longwave instability the jump in the normal stresses across the interface leads to a perturbation back flow in the bulk resulting in either accumulation (destabilizing) or depletion (stabilizing) of the fluid below the crest of the perturbed interface. In the case of purely elastic shortwave instability, coupling of the jump in the normal stresses across the interface and the perturbation velocity leads to perturbation vorticities adjacent to the interface which drive the instability.

A theoretical investigation of interfacial instabilities in the three layer superposed channel flow of viscoelastic fluids

The effects of viscosity and elasticity stratification on interfacial instabilities of symmetric and asymmetric three-layer superposed pressure driven channel flows have been investigated theoretically using linear stability analysis. The results indicate that the presence of an additional interface has a significant effect on the stability of the flow. Specifically, it has been shown that resonant instabilities as well as finite wavenumber interfacial instabilities could occur in this class of flows. Moreover, it has been demonstrated that a thin layer of less viscous fluid adjacent to the solid wall can stabilize longwave disturbances in both symmetric and asymmetric superposed Newtonian flows. In addition, in asymmetric superposed Newtonian flows it has been shown that when the jump in the shear rate across both the interface is positive then both the interfacial modes are unstable. In case of purely elastic instabilities, longwave disturbances can be stabilized if the more elastic fluid occupies the majority of the channel, irrespective of its position. Moreover, in both purely elastic and viscous instabilities the dominant mode of the instability has been shown to have wavenumbers of O(1). It has also been demonstrated that stability of viscoelastic flows can not be deduced based on the guidelines developed for purely elastic and viscous instabilities (i.e., the effects are non-additive). Furthermore, nonlinear constitutive equations which accurately depict the steady as well as transient viscoelastic properties of typical polymeric melts and solutions with various degrees of flexibility and accuracy have been used to assess the constitutive complexity required to accurately describe the stability characteristics of this class of flows by comparing the results of the stability analysis with the experimental results of Khomami and Ranjabaran [9]. It is shown that the multimode Giesekus model, which can accurately describe the steady as well transient behavior of the polymeric test fluids used in the experiments, can quantitatively describe the interfacial instability phenomenon in terms of the neutral stability contour as well as the growth/decay rate behavior when the effect of interfacial tension is taken into account. A rigorous energy analysis based on a disturbance–energy equation for viscoelastic flows has also been performed to investigate the mechanism of the purely viscous and purely elastic interfacial instabilities in pressure driven channel flows. The mechanisms of purely viscous and purely elastic instabilities of the three-layer flows are found to be identical to those of the two-layer flows. Namely, the mechanism of shortwave purely viscous instability is found to be due to the viscosity mismatch and the subsequent perturbation vorticity mismatch at the interface (i.e., interfacial friction), whereas the mechanism of the longwave purely viscous instability is found to be due to the bulk Reynolds stresses. The mechanism of purely elastic instability is found to be due to the coupling between the perturbation velocity and the jump in normal stresses across the interface at longwaves as well as shortwaves. Finally, the possibility of non-normal interactions between the two interfacial modes or interfacial and bulk modes has been demonstrated and it has been shown that these interactions should be carefully considered when the disturbance growth/decay rates are experimentally determined.

Effects of physical aging on dynamic mechanical and transient properties of polyetheretherketone

AbstractThe effects of physical aging on dynamic mechanical and transient stress‐relaxation properties of a high performance thermoplastic, polyetheretherketone (PEEK), are reported in this study. The aging effects are shown to be physical effects (and not chemical degradation) by establishing thermoreversibility of the aging process. The unique aspect of the present study is the simultaneous investigation of both dynamic and transient properties in the same mode of loading, namely, in shear. The influence of continuous dynamic deformation on the aging process is also reported. The results for the aging rate, obtained from dynamic and transient testing under shear load, agree with one another and verify the equivalence of dynamic and transient viscoelastic properties. However, these results differ significantly from the aging rate results reported in an earlier study for tensile loading. A possible explanation for the difference has been proposed in terms of the volume change that accompanies tensile creep deformation.

Time Dependent Nonlinear Viscoelastic Behavior of Polymer Fluids—A Review of Current Understanding

Abstract The behavior of concentrated polymer solutions and melts in transient flows has been under active investigation in recent years. This research interest stems from the realization that a better understanding of these time dependent phenomena has the potential to greatly improve existing industrial processes and to provide new insights to polymer dynamics. In this article, various aspects of the transient viscoelastic properties are discussed. Special emphases are placed on some important considerations in acquiring reliable transient data. Several promising network theories capable of interpreting these experimental results are reviewed. Future research challenges, such as the design of more complicated and stringent rheological tests for the established models, have also been identified.

Measurement of viscoelastic properties by strain response of a piezoelectric plate

A piezoelectric plate device called the “duomorph” has been developed for use in measuring dynamic and transient viscoelastic properties of both soft and hard materials. This device consists of two crystalline piezoelectric plates which are bonded together to form a driver‐sensor pair or a pair which produces pure bending and on which are mounted electrical resistance strain gauges. Electrical excitation of the duomorph causes bending and/or in‐plane straining which is affected by any material with which it makes contact. Through analysis and comparison of the response to that measured when the gauge is tested by itself. the dynamic and transient viscoelastic properties of the contacting material are found. Both phase angle and amplitude are used in determining properties. The duomorph was originally developed by Aerojet and Texas A&M several years ago for the purpose of measuring in situ complex modulus during aging of solid rocket fuel. Since then we have employed several different versions of the duomorph when fully embedded or when on the surface of various materials, including clay, asphalt, and rubber. Its most recent application consists of measuring viscoelastic properties of resin during and after curing. It has potential application to in‐process monitoring of polymeric laminates, and this application is presently under study. In this lecture we describe the device, its fabrication, calibration, and various applications. [Sponsored by the Air Force Office of Scientific Research and the Office of Naval Research.]

Stress Relaxation of Carbon Black-Linseed Oil Suspensions

Stress relaxation under large deformation and transient viscoelastic properties of carbon black-linseed oil suspensions were investigated using a cone and plate type rheometer.The equilibrium moduli in the long time region were observed in stress relaxation curves even under a large shear strain such as 6.0 shear unit, and these values exhibited a remarkable strain dependence. These phenomena may be attributed to the formation of close structural networks of carbon black particles in linseed oil.Shear stress development after sudden initiation of steady shear flow showed a overshoot and approached a steady-state value. This transient phenomenon seems to be caused by the break down of the structure of dispersed particles by shearing stress. Under steady flow conditions, the structural networks were destroyed completely. Therefore, the equilibrium values could not be observed in the stress relaxation curves after cessation of the steady flow.

Molecular Theories of Rubberlike Elasticity and Polymer Viscoelasticity

Abstract The foundation for nearly all the molecular theories of the physical properties of polymers was laid in 1934 when Guth and Mark and Kuhn first recognized the role of configurational entropy of polymer chains. By virtue of ability of their segments to rotate with respect to each other along the chain backbone, macromolecules are capable of assuming a myriad of conformations. It is this long, flexible chain nature of polymer molecules that provides us a link in interpreting the macroscopic physical properties in terms of microscopic molecular dynamics. In this review we shall demonstrate the utilization of conformational statistics in the formulation of molecular theories for two important aspects of the mechanical properties of polymers. The first part deals with the equilibrium elastic response of a crosslinked rubberlike network. A simplified derivation will be given on the basis of the entropic approach. Some of the underlying assumptions will then be examined, and the contribution of internal energy to rubber elasticity scrutinized. The second part describes the transient viscoelastic properties of linear polymers in solution and in bulk. Limitations of the model will be assessed and its applications to experimental data explored. It should be pointed out that a number of reviews are available in the literature both for elasticity and viscoelasticity of polymers. The present work is not an exhaustive review of these fields, but rather concentrates on the more recent developments not previously discussed. The emphasis will be placed upon polymers in the bulk state, although solution properties will be mentioned where appropriate.