We study the rheological characteristics of nanocomposites containing nano-sized plate like particles in a viscoelastic fluid at the startup of steady state in the simple shear flow mode. The nanocomposites of organoclay-polypropylene with different nanoclay contents were prepared by melt mixing. A rheological equation of state, originally formulated to predict the orientation state and viscoelastic behavior of suspensions of ellipsoidal particles in polymer melts, has been modified to describe the observed phenomena for the nanoclay/poly(propylene) composites. The rotational particle motion and alignment for a group of symmetric ellipsoids with the applied flow field are investigated. Additionally, model calculations of the macroscopic rheological properties for a simple flow case suggest the presence of nano-particles significantly modify the suspended fluid at volume concentrations as low as 0.5%. The model calculations for the startup viscosity are reasonably in agreement with the experimental results at the experimental range covered in this study. At the shear rate of , we observe pronounced stress overshoots at the three nanoclay loadings level tested which are found to be related to the fast alignment of the silicate layers with the shear direction in the polymer melt. 相似文献
<正>The effect of pre-shear flow on the subsequent crystallization process of polymeric melt was investigated and a flow-induced crystallization(FIC) model based on the conformation tensor incorporating the pre-shear effect was proposed. The model is capable of predicting the overshoot phenomena of the stress and the flow-induced free energy change of the polymeric system at high pre-shear rates.Under the condition of flow,the increase in the activated nuclei number was contributed by the flow-induced free energy change,which showed an overwhelming effect on the nuclei formation during the pre-shear process at high shear rates.The half crystallization time(f_(1/2)) of polypropylene(PP) as functions of pre-shear rate and pre-shear time at different crystallization temperatures was predicted and compared with the experiment data.Both numerical and experimental results showed that t_(1/2) of PP decreased dramatically when the flow started but leveled off at long times.It was found that two transformation stages in t_(1/2) existed within a wide range of shear rates.For the first stage where the melting polymer experienced a relatively weak shear flow,the acceleration of crystallization kinetics was mainly contributed by the steady value of free energy change while in the second stage for high shear rates,strong overshoot in flow-induced free energy change occurred and the crystallization kinetics was thus significantly enhanced.The overshoots in stress and flow-induced free energy change reflected an important role of flow on the primary nucleation especially when the flow was strong enough. 相似文献
IntroductionOwing to their special structures or morphologies,polymer-based montmorillonite nanocomposites exhibitsome particular characteristics,such as,physical,thermal,and mechanical properties,induced by theaddition of very small amounts of inorganic … 相似文献
Conceptually, an imagined conformation ellipsoid is supposed to represent the shape of a polymerchain for polymer melts in flow fields and to be equivalent to the volume element in a mathematical sense incontinuum mechanics. A power law dependence of shear modulus of polymer melts on detC, referred to asenvelope volume, is proposed. Based on those assumptions and the non-linear relation of shear modulus, aphenomenological viscoelastic model is derived. The model is tested in simple shear flow, simpleelongational flow, oscillatory shear flow, and relaxation process after flow suddenly stopped. The resultsshow that the model works well to predict the change of internal structure and viscoelastic performance ofpolymer melts in flow fields. 相似文献
A new method of treating experimental data on the viscous and viscoelastic properties of various polymer melts is suggested. The dependence of the apparent viscosity on the molecular weight, temperature and shear stress can be represented as the product of three independent functions, each of them having a single argument. All three functions are universal, at least in first approximation, and the dependence of the apparent viscosity on the variables indicated is determined by two parameters (glass transition temperature and critical molecular weight), characteristic of each homologous polymer series. The viscoelastic characteristics (dynamic, relaxation, creep, as well as relaxation and retardation spectra) of polymer melts are universal in shape in the linear region and contain only one individual polymer parameter, viz., maximum Newtonian viscosity. It is shown that upon normalization of certain nonlinear characteristics with respect to the maximum Newtonian viscosity, they can also be represented in the universal form. 相似文献
Summary: In this work, the behavior of some internal microstructural models with different mobility tensors has been studied for polymer melts and solutions under steady and transient simple shear and elongational flows. The time evolution equations for conformation and stress tensors in the models reviewed have their root in the Generalized Poisson bracket formalism. Two different families of conformational models have been selected for this study. The first family is based on the Modified Finitely Extensible Nonlinear Elastic (FENE‐P) energy while the second uses a Volume Preserving Conformational Rheological (VPCR) model based on the Hookean Helmholtz free energy function. Several expressions for the mobility tensor based on the previously mentioned energy functions are used to obtain the models. The sensitivity of both families of models to the choice of the mobility tensors on the prediction of material functions in the transient and steady flows is discussed. Also, effects of shear rate on the material functions in start‐up and relaxation shear flows for both models are studied. The predictions of both models are compared with experimental data taken from the literature for some polymer melts. These results show that the family of VPCR models is able to predict the steady shear and elongational flow material functions in an extended range of deformation rates whereas the family of FENE‐P models can predict the behavior of only some specified polymer melts.
Experimental data and VPCR model predictions for steady and elongational viscosity for PS [data of H. Munstedt, 1980]. 相似文献
A director theory of solutions and melts of flexible-chain polymers that is based on a simple law of anisotropic stress relaxation is developed. A spectral approach is used to study its structure. The simplest equation for the director that describes the dependence of the orientation of anisotropic viscoelastic fluids on shear rate is presented. The most appropriate flows for determining the material properties of polymer fluids with a single preferred direction are discussed. In addition, a new classification of anisotropic fluids is proposed. 相似文献
A quantitative theory of hydrodynamic interactions in unentangled polymer melts and concentrated solutions is presented. The study is focussed on the pre-Rouse transient time regimes (t < τ(R), the Rouse relaxation time) where the hydrodynamic response is governed mainly by the viscoelastic effects. It is shown that transient viscoelastic hydrodynamic interactions are not suppressed (screened) at large distances and are virtually independent of polymer molecular mass. A number of transient regimes of unusual and qualitatively different behavior of isotropic and anisotropic hydrodynamic response functions are elucidated. The regimes are characterized in terms of two main length-scale dependent characteristic times: momentum spreading time τ(i) ∝ r(4∕3) and viscoelastic time τ(?) ∝ r(4). It is shown that for t > τ(i) the viscoelastic hydrodynamic interactions can be described in terms of the time or length scale dependent effective viscosity which, for t < τ(R) and/or for r < R(coil), turns out to be much lower than the macroscopic "polymer" viscosity η(m). The theory also involves a quantitative analysis of the length-scale dependent stress relaxation in polymer melts. The general predictions for hydrodynamic interactions in thermostated systems with Langevin friction are obtained as well. 相似文献