Pavlos Stephanou

Cyprus University of Technology · Department of Chemical Engineering

The topological state of entangled polymers has been analyzed recently in terms of primitive paths which allowed obtaining reliable predictions of the static (statistical) properties of the underlying entanglement network for a number of polymer melts. Through a systematic methodology that first maps atomistic molecular dynamics (MD) trajectories o...

Atomistic configurations of model unentangled ring polyethylene (PE) melts ranging in chain length from C24 up to C400 have been subjected to detailed molecular dynamics (MD) simulations in the isothermal−isobaric statistical ensemble at temperature T = 450 K and P = 1 atm. Strictly monodisperse samples were employed in all cases. We present and di...

We provide a description of the Marrucci−Ianniruberto constitutive equation [Philos. Trans. R. Soc. London, A 2003, 361, 677−688] for the rheology of entangled polymer melts in the context of nonequilibrium thermodynamics and we properly extend it to account for a second normal stress difference by introducing a second order term in the relaxation...

We address the issue of flow effects on the phase behaviour of polymer nanocomposite melts by making use of a recently reported Hamiltonian set of evolution equations developed on principles of non-equilibrium thermodynamics. To this end, we calculate the spinodal curve, by computing values for the nanoparticle radius as a function of the polymer r...

We introduce a continuum model for polymer melts filled with nanoparticles capable of describing in a unified and self-consistent way their microstructure, phase behavior, and rheology in both the linear and nonlinear regimes. It is based on the Hamiltonian formulation of transport phenomena for fluids with a complex microstructure with the final d...

The balance equation of angular momentum in anisotropic fluids includes a couple stress contribution, also responsible for an antisymmetric contribution to the force stress tensor. We herein derive all balance equations for the simplest anisotropic fluid, i.e., a polar fluid, using the GENERIC formalism of non-equilibrium thermodynamics. In doing s...

A variable-entanglement density constitutive model is developed for the description of the rheological properties of entangled polymer melts and concentrated polymer solutions using non-equilibrium thermodynamics (NET). It proposes two evolution equations: one for the average number of entanglements per chain and one for the orientation of entangle...

In vitro testing methods offer valuable insights into the corrosion vulnerability of metal implants and enable prompt comparison between devices. However, they fall short in predicting the extent of leaching and the biodistribution of implant byproducts under in vivo conditions. Physiologically based toxicokinetic (PBTK) models are capable of quant...

The rheological behavior of styrene-butadiene rubber (SBR) compounds filled with silica are investigated as a function of silica volume fraction. To predict the mechanical response, a continuum model for entangled polymer melts filled with nanoparticles is herein introduced. This model is capable of describing the rheological response in both the l...

Local and systemic contamination caused by metal ions leaching from medical device materials is a significant and continuing health problem. The increasing need for verification and validation, and the imposition of stringent government regulations to ensure that the products comply with the quality, safety, and performance standards, have led regu...

Based on the Generalized bracket, or Beris–Edwards, formalism of non-equilibrium thermodynamics, we recently proposed a new differential constitutive model for the rheological study of entangled polymer melts and solutions. It amended the shortcomings of a previous model that was too strict regarding the values of the convective constraint release...

Based on the Generalized bracket, or Beris-Edwards, formalism of non-equilibrium thermodynamics, we have recently proposed [Stephanou et al. Materials, 13, 2867 (2020)] a new differential constitutive model for the rheology of entangled polymer melts and solutions. It has amended the shortcomings of a previous model that was too strict in the value...

Undoubtedly, cement is one of the most important materials in the construction industry. For its effective use, it is particularly important to fully comprehend the rheological behavior of cement paste. When cement is mixed with water, a suspension is initially formed and the rate of hydrolysis reactions accelerates leading to the formation of a ne...

In “soft–soft nanocomposites” based on film formation of latexes with structured particles, the combination of particle structure and interparticle crosslinking leads to materials that behave as nonlinear viscoelastic fluids at small strains and as highly elastic networks at larger strains. Similarly, in studies of flow-induced crystallization in p...

During crude oil extraction, crude oil is often mixed with water, leading to the formation of water-in-oil (W/O) emulsions. Since these emulsions pose severe flow resistance, such as higher pressure drops, due to their complex fluid rheology, it is important to have in our arsenal a rheological constitutive model that accurately predicts their rheo...

Since its introduction, back in the late 1970s, by Gordon and Schowalter (GS) and later by Johnson and Segalman (JS), the non-affine or slip parameter, ξ, has been routinely employed by numerous constitutive models. Its use results in spurious oscillations in the transient shear viscosity in start-up shear flow. Recent experimental evidence has sho...

Since its introduction in the late 1970s, the non-affine or slip parameter, ξ, has been routinely employed by numerous constitutive models as a constant parameter. However, the evidence seems to imply that it should be a function of polymer deformation. In the present work, we phenomenologically modify a constitutive model for the rheology of unent...

Correction for ‘A constitutive hemorheological model addressing the deformability of red blood cells in Ringer solutions’ by Pavlos S. Stephanou et al. , Soft Matter , 2020, 16 , 7585–7597, https://doi.org/10.1039/D0SM00974A.

Since its introduction, back in the late 1970s, the non-affine or slip parameter, ξ, has been routinely employed by numerous constitutive models as a constant parameter. However, the evidence seems to imply that it should be a function of polymer deformation. In the present work, we phenomenologically modify a constitutive model for the rheology of...

Since its introduction, back in the late 1970s, by Gordon and Schowalter (GS) and later by Johnson and Segalman (JS), the non-affine or slip parameter, ξ, has been routinely employed by numerous constitutive models. Its use results in spurious oscillations in the transient shear viscosity in start-up shear flow. Recent experimental evidence has sho...

During crude oil extraction, crude oil is often mixed with water, leading to the formation of water-in-oil (W/O) emulsions. Since these emulsions pose severe flow resistance, such as higher pressure drops, due to their complex fluid rheology, it is important to have in our arsenal a rheological constitutive model that accurately predicts their rheo...

In the past few decades, nanotechnology has been employed to provide breakthroughs in the diagnosis and treatment of several diseases using drug-carrying particles (DCPs). In such an endeavor, the optimal design of DCPs is paramount, which necessitates the use of an accurate and trustworthy constitutive model in computational fluid dynamics (CFD) s...

During the past few decades, the interest in understanding the peculiar rheological behavior of shear-thickening fluids has increased due to their potential use in various commercial applications. In such an endeavor, the optimal design of these fluids is essential, which necessitates our in-depth understanding of their properties from a modeling p...

Red blood cells (RBCs) in physiological conditions are capable of deforming and aggregating. However, both deformation and aggregation are seldom considered together when modeling the rheological behavior of blood. This is particularly important since each mechanism is dominant under specific conditions. To address this void, we herein propose a ne...

Red blood cells (RBCs) can deform substantially, a feature that allows them to pass through capillaries that are narrower than the diameter of an undeformed RBC. Clearly, to understand how they transport through our microcirculation, we need a constitutive model able of accurately predicting the deformability of red blood cells, which seems current...

We show how two-species models, already proposed for the rheology of networks of associative polymer solutions, can be derived from nonequilibrium thermodynamics using the generalized bracket formalism. The two species refer to bridges and (temporary) dangling chains, both of which are represented as dumbbells. Creation and destruction of bridges i...

We show how two-species models, already proposed for the rheology of networks of associative polymer solutions, can be derived from nonequilibrium thermodynamics using the generalized bracket formalism. The two species refer to bridges and (temporary) dangling chains, both of which are represented as dumbbells. Creation and destruction of bridges i...

In a recent reformulation of the Marrucci-Ianniruberto constitutive equation for the rheology of entangled polymer melts in the context of nonequilibrium thermodynamics, rather large values of the convective constraint release parameter βccr had to be used in order for the model not to violate the second law of thermodynamics. In this work, we pres...

In a recent reformulation of the Marrucci-Ianniruberto constitutive equation for the rheology of entangled polymer melts in the context of non-equilibrium thermodynamics, rather large values of the convective constraint release parameter \beta_{ccr} had to be used in order not to violate the second law of thermodynamics. In this work, we present an...

We have recently solved the tumbling-snake model for concentrated polymer solutions and entangled melts in the academic case of a monodisperse sample. Here, we extend these studies and provide the stationary solutions of the tumbling-snake model both analytically, for small shear rates, and via Brownian dynamics simulations, for a bidisperse sample...

We propose a new description of elasto-viscoplastic fluids by relating the notion of thixotropy directly to internal viscoelasticity and network structures through a general, thermodynamically consistent, approach. By means of non-equilibrium thermodynamics, a thermodynamically-admissible elasto-viscoplastic model is derived which introduces self-c...

The steady-state extensional viscosity of dense polymeric liquids in elongational flows is known to be peculiar in the sense that for entangled polymer melts it monotonically decreases—whereas for concentrated polymer solutions it increases—with increasing strain rate beyond the inverse Rouse time. To shed light on this issue, we solve the kinetic...

We have recently solved the tumbling-snake model for concentrated polymer solutions and entangled melts in the presence of both steady-state and transient shear and uniaxial elongational flows, supplemented by a variable link tension coefficient. Here, we provide the transient and stationary solutions of the tumbling-snake model under biaxial elong...

We have recently solved the tumbling-snake model for concentrated polymer solutions and entangled melts in the presence of both steady-state and transient shear and uniaxial elongational flows, supplemented by a variable link tension coefficient. Here, we provide the transient and stationary solutions of the tumbling-snake model under biaxial elong...

Red blood cells tend to aggregate in the presence of plasma proteins, forming structures known as rouleaux. Here, we derive a constitutive rheological model for human blood which accounts for the formation and dissociation of rouleaux using the generalized bracket formulation of nonequilibrium thermodynamics. Similar to the model derived by Owens a...

We describe a methodology for parameterizing and validating a continuum model for the rheological behaviour of polymer nanocomposites derived on principles of nonequilibrium thermodynamics based on numerical data collected from large-scale, fully-atomistic equilibrium (MD) and nonequilibrium molecular dynamics (NEMD) simulations. As a model system,...

The authors of the present study have recently presented evidence that the tumbling-snake model for polymeric systems has the necessary capacity to predict the appearance of pronounced undershoots in the time-dependent shear viscosity as well as an absence of equally pronounced undershoots in the transient two normal stress coefficients. The unders...

The rheological behavior of drillings fluids is usually described by the Casson or the Herschel- Bulkley models. Despite the overwhelming data highlighting the significance of their use in numerous fields, they fail to produce normal stresses, whose importance in drilling operations has only recently attracted attention. We herein introduce a conti...

Our experimental data unambiguously show (i) a damping behavior (the appearance of an undershoot following the overshoot) in the transient shear viscosity of a concentrated polymeric solution, and (ii) the absence of a corresponding behavior in the transient normal stress coefficients. Both trends are shown to be quantitatively captured by the bead...

We introduce a continuum model for polymer melts filled with nanoparticles capable of describing in a unified way their microstructure, phase behavior, and rheology in both the linear and non-linear regimes. The model contains several parameters, all of which can be determined from equilibrium and nonequilibrium atomistic molecular dynamics simulat...

Our experimental data unambiguously show the appearance of a dumping behavior in the transient shear viscosity of a concentrated polymeric solution, and the absence of dumping behavior in the transient viscometric functions. Such a behavior is shown to be quantitatively captured by the beadlink chain kinetic theory model for concentrated polymer so...

Our experimental data unambiguously show the appearance of a dumping behavior in the transient shear viscosity of a concentrated polymeric solution, and the absence of dumping behavior in the transient viscometric functions. Such a behavior is shown to be quantitatively captured by the beadlink chain kinetic theory model for concentrated polymer so...

We reconsider a recently proposed reptation-based constitutive model for the rheology of entangled polymer melts (derived in the context of nonequilibrium thermodynamics) by accounting for a variable chain entanglement density due to applied flow as indicated by recent atomistic nonequilibrium molecular dynamics simulations.

We reconsider a recently proposed reptation-based constitutive model for the rheology of entangled polymer melts (derived in the context of non-equilibrium thermodynamics) by accounting for a variable chain entanglement density due to applied flow as indicated by recent atomistic non-equilibrium molecular dynamics simulations.

We reformulate the Marrucci-Ianniruberto constitutive equation for the rheology of entangled polymer melts on the basis of the generalized bracket and the GENERIC frameworks of non-equilibrium thermodynamics (NET), and we properly extend it to account for a second normal stress difference by introducing a second order term in the relaxation tensor...

The complete constitutive model for concentrated polymer solutions and melts proposed by Curtiss and Bird is solved for shear flow: a) analytically by providing a solution for the single-link (or configurational) distribution function as a spherical harmonics expansion and then calculating the materials functions in shear flow up to second order in...

The complete kinetic theory model for concentrated polymer solutions and melts proposed by Curtiss and Bird is solved for shear flow: (a) analytically by providing a solution for the single-link (or configurational) distribution function as a real basis spherical harmonics expansion and then calculating the materials functions in shear flow up to s...

We present a constitutive model for polymer nanocomposite melts from non-equilibrium thermodynamics that can describe in a unified way their microstructure, phase behavior, and rheology. The model can reliably fit experimental rheological data for the shear viscosity of several polymer nanocomposites over broad ranges of volume fractions and deform...

We present a hierarchical computational methodology which permits the accurate prediction of the linear viscoelastic properties of entangled polymer melts directly from the chemical structure, chemical composition, and molecular architecture of the constituent chains. The method entails three steps: execution of long molecular dynamics simulations...

Design a multi-scale approach to predict the key material properties of higher-MW polymer melts

We present a hierarchical, three-step methodology for predicting the linear viscoelastic properties of entangled polymer melts. First, atomistic trajectories accumulated in the course of long molecular dynamics simulations with moderately entangled polymer melts are self-consistently mapped onto the tube model to compute the segment survival probab...

s,t) το σημείο s του πρωτογενούς δρόμου κατά μήκος της αλυσίδας (primitive path, PP) να παραμένει μετά από χρόνο t εντός του αρχικού δραστικού σωλήνα (όπως δηλαδή αυτός ορίστηκε σε χρόνο t=0) και ο οποίος περιορίζει την κίνηση της αλυσίδας κάθετα στο περίγραμμά της σε μια θεώρηση μέσου πεδίου. Με αυτό τον τρόπο μπορέσαμε να ποσοτικοποιήσουμε τη συν...

Recent atomistic simulations of polyethylene melt afford a close look at the entanglement dynamics of a real polymer. We analyze these results in a new way, by defining the tube primitive path as the mean path of consecutive molecular dynamics trajectories. The result suggests that tube is semiflexible, the persistence length being about half the e...

The prevailing theory of polymer rheology rests on a careful analysis of tube dynamics, tested by comparing predicted rheological response functions to experimental measurements. We provide a direct test of this theory by analyzing the tube dynamics of recently simulated mildly entangled polyethylene melt. The tube dynamics is obtained by defining...

By mapping atomistic trajectories accumulated in the course of long molecular dynamics simulations onto the tube model for model mono- and bidisperse polyethylene and cis-1,-4 and trans-1,4-polybutadiene melts, we have computed contributions to chain dynamics in entangled polymers from pure reptation, contour length fluctuation, and constraint rele...

State-of-the-art tube models for the dynamics of entangled polymer melts are usually validated on the basis of the agreement of their predictions for the linear viscoelastic properties (LVE data) of the system against experimentally measured data. We present here a more direct and fundamental test of these models based on their comparison against m...

Polymeric systems exhibit a plethora of interesting (and sometimes intriguing) phenomena which have their origin to the complex macromolecular structure of these materials. Polymeric chains are characterized by a broad spectrum of length and time scales, which gives rise to properties that are totally different from those of the simple Newtonian li...

Motivated by recently reported experimental data [1] on the peculiar rheological behaviour of polymer rings, we have undertaken a systematic and detailed investigation of the structural and mainly the dynamical and rheological properties of model ring polyethylene (PE) melts, ranging in molecular length from N =24 up to N=400 carbon atoms per molec...

We study dynamics in bidisperse melts of linear cis-1,4-polybutadiene composed of probe and matrix chains at the level of the segment survival probability function ψ(s,t) which is computed directly in the course of long atomistic molecular dynamics simulations [Stephanou et al. J. Chem. Phys. 2010, 132, 124904]. By controlling precisely the matrix...

Based on principles of nonequilibrium thermodynamics, we derive a generalized differential constitutive equation for polymer melts which incorporates terms that account for anisotropic hydrodynamic drag in the form suggested by Giesekus, finite chain extensibility with nonlinear molecular stretching, nonaffine deformation, and variation of the long...

Study the PP dynamics by mapping configurations from atomistic simulations onto ensembles of PPs

To develop a more accurate and more reliable single conformation tensor model