Flow-induced phase separation in polymer solutions

Macromolecules, 1984

The stationary flow of polymer solutions is treated as a near-equilibrium process, to which the normal tools of thermodynamics can be applied if the energy stored in the sheared state is added to the Gjbbs energy of the system at rest. The first part of the paper deals with the mathematical description of the experimental information concerning the two contributions to the Gibbs energy of sheared solutions, in particular with the calculation of the stored energy from measured flow curves. In the second part, these general considerations are applied to the phase separation of the flowing system trans-decalinlpolystyrene. The results of the latter calculations confii the experimental finding that the region of homogeneity is extended by shear and that the demixing curves of the flowing solutions normally exhibit two maxima instead of one. Furthermore, a new phenomenon is predicted, namely a shear-induced coexistence of three liquid phases at the temperature at which the two neighboring branches that evolve out of these maxima of the demixing curve cross. In addition, this intersection represents the largest extension of the dissolved state of the polymer that can be achieved by a given shear rate. It is called the eulytic point by analogy with the eutectic point. According to the present calculations, the eulytic point is shifted toward lower polymer concentrations as the shear rate is increased.

Macromolecules, 2020

Flow-induced phenomena in entangled solutions of linear C 1000 H 2002 polyethylene dissolved in n-hexadecane and benzene solvents were simulated via nonequilibrium molecular dynamics at concentrations of 14.5C* and 13.5C*, respectively, of the coil overlap concentration, C*. The simulations revealed that both solutions undergo a chemical phase separation when subject to planar extensional flow at extension rates faster than the inverse Rouse time of the solution. The onset of phase separation initiated after roughly two Hencky strain units of deformation for both solutions and attained a stationary state at about ten Hencky strain units. Furthermore, the simulations revealed that at very high extension rates the polymer phase forms semicrystalline domains regardless of the solvent; however, the critical extension rate for flow-induced crystallization appeared to be affected by a number of variables, including solution temperature and the chemical nature of the solvent.

1998

The dynamics of phase separation of a quenched polymer solution is studied using a stochastic molecular dynamics simulation. At early times, the elastic nature of the polymer chains generates a networklike domain morphology for sufficiently dense polymer solutions. However, at late times, this network structure breaks up into disconnected polymer-rich domains to minimize the interfacial energy. Our simulations explain why different experiments carried out to different time regimes seem to produce contradictory results, and strongly indicate that the true late-time growth kinetics of quenched polymer solutions belong to the same universality class of small molecular mixtures. [S0031-9007(97)04972-7]

Polymer, 2003

Recently, a method was presented to model phase separation behaviour of polymer-solvent mixtures around the lower critical solution temperature using dissipative particle dynamics (DPD) [Macromol Theory Simul 9 (2000) 698]. In the current article, a refined version of this method is described, yielding good agreement with the classic Flory-Huggins model. However, the great advantage of DPD, over such classical theories, is its ability to incorporate structural properties without additional parameters. This will be demonstrated on the case of branched polymers, for which DPD correctly predicts the cloud point pressure lowering experimentally observed.

2002

Thee subject of this thesis is polymer-solvent liquid-liquid phase separation. Thiss phenomenon is interesting both from an academic and practical point off view. In the oil industry the separation of a mixture into in a heavy oilrichh and a solvent-rich phase is a well-known and often applied purification technique.. However, in the current (and comparable) polymer production processess this is not the case. IX X thee degree of branching has a pronounced effect on the radius of gyration andd the centre of mass diffusion of the polymer. The effect on the demixingg behaviour has been mostly attributed to the increase in mobility of the polymerr due to the decrease in radius of gyration. In contrast, it was found thatt the effect of branching on the difference in chemical potential is the reall cause for the observed phase separation behaviour. XIII I uitgebreidee Flory-Hugginstheorie toegepast. Met de introductie van een temperatuur-en drukafhankelijke repulsieparameter kan het oplossend vermogenn worden gecontroleerd. Daarmee is het DPD-model in staat zowel UCST-als LCST-gedrag te simuleren. Opp basis van experimentele bevindingen was bekend dat een verandering in dee vertakkingsgraad van een polymeer het ontmenggedrag benvloedt. Het grotee voordeel van DPD boven de klassieke theorien is dat het mogelijk is, zonderr het gebruik van additionele parameters, structurele eigenschappen vann het polymeer in het model onder te brengen. Hett blijkt dat het model op de juiste manier de verlaging van vlokpuntsdrukk voorspelt van vertakte polymeren ten opzichte van lineaire polymeren. Verderr wordt aangetoond dat de vertakkingsgraad een duidelijk effect heeft opp de gyratiestraal en massamiddelpuntsdiffusie van het polymeer. Dit effectt op het ontmenggedrag wordt meestal toegeschreven aan de verhoogde mobiliteitt van het polymeer als gevolg van de afgenomen gyratiestraal. Echter,, op basis van de DPD-simulaties kan geconcludeerd worden dat hett vertakkingseffect op het verschil in chemische potentiaal de werkelijke oorzaakk is voor de gemeten verandering in fasescheidingsgedrag.

Physical Review Letters

Manufacturing of plastics is typically performed via flow processing of a molten polymeric fluid. Until recently, conventional knowledge has maintained that the deformation of the constituent molecules under flow is homogeneous and obeys Gaussian statistics. In this study via virtual experimentation, an entangled polyethylene melt subjected to planar elongational flow displayed an unanticipated microphase separation into a heterogeneous liquid composed of regions of either highly stretched or tightly coiled macromolecules, thus providing a natural realization of a bi-phasic coil-stretch transition.

Physical Review Letters, 1989

We introduce a tensor variable to describe chain deformations in the continuum limit. Then we set up a dynamic model and apply it to the problem of phase separation in polymer solutions under shear. EAects of chain deformations are shown to be crucial in the entangled case. A large shear-demixing eA'ect is found in semidilute solutions in agreement with experiments.

Rheologica Acta, 1993

Mechanical and optical rheometric measurements are reported on solutions of polystyrene dissolved in dioctyl phthalate, a solution that can undergo an apparent phase separation upon the application of shear. Solutions prepared using three molecular weights ranging from one to four million were studied. Time-temperature superposition was observed to apply for these solutions up to and including the onset of an apparent shear thickening of the steady shear and first normal stresses. Optical measurements employing turbidity and scattering dichroism determined that concentration fluctuations were enhanced by flow and grew parallel to the vorticity axis below the critical velocity gradient for the onset of the apparent shear thickening effect. Prior to the onset of thickening, the fluctuations were observed to rearrange and orient parallel to the flow direction. Second normal stress difference measurements indicate these solutions have a negative ratio of the second to the first normal stress differences. It is interesting to point out that the ratio tends to zero in the vicinity of the shear rate range at which shear thickening occurs.

ABSTRACTUnderstanding the thermodynamics that drives liquid-liquid phase separation (LLPS) is quite important given the many numbers of diverse biomolecular systems undergoing this phenomenon. Regardless of the diversity, the processes underlying the formation of condensates exhibit physical similarities. Many studies have focused on condensates of long polymers, but very few systems of short polymer condensates have been observed and yet studied. Here we study a short polymer system of various lengths of poly-Adenine RNA and peptide formed by the RGRGG sequence repeats to understand the underlying thermodynamics of LLPS. We carried out MD simulations using the recently developed COCOMO coarse-grained (CG) model which revealed the possibility of condensates for lengths as short as 5-10 residues, which was then confirmed by experiment, making this one of the smallest LLPS systems yet observed. Condensation depends on polymer length and concentration, and phase boundaries were identif...

We review the basic ideas and main results of our analysis of shear-induced effects in polymer solutions and blends. The analysis combines thermodynamic and hydrodynamic descriptions. The flow contribution to the free energy of the solution is described in the framework of extended irreversible thermodynamics, which relates the viscoelastic constitutive equations to a non-equilibrium entropy that depends on the viscous pressure tensor. This yields, by differentiation, the corresponding non-equilibrium equation of state for the chemical potential, which couples diffusion flux to viscous pressure in the presence of a flow. In some conditions, the one-phase system becomes unstable and splits into two phases, leading to a shift in the spinodal line. The theoretical analysis is based on the stability of the mass and momentum balance equations, the constitutive equations for viscous pressure tensor and diffusion flux, and the equation of state for the chemical potential. The resulting predictions corroborate qualitatively the known experimental observations. Results for dilute and entangled polymer solutions and for polymerblends are given