P B Sunil Kumar

Rheology of Complex Fluids Abhijit P. Deshpande, J. Murali Krishnan, P. B. Sunil Kumar This book provides an in-depth treatment of complex fluids. A clear understanding of the flow and rheological behavior of such fluids is crucial while carrying out the processing operations, designing equipments which handle/transport these fluids, and in their end-use applications. This book: • Discusses multicomponent-multiphase systems of which most of complex fluids are examples •Covers a wide variety of application areas from polymers to biological systems. •Introduces active fluids, with internal energy generation, and their rheology •Involves multidisciplinary tools, and brings together contributors from different backgrounds Rheology of Complex Fluids is a must-read for researchers and practicing engineers in the complex fluid industry. Selected portions of the book can be used as supplementary teaching material. © Springer Science+Business Media, LLC 2010.

A model consisting of short polymer segments, capable of self assembling to form a branched network through the interaction of their ends, is considered. We investigate the mechanistic origins of a shear induced isotropic-lamellar- columnar transition, reported earlier in this model for living polymers. We find that these structures arise only in the presence of a potential barrier in the end point interaction between polymer segments. To elucidate the role played by long range solvent mediated forces, we have carried out simulations with and without the hydrodynamic interactions between the polymers and have seen that these structures are present in both these cases. We also show that the shear induced layering in these systems is a bulk phenomena and boundary interactions have little effect on the transition. The monomer volume fraction and viscosity of the background solvent are shown to have significant effects on determining the morphology of the flow-induced structures. We demonstrate that the structures obtained are indeed non-equilibrium states induced by the shear. © 2013 The Royal Society of Chemistry.

The mechanism of the lower critical solution temperature (LCST) in thermoresponsive polymer solutions has been studied by means of a coarse-grained single polymer chain simulation and a theoretical approach. The simulation model includes solvent explicitly and thus accounts for solvent interactions and entropy directly. The theoretical model consists of a single chain polymer in an implicit solvent where the effect of solvent is included through the intrapolymer solvophobic potential proposed by Kolomeisky and Widom. The results of this study indicate that the LCST behavior is determined by the competition between the mean energy difference between the bulk and bound solvent, and the entropy loss due to the bound solvent. At low temperatures, solvent molecules are bound to the polymer and the solvophobicity of the polymer is screened, resulting in a coiled state. At high temperatures the entropy loss due to bound solvent offsets the energy gain due to binding which causes the solvent molecules to unbind, leading to the collapse of the polymer chain to a globular state. Furthermore, the coarse-grained nature of these models indicates that mean interaction energies are sufficient to explain LCST in comparison to specific solvent structural arrangements. (Figure presented.).

For mesoscale structural studies of polymers, obtaining maximum level of coarse-graining that maintains the chemical specificity is highly desirable. Here we present a systematic coarse-graining study of sulfonated poly(ether ether ketone), sPEEK, and show that a 71:3 coarse-grained (CG) mapping is the maximum possible map within a CG bead-spring model. We perform single chain atomistic simulation on the system to collect various structural distributions, against which the CG potentials are optimized using iterative Boltzmann inversion technique. The potentials thus extracted are shown to reproduce the target distributions for larger single chains as well as for multiple chains. The structure at the atomistic level is shown to be preserved when we back-map the CG system to re-introduce the atomistic details. By using the same CG mapping for another repeat unit sequence of sPEEK, we show that the nature of the effective interaction at the CG level depends strongly on the polymer sequence and cannot be assumed based on the nature of the corresponding atomistic unit. These CG potentials will be the key to future mesoscopic simulations to study the structure of sPEEK based polymer electrolyte membranes. (Figure presented.) .

We present a coarse-grained model for the dynamics of living polymer solutions which explicitly includes solvent hydrodynamics. We show that lamellar and columnar structures emerge when the solution is subjected to simple shear. In the absence of shear, the model predicts a fluid-gel transition as a function of polymer concentration. At a threshold concentration, the self-intermediate scattering function indicates Zimm-like dynamics at large wave vectors and diffusive dynamics at small wave vectors. The kinetics of scission and recombination clearly demonstrates the existence of mean field and diffusion controlled regimes for the dynamics of living polymers at low and high concentrations respectively. © 2010 The Royal Society of Chemistry.

We study the phenomena of decrease in lower critical solution temperature (LCST) with addition of kosmotropic (order-making) cosolvents in thermoresponsive polymer solutions. A combination of explicit solvent coarse-grained simulations and mean-field theory has been employed. The polymer-solvent LCST behavior in the theoretical models has been incorporated through the Kolomeisky-Widom solvophobic potential. Our results illustrate how the decrease in the LCST can be achieved by the reduction in the bulk solvent energy with the addition of cosolvent. It is shown that this effect of cosolvent is weaker with an increase in polymer hydrophilicity which can explain the absence of a LCST decrease in poly(N,N-diethylacrylamide), water, and methanol systems. The coarse-grained nature of the models indicates that a mean energetic representation of the system is sufficient to understand the phenomena of LCST decrease.

The extent of phase separation and water percolation in sulfonated membranes are the key to their performance in fuel cells. Toward this, the effect of hydration on the morphology and transport characteristics of sulfonated poly(ether ether ketone), sPEEK, membrane is investigated using atomistic molecular dynamics simulation at various hydration levels(λ: number of water molecules per sulfonate group). The evolution of local morphology is investigated using structural correlations and minimum pair distances. Transport properties are probed using mean squared displacements and diffusion coefficients. The water-sulfonate interaction in sPEEK is found to be stronger than that in Nafion, as observed in experiments. Analyses indicate the presence of narrow connected path of water and hydronium at λ = 4 and large domains, spanning half the simulation box, at λ = 15. The behavior of membrane water remains far from bulk as indicated by its diffusion coefficient. The persistence of small isolated water clusters demonstrates the extent of phase separation in sPEEK to be lesser than that in Nafion. Analyses at molecular and collective levels suggest the occurrence of a percolation transition between λ = 8 and 10, which leads to a connected network of water channels in the membrane, thereby boosting the hydronium mobility.

Sulfonated polyether ether ketone (SPEEK), an ionic polymer, has been shown to be a potential candidate for fuel cell electrolyte as a proton exchange membrane. Rheological behavior of SPEEK solutions is of great interest to understand the molecular associations as well as due to implications in membrane processing. In this work, SPEEK of various degrees of sulfonation (58-80) was prepared and rheology of concentrated solutions of SPEEK was studied. The rheological properties were evaluated using steady and oscillatory shear. It was found that steady shear viscosity and storage modulus at any given concentration, is the highest for the lowest degree of sulfonation SPEEK solutions in N-methyl-2-pyrrolidone. The low frequency plateau in storage modulus was observed at some combinations of degrees of sulfonation and concentrations, indicating gel-like behavior in these SPEEK solutions. No significant change in rheological behavior was observed with different polar solvents. Increase of several orders of magnitude in viscosity, storage and loss moduli were observed with increasing concentrations. The role of hydrophobic aggregation and inter-chain associations in determining rheology of SPEEK solutions is argued based on comparisons with other material systems. The rheological behavior of SPEEK solutions with 70 as the degree of sulfonation, suggests crossover from hydrophobic-hydrophilic balance. © 2013 Wiley Periodicals, Inc.