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The objectives of this project are to develop models and simulation approaches for addressing some specific issues in the context of dynamics of multicomponent polymer mixtures. We proposed to use models and simulation approaches which focus on the problems from a multiphase flow perspective and our aim was to shed light on phenomena and issues which have caught the recent attention of experimentalists.

Progress to Date: Broadly speaking, our work has evolved in two complementary paths which contribute towards the overall objectives above. These complementary directions relate to: (i) Equilibrium aspects; and (ii) Dynamical aspects of multicomponent polymers. The need to undertake such an approach has arisen from the fact that complex fluids such as multicomponent polymer solutions and melts typically exhibit novel rheological and flow behavior which often couple with their structural characteristics leading to intriguing morphological transformations. Hence understanding the mechanistic origins of flow behavior of multicomponent polymers requires a strong foundation for predicting their equilibrium structural characteristics as well as the response of the structure to applied flow fields. Our progress in these two areas is briefly described below:

Equilibrium Aspects: Our pursuits in this direction have focused on developing numerical methodologies which can accurately predict the equilibrium structure of multicomponent polymer solutions in a variety of situations. One such study considered the structure in semicrystalline multiblock copolymers. We modeled the non-crystalline block (A) as a flexible Gaussian chain and the crystalline block (B) as a semiexible chain with a temperature dependent rigidity and interactions which favor the formation of parallel oriented bonds. Using an approach termed as the self-consistent field theory, we developed predictions for the structure and conformational characteristics of such copolymers. The results were again shown to be in very good agreement with experiments. Finally, in a recent study developed a mean-field theory for the structure of semiflexible polymer solutions near spherical surfaces, and used the framework to study the depletion characteristics of semiflexible polymers near colloids and nanoparticles. Our results suggested that the depletion characteristics depend sensitively on the polymer concentrations, the persistence lengths and the radius of the particles. In a recent work, we used the preceding results to quantify the influence of filler-induced polymer matrix perturbations upon the barrier properties of PNCs. As a first step, we used a coarse-grained model for the polymer melt (devoid of chemical details) to deduce the packing features of the polymers around a single spherical particle. Subsequently, we then generated a random configuration of such particles and superposed these density characteristics using a simple model to obtain a representative density map of the polymers in the multiparticle system. Subsequently, this density map was transformed to a local diffusivity map  using free volume theories, which was subsequently used within a kinetic monte carlo scheme to obtain the effective diffusivities for penetrants. Our results mirrored some novel trends noted in experimental studies on permeability of penetrants.

Dynamical Aspects: In this direction, we have developed models and a new simulation approach which can predict the response of a multicomponent polymer system consisting of a mixture of flexible polymers + rod-like polymers (as models of nanofillers) + solvent, under the combined actions of flow, electric and magnetic fields. We have used these models to provide answers to the following questions: “How can one couple the external fields such as shear, electric and/or magnetic fields along with chemical influences such as surface modification, to aid in the dispersion of rod-like polymers ?” “What are the electric/magnetic fields (frequency, amplitude) and shear rate dependent mechanical and rheological properties of such blends of polymers?” We have developed a mesoscopic simulation method capable of addressing the dynamics of rod-like fillers in simple and  polymeric matrices. We  applied the above simulation methodology to study the combined effect of E-field and flow on dispersion and alignment of nanotubes. Efficient strategies for dispersion of carbon nanotubes in polymeric and solvent matrices constitutes an area of active interest. In a recent work, we examined from a theoretical perspective the hypothesis that a combination of AC electric and shear fields oriented at an angle may be used to enhance the dispersion of aggregated rod solutions. We presented a deterministic and a Smoluchowski equation based analysis of the dynamics of homogeneous rod suspensions in a configuration involving combined electric and shear fields. We used these analyses to suggest that a cross-field shear and electric field configuration may potentially disperse aggregated nanotube suspensions. We tested the predictions of our analytical models through Brownian dynamics simulations to analyze the dynamics of rod suspensions in a cross-field configuration. The results of our simulations displayed good agreement with our analytical results and served to delineate the parametric regimes in which the use of a combination of electric and shear fields may enhance the dispersion of aggregated  nanotubes.