46531-AC7
Dynamics of Ternary Polymer Blends
Broad Objectives of this Project: 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 phase behavior of a class of multicomponent polymers termed the rod-coil block copolymer. In this context, we developed a theory for the structure of a ternary blend of a mixture of rod-coil copolymer+rod-homopolymers+coil homopolymers, and deduced the interfacial segregation one may expect in such systems. These theories were directly applied to experiments and allowed for the first time a measurement of the Flory-Huggins parameter for the rod and coil blocks. In another study, we developed a new theoretical description of 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 suggest that the depletion characteristics depend sensitively on the polymer concentrations, the persistence lengths and the radius of the particles. Broadly, two categories of features were identified based on the relative ratios of the persistence lengths to the correlation length of the polymer solution. For the limit where the correlation length is larger than the persistence length, the correlation length proves to be the critical length scale governing both the depletion thickness and the curvature effects. In contrast, for the opposite limit, the depletion thickness and the curvature effects are dependent on a length scale determined by an interplay between the persistence length and the correlation length.
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?” To address these issues, we have developed a Brownian dynamics simulation approach which can predict the flow-induced dynamics of the different components in the system, while being able to accommodate the possibility of an applied electric/magnetic field oriented at an angle to the flow direction. The results of the simulations have been complemented by an analytical theory which considers the Smoluchowski equation for the dynamics of the rodlike polymer and its coupling to the shear and electric fields. The results of these combined studies suggest that appropriate combination of shear and electric fields provide an attractive strategy for achieving well-dispersed nanorods in polymeric matrices. Indeed, while application of just either shear or electric fields just orient the aggregates, we show that a combination of these fields in a cross-field manner can serve the dual purpose of well-dispersed nanorods. We have characterized this behavior quantitatively for different flow and electric field effects, and have developed a “pseudo” phase diagram characterizing this novel strategy.
