Morphogenesis and Shear Flow Dynamics of Copolymer Vesicles with Dr. Radhakrishna Sureshkumar

Understanding the morphogenesis of tubular, lamellar and vesicular assemblies of amphiphilic copolymers in aqueous solutions is of fundamental and practical interest. For instance, functionalized copolymer vesicles have been successfully employed in medical diagnostics, therapeutics and chemical synthesis. Molecular dynamics simulations are employed to probe the thermodynamic motifs underlying the evolution of vesicular morphologies in initially homogeneous solutions of diblock and triblock copolymers. Size, shape, aggregation number, body curvature, surface area of the polymer-solvent interface and configurational entropy of the intermediate structures are tracked along the vesiculation pathway. For diblocks, two mechanisms of vesicle formation are observed. One of these mechanisms is facilitated by the following pathway: formation of spherical micelles and their merger to form rodlike micelles, flattening of such cylindrical structures to form rectangular lamellae (flat bilayers) which further reorganize into disk-shaped lamellae, and bending/curving of disks into cavities which close to form vesicles. Changes in information entropy, quantified  by the expectation of the logarithm of the probability distribution function of segmental stretch, are statistically insignificant along the transition pathway. The process is facilitated by a partial exchange between the energies associated with hydrophobic interactions and curvature. In another mechanism, vesicle formation is enabled by solvent diffusion into spherical micelles, through an ostensibly activated process.  Selection of the vesiculation mechanism depends on polymer concentration and chain flexibility. In triblock solutions, self-assembly of hairpin-shaped copolymers leads first to the formation of spherical aggregates that further organize into polygonal topologies with perforated sides. Such polyhedral shapes reorganize at nearly constant aggregation number to form unilamellar vesicles. The surface area of the polymer-solvent interface is seen to decrease exponentially with time during the vesiculation process. The influence of mechanical forces caused by shear flow deformation on equilibrium morphologies will also be discussed in the presentation.