Comparison of Simulations of Lipid Membranes with Membranes of Block Copolymers

An overview of molecular models and computer simulation techniques for amphiphilic vesicles formed either by lipid or block copolymer molecules is presented. First, system-specific, atomistic or coarse-grained representations of amphiphilic vesicles, which account for the detailed, chemical structure of the system, are briefly considered. The common features of collective phenomena on the mesoscale (e.g., the self-assembly into vesicles, their rupture or fusion), observed in a broad class of amphiphilic systems, suggest a universal underlying mechanism. This observation forms the basis for modeling large-scale properties of amphiphilic vesicles by minimal models. These coarse-grained models describe the underlying atomistic structure through a few relevant interactions, whose strength is characterized by coarse-grained parameters. The discussion of these coarse-grained models particularly focuses on how their parameterization can be related to the material properties of specific systems. In this context, the concept of combining density functional representations of amphiphilic systems with particle-based simulation techniques is introduced. As an illustration, a solvent-free model based on a virial expansion functional is elaborated and applied to investigate the behavior of polymersomes loaded with long homopolymers. Although the interactions are cast in a density functional language, the model is a particle-based one and its equilibrium properties are obtained from a straightforward Monte-Carlo scheme. The mechanical properties of the vesicles are established and compared to the properties of a planar bilayer. Selected results concerning the effect of loading on vesicle stability and mechanical properties of its bilayer shell are presented.