Modeling Lipid-Block Copolymer Phase Separation and Mixing Behavior

Hybrid phospholipid block-copolymer membranes have unique material properties when compared to their constituent parts. This can be due to the (1) synergy of mechanically robust polymer domains and near-equilibrium lipid domains which can incorporate proteins and peptides or (2) the presence of interfaces. Using coarse-grained molecular dynamics, we have defined four morphologies of hybrid lipid--polymer membranes, controlled via the hydrophobic mismatch of polymer to lipid and the concentration of polymer: mixing, lateral phase separation, unzipping, and polymer rich membranes; each of which has been experimentally observed and has distinct properties. In this talk, we examine lipid--polymer mixing and lateral phase separation using coarse grained molecular dynamics, simulating mixtures of lipids (DPPC, DOPC, and Cholesterol + Lipid Mixtures) and PBD-b-PEO polymers of varying length. We show how the length of both the hydrophobic and hydrophilic blocks in the polymer determine the resulting phase-separated morphology, which is dictated by the confinement of the polymer.  We also show how the features of the lipid bilayer (fluidity, thickness, phase) affect the resulting morphology. The dynamics of phase separation are compared to nucleation and growth and spinodal decomposition. Lastly, we discuss methods for kinetically or thermodynamically stabilizing finite sized domains which are of importance for hybrid membrane applications.  

Through comparing the models for lipid-lipid phase separation to lipid--polymer membranes (and understanding where this analogy falters), we increase the ability to design the properties of novel synthetic systems.