Molecular Organization and Dynamics in Polymersome Membranes: A Lateral Diffusion Study

Amphiphilic block copolymers self-assemble into artificial membranes of enhanced strength and stability compared to lipid membranes and are still able to incorporate biological membrane proteins. Membrane fluidity is a key parameter for retaining function of incorporated proteins. In this study, lateral diffusion properties of membranes of diblock and triblock copolymers based on poly(2-methyl-2-oxazoline) and poly(dimethylsiloxane), with thicknesses between 6 and 21 nm, were systematically investigated. Z-scan fluorescence correlation spectroscopy was used to obtain highly accurate diffusion coefficients. The lateral diffusion coefficients (D) scale with the molecular weight of the hydrophobic block (M-h) for both diblock and triblock configurations as D proportional to M-h(1.25). A significant diffusion increase of diblocks compared to triblocks revealed that diffusion is primarily related to the different structural conformation of the macromolecules assembled in the membrane. Moreover, hindered diffusion for higher molecular weight copolymers was observed, indicating formation of domains due to interdigitation and entanglement, whereas free 2-D diffusion was detected for low molecular weight copolymers. These results represent a further step to understand structure-related membrane properties, i.e., density, stability, fluidity, permeability, etc. Additionally, the tracking of labeled membrane constituents embedded in artificial membranes offers crucial information about the desired functionality of bio-inspired supramolecular 3-D nanoassemblies.