Self-assembly and properties of diblock copolymers by coarse-grain molecular dynamics

Vesicles are microscopic sacs that enclose a volume with a molecularly thin membrane. The membranes are generally self-directed assemblies of amphiphilic molecules with a dual hydrophilic-hydrophobic character. Biological amphiphiles form vesicles central to cell function and are principally lipids of molecular weight less than 1 kilodalton. Block copolymers that mimic lipid amphiphilicity can also self-assemble into vesicles in dilute solution, but polymer molecular weights can be orders of magnitude greater than those of lipids. Structural features of vesicles, as well as properties including stability, fluidity, and intermembrane dynamics, are greatly influenced by characteristics of the polymers. Future applications of polymer vesicles will rely on exploiting unique property-performance relations, but results to date already underscore the fact that biologically derived vesicles are but a small subset of what is physically and chemically possible.

Vesicles were made from amphiphilic diblock copolymers and characterized by micromanipulation. The average molecular weight of the specific polymer studied, polyethyleneoxide-polyethylethylene (EO40-EE37), is several times greater than that of typical phospholipids in natural membranes. Both the membrane bending and area expansion moduli of electroformed polymersomes (polymer-based liposomes) fell within the range of lipid membrane measurements, but the giant polymersomes proved to be almost an order of magnitude tougher and sustained far greater areal strain before rupture. The polymersome membrane was also at least 10 times less permeable to water than common phospholipid bilayers. The results suggest a new class of synthetic thin-shelled capsules based on block copolymer chemistry.

The observation by transmission electron microscopy of six different stable aggregate morphologies is reported for the same family of highly asymmetric polystyrene-poly-(acrylic acid) block copolymers prepared in a low molecular weight solvent system. Four of the morphologies consist of spheres, rods, lamellae, and vesicles in aqueous solution, whereas the fifth consists of simple reverse micelle-like aggregates. The sixth consists of up to micrometer-size spheres in aqueous solution that have hydrophilic surfaces and are filled with the reverse micelle-like aggregates. In addition, a needle-like solid, which is highly birefringent, is obtained on drying of aqueous solutions of the spherical micelles. This range of morphologies is believed to be unprecedented for a block copolymer system.