Structural and mechanical properties of polymersomes formed by rod–coil diblock copolymers

Abstract

Polymersomes formed by rod–coil diblock copolymers (R_xC_y) are fundamentally different from those of coil–coil diblock copolymers. R_xC_y denotes the polymer comprising of x rod-like beads and y coil-like beads. The morphological phase diagram of R_xC_y in selective solvents and the essential physical properties of the R_xC_y-polymersomes are explored by using dissipative particle dynamics. Our simulation results show that small-sized polymersomes can only take shape for short coil-block lengths. Moreover, the rod-block length cannot be too long and π–π stacking must be weak because anisotropic rod packing actually resists membrane bending and thus vesicle formation. The detailed membrane structures of polymersomes are also investigated and it is found that the rods within the membrane are highly interdigitated which is intrinsically different from the ordered bilayer of the liposomes formed by triblock copolymers (R_x)₂C_y. The structural and mechanical properties of R_xC_y-polymersomes are studied as well. As the coil-block length is increased, both the thickness and area density of the rod domain decline. Since the coil stretching in the inner corona is more distinct than that in the outer one, significant interdigitation results due to the outward movement of the inner leaflet for relaxing overcrowded coil-blocks. The membrane tension exhibits a maximum while the stretching and bending moduli display a minimum at the intermediate coil-block length as y varies from 1 to 3. Fusion between two polymersomes of R₅C_y is investigated and the outcome relates to the membrane tension. R₅C₂-polymersomes fuse most easily. However, R₅C₁-polymersomes do not proceed beyond the hemifusion stage and R₅C₃-polymersomes can not even move past the initial kissing stage.