Structural Characteristics and Fusion Pathways of Onion-Like Multilayered Polymersome Formed by Amphiphilic Comb-Like Graft Copolymers

The self-assembly behavior of amphiphilic comb-like graft copolymers bearing pH-responsive hydrophilic side-chains on hydrophobic backbone in a selective solvent is investigated by dissipative particle dynamics. The aggregate exhibits a rich variety of the morphological conformations dependent on pH, polymeric concentration, side-chain length, and grafting density. The morphological phase diagram shows that micellar aggregates take shape at low pH. As pH increases, unilamellar vesicle (ULV) can form at low polymer concentration and the vesicle size grows with increasing concentration. Further increment of pH to neutral leads to the formation of multilamellar vesicles (MLV) with the layer-by-layer structure similar to that of an onion. The total number of layers rises with increasing polymer concentration. Our simulation outcomes are consistent with the experimental observations. The simulation results also reveal that as the grafting density is decreased, the thickness of the hydrophobic layer grows and thus the total number of layers declines for MLVs. The water permeation process through ULV is monitored to explore the effects of grafting density and pH on the membrane permeability. As the grafting density is decreased or the neutral pH is approached, the permeability of the vesicle membrane descends. Controlled releases of two types of drugs with different hydrophobicity situated at different layers of MLV are also examined. The release rate of hydrophilic drug is faster than that of hydrophobic drug. Polymersomes tend to fuse but the fusion mechanism is different from that of liposomes. The fusion pathway of ULVs follows the anisotropic stalk-pore scenario and both stalk and pore formation prefer to occur on the edge of the contact zone due to significant deformations of comb-like polymers in this region. The fusion pathway of MLVs is layer-by-layer and the fusion time of the inner layer is faster than that of the outer layer. The fusion process is expedited at about neutral pH but is prolonged substantially as the side-chain length is increased. Finally, it is interesting to observe that the total number of layers of the fused MLV can be greater than that of MLVs before fusion.