Molecular Insight into the Effect of Polymer Topology on Wettability of Block Copolymers: The Case of Amphiphilic Polyurethanes

The microstructure design of multiblock copolymers is essential for achieving desired interfacial properties in submerged applications. Two major design factors are the chemical composition and polymer topology. Despite a clear relationship between chemical composition and wetting, the effect of polymer topology (i.e., linear vs cross-linked polymers) is not very clear. Thus, in this study, we shed light on the molecular origins of polymer topology on the wetting behavior. To this end, we synthesized linear and three-dimensional (3D) cross-linked network topologies of poly(ethylene glycol) (PEG)-modified polycarbonate polyurethanes with the same amount of hydrophilic PEG groups on the surface (confirmed by X-ray photoelectron spectroscopy (XPS)) and studied the wetting mechanisms through water contact angle (WCA), atomic force microscopy (AFM), and molecular dynamics (MD) simulations. The linear topology exhibited superhydrophilic behavior, while the WCA of the cross-linked polymer was around 50 degrees. AFM analysis (performed on dry and wet samples) suggests that PEG migration toward the interface is the dominant factor. MD simulations confirm the AFM results and unravel the mechanisms: the higher flexibility of PEG in linear topology results in a greater PEG migration to the interface and formation of a thicker interfacial layer (i.e., twice as thick as the cross-linked polymers). Accordingly, water diffusion into the interfacial layer was greater in the case of the linear polymer, leading to better screening of the underneath hydrophobic (polycarbonate) segments.