Modeling surface segregation of smart PU coatings at hydrophilic and hydrophobic interfaces via coarse-grained molecular dynamics and mesoscopic simulations

Developing adaptive coatings having desired functionalities at targeted interfaces is one of the major efforts in the coatings science area. The adaptation of the surface functionality to the changing surface conditions can be maintained by introducing dangling chains with different properties to the cross-linked polymer coatings. In this work, we strive to investigate the change in interfacial morphology of PU coatings as exposed to hydrophilic (HPI) and hydrophobic (HPB) interfaces by employing molecular simulations at the coarse-grained and mesoscopic levels. The molecular structure and surface segregation dynamics are studied for PU coatings having pure HPI, mixture of HPI and HPB, and amphiphilic dangling chains. The dual-scale simulations, Dissipative Particle Dynamics (DPD) and MARTINI model, yield results about the dangling chain structures at the interface in terms of their end-to-end distances, where HPI chains adopt a more extended conformation in water in comparison to oil interfaces. The reverse is observed to be valid for the HPB chains. Regarding the dangling chain dynamics, a swift migration towards the interfaces is noticed at about 10 ns for both of the simulation methods. The structures of the dangling chains and their interaction with the interfaces are also characterized by computing the radial distribution function (RDF) profiles. Preferential interactions between the HPI/water and HPB/oil are clearly noted. The switchability of the surfaces is also studied by simulating the system in cycles, such that the interface is changed from water to oil and back to water. The migration of HPI groups in the dangling chains towards water and vice versa in each cycle is clearly shown by the simulations. In all, the inherent structure and dynamics of the dangling chains is obtained at the molecular level by the dual-scale molecular simulations. Our findings reveal a significant level of understanding about interfacial morphology of thermoset coatings modified by dangling chains, where the results can be extended to find applications in guiding the experimental studies.