Polymer/metal interface structure and adhesion
Cohesively bonded polymer-solid interfaces are essentially required for all kinds of applications such as protective coatings and metal-polymer compound systems. A feature that characterizes these interfaces is the substrate surface roughness. Typical rough substrates have surface undulations ranging from few nanometers to several micrometers. While polymer conformation is essentially independent of the details of the corrugation in the case of micrometer sized roughness patterns, it may be strongly influenced in the nanometer limit, where the roughness scale becomes comparable to the characteristic dimension of polymer (determined via, e.g. radius of gyration, Rg). It is known that the surface roughness strengthens the polymer-solid bonding either by increase in the effective contact area (quantified as roughness factor, RF=Arearough/Areaplanar) or by mechanical interlocking of polymer between surface undulations.
However, little is known regarding the role of relative dimensions of polymer chains with respect to surface undulations in effecting the polymer bonding. Such information can be used in substrate surface engineering for improving bonding without changing the interface chemistry. To this end, we have performed extensive molecular dynamics (MD) simulations to access this information. A coating system is obtained by bonding the coarse-grained polymer molecules with the solid substrate via a Lennard-Jones potential. A planar substrate and several rough substrates with periodic surface undulation are used to quantify the role of roughness. For different rough substrate samples, surface undulation features are varied in comparison to the average Rg of the polymer. The coating systems are subjected to different loading modes while monitoring their stress-strain behavior and the work of separation. We find that the roughness features with dimensions of the order of Rg cause confinement of polymer. At these dimensions, mechanical interlocking in place of increase in effective contact area appears to play an important role in improving polymer bonding. The increasing level of confinement with roughness also shows the ability to switch the failure mode of coating from adhesive to cohesive.