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Abstract

Cohesively bonded polymer-solid interfaces play an important role in coatings and composite materials, applications of which include renewable energy production structures, sandwich panels used in automobiles and food packaging. These interfaces are generally characterized by substrate roughness scale of few nanometers to several micrometers that leads to strengthening of polymer-solid bonding either by increase in the effective contact area (quantified as roughness factor: RF = Area_rough/Area_planar) or by mechanical interlocking of polymer between surface undulations. While polymer conformation is essentially independent in the case of micrometer sized roughness patterns [1], it is expected to be strongly influenced in the nanometer limit, when the roughness scale becomes comparable to the characteristic dimension of polymer (determined via, e.g. radius of gyration, R_g). Very little is known at present regarding the role of relative dimensions of polymer chains with respect to surface undulations in effecting the static, dynamic and mechanical properties of the polymers at interface. Such information can be used in substrate surface engineering for improving bonding without changing the interface chemistry. To understand the elementary mechanisms of adhesion, we have performed molecular dynamics (MD) simulations to access this information. Coating systems are realized by confining the polymer between a planar substrate and a rough substrate of varying roughness scale. Periodic boundary conditions are used along directions parallel to the substrate. To quantify the role of roughness, the undulation features are varied in comparison to the average R_g) of the polymer. The coating systems are subjected to different loading modes while monitoring their stress-strain behavior and the work of separation. As obtained by static and dynamic properties of the polymer close to the interface, we find that confinement is caused by roughness features with dimensions of the order of R_g. At these dimensions, mechanical interlocking appears to play a role in improving polymer bonding in place of increase in the effective contact area. Furthermore, confinement also shows the ability to switch the mode of failure from adhesive to cohesive type [2]. References [1] Redmer van Tijum. Interface and surface roughness of polymer-metal laminates. PhD thesis, Rijks Universiteit Groningen, 2006. [2] D. K. Mahajan, F. Varnik, and A. Hartmaier. Roughness effects on the failure mode of glassy polymers at solid interfaces (in preparation).