Time: 4:15 p.m.
Place: Kolloquium der Materialwissenschaft und Werkstofftechnik, University of Saarbrücken, Germany
In this seminar, I will describe the application of Molecular Dynamics (MD) simulations for understanding the failure behavior of glassy polymers in bulk as well as at rough substrate surface interface. First, I will discuss the mechanisms responsible for craze initiation in bulk polymers. I will then explain our recent work on surface roughness effects on the failure behavior of polymers at interface.
Plastic yielding of bulk glassy polymers leads to extreme localized deformation in the form of constant volume shear bands or zones of nano-scale fibrillation (crazing) triggered by cavitation. At macroscopic scale, both these form of localizations are controlled by the prevailing hydrostatic stresses and shear stresses. However, in particular, the mechanisms leading to initiation of crazing type failure in a glassy polymer are not clearly understood. This is mainly due to the difficulty in characterizing the stress state and polymer configuration sufficiently locally at the craze initiation site. Using MD simulations, we have now been able to access this information and have shown that the local heterogeneous deformation leads to craze initiation in glassy polymers. We found that zones of high plastic activity are constrained by their neighborhood and become unstable, initiating crazing from these sites. Furthermore, based on the constant flow stresses observed in the unstable zones, we conclude that microcavitation is the essential local deformation mode to trigger crazing in glassy polymers . Failure of polymer-substrate coating system is controlled by interface properties. These interfaces are generally characterized by substrate surface roughness scale of few nanometers to several micrometers that leads to strengthening of polymer-substrate 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. Very little is known at present regarding the role of relative dimensions of polymer chains with respect to surface undulations in effecting the failure of polymers at interface. MD simulations are used to access this information and to understand the elementary mechanisms of adhesion. Coating systems are realized by confining the polymer between a planar substrate and a rough substrate of varying roughness scale. To quantify the role of roughness, the undulation features are varied in comparison to the average radius of gyration (Rg) 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 Rg. 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.
 D. K. Mahajan and A. Hartmaier. Mechanisms of crazing in glassy polymers revealed by molecular dynamics simulations, Phys. Rev. E, 86:021802, 2012.
 D. K. Mahajan, F. Varnik, and A. Hartmaier. Roughness effects on the failure mode of glassy polymers at solid interfaces (in preparation).