Bekele Gurmessa

Onset of Plasticity in Thin Polystyrene Films

Polymer glasses have numerous advantageous mechanical properties in comparison to other materials. One of the most useful is the high degree of toughness that can be achieved due to significant yield occurring in the material. Remarkably, the onset of plasticity in polymeric materials is very poorly quantified, despite its importance as the ultimate limit of purely elastic behavior. Here, we report the results of a novel experiment which is extremely sensitive to the onset of yield and discuss its impact on measurement and elastic theory. In particular, we use an elastic instability to locally bend and impart a local tensile stress in a thin, glassy polystyrene film, and directly measure the resulting residual stress caused by the bending. We show that plastic failure is initiated at extremely low strains, of the order 10(-3) for polystyrene. Not only is this critical strain found to be small in comparison to bulk measurement, we show that it is influenced by thin film confinement--leading to an increase in the critical strain for plastic failure as film thickness approaches zero.

Influence of Thin Film Confinement on Surface Plasticity in Polystyrene and Poly(2-vinylpyridine) Homopolymer and Block Copolymer Films

Delamination is used to impart a quantifiable local stain on thin films of homopolymer polystyrene and poly(2-vinylpyridine), as well as block copolymers made of styrene and 2-vinylpyridine. Direct observation of the damage caused by bending with atomic force microscopy and laser scanning confocal microscopy leads to the identification of a critical strain for the onset of plasticity. Moving beyond our initial scaling analysis, delamination shapes are fit to two model functions and a more precise value for curvature is used in the calculation of surface strain. The analysis presented here shows strain levels in thick films to be comparable to bulk measurements. Monitoring the critical strain leads to several observations: (1) as-cast PS-P2VP has a low critical strain, (2) annealing slowly increases critical strain as microstructural ordering takes place, and (3) similar to the homopolymer, both as-cast and ordered films both show increasing critical strain under thin-film confinement.

Experimental evidence and structural mechanics analysis of force chain buckling at the microscale in a 2D polymeric granular layer

The force chain concept underlies a significant fraction of our current understanding of the mechanics and stability of granular solids. One of the more interesting physical questions that remain unanswered relates to how a granular solid traverses the transition in state to that of a granular fluid. One possibility is that the transition is initiated by the buckling of loaded columns of particles. Here we explore the physics of force chain buckling through a combination of experiment and theory, for the first time explicitly verifying the phenomena. In our idealized experiments, a monolayer of micron scale particles is adhered to a soft elastomer foundation and a uniaxial stress is applied to the system. Above a critical threshold stress, the particles buckle and form a dramatic sinusoidal topography reminiscent of a common elastic instability involving a continuum plate (wrinkling). We use Laser Scanning Confocal Microscopy (LSCM) to make a complete observation of the buckled film in three dimensions, which allows us to show the subtle differences between a continuum and granular film. We go on by comparing measurements to recent theoretical predictions and discuss some simple scaling relations relating the emergent sinusoidal structure to the details of the underlying particles.