How Do Ultra-thin Glassy Polymer Films Fail?

The physical properties of polymers confined to thin films can be vastly different from their bulk behavior. For confined polystyrene (PS) films, previous work has found a reduction in interchain entanglement density and an increase in average molecular mobility. In our work, we determine how these two effects influence the failure mechanism of PS films in the ultra-thin regime (15 nm – 60 nm) as a function of molecular weight (MW= 132 kDa, 345 kDa, 592 kDa). We directly measure the complete uniaxial stress-strain relationship by holding a film between a flexible cantilever and a movable rigid boundary, measuring force-displacement from the cantilever deflection. In addition, we use dark-field optical microscopy to observe the failure mechanisms in-situ and transmission electron microscopy to examine the strain localization morphology. We observe a dramatic decrease in yield stress and modulus for films less than 30 nm in thickness. In addition, most surprisingly, we find a sharp transition in observed strain localization mode for films with thickness changing from 30 nm to 20 nm. We present these results and our hypothesis for how average molecular mobility trades off with the decrease in entanglement density to cause these transitions.