Confinement Effect on Strain Localizations in Glassy Polymer Films

R. Konane Bay, Shinichiro Shimomura, Yujie Liu, Mark Ilton, and Alfred J. Crosby

Macromolecules 51 (2018) 3647-3653


The physical properties of glassy polymer films change as they become confined. These changes are often attributed to increased average molecular mobility and reduction in entanglement density. Both are known to alter mechanical behavior, including the formation of strain localizations, e.g., crazing and shear deformation zones. Here, we determine how the entanglement density and surface mobility change the mechanical behavior of a glassy polymer film when it becomes confined. We utilize a custom-built uniaxial tensile tester for ultrathin films and dark-field optical microscopy to characterize the complete stress strain response and the associated strain localizations for ultrathin polystyrene films of varying thickness (h(F) = 20-360 nm). These experiments provide direct measurement of the stress in a craze as well as the stresses involved through the transition from crazing to shear deformation zones. Most significantly, we observe a transition in strain localization from crazing to shear deformation zones as film thickness changes from 30 to 20 nm, providing new insights into how the surfaces alter the mechanical behavior in confined polymer films.