What controls failure in ultrathin glassy polymer films?

As polymer glasses are processed into ultrathin films, neighboring polymer chains become less entangled, and surface-bound chains with altered states of mobility play an increasingly important role. Such changes in physical properties have long been studied, but changes in mechanical strength and deformation processes have remained difficult to quantify. We have developed a method to directly measure the uniaxial stress-strain response of ultrathin glassy polymer films. Here, we quantify the influence of thickness (10nm-360nm), and molecular weight across a broad range (61kDa-2135kDa) on the deformation and failure response of ultrathin polystyrene films. We observe a molecular weight independent thickness-transition in strain localization and a molecular weight dependent decrease in maximum stress. We develop a model that provides new fundamental insights into how polymer behavior is altered due to changes in the entanglements and mobility in a polymer network upon dimensional confinement.