Decoupling the role of entanglements and mobility in the mechanics of ultrathin polymer glasses

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 of both liquid supported and freestanding films. In our work, we quantify the influence of thickness (10nm - 360nm), and molecular weight (61kDa - 2135kDa) on the deformation and failure response of ultrathin polystyrene films. We observe a molecular weight independent thickness-transition in strain localization and elastic modulus, and a molecular weight dependent decrease in maximum stress. We associate the changes in strain localization and elastic modulus to the surface mobile layer, and we form a model to capture the role of the average molecule size and the number of entanglements per chain on the decrease in maximum stress. These results provide new fundamental insights into how polymer behavior is altered due to changes in the entanglements and mobility in a polymer network upon confinement.