R. Konane Bay
The physical properties of polymer thin films change as the polymer chains become confined. Similar changes in mechanical properties have been observed, though these critical properties have only been explored a limited extent and with indirect methods. Here, we use a recently developed method to measure the complete uniaxial stress strain relationship of polymer thin films of polystyrene films (PS, Mw=130kg/mol) as a function of thickness (20nm-300nm). In this method, we hold a dog-bone shaped film on water between a flexible cantilever and a movable rigid boundary, measuring force-displacement from the cantilever deflection. From our measurements, we find that the modulus decreases as the PS chains become confined. The PS thin films exhibit ideal perfectly plastic behavior due to crazing, which differs from the typical brittle response of bulk PS. The failure mechanism transitions from 3D crazes to 2D crazes to shear deformation zones with decreasing thickness. These results provide new fundamental insight into how polymer behavior is altered due to structural changes in the entangled polymer network upon confinement.