Decoupling the Impact of Entanglements and Mobility on the Failure Properties of Ultrathin Polymer Films

The mechanical properties of glassy polymer thin films change as film thickness decreases below the average polymer molecule size. These changes have been associated with a reduction in interchain entanglements due to confinement and an increase in molecular mobility from the mobile surface layer. Here, using experiments and simulations, we determine how entanglements and surface mobility each individually impact the failure behavior of a glassy polymer film as the film 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 thicknesses (h = 10 to 150 nm) for a range of molecular weights M_nof 61 to 2135 kDa. To directly correlate the changes in the molecular network to changes in the failure properties of ultrathin films, we perform nonequilibrium molecular dynamics simulations on N = 250, N = 60, with h = 10 to 30 films. From our results, accounting for both the changes in entanglements and mobility, we propose a semiempirical model that captures the failure response in both simulated and experimental glassy polymer thin films.