Cohesion Mechanisms of Polystyrene-Based Thin Polymer Films

The cohesion mechanisms of end-functionalized high molar mass polystyrene with very low polydispersity (PS, M_n = 1.26 × 10⁶ g mol^–1, PDI = 1.06) and polylysine (PLL, 150–130 × 10³ g mol^–1) on silicon (Si) supported thin PS films are investigated by desorbing single polymers covalently bound to an atomic force microscope (AFM) cantilever tip. The influence of film preparation conditions and film architecture on polymer cohesion mechanisms is probed by comparing spin-coated PS films (scPS) with a thickness range of 6–52 nm and covalently surface-attached PS films (saPS) with a thickness of 15–83 nm. Annealed scPS prevents cohesion of further PS polymers unless the scPS partly dewets. In all other cases, two different cohesion mechanisms are observed: first, a previously described equilibrium desorption similar to hydrophobic solid substrate desorption, represented by a plateau of constant force in the force–extension curve, and second, a nonequilibrium mechanism with nonlinear force–extension behavior. The second requires a geometrical interlock between the tip bound single molecule and the PS film. Remarkably, this mechanism is observed below the glass transition temperature of PS films and is promoted by good solvent conditions. These findings contrast many bulk measurements assuming a glassy state of the complete polymer film, but they are consistent with fluid like boundary layers having a high mobility. Our results further underline the decisive influence of polymer film conformation and mobility close to its solvent exposed boundary layer for the cohesion of polymer coatings.