Entanglement Effect on Mechanical Properties of Ultrathin Glassy Polymer Films

Entanglement density is known to influence the large strain and failure responses of glassy polymer films. For ultrathin films, recent extension measurements show that failure strength decreases severely as film thickness decreases below the average size of an unconfined chain. The hypothesized cause for this effect is the loss of interchain entanglements as polymers statistically interact with themselves more than neighbors when confined. To provide more insight into how entanglements control the failure mechanisms of glassy polymers, especially in ultrathin films, we introduce polymer chains shorter than the entanglement molecular weight to control the entanglement density by effectively swelling the entanglement network. Specifically, we blend short (10.5 kDa or 61.8 kDa) and long (151.5 kDa) polystyrene chains and measure the changes in mechanical properties using The Uniaxial Tensile Tester for UltraThin films (TUTTUT). For 100 nm films, we observe a decrease in yield stress and work to failure with increasing diluent concentration that is diluent length dependent. These results establish the framework of how entanglement density affects mechanical properties of ultrathin polymer films.