Entanglement Effect on Mechanical Properties in Ultra-thin Glassy Polymer Films

Presenter: 

Cynthia Bukowski

Authors: 

Cynthia Bukowski, R. Konane Bay, Alfred J. Crosby

Author Affiliation: 

Polymer Science and Engineering, University of Massachusetts Amherst

Abstract: 

Entanglement density can influence the large strain and failure responses of ultra-thin glassy polymer films. For model films of high molecular weight polystyrene (PS), recent results show severe embrittlement 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 in dimensionally-confined geometries. Introducing polymer chains shorter than the entanglement molecular weight can also lower the entanglement density, effectively swelling the entanglement network. Here, we blend short (10 kDa) and long (151.5 kDa) PS chains and measure the changes in mechanical properties using the recently developed TUTTUT (The Uniaxial Tensile Tester for Ultra-Thin films). We measure the complete uniaxial stress-strain response of 100 nm PS films as a function of blend concentration. We observe a decrease in yield stress and strain with increasing diluent concentration. Above a critical diluent concentration, the entanglement density reaches a limit where films are too brittle to manipulate. These results establish the framework of how entanglement density affects mechanical properties of ultra-thin polymer films. NSF DMR-1608614