Polymer Engineering Faculty Research


Influence of Molecular Mass on the Modulus of Thin Polystyrene Films

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Publication Date

Fall 2010


In this work, we examine the influence of relative molecular mass (Mn, or the number average molecular mass) of PS on the thickness-dependent moduli using a wrinkling based instability of a stiff film on an elastic substrate. This wrinkling based metrology allows for the determination of elastic moduli for film thicknesses down to ≈5 nm. We have previously shown the modulus of PS being independent on molecular mass, for Mn>100 kg/mol. However, lower molecular masses have not been examined. It might be expected that confinement effects would be reduced for low molecular weight films (as Rg is reduced) if the intrinsic size of the polymer is partially responsible for the size dependent behavior. Conversely, molecular simulations suggest the critical length scale is proportional to quench depth (Tg-T) into the bulk glass . Quench depth is decreased for low molecular mass PS . We demonstrate that above a critical threshold of Mn>3.1 kg/mol the modulus remains statistically independent of molecular mass, while lower molecular mass PS films exhibit a decrease in modulus at a larger film thickness, which appears to scale with quench depth into the bulk glass.

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