Schedule Aug 20, 2003
Spatio-temporal patterns in ultra-thin polymeric films
Arlette Baljon (San Diego State University)

Molecular dynamics simulations were used to study the response of a nanometer thin polymer film to oscillatory shear. At low frequencies smooth viscoelastic flow is observed. At high frequencies several types of response occur depending on the amplitude of the shear. At low amplitude, the film deforms elastically. At higher ones the stress induces microstructural rearrangement. Crystalline domains appear. This state is very dynamic. The crystalline domains oscillate with the applied stress. In addition slower oscillations with a frequency and direction unrelated to that of the applied stress can be observed at their periphery. In the cause of repeated cycling, crystalline domains slowly increase in size. Upon cessation of flow this state is stable for long time. This may account for experimentally observed memory behavior. Ultra-thin polymer films typically possess relaxation times that are orders of magnitudes larger than those of the individual polymers.

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