Transitory response of confined polymer films subjected to oscillatory shear.

Molecular-dynamics simulations were used to study the response of a nanometer thin polymer film to oscillatory shear. Several types of response occur, depending on the amplitude of the shear. At low amplitude, the film deforms elastically. At intermediate ones it deforms plastically. Short-range stress-induced structured crystalline domains occur. This flexible elastic state is very dynamic. The crystalline domains oscillate with the applied stress. In the course of repeated cycling, they slowly increase in size. These mesoscopic domains 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. When oscillated at even higher amplitude, stick-slip is observed. In our constant pressure simulations, the film yields when wall spacing is increased to a value at which the polymer segments can smoothly rearrange and hence relax the internal stress.