Surface-rheological measurements on glass forming polymers based on the surface tension driven decay of imprinted corrugation gratings

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Abstract
We report on surface-rheological experiments on glass forming polymers making use of the decay of imprinted surface corrugation gratings. Whereas the grating is frozen in at temperatures below the glass temperature Tg, surface tension drives its decay once the sample is heated to above Tg. From the balance between surface tension and viscous stress, the near-surface shear modulus G(ω) is derived. For polystyrene a marked dependence on chain length is found. While the near-surface moduli agree with the bulk values for highly entangled samples, an increase of the apparent stiffness by a factor of 10 compared to the bulk is found for chains shorter than the entanglement length.