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

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 T-g, surface tension drives its decay once the sample is heated to above T-g. From the balance between surface tension and viscous stress, the near-surface shear modulus G(omega) 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. (C) 2000 American Institute of Physics. [S0021-9606(00)70308-4].