Measurements on the surface glass transition of PMMA from the decay of imprinted surface corrugation gratings: the influence of molecular weight

The near-surface mobility of poly-methylmethacrylate (PMMA) chains is probed by means of optical diffraction. Particularly, we monitor the surface tension-driven decay of shallow surface corrugation gratings. The probe depth of the technique is about 100 nm. We report on quasi-static experiments, in which the sample temperature was ramped slowly from room temperature to a temperature well above the glass temperature, T g. An estimate of the near-surface T g is derived from the temperature at which the decay sets in, T dec. For high molecular weight PMMA this decay temperature is much less than the bulk T g as determined by calorimetry on the same material. For low molecular weight PMMA, this effect is absent.