Glass transition of polystyrene near the surface studied by slow-positron-annihilation spectroscopy

We have measured the Doppler broadening energy spectra of positron annihilation using a variable monoenergetic positron beam as a function of positron energy and of temperature in polystyrene. The $S$ parameters from the energy spectra versus temperature show that onset temperatures significantly decrease as the positron energy decreases. This result gives direct evidence that the glass-transition temperature is suppressed at a magnitude of 57 °C at the depth of 50 Å from the surface.