Thermal stress and glass transition of ultrathin polystyrene films

The thermal stress of thin and ultrathin polystyrene (PS) films on Si subst rate has been studied and the glass transition temperature (T-g) is determi ned from the thermal stress data. T-g of PS turned out to be thickness inde pendent for thick films but decreases when the film thickness is comparable to the end-to-end distance of the polymer chains (< 100 nm). The thermal s tress level and the slope of the stress temperature curve of the film also decrease as the film thickness decreases. The slope reduction indicates tha t the product of the biaxial modulus E/(1-nu) and the coefficient of therma l expansion (CTE) of the film decreases with film thickness. Assuming that the CTE increases for ultrathin films, the modulus is found to decrease sig nificantly with respect to the bulk value. (C) 2000 American Institute of P hysics. [S0003-6951(00)03244-7].