The physical properties of rubbed polystyrene (PS) thin film were studied by using the technique of Reflectance Difference Spectroscopy (RDS). Through the rubbing process, optical anisotropy could be introduced into the PS films, which were spin coated either on the acid cleaned Si substrates with thermal oxide or on the hydrogen terminated silicon substrate. The molecular weights of the PS samples were varied from 12Kg/mol to 550Kg/mol, with thickness from 7nm to 30nm. In the Rubbing Dependence experiment, it is found that the induced anisotropy tends to increase with increasing the molecular weight or with increasing the thickness of the PS thin films. Based on the magnitude of the induced anisotropy of the PS samples, the relative change of the refractive index ( Δn ) for the two dif...[ Read more ]

The physical properties of rubbed polystyrene (PS) thin film were studied by using the technique of Reflectance Difference Spectroscopy (RDS). Through the rubbing process, optical anisotropy could be introduced into the PS films, which were spin coated either on the acid cleaned Si substrates with thermal oxide or on the hydrogen terminated silicon substrate. The molecular weights of the PS samples were varied from 12Kg/mol to 550Kg/mol, with thickness from 7nm to 30nm. In the Rubbing Dependence experiment, it is found that the induced anisotropy tends to increase with increasing the molecular weight or with increasing the thickness of the PS thin films. Based on the magnitude of the induced anisotropy of the PS samples, the relative change of the refractive index ( Δn ) for the two different polarizations due to the rubbing process is also determined. As the induced anisotropy is mainly introduced into the upper surface of the PS film, it is clear when we consider its relative change of the refractive index for the whole PS film. In the Temperature Dependence experiment, the rubbed PS samples were heated and cooled with a fixed rate (2℃/min). It is observed that the optical anisotropy disappears when it was above a certain temperature (T₀). Besides, Ellipsometry was used to determine the thickness of the PS thin films for different temperature, hence the glass transition temperature (T_g) for different PS samples can be found. This is due to the thermal expansion of the PS film is different when it is above the temperature of glass transition (T_g). The T_g is found to be higher in thicker PS films and in higher molecular weight of PS films. Also, The T_g of PS films grows on SiH substrate is higher than that on SiO₂ substrate.