Relaxations in thin polystyrene films

The relaxation dynamics of polystyrene (PS) films were probed by measuring the orientation induced birefringence in rubbed PS films on glass substrates. Isothermal measurements of the birefringence below the glass transition temperature (T_g) showed that the relaxation follows a Kohlrausch-William-Watts time dependence with an Arrhenius temperature dependence for the relaxation times. A more efficient experimental technique was devised wherein the rubbed PS films are heated at a constant rate, and a new relaxation model was developed to account for this constant heating rate. From a single constant heating rate measurement, useful physical parameters can be obtained such as the activation energy (ΔE), T_g, and the stretching exponent (β _KWW). Studies of the PS film thickness dependence of ΔE, T _g, and β_KWW reveal that ΔE and βKWW have no apparent dependence on film thickness. T_g, on the other hand, decreases by as much as 15 K as film thicknesses decrease to 5.8 nm.; A surface sensitive experimental technique, infrared-visible sum frequency generation spectroscopy (SFG), was used in an internal reflection geometry to probe the structure of a PS film's PS/air and PS/sapphire interfaces. Raman scattering and infrared absorption spectra from PS and liquid and vapor toluene were also acquired and analyzed to determine the aromatic C-H stretching vibrational mode frequencies, Raman activity, and infrared activity. Analysis of the SFG spectra reveals that the phenyl side-groups are nearly perpendicular to the plane of the interface at the PS/air interface and more tilted toward the plane of the interface at the PS/sapphire interface.; SFG was also used to characterize the in-plane orientation of the phenyl groups at a rubbed PS/air interface. The SFG intensity can be manipulated to yield an order parameter equivalent to that measured with birefringence. Analysis of the SFG intensity and birefringence from rubbed PS/air interfaces heated at a constant rate finds that a near surface region ∼15 nm thick has a lower ΔE, a larger β_KWW, and the same T_g as bulk PS. This means the PS/air surface is glassy and more mobile than bulk PS below T_g.