Relaxation dynamics of ultrathin polymer films investigated on the nanoscale by local dielectric spectroscopy

The dielectric relaxation of ultrathin polymer films has been investigated effectively by several techniques and most of them were used to measure relaxation properties on macroscopic regions of the sample. A possible approach to obtain highly local measurements of dielectric properties of ultrathin films is the local dielectric spectroscopy1. This technique has been implemented by means of an atomic force microscope (AFM)-based method named frequency-modulated electrostatic force microscopy2. In essence, an excitation voltage, V(t) = VAC cos (?t), is applied to the conductive tip of an AFM, inducing a variation of the tip resonant frequency, which is proportional to the gradient of the electrostatic force between the tip and the sample. This variation at the second harmonic, 2?, is measured in amplitude and phase by using dual-phase lock-in technique locked at the excitation voltage. In particular, the phase angle, ?v, is related to the dielectric relaxation properties of the sample. This method has been applied to study the change in dynamics of ultrathin poly(vinyl acetate) films as the film thickness is decreased. Dielectric loss spectra were obtained as a function of frequency at different temperatures in controlled atmosphere (relative humidity less than 5%) (Fig. 1). The influence of the polymer molecular weight on the dielectric relaxation of the films was also investigated. Fig.