Dielectric relaxation of amorphous polymer nano films

An alternating copolymer of vinylidene cyanide and vinyl acetate (VDCN/VAc) is known to exhibit large permittivity exceeding 100 associated with the micro-Brownian motion of noncrystalline segments. Polyvinyl acetate (PVAc) is known to show typical dielectric relaxation. Dielectric frequency spectra were measured for VDCN/VAc and PVAc nanofilms with special interest on the restriction of the correlation length. For VDCN/VAc 1000nm thick sample, two relaxation processes are observed at 2Hz and 1kHz which are assigned to the electrode polarization and segmental motion, respectively. Latter relaxation process shifts toward high frequency side with decreasing thickness accompanied by a reduction of relaxation strength. For PVAc, same thin film effect is observed. Relaxation strength and relaxation time may be reduced by the existence of surface layer. Thin film effect of PVAc is the case. However, the thickness dependence of the relaxation time in VDCNVAc below 200nm cannot be explained by the surface layer model. The thickness dependence of the relaxation time is larger than that expected by the model. It is an indication of the reduction of large correlation length resulting in faster segmental motion.