MOLECULAR-DYNAMICS IN LIQUID-CRYSTALLINE N-ACYLATED OLIGOETHYLENIMINES AS STUDIED BY BROAD-BAND DIELECTRIC-SPECTROSCOPY

The molecular dynamics and the charge transport in six liquid crystall ine N-acylated oligoethylenimines were investigated by dielectric spec troscopy. The dielectric spectra in the temperature range from 125 K u p to 400 K and in the frequency range between 10(-2) Hz and 10(7) Hz c ould be described by a conductivity contribution and three relaxation processes. The molecular assignment of the alpha-process is a relaxati on of the rigid benzamide unit. Its mean relaxation time is influenced by the length of the alkoxy side chains, the spacer length (ethylene or propylene groups) and the neighboring repeating units. In the colum nar mesophase this process is cooperative due to sterical hindrance. A normalized g-factor expressing the intermolecular cooperativity was c alculated. The cooperativity increases when approaching the glass tran sition temperature. The gamma-process is assigned to local librational motions of the alkoxy side chain involving the ether linkage. It has an Arrhenius-like temperature dependence (activation energy Delta E(a) = 22-32 kJ/ mel). The beta-process is presumably a libration at the e nd of the alkyl chains. The conductivity contribution to the imaginary part of the dielectric function epsilon '' is frequency independent e xcept for very low frequencies where electrode polarization causes a d ecrease in the measured conductivity. The charge transport is strongly influenced by the phase transitions isotropic to columnar and columna r to crystalline, but not by the glass transition.