Molecular dynamics in liquid crystalline N-acylated oligoethylenimines as studied by broadband dielectric spectroscopy

The molecular dynamics and the charge transport in six liquid crystalline N‐acylated oligoethylenimines were investigated by dielectric spectroscopy. The dielectric spectra in the temperature range from 125 K up to 400 K and in the frequency range between 10−2 Hz and 107 Hz could be described by a conductivity contribution and three relaxation processes. The molecular assignment of the α‐process is a relaxation 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 columnar mesophase this process is cooperative due to sterical hindrance. A normalized g‐factor expressing the intermolecular cooperativity was calculated. The cooperativity increases when approaching the glass transition temperature. The γ‐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 ΔEa = 22–32 kJ/mol). The β‐process is presumably a libration at the end of the alkyl chains. The conductivity contribution to the imaginary part of the dielectric function ε″ is frequency independent except for very low frequencies where electrode polarization causes a decrease in the measured conductivity. The charge transport is strongly influenced by the phase transitions isotropic to columnar and columnar to crystalline, but not by the glass transition.