Dielectric relaxation of liquid crystalline dendrimers and liquid crystalline polymers with pendant nitro groups

The dielectric relaxation was determined in a time-domain spectrometer between 100 and 295 K and from 0.1 to 1000 Hz for series of dendritic liquid crystalline polymers with scaffolds based on 2,2-bis(hydroxymethyl)propionic acid ( bis-MPA) of generations 0–3 with 4″-(( R)-1-methylheptyloxy)phenyl 4-{4′-[10-(hydroxy-carbonyl)decyloxy]-phenyl} benzoate as mesogen and for the second generation of dendrimers with 4″-[10-(hydroxycarbonyl)decyloxy]-biphenyl 4-[4′-(2-( R)-octyloxy)-3′-nitrophenyl] benzoate as mesogen. Dielectric data are also reported for a side-chain liquid crystalline polymer based on {4″-[( R)-(−)-2-octyloxy]-3″-nitro-phenyl 4-(4′-(11-acryloyloxyundecyloxy)phenyl}benzoate. The low temperature subglass process (γ) assigned to the spacer group was essentially insensitive to molecular architecture and to the presence of a pendant nitro group in the mesogen. The high temperature sub-glass process (β) assigned to a 180° flip of part of mesogen was only moderately sensitive to the molecular architecture. The introduction of a pendant nitro group in the mesogen led to a substantial increase in both activation energy and relaxation strength. The results suggest that the phenylene ring with the pendant nitro group participates in the β process.