NMR Relaxation study of molecular dynamics in the smectic a phase of PAMAM liquid crystalline dendrimers of generation 1 and 3

Abstract

We present a comparative study of the molecular dynamics in the smectic A phase (SmA) of two poly-(amidoamine) (PAMAM) liquid crystalline dendrimers of generation 1 and 3, respectively, using proton (1H) spin-lattice relaxation time dispersion. The dispersions are interpreted using two main relaxation mechanisms associated with collective motions and fast reorientational movements of the dendritic segments in the low and high frequency ranges, respectively. Our results can be interpreted on the basis of a decrease in the size of the coherent smectic domains that experience layer undulations compared to what is found in low molar mass liquid crystals, as a consequence of disorder induced by the dendritic core. At high frequency, the relaxation results reveal that the reorientational motions of the mesogenic units are conditioned by their covalent links with the dendritic core.