CRYSTAL AND LIQUID-CRYSTAL BEHAVIOR OF N-CYANOALKYL-N-ALKYL-N,N-DIMETHYLAMMONIUM BROMIDES - ROLE OF THE DIPOLE INTERACTIONS OF THE CYANO GROUPS

A homologous series of dialkyldimethylammonium bromides bearing a cyan o group at the end of one of the alkyl chains was synthesized. The cya noalkyl group ranges from monomethylene to hexamethylene and the cyano -free chain from dodecyl to octadecyl. The thermotropic liquid crystal behaviour of the series was investigated using differential scanning calorimetry, polarizing microscopy, and X-ray diffraction. The molar v olume of four selected compounds was determined as a function of tempe rature using dilatometry. The low-temperature crystal phases were foun d to have a lamellar structure in which the ammonium bromide groups ar e arranged within the layers in two distinct planes, alternately separ ated by single layers of upright, interdigitated alkyl chains and doub le layers of cyanoalkyl chains coupled through the cyano end groups. T he high-temperature smectic A phases were found to display a similar s tructure regarding the layering of the molecules, in which however the alkyl and cyanoalkyl chains have a disordered conformation. The role of the cyano end groups is discussed by comparing this smectic A struc ture with the known ordered smectic T structure of the cyano-free dial kyldimethylammonium bromides.