OPTICAL SUPPRESSION OF FERROELECTRICITY IN POLYSILOXANE COPOLYMERS WITH CHIRAL AND PHOTOCHROMIC SIDE-GROUPS

The ferroelectricity of surface-stabilized aligned films of polysiloxa ne copolymers containing chiral esters and photochromic azobenzene moi eties within the side groups is suppressed on UV irradiation and is re built on visible irradiation. The liquid crystalline polymer (S-c 64 S-A 80 i) shows ferroelectric behavior below 64 degrees C. The photoin duced modification of supramolecular order and the corresponding chang es in its dynamics are studied by polarized UV/vis spectroscopy and br oad band dielectric spectroscopy (10(-2)-10(5) Hz). The dielectric mea surements show that UV irradiation causes a loss of ferroelectricity w ithin an interval of 10K below the phase transition temperature. The p hotochemical change of intermolecular interactions results in the disa ppearance of the S-c phase and the Goldstone mode. Polarized absorpti on spectroscopy indicates the degree of order, the orientational direc tion as well as the establishment of different steady states of the EZ photoisomerization by UV and visible exposure. The photoisomerization of only a small amount of azobenzene moieties causes a decrease of th e phase transition temperature between the ferroelectric S-c and the nonferroelectric S-A mesophase. This is connected with a reversible mo dification of the degree of order and of the director orientation char acteristic for the S-c and S-A phases. Thus, the photoinduced modific ation of the intermolecular interactions results in an opto-dielectric effect. (C) 2998 John Wiley & Sons, Ltd.