Swelling equilibria in mixtures of isotropic gels and low molar weight smectic-A liquid crystals

Abstract

The phase behaviour of poly( n-butyl acrylate) gels swollen in smectic-A low molar weight liquid crystals (LMWLC) was investigated, in view of the increasing importance of such systems in advanced optical devices. Polyacrylate networks were prepared by ultraviolet (UV) irradiation of reactive formulations including a monomer ( n-butyl acrylate), a crosslinker (hexane diol diacrylate, HDDA) and a photoinitiator (2-hydroxy-2-methyl-1-phenyl-propane-1-one). The obtained dry polymer networks were characterized by a combination of physical techniques, and immersed in an excess of smectic-A LMWLC (4- n-octyl-4′-cyanobiphenyl (8CB) and 4- n-octyloxy-4′-cyanobiphenyl (8OCB)), forming polymer gels. A detailed study by polarized optical microscopy (POM) allowed us to determine the swelling degree of the gels and the phase behaviour of the solvent inside the polymer matrix in a wide range of temperature. It was found that the LC inside the swollen poly( n-butyl acrylate) networks remains isotropic, even if the surrounding solvent is liquid crystalline. The uptake of LMWLC inside the network preferentially takes place between the smectic to nematic and the nematic to isotropic transition temperatures of the solvent. Phase diagrams in the concentration–temperature framework were given and discussed as a function of crosslinking degree of the polymer network and temperature, and phase behaviour of the solvent. Finally, results were critically examined and compared to the few studies found in the literature.