Lyotropic and thermotropic phase transitions in films of ionene-alkyl sulfate complexes

Five [X,Y]-ionenes [(CH2)(X)N+(CH3)2(CH2)(Y)N+(CH3)2]nBr-(2n) were prepared (X = 3, 5; Y = 10, 12, 22). Using new preparation methods, dry, salt-free complexes with three n-alkyl sulfates (octyl, decyl, and dodecyl) were obtained. The ionenes and their complexes were characterized in methanol solution by light scattering, viscometry, and fluorescent probe studies. The solid materials were investigated by means of DSC, TG, and ATR-IR. Transparent films were formed from stoichiometric [3,10], [3,12], and [5,10]-ionene complexes with decyl and dodecyl sulfate. In the films, mesogenic phase transitions could be induced: dry films were optically isotropic; when exposed to elevated humidity, the films slowly became optically anisotropic because of a lyotropic transition to a hexagonal mesogenic phase. The relative humidity, at which the hexagonal phase developed, was distinct for each complex. The anisotropic phases were converted to isotropic in a thermotropic transition under controlled relative humidity at specific clearing temperatures, which were higher for dodecyl complexes than for decyl complexes. This thermotropic isotropic-anisotropic transition could be cycled several times, but partial hydrolysis of the alkyl sulfates reduced the reproducibility of transition points.