RELATIONSHIP BETWEEN PYROELECTRICITY AND MOLECULAR-ORIENTATION IN ALTERNATE LANGMUIR-BLODGETT-FILMS

The relationship between pyroelectricity and molecular orientation in alternate Langmuir-Blodgett (LB) films consisting of 5-(p-dodecyloxyphenyl)pyrazine-2-carboxylic acid and deuterated stearic acid was investigated. The same studies were also performed for alternate LB films of their barium salts. Pyroelectricity of the alternate film was measured by a static method in the temperature range from -30 to 80-degrees-C. The maximum values of the pyroelectric coefficient due to a decay of spontaneous polarization were obtained to be 1.2-mu-C m-2 K-1 at 37-degrees-C for the acid alternate film and 1.8-mu-C m-2 K-1 at 43-degrees-C for the barium salt alternate film. Rapid increases of positive currents due to a depolarization in the films were observed above 50-degrees-C. The molecular orientation in the film was evaluated by Fourier-transform infrared transmission and reflection absorption spectroscopy. The temperature dependence of the orientation angles of the hydrocarbon chain axes of the constituent molecules indicates that the depolarization is due to the conformational disorder of the hydrocarbon chains. Furthermore, the temperature dependence of the COO- stretching frequencies of the barium salt alternate film suggests that the spontaneous polarization is generated by a structural modification of the carboxylate groups. It is concluded that the alternate film with the more highly oriented molecules gave the larger pyroelectricity and that if the molecular orientations are in the same order, the acid alternate film has greater pyroelectricity thant the corresponding barium salt film. The latter finding may be due to weaker intermolecular interactions in the acid alternate film than in the barium salt film.