Cyclic polysiloxanes in polar LB assemblies: synthesis, evaluation and pyroelectric behaviour

Abstract

A family of cyclic polysiloxanes, of general structure ((CH 3)Si(X)O) 4, substituted at position X with either aliphatic acid or aromatic acid side-chains has been synthesized and evaluated. These compounds form monolayer films when spread from solution onto a pure water subphase. They can be transferred using the Langmuir-Blodgett (LB) deposition technique onto glass and aluminum-coated glass substrates to form Y-type multilayer films, but they also form alternate layer assemblies when co-deposited with monomeric eicosylamine monolayers. Such acid/amine multilayers exhibit the pyroelectric effects since the interaction of carboxylic acid and amine groups yields a temperature-dependent spontaneous polarisation. The quasi-static pyroelectric coefficients of LB films containing 11 monolayers (six monolayers of cyclic polysiloxane/five monolayers of eicosylamine) have been measured over a range of temperatures for each cyclic polysiloxane/eicosylamine assembly. The coefficient is significantly larger for the cyclic polysiloxanes substituted with ω-(4-alkoxyphenyl)alkanoic acid moieties than for those substituted with entirely aliphatic side-chains. This suggests that the increased dipolar nature of the former compounds plays an important role in the pyroelectric behaviour of these alternate layer LB structures. Dramatic changes in the magnitude of the pyroelectric coefficients have been observed when relatively subtle variations to the structure of the aromatic side-chains have been made. In particular, the pyroelectric activity depends on the length of the spacer groups separating the oxyphenyl moiety from the siloxane ring and the carboxylic headgroup.