Langmuir-Blodgett films of linear polysiloxanes incorporating aromatic side-chains: structure-property relationships

Abstract Four aromatic acidside-chain polymers on two different polysiloxane backbones, namely poly(hydrogen-methylsiloxane) homopolymer and poly(hydrogenmethylsiloxane-dimethylsiloxane) copolymer, have been synthesised and characterised. Two different ω-(4-alkoxyphenyl)alkanoic acid side-groups have been studied whose primary spacer groups (the alkyl chain that separates the siloxanebackbone from the oxyphenyl group) are -(CH 2 ) 8 - and -(CH 2 ) 4 - and whose secondary spacer groups (the alkyl chain that separates the oxyphenyl group from the carboxylic acid group) are both -(CH 2 ) 2 -. All of these compounds form stable Langmuir films on a pure water subphase and can be co-deposited with monomeric eicosylamine (CH 3 (CH 2 ) 19 NH 2 ) to form alternate layer ABABA… superlattices. Such assemblies exhibit the pyroelectriceffect. Indeed, one such alternate layer LB film containing monolayers of the polysiloxane copolymer backbone substituted with aromatic side-chains has yielded one of the highest quasi-static pyroelectriccoefficients ever reported for LB films. Its value of 4.6 microCm −2 K −1 is a factor of 3 greater than the coefficient measured for a similar polysiloxane copolymer/eicosylamine multilayer in which the siloxanebackbone is substituted with purely aliphatic acid side-groups. Therefore the presence of a molecular dipole (oxyphenyl) within each side-chain, in addition to the ionic dipole that arises from proton transfer between acid and amine groups in adjacent monolayers, has led to a dramatically improved pyroelectricresponse.