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

Abstract

Four aromatic acid side-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 siloxane backbone 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) 19NH 2) to form alternate layer ABABA… superlattices. Such assemblies exhibit the pyroelectric effect. 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 pyroelectric coefficients ever reported for LB films. Its value of 4.6 microC m −2 K −1 is a factor of 3 greater than the coefficient measured for a similar polysiloxane copolymer/eicosylamine multilayer in which the siloxane backbone 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 pyroelectric response.