LANGMUIR-BLODGETT-FILMS OF LINEAR POLYSILOXANES INCORPORATING AROMATIC SIDE-CHAINS - STRUCTURE-PROPERTY RELATIONSHIPS

Four aromatic acid side-chain polymers based on two different polysilo xane backbones, namely poly(hydrogenmethylsiloxane) homopolymer and po ly(hydrogenmethylsiloxane-dimethylsiloxane) copolymer, have been synth esised and characterised. Two different omega-(4-alkoxyphenyl)alkanoic acid side-groups have been studied whose primary spacer groups (the a lkyl chain that separates the siloxane backbone from the oxyphenyl gro up) are -(CH2)8- and -(CH2)4- and whose secondary spacer groups (the a lkyl chain that separates the oxyphenyl group from the carboxylic acid group) are both -(CH2)2-. All of these compounds form stable Langmuir films on a pure water subphase and can be co-deposited with monomeric eicosylamine (CH3(CH2)19NH2) to form alternate layer ABABA ... superl attices. 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 reporte d for LB films. Its value of 4.6 muC m-2 K-1 is a factor of 3 greater than the coefficient measured for a similar polysiloxane copolymer/eic osylamine multilayer in which the siloxane backbone is substituted wit h purely aliphatic acid side-groups. Therefore the presence of a molec ular dipole (oxyphenyl) within each side-chain, in addition to the ion ic dipole that arises from proton transfer between acid and amine grou ps in adjacent monolayers, has led to a dramatically improved pyroelec tric response.