Langmuir–Blodgett films of side-chain polysiloxanes: synthesis, characterisation and pyroelectric behaviour

Four aliphatic acid side-chain polymers have been synthesized and evaluated, based upon two different polysiloxane backbones, namely poly(methylsiloxane) and poly(methylsiloxane-dimethylsiloxane). A study of their behaviour at the water/air interface has shown that stable, fluid molecular films are formed that can be transferred as monolayers onto hydrophilic substrates. Their surface pressure-area isotherms indicate that bunching of the polysiloxane backbones may occur at relatively high surface pressures. The alternate layer deposition process has been used, in which the polysiloxane is co-deposited with a monomeric aliphatic amine (icosylamine). This ABABA... sequential transfer has been monitored using the piezoelectric quartz crystal microgravimetric technique in which the alternate layer is assembled on top of a bulk quartz crystal, the oscillation frequency of which is measured precisely. The relationship between the frequency change and the number of transferred polysiloxane/monomer icosylamine bilayers is linear for the three materials that could be deposited, indicating that the deposition process is highly reproducible. The pyroelectric effect in these films has been studied. These data indicate that the level of activity is not simply related to the fraction of the silicon units along the backbones which are substituted with aliphatic acid chains. There is likely to be a trade-off between the density of acid/amine interactions and the free volume in which molecular tilting can occur.