Qualitative evaluation of pyroelectric mechanisms in Langmuir-Blodgett films containing a cyclic polysiloxane substituted with aliphatic side chains using Fourier transform infrared (FTIR) spectroscopy

Alternate layer Langmuir-Blodgett (LB) films containing cyclic polysiloxane substituted with aliphatic side chains alternated with monomeric eicosylam ine have been shown to exhibit pyroelectric activity. The pyroelectric acti vity for 5, 11, 15, 21, 25 and 31 monolayer LB films, is recorded at 21 deg reesC and shows highest activity for 5 monolayers with a pyroelectric coeff icient of 0.5 muC m(-2) K-1. The coefficient decreases as the thickness of the film increases from 5 to 15 monolayers, increases in the thickness rang e of 15-25 monolayers and decreases again for 31 monolayers film. Fourier t ransform infrared (FTIR) spectroscopy shows that proton transfer between th e acid headgroups at the terminal end of the cyclic polysiloxane and the he adgroups of the eicosylamine increases as the number of monolayer increases . However, the angle at which the molecular axis of the LB film tilts from the surface normal, or (as indicated by the overall ratio of symmetric stre tching of CH2 to CH3, or asymmetric stretching of CH2 to CH3) increases as the number of monolayer increases from 5 to 15 layers and remains stable fo r 21-31 layers. (C) 2000 Elsevier Science S.A. All rights reserved.