Persistent In-Plane Order Through 90 Bilayers in an Accordion Polymer LB Film

摘要

Processing-assisted ordering techniques that do not require electric field poling are gaining increased interest in nonlinear optical (NLO) polymer research. Langmuir Blodgett (LB) processing offers advantages over electric- field poling in that it can be done at room temperature (hence the kT Brownian motion is much less) without the strong electric fields that can lead to film damage during corona poling. We report second harmonic generation (SHG) measurements on multiple heterolayer films of two interleaved amphiphilic accordion polymers fabricated by LB deposition. Accordion NLO polymers have the chromophores arranged in a syndioregic (head-to-head) fashion in the mainchain and are inherently more stable than sidechain polymers because both ends of the chromophore are attached to the polymer backbone. The two polymers in the alternating layers contain the same cinnamamide chromophore but differ in that the hydrophilic group (OH) and hydrophobic group (aliphatic chains) on either side of the chromophore are reversed from one polymer to the other in order to yield a net dipolar order with heterolayer Y-type deposition. in earlier work, we reported SHG results on a similar pair of polymers, except one had a 12-C aliphatic chain and the other a 16-C chain. Alternating ABAB... type LB films of these polymers exhibited mm2 point group symmetry at 24 bilayers, but (infinity) mm symmetry by 92 bilayers. Here we present SHG azimuthal data on 20,40, and 90 bilayer ABAB... Y-type LB films where both polymers contain 12-C aliphatic chains. We find in-plane order associated with the mm2 point group that persists even at 90 bilayers, as well as a quadratic dependence of the generated second harmonic on the number of bilayers.