A soluble, asymmetric polydiacetylene has been shown to form
stable monolayers at the
air−water interface of a Langmuir trough. The monolayers of this
unconventional amphiphilic polymer
could be repeatedly deposited onto hydrophobic substrates during every
upstroke cycle of the vertical
dipping method to form uniform Z-type multilayers.
Characterization of the precise orientation of the
polymer main chain has been carried out to determine the spatial
correlation between the nonlinear
optical chromophore responsible for second-order nonlinear optical
properties and the backbone. Polarized
UV/vis and IR spectroscopic studies indicate a preferential orientation
of the main chain along the dipping
direction with a mean fluctuation of ±30° between director axis and
chain trajectories. Polarization-dependent second-harmonic generation (SHG) in multilayers indicates a
high degree of orientational
anisotropy of the backbone. As expected, SHG intensities increase
with the number of layers due to the
noncentrosymmetric Z-type structure. The second-order nonlinear
coefficient (d
33) was estimated to
be
1.52 pm/V at 1064 nm of Nd:YAG laser after correcting for absorption.
Surface characterization and
alignment of the extended chain backbones is further established using
atomic force microscopy (AFM).
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