2D Photopolymerization of Liquid Crystalline Langmuir Monolayers: In Situ Observation by Reflected Polarizing Microscopy

Photopolymerization of Langmuir monolayers
composed of bifunctional
acrylic liquid crystalline (LC) compounds was observed in situ by
polarizing optical microscopy. In a dark state, monolayers of the
LC compounds formed at an air–water or liquid–liquid
interface exhibited liquid-like fluidity and in-plane optical anisotropy
because of the coherent molecular tilt from the surface normal. Irradiated
by UV light, the in-plane anisotropy and the liquid fluidity gradually
disappeared with time, indicating the formation of the polymerized
monolayers. Because the constituent molecules possess polymerizable
acryloyl groups, under UV light, they are combined by acrylic polymer
chains grown on the interface, which decreases the intermolecular
distance and disturbs the coherent molecular tilt, resulting in the
evanescence of the in-plane optical anisotropy and the fluidity. In
contrast to the classical model of radical polymerization, the time
taken for the monolayers to be photopolymerized was inversely proportional
to the UV intensity, which is ascribed to the ideal two dimensionality
of the reaction field. Because the polymerization degree is quantitatively
estimated from the in-plane optical anisotropy of the LC monolayers,
the process is traced, from moment to moment, by in situ microscopy
observation.