Optical storage in mixed Langmuir-Blodgett (LB) films of disperse red-19 isophorone polyurethane and cadmium stearate

The optical storage capability of mixed Langmuir-Blodgett (LB) films of disperse red-19 isophorone polyurethane (DR19-IPPU) and cadmium stearate has been demonstrated by measuring their optically induced birefringence that originates from the trans-cis isomerization of the azobenzene chromophore. Isomerization was achieved because the azobenzene chromophore could be introduced in the polyurethane main chain in a way that would allow for the free volume necessary for the molecular reorientation inherent in the isomerization process. The amplitude of induced birefringence decreased with the number of layers in the composite LB films, but the film thickness did not affect the time required to achieve maximum birefringence. Stored information could be optically erased by overwriting with a circularly polarized light. The identification of optimized conditions for producing LB films with the required optical properties was based on an extensive characterization process for the monolayers, using surface pressure and surface potential measurements, and for the deposited LB films. The mixed-film approach had to be used because pure monolayers of DR19-IPPU could not be transferred uniformly onto solid substrates. UV-vis and FTIR spectroscopy studies indicated the presence of both cadmium stearate and DR19-IPPU in the films, with a possible J-type aggregation of azobenzene chromophores, whereas X-ray diffraction patterns revealed that the cadmium stearate molecules form separate domains in a DR19-IPPU matrix.