Aggregation in Langmuir and Langmuir–Blodgett films of azopolymers and its role for optically induced birefringence

Aggregation in azopolymers is reported to affect the Langmuir monolayer characteristics and the optically induced birefringence of Langmuir–Blodgett (LB) films from DR19 isophorone polyurethane (PIPDI) and DR19 4,4′ diphenylmethane polyurethane (PMDI). In mixed monolayers with cadmium stearate (CdSt), the folding of PMDI molecules appears to be substantially changed compared to the monolayer of the pure polymer, leading to a surface potential that is higher than observed for monolayers of pure PMDI and of pure CdSt. UV–Vis spectroscopy data of the deposited mixed LB films indicate H-type aggregation for PMDI/CdSt, in contrast to PIPDI/CdSt and other azopolymers investigated earlier. The H-type aggregation precludes photoisomerization, thus requiring a higher laser power for the maximum induced birefringence to be achieved in LB films of PMDI/CdSt.