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Nanoparticle self-assembly assisted by polymers: the role of shear stress in the nanoparticle arrangement of Langmuir and Langmuir-Blodgett films.

We propose to use the self-assembly ability of a block copolymer combined with compression-expansion cycles to obtain CdSe quantum dots (QDs) structures of different morphology. The methodology proposed consists in transferring onto mica mixed Langmuir monolayers of QDs and the polymer poly(styrene-co-maleic anhydride) partial 2-butoxyethyl ester cumene terminated, PS-MA-BEE, previously sheared by 50 compression-expansion cycles. Results indicate that the shear stress takes out nanoparticles at the air-water interface from metastable states and promotes a new equilibrium state of the Langmuir monolayer. This new state was transferred onto mica by the Langmuir-Blodgett (LB) methodology, and the morphology of the LB films was analyzed by atomic force microscopy and transmission electron microscopy measurements. Our results show that when the amplitude strain increases, the QDs domain size decreases and the QDs LB film arrangement becomes more ordered. The dynamics of the monolayer relaxation after cycling involves at least three time scales which are related to the damping of surface fluctuation, raft rearrangement, and component movements inside each raft. Brewster angle microscopy allowed visualizing in situ the raft rearrangement at the air-water interface.