In-situ studies of flow-induced phenomena in Langmuir monolayers

Flow-induced orientation of the hairy-rod polymer poly(phthalocyaninato siloxane) (PcPS) dissolved at 5 mol.% in docosanoic acid and residing in a monolayer at the air-water interface is reported. The how was induced through two routes. In one, a four-roll mill was immersed through the monolayer to produce a well-defined extensional Row. In the second, a Langmuir-Blodgett (LB) deposition process was carried out simultaneously onto three parallel, glass plates, thereby generating a stagnation point, with extensional flow between each pair of slides. Linear dichroism was used to obtain in situ the measurements of orientation in the Langmuir films themselves, and on the glass substrates. It was found that the flows generated by the LB process induced substantial orientation within the Langmuir film, and this anisotropy was also evident in the LB films that were deposited. The velocity fields were measured using a particle tracking technique and compared against the predictions of a model assuming that the glass substrates act as line sinks of material during the LB process. It was found that the meniscus region distorts this velocity profile and leads to decreased alignment of the polymer chains.