On the smectic order of polymer monolayers at the air–water interface

The McMillan theory of the smectic-A phase is extended to two dimensions. Its predictions, which strongly differ from those in three dimensions, are found to be in disagreement with the experiment on monolayers of the polymer PLA [poly(l-lactic acid)] at the air–water interface. In contrast to the latter, the theory predicts that the smectic A-phase does not melt into the isotropic phase but first goes through a nematic phase. In addition, the effect of CdSe nanoparticles and Fe2O3 beads on the phase transition from the isotropic liquid-expanded phase to the smectic liquid-condensed phase of PLA monolayers at the air–water interface is investigated experimentally using the Langmuir–Blodgett method. It is found that depending on the size and concentration of the nanoparticles and beads, this transition can be shifted or suppressed altogether.