Bridging the gap between the mesoscopic 2D order–order transition and molecular-level reorganization in dot-patterned block copolymer monolayers

Abstract

Langmuir–Blodgett (LB) films of amphiphilic block copolymers (BCs) form well-defined nanostructures with long-range order useful, for instance, for nanolithography applications. Nanostructures with a 2D circular micelle (“dot”) morphology are known to present a constant pressure plateau in their Langmuir isotherm (surface pressure vs molecular area curve at the air/water interface), indicative of a first-order transition. We have previously shown, with LB films of polystyrene-b-poly(4-vinylpyridine) (PS-P4VP) and its supramolecular complex with 3-n-pentadecylphenol (PDP), that there is an order–order transition of the dots from hexagonal to square at the plateau. However, various literature results indicate that the molecular-level understanding of the transition is poorly understood. Here, using polarized infrared spectroscopy on the PS-P4VP/PDP system, we identify what molecular changes occur at the plateau. The only changes found are an edge-on to isotropic orientation of the pyridine rings and an increase in the level of P4VP hydrogen bonding with PDP; no changes are found in alkyl chain conformation or orientation. On the basis of these results and AFM observations of the film morphologies, we propose a new mechanism involving 2D to 3D folding of the P4VP chains on the water surface at the plateau pressure that connects the molecular changes with the hexagonal-square reorganization of the dot array. We further show how the model can be generalized to dot-forming LB films of pure PS-P4VP and other BC systems and even to BC monolayers obtained by spin-coating from dilute solutions. This study thus lays the foundation for a generalized fundamental understanding of dot-forming LB block copolymer films and enables predicting their behavior in order to precisely control their organization and, ultimately, their properties at different length scales.