Role of Intermolecular Association in Monolayer-Level Assembly and Morphogenesis of Conjugated Polymers at the Air–Water Interface

Abstract

How molecular architecture is translated through intermolecular association into film structure and mechanical behavior remains a central question in conjugated-polymer self-assembly. Here, we compare two luminescent polymers with distinct backbone and side-chain structures: poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) and poly(2,5-di(3′,7′-dimethyloctyl)phenylene-1,4-ethynylene) (PPE). Langmuir–Blodgett processing provides a controlled two-dimensional environment at the air–water interface, while the intrinsic fluorescence of both polymers allows interfacial organization to be followed from the earliest stages of spreading. PFO rapidly forms discrete domains. PPE instead develops a laterally interconnected network. Molecular dynamics simulations relate these morphologies to different modes of intermolecular association. In PFO, fluorene units associate more locally, favoring compact aggregation. PPE instead forms more spatially distributed contacts along the phenylene–ethynylene backbone. This distinction persists during mechanical deformation. Compression brings pre-existing PFO domains into contact, but decompression rapidly fragments the film. PPE remains laterally connected and undergoes extensive expansion. Deformation is accommodated predominantly through the ethynylene linkages of the conjugated backbone. Beyond the monolayer stability limit, both polymers develop anisotropic semicrystalline structures. PFO forms isolated needle-like domains, whereas PPE develops an interconnected fibrillar network. The results thus establish a hierarchical link between local intermolecular association and the subsequent evolution of monolayer structure and mechanical behavior. This relationship remains reflected in the ordered structures formed after monolayer collapse and provides a basis for designing ultrathin conjugated-polymer films with controlled lateral connectivity and anisotropic organization.