Kinetically Imprinted In-Plane Orientational Ordering of Flexible Polymer Chain at a Solid-Liquid Interface

Flexible, nonpolar polymer molecules are generally considered incapable of forming lateral molecular order at solid-liquid interfaces because their large conformational entropy is expected to overwhelm weak and nondirectional substrate interactions. Here we show that this limitation is not intrinsic to polymer flexibility itself but instead reflects the solvent conditions and formation pathways that have been predominantly explored. Using frequency-modulation atomic force microscopy, we investigate the adsorption structure of polydimethylsiloxane (PDMS) at the interface between a poor solvent, N,N-dimethylformamide (DMF), and highly oriented pyrolytic graphite (HOPG). We observe the formation of a substrate-parallel PDMS monolayer extending laterally over several hundred nanometers as a single domain. Within this monolayer, a 1D stripe pattern emerges with a periodicity of 0.80 ± 0.08 nm, comparable to the effective molecular diameter of locally extended PDMS chain segments. Furthermore, this stripe periodicity is also commensurate with 3 times the lattice constant of the HOPG substrate within experimental uncertainty, indicating substrate-registered in-plane orientational alignment of locally extended PDMS chain segments under interfacial confinement conditions. This observed PDMS monolayer does not represent a thermodynamically stable adsorption phase but is kinetically imprinted during the flattening of PDMS nanodroplets under poor-solvent conditions. Repeated AFM scanning causes gradual shrinkage and eventual disappearance of the PDMS monolayer, demonstrating that it is a metastable adsorption phase stabilized under interfacial confinement.