Elastic Modulus of Ultrathin Films Prepared via Interfacial Polymerization: Asymmetric Behavior and the Effect of Tip Radius

Poly(ethylene glycol)-based ultrathin films were prepared using hydrazide-poly(ethylene glycol)₁₂-hydrazide (HZ-PEG-HZ) and 4-[[4,6-bis(4-formylphenoxy)-1,3,5-triazin-2-yl]oxy]benzaldehyde (TOB) at the air/dimethyl sulfoxide (DMSO) interface via interfacial polymerization (IP). The resulting ultrathin PEG-TOB films exhibited asymmetric properties, with lower wettability and roughness on the air side (AS) compared to those on the DMSO side (DS). X-ray photoelectron spectroscopy (XPS) results indicate a higher PEG content on the DS. The mechanical properties of the films were studied by atomic force microscopy (AFM) nanoindentation mapping using small (radius, <i>R</i> = 10 nm) and large (<i>R</i> = 29 nm) tips. The AS has a higher elastic modulus than the DS in both air and water. These asymmetries are attributed to preferential enrichment of hydrophobic aromatic groups on the AS and greater exposure of hydrophilic PEG chains on the DS during IP. Additionally, a higher elastic modulus was observed with the large tip in air at small indentations (less than 10 nm), attributed to contact stiffening due to the formation of an interface region upon compression. Furthermore, due to the substrate effect, the elastic modulus increased markedly at larger indentations. The present study provides a new understanding of the asymmetric mechanical properties of ultrathin interfacial films, improving the design of functional thin films.