Impact of the α‑Methyl Group (α-CH₃) on the Aggregation States and Interfacial Isotherms of Poly(acrylates) Monolayers at the Water Surface

Polymer monolayers at liquid surfaces have attracted considerable attention as tools to construct molecular devices with unique properties. The aggregation states of chains within the monolayers must be understood as the first benchmark to establish molecular designs for the devices. It has been reported that large differences exist in surface pressure–area isotherms of polyacrylates and polymethacrylates at the water surface, although they differ chemically only by the absence of an alpha methyl group (α-CH₃) in the former. Herein, we carried out experiments using sum-frequency generation vibrational spectroscopy, atomic force microscopy and infrared reflection absorption spectroscopy to explore the changes of the aggregation state and interaction of four poly­(acrylates) (i.e., poly­(methyl acrylate), PMA; poly­(ethyl acrylate), PEA; poly­(methyl methacrylate), PMMA; poly­(ethyl methacrylate), PEMA) at the water surface during compression. In the case of PMA and PEA without α-CH₃, the polymer chains adopt an expanded 2-D random-coil conformation. The methyl group in the ester side chain (s-CH₃), which is the origin of the surface pressure variation, is increasingly ordered with compressing of the surface, leading to the expanded-type isotherms. Nevertheless, since chains of PMMA and PEMA were collapsed into compact globules, the s-CH₃ orientation becomes invariant on compression, giving rise to more condensed isotherms with faster increase of surface pressure with compression. Weaker hydration effects and suppressed segmental mobility due to introduction of an α-CH₃ group was responsible for the condensed molecular conformation and reduced structural compressibility of the monolayers of PMMA and PEMA.