Fluid to soft-glass transition in a quasi-2D system: thermodynamic and rheological evidences for a Langmuir monolayer

We report an experimental study that points out the existence of a fluid to soft-glass transition in Langmuir polymer monolayers of poly(methyl methacrylate) (PMMA), for which the water/air interface behaves as a poor-solvent. The temperature dependence of surface pressure vs. surface area equilibrium isotherms shows a glass-like transition temperature at Tg,2D ≈ 298 K, significantly lower than the value for bulk PMMA (Tg,bulk ≈ 378 K). The plot of the film thickness h vs. temperature shows a sharp change of slope at about the same temperature, 298 K, which is a typical hallmark of a glass transition in thin polymer films [J. L. Keddie, R. A. L. Jones, R. A. Cory, Europhys. Lett., 1996, 27, 59–64]. Furthermore, slightly above Tg,2D, the temperature dependence of the dilational viscosity does not follow an Arrhenius law, but instead can be described by a Vogel–Fulcher–Tamman equation with parameters that are typical of a fragile glass. Not only the qualitative behavior of three distinct equilibrium and dynamic properties, but also the quantitative agreement of the values of Tg obtained, are a strong evidence of the existence of a fluid to soft-glass transition in this quasi-2D system.