Behavior of Syndiotactic Poly(methyl methacrylate) Monolayers at the Air/Water Interface: Influence of Temperature and Molecular Weight on the Surface Pressure−Area Isotherms and Brewster Angle Microscopy Images

Abstract

On aqueous subphases of pH 6, the behavior of syndiotactic poly(methyl methacrylate) (synd-PMMA) (average molecular weight, Mw =120 000 g/mol) monolayers at the air/water interface was investigated in the range of temperatures between 15 and 50 °C. The monolayer characteristics of synd-PMMA stereoisomer were studied and compared in terms of surface pressure−area per residue (π−A) isotherms, surface compressional modulus−surface pressure (Cs−1− π) curves, hysteresis phenomena, film thickness, and the phase images observed from Brewster angle microscopy (BAM). The results show that synd-PMMA (120 000) monolayer exhibits a phase transition LE−L′E (from a liquid-expanded to another liquid-expanded state) at surface pressures of ca. 15−17 mN/m when the temperature is raised from 25 to 50 °C, which is attributed to the formation of reversible loops and tails in the monolayer. On the other hand, the study of the effect of the molecular weight on the PMMA monolayer behavior showed that the polymer of Mw = 15 000 g/mol exhibits a limiting area of 19.7 Å2 per monomer repeating unity at 30 °C, which is close to the value for the trans conformation of PMMA at the air−water interface. Nevertheless, the limiting area of PMMA (120 000) was lower (14.1 Å2/monomer), suggesting that not all monomers are located at the interface. The morphology and thickness of monolayers confirm at microscopic level the structural characteristics deduced from the π−A isotherms.