THIN-FILM BEHAVIOR OF POLY(METHYL METHACRYLATES) .1. MONOLAYERS AT THE AIR-WATER-INTERFACE

The monolayer behavior of PMMA of varying tacticities at the air-water interface was studied. A difference in lateral cohesive energy is argued to be responsible for the fact that the pressure area isotherms of isotactic PMMA deviate strongly from those of syndiotactic PMMA. At low surface pressures the isotactic PMMA monolayer can be characterized as an expanded type monolayer, whereas syndiotactic PMMA forms a condensed type monolayer. At areas of about 20 angstrom2/monomeric unit a transition can be observed in the pressure-area isotherm of isotactic PMMA. This transition was studied as a function of molecular weight, temperature, and compression speed. From the results it is deduced that the transition corresponds to a two-dimensional pseudocrystallization process in which the isotactic PMMA assumes a double-helix conformation in the monolayer, similar to the three-dimensional crystal structure. From an Avrami analysis it is inferred that the kinetics of this crystallization can be described by an activated nucleation, followed by a one-dimensional growth.