Temperature and concentration effects on the equilibrium and dynamic behavior of a Langmuir monolayer: from fluid to gel-like behavior.

The equilibrium isotherms of monolayers of poly(4-hydroxystyrene) on the air/water interface have been studied in the 5-60 degrees C range. The results indicate that the interface is a poor solvent for the monolayers over the whole temperature range. For surface pressures within the semidilute regime, the plot of the area occupied by the polymer coils versus temperature at constant surface pressure shows a sharp change of slope near 30 degrees C. Also, the surface excess entropy shows a similar change of slope at the same temperature. The surface shear viscosity can be described by a power law of the surface concentration. Within the semidilute regime, the exponent of the power law changes. The temperature dependence of the viscosity points out a change from independent particle to collective dynamics.