BEHAVIOR OF POLY(D,L-LACTIC ACID) MONOLAYERS AT THE AIR-WATER-INTERFACE - EFFECT OF SPREADING SOLVENTS

The properties of poly(D,L-lactic acid) monolayers spread at the air-water interface were shown to be strongly dependent of the nature of the spreading solvent. The monolayers spread from acetone and tetrahydrofuran exhibited typical reversible collapse behavior in the compression-expansion cycle with a quasi-plateau at large areas followed by a steep rise in the surface pressure at small areas. Conversely, the monolayers spread from chloroform, dichloromethane, and ethyl acetate displayed large hysteresis characteristic of irreversible collapse and high surface pressures throughout the compression cycle. A marked difference in interfacial behavior between polymer monolayers spread from these two groups of solvents has been attributed to the difference in orientation of hydrophobic groups of lactic acid units at the air-water interface. For the first group of solvents (represented by acetone), the calculated free energy of monolayer-water interaction, assessed from contact angle measurements on LB films sampled from these monolayers, was highly negative and resulted mainly from the high value of electron-donor parameter of the polymer surface free energy. The second group of solvents (represented by chloroform) produced monolayers characterized by a weaker adhesion to the water interface. The viscosity measurements of solutions of the polymer in the studied spreading solvents reveal that chloroform is a much better solvent for the polymer than acetone. It has been inferred that while monolayers spread from acetone, due to the strong intersegment interactions, formed microdomains capable of respreading, chloroform favored at least a partial deployment of polymer chains from their random coil conformation to a more unfolded structure at the air-water interface.