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-w ater interface were shown to be strongly dependent of the nature of th e spreading solvent. The monolayers spread from acetone and tetrahydro furan exhibited typical reversible collapse behavior in the compressio n-expansion cycle with a quasi-plateau at large areas followed by a st eep rise in the surface pressure at small areas. Conversely, the monol ayers spread from chloroform, dichloromethane, and ethyl acetate displ ayed large hysteresis characteristic of irreversible collapse and high surface pressures throughout the compression cycle. A marked differen ce in interfacial behavior between polymer monolayers spread from thes e two groups of solvents has been attributed to the difference in orie ntation of hydrophobic groups of lactic acid units at the air-water in terface. 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 electro n-donor parameter of the polymer surface free energy. The second group of solvents (represented by chloroform) produced monolayers character ized by a weaker adhesion to the water interface. The viscosity measur ements of solutions of the polymer in the studied spreading solvents r eveal that chloroform is a much better solvent for the polymer than ac etone. It has been inferred that while monolayers spread from acetone, due to the strong intersegment interactions, formed microdomains capa ble of respreading, chloroform favored at least a partial deployment o f polymer chains from their random coil conformation to a more unfolde d structure at the air-water interface.