COMPRESSION EXPANSION CURVES OF POLY(DL-LACTIC ACID-CO-GLYCOLIC ACID) MONOLAYERS

The copolymer poly(DL-lactic acid-co-glycolic acid), used in the design of nanoparticles as drug delivery systems, forms stable monolayers at the air/water interface when spread from solutions of chloroform or 4:1 (v:v) chloroform-ethanol mixtures. The PI-A curves obtained show a plateau at a surface pressure of about 9 mN.m-1, corresponding to a change from high to low compressibility monolayer states. Since the surface pressure in the plateau region was hardly altered by increasing temperature from 10 to 35-degrees-C, the plateau is interpreted as reflecting compression-induced reorientation of the polar groups of the polymer. There was marked hysteresis when the film was compressed to the collapse surface pressure (60 mN.m-1) and then expanded to its initial area: the first compression-expansion cycle did not return the monolayer to its original state, probably due to irreversible loss of molecules during collapse. Below the collapse surface pressure, however, the film showed reversible behavior.