(236d) A Study of the Air-Water Interfacial Properties of Biodegradable Polyesters and Their Block Copolymers with Poly(ethylene glycol): Toward Rational Design of a Polymeric Lung Surfactant

It has been reported that the surface pressure-area isotherm of poly(D,L-lactic acid-ran-glycolic acid) (PLGA) at the air-water interface exhibits several interesting features: (1) a plateau at intermediate compression levels, (2) a sharp rise in surface pressure upon further compression, and (3) a marked surface pressure-area hysteresis during compression-expansion cycles. In order to investigate the molecular origin of this behavior, we conducted an extensive set of surface pressure and AFM imaging measurements with PLGA materials having several different molecular weights and also a poly(D,L-lactic acid-ran-glycolic acid-ran-caprolactone) (PLGACL) material (in which the caprolactone monomers were incorporated as a plasticizing component). The results suggest that: (i) the plateau in the surface pressure-area isotherm of PLGA (or PLGACL) occurs due to the formation (and collapse) of a continuous water-free monolayer of the polymer under continuous compression; (ii) the PLGA monolayer becomes significantly resistant to compression at high compression because at that condition the collapsed domains become large enough to become glassy (such behavior was not observed in the non-glassy PLGACL sample); and (iii) the isotherm hysteresis is due to a deformation (i.e., coarsening) of the collapsed domains that occurs under high compression conditions. We also investigated the monolayer properties of PEG-PLGA and PEG-PLGACL diblock copolymers. The results demonstrate that the tendency of PLGA (or PLGACL) to spread on water allows the polymer to be used as an anchoring block to form a smooth biodegradable monolayer of block copolymers at the air-water interface. These diblock copolymer monolayers exhibit protein resistance.