(574i) Surface Mechanical Behavior of Biocompatible Poly((D,L-lactic acid-ran-glycolic acid)-Block-Ethylene Glycol) (PLGA-PEG) Block Copolymers at the Air-Water Interface
The surface pressure-area isotherms of poly((D,L-lactic acid-ran-glycolic acid)-block-ethylene glycol) (PLGA-PEG) monolayers at the air-water interface exhibit mixed patterns of behavior that seemingly represent both the glass transition of the condensed PLGA film and the lateral repulsion of the PEG brush chains that could occur simultaneously under lateral compression of the monolayer. A study was conducted to investigate which one of the above two mechanisms is more responsible for the observed PLGA-PEG isotherm behavior. Particularly, this study is focused on the isotherm behavior of PLGA-PEG monolayers at small monolayer areas. In this regime, the PLGA-PEG isotherms show a surface pressure upturn during continuous monolayer compression, similarly to the isotherms of PLGA homopolymers. However, unlike PLGA, the surface pressure vs. area behavior of the PLGA-PEG diblock copolymer exhibits little dependence on the compression rate. Also, the high surface pressure generated due to the monolayer compression and the molecular morphology of the monolayer developed under high compression were seen to relax very little over time. These observations suggest that in the diblock situation, the high monolayer surface pressure observed at high surface polymer concentrations is mainly due to the lateral repulsion of the PEG chains in the brush layer. These experimental results also compare favorably with predictions of the self-consistent field polymer brush theoretical model. A comparison of the interfacial rheological properties of the air-water monolayers formed by the PLGA-PEG diblock copolymer vs. a non-glassy analogue of this diblock copolymer, poly((D,L-lactic acid-ran-glycolic acid-ran-caprolactone)-block-ethylene glycol) (PLGACL-PEG) also support this conclusion. PLGA-PEG monolayers exhibit protein resistance and also reasonable chemical stability at the air-water interface, and therefore seem to have the potential to be used in lung surfactant applications.