Air-Water Interfacial Properties of Chloroform-Spread versus Water-Spread Poly((D,L-lactic acid-co-glycolic acid)-block-ethylene glycol) (PLGA-PEG) Polymers

Polymers at fluid interfaces are used for a number of applications that include coatings, electronics, separation, energy, cosmetics, and medicines. Here, we present a study on an amphiphilic block copolymer, poly((D,D-lactic acid-co-glycolic acid) block-ethylene glycol) (PLGA-PEG), at the air water interface. PLGA-PEG at the air-water interface prepared by using an organic spreading solvent exhibits an extremely high surface pressure without the occurrence of desorption, making it an attractive candidate for a variety of uses in the areas mentioned above. The origin of this high surface pressure increase was shown to be due to the glass transition of the PLGA segments. The temperature at which this glass transition occurs for the PLGA segments of PLGA-PEG at the air-water interface was measured to be about 290 K by thermodynamic analysis based on the two-dimensional Maxwell relations. However, from an applications standpoint, spreading by an organic solvent greatly limits its scope of feasible uses. To explore the possibility of maintaining the excellent surface mechanical properties of the PLGA-PEG at the air-water interface while not using an organic solvent, we investigated the air-water interfacial properties of water-spread PLGA-PEG. When spread with water, it was shown that the initial micelles that form in the aqueous spreading solution remain intact even after being spread onto the air-water interface. Due to this different morphology, the surface pressure and monolayer stability were greatly reduced for the water-spread PLGA-PEG at the air-water interface. We used the Daoud and Cotton's blob scaling model to describe the desorption process of the water-spread PLGA-PEG at the air-water interface. From the scaling concept, it was shown that with higher PEG molecular weight and larger micelle size, the adsorption energy of the water-spread PLGA-PEG to the air-water interface was increased.