Structural Transition of Poly(L-lactic acid) Film Induced by Compression at Air/Water Interface

Abstract

Poly(lactic acid) (PLA) has attracted considerable interest as an environmentally friendly and biodegradable polymer. The properties of poly(L-lactic acid) (PLLA) at an air/water interface were studied based on the Langmuir-Blodgett (LB) film balance and atomic force microscopy (AFM). The surface pressure-area (π-A) isotherm indicated that the surface pressure of PLLA initially increased as the interfacial film was compressed; at π=9.0 mN•m-1 , a plateau was observed in the π-A isotherm, in which the area of the repeat unit was in the approximate range 0.11-0.17 nm 2. The AFM results showed that there is a clear structural transition in the PLLA film during the compression: (i) at the beginning of the plateau, a number of fibrils are present at the air/water interface and (ii) multilayer structures (at least bilayer, i.e., the underlying layer and top layer consisting of fibrils) is formed in the plateau region. In particular, when π=20.0 mN•m-1 , a thin film of PLLA of thickness about 6.0 nm was fabricated. Our findings suggest that the plateau in the PLLA π-A isotherm is closely related to a change in the film structure from monolayer to multilayer at the air/water interface. This is significantly different from the behavior of conventional amphiphiles, because the plateau in amphiphiles π-A isotherm is equivalent to a phase transition of monolayers derived from amphiphiles in a two-dimensional plane.