Characterization of the morphology of poly(alpha-hydroxy acid)s Langmuir-Blodgett films by atomic force microscopy measurements

Isotherms measurements, X-ray reflectivity measurements at the air/water in terface and structural analysis of polymer films have shown that phase tran sitions appears on compression of poly(alpha-hydroxyacid)s (biodegradable p olymers) monolayers. Atomic force microscopy (AFM) permits one to follow th e growth of quasi-crystalline domains with morphology depending on the poly mer. For PLA50, the homopolymer of lactic acid, isotherm measurements have shown a transition observed at 15.5 mN m(-1). AFM analysis revealed that th is transition was accompanied by a nucleation process. The structure of the films appeared influenced by the rate of the compression at the air/water interface. For PLA25GA50, a copolymer that contains half glycolic acid and lactic acid, AFM shows a different morphology. For low pressure, one observ ed the formation of a rough pattern. During the phase transition, one obser ved the formation of a continuous wormlike structure corresponding to the c ondensed phase of the polymer monolayer. The difference in morphology of PL A50 and PLA25GA50 monolayers are explained by hydrophobic interactions modu lated by the proportion of lactic and glycolic segments in the polymer back bones. AFM appeared very convenient to follow the evolution of the polymer firms on the mica. Condensation effects and degradation processes lead to v arious kind of structures depending on the nature of the polymer film. (C) 1999 Elsevier Science B.V. All rights reserved.