Role of the molecular weight and the composition on the hydrolysis kinetics of monolayers of poly(alpha-hydroxy acid)s

The hydrolysis kinetics of spread insoluble monolayers of poly(alpha-hydrox y acid)s with various molecular weights and lactic acid-glycolic acid molar ratios was followed by measuring simultaneously the decrease in the surfac e area at constant surface pressure and the evolution of the surface potent ial. The interfacial hydrolysis at alkaline pH leads to the progressive fra gmentation of the polymer molecules and the appearance of charged insoluble products (detected by measuring the surface potential) and small soluble f ragments (detected by measuring the decrease in the surface area). The data obtained by both approaches were interpreted in the framework of the rando m scission model. The rates of hydrolysis are larger for polymers with smal ler initial polymerization numbers and increase with the decrease in the mo lar ratio of lactic acid units.