Langmuir monolayers as tools to study molecular degradation of aliphatic polyesters


The great challenge in designing future polymeric materials is tailoring their degradation profile to create consumables and medical devices that exist just long enough to fulfil their purpose before disintegrating into non-toxic fragments both in nature and in the human body. Studying the degradation of insoluble polymeric materials under physiological or environmental conditions requires long time scales since water uptake and bond hydrolysis rates in these materials are low. Moreover, in bulk materials, identifying the individual contributions of diffusion of reactants, reaction products and molecular reaction kinetics to the overall degradation rate as observed by mass loss and reduction of molecular weight is challenging. Therefore, the Langmuir monolayer degradation technique has been introduced for fast assessment of the molecular degradation kinetics of biodegradable macromolecules. To this end, insoluble polymer films are spread at the air-water interface of a Langmuir trough and degraded at constant surface pressure. Fragmentation of the macromolecules by hydrolysis produces water soluble fragments, resulting in reduction of the film area. Due to the fast mass exchange between the water surface and the aqueous bulk phase, the degradation velocity of the film as observed by area reduction is determined solely by the molecular degradation kinetics. Extracting kinetic data from the area reduction curves requires adequate model of the fragmentation and dissolution of the Langmuir film. Here, we present new models that allow for quantitative analysis of complex molecules undergoing both random fragmentation and chain-end scission simultaneously. The models are used to analyse the experimental area reduction curves and hydrolysis rate constants of different biodegradable macromolecules like polyhydroxyalkanoates, polyanhydrides and copolyesterurethanes. For the latter, we found that the hydrolysis rate constant of degradable blocks decreased with increasing content of non-degradable blacks. By adjusting parameters like molecular weight or block length in the models, predictions of the interplay between molecular architecture and degradation rate are made. Since our models consider the concentration of endgroups in the Langmuir film, the number average molecular weight of the degrading chains can be calculated from the area reduction curves.
QR Code: Link to publication