Abstract
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.
