The use of SIMS, XPS and in situ AFM to probe the acid catalysed hydrolysis of poly(orthoesters)

Abstract

ue to poly(orthoesters) being susceptible to acid catalysed hydrolysis, these polymers have attracted considerable interest for the controlled delivery of therapeutic agents within biodegradable matrices. The pH-sensitivity of the poly (orthoesters) has lead to several drug delivery systems being developed, whose rate of drug release is predominantly controlled by the rate of polymer hydrolysis. This study reports on the use of X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), and atomic force microscopy (AFM) in a multitechnique approach to probe the effect of acid catalysed hydrolysis at the interface of poly(orthoesters). The molecular specificity of SIMS was successfully employed, suggesting that the preferred mechanism for hydrolysis was via the cleavage of an exocyclic alkoxy bond in the 3,9,-diethylidene-2,4,8,10-tetraoxaspiro [5,5] undecane(DETOSU) unit. The resulting change in the surface chemical structure of the partially hydrolysed poly(orthoester) is such that it was not detectable by XPS analysis. Images acquired from an in situ AFM study of the hydrolysis of a poly(orthoester), showed changes in the surface morphology, seen as the formation of pits, and an overall thinning of the polymer film. The use of SIMS, XPS and AFM has enabled changes in surface chemistry to be compared with changes in surface morphology. These complementary data, on the behaviour of the polymer during degradation have important implications for the further design of novel biodegradable materials.