Esca Detection of Surface Carboxyl Group Concentration of Polyanhydrides
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ABSTRACT We present in this paper the surface derivatization of polyanhydrides with triethylamine for labeling the hydroxyl group. The reaction was verified with diffuse reflectance FTIR spectroscopy. Surface derivatization of poly(fumaric-co-sebacic)anhydride of 50:50 and 20:80 compositions was performed and ESCA measurements showed the surface nitrogen concentration in good agreement with stoichiometry values. INTRODUCTION Polyanhydrides have shown strong potential as support materials for controlled drug release devices and for tissue regeneration.lx Water labile linkages in the backbone of polyanhydrides make this class of polymers desirable for controlled drug releaser' since the anhydride bond undergoes hydrolysis in the physiological environment. The hydrolytic reaction rate can be adjusted over three orders of magnitude by changing the backbone structure.9 The degradation produces diacids and shows favorable biocompatibility in tissue response and toxicological studies."' The degradation reaction could occur both on the surface and in the bulk phase of the device." Depending on the particular application, each might be desirable. The hydrolysis mechanisms depend on both the polymer structure and the material morphology. Electron Spectroscopy for Chemical Analysis (ESCA) (or X-ray Photoelectron Spectroscopy) has been widely used in surface chemical composition analysis, and proven to be a powerful tool in surface chemical structure analysis of biodegradable polymers.7' 2' In the study of surface analysis of polyanhydrides, the hydroxyl group, which is the product of the hydrolysis of backbone anhydride linkages, can not be differentiated by ESCA from the backbone oxygen and the carbonyl oxygen. This is because the core-level binding energy (BE) shifts are not large enough. The strong element sensitivity factor of oxygen and small chemical shifts of oxygen in different functional groups lead to strong overlap in oxygen BE peaks. Derivatization reactions for surface analysis have been developed as a means to overcome this problem by tagging different functional groups in polymer molecules."2 -4 In order for ESCA to provide an accurate estimate of the relative concentration of the functional groups, the derivatization reaction must be highly selective and quantitative with good reproducibility. The selectivity in the derivatization reactions for functional groups in polyanhydrides is extremely important since the anhydride bond is chemically more reactive than carboxylic acid". Trifluoroacetic anhydride (TFAA) had been routinely used to label the hydroxyl oxygen in oxygen containing multi-functional group in polymers until it was found that TFAA shows a comparable reactivity with epoxide groups as with hydroxyl groups." Trifluoroethanol (TFE) derivatization was developed to tag carboxyl groups specifically in multi oxygen containing functional groups." Our study shows that when anhydride bond is the backbone linkage, it is not safe to tag the carboxyl group with TFAA or TFE since the anhydride bond could break
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