Polyglycolide: degradation and drug release. Part I: Changes in morphologyduring degradation

The changing morphology of quenched polyglycolide (PGA) is investigated dur ing hydrolytic degradation in phosphate buffered saline at pH 7.4. Analysis techniques include small and wide-angle X-ray scattering (SAXS and WAXS), mass measurements, DSC, pH measurement and UV-spectrophotometry. It is post ulated that the degradation process can be separated into four distinct sta ges. In stage I, water diffuses quickly into the sample. During stage II, t he polymer crystallizes by insertion crystallization, whilst the molecular weight gradually falls. This stage is characterized by a dramatic fall in t he long period together with an increase in the crystallinity, minimal mass loss and minimal water uptake. At the onset of stage III, at around 10 day s, a critical molecular weight is reached. Degradation products are now sma ll enough to diffuse from the surface of the sample which begins to swell, water diffuses into the space created, and the crystals are freed from cons traint. A co-operation between degradation products diffusing out of the sa mple and the water diffusing in causes "reaction-erosion" fronts to develop inside the sample. Ahead of these fronts, the trapped acidic degradation p roducts remain to catalyze the hydrolysis. Stage III is characterized by sw elling and an increase in the long period, together with mass loss and furt her water uptake. It is postulated that these reaction-erosion fronts move through the sample and meet in the centre at the beginning of stage IV, at which point the degradation again becomes homogeneous throughout the sample . (C) 2001 Kluwer Academic Publishers.