EFFECT OF HYDROLYTIC DEGRADATION ON THE MICROSTRUCTURE OF POLY(GLYCOLIC ACID) - AN X-RAY-SCATTERING AND ULTRAVIOLET SPECTROPHOTOMETRY STUDYOF WET SAMPLES ULTRAVIOLET

The effect of in vitro hydrolytic degradation on the microstructure of unoriented semicrystalline poly(glycolic acid) (PGA) was examined usi ng simultaneous small-and wide-angle synchroton x-ray scattering (SAXS /WAXS) and ultraviolet (UV) spectrophotometry. Samples were degraded i n buffer solutions at 37 degrees C and were examined wet to avoid the structural changes that occur on drying. During degradation, the cryst al density remained constant, and little change was seen in the latera l extent of the crystal lamellae. The transition layer between the cry stalline and amorphous phases roughened slightly. More dramatic change s were seen in the amorphous phase, resulting in sharp increases in th e crystallinity, the amount of glycolic acid in the buffer solution, a nd in the density difference between the crystal lamellae and the laye rs separating them. These changes indicated a loss of amorphous materi al that leveled off after 30 days. The lamellar repeat distance fell f rom around 95 to 80 Angstrom in the first 20 days before slowly rising again towards its initial value, changes which are interpreted as ref lecting a two-stage loss of amorphous material, in which highly coiled loops and tie chains are degraded faster than taut tie chains. Once t he coiled material is removed, the taut chains are able to adopt entro pically favorable conformations, pulling the crystals towards each oth er, lowering the lamellar repeat, and creating internal stresses withi n the spherulite. As these newly coiled chains degrade, the crystals a re released and slowly separate. The changes in long period are also c onsidered in the light of chemical changes during degradation. Such ch ange of chemical environment could affect the affinity for water of th e amorphous inter-lamellar regions and affect the swelling. The observ ed changes in the long period may be a consequence of either or both o f these factors. These findings give microstructural information of im portance in the prediction and control of mechanical properties during degradation and the diffusivity of other molecules through degrading PGA. (C) 1997 John Wiley & Sons, Inc.