The influence of the initial preparation and crystallinity on the free radical evolution in gamma irradiated PLLA

Abstract

Poly-L-lactide (PLLA) is a well-known biodegradable and biocompatible semi-crystalline polymer, used in a wide variety of applications, from implantable medical devices and drug release matrices to environmentally friendly packaging materials: diversity in the initial preparation, morphology and crystallinity plays a significant role in most of these applications. On the other hand, gamma radiation, as a processing tool, has often been used for the sterilisation of sensitive polymeric materials. This study presents the influence of the initial preparation and crystallinity on the gamma radiation-induced evolution of free radicals in PLLA. For this purpose, PLLA samples with a large variation in crystallinity (below 20% and over 70%), prepared by two different procedures, were exposed to gamma radiation in air to absorbed doses of 25, 50 and 100 kGy. The annealing treatment was applied to part of the samples, too. The presence and evolution of free radicals were followed using electron spin resonance (ESR) spectroscopy for three weeks. Further characterisation was performed by scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and wide angle X-ray diffraction (WAXD) measurements. Presented results show that depending on the initial preparation conditions, the radiation-induced changes in structure and properties of PLLA, as well as the evolution of free radicals, can differ significantly. Furthermore, the annealing treatment substantially reduces the concentration of long-lived free radicals, but can also introduce significant crystallisation. (C) 2011 Elsevier Ltd. All rights reserved.