Radiation-Induced Effects in Gamma-Irradiated PLLA and PCL at Ambient and Dry Ice Temperatures

The effect of gamma-rays sterilization at ambient (23 degrees C) and at dry ice temperatures (-78 degrees C) on two biodegradable semicrystalline polyesters, namely poly(L-lactide) (PLLA) and poly(e-caprolactone) (PCL), was studied. The radical processes generated by irradiation were investigated by electron paramagnetic resonance (EPR) spectroscopy, whereas postradiation changes were monitored for 5 months by scanning calorimetry, thermogravimetric analysis, and apparent viscosity measurements. It was confirmed that the radiation sterilization initiates degradation in both studied polymers, however, the effect is smaller in PCL than in PLLA, and that the range of changes might be limited by lowering temperature of the exposure to gamma-rays. Newly formed oxygen containing functional groups confirmed by EPR and thermogravimetric analysis methods are produced in the irradiated polyesters. In PCL, the process might give rise to thermally induced cross-linking via oxygen containing intermolecular bridges. During storage, in both irradiated materials, morphology determined by differential scanning calorimetry is changing - PCL ability toward crystallization decreases insignificantly, whereas in PLLA the tendency is opposite - on irradiation a contribution of crystalline phase is growing considerably, from 18 and 19% to 41 and 31% for PLLA irradiated at 23 degrees C and at -78 degrees C, respectively. (c) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 122: 375-383, 2011