Early stages of X-ray induced molecular unit modifications in poly(lactic acid)

Abstract

Exposure of polymers to ionizing radiation and high-energy particle beams is involved in many processes – from food sterilization to radiotherapy protocols. The resulting effects, at the final stages, determine the functional, structural, and morphological alteration of the material. However, the processes responsible for cumulative chain scission, crosslinking, and reactions with the environment are often difficult to disentangle as a sequence of elementary events. Here we report a spectroscopic study – by attenuated-total reflection Fourier transform infrared spectroscopy and powder x-ray diffraction measurements, together with differential scanning calorimetry and thermogravimetric analyses – that sheds light on the very initial stages of the mechanisms involved in radiation-induced modifications of poly(lactic acid) (PLA). The results, collected on samples from biobased processes and exposed to low x-ray doses in the range of 100–103Gy after different thermal treatments, show spectroscopic evidence of the emergence of molecular unit perturbations – mainly related to methyl and carbonyl groups with the formation of C=C double bonds – with a clear-cut dependence on the polymer conformation. Understanding the mechanisms involved and the role of crystallinity provides information potentially useful for the development of PLA with a suitable radiation hardness for applications in radiotherapy.