On the Mechanism of Electron Beam Radiation-Induced Modification of Poly(lactic acid) for Applications in Biodegradable Food Packaging

Abstract

Poly(lactic acid) (PLA) is a biodegradable polymer used for food packaging. The effects of electron beam radiation on the chemical and physical properties of amorphous PLA were studied. In this study, amorphous, racemic PLA was irradiated at doses of 5, 10, 15, and 20 kGy in the absence of oxygen. Utilizing electron paramagnetic resonance spectrometry, it was found that alkoxyl radicals are initially formed as a result of C-O-C bond scissions on the backbone of the PLA. The dominant radiation mechanism was determined to be H-abstraction by alkoxyl radicals to form C-centered radicals. The C-centered radicals undergo a subsequent peroxidation reaction with oxygen. The gel permeation chromatography (GPC) results indicate reduction in polymer molecular mass. The differential scanning calorimetry and X-ray diffraction results showed a subtle increase in crystallinity of the irradiated PLA. Water vapor transmission rates were unaffected by irradiation. In conclusion, these results support that irradiated PLA is a suitable material for applications in irradiation of food packaging, including food sterilization and biodegradation.

Electron-Induced Alteration of PLA Chain Structure

“…In other words, irradiated PLA has a smaller area of cold crystallization peak than nonirradiated neat PLA is a reflection of cross-linking occurrences during irradiation. This is important evidence of cross-link induction, while chain scission normally results in the opposite phenomenon, i.e., an obvious increase can be seen in the area of the cold crystallization peak in EB-degraded polymers. ,− Additionally, in Figure c, it can be seen that in the second heating scan, the nonirradiated neat PLAs only have one melting peak, while the irradiated PLAs have double melting peaks. However, the reason for this phenomenon is unclear.…”

“…Due to the increased chain mobility above T g , a rearrangement to an appropriate spatial conformation ( 16) can take place, and this radical type can recombine with a radical (17). As a result, a SCB will be obtained by such a radical termination reaction, structure (18).…”

“…Based on the above results and discussion, two mechanisms for the complex system that contained irradiation-induced chain scission, cross-linking, and LCB in additive-free PLA were proposed: (I) hydrogen abstraction and radical transfer mechanism and (II) radical disproportionation and addition mechanism, Figure a–e. Initially, accelerated electrons interact with the PLA polymer chains and generate radicals through bond cleaving at four different sites: ①, ②, ③, and ④, resulting in structures (1)–(4), − , Figure a. These radicals are highly reactive species that will be transformed into a thermodynamically stable state by various chemical reactions.…”

“…Since most researchers obtained cross-linked PLA by irradiation with additives as cross-linking promoters (e.g., functional monomers), the proposed mechanisms for irradiation-induced cross-linking of PLA involved such additives. In the case of additive-free PLA irradiation, most literature only discussed the mechanism at room temperature, and due to PLA primarily undergoing degradation at temperatures below T g , chain scission reactions are discussed in detail in that literature. − Some researchers expected the irradiation-induced cross-linking of PLA to be based on a direct H-type cross-linking, in which two chains are linked in the middle by one covalent bond . As far as we know, no research has discussed the EB-induced cross-linking mechanism for PLA irradiation without using a cross-linking agent at elevated temperatures (above T g ).…”

Effect of nanoclay and nano-calcium carbonate content on the properties of polybutylene succinate/nanoparticle composites

“…Increased chain scission increases the polymer's WVTR due to reduced tortuosity. 37 Although the added nanoCaCO 3 increased crystallinity and tortuosity of the water diffusion path, it may not be adequate to counter the effect the CaCO 3 high porosity and PBS chain scission. Furthermore, considering the different WVTR trends for the added nanoclay with a nanoplatelet geometry and nanoCaCO 3 with a cubic geometry, the nanofiller shape may influence the water barrier properties.…”