Electron beam irradiation effects on poly(ethylene terephthalate)
Introduction
There are several studies on the effect of radiation such as fast electron, gamma ray, and low-energy ions on poly(ethylene terephthalate) (PET); Buttafava et al. (2002) studied the effects of gamma irradiation on PET by positron annihilation lifetime spectroscopy at doses up to 1.1 MGy and dose rate of 1.08 kGy/h. Recently high-energy ion irradiation of polymers has attracted more interest (Wong et al., 2001; Liu et al., 2000), but there are no, to our knowledge, papers on the study of radiochemical yield degradation in the electron beam irradiation of PET at the high dose rate of 1.65 MGy/h under different conditions.
Radiation-modified PET can be applied in many fields, including biomedicine as well as to improve adherence for metalized polymers (Bodino et al., 1994).
In the present paper, we focus on the changes of molecular weight of PET irradiated by electron beam at high doses and dose rate, under different conditions of pressure, radiation atmosphere, and temperature, and the determination of radiochemical yield of scission. PET is a polymer which shows a good resistance to radiation due to the aromatic groups. It is known that 1.5 MGy produces noticeable changes in PET and severe damage is achieved at doses higher than 30 MGy (Woods and Pikaev, 1993). In this paper radiation effects of PET at doses from 0.2 to 15 MGy were studied, and samples were characterized by differential scanning calorimetry (DSC), contact angle, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and molecular weight.
Section snippets
Experimental
Pellets of commercial PET with molecular weight (Mv) of 103,600 were irradiated under nitrogen atmosphere in a sealed polyethylene bag at doses from 0.2 to 15 MGy. PET samples of compressed disks of 1×0.2 cm cylinders, in a pressure range of 0–15 t/cm2, were irradiated at 0.5 MGy, and PET films, from God Fellow Co., biaxially oriented, 0.25 mm thick, with a molecular weight of 52,200 were irradiated at doses from 0.2 to 15 MGy, at temperature range of 22–80 °C. All samples were irradiated in a Van de
Results and discussion
Changes in molecular weight with radiation dose in PET films and pellets, at room temperature, are shown in Fig. 1. Scission is the main effect of the electron beam irradiation of PET with a maximum degradation at 5 MGy. After this dose, the molecular weight increases again, by recombination and branching of the macromolecular fragments of scission, but without gel formation at the maximum radiation dose studied. For pressures used to form disk samples up to 4 t/cm2, the molecular weight after
Conclusions
The molecular weight of PET samples decreases with dose up to 5 MGy, because the chain scission is the predominant effect. After this dose an increase with dose was observed until 15 MGy. In the range of 5–15 MGy several processes could occur, including chain scission, oxidative degradation, radiolysis, and crosslinking. Further studies must be done to understand the reactions, which take place after 5 MGy. Crystallinity decreases with increase in dose because of the formation of a branching
Acknowledgments
The authors thank Araceli Ordoñez from IIM UNAM and M Vásquez from IF UNAM for their technical support.
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