Muhammad Shah Jahan

The effect of high dose of gamma-irradiation on residual radicals concentration in ultra-high molecular weight polyethylene (UHMWPE) in the presence of vitamin E

Powder samples of UHMWPE (GUR 1020) containing 0.1% by wt. vitamin E (α-tocopherol, α-T) were irradiated at room temperature in air for doses of 30-kGy, 65-kGy or 100-kGy (60Co). After irradiation, they were stored at −78.5°C (dry ice temperature) for one year and then opened to air at room temperature. Following the decay of the primary alkyl and allyl radicals (at room temperature in air), growth of the carbon-centered polyenyl R1 (−·CH⁅CH=CH⁆, m ≥ 3), and the oxygen-centered dior tri-enyl R2 (−·OCH⁅CH=CH⁆m, m ≤ 3) residual radicals were measured for eight weeks. An X-band electron spin resonance (ESR) spectrometer was used for radical measurements. The initial relative radical concentrations (R2/R1) were found to be 10.13, 4.6 and 3.7 for the 65-kGy, 30-kGy and 100-kGy samples, respectively. R1 and R2 were both found to grow significantly in the 65-kGy samples while they grew only slightly in the 30-kGy and 100-kGy samples. In 65-kGy sample, R1 grew faster than R2 and the relative concentration R2/R1 was reduced from 10.13 to 2.9 for the 65-kGy sample while that for the 30-kGy and 100-kGy samples reduced only slightly, from 4.6 to 3.5 and 3.7 to 3.2, respectively. The behavior of the residual radicals can be explained by Raman spectroscopic data which suggest that the 65-kGy samples had a higher percentage of amorphous regions when compared to the 30-kGy or 100-kGy (21.7% compared to 15.7% or 17.9%) and also suggest a lower percentage of inter-phase regions (16.4% compared to 25.6% or 17.5%) and a lower level of structural disorder (0.26% compared to 0.44% or 0.27%).

ESR Insights into Macroradicals in UHMWPE

Electron spin resonance (ESR) is a technique that can directly detect and quantify unpaired or odd electrons in atomic or molecular systems. The materials that contain unpaired electrons are known as paramagnetic materials since they exhibit a net magnetic moment in an external magnetic field. For this reason, ESR is also known as electron paramagnetic resonance (EPR). Although both names are used in practice, a third name has been introduced to replace them. In line with nuclear magnetic resonance (NMR), the new name is electron magnetic resonance (EMR) since the magnetic resonance in ESR/EPR results from the electron magnetic moment. In this chapter, the term ESR is used.

On the Structural Analysis of -Ray Induced Primary Free Radicals in UHMWPE and Vitamin E Stabilized UHMWPE by ESR Spectroscopy

Oriented allyl radicals are detected at room temperature in gamma irradiated UHMWPE. The effects of vitamin E and storage at room temperature on this oriented structure are also investigated during the study. While testing powder as well as compression-molded solids, with or without vitamin E, a typical ESR spectrum was recorded at room temperature following 100 kGy gamma dose and subsequent storage at −78.5°C for one year. The simulated results show that the relative abundance of 5% alkyl, 68% allyl, and 27% polyenyl produced a 98.7% best fit of experimental ones. Furthermore, the allyl radical signal gives approximately 20% of random orientations and 80% of oriented molecules. In oriented PE, measured at −196°C, Ohnishi et al. (1916) observed 25 lines within a total magnetic field width of approximately 133 G. Our spectra also show 25 lines spread over 136 G in UHMWPE powder samples and at room temperature after one year of storage.

Thermally Stimulated Luminescence in Powdered Soy Proteins

Heating powder isolated soy proteins (ISPs) in a N₂ environment produced thermally stimulated luminescence (TSL), in 2 major temperature regions, 50 to 250°C (region R1) and 250 to 350°C (region R2). In soy protein 7S fraction, strong TSL was detected in both regions with glow peak maximum (T(m)) at 150 ± 15°C and at 300 ± 10°C. Two additional satellite or shoulder peaks were detected from the ISP and 7S protein fraction within region R1 at T(m) = 90°C and T(m) = 210°C. The soy protein 11S fraction produced a broad, poorly defined TSL peak in the low-temperature region. Electron paramagnetic resonance spectroscopy data from the control ISP sample, deuterium sulfide-treated ISP, ISP stored in either N₂ or O₂, and defatted soy flour, indicated that the trapped radicals present in ISP is associated with the production of the primary TSL peak at 150 ± 15°C. Activation energies required to release the trapped charges (for luminescence to occur) are approximately 0.70, 0.78, 1.50, and 1.8 eV for TSL at Tm = 100, 150, 200, and 300°C, respectively. The reaction mechanism that leads to the release of the trapped charges for TSL to occur followed a mixed order kinetic, between 1.5 and 1.8. The frequency factor varied between 10⁷/s and 10¹⁷/s.