M.S. Jahan

Measurements of free radicals over a period of 4.5 years in gamma-irradiated ultra-high molecular weight polyethylene

Abstract Gamma irradiation-induced free radicals have been investigated for 4.5 years in medical grade ultra-high molecular weight polyethylene resins, GUR 4120, GUR 4150 and Himont 1900, at room temperature (23 °C) in absence (vacuum) or presence of oxygen (air), and at 75 °C in vacuum. In vacuum, the primary radicals (predominantly allyl) decayed following Y=YR+ae−bt and, in 30 days at 75 °C or 100 days at 23 °C, their concentration was reduced from ∼1017 radical/g (initial) to a minimum YR∼1015 radical/g and remained at this level as long as the samples were kept in vacuum. When the samples were exposed to air, at any time during this study, these residual radicals were found to decay to a very stable, polyenyle-type terminal radical.

Observation of a non-radical intermediate in the oxidation pathway of free radicals in gamma-irradiated medical grade polyethylene

Abstract Free-radical-depth profile of a large cube (side, 25.4 mm) of UHMWPE was determined following gamma irradiation (25 kGy, 60Co) in air and subsequent aging for 120 days in ringer solution, bovine serum or ethyl alcohol at 37 °C, or in air at room temperature (RT). The initial electron spin resonance (ESR) data showed the presence of predominantly allyl radicals in the core specimens (>6 mm from the surface), alkoxy (PO ) or peroxy ( PO 2 ) in the surface (1–3 mm from the surface), but none or very weak radicals in the sub-surface region (3–5 mm from the surface). Ten months later, the same samples, including those in the sub-surface region, produced identical surface radicals. The results of this study led to the conclusion that the transient non-radical species observed, for the first time by ESR technique, could be the well-known hydroperoxy (POOH) group.

Structural changes of ultra-high molecular weight polyethylene exposed to X-ray flux in X-ray photoelectron spectroscopy detected by valence band and electron spin resonance spectroscopy

The effect of X-ray flux in an X-ray photoelectron spectroscopy (XPS) instrument on the chemical structure of ultra-high molecular weight polyethylene (UHMWPE) has been examined. The UHMWPE samples were exposed in vacuum to radiation from both a standard (non-monochromatic) source and a monochromatic source. For samples exposed to the standard source for up to 5 h, we observed very little change in the core level spectra but observed significant changes in the valence band (VB) spectra. We also observed the production of free radicals with an electron spin resonance (ESR) spectrometer which confirm radiation-induced structural changes and which correspond to the VB spectral changes. For samples exposed to the monochromatic source for up to 18 h, we see changes similar to the standard source, and very little free radical production compared to the standard source. Our results show: (1) that structural changes occur in polyethylene under X-irradiation with energies as low as those in the XPS. These structural changes are initially free radicals and lead to structural changes. (2) the structural changes cause very small changes in core level spectra, and (3) structural changes cause relatively large, easily identifiable VB spectral changes, which increase along with the free radical concentration as a function of exposure time. VB spectra can be an important indicator of radiation damage in purely hydrocarbon polymers.

Effect of X irradiation on optical properties of Teflon-AF

Abstract Radiation effects in optical-grade amorphous fluoropolymer, Teflon-AF, is investigated by UV-visible absorption and electron spin resonance (ESR) measurements. When irradiated with low-energy (40 kVp) X-rays at room temperature in air, Teflon-AF is found to develop a broad, structureless UV-absorption band in the wavelength interval 200–350 nm. While the UV absorption increases as a function of X-ray dose, with relative rates of approx 2 × 10-5 Gy-1 (1 × 10-5 Gy-1) in Teflon-AF 1600 (Teflon-AF 2400), its optical transparency for a given dose of 67.5 kGy, however, remains unaffected. Additional measurements conducted using electron spin resonance (ESR) technique reveal that the observed UV absorption is caused by the X-ray induced peroxy radical (POO . ). The results also suggest that the inclusion of dioxole monomer in the PTFE chain not only improves the optical clarity of Teflon-AF, as reported, but also increases its radiation tolerance. During a post-irradiation storage in air at RT for about 30 days the peroxy radical is observed to decay, with a concomitant decrease in UV absorption. A tentative model is proposed to explain the radiation damage and recovery mechanisms.

Electron spin resonance study of oxidation in X-irradiated poly(ester urethane) containing nitroplasticizer

Abstract The effect of oxidation on X-irradiated Estane®5703 containing nitroplasticizer (NP) has been examined by electron spin resonance (ESR) spectroscopy, and the results are compared to similar data previously obtained on pristine Estane®5703. Although both specimens exhibit similar spectra immediately following X-irradiation, their decay upon exposure to air is quite different. The free radical concentration of the pristine specimen continuously decreases with time whereas the NP sample exhibits an initial decrease followed by a significant increase due to the growth of a newly-formed radical. Terminal species of the pristine and NP-Estane®5703 samples are identified as peroxy and nitroxide radicals, respectively. Hyperfine coupling constants and g-values are extracted for the nitroxide radical and a tentative model is proposed to explain the reaction pathway leading to its production.

Recovery of radiation-damaged plastic light-guide materials

Abstract Radiation damage and subsequent recovery of PMMA-based and amorphous fluoropolymer (Teflon-AF) light guides (LG) were studied using uv-visible absorption, ESR, and thermally stimulated luminescence (TSL) techniques. No appreciable decay of the γ-ray-induced 420-nm band of the PMMA-based LG was observed in air at room temperature (RT) within a week after irradiation, while it was found to be annealable by isothermal heating at temperatures varying between 40 and 100°C or by heating in a microwave oven. Emission of light was also observed during the isothermal annealing of the LG. X- and γ-irradiated Teflon-AF showed a broad absorption band spreading from 200 to 350 nm with no observable degradation of its optical clarity. In conjunction with ESR measurements the uv absorption was attributed to the radiation-induced peroxy radicals formed at the polytetrafluoroethylene (PTFE) site of the main copolymer chain. The recovery of the Teflon-AF was obtained in a few days by post-irradiation storage in air at room temperature. However, a rapid recovery could be obtained by heating at higher temperatures (RT≤T≤95°C) as suggested by TSL result.

Investigation of long-lived free radicals in shelf-aged UHMWPE tibial liners

UHMWPE used in total joint arthroplasty has commonly been sterilized by gamma radiation. Irradiation and subsequent aging in air reportedly cause extensive oxidation and, thus, adversely affect this material. Long-lived free radicals in gamma-sterilized/shelf-aged UHMWPE tibial inserts were investigated in this study. Significant concentrations of peroxy and primary radicals were found in the surface layer and the core region, respectively. Based on this study, it is expected that ir radiation of UHMWPE in an inert environment would result in long-lived primary radicals, which could be a potential cause of long-term oxidation.

A study of free radicals in irradiated/aged UHMWPE materials

UHMWPE bar stock, resin powder and fibers that were irradiated with an electron beam and then stored in air for three years were investigated for long-lived free radicals. Significant concentrations of free radicals were found in all UHMWPE materials. Peroxy radicals are predominant in the UHMWPE materials that are highly accessible to oxygen, while more primary radicals remain in the UHMWPE materials that are isolated from oxygen. Based on this study, it is expected that irradiation of UHMWPE in an inert environment would result in long-lived primary radicals, which could be a potential cause of long-term oxidation. Furthermore, a highly crystalline UHMWPE may be more susceptible to long-term oxidation as more free radicals can be trapped in crystalline regions.

Sterilization condition of UHMWPE is measured by thermoluminescence technique

A thermoluminescence (TL) technique, which is simple but fast and sensitive, was used as a diagnostic tool to test short-term or initial (immediately following sterilization) oxidation of UHMWPE. More than 50% radical species decayed or oxidized in the first 10 min in an oxygen environment and in 25 min in a nitrogen environment. The glow-peak temperatures 130/spl deg/C and 160/spl deg/C of the TL glow curve were also found to be associated with the sterilization conditions, nitrogen and oxygen, respectively.