Thermal chemiluminescence from γ-irradiated polytetrafluoroethylene and its emission mechanism: Investigation by multichannel Fourier-transform luminescence spectroscopy
Introduction
Thermal chemiluminescence (TCL) measurement is a powerful technique to probe the thermal oxidative degradation of hydrocarbon polymers and γ-irradiated polymers [1], [2], [3], [4]. In most previous studies, only the total emission intensity, rather than the spectrum, was measured and analyzed. As a result, a variety of thermal oxidation reactions which lead to chemiluminescence emission were proposed [5], [6] but could not be clearly established. We have recently developed a new spectrometer using a CCD sensor and an interferometer composed of a Savart plate sandwiched between two linear polarizers and a quartz lens [7], [8], and reported time-dependent thermal TCL spectral changes in polyamides [9], γ-irradiated elastomers [10] and polypropylene (PP) heated in dry air [11], [12]. However, even with the spectral information, some emission mechanisms have been difficult to confirm because of the complex time evolution of the spectral profiles. This originates from thermal oxidative degradation which occurs during the heating required for spectral measurement.
In the present study, we have applied our new spectrometer to the TCL of γ-irradiated polytetrafluoroethylene (PTFE). This polymer has excellent thermal and chemical stability and is widely used in a broad range of applications [13], implying that no chemiluminescence of PTFE is expected to be caused by heating due to autoxidation. However, it is well-known from a number of ESR studies [14], [15], [16], [17], [18], [19], [20], [21], [22] that stable free radicals are easily generated in PTFE by exposure to a high-energy electron beam or γ-irradiation. Some research groups have already studied the effect of γ-irradiation on PTFE by measurement of TCL intensity [23], but their data were insufficient to elucidate the emission mechanism for TCL from PTFE.
Since our spectrometer contains neither a slit nor a grating, very weak emission from heated polymers can be observed as a spectrum. We have also examined the time evolution of the luminescence spectra of γ-irradiated PTFE powder heated isothermally, and analyzed it by a least-squares fitting process using Gaussian functions. From these data, the spectra measured at different temperatures for various different heating times are understandable in terms of only one kind of luminophore. This finding differs from our previous studies on hydrocarbon polymers, such as elastomers [10] and PP [11], [12], for which at least two kinds of luminophores were required to explain the complex time evolution of TCL spectral shapes. In the present letter, with the aid of IR and ESR spectroscopy, we propose a simple luminescence emission mechanism for PTFE, which is free from thermal oxidative degradation during heating because of its high thermal stability. These data are also useful for understanding the more complex emission mechanisms of hydrocarbon polymers.
Section snippets
Sample and γ-irradiation
The γ-irradiated sample was obtained from additive-free PTFE powder (Polyflon) supplied by Daikin Industries. The γ-treatment was carried out in air at room temperature using an irradiator (Nordion, Ottawa, Canada). The activity of the 60Co source was 74 PBq and the administered irradiation dose was 141 kGy, which was estimated by using a PMMA dosimeter (Harwell Dosimeters, Oxfordshire, UK). The spectroscopic experiments were carried out after storage for six months at room temperature. However,
Effect of γ-irradiation
Before measuring the TCL of the γ-irradiated sample, we examined whether a virgin PTFE sample exhibits luminescence by heating isothermally in dry air and in N2. The spectra converted from the data accumulated over the initial 120 s are represented by square symbols in Figure 1, resulting in no luminescence emission, where the red, green and blue symbols represent spectra measured at 200, 180 and 160 °C, respectively. Heating was continued for up to 360 min, but any luminescence emission was
Conclusion
We propose a new TCL emission mechanism (measured in dry air and N2) for additive-free PTFE irradiated with γ rays. Some fluorine atoms in PTFE were dissociated from main chains by γ-irradiation to yield carbon radicals; these carbon radicals combined with oxygen molecules in air during γ-irradiation to produce peroxy radicals. These peroxy radicals are extremely stable and persist in PTFE for a long time in air at room temperature. They exhibited intense signals in ESR spectra. In addition, a
Acknowledgements
The authors thank Daikin Industries for supplying the sample. This work was supported by a JSPS Grant-in-Aid for Scientific Research (C), Grant number 24550012.
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Thermal chemiluminescence from γ-irradiated polytetrafluoroethylene and its emission mechanism: Kinetic analysis and bond dissociation energy of fluoroperoxide group
2014, Chemical Physics LettersCitation Excerpt :On the other hand, the k2 constant, which represents the rate of a radical abstraction through electron rearrangements to produce an electronically excited carbonyl group by elimination of a fluorine atom from a main chain of PTFE, is almost constant, as expected. We reported in Ref. [7] that the sum of TCL intensities at each heating time, which is equivalent to the relative amount of TCL emission, was found to be nearly constant at each heating temperature. The sum can be calculated by integration of Eq. (9) between 0 and infinite time, resulting in A0/k2.
Relationship between crystal structures and thermoluminescence spectra of gamma-irradiated shells of shellfish measured with a multichannel fourier-transform luminescence spectrometer
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O bond in the fluoroperoxide group was determined to be 97 ± 4 kJ mol−1, being consistent with the corresponding value for small fluorocarbon model systems obtained by quantum chemical calculations. This strongly supports the emission mechanism [