Effect of Crosslinking and Doping on Thermoluminescence of U.V.‐Irradiated Polymers

Abstract

The thermoluminescence (TL) of elastomeric materials modified by doping and chemical or ~-induced crosslinking was measured following irradiation with u.v. light at 77 K and compared with the thermally stimulated depolarization currents (TSDC) obtained during warmup of samples previously polarized from 298 to 77 K, stressing the comparison of luminescence and current peaks observed in the glass ~ransition range. Significant.shiftings of the TL peaks were found as a function of degree of crosslinking and nature of the dophnt, even though the glass transition temperature and the position of the TSDC peaks were not affected, showing that the kinetics of the light-emitting process is not only governed by the thermal motions of the polymer units but also critically depends on the nature of traps and luminescent species. Generally speaking, the TL method appears to be much more sensitive than the classical relaxation methods to any structural, morphological, or environmental change of the macromolecular chains and seems, thus, particularly attractive to obtain valuable information on new aspects of polymer relaxation.The thermoluminescence (TL), which involves measuring and analyzing light emitted during warmup of materials previously irradiated at low temperature, is generally considered as a useful means for studying molecular motion and structural transitions occurring in polymeric solids (1-5). On the basis of significant correlations observed between the TL glow curves and the relaxation peaks obtained by mechanical or dielectric loss measurements, it is usually assumed that the luminescence emission depends on the molecular mobility either because the macromolecular chains can act as effective carrier traps (e.g., Via formation of dielectric cavity traps defined by the chains themselves) or because the release of charge carriers from other traps (such as free radicals or neutral molecules with positive electron affinity) can only occur as a result of the onset of local motion involving molecules located in the immediate vicinity of the trap (1-5). As an alternative to the detrapping phenomenon, it has been sometimes postulated that charge recombination can occur by physical approach and reaction of a radical ion with the parent ion or any other neighboring positive ion (3). In principle, such a process should be also closely related to structural changes in the polymeric matrix because it is likely to arise only in a temperature range where large scale motion sets in, i.e., in the vicinity of the glass transition temperature Tg. Whatever the exact trapping and detrapping mechanisms involved and the nature of luminescence centers, the macromolecular motions are, thus, expected to contribute Significantly to the light-emitting processes in such a way that the TL glow curve could be considered as reflecting the relaxation itself and could be used to determine the transition temperatures.We have recently shown, however, that, at least in certain polymers, the type of impurity present in the matrix can atso play a pre...

Visible Organic Thermoluminescence above Room Temperature in Small Molecule Crystals

“…Thermoluminescence in small molecules has garnered limited attention mainly due to its prevalence in low-temperature regimes [14][15][16][17][18][19][20] . Butterfield et al investigated the thermoluminescence of 42 organic small molecules 21 , and found that these molecules exhibited the highest thermoluminescent intensity between 100 K and 240 K when exposed to X-ray (or electron beams) radiation at 77 K. Interestingly, certain n-alkanes (n≥20) exhibit radiothermoluminescence above room temperature after being exposed to γ-ray 22 .…”

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Thermoluminescence (TL) is commonly observed in inorganic luminescent materials at elevated temperatures [1][2][3][4][5] , with applications in radiation dosimetry, archaeology, geological dating, and defect studies [6][7][8][9][10][11] . However, the understanding of TL in organic small molecules above room temperature is still limited [12][13][14][15][16][17][18][19][20][21][22][23][24] . Here, we experimentally discovered and theoretically analyzed the unusual TL exhibited by organic scintillation crystals in air.

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“…Moreover, certain polymers, after being exposed to γ-ray, demonstrate radiothermoluminescence above room temperature, regardless of whether in an inert gas or in atmospheric conditions 15,16 . TL spectra are closely related to sample fluorescence and phosphorescence, which can be attributed to the introduced dopants or natural impurities in the polymer matrix 14,17 . The TL in these polymers involves three processes: electron trapping, thermal release of trapped electrons at various temperatures depending on trap depth, and recombination of electrons/holes at glowing centers to generate excitons that undergo radiative transitions to the ground state [14][15][16][17][18][19] .…”