Isothermal recombination luminescence

The first study in which the data on low-temperature ITL have been interpreted in terms of electron tunneling seems to be that by Kieffer et al. [52], In this work the ITL at 4.2, 66, and 77K of y-irradiated methylcy-clohexane solutions containing biphenyl additives (Ph2) has been investigated. The samples were irradiated at the same temperature at which the kinetics of luminescence was studied. The ITL in this system was assumed to be due to the recombination process 

The main argument in favour of the tunneling mechanism of the reaction was the coincidence of the kinetics observed for the ITL process at three different temperatures (4.2, 66, and 77 K) (Fig. 12). Along with the ITL, the y-irradiated solutions of Ph2 in methyl-cyclohexane display two peaks of thermoluminescence at 90 and 95 K. They were accounted for by the existence of two different recombination processes reaction of Ph2 with etr captured at long distances from Ph2+ (peak at 90 K) and reaction of Ph2+ with Ph2 (peak at 95 K) [54]. Both these processes were shown to contribute to 

ITL observed at 77 K. The contribution from the reaction of Ph2 with Ph2 can be isolated by a rapid annealing of etr by irradiating the sample with IR light or by heating the sample for a short time [53], The contribution of this process was found to increase with increase in the Ph2 concentration, reaching virtually 100% at [Ph2] = 10 2 M. Since the diffusion of Ph2 and Ph2 at 77 K in methylcyclohexane seems to be too slow [the diffusion coefficient at 77 K does not exceed 10 26 cm2s 1 (see Chap. 5, Table 1)], the authors assumed the recombination of Ph2 with Ph2 to proceed by a tunneling mechanism. 

the kinetics of the ITL of /i-irradiated vitreous solutions of Ph2 in methylcyclohexane was studied [55] over a much wider time interval (10 6 -103 s). Within this whole time interval the kinetics of ITL was found to obey one and the same hyperbolic law, i.e. eqn. (7) with m 1. These results are difficult to interpret in terms of conventional kinetic models, but are easy to account for in terms of the electron tunneling model. Indeed, as shown in Chap. 4, the drop in the intensity of recombinational luminescence in the case of the tunneling mechanism of recombination obeys the equation 

The kinetics of recombination of the tetramethyl-p-phenylenediamine cation radical TMPD with etr and with the naphthalene anion radical Nh in vitreous squalane was studied in ref. 57. The studies were carried out at temperatures of 77 - 150K in two time ranges 10 4 to 10 1 s and 102 to 10 s. At low temperatures (e.g. at 77 K), for both recombination processes the decay of the luminescence intensity for both time ranges was found to be described by eqn. (7) with m = 1 (see the data for the reaction of TMPDf with et7 in Fig. 13), which is characteristic of the tunneling mechanism of recombination. At higher temperatures, however, the kinetics of the luminescence decay for the reactions with et and Nh turned out to be different. Thus, for example, at 98 K the kinetics for both reactions is described by eqn. (7) as before. But while for the reaction 

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This chapter discusses the intenelation between mechanical properties, molecular mobility and chemical reactivity of curing epoxy-amine thermosets, illustrated by examples of how the charge recombination luminescence (CRL), heat-capacity and rate constants of chemical reactions are influenced by gelation and vitrification during isothermal cure. A comparison of dynamic mechanical, CRL and modulated temperature DSC data shows that vitrification is accompanied by an increase in CRL and a decrease in heat-capacity, and that the heat-capacity and CRL continue to change after the viscoelastic properties have levelled out. It is also shown how the rate constant of an intermolecular secondary amine reaction, measured by near infirared spectroscopy, is sensitive to gelation, whereas the intramolecular rate constant instead is sensitive to vitrification. 

In this chapter the interrelation between mechanical properties, molecular mobility and chemical reactivity is discussed. Examples of how the changes in charge recombination luminescence, heat capacity and rate constants of chemical reactions can be related to the evolution of viscoelastic properties and the transitions encountered during isothermal cure of thermosetting materials are given. The possible application of the experimental techniques involved to in-situ cure process monitoring is also reviewed. 

Charge recombination luminescence was measured in a set-up described in detail elsewhere (75). Stoichiometric mixtures of DGEBF and DDM in aluminium pans were taken to the cure tenq)erature at 15 G/min and cured isothermally under nitrogen in a chamber covered by a quartz window. The sanq>le was intermittently irradiated with a Kulzer Duralex UV-3(X) fibre optic wand for 60 s. After each irradiation the shutter of the photomultiplier was opened with a delay of 5 s, and the initial intensity of emitted light, 7o, was recorded. 

Figure 3. Charge-recombination luminescence intensity during isothermal cure of DGEBF/DDM at three diffoent temperatures.

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