Photothermal initiation

Eggert [24] showed that the amount of light energy (millisecond exposure time to light of a flash lamp) needed to initiate nitrogen iodide decreased linearly with the initial temperature of the irradiated explosive and converges toward zero at the dark ignition temperature of the sample. Similar observations were obtained with lead azide and some other explosives. These results indicate a photothermal initiation mechanism. Roth [24] arrived at the same conclusion. The... 

The theory on light induced thermal explosions is limited. Some theoretical analysis can be found in [27-30]. The photothermal initiation mechanism is complicated because of phase changes, melting, sublimation and vaporation processes, which are not usually taken into account. The same holds for the change of the absorption and reflection coefficient by these processes. 

Photons, therefore, provide the activation energy needed for the initiation of a reaction. Whether or not the drug itself will absorb energy can be deduced from an examination of its UV-VIS spectrum. Since the incident photonic energy may be converted to heat, a photolytic reaction may be accompanied by a thermal reaction, resulting in a photothermal effect. Once a thermal reaction is initiated, it may proceed even in the dark (25). 

Photothermal dehydrochlorination of PVC as studied by derivative ultraviolet absorption and fluorescence analysis techniques has shown that carbonyl and hydroperoxide groups are more important in the initiation steps than... 

It will be clear that little can be said about the character of the initiation process. Photothermal, mechanical and photochemical processes may contribute to the initiation process. Reflection of the shockwave, by the wall of the sample holder, can be important too. Evidence of this to play a role is the successful initiation of mercury fulminate in one sample holder, while it cannot be initiated in another, with the irradiation conditions being the same. 

The thermal degradation of polypropylene is accelerated by 253.7 nm radiation (photothermal degradation), and methane and ethane appear as additional products [822] these are associated with photoinitiation of the radical process involved, as the mechanisms of the thermal and photothermal reactions are almost identical. The different overall activation energies of the two reactions, 50.1 and 11.7kcal/mol, respectively, may be associated with the different initiation steps. 

The use of soluble precursor-polymers, which can subsequently be converted to a conjugated form, offers a route to pure materials with controlled morphology. It has been known for many years that poly(vinyl chloride) (PVC) can be thermally, chemically and photothermally dehydro-chlorinated to produce polyene (PA) chain sequences. Initiation occurs at random, and continuous conjugated-sequences are not obtained. As a result the maximum conductivity for iodine-doped dehydrochlorinated PVC is ca. 10 Scm" ... 

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