Zero activation energy, radiation initiation

The zero activation energy for radiation initiation, E., leads to some interesting practical consequences. The overall activation for the rate, E = E + 1/2 E - 1/2 E, where E and E are the activation energies for propagation and termination, respectively. Since E. for most catalytic initiation is about 30 and E close to zero, this leads to a value of about 22 kcal per mole compared x ith about 7 kcal per mole for radiation for v/hich E.=0. The practical advantages of this were referred to earlier. 

These equations are commonly called pyrolysis relations, in reference to the thermal (as opposed to a possibly chemical or photonic) nature of the initiating step(s) in the condensed phase decomposition process. It can be seen that while the second, simpler pyrolysis expression with constant coefficient As) preserves the important Arrhenius exponential temperature dependent term, it ignores the effect of the initial temperature, condensed phase heat release and thermal radiation parameters present in the more comprehensive zero-order pyrolysis relation. These terms To, Qc, and qr) make a significant difference when it comes to sensitivity parameter and unsteady combustion considerations. It is also important to note the factor of 2, which relates the apparent "surface" activation energy Es to the actual "bulk" activation energy Ec, Es- E /1. Failure to recognize this factor of two hindered progress in some cases as attempts were... 

Bulk polymerization of tetrafluoroethene (TFE) by radiation was studied in the gas, liquid, and solid phase over a wide range of temperatures from —196 to 90 °C by a number of methods (e.g. NMR and FTIR spectroscopy). Volkova et al. [710] studied the radiation-induced polymerization in the gas phase from 12 to 90 °C. Different activation energies were found below and above 70 °C. Enslin et al. [711] reported that the rate of polymerization in the gas phase was a zero-order function of the monomer pressure. However, the rate of polymerization was profoundly influenced by the initial monomer pressure (4.6-order dependence) and on the radiation intensity (0.36-order dependence). 

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