The natural logarithm of Deff as a function of the reciprocal
of absolute temperature was plotted in Fig. 7. The result
shows a linear relationship derived from the Arrhenius-type
equation. From the line slope the values of activation
energy can be found to be 24.512 kJ/mol, 23.951 kJ/mol
and 25.492 kJ/mol for the whole falling rate period, the first