Random degradation, kinetics

The random degradation and reaction kinetics of high-molecular weight polymers can be determined by an approximate integral method (Ozawa 1965, Hirose and Hatakeyama 1986). Generally, the fractional weight, W, of a reacting material can be expressed as a function of the fraction of a structural quantity which is represented by x, i.e.,... 

All these data can be interpreted by a free radical mechanism including random initiation and inter- and intramolecular transfer [111]. The shape of the curve of rate of volatilization plotted as a function of percentage volatilization is in agreement with this kinetic scheme. Theoretical calculations (section 3) have in fact shown that, for polymers undergoing random degradation, the maximum rate of weight loss has to correspond to 26% conversion and not to the 10—20% observed for polymethylacrylate. The position of the maximum is, however, sensitive to chain transfer. 

Isothermal thermogravimetric measurements were made at 268,280,290 and 299 °C. The isothermal rate of volatilisation, da/dt, was calculated numerically as a function of conversion. A maximum in the rate of volatilisation is observed at 28 % conversion, which compares favourably with the 26% predicted by Simha and co-workers [3] and by Boyd [4] for random initiated degradation. Hence, a kinetic model based on random degradation was considered. 

Batycky et al. (1997) adopted population balances along with a pseudo-first order degradation kinetics to describe the behavior of eroding microparticles. The kinetic mechanism includes both random chain scission and chain-end scission. The change in matrix porosity is accounted for by considering the coalescence of small pores caused by the breakage of polymer chains. 

---