Thermal degradation of polypropylene

The molecular weight distributions of polypropylene samples subjected to various degrees of thermal degradation were experimentally determined and compared with those expected theoretically for random scission of the polymer chains. The comparison demonstrated that the chain breakage is predominately random and also indicates that current methods of determining molecular weight distributions are adequate for use in evaluating hypothesized degradation mechanisms. In terms of the free radical depolymerization theory, intermolecular transfer of the radical is the dominant process. A curve relating the degree of degradation to the intrinsic viscosity was obtained from calculations based upon a random degradation. With this curve, intrinsic viscosities measured as a function of degradation time could be used to obtain the time dependence of the degree of degradation. The observed degradation rates could not be described by a simple process in which the rate of bond rupture was proportional to the number of bonds present. Neither a similar process involving two bond types nor a dependence of the free radical diffusion rate upon the degree of degradation was by itself a satisfactory basis for interpreting the kinetics. The activation energy estimated for the polypropylene degradation at 250-300°C. was not significantly different from that determined by volatilization measurements at higher temperatures.