In p revious work on t he t hermal degradation of polystyrene of average molec ular weight of 230,000, carried out in a vac uum in the temperature range 318 to 348 C, t he rate curves exhibited distinct maximums when percentage loss of sample per minute was plotted as a function of per ce ntage volatili zation. These maximums correspond to a volatilization of abo ut 35 to 45 percent and te nd to flatten with decreased temperature of pyrolysis. A similar stud y of rates of thermal d eg radation at 307.5 C has now been made on two groups of polymers: (1) low molec ular weight, 24,000; 51,000 ; and 66,000 ; and (2) high molecular weight, 2,000,000 ; 2,250,000 ; and 5, 000,000. Whole polymers and also fractions of narrowrange molec ular weig ht were used. The rate curves for t he 24, 000 and 51,000 molecularweight samples exhibit very high initial rates, but no maximums; the 66,000 sample showed a maximum at 45 percent volatilization. The rate curves for the 2,000,000, 2,250,000, and 5,000,000 molecul ar-weight samples exhibit a gradual rise up to about 25 percent volatilizat ion; t hen, instead of formin g maximums, they follow plateaus to about 50 percent volatilizatio n. These plateaus are indicative of a zero-order reaction in the range that they cover, an d t he rates co rrespond in g to them fit well on t he Arrhenius activation-energy curve obtain ed previo usly for polystyren e samples at hi gher temperatures.
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Madorsky et al. (1962) studied this question.