Irradiation of polyethylene oxide and polypropylene

The solubility behavior of electron‐irradiated ( in vacuo ) polyethylene oxide indicates a ratio of main‐chain scission density to the density of crosslinked units of 0.6, independent of molecular weight. This value, derived from measurements at high doses, correlates well with the observed increase in intrinsic viscosity and invariance of number‐average molecular weight at low irradiation doses. Analogous measurements on polypropylene indicate a linear dependence of S + S 1/2 (where S is the sol fraction of the irradiated polymer) on the reciprocal irradiation dose. This treatment leads to a ratio of scission‐to‐crosslinking of 0.8. Increased scission in polypropylene as compared to polyethylene oxide is evidenced by a gradual decrease in the intrinsic viscosity as a function of dose. This decrease is not indicative of an initial irradiation degradation of the polymer, because the number‐average molecular weight is relatively unaffected by irradiation until quite close to the gel point. The intrinsic viscosity of the progressively crosslinked polymer is apparently depressed by branching. Direct comparison of the results on polypropylene with theory is uncertain because of a nonrandom initial molecular weight distribution. It is reemphasized that the intrinsic viscosity is not a simple indication of molecular weight changes in crosslinking polymers.