Atomistic modeling of the formation of radical pairs in irradiated amorphous poly(propylene)

Radiation-induced radical-pair formation has been modeled in a simple atomi stic study, utilizing Molecular Simulations Inc., (MSI) amorphous macrocell s, containing the three poly(propylene) tacticities; isotactic (i-PP), synd iotactic (s-PP), and atactic (a-PP). For all three tacticities, the ratios of secondary "partner" radical probabilities, generated following hydrogen atom ejection from the primary radical site, were found to deviate only a l ittle from the availability of hydrogen atoms of the PP "a"-, "b"-, and "c" -type carbons (-(CH2CHCH3)-H-a-H-b-H-c-). It is demonstrated that b-type pa rtners have the lowest probability of being formed, and therefore, b-type p rimary radical initiation is required to produce the ESR spectra generally observed, which is predominantly composed of b-type radicals. It is reasone d that the hydrogen atom on the b-carbon is most easily ejected, either fol lowing ion-molecule-electron recombination, or as proton ejection. Radical- pair distance distribution functions demonstrate that most partners are for med on immediately adjacent chains, because ejected hydrogen atoms or proto ns have little possibility to travel much further. Mean radical-pair distan ces were found not to differ greatly at rho approximate to 0.90 x 10(3) kg m(-3), being 6.15, 6.20, and 6.26 Angstrom, from b-type radical initiation in i-PP, s-PP, and a-PP, respectively. Density variation had only a small e ffect on the mean radical-pair distance in i-PP, except when the polymer wa s completely amorphous, a condition which never occurs in reality. Similarl y, only small mean radical-pair distance variations are expected in s-PP an d a-PP, over the range of densities normally encountered.