Selectivity and efficiency of pyrene attachment to polyethylene films by bombardment with MeV-range protons

We demonstrate the attachment of pyrene molecules to chains of polyethylene (PE) films using 1-4.5 MeV-range protons (H+). Despite the enormous energy available fur reaction, the attachment is exceedingly selective; MeV-range protons lead to photochemistry as selective as that initiated by eV-range photons (>300 nm). The effects of proton kinetic energy, proton dose, polymer crystallinity, and initial pyrene concentration within the films on the selectivity of the attachment process are analyzed on the basis of UV/vis absorption spectroscopy, steady stale and dynamic fluorescence measurements, and differential scanning calorimetry. Attachment selectivity is independent of proton kinetic energy but increases with decreasing proton dose. The efficiency of pyrenyl attachment increases with decreasing polymer crystallinity. In polyethylene of 42 % crystallinity, the efficiency of attachment is only slightly dependent on pyrene dopant concentration (in the range 10(-4)-10(-2) mol/kg) at low doses but increases significantly with concentration at higher doses. Time-resolved fluorescence studies on films subjected to high doses indicate either a drastic change in local environments around attached 1-pyrenyl groups or that the attached groups have suffered secondary transformations. Extensive cross-linking of proton-bombarded PE samples, indicative of C-H and C-C bond scission, was observed in both the absence and presence of pyrene. At all proton energies, films containing 10(-2) mol/kg pyrene and exposed to 10(13) H+/cm(2), the highest fluence employed, exhibited a pronounced, broad emission band centered at similar to470 net, attributed to excimer. Dissolution of these samples led to loss of the excimer band, indicating that the aggregated pyrenyl groups are attached to neighboring polyethylene chains. A mechanism for attachment, based on the addition of preformed carbon-centered polymer radicals or cations to ground-state pyrene molecules, is proposed. It is contrasted with the mechanism for attachment initiated by eV-range photons.