Selectivity and Efficiency of Pyrene Attachment to Polyethylene Films by Bombardment with MeV-Range Protons

Abstract

We demonstrate the attachment of pyrene molecules to chains of polyethylene (PE) films using 1-4.5 MeVrange protons (H + ). Despite the enormous energy available for 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 state 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 ∼470 nm, 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.

Unexpected Absorbance Enhancement upon Clustering Dyes in a Polymer Matrix

“…The kinetics of pyrene excimer formation for pyrene-labeled macromolecules are well understood in organic solvents where the Birks scheme, , compartmental analysis, − Fluorescence Blob Model, or Model Free analysis , can be applied depending on the selected macromolecule and mode of pyrene attachment. Pyrene aggregation in organic solvents or aqueous solution has been investigated, and a few mathematical treatments have been proposed to handle the kinetics of excimer formation under such conditions. ,,− In comparison, quantitative analysis of the kinetics of pyrene excimer formation in solid polymer films labeled with pyrene is poorly developed, and studies presenting fluorescence decays of pyrene-labeled polymer matrices provide typically a qualitative interpretation of the sum of exponentials analysis of the decays. ,− This state of affair is probably a result of the absence of long-range chain motions in solid-state samples which prevents diffusive excimer formation. Diffusive excimer formation in solution enables one to couple parts of the monomer decay with the rise time of the excimer decay resulting in retrieval of similar decay times in the sum of exponentials analysis of the monomer and excimer decays.…”

Investigation of Temperature-Dependent Relaxation Processes in Polyethylene Films by Covalently Attached Anthryl Probe

“…See Figure 6 for an explanation of the slope changes and their assignments. range of crystallinities 51 (an indicator of the frequency and length of branches, 62,63 the molecular weight, and the sample history 64 ). For instance, polyethylenes containing few chain branches usually have higher degrees of crystallinity and higher melting points than polyethylenes with higher branching ratios.…”

“…Some of the physical properties and structural characteristics of these and the other films are summarized in Table 1. 51,52 Films with covalently attached groups are differentiated by lumophore type; for example, EXACT/1 is an EXACT-type film containing 9-anthryl groups attached via a methylene tether (i.e., R ) AnCH2in Scheme 1). The NDLDPE/1 films are designated h when [1] ) 9 × 10 -3 mol/kg and l when [1] < 10 -4 mol/kg.…”

Covalently-attached 1-pyrenylmethyl groups as a probe of temperature-dependent relaxation processes in polyethylene films

“…We have also attached 1-pyrenyl (Py-) lumophores to polyethylene chains by irradiating doped pyrene molecules with eV-range (UV/Vis) photons [26,27] or bombarding them with MeV-range neutrons, γ -rays, fast electrons, protons, or alpha particles [28,29]. The temperature-induced variations in integrated emission intensities from these Py-probes correlate well with reported onset temperatures for relaxation processes of PE (data not shown).…”

“…In general, attachment to polymer chains facilitates analyses of luminescence results because translocation of lumophores upon macroscopic perturbations such as temperature changes or film stretching are avoided; doped lumophores can (and frequently do) change their locations within a polymer as a perturbation is applied. The PyCH 2 -attachment technique, irradiation of 1-pyrenyldiazomethane [46,52] is more selective and yields fewer side products than our more commonly used method to introduce covalently-attached Py-groups, direct UV/Vis irradiation [26,27] or bombardment with MeV range particles [28,29] of pyrene molecules.…”