Solid-State ¹³C NMR Investigation of the Oxidative Degradation of Selectively Labeled Polypropylene by Thermal Aging and γ-Irradiation

Abstract

Unstabilized polypropylene (PP) films having selective 13 C isotopic labeling were subjected to thermal aging at 50, 80, and 109°C and to γ-irradiation at 24 and 80°C. The oxidized films were examined using solid-state 13 C nuclear magnetic resonance (NMR) spectroscopy. Dramatic differences were found in the type and distribution of oxidation products originating from the three carbon atom sites within the PP macromolecule (tertiary carbon, secondary carbon, and methyl side group). Most of the oxidation products that formed on the polymer chain originated through chemical reactions at the PP tertiary carbons. Under all of the aging conditions examined, tertiary peroxides (from the PP tertiary site) were the most abundant functional group produced. Also originating from the PP tertiary carbon were significant amounts of tertiary alcohols, together with several more minor products that included "chain-end" methyl ketones. No significant amount of peroxides or alcohols associated with the PP secondary carbon sites was detected. A substantial yield of carboxylate groups was identified (acids, esters, etc.). The majority of these originated from the PP secondary carbon site, from which other minor products also formed, including in-chain ketones. We found no measurable yield of oxidation products originating from reaction at the PP methyl group. Remarkably similar distributions of the major oxidation products were obtained for thermal aging at different temperatures, whereas the product distributions obtained for irradiation at the different temperatures exhibited significant differences. Time-dependent concentration plots have been obtained, which show the amounts of the various oxidation products originating at the different PP sites, as a function of the extent of material oxidation.

A general kinetic model for the photothermal oxidation of polypropylene

“…in agreement with the distribution of carbonyl products determined in the literature in photothermal oxidation [30,[50][51][52][53], in particular from the results of Carlsson and Wiles [1][2][3]. First of all, g 1 was increased up to 0.6, since it was reported that more than half the alkoxy radicals undergo b scission [1,2].…”

Carbon dioxide evolution and carbonyl group development during photodegradation of polyethylene and polypropylene

“…[9]. Recently Mowery et al [10] have used solid state 13 C NMR to demonstrate that most of the products that formed on the polymer chain during the oxidation of PP originated at the tertiary carbon, and that tertiary peroxides were the most abundant functional group produced. A complementary mass spectrometric analysis of the gas phase products [11] demonstrated that the carboxyl group in acetic acid originates solely from the tertiary carbon, whereas in formic acid it originates from both secondary and tertiary carbons.…”

Influence of temperature, UV-light wavelength and intensity on polypropylene photothermal oxidation

“…The spectrum is reported below, but it looks more like that of 4-methyl-2-pentanone than 2,4-dimethyl-3-pentanone in gas phase. Anyway, the former compound is predominant since oxidation is known to occur mainly on tertiary carbons in PP [44]. The Carlsson's spectrum was chosen for this study because it seems rather consistent with all spectra recorded for ketones, but also with data measured on polyethylene-co-carbon monoxide polymer.…”