E.s.r. studies of peroxy radicals in polyethylene: 1. Temperature dependence of spectra and molecular motion of radical sites

The temperature dependence of the e.s.r. spectrum of the peroxy radicals in polyethylene was successfully observed. Two kinds of radical site were found in the amorphous region of polyethylene, one bearing the radicals corresponding to the usual e.s.r. spectrum of the amorphous pattern as for the usual polymer peroxy radicals, and the other presenting the singlet-like pattern which has never actually been observed. Computer simulation of these spectra was carried out in order to determine the anisotropic g-values as well as the ratios of the amounts of the radicals located at respective radical sites. The temperature dependence of these values made it possible to discuss the difference of the radical sites and the molecular motion of these radical sites. The motion around the chain axis was found to be much more rapid than that around the CO bond axis in the peroxy raidcals.     

A study of the interaction of mechanically generated polypropylene radicals with phenolic antioxidant in the presence of oxygen by EPR spectroscopy

Isotactic polypropylene (iPP) was mechanically degraded in the absence and presence of a phenolic antioxidant at liquid nitrogen temperature in the presence of oxygen. The interaction of the primarily formed iPP macroradicals with oxygen and phenol was studied by EPR spectroscopy in the temperature region between 77 and 303 K at antioxidant concentrations of 0.1, 0.5, and 3.0 wt%. It was found that iPP macroradicals react at low temperature with oxygen, yielding peroxy radicals, and with antioxidant, with a simultaneous formation of phenoxy radicals. The development and decay of all kinds of radicals was studied. The rate constants for peroxy radical decay were determined and the apparent activation energies for the regions of slow and fast decay were calculated. © 1993 John Wiley & Sons, Inc.     

Radical formation and deformation in poly-p-(2-hydroxyethoxy) benzoic acid fibers

Radical generation during tensile deformation of poly-p-(2-hydroxyethoxy) benzoic acid (PEOB) fibers was studied by electron spin resonance techniques under various stretching conditions. Primary phenoxy radicals were detected at room temperature, indicating main-chain rupture during mechanical loading. The kinetics of phenoxy radical formation were observed at constant strain, constant load, constant strain-rate, and in constant load rate experiments. The effect of strain rate on the radical formation was found to be larger in this sample than was observed in Nylon 6. These results coupled with other experimental observations have yielded information on a basic structural model and improved insight into the basic molecular mechanism responsible for deformation and fracture.     

An e.s.r. study of the motion of peroxy radicals in isotactic and atactic polypropylenes

E.s.r. spectra of peroxy radicals trapped in isotactic and atactic polypropylenes have been observed at various temperatures. The spectrum observed at 77 K was shown to be composed of one component by comparing it with that at 4 K. The spectra observed at higher temperatures comprised of two spectra arising from the rigid peroxy radicals and the mobile peroxy radicals. All of the observed spectra have been reconstructed by means of computer simulation and the changes in anisotropic g-values with temperature were estimated. The motion of the mobile fraction was shown to be rotation or rotatory vibration of the radicals around the chain axis both in the isotactic polypropylene and in the atactic polypropylene as well as in polyethylene and poly(tetrafluoroethylene) although the structures of the polymers are different, that is helical (polypropylene) or planar zigzag (polyethylene, poly(tetrafluoroethylene)).     

E.p.r. study of radicals produced mechanically in PGMA and their interaction with oxygen

The vibrational grinding of poly(ethyleneglycol methacrylate) (PGMA) in vacuo at the liquid nitrogen temperature gives rise to polymer radicals in high concentrations. Changes in the radical concentration as a function of temperature in the presence and absence of oxygen were followed by means of electron paramagnetic resonance. It was found that polymer radicals reacted at very low temperatures with oxygen with simultaneous formation of polymer peroxy radicals and of a non-paramagnetic polymer tetroxide. This polymer tetroxide, which has been proved indirectly, can decompose to yield polymer peroxy radicals and non-paramagnetic products; the observed anomalies on the curve of the thermal decomposition of radicals may be thus elucidated. The relative participation of polymer tetroxide depends on the oxygen concentration, on the temperature of the sample in contact with oxygen and on the concentration of polymer radicals arising by grinding predominantly on the surface of polymer particles.     

Model studies on the mechanism of hals stabilization

Hindered Amine Light Stabilizers (HALS) are know to inhibit the photo-oxidation of polymers. A key reaction in their stabilization mechanism is believed to be the conversion of a hindered aminoether into a nitroxyl radical. Several different possible mechanisms for this conversion were explored. One, the elimination of the aminoether to form an olefin and hydroxylamine (an intermediate in the formation of a nitroxyl), while possible at high temperatures, cannot account for the inhibitory activity we observed for secondary and primary aminoethers. Direct radical displacement by peroxy radicals was also considered. However, the products predicted by this reaction pathway were not observed. Finally, oxidation of the nitrogen by a peroxy radical, by either electron transfer or a radical attack on the nitrogen, was investigated. While electron transfer was shown to be unlikely, direct oxidation of the aminoether nitrogen was supported by our results. A detailed mechanism for the reaction of both alkyl- and acyl-peroxy radicals with aminoethers is proposed.     

E.s.r. studies of peroxy radicals in polyethylene: 2. Peroxy radicals in urea — polyethylene complexes

Temperature dependent e.s.r. spectra of peroxy radicals in urea—polyethylene complex materials (UPEC) were discussed. Peroxy radicals in UPEC were much more stable than in normal polyethylene. Change of anisotropic g-values of the radicals with temperature were investigated and it was found that the g₁-axis, which was assigned to be parallel to the chain axis, seemed to be almost unchanged throughout the temperature range in the present study, but the g₂- and g₃-axes changed drastically. Using these results, it was concluded that the main motion of the radical sites was rotation around the chain axis and this motion was found to be much faster than in normal polyethylene, which we have already studied and which we reported recently. This is a reflection of the properties of the materials studied, i.e. interaction between molecular chains in UPEC must be much weaker than in normal polyethylene because of the presence of the wall of urea molecules in the former.     

ESR study of pressure dependence of the peroxy radical decay in irradiated isotactic polypropylene

By the use of gamma radiation the free radicals were generated in isotactic polypropylene. The polymer was exposed to the action of atmospheric oxygen and the peroxy radicals were formed. Then the decay of peroxy radicals was investigated at varying temperatures as a function of pressure. The rate constants of the free radical decay were determined, and the corresponding activation volumes were calculated. The kinetic characteristics imply that the peroxy radicals predominantly occured in amorphous region the polymer. The results suggest that useful information on molecular mechanism of radical reactions in the solid phase may be obtained from the determination of activation volumes.     

Generation of Free Radicals in RDX and HMX Compositions

Efforts to generate detectable concentrations of free radicals in explosives, binders, and their mixtures are described. Radicals were readily produced in polycaprolactone and polyethylene glycol binders at liquid nitrogen temperature using stresses as low as 0.4 kbar. These radicals were all of the peroxy type, and presumably formed by reaction of mechano-radicals with oxygen present in the polymer. No mechano-radicals were observed from HMX or RDX using samples cooled to liquid nitrogen temperature and applied stresses up to 4 kbar. In neither impacted samples that failed to explode nor the residues remaining after a partial explosion were radicals detected by ESR. Low temperature γ-irradiation of these materials was also carried out. Free radical signals originating in both the polymer and the explosive could be identified. The reactivity of NO₂ radicals from γ-irradiated HMX is enhanced in the presence of binder. In γ-irradiated HMX/polycaprolactone mixtures, the NO₂ radical anneals rapidly at 150 K, versus 240 K for HMX alone. Evidence is presented to suggest that the relatively stable NO₂ radical (such as produced by γ-irradiation) in HMX does not play a dominant role in mechanical initiation.     

Effects of Stabilized Criegee Intermediate and OH Radical Scavengers on Aerosol Formation from Reactions of β-Pinene with O 3

The formation of secondary organic aerosol (SOA) from reactions of O 3 with g -pinene, an exocyclic monoterpene prominent in the ambient atmosphere, was studied in an environmental chamber using a thermal desorption particle beam mass spectrometer for chemical analysis and a scanning mobility particle sizer for aerosol yield measurements. Potential reaction pathways for SOA formation were investigated in a series of experiments conducted using various scavengers for stabilized Criegee intermediates (SCI) and OH radicals, both of which are formed in the reaction. The major particulate products were compounds less volatile than pinic acid, a low-volatility dicarboxylic acid that was identified but was a minor component of the aerosol. The aerosol mass spectrum and yield were relatively insensitive to the identity of the SCI scavenger, indicating that association reactions of scavengers with SCI were not important in SOA formation. The mass spectrum of the aerosol also did not depend on the identity of the OH radical scavenger. SOA yields, on the other hand, were significantly larger when cyclohexane was used as an OH radical scavenger, compared to those measured for reactions conducted using alcohols or aldehydes. This dependence indicates that radical pathways play a major role in SOA formation in this reaction. Furthermore, the results show that reaction of OH radicals with scavengers used in laboratory studies can perturb the radical chemistry in such a way as to significantly impact SOA yields. We propose that this effect is due to increases in the ratio [hydroperoxy radicals]/[organic peroxy radicals] when alcohols or aldehydes are used as OH radical scavengers. This apparently enhances the rate of reaction of hydroperoxy radicals with key radical intermediates in SOA formation, effectively short-circuiting the reaction system into pathways leading to more volatile products.     

Electron spin resonance studies of γ-irradiated cellulose. I. Free radicals in decrystallized cellulose

The types of free radicals formed in decrystallized cellulose prepared from cellulose I and II after γ-irradiation in nitrogen atmosphere at room temperature were studied by ESR spectroscopy. X-Ray diffraction revealed that decrystallized cellulose I and II have the same microstructure. The ESR spectra obtained with the γ-irradiated decrystallized samples are simple. By contacting the irradiated sample with moisture in nitrogen atmosphere, the ESR spectrum changed to a narrow singlet, which gradually decreased in intensity until the spectrum completely disappeared. It was found that the types of free radicals generated in the decrystallized cellulose by γ-irradiation consist of the overlap of singlet and doublet. The singlet spectrum is mainly attributed to alkoxyl radical formed by the rupture of glycosidic linkage at the C 1 or C 4 position, and the doublet spectrum is ascribed to radical formed by hydrogen abstraction from the C 1 position in cellulose molecule.     

Light-Induced Yellowing of Mechanical and Ultrahigh Yield Pulps. Part 2. Radical-Induced Cleavage of Etherified Guaiacylglycerol-β-Arylether Groups is the Main Degradative Pathway

Bleached softwood mechanical pulp was treated with sodium borohydride to reduce carbonyl groups and dimethyl sulphate to block phenolic hydroxyl groups. Upon irradiation with near ultra-violet light, new phenolic hydroxyl groups and aromatic carbonyl groups formed. Existing mechanisms for photoyellowing require that either aromatic ketones or phenolic hydroxyls be initially present in the pulp, and cannot explain these observations. We interpret our results in terms of light-induced breakdown of etherified arylglycerol-β-arylether structures in lignin. Degradation occurs via the corresponding ketyl radicals, which cleave rapidly at the β-O-4 bond to give a phenoxy radical and an acetophenone enol. The enol tautomerizes to a ketone, while the phenoxy radical is oxidized to coloured groups. We believe that the ketyl route is the main lignin degradative pathway, and estimate that up to 70% of the colour formed during light-induced yellowing is attributable to this reaction.     

ESR study on molecular motion of peroxy radicals of polytetrafluoroethylene

Temperature variations of the line shapes of e.s.r. spectra from the peroxy radicals of polytetrafluoroethylene ( ) were investigated in detail and the results were quantitatively analysed based on Kneubühl's theory on the line shape. It was found that there were two kinds of rapid molecular motions, of which the correlation times were about 10⁻⁹ s, for the peroxy radicals of ; one was the rotational motion related to the peroxy chain radical, the other was the three dimensional motion associated with the peroxy end radical. The activation energies of these molecular motions were estimated as 0·52 kcal/mol and 0·26 kcal/mol for the rotational motion and the three dimensional motion, respectively. Origins of such rapid motions were discussed in connection with the other molecular motions of polymers, which were found by other experimental methods.     

Correlated electron spin-resonance and infrared spectroscopic study of the postformation auto-oxidation phenomenon in plasma-polymerized 4-vinyl pyridine films

Plasma polymerization of 4-vinyl pyridine (4-VP) proceeds through a gas-phase free radical mechanism to yield a film that retains much of the organic functionality of the monomer. During the deposition process, free radicals, which have been shown to quickly react with oxygen, are trapped to yield a film with a nascent peroxy radical density of 2.9 × 10¹⁸ spins/gas quantified by electron spin resonance (ESR) spectroscopy. In air at room temperature, peroxy radicals in the film react to produce carbonyl, hydroxyl, and ether structures in the polymer that was monitored using infrared (IR) spectroscopy. The free radical population was found to decay rapidly at first and then reach an apparent steady state after 30 hr. As the spin density decreases, a concomitant growth of vibrational modes associated with oxygen-containing functional groups was observed in the IR spectrum of the film. The relative population of oxygen-containing groups continued to increase even after the free radical population reached steady state. This slow, auto-oxidative effect may be attributed, in part, to free radical centers that are anchored to the polymer chain in regions of high crosslinking. In such regions, limited segmental mobility may limit the rate of radical-radical recombination (termination) proceses relative to oxidative radical-center. © 1996 John Wiley & Sons, Inc.     

Facile synthesis and promising antibacterial properties of a new guaiacol-based polymer

A new acrylamide-type monomer (N-(4-hydroxy-3-methoxy-benzyl)-acrylamide) derived from guaiacol was successfully synthesized. Polymers containing guaiacol moiety were obtained via conventional radical polymerization of this monomer with AIBN as initiator. The influence of reaction time, initiator concentration and temperature on polymers characteristics was studied. Evaluation of the termination mode in free-radical polymerization was performed by MALDI-TOF mass spectrometry. Termination occurs mainly by disproportionation reaction. Additional peaks in the spectrum were attributed to side chain reactions implying phenoxy radicals. This new polymer exhibits a potential antibacterial activity against Bacillus subtilis by using anti-adhesion and anti-biofilm tests. After an adhesion time of 3 h, compared to a non-coated glass slide, there was a decrease of bacteria of 99% on the polymer coated glass slide. After three days of culture in a bacterial suspension, no biofilm was observed on the polymer coated surface.     

Synthesis of degradable network polymers containing peroxy units in the main chain or the cross-linking point

We have synthesized degradable network polymers containing readily labile peroxy bonds as the repeating units in the main chain or as the cross-linking point in the side chain. Poly(2-hydroxyethyl sorbate-alt-O₂) was obtained by the radical copolymerization of 2-hydroxyethyl sorbate with oxygen, and then cross-linked using tolylene 2,4-diisocyanate as the cross-linking agent to obtain a new type of degradable gel. We also investigated the cross-linking of conventional polymers by the reaction of the diene moieties introduced into the polymer side chain with oxygen. The hydroxy group in the side chain of poly(vinyl alcohol) (PVA) was reacted with 1-isocyanate-1,3-pentadiene in order to introduce a diene moiety into the side chain of the PVA. The dienylated PVA was reacted with oxygen in the presence of a radical initiator, leading to the formation of a gel containing a labile peroxy linkage at the cross-linking point. These gels containing peroxy units in the main chain or at the cross-linking points degraded upon heating.     

Electron spin resonance studies on graft copolymerization of gaseous styrene onto preirradiated polypropylene. III. Preirradiation under vacuum followed by exposure to oxygen at −78°C

Grafting of styrene vapor was carried out onto polypropylene γ-irradiated under vacuum followed by exposure to oxygen at ?78°C. A comparison with the results on polypropylene irradiated in an oxygen atmosphere leads to an interesting result. Two samples were irradiated at room temperature under vacuum and in an oxygen atmosphere, respectively, the former being followed by exposure to oxygen. Although irradiation was carried out at room temperature for either sample and nearly the same amounts of the peroxy radicals are trapped, the former showed more grafting than the latter. The origin of this difference was investigated by means of electron spin resonance. Significant differences were observed in ESR spectra as well as in isothermal decays of the trapped peroxy radicals. In the former sample, the trapped peroxy radicals had a higher mobility, and consequently a significant hydrogen abstraction by the peroxy radicals was observed during storage at 40°C. This result supports the conclusion of the previous paper that the radicals effective in the grafting reaction of polypropylene preirradiated in an oxygen atmosphere are the carbon radicals which are produced by hydrogen abstraction by the peroxy radicals.     

Free radical formation in oriented polybutadiene during mechanical deformation below Tg

The tensile behavior of oriented polybutadiene at 83°K is systematically studied as a function of strain rate and pre-test orientation. Electron spin resonance studies of radical formation are made in conjunction with the mechanical tests. Three different modes of mechanical behavior are observed (brittle, crazing and a second ductile behavior without crazing), depending on test conditions. Radical formation is observed in association with the two ductile modes of behavior. The ESR spectra obtained are attributed to a combination of allyl radicals formed by chain scission between the α-methylene groups and peroxy radicals. The relative quantity of the two radical species present is thought to be related to the ratio of cis/trans-isomerism. Stability of the radicals observed with time and with an increase in temperature is studied. Further studies are made of the quantity of environmental test gases absorbed during crazing.     

The inhibition of autoxidation by aromatic amines

Recent work on the inhibition of autoxidation by aromatic amines is reviewed. The rate controlling step for the inhibition process is abstraction of the amino hydrogen by a peroxy radical. The rate of this reaction is increased by electron donating substituents attached to the aromatic ring. Nitroxide radicals may be produced in the reaction of peroxy radicals with secondary amino radicals. The efficiency of nitroxide formation is greater for tertiary than for secondary peroxy radicals because, in the latter case, the caged nitroxide and alkoxy radical may disproportionate to give a hydroxylamine and a carbonyl compound. Primary aromatic amines do not give nitroxides, presumably because the peroxy radical-amino radical reaction gives an alcohol and a nitroso aromatic. The tertiary amine, N,N,N′,N′-tetramethyl-p-phenylenediamine is shown to be a catalyst of autoxidation rather than an inhibitor, as has been previously reported. The products of the N-phenyl-2-naphthylamine-peroxy radical reaction have been identified. Presented at the AOCS Meeting, San Francisco, April 1969.     

Nitroxyls for scorch suppression,cure control,and functionalization in free-radical crosslinking of polyethylene

This paper describes the use of 2,2,6,6-tetramethylpiperidinyl-oxy (TEMPO) derivatives for scorch suppression, cure control, and functionalization in peroxide crosslinking of polyethylene. When 4-hydroxy 2,2,6,6-tetramethylpiperidin-1-oxyl was used for scorch suppression, there was often a loss in ultimate degree of crosslinking. In contrast, with bis(1-oxyl-2,2,6,6-tetramethylpiperidine-4-yl)sebacate, both scorch suppression and ultimate degree of crosslinking were enhanced. A model study in hexadecane showed that TEMPO radicals terminate with carbon-centered radicals formed as a consequence of peroxide homolysis and propagation steps. This termination occurs preferentially over peroxide-initiated crosslinking and results in TEMPO-grafted polymer. In addition to polymer radical formation, several additional reaction pathways are available following thermal homolysis of the peroxide, including unimolecular disassociation of the peroxy radical to yield a methyl radical and a ketone, and proton extraction from one of several substrates by the peroxy radical to yield an alkyl radical and an alcohol. This study reveals that the reaction rate is limited by the rate of peroxide homolysis, and proceeds to statistical products with little or no preference for any specific species. The implication is that choice of peroxide is a dominant controlling factor over whether the TEMPO derivatives are ultimately grafted to the polymer or are bound to small alkyl radicals. POLYM. ENG. SCI., 47:50–61, 2007. © 2006 Society of Plastics Engineers