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.     

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.     

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)).     

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.     

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.     

E.s.r. studies on free radicals generated in poly[p-(2-hydroxyethoxy) benzoic acid] fibres at low temperatures

Free radicals generated in stretched and ground poly [p-(2-hydroxyethoxy) benzoic acid] (PEOB) were studied by e.s.r. spectroscopy in an effort to isolate those formed by main chain scission. PEOB fibres stretched at ?86°C in a dry nitrogen atmosphere gave an asymmetric spectrum, which had patterns characteristic of both phenoxy and peroxy radicals in addition to some unknown peaks. The shape of the spectrum changed with increasing temperatures, finally becoming identical to that of phenoxy radicals at room temperature. The phenoxy radical was observed in all PEOB films ground or γ-irradiated in liquid nitrogen. In some cases a small peak of peroxy radical was observed. Theoretical spectra calculated for possible radical species or combinations of them were compared with those observed. No direct evidence was observed of the formation of alkyl type and radicals which were assumed to be formed with phenoxy radicals upon main chain rupture of PEOB. However, the relatively unstable peroxy radical observed in these experiments is thought to arise from them.     

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.     

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.     

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. II. Preirradiation under vacuum

Graft copolymerization of gaseous styrene was carried out at 40°C onto polypropylene preirradiated under vacuum. In order to investigate the origin of graft initiation activity, ESR spectra of irradiated polymers were measured and discussed. Although there are about three times as many radicals trapped in the sample irradiated at ?78°C as in the sample irradiated at room temperature, both the samples showed nearly same degree of grafting. This is due to the ineffectiveness of radicals trapped in the amorphous region for grafting. The samples irradiated under vacuum showed more grafting than those irradiated in an oxygen atmosphere. This is compatible with the conclusion in the previous paper that in grafting of polypropylene irradiated in an oxygen atmosphere the carbon radicals produced by hydrogen abstraction by trapped peroxy radicals are the effective active centers.     

The photodegradation of polypropylene. IV. UV stabilizer decomposition

The destruction of various UV stabilizers in polypropylene films during irradiation has been examined both in the presence and absence of air and oxidation products. Although some stabilizers were destroyed in the absence of oxygen, all were more sensitive when irradiated in the presence of air. Several stabilizers were highly effective but were destroyed so rapidly that photoprotection of the polymer must have resulted from stabilizer products. Work with preoxidized polymer and with model hydroperoxides and peroxides indicated that most stabilizers were destroyed by the attack of oxygen-centered radicals, with peroxy radical attack appearing to dominate.     

Oxidation of gamma‐irradiated ultrahigh molecular weight polyethylene

Ultrahigh molecular weight polyethylene (UHMWPE), the current polymer of choice in orthopedic prosthetic devices, is typically sterilized by exposure to Co‐60 gamma irradiation prior to packaging for long‐term storage. However, the exposure to Co‐60 irradiation generates free radicals along the polymer chain that can participate in a series of reactions commencing with the oxidation of the free radicals to form reactive peroxy radicals. This study was undertaken to identify the role of hydroperoxide species in shelf‐aged and accelerated aged UHMWPE samples by using a nitric oxide derivatization technique. It is shown that the concentration of hydroperoxides did not change appreciably with shelf aging. However, during accelerated aging the hydroperoxide concentration increased to a plateau and then decreased, suggesting its role as an intermediate in the process. By contrast, the concentrations of carbonyl species continued to increase during shelf aging and accelerated aging. The effects of several packaging materials on the oxidation characteristics were also investigated. A vacuum foil package is shown to be effective in preventing oxidation to a significant extent during accelerated aging. However, accelerated aging after removal from the foil pack resulted in oxidative degradation. Extended vacuum to remove dissolved oxygen and a 5‐week room‐temperature healing process in the foil pack were shown to be ineffective in reducing oxidative degradation. It also was shown that increased moisture content in the aging environment did not affect the degradation process. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2525–2542, 2000     

Ion exchange resins and ethyleneimine polymer as antioxidants: I. Activity and mechanism

The antioxidant activity of ion exchange resins and ethyleneimine polymer was determined in methyl linoleate free from natural antioxidants and metals. Superior antioxidant activities were recognized in an ion exchange resin with NH groups and in the ethyleneimine polymer. The antioxidant activities in these heterogeneous reaction systems increased in linear proportion to the amount present. These antioxidant activities in the heterogeneous reaction system were mainly owing to their ability to donate hydrogen to the peroxy radicals produced in the autoxidation of methyl linoleate. The antioxidant radicals produced in these cases were very stable and generally couldn’t participate in the increase of the induction periods in the autoxidation of methyl linoleate by terminating the peroxy radicals. These results have never been obtained in the homogeneous reaction system but are characteristic in the heterogeneous one.     

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.     

Electrochemical oxidation of aliphatic hydrocarbons promoted by inorganic radicals. II. NO3 radicals

The anodic electrolysis of linear alkanes in tert-butanol/H₂O solutions containing HNO₃ and saturated with oxygen at P_{O 2}=1 atm results in the partial oxidation of the hydrocarbons to the corresponding isomeric ketones. The experimental data are in support of a mechanism in which the first steps are: (1) one-electron transfer from the nitrate ion to the anode to give a nitrate radical; (2) hydrogen abstraction by the nitrate radical on the aliphatic substrate to give an alkyl radical; (3) molecular oxygen addition by the alkyl radical and formation of peroxy radicals. These species decay by bimolecular reactions and a carbonyl function is formed. The isomer distribution is in excellent accord with a statistical H abstraction from the secondary C–H bonds.     

Cellulose graft copolymers. III. Effects of oxygen on graft copolymerization of ethyl acrylate with γ-irradiated cellulose

The effects of oxygen on graft copolymerization of ethyl acrylate from methanol–water systems with γ-irradiated fibrous cotton cellulose were investigated by electron spin resonance spectroscopy and by the formation of cellulose–poly(ethyl acrylate) copolymer. The concentrations of free radicals in cellulose irradiated dry in an atmosphere of nitrogen at 25°C decreased during postirradiation storage in nitrogen or oxygen. The concentration of free radicals in the irradiated cellulose, moisture regain of the irradiated cellulose, and formation of cellulose–poly(ethyl acrylate) copolymer decreased with increase in temperature and time of postirradiation storage and to a greater extent when stored in oxygen rather than in nitrogen. From the decrease in moisture regain of irradiated cellulose during postirradiation storage, it was concluded that increased intermolecular bonding occurred in irradiated cellulose during storage in both nitrogen and oxygen atmospheres. When irradiated celluloses which had been stored in either oxygen or nitrogen were copolymerized with ethyl acrylate at 60°C, less formation of copolymer was observed than when the copolymerization reactions were conducted at 25°C. It was concluded that there was no evidence for the formation or decomposition of cellulose peroxides during these reactions and that formation of graft copolymer depended primarily on the concentration of free radicals in the irradiated cellulose at the time of copolymerization.     

Formation and behavior of mechanoradicals in pulp cellulose

The mechanical degradation of pulp cellulose fiber was studied at ambient temperature and at 77°K. ESR findings reveal that mechanical degradation occurs via free-radical routes. Three types of mechanoradicals contributing singlet, doublet, and triplet ESR signals are identified. The singlet signals are derived from the alkoxy radicals at C₄ positions as a consequence of the cleavage of glucosidic bonds, the radical pairs generated at C₁ positions contributing the doublet signals. Triplet signals are derived from the C₂ and C₃ positions due to the cleavage of C₂ and C₃ bonds. Of these radicals, alkoxy radicals are the most stable at ambient temperature. Carbon radicals are capable of interacting rapidly with oxygen molecules to produce peroxy radical intermediates, where alkoxy radicals are inert toward oxygen molecules. ESR study also reveals that cellulose mechanoradicals are capable of initiating vinyl polymerization. MMA propagating radicals are identified when the monomers are in contact with cellulose mechanoradicals. The ability of mechanoradicals to initiate graft copolymerization from cellulose fiber is discussed.     

Simplified Free‐Radical Reaction Kinetics for p‐Xylene Oxidation to Terephthalic Acid

The kinetic models based on complex free‐radical mechanisms always involve lots of parameters, which result in model overparameterization. In this work, on the basis of free‐radical reaction mechanisms, a simplified kinetics for liquid‐phase catalytic oxidation of p‐xylene (PX) to terephthalic acid (TPA) was developed. By assuming that different peroxy radicals have equivalent reactivity, all the initiation rate constants are identical, and the differences in the rates of termination between various peroxy radicals are neglected, the kinetic model is simplified to include only six parameters that are to be determined by experiment. The kinetic model established in this paper was shown to have satisfactory precision in predicting the concentration profiles. The kinetic model proposed is even simpler than the first ‐ order kinetic model because the rate constants concerning chain propagation and termination are independent of temperature within the range investigated.     

Grafting of N‐carbamyl maleamic acid onto a styrene–butadiene–styrene copolymer

A styrene–butadiene–styrene block copolymer (SBS) was functionalized with N‐carbamyl maleamic acid (NCMA) using two peroxide initiators with the aim of grafting polar groups onto the molecular chains of the polymer. The influence of the concentration of benzoyl peroxide (BPO) and 2,5‐dimethyl, 2,5‐diterbuthylperoxihexane (DBPH) was studied. The concentration of peroxy groups ranged between 0.75 and 6 × 10^?4 mol % while the concentration of NCMA was constant at 1 wt %. The reaction temperature was chosen according to the type of peroxide employed, being 140°C for BPO and 190°C for DBPH. FTIR spectra confirmed that NCMA was grafted onto the SBS macromolecules. It was found that the highest grafting level was achieved at a concentration of peroxy groups of about 3 × 10^?4 mol %. Contact angle measurements were used to characterize the surface of the SBS and modified polymers. The contact angle of water drops decreased with the amount of NCMA grafted from 95°, the one corresponding to the SBS, to about 73°. T‐peel strength of polymer/polyurethane adhesive/polymer joints made with the modified polymers was larger than those prepared with the original SBS. The peel strength of SBS modified with 1.5 and 3 × 10^?4 mol % of peroxy groups from BPO were five times larger than that of the original SBS. The materials modified using BPO showed peel strengths higher than the ones obtained with DBPH. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 4468–4477, 2006     

The synergistic mechanism of thermally reduced graphene oxide and antioxidant in improving the thermo-oxidative stability of polypropylene

The feasibility of using thermally reduced graphene oxide (TrGO) to improve the antioxidative efficiency of commercial antioxidant (3,5-di-tert-butyl-4-hydroxycinnamic acid, AO) on unstabilized polypropylene (PP) was evaluated in terms of the oxidation induction time (OIT) and the initial degradation temperature (T_i). It was found that a synergistic effect exist between AO and TrGO in retarding the degradation of PP in oxygen-containing atmosphere. Compared with that of PP/AO (100/0.5) sample, the OIT value of PP/AO/TrGO (100/0.5/1) composite was almost doubled at 180 °C and the value of T_i in air was improved by 37.2 °C. As verified by using radical scavenging assay with the 1,1-diphenyl-2-picrylhydrazyl radical, the TrGO sheets exhibited a good radical scavenging capacity. The synergistic stabilization mechanism was attributed to the enhanced dispersion of TrGO sheets in the PP matrix in the presence of 0.5 wt.% AO, which could improve the oxygen barrier effect and radical scavenging efficiency of TrGO. This synergistic effect between AO and TrGO can efficiently reduce the concentrations of oxygen and peroxy radicals in the PP matrix, leading to the significant improvement in thermo-oxidative stability of PP/AO/TrG composite.