Electron spin resonance study of γ-irradiated poly(acrylic acid)

Electron spin resonance (ESR) spectra of poly(acrylic acid) (PAA) γ-irradiated in air at room temperature and recorded at room temperature and at liquid-nitrogen temperature have been studied to identify the radiation products. The ESR spectra are composed of eight lines with hyperfine splittings of 23 ± 1 G and 11 ± 1 G. The method of least-squares total curve fitting, employing the Lorentzian line shape function, to the observed spectra enabled the assignment of the spectra. Computed spectra obtained by the superposition of a singlet and the spectra due to chain radicals are considered to give the best fits to the observed ESR spectra. The singlet is assigned to the radicals COOH, and the component 10-line spectra are assigned to the chain radicals CH₃? CH? CH₂ ～ and/or ～ CH₂? CH? CH₂ ～. The observed change in line shape with temperature of the ESR spectra is attributed to the hindered oscillations of the methyl groups about the C_α? C_β bond axis of the chain radicals. The existence of the methyl groups is confirmed by the measurement of infrared absorption.     

Electron spin resonance study on plasma-induced surface radicals of poly(ethylene naphthalate)

We have undertaken Ar plasma irradiation on poly(ethylene naphthalate) (PEN) powder, and the radicals formed were studied by electron spin resonance (ESR). The room temperature ESR spectrum of plasma-irradiated PEN shows a five-line spectrum separated with nonbinomial intensity distribution, indicating that the spectrum is an outline of multicomponent spectra. The systematic computer simulation of the observed ESR spectra disclosed that the spectra consist of two types of radicals in structural term: the major radicals formed were assigned to dihydronaphthalenyl-type radicals generated by a nearly random addition of a hydrogen atom to the naphthalene ring, and immobilized dangling bond sites at the surface crosslinked moiety.     

Nature of plasma-induced radicals on crosslinked methacrylic polymers studied by electron spin resonance

Plasma-induced radicals of several crosslinked methacrylic polymers such as poly(ethyleneglycol dimethacrylate) (PEDMA), poly(2-hydroxyethylmethacrylate) (PHEMA) and polymethacrylamide (PMAAm) were studied by electron spin resonance (ESR). The observed ESR spectra did not exhibit a drastic difference in the spectral feature caused by the effect of crosslinking. All the spectral features can be represented by “nine-line spectra” as a major spectral component similar to those of linear methacrylic polymers such as polymethacrylic acid (PMAA). A pronounced effect of crosslinking, however, has emerged on the specific formation in the radical structure and the stability of radicals formed, especially in PEDMA. The formation of fewer kinds of radical in PEDMA is apparently caused by the high degree of crosslinking which leads to a suppression of the occurrence of depolymerization on plasma irradiation.     

Electron spin resonance of γ-irradiated poly(ethylene 2,6-naphthalene dicarboxylate)

The two types of radicals trapped in γ-irradiated poly(ethylene 2,6-naphthalene dicarboxylate) (PEN 2,6) have been identified by ESR as ? O? CH? CH₂? O? (radical I) and a radical located on the naphthalene ring (radical II). The relative concentrations of radicals in the gross polymer are 10–20% radical I and 80–90% radical II. Similar trapped radicals have been identified in γ-irradiated poly(ethylene terephthalate) (PET), a structurally related polymer which differs only in the aromatic moiety, but the relative radical concentrations are quite different. These results are discussed in relation to the radiation resistance of the two polymers.     

An electron spin resonance study on γ-irradiated poly( -lactic acid) and poly( -lactic acid)

The effect of gamma irradiation on poly( -lactic acid) ( -PLA) and poly( -lactic acid) ( -PLA), has been examined using ESR spectroscopy and through analysis of the changes in molecular weight. The G values for radical formation of both polylactic acids have been calculated at 77 and 300 K; G(R) = 2.0 at 77 K and G(R) = 1.5 at 300 K for -PLA and G(R) = 2.4 at 77 K and G(R) = 1.2 at 300 K for -PLA. The ESR spectrum at 300 K for the polymers was assigned to one radical, resulting from H atom abstraction from the quaternary carbon atom. The G values for crosslinking and scission have also been determined for the polymers at 300 K; G(S) = 2.3 and G(X) = 0.0 for -PLA, G(S) = 2.4 and G(X) = 0.28 for -PLA.     

ESR studies in γ‐irradiated and PS‐radiation‐grafted poly(vinylidene fluoride)

The fluoropolymer poly(vinylidene fluoride) was irradiated with γ rays to induce a polystyrene (PS) radiation grafting via an indirect method. Electron spin resonance and Fourier transform infrared studies were performed to identify the species that initiated the PS grafting. Specific experiments were performed to prove separately the importance of each kind of radical. A localization of the radicals in this irradiated polymer is proposed, and a kinetic model of the grafting is given. © 2001 John Wiley & Sons, Inc. J Polym Sci B Part B: Polym Phys 39: 1437–1448, 2001     

Electron spin resonance study on chemical‐crosslinking reaction mechanisms of polyethylene using a chemical agent. VI. Effect of α‐methyl styrene dimer

The effect of α‐methyl styrene dimer (AMSD), which is used as a scorch retarder, on the reaction mechanisms of the chemical crosslinking of polyethylene (PE) with dicumyl peroxide (DCP) at high temperatures was investigated using electron spin resonance. When AMSD was added to PE containing DCP, the AMSD radical was observed; however, the PE alkyl radical or allyl radical presence was not detected. At 145 °C, crosslinking was obstructed as a result of the reaction between AMSD and alkyl radicals. As the temperature increased, AMSD fragmented to form 2‐phenyl‐2‐propyl and double bonds in PE. This generation of double bonds, however, accelerated crosslinking at 180 °C and was more effective than when AMSD was not present. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2151–2156, 2001     

Electron spin resonance on carbon nanotubes–polymer composites

Electron spin resonance investigations on styrene–polyisoprene–polystyrene block copolymer loaded with various amounts of multiwalled carbon nanotubes are reported. The temperature dependence of resonance line parameters in the range 290–425 K was analyzed. It was proved that the main resonance line represents a bottleneck of localized and delocalized electrons residing on carbon nanotubes. The temperature dependence of the g-factor and double integral of resonance spectra confirmed this interpretation. The temperature dependence of the resonance linewidth of composites containing various concentrations of nanotubes was explained by the thermally activated narrowing of the resonance spectra. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3406–3412, 2005     

Microphase separation in poly(acrylonitrile–butadiene–styrene) (ABS) studied with paramagnetic spin probes. I. Electron spin resonance spectra

Microphase separation in poly(acrylonitrile–butadiene–styrene) (ABS) was studied as a function of the butadiene content and method of preparation with electron spin resonance (ESR) spectra of nitroxide spin probes. Results for the ABS polymers were evaluated by comparison with similar studies of the homopolymers polybutadiene (PB), polystyrene (PS), and polyacrylonitrile (PAN) and the copolymers poly(styrene‐co‐acrylonitrile) (SAN) and poly(styrene‐co‐butadiene) (SB). Two spin probes were selected for this study: 10‐doxylnonadecane (10DND) and 5‐doxyldecane (5DD). The probes varied in size and were selected because their hydrocarbon backbone made them compatible with the polymers studied. The ESR spectra were measured in the temperature range 120–420 K and were analyzed in terms of line shapes, line widths, and hyperfine splitting from the ¹⁴N nucleus; the appearance of more than one spectral component was taken as an indication of microphase separation. Only one spectral component was detected for 10DND in PB, PS, and PAN and in the copolymers SAN and SB. In contrast, two spectral components differing in their dynamic properties were detected for both probes in the three types of ABS samples studied and were assigned to spin probes located in butadiene‐rich domains (the fast component) and SAN‐rich domains (the slow component). The behavior of the fast component in ABS prepared by mass polymerization suggested that the low‐T_g (glass‐transition‐temperature) phase was almost pure PB. The corresponding phase in ABS prepared by emulsion grafting also contained styrene and acrylonitrile monomers. A redistribution of the spin probes on heating occurred with heating near the T_g of the SAN phase, suggesting that the ABS polymers as prepared were not in thermodynamic equilibrium. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 415–423, 2002; DOI 10.1002/polb.10109     

Electron spin resonance analysis of paramagnetic Pd(I) species in Pd(II)-exchanged H-rho zeolite

Zeolite rho was synthesized and Pd(II) exchanged into it. Pd(II) was reduced to paramagnetic Pd(1) by a thermal activation process. The interactions of Pd(I) in zeolite H-rho with oxygen, water, methanol, ammonia, carbon monoxide and ethylene have been studied by electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopies. The ESR spectrum of an activated sample shows the formation of one Pd(I) species. Pd(I) interacts with water vapor or molecular oxygen to form Pd(II)–O₂, indicating decomposition of water. Equilibration with methanol results in a broad isotropic ESR signal which is attributed to the formation of small palladium clusters. ESEM shows that the Pd clusters coordinate one molecule of methanol. Adsorption of ammonia produces a Pd(I) complex containing four molecules of ammonia based upon resolved nitrogen superhyperfine coupling. Adsorption of carbon monoxide results in a Pd(I) complex containing two molecules of carbon monoxide based upon resolved¹³C superhyperfine coupling. ESR and ESEM results indicate that exposure to ethylene leads to two new Pd(I) species each of which coordinates one molecule of ethylene.     

Activation parameters associated with the β relaxation of poly(cyclohexyl acrylate)

The interpretation of the mechanical and dielectric β relaxations of poly(cyclohexyl acrylate) by the coupling scheme suggests that the dielectric relaxation process is more complex than the mechanical one, contrary to what occurs in the glass–rubber relaxation. The distribution of activation energies for the dielectric process, obtained from isochronal and isothermal loss curves, increases with temperature and frequencies. The determination of the distribution of the activation-free energy indicates a distribution of the activation entropy for both the mechanical and dielectric β process, suggesting that a distribution of preexponential factors in the Arrhenius equation also exists. © 1992 John Wiley & Sons, Inc.     

Advanced protocol for the detection of irradiated food by electron spin resonance spectroscopy

Using ESR (electron spin resonance) spectroscopy, we found various free radicals in a pepper before and after irradiation. The representative ESR spectrum of the pepper composed of a sextet centered at g=2.0, a singlet at the same g-value and a singlet at g=4.0. This reflects the evidence of three independent radicals in the pepper before irradiation. Upon gamma ray irradiation, a new pair of signals appeared. The progressive saturation behavior (PSB) at various microwave power levels indicates quite different relaxation behaviors of those signals. For the evaluation of radiation-induced radicals and irradiation effects we propose a new protocol using the PSB method. This would call for an advanced protocol for the detection of irradiated foods.     

Nitroxide spin probe study of probe size,hydrogen bonding and polymer matrix rigidity effects on poly(acrylic acid)/poly(ethylene oxide) complexes

An electron spin resonance (ESR) spin probe study was performed on 1 : 1 by weight poly(acrylic acid) (PAA)/poly(ethylene oxide) (PEO) complex over the 100–450 K temperature range with a series of tetramethylpiperidyloxy‐based spin probes. Measurements of the parameters T_5mT, Ta and Td demonstrated the effects of probe size and the strength of hydrogen bonding. The probes in the series Tempone, Tempo, Tempol and Tamine (respectively 4‐oxo‐, unsubstituted, 4‐hydroxy‐ and 4‐amino‐2,2,6,6,‐tetramethylpiperidine ‐1‐oxyl) displayed noticeable increases in the hydrogen‐bonding effect, as indicated by Ta and Td. These increases correlated with increasing hydrogen bond acceptor strength. On the other hand, as the probe size became larger, T_5mT gradually increased due to the free volume decrease. These effects were analyzed using the established theoretical relationship of T_5mT to probe volume expressed by f. Meanwhile, in order to investigate the effect of polymer matrix rigidity, a similar study was performed with a nitroxide spin probe, 2,2,6,6‐tetramethyl‐1‐piperidine‐1‐oxyl (Tempo), on PAA/PEO complexes of different weight compositions. The quantitative fast motion fraction in the composite ESR spectrum was calculated. The influence of changes in the composition of PAA on the molecular mobility was characterized by changes of the spectral parameters and τ_c. The molecular mobility was shown to diminish with increasing content of PAA in PAA/PEO blends duo to the restriction of the polymer matrix rigidity increase. Copyright © 2003 John Wiley & Sons, Ltd.     

γ‐Irradiation of ultrahigh‐molecular‐weight polyethylene: Electron paramagnetic resonance and nuclear magnetic resonance spectroscopy and imaging studies of the mechanism of subsurface oxidation

The shelf aging of irradiated ultrahigh‐molecular‐weight polyethylene (UHMWPE) causes subsurface oxidation, which leads to failure in UHMWPE orthopedic components, yet the mechanisms causing subsurface oxidation remain unclear. The shelf aging of γ‐irradiated UHMWPE bars has been studied with electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) imaging and with microtoming and Fourier transform infrared microscopy. The bars initially contained only allyl radicals, and upon air exposure, a surface layer of peroxyl radicals formed through the reaction of allyl radicals with oxygen. Importantly, a band of low radical intensity just beneath the peroxyl layer became apparent. NMR imaging showed a zone of altered proton relaxation in this zone. With increasing time, surface peroxyl radicals persisted in comparison with the interior allyl radicals, although oxygen did not appear to penetrate any more deeply into the bar. The area of maximal oxidation and mechanical disruption, measured after 3 years, was at the interface between the zone of exterior peroxyl radicals and the zone of low radical intensity. We present a mechanism involving the intermediacy of sterically strained reactive dialkyl peroxides at this interface to explain subsurface oxidation. We also demonstrate that EPR and NMR imaging provides information that could potentially be used to identify subsurface oxidized UHMWPE components before failure. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5929–5941, 2004     

Spin label study of phase morphology and polymer segmental motion in poly(acrylic acid)–poly(ethylene oxide) complex

The ESR lineshapes of nitroxide radical end‐labeled on poly(ethylene oxide) (SLPEO) for the pure polymer and for different weight ratio complexes with poly(acrylic acid) (PAA) were studied as a function of temperature. For SLPEO one spectral component was detected in the entire temperature range, indicating that the spin label was in the homogeneous phase domain. For all PAA–PEO complexes two spectral components with different rates of motion, a ‘fast’ and a ‘slow’ component, were observed, which indicates the existence of microheterogeneity at the molecular level: the more mobile the PEO‐rich microphase, the more rigid is the PAA‐rich microphase. On the other hand, the SLPEO polymer segmental motion was restricted owing to the hydrogen bond interaction between the carboxyl proton in PAA and the ether oxygen in PEO. This restriction was exacerbated with increasing the PAA content in the complex, which could be further substantiated through the calculated S and τ_c values. Copyright © 2003 John Wiley & Sons, Ltd.     

The grafting of acrylamide onto poly(ε‐caprolactone) and poly(1,5‐dioxepan‐2‐one) using electron beam preirradiation. II. An in vitro degradation study on grafted poly(ε‐caprolactone)

Acrylamide was graft polymerized onto the surface of a biodegradable semicrystalline polyester, poly(ε‐caprolactone). Electron beam irradiation at a dose of 5 Mrad was used to generate initiating species in the polyester. The degradation in vitro at pH 7.4 and 37°C in a phosphate buffer solution was studied for untreated, irradiated and acrylamide‐grafted polymers. In the case of poly(ε‐caprolactone), all materials showed similar behavior in terms of weight loss. No significant decrease in weight was observed up to 40 weeks, after which the loss of weight accelerated. The main differences in degradation behavior were found for the average molecular weights, M̄_n and M̄_w. Virgin poly(ε‐caprolactone) maintained M̄_n and M̄_w up to about 40 weeks, whereas the irradiated and grafted poly(ε‐caprolactone) showed similar continuous declines in M̄_n and M̄_w throughout the degradation period. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1651–1657, 1999     

Microphase separation in poly(acrylonitrile–butadiene–styrene) (ABS) studied with paramagnetic spin probes. II. Simulation of electron spin resonance spectra

We recently presented electron spin resonance spectra of poly(acrylonitrile–butadiene–styrene) (ABS) doped with 10‐doxylnonadecane (10DND) and 5‐doxyldecane (5DD) as spin probes. The spectra were measured in three types of ABS that differed in their butadiene contents and methods of preparation. Results for the ABS polymers were evaluated by comparison with similar studies on the homopolymers polybutadiene (PB) and polystyrene (PS) and the copolymers poly(styrene‐co‐acrylonitrile) (SAN) and poly(styrene‐co‐butadiene) (SB). Only one spectral component was detected for 10DND in PB, PS, SAN, and SB. In contrast, two spectral components differing in their dynamic properties were detected in the ABS samples and were assigned to spin probes located in butadiene‐rich domains (the fast component) and SAN‐rich domains (the slow component). The presence of two spectral components was taken as an indication of microphase separation. In this study, we present details on the dynamics and microphase separation by simulating spectra of 10DND in ABS, PB, PS, and SAN. The simulations are based on a dynamic model defined by the components of the rotational diffusion tensor and the diffusion tilt angle between the symmetry axis of the rotational diffusion tensor and the direction of the nitrogen 2p_z atomic orbital. The jump diffusion model led to good agreement with experimental spectra. In this model, the spin probe has a fixed orientation for a given time and then jumps instantaneously to a new orientation. The temperature variation of the rotational correlation time in PB and PS consisted of two dynamic regimes, with different activation energies. The transition temperature at which the change in dynamics occurs (T_tr) is 380 K for PS and 205 K for PB, essentially the same as the corresponding glass‐transition temperatures measured by differential scanning calorimetry. We suggest that T_tr is a better indicator of the glass transition than the temperature at which the total spectral width is 50 G, especially for large probes. The simulation program allowed the determination of the relative intensities of the fast and slow spectral components as a function of temperature; this information was used to clarify the redistribution of the probe above the glass transition of the SAN‐rich component in ABS systems. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 424–433, 2002; DOI 10.1002/polb.10110     

Rotational vibration of poly(tetrafluoroethylene) chains by the electron spin resonance method

Temperature‐dependent electron spin resonance spectra of main‐chain free radicals, ? CF₂(β)? C · F(α)? CF₂(β)? , in poly(tetrafluoroethylene) (PTFE) were analyzed by the change in the hyperfine splitting due to β‐fluorines, which was a decreasing function of the observation temperature. The results were interpreted in terms of the rotational vibration around the C_α? C_β bond. The amplitude of the vibration was estimated on the assumption of its harmonic oscillation. The vibration of the PTFE chain was found to have a large amplitude in comparison with that of a polyethylene chain in single crystals. The vibration of the large amplitude was caused by a weak interchain interaction in the PTFE matrices. The amplitude of the vibration in crosslinked PTFE was much larger than that in noncrosslinked PTFE. This result indicated that the free radicals in crosslinked PTFE were trapped in the amorphous region, which had the disordered sites of crosslinking, whereas the free radicals in noncrosslinked PTFE were mainly trapped in the paracrystalline region. The decay reaction of the free radicals in the PTFE matrices was also related to the heterogeneity in the structure and the rotational vibration. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1539–1547, 2004