Electron spin resonance investigation of small spin probes in aqueous and vesicle phases of mixed membranes made from poly(oxyethylene) hydrogenated castor oil and hexadecane

Small spin probes in the vesicle and aqueous phases of mixed membranes made from poly(oxyethylene) hydrogenated castor oil (HCO) and hexadecane were investigated by ESR (electron spin resonance) spectroscopy. Partitioning (partition coefficients), rotational correlation times, and activation energies of the spin probes in the vesicle phase were determined. The quantitative results obtained by an ESR simulation indicated that the spin probe DTBN (di-tert-butyl nitroxide), in the vesicle of the mixed membrane, was partitioned similar to that of TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) at 50 degrees C, and the partitioning for both probes showed no abrupt change across the temperatures examined. The results differed considerably from conventional analyses of the high-field signal intensities. Activation energies obtained by the simulation for DTBN and TEMPO were 21 and 20 kJ/mol, respectively. In addition, the hyperfine values of 15.8 (DTBN) and 16.1 G (TEMPO) in the vesicle phase were consistent with the activation energies and supported the probe environments in the membrane. Thus, the present ESR results provided detailed information regarding the probe behavior in the membrane.     

ESR studies of chemically modified and photocrosslinked poly(vinyl alcohol) gels

The hydrophilicity of poly(vinyl alcohol) (PVA) was lowered by acetalization with propionic aldehyde. After UV irradiation of pendent styrylpyridinium groups, these prepolymers form networks of different hydrophobicity depending on the degree of acetaization. PVA films containing different amounts of propionic acetals were doped with 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl. Then, the ESR spectra of the spin probe entrapped in the polymeric network were monitored while allowing different solvents to diffuse into the polymer. The rate of spin probe mobilization depends on the hydrophilicity/hydrophobicity of both the polymer network and the solvent. However, the modified polymers, even the more hydrophobic ones, are hydrophilic in comparison to “really” hydrophobic polymers. This is probably caused by the character of the parent polymer (PVA), unreacted hydroxy groups. S++uiations of the ESR spectra have shown that in polymers with a higher degree of acetalization domains of different hydrophobicity are built in which the spin probes have a different mobility. © 1995 John Wiley & Sons, Inc.     

Spatially resolved degradation in heterophasic polymers by ESR imaging and FTIR: The case of propylene-ethylene copolymers

Heterophasic propylene-ethylene copolymers (HPEC) containing bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate (Tinuvin 770) as a hindered amine stabilizer (HAS) were thermally aged at 393 and 433 K. Two types of HPEC were examined, containing 25% and 10% ethylene (E), respectively, as ethylene/propylene rubber (EPR). Electron spin resonance (ESR) spectra of nitroxide radicals in HPEC were studied in the temperature range 100–433 K; the nitroxides were derived from the HAS and are termed HAS-NO. The results were compared with ESR spectra of the same radicals obtained first by oxidation of Tinuvin 770 and then were doped in HPEC and related homopolymers, polyethylene (PE) and polypropylene (PP); these nitroxides are termed “spin probes.” ESR spectra indicated that HAS-NO and the spin probes reside in a range of amorphous sites differing in their dynamic properties. The relative population of the sites was explained by assuming that the crystalline domains exert a restraining effect on chains located in vicinal amorphous domains. Spatial and temporal effects of the aging process were studied by ESR and ESR imaging (ESRI) of HAS-derived nitroxide radicals, and by FTIR of films prepared by compression molding. 1D ESRI enabled the visualization of an outer region of thickness ≈100 μm that contained a lower amount of nitroxides, and is believed to result from the loss of the stabilizer by diffusion (“blooming”) and possibly also in chemical reactions during aging. Two-dimensional spectral-spatial ESRI indicated the presence of nitroxide radicals in two amorphous sites, fast and slow; the corresponding relative intensity varied with sample depth. Both ESRI and FTIR experiments suggested a faster degradation rate in HPEC containing 25% E, as compared to 10% E; moreover, a larger Tinuvin 770 content in the polymers led to less efficient stabilization. FTIR spectra indicated increased ordering of polypropylene segments in HPEC during aging at 433 K.     

Spin Probe Study of IPN-like Systems based on Polyethylene and Polymethacrylates

A system similar to an interpenetrating polymer network (IPN), based on polyethylene/poly(methylmethacrylate-co-dodecyl methacrylate), was studied by the spin probe method with a nitroxide spin probe, 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO). The influence of changes in the composition of the copolymer (methyl methacrylate-co-dodecyl methacrylate) on the molecular mobility and composition of the IPNs is characterized by changes of the empirical spectral parameters T_50G and τ_R. The molecular mobility was shown to diminish with rising content of methyl methacrylate in the copolymer. © of SCI.     

Plasticizer diffusion into PVC particles as studied by ESR spin probe method

Summary The spin probe technique of electron spin resonance (ESR) spectroscopy has been applied for studying the plasticizer diffusion, migration, and redistribution processes in suspension polymerized PVC particles. In the first series of experiments six PVC powder samples with different K values (58, 61, 64, 67, 70, and 72) were mixed with diisooctyl phthalate (DOP) containing 10^–4 M 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO) as stable free radical spin probe. In the second run TEMPO-doped dry PVC powders were mixed with DOP plasticizer. Finally we studied the plasticizer transport phenomena between plasticized and non-plasticized polymer particles in powder mixtures. The observed diffusion properties of different samples were interpreted in terms of the different porosities of powders.     

Investigation of liquid diffusion into contact lenses using an electron spin resonance technique

In this study, electron spin resonance (ESR) spectroscopy was used for the first time to investigate liquid diffusion into contact lenses. As contact lenses are not paramagnetic substances, they were labeled with nitroxide spin probes to get an ESR spectrum. Thus, it gives a solid spin‐labeled ESR spectrum. The shape and intensity of the ESR signals depend on the environment of these spin probes. The spin probe environment began to change from solid to liquid if liquid were dropped into the system. Consequently the ESR spectra began to change with time, too. By following these changes, three distinct steps were found. Their diffusion coefficients were determined to be 6.38 × 10^?8 cm²/s for the first step (rapid decay region) and 0.37 × 10^?8 cm²/s for the second step (slow decay region), and 2.50 × 10^?8 cm²/s for the third and last step (desorption region). © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2942–2946, 2006     

Segment size in synthetic polymers by the spin-probe method

Following the approach of Kusumoto an equation is derived which relates the correlation time for the tumbling of a nitroxide spin probe in a polymer matrix with the parameter f, the ratio of the volumes of the probe and the polymer segment undergoing motion at T>T_g. For poly(vinyl acetate) probed with a series of nitroxides the correlation between f and the molecular volumes of the probes is poor, possibly because of wide variations in probe flexibility and polarity. An approximate version of the equation also permits evaluation of f from the parameter T_50G. Values of f calculated in this manner for the probe 2,2,6,6-tetramethyl-4-hydroxypiperidin-1-oxyl-benzoate in nine different polymers suggest that polymers with high glass transition temperatures have relatively bulky segments.     

Motional heterogeneity of segmented polyurethane-polymethacrylate mixtures: an influence of functional groups concentration

The motional heterogeneity in the polymer mixtures of segmented polyether polyurethane (PU) with carboxylic groups and methacrylic copolymer (PM) with tertiary amine groups was studied by the electron spin resonance (ESR)—spin probe and spin label methods. Pure polymer components containing varying amounts of functional groups and their 1:1 mass ratio mixtures have been analysed. The results of motional heterogeneity and polymer interaction were complemented with the glass transition temperatures, structural and morphological characteristics. Spin probed PU/PM mixtures indicate that the probe motion and the phase separation deduced from the temperature-dependent ESR spectra are sensitive to a free volume determined by the polymer-polymer interactions. The interaction between the two components in PU/PM mixture with the highest functional groups concentration disorganizes hard segment domains with spherulitic character at the microscopic level as compared with the ordered hard phase in the corresponding pure PU sample. The influence of PU hard and soft segments on the motional dynamics of PM chains is analysed from the ESR spectra of spin labelled PM chains in the mixtures with unlabelled PU components. The fractional amount of the PM fast motion depends on the temperature and concentration of functional groups.     

Motional heterogeneity and phase separation of semi-interpenetrating networks and mixtures based on functionalised polyurethane and polymethacrylate prepolymers

Semi-interpenetrating polymer networks (SIPNs) and polymer mixtures (1:1 mass ratio) based on segmented polyester polyurethane (PU) with carboxylic groups and methacrylic copolymer (PM) with tertiary amine groups were studied by the electron spin resonance (ESR) spin label method. The concentration of functional groups varied from 0 to 0.45 mmol g⁻¹ in both prepolymers. The ESR spectra of spin labelled PM component were used to characterise the heterogeneity of segmental motion and transitions due to the additional polymer interactions imposed by complementary functional groups. The results were deduced from the temperature dependent ESR spectra. Two component spectra reflect the effect of PU chains on segmental motion of the PM component below the macroscopic glass transition temperature, T_g. The ratio of the fast and slow component was related to the complex polymer-polymer interaction or extent of miscibility. Restrictions of segmental motion of PM chains increase with functional groups concentration and above certain concentration (0.25 mmol g⁻¹) PM segments in the network assess faster motion suggesting a change in the local packing density and domain structure. An increased miscibility and disorganisation of the ordered domains are confirmed by the loss of spherulitic morphology and crystallinity at higher functional groups concentration. PU/PM mixtures reveal similar motional behaviour as SIPNs of the same composition. However, the differences in the fractional amount of fast and slow motions confirm better interpenetration and interaction of the two polymers in the SIPNs. The results of motional heterogeneity and polymer interactions were complemented with the T_gs.     

Carbon radicals generated by solid polymers: Electron spin resonance spectroscopy for detection of species in water

Radicals generated in plastic medical devices (solid phase) by γ-rays or electron-beam irradiation during sterilization are known to cause oxidation of protein drugs, resulting in a loss or reduction in drug efficacy. The generation of radical species in water by the radical species in solid polymers has not been proved. Using electron spin resonance (ESR) spectroscopy, we confirm the generation of new radicals in water by γ-ray irradiated cyclic olefin polymers (COP). ESR measurements are obtained using 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL) as a spin probe and 5-(2,2-dimethyl-1,3-propoxycyclophosphoryl)-5-methyl-1-pyrroline N-oxide (CYPMPO) as a spin trap, in which the irradiated COP was immersed. The ESR signals indicate the TEMPOL radicals decline over time, suggesting the generation of new radicals. Conversely, the characteristic ESR signals of the adduct formed by the reaction between CYPMPO and the hydroxyl radical are observed. Thus, hydroxyl radicals are generated because of the migration of the radicals from COP to water. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48604.     

Main chain and segmental dynamics of semi interpenetrating polymer networks based on polyisoprene and poly(methyl methacrylate)

Main chain and segmental dynamics of polyisoprene (PI) and poly(methyl methacrylate) (PMMA) chains in semi IPNs were systematically studied over a wide range of temperatures (above and below T_g of both polymers) as a function of composition, crosslink density, and molecular weight. The immiscible polymers retained most of its characteristic molecular motion; however, the semi IPN synthesis resulted in dramatic changes in the motional behavior of both polymers due to the molecular level interpenetration between two polymer chains. ESR spin probe method was found to be sensitive to the concentration changes of PMMA in semi IPNs. Low temperature spectra showed the characteristics of rigid limit spectra, and in the range of 293-373 K, complex spectra were obtained with the slow component mostly arising out of the PMMA rich regions and fast component from the PI phase. We found that the rigid PMMA chains closely interpenetrated into the highly mobile PI network imparts motional restriction in nearby PI chains, and the highly mobile PI chains induce some degree of flexibility in highly rigid PMMA chains. Molecular level interchain mixing was found to be more efficient at a PMMA concentration of 35 wt.%. Moreover, the strong interphase formed in the above mentioned semi IPN contributed to the large slow component in the ESR spectra at higher temperature. The shape of the spectra along with the data obtained from the simulations of spectra was correlated to the morphology of the semi IPNs. The correlation time measurement detected the motional region associated with the glass transition of PI and PMMA, and these regions were found to follow the same pattern of shifts in α-relaxation of PI and PMMA observed in DMA analysis. Activation energies associated with the T_g regions were also calculated. T_50G was found to correlate with the T_g of PMMA, and the volume of polymer segments undergoing glass transitional motion was calculated to be 1.7 nm³. C-13 T_1ρ measurements of PMMA carbons indicate that the molecular level interactions were strong in semi IPN irrespective of the immiscible nature of polymers. The motional characteristics of H atoms attached to carbon atoms in both polymers were analyzed using 2D WISE NMR. Main relaxations of both components shifted inward, and both SEM and TEM analysis showed the development of a nanometer - sized morphology in the case of highly crosslinked semi IPN.     

Spinmarkierte Oligopeptide als substratanaloge Inhibitoren der Leucinaminopeptidase

Spin Labeled Oligopeptides as Substrate Analogous Inhibitors of Leucine Aminopeptidase The synthesis of L-Thr(Bu^t)-(L-Phe)_n-SL peptides C-terminal labeled with the nitroxide spin label 2,2,5,5-tetramethyl-3-amino-pyrrolidin-1-oxyl, is described and e.s.r. parameters of the labeled compounds are presented. As shown by e.s.r. spectroscopy studies Thr(Bu^t)-Phe-SL is a competitive inhibitor of the leucine aminopeptidase and is bound by the enzyme with high affinity.     

Study of miscibility enhancement in poly(styrene‐co‐4‐vinylphenol)/poly(ethylene oxide) blends by the spin labelling method

The electron spin resonance (ESR) spectra of end‐group spin labelled poly(ethylene oxide) (SLPEO) using 2,2,6,6‐tetramethyl‐piperdine‐1‐oxyl nitroxide and its blends with poly(styrene‐co‐4‐vinylphenol) (STVPhs) of different hydroxyl contents were recorded over a wide temperature range. For a blend of SLPEO and pure polystyrene (PS), the ESR spectrum was composed of a single motion component, indicating that PS was immiscible with PEO. For blends composed of SLPEO and different‐hydroxyl‐content STVPhs, two spectral components with different motion rates were observed over a certain temperature range. The difference between the motion rates should be attributed to micro‐heterogeneity in the blends, with the faster rate corresponding to a nitroxide radical motion trapped in the PEO‐rich domain and the slower rate corresponding to a nitroxide radical motion trapped in the STVPh‐rich domain. Variations in the values of a number of the ESR parameters (T_a, T_d and T_50G) and the apparent activation energy (E_a) with hydroxyl content in the blends indicated that the miscibility of the blends increased with increasing hydrogen‐bonding density due to specific interactions between the hydroxyl groups in STVPh and the ether oxygens in PEO. Copyright © 2004 Society of Chemical Industry     

Antioxidant action of a lipophilic nitroxyl radical,cyclohexane-1-spiro-2′-(4′-oxyimidazolidine-1′-oxyl)-5′-spiro-1″-cyclohexane,against lipid peroxidation under hypoxic conditions

Nitroxyl radicals are known to act as radical scavenging antioxidants. In the present study, a lipophilic nitroxyl radical, cyclohexane-1-spiro-2′-(4′-oxyimidazolidine-1′-oxyl)-5′-spiro-1″-cyclohexane (nitroxyl radical I) was synthesized and its antioxidant capacity was assessed in comparison with a hydrophilic nitroxyl radical, 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (Tempol). Both nitroxyl radical I and Tempol inhibited methyl linoleate oxidation induced by free radicals, and the efficacy increased with decreasing partial pressure of oxygen, the effect being more pronounced for nitroxyl radical I than Tempol. Their hydroxylamines inhibited lipid peroxidation more effectively than their corresponding parent nitroxyl radicals. In liposomal membranes, a synergistic effect was observed in the combination of nitroxyl radical I with ascorbic acid, whereas only an additive effect was observed between Tempol and ascorbic acid. The present study suggests that nitroxyl radical I and its hydroxylamine may act as potent antioxidants, especially in combination with ascorbic acid under hypoxic conditions.     

Paramagnetic conjugated polymers with stable radicals in side groups

Summary Poly(1,3-dihydroxy-2-(4-ethynylphenyl)-4,4,5,5-tetramethylimidazolidine), poly(2-(4-ethynylphenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl-3-oxide), poly(4-(4-ethynylbenzylideneamino)-2,2,6,6-tetramethylpiperidine) and poly(4-(4-ethynylbenzylideneamino)-2,2,6,6-tetramethylpiperidine-1-oxyl) were prepared from poly(4-ethynyl-benzaldehyde). The products were characterized spectroscopically (IR, UV, ¹H and ¹³C NMR) and with respect to the magnetic susceptibility. The polyradicals (1-oxyl compounds) showed paramagnetic properties obeying Curie's law.     

Study of the structure and transport of asymmetric polyamide membranes for reverse osmosis using the electron spin resonance (ESR) method

The electron spin resonance technique (ESR) was used to study the structure and transport of asymmetric aromatic polyamide membranes. TEMPO (2,2,6,6-tetramethyl-1-piperridinyloxy free radical) was used as a spin probe that was brought into the membrane either by (a) immersion ofthe membranes in aqueous TEMPO solutions, (b) reverse osmosis (RO) experiments with feed solutions involving TEMPO or (c) blending TEMPO in casting solutions. The membranes were further tested for the separation of sodium chloride and TEMPO from water by RO. It was concluded that aromatic polyamide membranes contain water channels in the polymer matrix like cellulose acetate membranes. The presence of such water channels allows aromatic polyamide membranes to be used as RO membranes. The diffusion of organic solutes through the water channels seems much slower in aromatic polyamide membranes than in cellular acetate membranes, which probably causes a higher separation of organic solutes by aromatic polyamide membranes than cellulose acetate membranes. A comparison was made with other RO membranes (cellulose acetate, CA) and ultrafiltration membranes (polyethersulphone, PES). It was observed that the ESR technique can be used to study the structure of OF and RO membranes. The presence of water channels in the polymer matrix seems indispensable for the RO membrane.     

Slow molecular motion in uniaxially stretched poly(ethyl methacrylate) as observed by double modulation electron spin resonance

Summary The double modulation ESR (DMESR) technique has been applied to investigate the change of very slow molecular dynamics ({ei235-1} >10^-7 s) of nitroxide spin probe in amorphous poly(ethyl methacrylate) (PEMA) at low drawing ratios. The polymer has been characterized by means of size exclusion chromatography (SEC) apparatus fitted with three detectors in series with continuous viscometer, low angle light scattering photometer and differential refractometer. The side chain group motion and segmental motion well below the glass transition temperature are discussed in terms of local density changes induced by orientational anisotropy of the chain segments.     

Spin probe study of semi‐interpenetrating polymer networks based on polyurethane and polymethacrylate functional prepolymers

The motional transition and heterogeneity of semi‐interpenetrating networks (SIPNs) based on polyurethane (PU) with carboxylic groups and methacrylic copolymer (PM) with tertiary amine groups were studied by the electron spin resonance (ESR) spin probe method. The concentration of functional groups in both prepolymers varied from 0 to 0.45 mmol g^?1. Spin‐probed SIPNs show that the temperature‐dependent spectra are sensitive to polymer interactions imposed by functional groups. These interactions determine the free volume distribution in the matrix and temperature at which motional transition takes place. The fraction of free volume increases with functional group concentration and reaches its maximum at 0.25 mmol g^?1. Further increases in the functional group concentration reduce the free volume. The results of the networks with strong interactions are discussed in terms of the interference of the plasticizing effect of the PU component and the formation of possible cluster cross‐links, which restricts segmental motions. Copyright © 2003 Society of Chemical Industry     

Distribution in correlation times for rotational diffusion of spin probes in polymers

Electron spin resonance spectra of poly(vinyl acetate) (PVAc) and plasticised PVAc doped with nitroxide spin probes show a double-peak feature at temperatures immediately below T_{50 G}. This can be explained on the basis that probe tumbling occurs with a distribution of correlation times τ_R arising from a distribution of free volume within the polymeric matrix. The two-component e.s.r. spectra arise because the distribution in τ_R of the probes straddles the value (c. 3 × 10^?9 s) which marks the change-over from fast- to slow-motion spectra. The implications of the existence of a distribution of τ_R values for calculations based on the assumption of a single value are examined. It is concluded that when probe tumbling is slow the error in τ_R could be significant, but for rapid tumbling (motionally narrowed spectra) the error is likely to be insignificant.     

New adducts of 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX)

Relatively stable equimolar adducts of 1,3,5-trinitro-1,3,5-triazacyclohexane were prepared with 2,6-lutidine-N-oxide, 4-hydroxy-1-butanesulfonic acid δ-sultone (1,4-butane sultone), and 2,2,6,6-tetramethyl-4-piperidone-1-oxyl. Only the last adduct is selective for RDX; the others also form adducts with 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane. A very labile RDX adduct is formed with tetrahydrothiophene-1-oxide. Some properties of the new adducts are presented.