EPR Investigations of Spin-Probe Dynamics in Aqueous Dispersions of a Nonionic Amphiphilic Compound

Dynamics of various spin probes in aqueous dispersions of nonionic amphiphilic compound, [poly(oxyethylene) hydrogenated castor oil, HCO], were investigated by EPR (electron paramagnetic resonance) and saturation recovery (SR) spectroscopies. Partitioning, rotational correlation time (τ_R), rotational diffusion coefficient, and electron spin-lattice relaxation time (T _1e) in dispersions of the HCO membrane were obtained. The partitioning of water soluble spin probes, DTBN and TEMPO, in the aqueous and vesicle phases was determined by an EPR linewidth simulation as a function of temperature. The results suggest that DTBN and TEMPO have a similar partitioning in the vesicle phase throughout the temperatures studied. The longer τ_R and shorter T _1e (~0.33 μs) values of DTBN in the vesicle phase were obtained, and could be attributed to the probe environment in the membrane. The simulation results for fast tumbling probes were quite different from those of conventional intensity analysis (spectral parameter, f). Thus, the simulation and T _1e analyses have provided a quantitative understanding of the probe dynamics in both phases. Aliphatic spin probes, doxylstearic acids (DSAs) and 3β-doxyl-5α-cholestane (CHL), were used for monitor of various membrane motions. The EPR spectra were quantitatively analyzed by a slow tumbling simulation. The rotational diffusion coefficients and order parameter were obtained by the simulation. In addition, the direct observations of the behavior of the probes were measured by SR method. The results were consistent with T _1e obtained for spin probes. Thus, the quantitative results regarding EPR, SR method, various simulation analyses have provided detailed information regarding physicochemical properties of the various moieties of the probe region in the amphiphilic compound.     

Permeable domains of segmented polyurethanes studied with paramagnetic spin probe

Studies on the permeable regions of the dense polyurethane-based membranes were performed by electron spin resonance spectroscopy (ESR) using TEMPO spin probe incorporated into the membrane via diffusion from the vapour phase. The ESR spectra were measured as a function of temperature and microwave power for polyurethanes (PU) varying in the molecular structure and morphology. It was found that the TEMPO spin probe exhibited anisotropic rotation whose anisotropy increased as temperature decreased and was more pronounced for PU with shorter soft segments. The simplified method was used to obtain apparent correlation time τ_c enabling the comparison of the polyurethanes studied. This approach was based on the Arrhenius relation of τ_c vs. 1/T determined from motionally narrowed ESR spectra and on the assumption that this behaviour prevails over a broader temperature range at temperatures generally greater than T_g of a given polymer.     

Relationships between permeation properties of the polyurethane-based pervaporation membranes and their structure studied by a spin probe method

The cyclohexane and water permeability properties as well as the structure of the polyurethane-based pervaporation membranes were investigated. The polyurethanes (PU) were synthesised from poly(oxytetramethylene) diols (PTMO) of various molar masses, 2,4-tolylene diisocyanate (2,4-TDI), and different chain extenders used in various molar ratios. The microstructure of the synthesised PU was investigated by means of the density measurements, DSC method, and ESR technique with a spin probe TEMPO incorporated into PU via diffusion. The results show that only the ESR method was sensitive to all structural modifications applied enabling the correlations between the PU molecular structure and the mobility of the spin probe to be established. The diffusion coefficients (D) of cyclohexane and water were calculated based on the pervaporation results and the sorption data, and were correlated with the rotation correlation time (τ) of the spin probe as a measure of the microstructure of the permeable domains. The exponential and linear empirical equations were obtained for the PU/cyclohexane and PU/water systems, respectively, as a result of the fitting procedure applied. These correlations demonstrate that a spin probe method might be useful to characterise free volume, especially in case of complex membrane materials, and to predict the flux of permeant through the pervaporation membranes.     

ESR spectra of spin probe in PPO membrane

Poly(phenylene oxide) (PPO) polymer's membranes (dense) were prepared by blending spin probes (TEMPO, 5-, 12- and 16-doxylstearic aid) in the casting solution used for the preparation of membranes. It was noticed that the shape and size of the probe influence the ESR spectra of the NO radical in the poly(phenylene oxide)membrane. Unexpectedly, from the shape of the ESR signal it was noticed that of the NO radical of TEMPO in PPO membrane was more mobile than in water media. However, the motion of the NO radical of 16-doxylstearic acid was higher than NO of 5- and 12-doxylstearic acid when the radicals were in the PPO membrane. This could be due to the inductive effect from COOH group. The Hamiltonian parameters of the ESR signal indicated that all the probes were not randomly distributed in PPO membrane, but some probes were in orderly fashion.     

Motional heterogeneity of polystyrene-block-polybutadiene: a spin probe study

Polystyrene-block-polybutadiene copolymers (SB) with 0.5 mass fraction of styrene were studied by electron spin resonance (ESR) of nitroxide spin probes. The influence of the block length ( and ) and the solvation power of casting solvents on the motional dynamics of spin probe were measured over a wide temperature range. Two nitroxide radicals as spin probes were selected: 2,2,6,6-tetramethyl-4-hydroxypiperidine-1-oxyl benzoate (BzONO) and 2,2,6,6-tetramethyl-4-hydroxypiperidine-1-oxyl (Tempol). Irrespective of the spin probe used two ESR spectral components differing in their motional properties above the phase transition of polybutadiene blocks (PB) were observed. The fast component was assigned to spin probes located in polybutadiene-rich domains and the slow component to spin probes in polystyrene-rich domains. The range of two spectral components and the phase transition of the slow ESR component, T_5mT, depend on the block length. The influence of the interphase and accumulation of free volume in the interphase on the Tempol probe motion was investigated by changing copolymer morphology in the films casted from selective and nonselective solvents. The analysis of the motional heterogeneity from the ratio of the fast and slow motional component presents evidence that in the selective solvent for polystyrene (PS) blocks (2-butanone) the most irregular structure with a large interphase is formed. The difference in fast motion of spin probes indicates that the motional dynamic is related to the change of domain structure.     

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.     

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.     

Mechanism of migration from agglomerated cork stoppers: I. An electron spin resonance investigation

Related to the study of interactions between food and packaging, migration from agglomerated cork stoppers was studied by electron spin resonance. Paramagnetic probes were incorporated separately in the adhesive or in the cork granules. The finished cork was obtained by an individual molding procedure. The behavior of aminoxyl probes, differing by their functional groups, was studied. Free probes, initially incorporated into the adhesive phase, partitioned into cork during processing. Their migration occurred from both the cork and the adhesive phases. With amino‐TEMPO, a probe covalently bonded to the adhesive phase, it was possible to study the penetration of the alcoholic simulant of wine (12% ethanol) into the whole structure of the cork and to demonstrate that strong swelling took place. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2644–2654, 2002     

Effects of Cisplatin and its analogues on the permeability of human erythrocyte membrane

The biomembrane is postulated as the initial target when Platinum(II) complexes attack cells. In this work, a spin-labeling ESR technique has been used to study the effects of cis-DCDP, cis-DBDP, cis-DIDP, trans-DCDP, and cis-DADP on the permeability of human erythrocyte membrane. We monitored the reduction processes of the ESR signal of a nitroxide spin label, (TEMPO), which leaks out through the membrane and is reduced by the external ascorbate. Our results indicate that cisplatin and its analogues can enhance the permeability of membranes to small moieties such as TEMPO and ascorbate, and the differences between these compounds are related to features of the leaving group. In addition, changes in the order parameter of 5DS spin label in membrane indicate that hydrolysis of these Pt(II) complexes result in membrane damage.     

Polyisoprene matrix inhomogeneity studied by nitroxide spin probe motion

Summary The spin probe ESR method was applied to study natural rubber and synthetic polyisoprenes with different content of cis-configuration over a wide temperature range. The ESR spectra of natural rubber near and above the glass transition indicate the existence of two distinctly different mobilities as a consequence of the spin probe distribution in sol and gel phase. The results show that the spin probe method can yield information about the inhomogeneity of the polyisoprene matrix and the character of gel phase.     

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     

Temperature dependence of molecular motions in the polyurethane-based membranes studied with paramagnetic spin probe

This third paper in this series regarding structure and dynamics of the polyurethane-based membranes studied with TEMPO spin probe presents the results of the ESR measurements performed to characterise mobility of segments in the permeable regions of the membranes. The variations of the spin probe motions with temperature have been analysed for the series of polyurethanes (PU) differing in molecular structure and compared with the results of the DSC studies. Along with T_50 G, the other temperatures, T_n, T_i, T_τ, at which the significant change in dynamics occurs have been determined and correlated with the length of the PU soft and hard segments, and then discussed with regard to the respective relationships of T_g. The results have demonstrated the sensitivity of the ESR method to the segmental motions, which have not been detected by the DSC technique. From the DSC and ESR data the size of the motional chain segment in various PUs has been estimated, which has been found to follow the trend that polymers with higher T_g have bulkier segments. Two unusual observations have been made, concerning the deviation from the Arrhenius relation at high temperature for some PUs, and the increased mobility of the TEMPO molecules in some poly(urethane-urea)s after their thermal treatment. These results have been interpreted so far either in terms of the possible translational diffusion of the TEMPO molecules for those PUs showing lesser amount of the hard segments, or in terms of the increased free volume resulting from the temperature induced structural changes within the permeable regions of poly(urethane-urea)s.     

Molecular Dynamics and Partitioning of Di-<Emphasis Type="Italic">tert</Emphasis>-butyl Nitroxide in Stratum Corneum Membranes: Effect of Terpenes

In this work, we have used electron paramagnetic resonance (EPR) spectroscopy of the small spin label di-tert-butyl nitroxide (DTBN), which partitions the aqueous and hydrocarbon phases, to study the interaction of the terpenes α-terpineol, 1,8-cineole, l(−)-carvone and (+)-limonene with the uppermost skin layer, the stratum corneum, and the membrane models of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). The EPR spectra indicated that the terpenes increase both the partition coefficient and the rotational correlation time of the spin labels in the stratum corneum membranes, whereas similar effects were observed in the DMPC and DPPC bilayers only at temperatures below the liquid-crystalline phase. The EPR parameter associated to probe polarity inside the membranes showed thermotropically induced changes, suggesting relocations of spin probe, which were dependent on the membrane phases. While the DMPC and DPPC bilayers showed abrupt changes in the partitioning and rotational correlation time parameters in the phase transitions, the SC membranes were characterized by slight changes in the total range of measured temperatures, presenting the greatest changes or membranes reorganizations in the temperature range of ~50 to ~74 °C. The results suggest that terpenes act as spacers, weakening the hydrogen-bonded network at the polar interface and thus fluidizing the stratum corneum lipids.     

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.     

Spin-Probe Investigations of Head Group Behavior in Aqueous Dispersions of a Nonionic Amphiphilic Compound

Head group behavior of nonionic amphiphilic compound, (poly(oxyethylene) hydrogenated castor oil, HCO), in aqueous dispersions were investigated by EPR (electron paramagnetic resonance) in conjunction with a modern slow-tumbling simulation. The aliphatic spin probes, 5-doxylstearic acid (5-DSA) and 3β-doxyl-5α-cholestane (CHL), were used to obtain fluidity of the surface region of the membrane. The order parameter (S ₀) using the simulation for 5-DSA and CHL in the region were approximately 0.4 and 0.2, respectively. The ordering results suggest that the head group region of the membrane is somewhat fluid. The rotational diffusion coefficients (R _⊥ ≈ 1/(6τ_R)) for the probes were 3.4 × 10⁷ and 7.1 × 10⁷ s⁻¹, respectively. Activation energies, calculated using the temperature dependence of diffusion coefficients, were 18 and 17 kJ/mol for the probes. The EPR results imply that the CHL probe in the HCO membrane has quite different behavior in comparison with that of PC (phosphatidylcholine) from egg. Thus, the present EPR analyses have provided quantitative insight into the surface region of the amphiphilic membrane.     

The study of poly(styrene-co-p-(hexafluoro-2-hydroxylisopropyl)-α-methyl-styrene)/poly(propylene carbonate) blends by ESR spin probe and Raman

Different hydroxyl content poly(styrene-co-p-(hexafluoro-2-hydroxyisopropyl)-α-methylstyene) [PS(OH)] copolymers were synthesized and blends [noted for PP-X] with poly(propylene carbonate) [PPC] were prepared by casting from chloroform solution. The miscibility, micro heterogeneity and hydrogen bonding interaction of the component polymers were investigated by Differential Scanning Calorimetry (DSC), Electron Spin Resonance (ESR) spin probe method and Micro Raman spectroscopy. DSC results showed that the PP-2, PP-5, PP-8, PP-12 blends exhibited two distinct T_gs, indicating immiscibility, while the PP-20 and PP-27 blends were miscible with the existence of a single T_g. ESR results indicated that the probe molecule: Tempo couldn't give clear micro phase separation or miscibility information and thus was not sensitive to the investigated polymer blends system. On the contrary for all the blends spin probed with the probe molecules: Tempol and Tamine, two spectral components with different rates of motion: ‘fast’ and ‘slow’ motion were observed in different temperature range, which indicated the existence of micro heterogeneity on the molecular level; the more mobile PPC-rich micro phase and the more rigid PS(OH) rich micro phase. In addition, the scale of miscibility was progressively enhanced due to the increasing hydrogen bonding interaction between the hydroxyl in PS(OH) and the oxygen atoms in PPC. Meanwhile it was found that the degree of the probe molecule rotation detectable in the ESR spectrum was dependent on the polymer matrix rigidity and the strength of the hydrogen bonding between the probe molecule and the polymer matrix. Micro Raman substantiated the existence of the PS(OH)-rich micro phase and the PPC-rich micro phase. The hydrogen bonding strength between PS(OH) and PPC and the mixing level of the component polymers were increased gradually with the increase of hydroxyl content in the PS(OH) copolymer.     

Structural study of a varnish by electron spin resonance

Related to the study of interactions between food and aluminum cans, structural features of a vinylic organosol coating were studied by electron spin resonance (ESR). Paramagnetic probes were incorporated separately in the normal formulation of a widely used varnish, which was then cured at high temperature for different times. The behavior of aminoxyl probes, differing by their volatility and their functional groups, was studied. When 4-amino-2,2,6,6-tetramethylpiperidionxy (3) was used as probe, it grafted onto the polymeric chains during curing. Two transitions could then be observed, corresponding to NO^˙ in two environments, which were assigned to the PVC (T_fs, 132–142°C) and to the epoxyphenolic (EP) (T_50G 142–152°C) phases of the varnish. This was confirmed by a separate study of the PVC and EP constituents. Both transition temperatures depended on the extent of curing, suggesting that the PVC phase was plasticized by reactive constituents of the coating. The transition temperatures were also influenced by the extent of penetration of the PVC in the EP phase. Probes which could not graft on the polymer were lost by volatilization during curing. © 1996 John Wiley & Sons, Inc.     

Molecular mobility in starchy materials studied by electron spin resonance

Phase transitions in hydrated starch–sorbitol system were investigated both by molecular (electron spin resonance, ESR) and macroscopic (differential scanning calorimetry, DSC) methods. In rapid‐tumbling region, one did not observe the same phenomena by DSC and by ESR. The transitions observed by ESR, which seemed to reflect more the interactions probe–matrix than plasticization, probably corresponded to the interactions probe–sorbitol. For system concentrated in plasticizer, it was conceivable to admit that a demixion of sorbitol occurred. In slow‐tumbling region, a correspondence in temperature mobility changes measured by mechanical spectroscopy and ESR was observed. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 990–997, 2003     

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.