Anthony T. Bullock

Segment size in synthetic polymers by the spin-probe method

Abstract 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>Tg. 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 T50G. 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.

E.s.r. and saturation transfer e.s.r. spectroscopy of spin-probed poly(vinyl acetate): A new spectral transition correlating with glass transition temperature

Abstract From the temperature dependence of the rotational frequency of a nitroxide spin probe dispersed in poly(vinyl acetate) the size of the polymer segment involved in the glass to rubber relaxation can be estimated. The calculation is based on a free volume model which relates f , the ratio of the volumes of the spin probe and the polymer segment, to the rotational frequency of the probe. An increase in f with increasing size of the probe is demonstrated. Saturation transfer electron spin resonance spectra of two nitroxide probes are shown to undergo a rapid change with temperature at a temperature T R which is characteristic of both the probe and its environment. T R corresponds to a pseudo-isofrequency point and is analogous to, though at a lower characteristic frequency than, the widely-used parameter T 50G . For the systems examined, T R correlates with the glass to rubber relaxation.

Electron spin resonance studies of spin-labelled polymers—VIII: Relaxation processes in low density polyethylene

Abstract Low density polyethylene was spin-labelled with an oxazolidine- N -oxyl group. Correlation times for the motion of the label were calculated both in the slow-motion region and in the motionally-narrowed region. In the latter situation, the central line shape was successfully simulated. A correlation map revealed the α- and β-relaxation processes in solid polyethylene, with activation energies of 48 and 28 kJ mol −1 , respectively. The segmental reorientation processes of polyethylene in the melt and in solution had activation energies of 15·2 and 22·9 kJ mol −1 , respectively. Taking into account the effect of viscous drag of the solvent, the two activation energies were almost the same suggesting that the same relaxation process occurs in the bulk molten polymer and in the isolated molecule. The model compound 2,2′,5,5′-tetramethyl-3-oxazolidinyloxy was synthesized and used as a spin-probe in the polymer. The spin-probe experiments gave a transition temperature attributed to the γ -relaxation.

Electron spin resonance studies of spin-labelled polymers. Part 7.—Dependence of rotational correlation times on solvent properties and polymer concentration

Rotational correlation times τc, for seven narrow fractions of spin-labelled polystyrene have been measured in cyclohexane and α-chloronaphthalene. Within the temperature range studied τc is greater in both of these solvents than in toluene, due to the higher viscosity of chloronaphthalene and the poor solvent properties of cyclohexane. In high molecular weight polymers, segmental reorientation is the main relaxation process having an activation energy of 25.1 ± 0.8 kJ mol–1 in cyclohexane and 26.4 ± 1.3 kJ mol–1 in chloronaphthalene, compared with our earlier figure of 18.0 ± 0.8 kJ mol–1 in toluene. When the activation energies for viscous flow of the solvent are deducted from these figures the internal energy barriers to rotation are obtained. These barriers are 8.8 kJ mol–1 in toluene and chloronaphthalene and at least 12.6 kJ mol–1 in cyclohexane. The higher internal barriers to segmental rotation in cyclohexane are probably due to the more tightly coiled conformation of the polymer chains. τc increases slowly with polymer concentration up to a critical value beyond which τc rises steeply. The critical concentration is close to the “entanglement” concentration as obtained by viscosity measurements. The entanglement concept is discussed in the light of these results.

Electron spin resonance detection of spin-trapped radicals formed during the glow-discharge electrolysis of aqueous solutions

Hydrogen atoms and hydroxyl radicals produced in the glow-discharge electrolysis of aqueous solutions have been identified by e.s.r. spectroscopy using 5,5-dimethylpyrroline-1-oxide (DMPO) as a spin trap. The trap is able to penetrate the primary reaction zone (at the discharge/solution interface) to scavenge for free radicals. No evidence for the presence of hydrated electrons (e–aq) was found.

Electron spin resonance studies of spin-labelled polymers: Part 6. End-labelled poly(methyl methacrylate)

Abstract The anionic synthesis of poly(methyl methacrylate) (PMMA) carrying a nitroxide spin-label at the chain end is described. With n-butyl lithium as initiator it is necessary to polymerize at −70°C and add the labelling agent, 2-methyl-2-nitrosopropane, to the active polymer anion at the same temperature. At higher temperatures initiator attack at the ester groups yields spin-labelled side chains. The correlation time for rotational diffusion of an end-labelled PMMA sample is 2·8×10−10sec which is shorter than that from fluorescence depolarization studies. The probable reasons for this discrepancy are discussed.

Electron spin resonance studies of spin-labelled polymers—18: Adsorption of spin-labelled poly(vinyl acetate) on silica surfaces

Abstract The adsorption of spin-labelled poly(vinyl acetate) from dilute solutions in ethyl acetate, chloroform and toluene onto three silica adsorbents of different surface silanol contents was studied. The adsorption capacities of the three silica samples, which decreased with decreasing surface silanol content, were dependent on the nature of the solvent, being greatest in the poor solvent toluene and least in the good solvent ethyl acetate. The ESR spectra of the polymer adsorbed on the silica of highest silanol content suggested that the polymer had a relatively flat conformation when toluene or chloroform was solvent and a more looped conformation when ethyl acetate was solvent. With the silica of intermediate silanol content, the polymer adsorbed from chloroform solution also had a loopy conformation. The silica of lowest silanol content was prepared by treating the first silica absorbent with trimethylchlorosilane. The line-shapes in the ESR spectrum of the labelled polymer adsorbed on this modified silica indicated a change in mode of adsorption.

Temperature-dependent hyperfine coupling constants in electron spin resonance. Part 6.—Planar and non-planar nitroxide radicals

The coupling constant to 14N in di-t-butyl nitroxide has a temperature coefficient which is negative and which decreases with increasing temperature. It is argued that the source of this temperature dependence is out-of-plane, or umbrella, vibrations about the non-planar nitrogen centre. Measurements of daN/dT have also been made for a series of nitroxides having well-defined geometries.

Temperature-dependent hyperfine coupling constants in electron spin resonance. Part 3.—Amino group rotation in the cation of p-phenylenediamine

The electron spin resonance spectrum of the radical cation of p-phenylenediamine has been recorded over a wide range of temperatures in the solvent methanol, ethanol, dimethyl sulphoxide and dimethylformamide. The variations with temperature of aHCH, aHNH2 and aN were measured and the results analysed in terms of restricted rotation of the amino groups using McLachlan SCF–MO calculations and numerical solutions to the Mathieu equation. A value of 9.9 ± 0.8 kJ mol–1 for V0, the two-fold barrier to rotation, fits the results for all four solvents. Values for the σ–π parameters QNN, QHCN and QHNH were in good agreement with literature values whilst the hyperconjugation parameter QNCNH was found to be 3.80 ± 0.20 mT. Evidence for the near-planarity of the nitrogen centres is presented.

Temperature-dependent hyperfine coupling constants in electron spin resonance. Part 4.—Out-of-plane vibrations of the ring protons in the cation of p-phenylenediamine

The effect of out-of-plane vibrations on the temperature coefficient of the coupling constant of the ring protons in the radical cation of p-phenylenediamine is considered. In accordance with an earlier suggestion by the authors it is found that this mechanism adequately accounts for the experimental values of daHCH/dT in four different solvents.