Nonlinear Electron Spin Resonance Research Articles

On the Influence of the Backbone Conformations of a Side Chain Liquid Crystal Polymer on the Fast Rotational Dynamics of a Probe Molecule

Dynamical processes effective on the nanometer length scale are of interest in polymeric systems due to their importance on the structural properties of the materials. In this study, a nm scale molecular probe is dissolved in a polymeric liquid crystal and its reorientation is studied by non-linear Electron Spin Resonance spectroscopy. Different dynamic behaviours as a function of the temperature are observed. Moreover it is found that the rotational motion of the molecular probe is sensitive to the different conformations of the polymer chain, which strongly depend on the thermal history of the material.

Short-time rotational relaxation in supercooled and glassy o-terphenyl: a nonlinear electron spin resonance study by using molecular probes with different symmetries

Short-time rotational relaxation in supercooled and glassy o-terphenyl: a nonlinear electron spin resonance study by using molecular probes with different symmetries

Scaling of the Rotational Relaxation of Tracers in o-Terphenyl: A Linear and Nonlinear ESR Study

The rotational dynamics at long and short times of nearly spherical and cylindrical tracers in supercooled and glassy o-terphenyl is investigated by linear and nonlinear electron spin resonance spectroscopy, respectively. The short-time and long-time dynamics are characterized by the effective correlation times τs and τl, respectively. At higher temperatures, the relaxation is exponential and the Debye−Stokes−Einstein law (DSE) holds, τs ≈ τl ∝ η. η is the shear viscosity. On cooling, the rotational dynamics is partially coupled to the viscosity. At lower temperatures the reorientation is activated. Interestingly, on the short time scale the rotational correlation times of the two tracers (τs) in a wide temperature range down to the activated regime collapse in a single curve by proper scaling. The scaling does not work at long times (τl).

Short-time rotational relaxation in supercooled and glassy o-terphenyl: A nonlinear electron spin resonance study by using molecular probes with different symmetries

Abstract The short-time rotational dynamics of nearly spherical and cylindrical spin probes in supercooled and glassy o-terphenyl is investigated by nonlinear electron spin resonance spectroscopy. At higher temperatures, the relaxation is exponential and the Debye-Stokes-Einstein law holds, τ α(η/T). τ is the rotational correlalion time, η the shear viscosity and T the temperature. On cooling, the coupling of the short-time rotational dynamics and the shear viscosity weakens, and a fractional Debye-Stokes-Einstein law τ α(η/T)ξ, with ξ constant, is observed. At the lower temperatures, the rotation is activated. A scaling of the short-time rotational dynamics of the different molecular probes is obtained for the overall range of temperature.

Linear and non-linear electron spin resonance study of the rotational diffusion of a molecular tracer in supercooled o-terphenyl

The long-time and the short-time rotational relaxation of a molecular tracer in supercooled o-terphenyl (OTP) have been studied by linear and non linear electron spin resonance (ESR) spectroscopy, respectively. At higher temperatures the related correlation times, τ l and τ s, are equal within the experimental errors, i.e. the relaxation is exponential, and well coupled to the viscosity, η. On cooling, τ s, and, at lower temperatures, τ l, are partially coupled to the viscosity in agreement with the fractional Debye–Stokes–Einstein (DSE) law τ i ∝( η/ T) ξ i , i=l,s with ξ i constant. For T < 330 K the rotational correlation function is not exponential.

Measurement of the Longitudinal Relaxation Time by Continuous-Wave, Nonlinear Electron Spin Resonance Spectroscopies

We characterize a continuous-wave, nonlinear electron spin resonance spectroscopy which detects the longitudinal component of the magnetization. It is demonstrated that the signal is proportional to the Laplace transform of a relaxation function with decay time equal to the longitudinal relaxation timeT1. The conclusion is reached by comparingT1to the effective timeT(eff)1being drawn by progressive saturation for a nitroxide radical dissolved in supercooledo-terphenyl.

Debye-Stokes-Einstein Fractional Law in a Polymeric Liquid Crystal: A Non-Linear ESR Study by Using Spin Probes with Different Symmetries

Abstract The reorientation of spin probes with very different aspect ratios, dissolved in a polymeric liquid crystal, is studied by non-linear Electron Spin Resonance. A scaling law between the rotational correlation time τ and the macroscopic viscosity, as measured via the dynamic shift factor aT , is found. A discussion on the current interpretations of the scaling, which takes the form τ a ξ T with 0 ≤ ξ ≤, is given.

Rotational Dynamics of a Molecular Probe in Tri-Cresyl Phosphate: From “Stick” to “Slip” Boundary Conditions

ABSTRACTThe rotational dynamics of a molecular probe dissolved in the glass-former tri-cresyl phospate has been studied by linear and non-linear Electron Spin Resonance spectroscopy. At the highest temperatures the Debye-Stokes-Einstein (DSE) law with stick boundary conditions holds. On cooling, a transition region is crossed and then a new regime sets in which is accounted for by the DSE law provided that slip boundary conditions are assumed. At the lowest temperatures the rotational diffusion and the shear viscosity are decoupled. The onset of the decoupling takes place close to Tc, the critical temperature predicted by the Mode Coupling Theory.

Microscopic transport properties in liquid crystalline polymeric matrices: dependence on the thermal history

The dependence on the thermal history of the dynamical and structural properties of a polymer liquid crystal has been evidenced by differential scanning calorimetry, infrared dichroism and non-linear electron spin resonance (ESR) spectroscopy in a wide range of temperatures. The behaviour of the microscopic viscosity as a function of both the isothermal annealing and the temperature is studied and its salient features are interpreted in terms of cooperativity. The multiexponential decay of the orientation correlation functions, relevant in the relaxation processes of linear and non-linear ESR spectroscopies, occurs in the supercooled and glassy phases, whereas the single exponential decay recovers above the isotropization temperature.

A linear and non-linear electron spin resonance study of orientational relaxation in semi-crystalline polymeric liquid crystals

Linear and non-linear Electron Spin Resonance spectroscopies have been designed to study the decay of the orientation correlations on different time scales. The reorientation of radicals («spin probes») dissolved in host phases (in the present case a polymeric liquid crystal (PLC)) may be investigated in the time window 10−11–10−5 s. Evidence of the non-exponential regression of fluctuations is given. The temperature dependence of the correlation time of the spin probe orientation is discussed in the case of semi-crystalline PLCs and contrasted with the case of amorphous PLCs.

The characterization of the orientational correlation decay in polymeric fluids by linear and non-linear ESR

The reorientational motion of probe molecules in polymeric mesophases is studied by linear and non-linear electron spin resonance. A non-exponential decay of the orientational correlations, which is homogeneous for observation times longer than 10 mu s, is found. In the undercooled phase of the polymer the apparent distribution of correlation times tau is extended over two decades. The temperature dependence of tau is well represented by a Vogel-Fulcher law and is consistent with the frequency-temperature superposition principle.

Nonlinear electron spin resonance techniques for the study of inhomogeneously broadened spectra

The application of nonlinear multiple irradiation electron spin resonance techniques to the case of inhomogeneously broadened spectra is studied. A detailed theoretical analysis within a unified method shows that the longitudinally detected electron spin resonance (LODESR) and the double modulation electron spin resonance (DOMESR) techniques represent two different aspects of the same physical effect and, under the same conditions, both give information on the longitudinal relaxation time T1 of the single spin packet. Previous papers, giving different interpretation for the double modulation spectrum, are critically reviewed. The usefulness of the two techniques in the case of inhomogeneously broadened lines is put into evidence by experiments with dextrose chars pyrolyzed at different temperatures. The results are in excellent agreement with theoretical results. The optimum application ranges of these nonlinear techniques are discussed and compared.

Non-linear electron spin resonance study of archaebacteria lipid dynamics

The spectral diffusion occurring in non-linear electron spin resonance experiments performed at a suitable frequency of field modulation is exploited to gain dynamical information on an unusual class of lipids extracted from bacteria living in extreme physical conditions (high temperature and very low pH). These lipids are quite different from the usual fatty acid lipids and their molecular structure seems to play a particular role in the stability of the plasma membrane of these bacteria. An accurate and extensive determination of the magnetic instrumental parameters allowed a reliable calculation of the rotational correlation time of suitable spin labelled lipids which mimic the dynamical behaviour of the lipids under investigation. The lipids investigated have been shown to undergo an anisotropic molecular motion characterized by a very slow correlation time even at very high temperatures. The very slow anisotropic motion of these lipids indicates the ability of their nonitol polar heads to establish strong cooperative interactions which, from a biophysical point of view, could be invoked to explain the great stability of the plasma membrane even under those extreme environmental conditions.