Field-cycling 31P and 1H NMR relaxometry studying the reorientational dynamics of glass forming organophosphates.

We apply field-cycling (FC) 31P nuclear magnetic resonance (NMR) to access the reorientational susceptibility of two glass formers, m-tricresyl phosphate (m-TCP) and tri-butyl phosphate (TBP). Although FC 31P studies are still instrumentally demanding, together with FC 1H data, they provide site-resolved information. A crossover from dipolar relaxation at low frequencies to relaxation determined by chemical shift anisotropy at high frequencies and probed by conventional NMR is identified. A comparison is made between dielectric (DS) and depolarized light scattering (DLS) relaxation spectra demonstrating similar behavior close to Tg, including an excess wing contribution for m-TCP. The time constants of 31P NMR and DLS, probing the molecular core, agree. The 1H data monitoring the dynamics of the phenyl groups yield slightly shorter correlation times. At high temperatures, the DS relaxation spectra show a bimodal character: a fast component in agreement with 1H data, and a slow component much slower than 31P NMR and DLS suggest. We discuss the possible origins of the slow component. All time constants tend to merge toward Tg. Hence, we propose that site-specific dynamics disappear and a common α-relaxation establishes near Tg. In addition, we compare the diffusion coefficient D(T) determined by FC and static field gradient 1H NMR. Concerning TBP, we present FC 31P data of both α- and ß-processes. Regarding the latter, we compare the DS and NMR susceptibility on absolute scale, yielding a significantly stronger ß-relaxation in the 31P NMR spectra.

Molecular dynamics simulations vs field-cycling NMR relaxometry: Structural relaxation mechanisms in the glass-former glycerol revisited.

We combine field-cycling (FC) relaxometry and molecular dynamics (MD) simulations to study the rotational and translational dynamics associated with the glassy slowdown of glycerol. The 1H NMR spin-lattice relaxation rates R1(ω) probed in the FC measurements for different isotope-labelled compounds are computed from the MD trajectories for broad frequency and temperature ranges. We find high correspondence between experiment and simulation. Concerning the rotational motion, we observe that the aliphatic and hydroxyl groups show similar correlation times but different stretching parameters, while the overall reorientation associated with the structural relaxation remains largely isotropic. Additional analysis of the simulation results reveals that transitions between different molecular configurations are slow on the time scale of the structural relaxation at least at sufficiently high temperatures, indicating that glycerol rotates at a rigid entity, but the reorientation is slower for elongated than for compact conformers. The translational contribution to R1(ω) is well described by the force-free hard sphere model. At sufficiently low frequencies, universal square-root laws provide access to the molecular diffusion coefficients. In both experiment and simulation, the time scales of the rotational and translational motions show an unusually large separation, which is at variance with the Stokes-Einstein-Debye relation. To further explore this effect, we investigate the structure and dynamics on various length scales in the simulations. We observe that a prepeak in the static structure factor S(q), which is related to a local segregation of aliphatic and hydroxyl groups, is accompanied by a peak in the correlation times τ(q) from coherent scattering functions.

On the theory of deuteron NMR free induction decay of reptating polymer chains: Effect of end segment dynamics.

A self-consistent approximation beyond the Redfield limit and without using the Anderson-Weiss approximation for the Free Induction Decay (FID) of deuteron spins belonging to polymer chains undergoing reptation is formulated. The dynamical heterogeneity of the polymer segments created by the end segments is taken into account. Within an accuracy of slow-changing logarithmic factors, FID can be qualitatively described by a transition from an initial pseudo-Gaussian to a stretched-exponential decay at long times. With an increase in observation time, the contribution from end effects to the FID increases. In the regime of incoherent reptation, contributions to the FID from central segments yield an exponent of 1/4 for the stretched decay and contributions from end segments yield an exponent of 3/16. In the regime of coherent reptation, the central segments generate a stretching exponent of 1/2, whereas the end segments contribute with an exponent of 1/4. These predictions are shown to be in qualitative agreement with the experimental FIDs of perdeuterated poly(ethylene oxide) with molecular masses of 132 kg/mol and 862 kg/mol.

Dynamically asymmetric binary glass formers studied by dielectric and NMR spectroscopy.

We investigate the component dynamics in asymmetric binary glass formers. Focusing on the dielectric spectra of the high-Tg components m-tricresyl phosphate and quinaldine mixed with toluene as low-Tg component, the broadend spectra cannot be described by Kohlrausch or Cole-Davidson (CD) functions. Instead, we apply a generalized CD function which allows to control the width of the susceptibility independently of its high-frequency flank. The spectra show a common broadening and failure of the frequency-temperature superposition with increasing toluene concentration. This is confirmed by stimulated echo experiments showing an increased stretching of the probed orientational correlation function. In analogy to the definition of Tg, we consider "isodynamic points". For each component, a different but linear concentration dependence of 1/Tiso is revealed, indicating different time scales. Qualitativly, we do not find significant differences for the present mixtures with Tg-contrasts of 63-89K compared to those with larger Tg-contrast ( [Formula: see text] K): Whereas the high-Tg component shows relaxation features similar to those of neat glass formers, yet, with "atypical" weak relaxation broadening, the faster low-Tg component displays pronounced dynamic heterogeneities. This is supported by scrutinizing NMR relaxation data of several mixtures investigated previously as a function of concentration. A universal evolution of the dynamics of the high-Tg as well as the low-Tg component is suggested for mixtures with high [Formula: see text]Tg .

The dynamics of the plastically crystalline phase of cyanoadamantane revisited by NMR line shape analysis and field-cycling relaxometry.

The dynamics of cyanoadamantane (CN-ADA) in its plastically crystalline phase encompasses three processes: overall tumbling of the rigid molecule, rotation around the molecular symmetry axis, and vacancy diffusion. This makes CN-ADA a prototypical case to be studied by field-cycling as well as by conventional NMR relaxometry. Data are collected from 430 K down to about 4 K and frequencies in the range of 10 kHz-56 MHz are covered. The overall tumbling is interpreted as a cooperative jump process preceding along the orthogonal axis of the cubic lattice and exhibiting a temperature independent non-Lorentzian spectral density. Consequently, a master curve is constructed, which yields model-independent correlation times, which agree well with those reported in the literature. It can be interpolated by a Cole-Davidson function with a width parameter ßCD = 0.83. The uniaxial rotation persisting in the glassy crystal (T < Tg = 170 K) is governed by a broad distribution of activation energies, g(E). In this case, the standard master curve construction applied for the overall tumbling, for example, fails, as the actually probed distribution of correlation times G(ln τ) strongly changes with temperature. We suggest a scaling method that generally applies for the case that a relaxation process is determined by a distribution of thermally activated processes. Frequency as well as temperature dependence of the relaxation rate can be used to reconstruct g(E). In addition, g(E) is extracted from the proton line-shape, which was measured down to 4 K. Vacancy diffusion governs the relaxation dispersion at highest temperatures; yet, a quantitative analysis is not possible due to instrumental limitations.

Scaling analysis of the viscoelastic response of linear polymers.

Viscoelastic response in terms of the complex shear modulus G*(ω) of the linear polymers poly(ethylene-alt-propylene), poly(isoprene), and poly(butadiene) is studied for molar masses (M) from 3k up to 1000k and over a wide temperature range starting from the glass transition temperature Tg (174 K-373 K). Master curves G'(ωτα) and G″(ωτα) are constructed for the polymer-specific relaxation. Segmental relaxation occurring close to Tg is independently addressed by single spectra. Altogether, viscoelastic response is effectively studied over 14 decades in frequency. The structural relaxation time τα used for scaling is taken from dielectric spectra. We suggest a derivative method for identifying the different power-law regimes and their exponents along G″(ωτα) ∝ ωε″. The exponent ε″ = ε″(ωτα) ≡ d ln G″(ωτα)/d ln(ωτα) reveals more details compared to conventional analyses and displays high similarity among the polymers. Within a simple scaling model, the original tube-reptation model is extended to include contour length fluctuations (CLFs). The model reproduces all signatures of the quantitative theory by Likhtman and McLeish. The characteristic times and power-law exponents are rediscovered in ε″(ωτα). The high-frequency flank of the terminal relaxation closely follows the prediction for CLF (ε″ = -0.25), i.e., G″(ω) ∝ ω-0.21±0.02. At lower frequencies, a second regime with lower exponent ε″ is observed signaling the crossover to coherent reptation. Application of the full Likhtman-McLeish calculation provides a quantitative interpolation of ε″(ωτα) at frequencies below those of the Rouse regime. The derivative method also allows identifying the entanglement time τe. However, as the exponent in the Rouse regime (ωτe > 1) varies along εeRouse = 0.66 ± 0.04 (off the Rouse prediction εRouse = 0.5) and that at ωτe < 1 is similar, only a weak manifestation of the crossover at τe is found at highest M. Yet, calculating τe/τα= (M/Mo)2, we find good agreement among the polymers when discussing ε″(ωτe). The terminal relaxation time τt is directly read off from ε″(ωτα). Plotting τt/τe as a function of Z = M/Me, we find universal behavior as predicted by the TR model. The M dependence crosses over from an exponent significantly larger than 3.0 at intermediate M to an exponent approaching 3.0 at highest M in agreement with previous reports. The frequency of the minimum in G″(ωτα) scales as τmin ∝ M1.0±0.1. An M-independent frequency marks the crossover to glassy relaxation at the highest frequencies. Independent of the amplitude of G″(ω), which may be related to sample-to-sample differences, the derivative method is a versatile tool to provide a detailed phenomenological analysis of the viscoelastic response of complex liquids.

On the theory of the proton dipolar-correlation effect as a method for investigation of segmental displacement in polymer melts.

A thorough theoretical description of the recently suggested method [A. Lozovoi et al. J. Chem. Phys. 144, 241101 (2016)] based on the proton NMR dipolar-correlation effect allowing for the investigation of segmental diffusion in polymer melts is presented. It is shown that the initial rise of the proton dipolar-correlation build-up function, constructed from Hahn Echo signals measured at times t and t/2, contains additive contributions from both inter- and intramolecular magnetic dipole-dipole interactions. The intermolecular contribution depends on the relative mean-squared displacement of polymer segments from different macromolecules, which provides an opportunity for an experimental study of segmental translational motions at the millisecond range that falls outside the typical range accessible by other methods, i.e., neutron scattering or NMR spin echo with the magnetic field gradients. A comparison with the other two proton NMR methods based on transverse spin relaxation phenomena, i.e., solid echo and double quantum resonance, shows that the initial rise of the build-up functions in all the discussed methods is essentially identical and differs only in numerical coefficients. In addition, it is argued that correlation functions constructed in the same manner as the dipolar-correlation build-up function can be applied for an experimental determination of a mean relaxation rate in the case of systems possessing multi-exponential magnetization decay.

Non-polymeric asymmetric binary glass-formers. II. Secondary relaxation studied by dielectric, 2H NMR, and 31P NMR spectroscopy.

We investigate the secondary (ß-) relaxations of an asymmetric binary glass former consisting of a spirobichroman derivative (SBC; Tg = 356 K) as the high-Tg component and the low-Tg component tripropyl phosphate (TPP; Tg = 134 K). The main relaxations are studied in Paper I [B. Pötzschner et al., J. Chem. Phys. 146, 164503 (2017)]. A high Tg contrast of ΔTg = 222 K is put into effect in a non-polymeric system. Component-selective studies are carried out by combining results from dielectric spectroscopy (DS) for mass concentrations cTPP ≥ 60% and those from different methods of 2H and 31P NMR spectroscopy. In the case of NMR, the full concentration range (10% ≤ cTPP ≤ 100%) is covered. The neat components exhibit a ß-relaxation (ß1 (SBC) and ß2 (TPP)). The latter is rediscovered by DS in the mixtures for all concentrations with unchanged time constants. NMR spectroscopy identifies the ß-relaxations as being alike to those in neat glasses. A spatially highly restricted motion with angular displacement below ±10° encompassing all molecules is involved. In the low temperature range, where TPP shows the typical 31P NMR echo spectra of the ß2-process, very similar spectral features are observed for the (deuterated) SBC component by 2H NMR, in addition to its "own" ß1-process observed at high temperatures. Apparently, the small TPP molecules enslave the large SBC molecules to perform a common hindered reorientation. The temperature dependence of the spin-lattice relaxation time of both components is the same and reveals an angular displacement of the SBC molecules somewhat smaller than that of TPP, though the time constants τß2 are the same. Furthermore, T1(T) of TPP in the temperature region of the ß2-process is absolutely the same as in the mixture TPP/polystyrene investigated previously. It appears that the manifestations of the ß-process introduced by one component are essentially independent of the second component. Finally, at cTPP ≤ 20% one finds indications that the ß2-process starts to disintegrate. More and more TPP molecules get immobilized upon decreasing cTPP. We conclude that the ß-process is a cooperative process.

Non-polymeric asymmetric binary glass-formers. I. Main relaxations studied by dielectric, 2H NMR, and 31P NMR spectroscopy.

In Paper I of this series of two papers we study the main relaxations of a binary glass former made of the low-Tg component tripropyl phosphate (TPP, Tg = 134 K) and of a specially synthesized (deuterated) spirobichroman derivative (SBC, Tg = 356 K) as the non-polymeric high-Tg component for the full concentration range. A large Tg contrast of the neat components is put into effect. Dielectric spectroscopy and different techniques of 2H nuclear magnetic resonance (NMR) as well as of 31P NMR spectroscopy allow to selectively probe the dynamics of the components. For all concentrations, two well separated liquid-like processes are identified. The faster α2-process associated with the low-Tg component TPP shows pronounced dynamic heterogeneities reflected by quasi-logarithmic correlation functions at low TPP concentrations. The slower α1-process involves the reorientation of the high-Tg component SBC. Its correlation function is Kohlrausch-like as in neat glass formers. The corresponding time constants and consequently their glass transition temperatures Tg1 and Tg2 differ more the lower the TPP concentration is. Plasticizer and anti-plasticizer effect, respectively, is observed. At low temperatures a situation arises that the TPP molecules isotropically reorient in an arrested SBC matrix (Tg2 < T < Tg1). At T < Tg2 the liquid-like reorientation of TPP gets arrested too. We find indications that a fraction of the TPP molecule takes part in the slower α1-process of the high-Tg component. All the features known from polymer-plasticizer systems are rediscovered in this non-polymeric highly asymmetric binary mixture. In Paper II [B. Pötzschner et al., J. Chem. Phys. 146, 164504 (2017)] we study the secondary (ß-) relaxations of the mixtures.

Communication: Proton NMR dipolar-correlation effect as a method for investigating segmental diffusion in polymer melts.

A simple and fast method for the investigation of segmental diffusion in high molar mass polymer melts is presented. The method is based on a special function, called proton dipolar-correlation build-up function, which is constructed from Hahn Echo signals measured at times t and t/2. The initial rise of this function contains additive contributions from both inter- and intramolecular magnetic dipole-dipole interactions. The intermolecular contribution depends on the relative mean squared displacements (MSDs) of polymer segments from different macromolecules, while the intramolecular part reflects segmental reorientations. Separation of both contributions via isotope dilution provides access to segmental displacements in polymer melts at millisecond range, which is hardly accessible by other methods. The feasibility of the method is illustrated by investigating protonated and deuterated polybutadiene melts with molecular mass 196 000 g/mol at different temperatures. The observed exponent of the power law of the segmental MSD is close to 0.32 ± 0.03 at times when the root MSD is in between 45 Å and 75 Å, and the intermolecular proton dipole-dipole contribution to the total proton Hahn Echo NMR signal is larger than 50% and increases with time.

Change of translational-rotational coupling in liquids revealed by field-cycling 1H NMR.

Applying the field-cycling nuclear magnetic resonance technique, the frequency dependence of the (1)H spin-lattice relaxation rate, R1ω=T1(-1)ω, is measured for propylene glycol (PG) which is increasingly diluted with deuterated chloroform. A frequency range of 10 kHz-20 MHz and a broad temperature interval from 220 to about 100 K are covered. The results are compared to those of experiments, where glycerol and o-terphenyl are diluted with their deuterated counter-part. Reflecting intra- as well as intermolecular relaxation, the dispersion curves R1ω,x (x denotes mole fraction PG) allow to extract the rotational time constant τrot(T, x) and the self-diffusion coefficient D(T, x) in a single experiment. The Stokes-Einstein-Debye (SED) relation is tested in terms of the quantity D(T, x) τrot(T, x) which provides a measure of an effective hydrodynamic radius or equivalently of the spectral separation of the translational and the rotational relaxation contribution. In contrast to o-terphenyl, glycerol and PG show a spectral separation much larger than suggested by the SED relation. In the case of PG/chloroform mixtures, not only an acceleration of the PG dynamics is observed with increasing dilution but also the spectral separation of rotational and translational relaxation contributions continuously decreases. Finally, following a behavior similar to that of o-terphenyl already at about x = 0.6; i.e., while D(T, x) τrot(T, x) in the mixture is essentially temperature independent, it strongly increases with x signaling thus a change of translational-rotational coupling. This directly reflects the dissolution of the hydrogen-bond network and thus a change of solution structure.

Dynamics of asymmetric non-polymeric binary glass formers-A nuclear magnetic resonance and dielectric spectroscopy study.

We study a dynamically asymmetric binary glass former with the low-Tg component m-tri-cresyl phosphate (m-TCP: Tg = 206 K) and a spirobichroman derivative as a non-polymeric high-Tg component (Tg = 382 K) by means of (1)H nuclear magnetic resonance (NMR), (31)P NMR, and dielectric spectroscopy which allow component-selectively probing the dynamics. The entire concentration range is covered, and two main relaxation processes with two Tg are identified, Tg 1 and Tg 2. The slower one is attributed to the high-Tg component (α1-process), and the faster one is related to the m-TCP molecules (α2-process). Yet, there are indications that a small fraction of m-TCP is associated also with the α1-process. While the α1-relaxation only weakly broadens upon adding m-TCP, the α2-relaxation becomes extremely stretched leading to quasi-logarithmic correlation functions at low m-TCP concentrations-as probed by (31)P NMR stimulated echo experiments. Frequency-temperature superposition does not apply for the α2-process and it reflects an isotropic, liquid-like motion which is observed even below Tg 1, i.e., in the matrix of the arrested high-Tg molecules. As proven by 2D (31)P NMR, the corresponding dynamic heterogeneities are of transient nature, i.e., exchange occurs within the distribution G(lnτα 2). At Tg 1 a crossover is found for the temperature dependence of (mean) τα 2(T) from non-Arrhenius above to Arrhenius below Tg 1 which is attributed to intrinsic confinement effects. This "fragile-to-strong" transition also leads to a re-decrease of Tg 2(cm - TCP) at low concentration cm - TCP, i.e., a maximum is observed in Tg 2(cm - TCP) while Tg 1(cm - TCP) displays the well-known plasticizer effect. Although only non-polymeric components are involved, we re-discover essentially all features previously reported for polymer-plasticizer systems.

Depolarized light scattering spectra of molecular liquids: Described in terms of mode coupling theory.

Depolarized light scattering spectra of eight molecular liquids as obtained from applying tandem-Fabry-Pérot interferometry and double monochromator are analyzed in the frame work of the mode coupling theory (MCT). The susceptibility spectra are fitted to the numerical solution of the schematic F12 model of MCT and the validity of the asymptotic laws is discussed. The model is able to quantitatively describe the spectra up to the boiling point, where the main (structural) relaxation and the contribution of the microscopic (vibrational) dynamics essentially merge, and down to the moderately super-cooled liquid where glassy dynamics establishes. The changes of the spectra with temperature are mapped to only two control parameters, which show a smooth variation with temperature. Strong correlation between experimental stretching parameters and extrapolated values from the model is found. The numerical solutions are extrapolated down to Tc, where the asymptotic scaling laws can be applied. Although the spectra apparently follow scaling relations, the application of the asymptotic laws usually overestimates Tc by up to 12 K. In all the cases, the experimental spectra are outside the applicability regime of the asymptotic laws. This is explained by more or less strong vibrational contributions. Within a phenomenological approach which extends the spectral analysis down to Tg and which allows for separating fast and slow dynamics, the strength of the fast dynamics 1 - frel is revealed. It shows the cusp-like anomaly predicted by MCT; yet, the corresponding critical temperature is significantly higher than that derived from the F12 model. In addition, we demonstrate that close to Tg, the susceptibility minimum is controlled by the interplay of the excess wing and the fast dynamics contribution.

Dynamics of asymmetric binary glass formers. I. A dielectric and nuclear magnetic resonance spectroscopy study.

Dielectric spectroscopy as well as (2)H and (31)P nuclear magnetic resonance spectroscopy (NMR) are applied to probe the component dynamics of the binary glass former tripropyl phosphate (TPP)/polystyrene (PS/PS-d3) in the full concentration (cTPP) range. In addition, depolarized light scattering and differential scanning calorimetry experiments are performed. Two glass transition temperatures are found: Tg 1(cTPP) reflects PS dynamics and shows a monotonic plasticizer effect, while the lower Tg 2(cTPP) exhibits a maximum and is attributed to (faster) TPP dynamics, occurring in a slowly moving or immobilized PS matrix. Dielectric spectroscopy probing solely TPP identifies two different time scales, which are attributed to two sub-ensembles. One of them, again, shows fast TPP dynamics (α2-process), the other (α1-process) displays time constants identical with those of the slow PS matrix. Upon heating the α1-fraction of TPP decreases until above some temperature Tc only a single α2-population exists. Inversely, below Tc a fraction of the TPP molecules is trapped by the PS matrix. At low cTPP the α2-relaxation does not follow frequency-temperature superposition (FTS), instead it is governed by a temperature independent distribution of activation energies leading to correlation times which follow Arrhenius laws, i.e., the α2-relaxation resembles a secondary process. Yet, (31)P NMR demonstrates that it involves isotropic reorientations of TPP molecules within a slowly moving or rigid matrix of PS. At high cTPP the super-Arrhenius temperature dependence of τ2(T), as well as FTS are recovered, known as typical of the glass transition in neat systems.

Dynamics of asymmetric binary glass formers. II. Results from nuclear magnetic resonance spectroscopy.

Various (2)H and (31)P nuclear magnetic resonance (NMR) spectroscopy techniques are applied to probe the component dynamics of the binary glass former tripropyl phosphate (TPP)/polystyrene-d3 (PS) over the full concentration range. The results are quantitatively compared to those of a dielectric spectroscopy (DS) study on the same system previously published [R. Kahlau, D. Bock, B. Schmidtke, and E. A. Rössler, J. Chem. Phys. 140, 044509 (2014)]. While the PS dynamics does not significantly change in the mixtures compared to that of neat PS, two fractions of TPP molecules are identified, one joining the glass transition of PS in the mixture (α1-process), the second reorienting isotropically (α2-process) even in the rigid matrix of PS, although at low concentration resembling a secondary process regarding its manifestation in the DS spectra. Pronounced dynamical heterogeneities are found for the TPP α2-process, showing up in extremely stretched, quasi-logarithmic stimulated echo decays. While the time window of NMR is insufficient for recording the full correlation functions, DS results, covering a larger dynamical range, provide a satisfactory interpolation of the NMR data. Two-dimensional (31)P NMR spectra prove exchange within the broadly distributed α2-process. As demonstrated by (2)H NMR, the PS matrix reflects the faster α2-process of TPP by performing a spatially highly hindered motion on the same timescale.

Determining diffusion coefficients of ionic liquids by means of field cycling nuclear magnetic resonance relaxometry.

Field Cycling Nuclear Magnetic Resonance (FC NMR) relaxation studies are reported for three ionic liquids: 1-ethyl-3- methylimidazolium thiocyanate (EMIM-SCN, 220-258 K), 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM-BF4, 243-318 K), and 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6, 258-323 K). The dispersion of (1)H spin-lattice relaxation rate R1(ω) is measured in the frequency range of 10 kHz-20 MHz, and the studies are complemented by (19)F spin-lattice relaxation measurements on BMIM-PF6 in the corresponding frequency range. From the (1)H relaxation results self-diffusion coefficients for the cation in EMIM-SCN, BMIM-BF4, and BMIM-PF6 are determined. This is done by performing an analysis considering all relevant intra- and intermolecular relaxation contributions to the (1)H spin-lattice relaxation as well as by benefiting from the universal low-frequency dispersion law characteristic of Fickian diffusion which yields, at low frequencies, a linear dependence of R1 on square root of frequency. From the (19)F relaxation both anion and cation diffusion coefficients are determined for BMIM-PF6. The diffusion coefficients obtained from FC NMR relaxometry are in good agreement with results reported from pulsed- field-gradient NMR. This shows that NMR relaxometry can be considered as an alternative route of determining diffusion coefficients of both cations and anions in ionic liquids.

1H relaxation enhancement induced by nanoparticles in solutions: influence of magnetic properties and diffusion.

Magnetic nanoparticles that induce nuclear relaxation are the most promising materials to enhance the sensitivity in Magnetic Resonance Imaging. In order to provide a comprehensive understanding of the magnetic field dependence of the relaxation enhancement in solutions, Nuclear Magnetic Resonance (1)H spin-lattice relaxation for decalin and toluene solutions of various Fe2O3 nanoparticles was investigated. The relaxation experiments were performed in a frequency range of 10 kHz-20 MHz by applying Field Cycling method, and in the temperature range of 257-298 K, using nanoparticles differing in size and shape: spherical--5 nm diameter, cubic--6.5 nm diameter, and cubic--9 nm diameter. The relaxation dispersion data were interpreted in terms of a theory of nuclear relaxation induced by magnetic crystals in solution. The approach was tested with respect to its applicability depending on the magnetic characteristics of the nanocrystals and the time-scale of translational diffusion of the solvent. The role of Curie relaxation and the contributions to the overall (1)H spin-lattice relaxation associated with the electronic spin-lattice and spin-spin relaxation was thoroughly discussed. It was demonstrated that the approach leads to consistent results providing information on the magnetic (electronic) properties of the nanocrystals, i.e., effective electron spin and relaxation times. In addition, features of the (1)H spin-lattice relaxation resulting from the electronic properties of the crystals and the solvent diffusion were explained.

Simultaneous measurement of very small magnetic fields and spin-lattice relaxation.

A field cycling (FC) NMR experiment is presented which allows for the simultaneous determination of very small magnetic fields down to about 3 µT and the concomitant measurement of nuclear spin-lattice relaxation times in these fields. The technique will enable broadband spin-lattice relaxation dispersion experiments down to about 100 Hz (1)H Larmor frequency. Limitations of its applicability are discussed.

Primary and secondary relaxation process in plastically crystalline cyanocyclohexane studied by 2H nuclear magnetic resonance. II. Quantitative analysis.

We analyze the results of our previously reported 2H nuclear magnetic resonance (NMR) experiments in the plastically crystalline (PC) phase of cyanocyclohexane (Part I of this work) to study the fast secondary relaxation (or ß-process) in detail. Both, the occurrence of an additional minimum in the spin-lattice relaxation T1 and the pronounced effects arising in the solid-echo spectrum above the glass transition temperature T(g) = 134 K, allow for a direct determination of the restricting geometry of the ß-process in terms of the "wobbling-in-a-cone" model. Whereas at temperatures below T(g) the reorientation is confined to rather small solid angles (below 10°), the spatial restriction decreases strongly with temperature above T(g), i.e., the distribution of cone angles shifts continuously towards higher values. The ß-process in the PC phase of cyanocyclohexane proceeds via the same mechanism as found in structural glass formers. This is substantiated by demonstrating the very similar behavior (for T < T(g)) of spin-lattice relaxation, stimulated echo decays, and spectral parameters when plotted as a function of (taken from dielectric spectroscopy). We do, however, not observe a clear-cut relation between the relaxation strength of the ß-process observed by NMR (calculated within the wobbling-in-a-cone model) and dielectric spectroscopy.

Secondary relaxation processes in binary glass formers: emergence of "islands of rigidity".

In addition to the primary α-process, some neat glass formers show a well resolved secondary ß-process (type-B) or solely an excess wing (type-A). We investigate two binary glass forming systems composed of a type-A and a type-B component. (2)H nuclear magnetic resonance (NMR) spectroscopy is selectively applied to the type-B component in order to characterize the ß-process over a large range of mole fractions x in the glassy state. We demonstrate that for x ≳ 0.75 the apparent relaxation strength is constant, i.e., all molecules of type-B participate in the ß-process and the time constant τß(T) is independent of concentration. For x < 0.75, however, the apparent relaxation strength decreases abruptly, which we interpret in terms of population: below this concentration a fraction ξ of type-B molecules still exhibits essentially the ß-process of the neat system (in terms of time scale and mechanism), while others have been immobilized. The arise of such a scenario is verified by 2D and spin-lattice relaxation (2)H NMR techniques. In selective (2)H NMR experiments on the type-A component we observe a contribution to the ß-process of the type-B molecules at medium concentrations. The latter finding and the rather sharp threshold occurring at x ≈ 0.75 may indicate that the ß-process is a cooperative process.