The use of DSC to determine the Curie temperature of metallic glasses

The subject of metallic glasses is introduced. These materials have technological potential, but only if their stability is adequate. It is, therefore, important to study their transformations on annealing, especially structural relaxation. The ferromagnetic Curie temperature, T_c, is a particularly useful parameter for monitoring structural relaxation and it can be measured by differential scanning calorimetry (DSC). The DSC method is convenient and accurate if temperature lags are corrected for. Ways of performing this correction in the DuPont DSC system are reviewed and a new method is proposed and tested. The experimental details of determining the T_c of metallic glasses are given. Finally, the relaxation behaviour of Fe₈₀B₂₀ glass, as studied by measuring its T_c, is reviewed. This behaviour is similar to that of conventional oxide glasses.     

NMR and nuclear spin relaxation of cement and concrete materials

Recent highlights of NMR works are reviewed for various cement-based materials. The emphasis is on nuclear magnetic relaxation methods at variable magnetic fields for continuous characterization of the evolving microstructure of various cementitious materials. Also, the recent applications of 2-dimensional T₁–T₂ and T₂–store–T₂ correlation experiments for characterization of water exchange in connected micropores of cement pastes are reported.     

Pulsed NMR of cis-polyisoprene solutions T1 and T2 relaxations,free volume,viscosity relationships

Previous pulsed NMR studies of polyisoprene have largely been concerned with entangled or crosslinked networks. This paper deals with (i) the relaxation of high molecular weight entangled; (ii) cross-linked; (iii) monodisperse low molecular weight; and (iv) high molecular weight polymer in the presence of tetrachloroethylene which, by increasing molecular mobility, can be expected to influence the NMR relaxation. For all four types of polyisoprene, the spin-lattice T₁, relaxation shows a minimum with position depending only on the free volume, as influenced by changes in temperature T and polymer concentration v₁,. For monodisperse polyisoprene of molecular weight 7200, insufficient to form an entangled network, the spin-spin relaxation decay constant T_2L is quantitatively related to the free volume ₁ by two parameters A′ and B″ when the free volume is altered by a change in temperature, or in polymer concentration (10–100/). This can also be expressed in the form where the parameter T_∞ at 100% concentration agrees with the value used to describe rheological properties. At other concentrations of polymer, T_∞ and B′ can be derived quantitatively from the coefficients of volume expansion of polymer and solvent. The variation of T_2L with molecular weight (T_2L ∝ M^?0.5) occurs via the A′ parameter. It is concluded that T_2L can be quantitatively related to the free volume available for molecular motion (as influenced by temperature and solvent concentration) as well as to molecular weight. Furthermore T_2L is simply related to viscosity n, over a wide range of temperatures and concentrations. T₂ can be used to analyse the molecular motions involved in theology.     

Pulsed NMR study of proton mobility in a hydrogen tungsten bronze

Crystalline hydrogen tungsten bronze H_0.46WO₃ was prepared by reduction of WO₃ single crystals. NMR relaxation times T₂, T₁, and T_1? were measured for 80 K < T < 450 K at 16 MHz and second moments for 160 K < T < 450 K at 100 MHz. The relaxation data were analyzed in terms of proton diffusion to give an activation energy of about 16 kJ mole^?1 and a correlation time preexponential factor of about 70 nsec for the process.     

Dynamics of [Zn(D2O)6] in [Zn(D2O)6][SiF6] crystal as studied by 1D, 2D spectra and spin-lattice relaxation time of H NMR

The dynamics of [Zn(D₂O)₆]²⁺ in [Zn(D₂O)₆][SiF₆] was investigated by ²H NMR one-dimensional spectra, two-dimensional exchange spectra and spin-lattice relaxation time (T₁). The lineshapes of those spectra and T₁ were dominated by the 180° flip of the water molecules and the reorientation of [Zn(D₂O)₆]²⁺ about the C₃ axis. The variation of lineshape of the one-dimensional spectrum below room temperature can be explained by only the 180° flip of the water molecules. The spectrum at room temperature showed a typical shape due to the rapid 180° flip of water molecules. The change in lineshape of the one-dimensional ²H NMR spectrum is caused by the three-site jump of [Zn(D₂O)₆]²⁺ about its C₃ axis above 333 K. Information of the reorientation of [Zn(D₂O)₆]²⁺ below 333 K could not be obtained from the one-dimensional spectrum and T₁. In this temperature range, the two-dimensional exchange spectrum was effective for analysis of molecular motion. The effects of multiple motions, the 180° flip of the water molecules and the reorientation of [Zn(D₂O)₆]²⁺ about the C₃ axis, on the lineshape of the two-dimensional exchange spectrum were studied using spectral simulation.     

Simultaneous measurements of T1 and T2 during fast polymerization reaction using continuous wave‐free precession NMR method

Continuous wave‐free precession (CWFP) pulse sequence employing time domain nuclear magnetic resonance spectroscopy (TD‐NMR) was used to measure longitudinal (T₁) and transverse relaxation times (T₂), during the cure of a commercial epoxy resin (Araldite^TM) with a 10‐min solidification time. The intensity of the NMR signal after the first pulse and in the CWFP regime were used to monitor the concentration of the monomers, and the relaxation times were used to monitor the chain mobility. The main advantage of CWFP over the standard methods to measure relaxation times, inversion recovery (inv‐rec) for T₁ and Carr‐Purcell‐Meiboom‐Gill (CPMG) for T₂, is that the measurement of both relaxation times can be performed in a fast and single NMR experiment and, therefore, using a single reaction batch. CWFP is also as fast as the CPMG measurement but at least fivefold faster than the method to obtain T₁ using null point approximation in the inv‐rec method. Therefore, the CWFP sequence can be used as a fast and general method to measure relaxation times in polymerization reactions, even with fast solidification time. As a TD‐NMR technique, CWFP can be employed in any low‐cost bench top TD‐NMR equipment commonly used in an academic or industrial laboratory. Copyright © 2012 John Wiley & Sons, Ltd.     

EFfect of alumina modification on the structure of cobalt-containing Fischer-Tropsch synthesis catalysts according to internal-field <Superscript>59</Superscript>Co NMR data

An internal-field ⁵⁹Co NMR study of cobalt-containing Fischer-Tropsch synthesis catalysts supported on different alumina modifications was reported. The Co/δ-Al₂O₃ sample was shown to contain single-domain fcc packing and stacking faults, whereas Co/γ-Al₂O₃ gave signals from the fcc domain walls, hcp and stacking faults, thus indicating differences in the particle size of the studied samples. T ₂ relaxation times were measured; their distribution in a spectrum is non-uniform, which allows signals to be distinguished by their relaxation times. Quantitative measurements of the relative atoms content in different packings revealed that the catalysts have mostly a defect structure. A brief historical background was presented to characterize the internal-field ⁵⁹Co NMR technique, the related problems, and different approaches to acquired data interpretation.     

A relaxation time study of the electron-beam induced polymerization of 1,6-hexanediol diacrylate

NMR measurements have been used to characterize the electron-beam (EB) induced polymerization of 1,6-hexanediol diacrylate. Hydrogen T₁, T_1θ, and T₂ values have been measured as a function of radiation dose. The T₂ signal consists of a Gaussian component and a longer component consisting of two exponentials. The sum of the Gaussian and the short exponential intensities is strongly correlated with the polymer fraction determined by gel extraction. T₂ values associated with the exponential component decrease rapidly with increasing dose. The T₁ relaxation consists of two exponentials for low dose and a single exponential for high dose. The T_1θ relaxation consists of from two to three exponential components, and the departure from a single exponential decreases with increasing dose. T₁ and T_1θ intensities are poorly correlated with gel extraction results. T₁ and T_1θ minima occur at intermediate radiation dose, and T_1θ depends on H₁ at high dose. Limited spin diffusion plays an important role in the T₁ and T_1θ relaxation. Hydroquinone has little effect on the EB-induced polymerization.     

Spin-lattice relaxations study of water mobility in natural natrolite

The mobility of water molecules in natural natrolite (Na₂Al₂Si₃O₁₀?2H₂O) is investigated by the ¹H NMR method. The spin-lattice relaxation times in the laboratory and rotating frames (T₁ and T_1ρ) are measured as a function of the temperature for a polycrystalline sample. From experimental T₁ data it follows that at T > 286 K the diffusion of water molecules along channels parallel to the c axis is observed. From experimental T_1ρ data it follows that at T > 250 K the diffusion of water molecules in transversal channels of natrolite is also observed. At a low temperature (T < 250 K) the dipolar interaction with paramagnetic impurities (presumably Fe³⁺ ions) becomes significant as a relaxation mechanism of ¹H nuclei.     

Molecular reorientation of liquid benzene. Anisotropic Raman scattering of the CH and CD stretching modes in the deuterated molecules

The anisotropic Raman spectra of the CH and CD stretching modes in seven deuterated benzenes of D_6h, D_3h, D_2h and C_2h symmetry are reported. The reorientational linewidths are interpreted within the model of anisotropic rotational diffusion. The data are consistent with NMR relaxation studies. The study covers the temperature range between T/T_c = 0.49 and T/T_c = 0.97.     

Experimental investigations of relaxation times of gel electrolytes during polymerization by MR methods

In this article, the properties of gel electrolytes based on the NaClO₄ inorganic salt are described. For electrolytes with various concentrations of inorganic salts, relaxation times T ₁ and T ₂ were measured by means of MR techniques at regular intervals during the polymerization. To measure relaxation time T ₁, the inversion recovery (IR) technique was used. The measurement of relaxation time T ₂ was performed using the spin–echo (SE) technique. The reduction of relaxation times during the polymerization, namely, T ₁ from the value of 5.5 to 1.7 ms and T ₂ from the value of 5.5 to 1 ms, indicates a change in the internal structure of the gels and a change in their chemical composition. The waveforms measured will contribute to further studies of gel electrolytes aimed at increasing their electrical conductivity and establishing the influence of water molecules on the conductivity.     

The effect of water on the physicochemical and mechanical properties of gelatin

Strips of gelatin have been prepared by extrusion at different water contents varying from 20 to 50% H₂O (dry weight basis, d.w.b.). The processes of subsequent hydration or dehydration of these strips were followed by dynamic mechanical thermal analysis (DMTA), wide-angle X-ray diffraction and NMR relaxation measurements. A comparison of the calculated dependence of theT _g of gelatin (T _g anhydrous, 200?C) on water content (using the Ten Brinke and Karasz equation) with experimental results derived from DMTA showed that freshly extruded material followed the theoretical plot below 25% H₂O (d.w.b.), but at higher water contents, the7 _g deviated positively, probably due in part to the effect of delayed re-equilibration of water content after thawing of separated ice crystals. The experimental results determined after storage for one week fell on a different line, with aT _g of 145?C for anhydrous gelatin Possibly, theT _g is elevated by crystallization — a view supported by the WAXS spectra. The NMR relaxation results also showed a profound mobilization of the gelatin protons at water contents greater than 25% d.w.b.     

Analysis of macromolecular structures by pulsed NMR

The T₂ spin-spin relaxation curves obtained by pulsed NMR techniques can readily be used to study important features of macromolecular systems quite distinct from their chemical structure. Such features refer to more physical properties such as molecular size, flexibility and mobility, the influence of solvent and temperature on this motion (which is related to viscosity), crystalline fraction and the rate of crystallization, polymerisation and other chemical reactions where there is a considerable change in dimensions etc. It can also serve to determine the degree of crosslinking, where this forms a partial or complete network. However it appears to indicate the presence of a network even when no permanent network is revealed by alternative and well-established techniques such as solubility and swelling which require much longer times. This difference is ascribed to the presence of some intermolecular binding somewhat akin to permanent crosslinks, but of a very shortlived dynamic nature, and this is referred to as due to entanglements between adjacent macromolecules. The T₂ measurements reveal their presence if the life-time of these entanglements is comparable or longer than the period ofmeasurement by pulsed NMR.The usual formulae used to determine network formation by permanent crosslinks can be applied to such systems with entanglements or with entanglements plus crosslinks, so that the elastic properties can be determined by NMR T₂ measurements. Over a long time only the permanent crosslinks will provide elastic recovery but for sufficiently short times the entanglements provide an additional restoring force and this may be taken to be the cause of the rheological property referred to as creep and viscosity. Since the entanglements but not the permanent crosslinks depend on temperature, many of these physical properties and their variation with temperature can be related directly to the effect of these entanglements as determined by these T₂ measurements and derived from pulsed NMR.Another feature which emerges from these investigations is their dependence on solvent where present. The total variation can be ascribed to molecular dimensions and the free volume available for their motion (and hence their freedom to become disentangled). This free volume is influenced by temperature and concentration of solvent where present.The meaning of these T₂ responses have been deduced from the changes in pulsed NMR responses to a series of macromolecular systems whose properties have been modified to known extents by known radiation doses. The interpretation of the T₂ relaxation patterns obtained from other macromolecular structures now becomes possible. We can therefore hope to see this technique used not only for polymers but especially for biological systems where considerable changes of molecular behaviour such as conformation and motion can result from very minute chemical modifications. Such sensitive features might be for example molecular entanglements and concentration, radiation or chemically-induced crosslinking or degradation (scission), disruption of a regular refolding sequence etc. This T₂ technique is particularly suitable for following such changes.     

Carbon-13 NMR relaxation time measurements of molybdenum carbonyl physisorbed on alumina

In order to obtain information about the motion of Mo(CO)₆ physisorbed on high surface area alumina, the temperature dependences of ¹³C NMR relaxation times, T₁ and T₂, have been measured at 7.05 T. Eight samples prepared under various conditions were studied. Two types of alumina (γ and η) were used and the alumina in two samples was pretreated with iron nitrate. A T₁ minimum of 0.4 s at 220 K observed for most samples corresponds to a correlation time of 2 × 10⁻⁹ s. The dominant relaxation mechanism is not chemical shift anisotropy since T₁ measurements at 3.5 T indicated no field dependence. An increase in the iron content of the alumina decreased T₁ above 273 K but caused only minor changes in T₁ and T₂ at lower temperatures. T₁ was 0.73 s and T₂ was 11 ms for Mo(CO)₆ on the η-alumina sample at 300 K.     

NMR study of water reorientation in molybdic acids: MoO3 · 2H2O and yellow MoO3 · H2O

Proton NMR relaxation times (T₂, T₁, T_1?) are reported for powder samples of MoO₃ · 2H₂O and yellow MoO₃ · H₂O in the temperature range 150–325 K and at 20 and 60 MHz. No translation of hydrogen atoms is detected but the spin-lattice relaxation behavior indicates reorientation of H₂O molecules. The waters coordinated to Mo atoms undergo 180° flips (about their C₂ axes) with similar motional parameters in both compounds. The interlayer waters in MoO₃ · 2H₂O undergo 180° flips with different parameters. An assumed Arrhenius-type temperature dependence of correlation times leads to preexponential factors which are “anomalously” low. The possible involvement of temperature-dependent activation barriers is discussed.     

NMR spin–spin relaxation in solid and molten polyethylene structures

The NMR spin-spin relaxation (T₂) spectra of high-density polyethylene (PE) has been investigated over a wide range of temperatures, both in the solid and molten states. Previous work in these laboratories has shown that the T₂ relaxation spectrum of molten polyethylene differs from that of other polymers studied in that (a) it cannot be decomposed into two relaxation spectra (T_2S and T_2L) and (b) there is some evidence of a memory effect. This paper attempts to elucidate these observations, and compare them with the spin-spin relaxation of polyethylene at lower temperatures. In the solid state, the T₂ decay comprises both a Gaussian distribution for the crystalline region, and an exponential decay for the amorphous component. The effects of crystallization conditions and of temperature were determined. In the molten state the T₂ decay is more complex, but can be resolved into three exponentials. The longest (T_2L) component arises as expected from the most mobile, low molecular weight fraction. The T_2S component is due to an entangled but mobile network, as in other polymers. In addition, a short relaxation component T_2X is observed, which is influenced by previous crystallinity and the processing history of the material, and is ascribed to some vestigial degree of structure in the molten phase.     

Specific Features of the Adsorption and Nuclear Magnetic Relaxation of the Water Molecules in Active Carbons: 2. The State of Water in Active Carbon with Relatively Large Pores According to the NMR Relaxation Data

The state of water upon adsorption on FAS-3 active carbon with relatively large micropores is studied by the NMR relaxation method. The dependences of the times of spin–lattice (T ₁) and spin–spin (T ₂) NMR relaxation of adsorbed water molecules on the adsorption value are established. The character of the dependences of T ₁ and T ₂ on the number of adsorbed water molecules per primary adsorption site reflects the specific features of the volume filling of micropores and the formation of a continuous adsorption layer on the mesopore surface due to cluster coalescence on the one wall of a pore. The results obtained are compared with the data for typical microporous active carbons, as well as with the data obtained by the adsorption method.     

Spin-chemical approach to photochemistry: reaction control by spin quantum operation

In this review, the contribution of spin chemistry (in particular, magnetic resonance-related chemistry) to the photochemical field is briefly introduced. First, the development of a time-resolved EPR method and its significant application to radical-related physical phenomena and chemical reactions are presented. Second, a reaction-control method by means of electron spin operations is introduced, and several reaction yield-detected magnetic resonance (RYDMR) methods are presented as applications of this concept. One of the most important physical conclusions is the introduction of the concept of “spin phase relaxation” termed singlet–triplet (S–T) and triplet–triplet (T–T) dephasing, instead of the traditional concepts of longitudinal (T₁) and transversal relaxations (T₂). The effects of strong microwave power on the RYDMR spectrum and time-domain data are analyzed according to this concept. Furthermore, a new detection method is introduced, termed “photoconductivity detected magnetic resonance” (PCDMR), which is applicable exclusively to the system of charge transfer reactions.     

Investigating the two inequivalent NH2(CH3)2 ions in [NH2(CH3)2]2CuCl4 using magic angle spinning nuclear magnetic resonance

The structural change near the phase transition temperatures of [NH₂(CH₃)₂]₂CuCl₄ is discussed in terms of the chemical shifts and the spin-lattice relaxation times T_1ρ in the rotating frame for ¹H MAS NMR and ¹³C CP/MAS NMR. The ¹H T_1ρ undergoes molecular motion near the phase-transition temperature (T_C2 = 253 K). In addition, the two inequivalent [NH₂(CH₃)₂] (1) and [NH₂(CH₃)₂] (2) sites were distinguishable by the ¹³C chemical shift. And, the most significant change was observed at T_C2 for the ¹³C CP/MAS NMR spectrum; this temperature corresponds to a ferroelastic phase transition with different orientations.