P.J. McDonald

A new approach to the NMR imaging of solids

Abstract A new nuclear magnetic resonance method to image solids with dipolar-broadened lines using solid echoes is described. The broad lines are effectively narrowed by exploiting the independence of the echo intensity from the dipolar interaction. The echo is encoded with spatial information using magnetic field gradients which remain static throughout the spin-echo pulse sequence. The method is demonstrated with one-dimensional profiles of a series of hexamethylbenzene samples.

A broad line NMR and MRI study of water and water transport in portland cement pastes

The results of a magnetic resonance spin-spin relaxation analysis and broad-line magnetic resonance imaging (MRI) (gradient-echo and stray-field imaging) study of water and water transport in Portland cement pastes are presented. The effect of varying the cure conditions and the water to cement (w/c) ratio of the sample of mix are discussed. The water sorptivity and the concentration dependence of the hydraulic diffusion coefficient are calculated for samples prepared with a 0.5 w/c ratio and, therefore, an open pore structure. In the case of 0.3 w/c ratio samples, little water transport is observed, and a closed pore structure is inferred.

Hydrogen bond dynamics in benzoic acid dimers as a function of hydrostatic pressure measured by nuclear magnetic resonance

The dynamics of hydrogen atoms in the hydrogen bonds of benzoic acid dimers have been studied as a function of hydrostatic pressure to pressures in excess of 4 kbar. This paper is primarily concerned with results up to 3.3 kbar. The temperature dependence of the correlation time for the motion at a series of pressures has been investigated using measurements of the proton spin–lattice relaxation time. Strong non‐Arrhenius behavior is exhibited and the data are in good agreement with a model which invokes phonon assisted tunneling at low temperature and thermally activated Arrhenius dynamics at high temperature. The parameters in the model include the asymmetry of the double minimum potential experienced by the hydrogen atoms and dynamical variables relating to the tunneling and hopping processes. The rate of phonon assisted tunneling is observed to increase exponentially with increasing pressure and this is attributed to the increase in the tunneling matrix element which occurs as the distance between the...

Fickian and Case II diffusion of water into amylose: A stray field NMR study

Abstract The ingress of liquid water and water vapour into glassy pellets of the starch polymer amylose has been studied using stray field nuclear magnetic resonance imaging (STRAFI), to produce one-dimensional water concentration profiles as a function of depth from the sorption surface, and time. For the vapour experiments the observed concentration profiles are characteristic of a system showing Fickian diffusion with a mutual diffusion coefficient, a function of the concentration of the penetrant. In contrast, the ingress of liquid water is non-Fickian; concentration profiles characteristic of Case II diffusion have been modelled semi-quantitatively using the Thomas-Windle model.

Stray field magnetic resonance imaging of the diffusion of acetone into poly(vinyl chloride)

Abstract Stray field nuclear magnetic resonance imaging has been used to study the diffusion of acetone vapour into poly(vinyl chloride) (PVC). For the first time in a n.m.r imaging experiment the penetrant, the swollen polymer and the rigid polymer have been simultaneously visualized. Results are presented for a range of acetone vapour activities and sample preparation temperatures, and also for a sample prepared with deuterated acetone. The profiles show that the diffusion is of Case II type. A sloping profile is observed behind the penetrant front and is interpreted in terms of a softening of the swollen polymer, which occurs over a period of hours after penetration by the acetone. The softening rate has been measured as a function of temperature.

Influence of drier combination on through-drying in waterborne alkyd emulsion coatings observed with magnetic resonance profiling

Achieving fast and uniform crosslinking in alkyd coatings poses a challenge to formulators that demands a fundamental understanding of drier efficiency. In recognition of this, we have examined the physical changes that accompany autooxidative crosslinking in alkyd films (cast from waterborne emulsions) in the presence of various combinations of metal carboxylate driers. A newly developed type of magnetic resonance (MR) profiling was used in conjunction with conventional techniques: Beck-Koller drying tests, pendulum hardness, and mass uptake. MR profiling noninvasively probes the molecular mobility of the alkyd as a function of depth (with a pixel resolution of about 9 µm), over drying times ranging from minutes to weeks. It thereby indicates drier efficiency via its sensitivity to viscosity build-up during drying and to subsequent film hardening. We show unequivocally that more uniform crosslinking is achieved using a combination of a primary (Co) and a secondary (Ca) drier, in support of conventional belief. Furthermore, these results yield new insight into the chemical mechanisms induced by the driers and are thus of clear benefit to coatings researchers and formulators. Notably, the secondary driers improve the efficiency of the hydroperoxide decomposition reactions, but they are only active during an initial period, after which crosslinking nonuniformity develops.

The correlation of methyl tunnelling and thermally activated reorientation

A strong correlation is demonstrated between two parameters connected with methyl group motion, and measured in a wide variety of solids. The parameters are h nu t, the tunnel splitting of the ground torsional state, and theta min, the temperature at which the rate constant tau -1 associated with thermally activated reorientation has the value 2*108s-1. The results suggest that there exists a universal law of the form f( nu , tau -1, theta )=0 governing the temperature ( theta ) dependence of tau -1. This is independent of the phonon spectrum of the host lattice, in conflict with much recent theoretical work but in harmony with the model of Das (1957).

A study of water exchange in wood by means of 2D NMR relaxation correlation and exchange

Abstract Two-dimensional T 1-T 2 1H NMR relaxation correlation spectra and T 2-T 2 relaxation exchange spectra for samples of spruce wood across the full moisture content range from 0% to >100% are presented. The T 1-T 2 spectra unambiguously identify water in different environments within the wood that overlap in the corresponding and more traditional one-dimensional T 1 or T 2 experiment. The T 2-T 2 spectra unambiguously reveal the presence of water exchange between the lumen and cell wall above the fibre saturation point in sapwood. An estimate is made of the exchange rate: 1/30 to 1/3 ms-1.

Stray field magnetic resonance imaging: a preliminary study of skin hydration

We report the first use of GARField (STRAFI-like) magnetic resonance profiling (1D-MRI) to study skin hydration, both in vitro and in vivo. It is shown that, for in vitro samples, a high degree of reproducibility is achievable for skin samples from a single donor and that the signal intensity correlates well with equilibrium moisture content in the skin. In vivo, the method allows measurement of the influence of a moisturizer cream on water levels in the skin as a function of time.

The correlation between hydrogen bond tunneling dynamics and the structure of benzoic acid dimers

We report a correlation between the rate of incoherent tunneling associated with proton transfer in hydrogen bonds and the structure of aromatic carboxylic acid dimers. The compressibility of the hydrogen bond in benzoic acid, specifically the oxygen–oxygen distance r(O⋅⋅O), has been measured as a function of hydrostatic pressure up to 3.2 kbar using neutron powder diffraction. All data were recorded at a temperature of 5 K. Using previously published pressure dependence NMR measurements, we have investigated the relationship between the dynamics in the quantum regime and r(O⋅⋅O) in the hydrogen bonds of benzoic acid. The incoherent tunneling rate increases exponentially with decreasing r(O⋅⋅O). This behavior is attributed to the increase in the tunneling matrix element as the potential wells and the localized eigenfunctions of the double minimum potential which characterize the system are brought into closer proximity. There is a quantitative agreement between this study, in which the hydrogen bonds are ...