Transverse NMR relaxation of water in wood

Abstract

SynopsisThe transverse NMR relaxation times of hydrogen nuclei of water absorbed in white spruce sapwood [Picea glauca (Meunch) Voss] were measured for moisture contents in the range from 5 to 176%.The spin echo amplitudes resulting from the Carr-Purcell sequence decay nonexponentially suggesting the possibility of a t least two different relaxation times for water in wood. A simplified structural model of the wood-water mixture is used to estimate the rates of chemical exchange at room temperature of hydrogen nuclei between various sites in the system. The high-resolution NMR line shape is discussed briefly in terms of this proposed model. INTRODUCTIONIt is well known that the macroscopic properties of cellulose and similar polymers are influenced profoundly by the presence of ~a t e r . l -~ Thus, measurement of the moisture content of wood is of considerable importance, and numerous methods for determining the mass fraction of water in wood and pulp have been p r o p o~e d .~ Measurement of the nuclear magnetic resonance signal of hydrogen nuclei in cellulose-water mixtures is one possible t e~h n i q u e .~-~ Magnetic resonance also has played a significant role in understanding the wood-water relationship,a12 because the technique is sensitive to the microscopic environment and dynamics of molecules containing spin-bearing nuclei such as hydrogen. The wide-line NMR absorption spectrum of hydrogen nuclei in wood-water systems exhibits a relatively narrow line attributable to water, superimposed upon a much broader line resulting from hydrogen nuclei within the wood fiber.1°J2 The intensity of the narrow absorption line gives an accurate measurement of the amount of water present in a given ample,^ but the line shape of this component was found to be much broader than that of pure water, possibly implying that water in wood is much less mobile than in the liquid phase.The NMR evidence indicates that water in cellulosic materials takes at least two different formsg: water that is associated intimately with wood cell walls and water that is relatively free to move about in the cell cavities. Pulsed nuclear magnetic resonance techniques are quicker, easier, and more readily analyzed in terms of models for the molecular dynamics than steady-state absorption methods, but in spite of this, few such studies have been undertaken for watercellulose systems. Therefore, as part of an ongoing program to investigate the properties of water and oils in wood, we have used pulsed NMR methods to measure the transverse relaxation time T2 as a function of the moisture content of white spruce sapwood. EXPERIMENTALCylindrical samples 0.5 cm in diameter and 1.5 cm long were cut from a block of green white spruce [Picea glauca (Meunch) Voss], dried for varying lengths of time, and sealed in glass tubes with picine vacuum wax.7 After the magnetic resonance experiments, moisture contents of the specimens were determined by weighing before and after drying at 105°C for 12 hr. The moisture contents ranged from 5 to 176% on an oven dry...

Cluster theory for water sorption in wood

“…More recently, pulse techniques have been used and are considered to be quicker than the wideline technique (Riggin et al 1979). The research using NMR has concentrated on derivative forms of cellulose, or natural hygroscopic materials such as cotton, wool and wood.…”

“…As the temperature decreased, the T 2 relaxation times of the water in the milled wood decreased. Riggin et al (1979) also analyzed Tz relaxation in wood using the CPMG sequence. They identified two different states of water within the wood structure each having its own relaxation time (Fig.…”

“…Menon et al (1987) and Hartley (1986), using the CPMG pulse sequence, found that the transverse decay curve of water protons in wood could be well described using three T 2 components. The additional third component magnetization had a shorter relaxation time than either of the two component magnetizations found by Riggin et al 1979. (Froix and Nelson, 1975), b signal intensity for milled wood at MC > 38 % (solid line) and for MC < 38% (dashed line), plotted against temperature (note reverse temperature axis).…”

Bound and free water distribution in wood during water uptake and drying as measured by 1D magnetic resonance imaging

“…Another important advantage of NMR over other methods is its ability to distinguish between bound and free water by relaxation analysis (T 2 ), i.e. the decay of the NMR signal (Riggin et al 1979;Menon et al 1987;Araujo et al 1992;Telkki et al 2013). For detailed information, please see section ''Relaxation analysis of water in wood''.…”

Proton Spin–Lattice Relaxation in Wood—T1Related to Local Specific Gravity Using a Fast-Exchange Model

“…[5] are compared with the T 2cww measured with the CPMG sequence in Table 1. For hemlock, the agreement between the measured and estimated where T 1 is the measured spin-lattice relaxation time for all protons in the cell, T 1cw and T 1free are successively the T 2cww was good; however, for red cedar, Eq.…”