Segmental anisotropy in strained elastomers detected with a portable NMR scanner

Single-side NMR is particularly suitable for measurements of segmental anisotropy induced in elastomers by uniaxial forces or local strain. Proton transverse nuclear magnetic relaxation was investigated with the NMR-MOUSE by recording the Hahn-echo decay in cross-linked natural rubber bands. This provided information on the dependence of the Hahn-echo decay on the angle between the direction of the uniaxial stretching force and the axis Z defined direction perpendicular to the magnet pole faces of the NMR-scanner. The anisotropy effect on the Hahn-echo decay is correlated with the extension ratio, and it is more evident in the liquid-like regime of the decay. A weaker segmental anisotropy is detected by 1H solid- and Hahn-echo decays recorded by multi-pulse sequences. A qualitative understanding of the angular dependence is obtained by an analytical theory of the Hahn-echo decay adapted to the case of stretched elastomers and to strongly inhomogeneous magnetic fields. Using angular-dependent 1H residual second van Vleck moments and correlation times reported previously [P.T. Callaghan and E.T. Samulski, Macromolecules 30, 113 (1997)] from stretched natural rubber bands the segmental anisotropy measured in inhomogeneous magnetic fields by the Hahn-echo decay was numerically simulated. As an example of a macroscopic distribution of local segmental anisotropy, 1H Hahn-echo decays were measured by the NMR-MOUSE sensor in a stretched cross-linked natural rubber plate with a circular cut in the center.     

NMR of multipolar spin states excitated in strongly inhomogeneous magnetic fields

The possibility of exciting and filtering various multipolar spin states in proton NMR like dipolar encoded longitudinal magnetization (LM), double-quantum (DQ) coherences, and dipolar order (DO) in strongly inhomogeneous static and radio-frequency magnetic fields is investigated. For this purpose pulse sequences which label and manipulate the multipolar spin states in a specific way were implemented on the NMR-MOUSE (mobile universal surface explorer). The performance of the pulse sequences was also tested in homogeneous fields on a solid-state high-field NMR spectrometer. The theoretical justification of these procedures was shown for a rigid two-spin 1/2 system coupled by dipolar interactions. Dipolar encoded longitudinal magnetization decay curves, double-quantum and dipolar-order buildup curves, as well as double-quantum decay curves were recorded with the NMR-MOUSE for natural rubber samples with different crosslink density. The possibility of using these multipolar spin states for investigations of strained elastomers by NMR-MOUSE is also shown. These curves give access to quantitative values of the ratio of the total residual dipolar couplings of the protons in the series of samples which are in good agreement with those measured in homogeneous fields.     

Application of the NMR-MOUSE to food emulsions

The application of the NMR-MObile Universal Surface Explorer (NMR-MOUSE) to study food systems is evaluated using oil-in-water emulsions, and the results are compared to those obtained using a conventional low-field NMR (LF-NMR) instrument. The NMR-MOUSE is a small and portable LF-NMR system with a one-sided magnet layout that is used to replace the conventional magnet and probe on a LF-NMR instrument. The high magnetic field gradients associated with the one-sided MOUSE magnet result in NMR signal decays being dominated by molecular diffusion effects, which makes it possible to discriminate between the NMR signals from oil and water. Different data acquisition parameters as well as different approaches to the analysis of the NMR data from a range of oil-in-water emulsions are evaluated, and it is demonstrated how the concentration of oil and water can be determined from the NMR-MOUSE signals. From these model systems it is concluded that the NMR-MOUSE has good potential for the quantitative analysis of intact food products.     

Nuclear magnetic resonance in inhomogeneous magnetic fields

The response of the spin system has been investigated by numerical simulations in the case of a nuclear magnetic resonance (NMR) experiment performed in inhomogeneous static and radiofrequency fields. The particular case of the NMR-MOUSE was considered. The static field and the component of the radiofrequency field perpendicular to the static field were evaluated as well as the spatial distribution of the maximum NMR signal detected by the surface coil. The NMR response to various pulse sequences was evaluated numerically for the case of an ensemble of isolated spins (1/2). The behavior of the echo train in Carr-Purcell-like pulse sequences used for measurements of transverse relaxation and self-diffusion was simulated and compared with the experiment. The echo train is shown to behave qualitatively differently depending on the particular phase schemes used in these pulse sequences. Different echo trains are obtained, because of the different superposition of Hahn and stimulated echoes forming mixed echoes as a result of the spatial distribution of pulse flip angles. The superposition of Hahn and stimulated echoes originating from different spatial regions leads to distortions of the mixed echoes in intensity, shape, and phase. The volume selection produced by Carr-Purcell-like pulse sequences is also investigated for the NMR-MOUSE. The developed numerical simulation procedure is useful for understanding a variety of experiments performed with the NMR-MOUSE and for improving its performance. Copyright 2000 Academic Press.     

Degradation of historical paper: nondestructive analysis by the NMR-MOUSE

The NMR-MOUSE is a mobile sensor for single-sided NMR inspection of organic materials which takes advantage of the principles of magnetic resonance and inside-out-NMR. Historical books dating from the 17th century were measured at different points by positioning the NMR-MOUSE on the paper. Different degrees of paper degradation can be discriminated from the regularized inverse Laplace transform of the envelope of the acquired echo signals. For the first time the degradation of historical paper was characterized entirely nondestructively by NMR. As a contribution to current preservation efforts, NMR shows great promise for future use in damage assessment of historical documents.     

Thermo-oxidative aging of elastomers: a temperature control unit for operation with the NMR-MOUSE

Temperature-controlled measurements of the transverse relaxation time were conducted with the NMR-MOUSE (nuclear magnetic resonance mobile universal surface explorer) at 50°C to investigate the heterogeneity of rubber sheets induced by thermo-oxidative aging. With temperature stabilization the relative error ofT ₂ decreased by a factor of 4 on the short time scale. Without temperature stabilization it may reach 30% in the long-time range because of temperature changes in the laboratory. Two different types of mobile temperature control units operating with the NMR-MOUSE are described. These temperature control units are small and portable like the NMR-MOUSE.     

Selective NMR excitation in strongly inhomogeneous magnetic fields

The NMR-MOUSE is a unilateral and mobile NMR sensor which operates with highly inhomogeneous magnetic fields. To produce a mobile NMR unit, RF excitation is sought, which can be produced with the most simple equipment, in particular nonlinear, low-power amplifiers, and to observe a free induction decay in strongly inhomogeneous fields, the excitation needs to be selective. The possibility to produce selective excitation by sequences of hard low-power radiofrequency pulses in the strongly inhomogeneous magnetic fields of the NMR-MOUSE is explored. The use of the DANTE sequence for selection of magnetization from parts of the sensitive volume was investigated for longitudinal and transverse magnetization by computer simulations and experiments. The spectra of the recorded FIDs and echo signals are in good agreement with those simulated for the excitation, which verifies the concept of the DANTE excitation. The results obtained are an important step towards a low-power operation of the NMR-MOUSE to improve its mobility.     

Analysis of historical porous building materials by the NMR-MOUSE

Samples of sandstone with and without deposits of silicon oxide stone strengthener as well as samples of historical brick material were analyzed by transverse NMR relaxation and mercury intrusion porosimetry. Relaxation times and relaxation time distributions of the protons from the water saturated samples were measured by low-field NMR using homogeneous and inhomogeneous fields. The measurements in inhomogeneous fields were performed with two different NMR-MOUSE sensors, one with a field gradient of 2 T/m and the other with an average field gradient of about 20 T/m. In the sandstone samples the application of stone strengtheners was shown to result in a confinement of the large pores within the outer layer of a few millimeters depth. Depending on the ferromagnetic contamination of the brick samples, the relaxation time distributions can be affected. The agreement of T2 relaxation time distributions and pore size distributions from mercury intrusion porosimetry was found to be better for the NMR-MOUSE sensors than for the homogeneous field measurements. This is true even for different brick samples, unless the content in ferromagnetic particles is very strong.     

Investigations of silicone breast implants with the NMR-MOUSE

Silicone breast implants are used for breast augmentation and breast reconstruction. The issues of concern associated with such implants are: (a) the quality control of each implant before implantation, and (b) the detection of implant bleeding after implantation. We have studied the use of the Nuclear Magnetic Resonance-MObile Universal Surface Explorer (NMR-MOUSE) for the nondestructive testing of (a) the quality of implant shells, and (b) changes in implant gel due to leakage of body fluid into the implant. Depth profiles measured nondestructively through implant shells at different positions of each implant by the Profile NMR-MOUSE assured good reproducibility of the quality and thickness of different shell layers. The leakage of implants upon rupture was mimicked by observing changes in the transverse NMR relaxation time of the implant gel upon ingress of physiological saline solution and safflower oil through the rupture. Results demonstrate that nondestructive testing with unilateral NMR is a potential method for use in the quality control of implants and for the screening of implants for rupture after implantation.     

Anisotropy in tendon investigated in vivo by a portable NMR scanner, the NMR-MOUSE

Ordered tissue like tendon is known to exhibit the magic-angle phenomenon in magnetic resonance investigations. Due to the anisotropic structure the transverse relaxation time T(2) depends on the orientation of the tendon in the magnetic field. In medical imaging, relaxation measurements of tendon orientation are restricted by the size of the object and the space available in the magnet. For humans, tendon orientation can only be varied within small limits. As a consequence, the magic-angle phenomenon may lead to a misjudgement of tendon condition. It is demonstrated that the NMR-MOUSE (mobile universal surface explorer), a hand-held NMR sensor, can be employed to investigate the anisotropy of T(2) in Achilles tendon in vivo. The NMR-MOUSE provides a convenient tool for analyzing the correlation of T(2) and the physical condition of tendon.     

NMR detection of thermal damage in carbon fiber reinforced epoxy resins

Composite materials of epoxy resins reinforced by carbon fibers are increasingly being used in the construction of aircraft. In these applications, the material may be thermally damaged and weakened by jet blast and accidental fires. The feasibility of using proton NMR relaxation times T1, T1rho, and T2 to detect and quantify the thermal damage is investigated. In conventional spectrometers with homogeneous static magnetic fields, T1rho is readily measured and is found to be well correlated with thermal damage. This suggests that NMR measurements of proton T1rho may be used for non-destructive evaluation of carbon fiber-epoxy composites. Results from T1rho measurements in the inhomogeneous static and RF magnetic fields of an NMR-MOUSE are also discussed.     

全开放式单边核磁共振探测器及其应用

全开放式单边核磁共振(nuclear magnetic resonance,NMR)系统具有成本低、易便携、可实时无损检测等特点.该文介绍了单边NMR探测器的发展,重点以德国亚琛工业大学Blumich教授团队研发的NMR-MOUSE(NMR mobile universal surface explorer)探测器为例,详细讨论了其硬件结构,探讨了单边NMR的测量方法,最后介绍了单边NMR探测器在波谱分析和生物医学等领域中的应用.     

Anisotropy in Tendon Investigated in Vivo by a Portable NMR Scanner, the NMR-MOUSE

Ordered tissue like tendon is known to exhibit the magic-angle phenomenon in magnetic resonance investigations. Due to the anisotropic structure the transverse relaxation time T₂ depends on the orientation of the tendon in the magnetic field. In medical imaging, relaxation measurements of tendon orientation are restricted by the size of the object and the space available in the magnet. For humans, tendon orientation can only be varied within small limits. As a consequence, the magic-angle phenomenon may lead to a misjudgement of tendon condition. It is demonstrated that the NMR-MOUSE (mobile universal surface explorer), a hand-held NMR sensor, can be employed to investigate the anisotropy of T₂ in Achilles tendon in vivo. The NMR-MOUSE provides a convenient tool for analyzing the correlation of T₂ and the physical condition of tendon.     

A unilateral NMR magnet for sub-structure analysis in the built environment: the Surface GARField

A new, portable NMR magnet with a tailored magnetic field profile and a complementary radio frequency sensor have been designed and constructed for the purpose of probing in situ the sub-surface porosity of cement based materials in the built environment. The magnet is a one sided device akin to a large NMR-MOUSE with the additional design specification of planes of constant field strength /B0/ parallel to the surface. There is a strong gradient G in the field strength perpendicular to these planes. As with earlier GARField magnets, the ratio G//:B0/ is a system constant although the method of achieving this condition is substantially different. The new magnet as constructed is able to detect signals 50mm (1H NMR at 3.2 MHz) away from the surface of the magnet and can profile the surface layers of large samples to a depth of 35-40 mm by moving the magnet, and hence the resonant plane of the polarising field, relative to the sample surface. The matching radio frequency excitation/detector coil has been designed to complement the static magnetic field such that the polarising B0 and sensing B1 fields are, in principal, everywhere orthogonal. Preliminary spatially resolved measurements are presented of cement based materials, including two-dimensional T1-T2 relaxation correlation spectra.     

Advances of unilateral mobile NMR in nondestructive materials testing

Unilateral mobile NMR employs portable instrumentation with sensors, which are applied to the object from one side. Based on the principles of well-logging NMR, a hand-held sensor, the NMR-MOUSE (MObile Universal Surface Explorer) has been developed for nondestructive materials testing. In the following, a number of new applications of unilateral NMR in materials science are reviewed. They are the state assessment of polyethylene pipes, the characterization of wood, the in situ evaluation of stone conservation treatment, high-resolution profiling of rubber tubes and 2-D imaging for defect analysis in rubber products.     

Hole-burning NMR in strongly inhomogeneous fields

Different pulse sequences for frequency-selective NMR in the highly inhomogeneous fields of single-sided NMR are explored. A modified Hahn-echo is used to burn a hole in the spectrum of the detected echo. The hole diminishes following molecular dynamics on the scale of the echo time. Preliminary experiments were performed on pure water and natural rubber with the NMR-MOUSE. The results demonstrate the feasibility of hole burning to study slow molecular dynamics by mobile NMR in strongly inhomogeneous magnetic fields.     

Surface UV aging of elastomers investigated with microscopic resolution by single-sided NMR

Depth profiles taken from the surface of UV irradiated natural rubber sheets have been measured with microscopic resolution using a Profile NMR-MOUSE. An NMR observable related to the sum of the spin echoes in the Carr-Purcell-Meiboom-Gill pulse sequence was used to characterize the cross-link density changes produced by the action of UV radiation in each sheet. The aging process was investigated as function of irradiation time and penetration depth. An exponential attenuation law with a space dependent absorption coefficient describes the change in the NMR observable with penetration depth. An Avrami model is used to describe the dependence of the absorption coefficient on the aging time. The method can be applied to investigate the effect of various aging agents on the surfaces of elastomers.     

Shaping the Sensitive Volume of a Single-Sided NMR-Sensor to Profile Cylindrical Samples with High Resolution

This work reports the construction of a single-sided magnet generating a sensitive volume with adjustable curvature. It is useful to profile cylindrical samples with high depth resolution. The sensor geometry is based on the Profile NMR-MOUSE, which generates a parabolic field profile with a quadratic coefficient that decreases with the distance from the magnet surface. Such field profiles approximate cylinder walls within limited angular range with negligible deviation. Then, by varying the working depth, shells of different diameter in the sample can be matched. Simulation and experiments conducted on phantoms confirm that a depth resolution of about 35 μm can be obtained. The sensor was used to profile the structure of a cylindric air spring bellows with high resolution. Besides resolving the fiber layers used to reinforce the rubber matrix, the ingress of hexane was detected via T ₂ changes of the material.     

Self-diffusion measurements by a constant-relaxation method in strongly inhomogeneous magnetic fields

The simple pulse sequence thetax-tau1-2thetay-tau1+tau2-2thetay-tau2-Hahn echo used to measure the self-diffusion coefficient D under constant-relaxation condition, i.e., for tau1+tau2=const. was investigated in the presence of strongly inhomogeneous static as well as radiofrequency magnetic fields. The encoding of the Hahn-echo amplitude by the pulse flip angle and diffusion was evaluated by taking into account the spatial distribution of the off-resonance field, the strength and orientation of the local field gradients, and the pulse flip angles by a computer simulation program. As input files, this program uses maps of static and radiofrequency fields, and the D coefficient can be evaluated from the time dependence of the Hahn-echo amplitude. The method was applied to a mobile one-sided NMR sensor, NMR-MOUSE with a bar magnet by measuring D for a series of liquids with different viscosities. The method was shown to be particularly useful for measuring D of solvents in elastomers without the need for measurements of the transverse relaxation rates. The self-diffusion coefficient of toluene in a series of crosslinked natural rubber samples was measured and correlated with the crosslink density. Finally, the method was applied to measure the diffusion anisotropy of free water in bovine Achilles tendon.     

Time-Resolved Study of the Photo-Curing Process of Dental Resins with the NMR-MOUSE

The photo-curing reaction of dental resins has been examined with unilateral nuclear magnetic resonance (NMR-MOUSE) allowing nondestructive high-resolution measurement of depth profiles as a function of time and space. The NMR signal is sensitive to both the monomer concentration and changes in molecular mobility. Upon irradiation with blue light, it first increases due to molecular mobility enhanced by the reaction heat and then decreases exponentially with the monomer concentration as the polymer signal is lost in the dead time of the instrument upon curing. The space and time dependence of the NMR signal can be described by the photo-polymerization reaction kinetics together with a heuristic approximation of the temperature dependence.