Real time pulsed NMR

Characterization of Polymer Systems by Real Time Pulsed NMR... 

Recently, we have developed a real time pulsed NMR(RTPNMR) system(2) controlled by a microcomputer as shown in Figure 3 to study dynamics of structure formation in polymer systems. It has been successfully applied to study crystallization process (3), gelation process(4) and so on. In this paper, application of RTPNMR for characterization of curing process of epoxy resins, polymerization crosslinking process of diacetylenes, and orientation process of liquid crystalline polymers(LCPs) under magnetic field are described. Papers related to the detailed analysis of these data will appear in the near future. EXPERIMENTAL... 

Figure 3. Block diagram of the real time pulsed NMR measurement system controlled by a microcomputer.

Tanaka H, Nishi T. Study of crystallization process of polymer from melt by a real-time pulsed NMR measurement. J Chem Phys 1986 85 6197. 

In 2006 Wimperis et al. proposed a method called satellite transitions acquired in real time by MAS (STARTMAS) [142, 202], which allows for the real-time acquisition of high-resolution NMR spectra of spin-3/2 nuclei under MAS. This method combines a train of pulses, similar to CPMG [109, 110], with sample rotation at the magic angle to refocus the quadrupolar broadening in a series of echoes, while allowing the isotropic quadrupolar shift and chemical shift to evolve. 

Since the sensitivity of pulse NMR is very high and H Ti values for usual polymers are less than 1 s due to the spin diffusion, rapid measurements with short repetition times are possible. This gives us the real time measurement of nonequilibrium phenomena such as crystallization in the polymer. The crystallization process of polymers has been studied by an optical microscope, dilatometry and X-ray diffraction. These methods only gives static information about the crystallization process. The pulse NMR measurements provide both the fraction and the molecular mobility of each phase. Figures 7.19 and 7.20 show the temperature change of the fractions and T2 values of crystalline, interfacial and amorphous components for poly(e-caprolactone)... 

Modem solid-state NMR involves the use of very short radio-frequency pulses (of variable duration from 1 to 200 ms) and can be complemented with real-time Fourier transform analysis and multiple scan capability. Standard NMR enhancements nowadays, such as scalar (low power, ca. 4 kHz) and dipolar (about 45 kHz) decoupling, magic angle spinning, spectra of multiple elemental isotopes beyond... 

Pulse NMR implies that a spectrum is obtained with an excitation pulse followed by detection of a free-induction decay (FID) and subsequent Fourier transformation. Pulse NMR methods are suitable for rapid and real-time measurements, which is the major requirement in manufacturing industries wishing to improve efficiency in quality and process control. The older field swept continuous-wave (CW) NMR technique - no longer used - carried considerable limitations for rapid measurements. 

With very fast computer control, the complex FID can be sampled and the transform calculated in real time for several reasons using the trick of sampling the FID at time delays that correspond to a phase shift to obtain the sin(tot) sample as sin((i)f) = cos (tor + ir/2). Another featore of the pulse acquisition is that it is repeated many times to average the results. Thus we have a sitoation where a computer controls the timing of RF pulses perpendicular to the z-axis of the main magnet and many programs are available to carry out a nmnber of experiments with the same spectrometer. As such modem NMR spectrometers are programmable experiments. ... 

Computers play a central part in modern NMR spectroscopy. Their use for the real-time control of pulsed NMR experiments has enabled the development of multiple pulse techniques such as two-dimensional NMR this article deals with the part played by computers in the acquisition, processing and presentation of experimental NMR data. 

In two-dimensional (2D) NMR a series of FIDs is acquired using a pulse sequence containing a variable evolution period, which is incremented regularly to map out the behaviour of the nuclear signal as a function both of real time t2 during the FID, and of the evolution time tj. A typical 2D NMR experiment might acquire 512 FIDs of 1024 complex points, which would then be doubly Fourier transformed as a function of the two time variables. 3D and 4D NMR extend the principle to two and three evolution periods respectively. 

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