# Time Domain NMR in Polymer Science: From the Laboratory to the Industry
## 2 Sequences
### Spins' Manipulation  
Building on the concepts presented above, experiments can be tailored or combined to manipulate specific spins interactions.  
A typical issue of TD-NMR is detecting the magnetization of rigid components, which is lost during the instrument dead-time, since it decays at most in 20 \(\upmu\)s (Figure 5) and cannot be refocused by Hahn echo or CPMG. The traditional approach to overcome the dead-time was the introduction of a solid echo (SE), constituted by two \(\pi\)/2 pulses, separated by a time \(\tau\), but it proved to be not quantitative in dense dipolar-coupled systems [26]. MSE (mixed magic sandwich echo) is a more advanced refocusing sequence that can be applied before FID detection. The double cycle of \(\pi\)/2 pulses attains \"time reversion\", recovering the lost signal as an echo. The so-obtained FID can be fitted with a multicomponent function that in its simplest version is made of a Gaussian, for the rigid part, and a Weibullian for the mobile component. The weight of each component is then translated into a rigid proton fraction and a mobile proton fraction. The value of the Weibullian exponent is generally associated with the degree of phase separation.  
Furthermore, the MSE sequence can be used as a building block and combined with other sequences. For example, an MSE block inserted at the end of a saturation recovery sequence allows phase-resolved measurement of T\({}_{1}\) relaxation time.  
Pulse sequence blocks can also be used as filters, being able to select the magnetization pertaining to only one materials' fraction and filtering out the other.  
One filter that can be used to select magnetization only for dipolar-coupled fractions is the DQ filter (Figure 6a). It consists of a BP, two-pulse or three-pulse sequence, with the excitation time fixed at a value that maximizes signal intensity, and it is preceded by a so-called z-filter, that is an inversion of the magnetization in the z-axis, to compensate for T\({}_{1}\) relaxation artifacts. It can be applied even prior to applying the same BP sequence, to preventively remove the contribution of liquidlike components and improving the I\({}_{\text{LDQ}}\)[27], but a shorter version is largely applied for spin diffusion experiments [28; 29; 30].  
Figure 5: Sequence for the magic sandwich echo (MSE) measurement.  
In order to select the magnetization arising from mobile fractions, an early approach was the Goldman-Shen (GS) sequence [31]. It is constituted by three \(\pi\)/2 pulses: the first two are separated by a time \(\tau_{0}\), after which the magnetization from hard segments has decayed to zero due to short effective transverse relaxation time T\({}_{2}\)*. The second pulse rotates the remaining magnetization onto the z-axis. Then, during a diffusion time \(\tau\), the magnetization diffuses from the soft to the hard component, allowing the magnetization arising from the hard fraction to increase; the third pulse tips the magnetization in the transverse plane, where it can be detected [32] for the spin diffusion measurements described below. A more sophisticated approach, that concerns the refocusing of the rigid fraction without relying on the T\({}_{2}\)*, led to the development of the MAPE (magic and polarization echo) filter (Figure 6b). It consists of an MSE filter applied directly on the longitudinal magnetization. Increasing the spacing between the pulses, the filter length increases, and the contribution of strong dipolar couplings can be completely suppressed [16].  
A different approach was proposed to bring out only signals of the mobile component with the use of a dipolar filter, based on the GS, followed by an MSE, to refocus the magnetization lost during the filter time [33].