Transient Behaviour

Figure 62 illustrates the response of the 19F spectra of PVDF for each of the selected excitation angles as the inter-pulse delay is varied such that the TF ranges over approximately two rotor periods (τr) in steps of 3/50 τr (referred to as the short time series). It is important to note that the use of both a 100 Hz and 5 kHz pulse offset did not significantly change the observed transient behaviour (this will be shown later in section 3.6.1). Refocused DIVAM has been implemented using a 1 to 5 kHz pulse offset and the effect of using a pulse offset larger than 5 kHz will be discussed in section 3.6. Similarly to Direct DIVAM, very little to no variation is seen in the signal intensity with respect to the inter-pulse delay for the excitation angle of 2.5ο. In contrast, variations in the signal intensity with respect to inter-pulse delay, or the rotor phase, did not become apparent in Direct DIVAM until the excitation angle was greater than 30ο; however, such behaviour is already quite pronounced in Refocused DIVAM at excitation angles of 15ο or greater. This indicates that the selection mechanism at much smaller angles is being driven by an orientationally dependent term. This further supports the earlier statement that the CSA is the main interaction driving the selection mechanism of Refocused DIVAM. The amorphous signal is seen to nutate through one cycle for an excitation angle of 15ο and multiple cycles for excitation angles greater than 15ο. Nutation of the crystalline signals

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can be seen, but is far less pronounced than that of the amorphous signal. The periodic behaviour of the amorphous signal, with respect to the rotor period, is very different from that seen in Direct DIVAM. The addition of 180ο refocusing pulses could be causing partial recoupling of the CSA term in the Hamiltonian. The oscillations in the intensity of the amorphous signal will be further explored with Simpson simulations in section 4.4.2.

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Figure 62 19F Refocused DIVAM spectra of PVDF over a series of inter-pulse delays, calibrated such that TF covers 3 to 4 rotor periods (TR). This array is shown for selected excitation

angles from 2.5 to 90 degrees using a MAS rate of 20 kHz, a 90ο pulse width of 2.5 µs, a 180ο pulse width of 5 µs (TF = 150 µs), and a pulse offset of 1 kHz.

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Figure 63 illustrates the response of the 19F MAS NMR spectra of PVDF for each of the selected excitation angles as the inter-pulse delay is varied such that the TF covers 500 rotor periods (τr) in a non-linear fashion, with the spacing increasing from 5, 10, to 100 τr (referred to as the long time series). For the small excitation angles, ca. 2.5o, the intensity of the amorphous signal appears to oscillate around its maximum and it will be shown later that this minor oscillation can be attributed to the CSA term of the Hamiltonian (see section 4.4.2.). For short delay times (τ = 17.06 µs), as the excitation angles increases (θ = 2.5 o to θ = 30o) the net rotation of the amorphous signal also increases and its intensity is reduced until it reaches the first null condition, as seen in the excitation angle array (figure 58). As the delay time increases none of the signals appear to recover intensity when exceeding the T2 time scale. In the case of Direct DIVAM, T2 recovery behaviour was apparent for the excitation angles of both 15o and 30o. This indicates that relaxation plays a lesser role in the domain selection of Refocused DIVAM. This is most likely due to the application of the refocusing pulses as they remove the effects of field in-homogeneity and effectively lengthen the T2 value. Beyond 30o the net rotation in the transverse plane becomes large enough that the saturation effect becomes increasingly pronounced, eventually reaching a point at which the signal does not recover over the long time range. The amorphous signal at large excitation angles (θ = 45o or larger) does appear to recovery for small inter-pulse delays that coincide with multiples of the rotor period. This again suggests that the use of 180o refocusing pulses may lead to some recoupling of the CSA term in the Hamiltonian.

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Figure 63 19F Refocused DIVAM spectra of PVDF over a series of inter-pulse delays, calibrated such that TF covers 10 to 500 rotor periods (TR). This array is shown for selected

excitation angles from 2.5 to 90 degrees using a MAS rate of 20 kHz, a 90ο pulse width of 2.5 µs, a 180ο pulse width of 5 µs (TF = 150 µs), and a pulse offset of 1 kHz.

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In summary, the long and the short time series illustrate aspects of the spin dynamics during the Refocused DIVAM experiment that differ from those seen in Direct DIVAM. The short time series for Direct DIVAM showed that transient oscillations are observed for excitation angles larger than 30o. In contrast, the analogous Refocused DIVAM series illustrated significant transient behaviour for angles of 15o or larger. The long time series of both Refocused and Direct DIVAM illustrate that the saturation effect is more pronounced with increasing excitation angle; however, in Refocused DIVAM this saturation does not appear to recover on the T2 time scale for small excitation angles, as was seen in Direct DIVAM. Both of these results suggest that coherent spin dynamics play a much larger role in the selection mechanism of Refocused DIVAM, while the role of relaxation seems to have decreased.