Bipolar gradient pulses stimulated

The bipolar gradient pulse stimulated echo sequence (BPPSTE) (Figure 1) was employed to selectively suppress the THF and ACN solvent peaks, as well as the small molecule impurities. A diffusion time (A) of 24 ms and a gradient of 30 G/cm with 2 ms duration time (5) were employed. The NMR spectrum for each polymer component was acquired with 256 scans, a spectral width of 8080 Hz, and a relaxation delay of 2 s. For the on-flow GPC-NMR, the WET sequence was employed to suppress THF solvent peaks, and each H spectrum was acquired with an acquisition time of 0.45s, 32 scans and no relaxation delay. This condition was used for qualitative detection of the various components at a given elution time of the GPC separation. 

For self-diffusion measurements, the HT PFG NMR probe is a 10 mm liquid probe (Bruker design) equipped with a gradient coil providing 5.5G/cm/A and with a gradient amplifier of 10 A. The thermal shielding of the radiofrequency and gradient coils is insured by several ceramic barriers. The NMR pulse sequence used here combines stimulated echoes and bipolar gradient pulses [17]. 

To allow for gradient pulse recovery delays Tg in the bipolar pairs, Eq. (9.8) for defining the signal amplitude of the stimulated-echo is modified slightly ... 

Figure 8 Pulse sequences commonly used for PGSE measurements sequences with (a) spin-echo (b) stimulated echo (c) stimulated echo and longitudinal eddy-current delay (LED) (d) stimulated echo with bipolar pulsed field gradients and LED. Narrow and wide black rectan es represent 7t/2 and tt radiofrequency pulses, respectively. Narrow and wide open rectangles are field-gradient pulses of duration /2 and...

Figure 9.4. The (a) BPP-STE and (b) BPP-LED diffusion sequences. The encoding gradients of the stimulated-echo are apphed as symmetrical bipolar pulse pairs of total duration 8 and the LED sequence is extended with an eddy current delay period T. Optional purging pulses Gpi and Gp2 may also been employed.

Figurel. Bipolar stimulated echo pulse sequence (BPPSTE). The gradient time <5 T, t1 and x2 delays were set to 2 ms, 20 ms, 68ps and 78ps, respectively. (Reproduced with permission from Polym. Prepr. 2001,42.1, 23-24.

Q-space diffusion MR was proposed as means to obtain structural information on a micron scale." The effect of diffusion pulse sequence on the signal decay was examined in q-space diffusion MR performed on micron-scale phantoms of different geometries and homogeneities. The effect of the pulse gradient stimulated-echo, the longitudinal eddy current delay (LED) and the bipolar LED (BPLED) pulse sequences was evaluated. 

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