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Abstract Summary: An improved proton NMR method for the real‐time measurement of the hard/soft ratio or the crystallinity, and the mobile‐fraction dynamics, in phase‐separated or semicrystalline polymers is presented. It avoids some difficulties associated with earlier approaches and can be applied on high‐ as well as inexpensive low‐field instrumentation. A pulsed mixed magic‐sandwich echo is shown to provide near‐quantitative refocusing of the rigid contribution to the initial part of the free induction decay. This essentially removes the need to account for signal loss during the receiver dead time, and the method should thus be useful for a variety of applications where the magnetization distribution over differently mobile fractions is to be determined. The overall decay of the mobile signal of a semicrystalline polymer was found to exhibit a significant field dependence, such that the apparent transverse relaxation function of the amorphous part is in a real‐time experiment best characterized by a subsequent Carr–Purcell–Meiboom–Gill pulse train. It is demonstrated to be mainly influenced by mobility, while instrumental effects play a minor role. The mobility of the amorphous fraction depends not only on the overall crystallinity, but also on the crystallization conditions, thus on the nanometer‐scale morphology. Isothermal crystallization of sPP monitored on a 20 MHz low‐field proton NMR spectrometer using the MSE‐CPMG experiment. image Isothermal crystallization of sPP monitored on a 20 MHz low‐field proton NMR spectrometer using the MSE‐CPMG experiment.
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