Analysis of FID shapes of polycrystalline polymeric solids

Knowledge of the molecular structure of a polymer is crucial for fundamental scientific studies and industrial applications, as it strongly influences its macroscopic properties.A wide variety of methods exist for the determination of polymer molecular structure, with Time-Domain NMR (TD-NMR) gaining increasing attention.However, this technique still lacks a unified analytical approach and usually requires complex analysis performed by educated specialists.This study proposes a framework for the analysis of Free Induction Decays (FIDs) of heterogeneous solid materials using TD-NMR.We address the limitations of traditional multicomponent fitting by introducing the approach based on the determination of the second moment of the NMR spectral line to unambiguously evaluate the characteristic apparent $T_2^*$.We performed and tested the preprocessing procedures to handle the issues of the ringing time, magnet inhomogeneity, and the possible presence of liquids.These procedures are supplemented by previously established FID reconstruction methods based on Solid Echo (SE) and mixed Magic Sandwich Echo (mixed-MSE) experiments.Experiments performed on various polyolefins demonstrate that FID reconstruction derived from the SE experiment with varied inter-pulse time yields amplitudes closest to the expected values.Importantly, we show the calibration method for rapid crystallinity assessment through the second moment and the observed dependence of echo attenuation on $T_2^*$.The proposed Second-Moment Gaussian Approximation (SMGA) approach enhances the accuracy, repeatability, and efficiency of TD-NMR analysis of solid and semicrystalline materials.