NMR determination of crystallinity for poly(ɛ-l-lysine)

Crystallinity of poly(ɛ-l-lysine) (ɛ-PL) was discussed by analyzing the differences in the 1H spin–spin relaxation times (T2H), the 13C spin-lattice relaxation times (T1C), and the 13C NMR signal shapes between the crystalline and the non-crystalline phases. The observed 1H relaxation curve (free induction decay followed by solid-echo method) showed the sum of Gaussian and exponential decays. Similarly, the observed 13C relaxation curves obtained from the Torchia method were double-exponential. The 13C NMR spectrum of ɛ-PL was divided into the narrow and the broad lines by utilizing the intrinsic differences in the 1H spin-lattice relaxation times in the rotating-frame between them, which are attributed to the crystalline and the non-crystalline phases, respectively. Even though the crystallinity is obtained from the identical NMR measurements, the estimated values are different with each other. The crystallinity estimated from the T2H differences was 75.8 ± 0.1% at 333 K and 60.7 ± 0.4% at 353 K. From the T1C differences, the value was estimated to be 62 ± 11%. Furthermore, the value estimated from the NMR signal separation was 54 ± 5%. In this study we have explained these discrepancies by the difference in susceptibility among the experiments for the inter-phase, which exists in-between the crystalline and the amorphous phases. Furthermore, the estimated crystallinity was ascertained by the X-ray diffraction experiment.