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

Abstract

Crystallinity of poly(ɛ- l-lysine) (ɛ-PL) was discussed by analyzing the differences in the 1H spin–spin relaxation times ( T 2 H), the 13C spin-lattice relaxation times ( T 1 C), 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 T 2 H differences was 75.8 ± 0.1% at 333 K and 60.7 ± 0.4% at 353 K. From the T 1 C 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.