One- and Two-Dimensional Solid-State 13C NMR Analyses of the Solid Structure and Molecular Motion of Poly(ε-caprolactone) Isothermally Crystallized from the Melt

Abstract

The crystalline−noncrystalline structure and molecular motion of poly(ε-caprolactone) (PCL) isothermally crystallized from the melt have been investigated by one- and two-dimensional solid-state 13C NMR spectroscopy. The 13C spin−lattice relaxation time (T1C) analysis reveals that the PCL sample contains three components with different T1C values, which are assignable to the crystalline, mobile crystalline, and noncrystalline components. By the 13C spin−spin relaxation time (T2C) analysis, it is found that the noncrystalline component can be further resolved into the crystalline−amorphous interfacial, and amorphous components. The mass fractions of the crystalline, interfacial, and amorphous components are finally determined to be 0.42, 0.30, and 0.28, respectively. In the crystalline region, different molecular mobilities along the methylene sequence are suggested through the difference in T1C value for the constituting carbons. More detailed molecular motion in the crystalline region has been characterized by the analysis of the 13C chemical shift anisotropy (CSA) in terms of the two-site exchange model. 13C CSA spectra of the individual carbons are successfully recorded by using the two-dimensional switching angle sample spinning technique. The CSA spectrum of the carbonyl carbon exhibit that the carbonyl carbon is almost in the rigid state or undergoes the jump motion around the molecular chain axis with a jump angle less than 30°. In contrast, methylene carbons exhibit almost axially symmetric CSA spectra, suggesting the rapid jump motions around the molecular chain axis with a jump angle of 60−90°. Further narrowed CSA spectra, which are still axially symmetric, are observed for the methylene carbons not directly attached to the ester group, suggesting the existence of additional enhanced jump motion around the C−C bonds. Such enhanced molecular motions of the methylene sequence may be due to the distorted nonplanar zigzag chain conformation of PCL in the crystalline region.