Distribution of Chain Ends in a Crystal-Fixed Polymer Elucidated by Solid-State NMR

The location of chain ends in the morphology of semicrystalline polymers has recently received renewed attention. In particular, chain ends at the crystal surface can help avoid density anomalies in the noncrystalline surface layers.¹ Here we present a detailed analysis of the distribution of chain ends across the ~ 15 nm repeat unit of semicrystalline poly(ε-caprolactone) (PCL)², a crystal fixed polymer³ of moderate crystallinity. An excellent signal-to-noise ratio and spectral resolution is achieved by isotopic labeling of chain ends via conversion of OH end groups with ¹³COO-acetyl groups. Through distinct (an)isotropic chemical shifts and relaxation times, one can distinguish chain ends in the crystaline, interfacial, and amorphous layers. 56% of the chain ends are found in the core amorphous layers, which exceeds the amorphous core fraction. About 20% are immobilized in the crystal lattice; many of these segments are found near the interface, according to fast ¹H spin diffusion from the amorphous core layer.
(1) Fritzsching, K. J.; Mao, K.; Schmidt-Rohr, K. Macromolecules 2017, 50 (4), 1521–1540.
(2) Kurz, R.; Thurn-Albrecht, T.; Saalwächter, K., et al. Macromolecules 2018, 51 (15), 5831–5841.
(3) Hu, W.-G.; Schmidt-Rohr, K. Acta Polym. 1999, 50 (8), 271–285.