Molecular Origins of Thermoplastic Elasticity of Highly Branched Polyethylene as Revealed by Solid-State NMR Spectroscopy

High Resolution Image Download MS PowerPoint Slide Highly branched polyethylenes (HBPEs) synthesized via α-diimine nickel catalysis exhibit excellent thermoplastic elasticity due to the unique combination of long and short branching structures. It is, however, challenging to analyze the crystalline structure, crystallinity, and crystal size distribution of HBPEs due to their low crystallinity. Therefore, the relationship between the branching structure, solid structure, and mechanical properties has not been established. In this work, we provide a detailed structural characterization of various HBPEs by using solid-state 13 C NMR spectroscopy combined with magnetic relaxation measurements. The combination of two 13 C spin–lattice relaxation methods quantitatively analyzes the crystallinity, as well as size distributions of both the orthorhombic and monoclinic phases. By quantitatively comparing the detailed branching microstructures with crystallinity, it is established that HBPEs with long chain branching (LCB < 12b/1000C) and short-chain branching (SCB < 85 b/1000C) exhibit semicrystalline features. Furthermore, it was found that high fractions of small crystallites relative to the large ones, rather than crystallinity, lead to excellent strain recovery behavior. In conclusion, solid structures, including crystallinity, polymorph, and crystalline size distribution, govern the unique thermoplastic elasticity of HBPEs. This research provides guidance for designing branching microstructures for tunable solid structures and the mechanical properties of HBPEs.