Summary: A systematic investigation of the microstructure of ethylene copolymers has been carried out as a function of the comonomer type and content, employing small‐angle X‐ray scattering (SAXS) experiments to study the lamellar structure, and solid‐state NMR to study the local conformational freedom. The SAXS results clearly show that the major consequence of incorporating comonomers (butene and octene) is the absence of lamellar thickening during the crystallization. From solid‐state NMR study, applying the rotor‐encoded rotational echo double resonance technique (REREDOR), it is found that the extent of the orientational order in the noncrystalline region of semicrystalline polyethylene is substantial but this order is not significantly affected by the incorporation of comonomer units. Therefore, the major cause for the significant improvements of mechanical properties observed for ethylene copolymers is not a change in the thermodynamic state of the noncrystalline region, but most likely a significant change in the intermolecular topological structure due to the blocking of disentanglement processes required by the lamellar thickening step during crystallization.
