Density of Physical Network Junctions in the Amorphous Phase of Polyethylene and Its Copolymers: The Effect of Crystallinity, Domain Sizes, and Molecular Weight

The 1H NMR T2 relaxation method is explored in the present study for the characterization of the effect of molecular weight, crystallinity, and comonomer units on the density of the network of physical junctions in the amorphous phase of polyethylene (PE) and its copolymers. The build-up of the network of physical junctions in the amorphous phase was studied during isothermal crystallization using real-time NMR experiments, which recorded the free induction decay of the crystalline phase and Hahn-echo decay of the amorphous phase. Two high-density polyethylenes (HDPEs) with similar Mn and different molecular weight distributions, ultrahigh-molecular-weight polyethylene (UHMWPE), and three ethylene copolymers with largely different amounts of short-chain branches were selected for the study. The density of crystal-induced physical junctions, which form tie molecules, increases linearly with the crystallinity increase in UHMWPE and PE copolymers. A partial disentangling is observed at the initial stage of the crystallization of low-molecular-weight HDPE. The density of tie molecules depends on the molecular weight of polymer chains and the ratio between the radius of gyration of polymer chains and the thickness of crystalline and amorphous domains. The lower the molecular weight and the smaller the ratio, the larger the fraction of chain end segments residing in interlamellar amorphous layers and the smaller the density of tie molecules. The increase in the fraction of short-chain branches in PE copolymers causes a large decrease in the long-range periodicity of the lamellar structure, a decrease in the lamellar thickness and the thickness of interlamellar amorphous layers, as shown by small-angle X-ray scattering experiments. The decrease leads to an increase in the relative fraction of network chains in the amorphous phase and the density of network junctions. The NMR method will be further explored in a follow-up study of model polyethylenes to establish the effect of molecular weight characteristics on the density of physical network junctions. Obtaining this knowledge could help in better understanding of stress–strain behavior and several other mechanical properties of polyolefins.