Emergence Of Chain Entanglement Of Ultra High Molecular Weight Polyethylene

The influence of chain entanglements on process is an interesting and fundamental issue of polymer science, but so far it is still an open question. Especially as the application of UHMWPE materials in our industries and lives, the excellent properties, such as impact and wear resistance won it a rapturous ovation from people all around the world. Meanwhile, the long scale chains of the commercially available ones adversely made it notoriously difficult to process(high viscous in melts) due to its incredibly high degree of entanglement between chains. Therefore, it always has been an exploration to make hypothesis. Some excellent works, including the theoretical and experimental, have been made to improve the understanding of constrained polymers. Serpentine scaling theory(the tube model), which regards the chain entanglements as tube model, is the most accepted. This model reveals the molecular weight dependence of the zero shear viscosity. However, it does not definitely describe the emergence of chain entanglement in processing. In the past few decades, polystyrene chain entanglement was paid more attentions in rheological study. But studying the flexible UHMWPE chain will be more helpful to guide the processing of other UHMWPE reinforced materials.In this study, we designed a simplified model system: blends of UHMWPE and PE matrix. PE matrix, with low molecular weight can freeze chain entanglement evolution with different shear history. It is easy to reveal the entangled behavior and evolution of UHMWPE chains in semi-dilute solution by regarding the PE matrix, including LLDPE and two PE wax, as a solvent in polymer solution.1. UHMWPE and linear low density polyethylene(LLDPE) were used to prepare UHMWPE /LLDPE blends with relatively low UHMWPE concentrations. The influence of different concentrations of UHMWPE on the formed structure of UHMWPE chain entanglement in two blends, especially the critical concentration of onset of entanglement, was investigated by applying small oscillatory shear rheology. It is suggested that when the concentration of UHMWPE in UHMWPE/LLDPE blends is higher than 1.68wt%, the blends perform as solid gel independent of frequency at region of low frequency in frequency sweep curves, at the same time, the storage modulus begins to increase to plateau modulus in time sweep curves. The UHMWPE chains begin to diffuse to form a stable network, which can be sufficient conditions of entanglement. UHMWPE forms entanglement structure in the blends with UHMWPE concentrations above a critical value of c = 0.84wt%, four times the critical overlap concentration.2. UHMWPE and PE wax(Mw=1000g/mol) were used to prepare UHMWPE /PE-wax blends with relatively low UHMWPE concentrations, and then begin to anneal for a time. The PE wax had a molecular weight slightly smaller than the entanglement molecular weight, Me. In such a system, the entanglements between the matrix PE chains and their contribution to the modulus could be neglected, which facilitated extracting the information of UHMWPE entanglement. During the annealing, with the diffusion of wax into the UHMWPE-rich domains, UHMWPE chain segments moved cooperatively, causing a more homogeneous distribution of UHMWPE oriented networks in the wax matrix and the fast building up of the modulus. After that, the reptation of UHMWPE chain begins to dominate the chain diffusion, leading to a slow increasing of modulus. Finally, the melt achieves the thermo-dynamic equilibrium and no further increase in the modulus occurs.3. Redesign a simple model: UHMWPE was seeded into PE wax(Mw=4000g/mol) to prepare another UHMWPE /PE-wax blends. Then rheological measurements was performed with the blends to trace UHMWPE chains mobility. During the annealing, with the inter-diffusion of wax and UHMWPE chains into each other, UHMWPE nascent particles begins to swell to sponge-like microgels. The building up of the modulus, fast at beginning, begins slowing down, resulting from the UHMWPE chain reptation begins to dominate the chain diffusion. Though the radius of initial nascent particles was 100 nm, the dynamic-mechanical master curves can still be cut up in a terminal, a plateau, and a transition zone, as is textbook knowledge for linear polymers for all larger microgels.In conclusion, the emergence of chain entanglement of UHMWPE include two steps: 1) Homogeneous distribution of UHMWPE chain, diffusion of UHMWPE chain dominating the chain mobility, and 2) Homogeneous distribution of chain entanglement, the reptation of UHMWPE chain beginning to dominate the chain diffusion. And finally, the melt achieves the thermo-dynamic equilibrium.