Nascent Ultra-high Molecular Weight Polyethylene Research Articles

Melting kinetics, ultra-drawability and microstructure of nascent ultra-high molecular weight polyethylene powder

For the production of high-performance polyethylene fibers and tapes by ultra drawing, using solid-state processing of disentangled nascent ultra-high molecular weight polyethylene (UHMWPE), the maximum draw ratio is an important design parameter, as it determines the maximum degree of chain alignment and, therewith, the properties of the final product. It would, therefore, be advantageous to have a fast scanning method to estimate the maximum drawability of reactor powders. In the present work, the melting behavior of nascent UHMWPE reactor powder was studied by differential scanning calorimetry, followed by isoconversional analysis to evaluate the apparent activation energy barrier of melting for the different UHMWPE grades of different level of disentanglement. Powder samples were solid-sate processed into tapes at elevated temperature. Ultra drawing of these tapes at the optimum drawing temperature allowed for the evaluation of the maximum draw ratio (λmax) that is determined by the entanglement density. The morphology of the lamellar structure of the nascent powder was studied by small-angle X-ray scattering and by scanning transmission electron microscopy analyses. A correlation between the melting kinetics, the microstructure and the maximum draw ratio was observed for all grades, suggesting that the apparent activation energy of melting can be used as a screening method to estimate the maximum draw ratio.

Particle morphology, structure and properties of nascent ultra-high molecular weight polyethylene.

The effects of particle morphology on the structure and swelling/dissolution and rheological properties of nascent ultra-high molecular weight polyethylene (UHMWPE) in liquid paraffin (LP) were elaborately explored in this article. Nascent UHMWPE with different particle morphologies was prepared via pre-polymerization technique and direct polymerization. The melting temperature and crystallinity of UHMWPE resins with different particle morphologies were compared, and a schematic diagram was proposed to illustrate the mechanism of UHMWPE particle growth synthesized by pre-polymerization method and direct polymerization. The polymer globules in the nascent UHMWPE prepared by using pre-polymerization technique are densely packed and a positive correlation between the particle size and the viscosity-averaged molecular weight can be observed. The split phenomenon of particles and the fluctuation in the viscosity of UHMWPE/LP system prepared by direct polymerization can be observed at a low heating rate and there is no correlation between particle size and viscosity-averaged molecular weight.

Entanglement Formation Mechanism in the POSS Modified Heterogeneous Ziegler–Natta Catalysts

Chain entanglement was very important for adjusting the processability and mechanical property of nascent ultrahigh molecular weight polyethylene (UHMWPE). So far, it is still a mystery to unravel ...

Crystal morphology and corresponding physical properties of nascent ultra-high molecular weight polyethylene powder with short-branched chains

Due to the linear and ultra-long molecular chains, ultra-high molecular weight polyethylene (UHMWPE) has been playing a vital role in many fields. Based on more and more demands for special UHMWPE resins, the relationship between microstructure of nascent powder and its corresponding physical properties was investigated in this work, and one special powder with high degree of short-chain branch was fully contrasted with one general-purpose UHMWPE resin. Based on series characterization results, it revealed that short-branched chains led to a decrease on the entanglement density and brought a significant increase in the processing performance and mechanical properties. In addition, the short-branched chains also made a huge difference on crystal morphology of nascent UHMWPE powders. By correlating high-resolution scanning electron microscopy, DSC curves and Raman spectroscopy characteristics, the slurry polymerization mechanism of UHMWPE was deduced, and the corresponding performance of nascent UHMWPE powders was fully explained.

Heterogeneous distribution of chain mobility in nascent UHMWPE in the less entangled state

The structure of chain entanglements in the solid state is important for revealing the relationship of structure and properties of polyolefin. In this work, low-field solid-state 1H NMR is used to study the chain dynamics of ultra-high molecular weight polyethylene (UHMWPE) in the solid state. It has been found that the relaxation distribution, analyzed by a multi-exponential inversion program, is an effective method to characterize the heterogeneous chain mobility. It is evidenced from the results that the UHMWPE in a less entangled state presents an obviously heterogeneous distribution of chain mobility in the non-crystalline phase, corresponding to its heterogeneous distribution of entangled points. In comparison, the commercial UHMWPE with a large number of entanglements shows a much more uniform mobility of the non-crystalline components. This heterogeneous distribution of chain mobility becomes even more critical after annealing the samples below the melting point, especially for the less entangled UHMWPE.

The unique effect of chain entanglements and particle morphology on the sintering of ultrahigh molecular weight polyethylene

Nascent ultrahigh molecular weight polyethylene (UHMWPE) is a thermoplastic material, consisting of nodular-fibrillar structure making it too much porous. At the present work, the effect of both particle morphology and chain entanglements on the sintering of UHMWPE was investigated. Neck growth for particles of three different commercial grades was determined as a function of time at isothermal sintering temperatures. A comparison between the experimental data and predictions from a theoretical model based on Maxwell constitutive equations revealed a complicated effect of particle size and morphology. It was observed that at early stages of sintering, Bellehumeur’s model was in an excellent agreement with the experimental data, while it was deviated at late stages when the neck growth ratio ( y/ a) exceeds 0.8. Unlike commercially entangled samples, melted disentangled powder particles did not flow toward each other even after a long time had elapsed. Disentangled UHMWPE pellets, sintered for 10 min near the melting temperature, showed great transparency indicating low air voids and developing the sintering process, while grain boundaries remained undeveloped for entangled pellets.

The morphology of nascent and moulded ultra-high molecular weight polyethylene. Insights from solid-state NMR, nitric acid etching, GPC and DSC

The differences in morphology between nascent ultra-high molecular weight polyethylene (UHMWPE) GUR415 grade powder and a ram-extruded moulded form have been studied by a variety of techniques. While it would be thought from DSC results that the powder has much thicker crystallites than the moulded material, it is shown quite clearly by nitric acid etching techniques followed by GPC that the crystal lamellae in both forms of the material have similar dimensions, close to 20nm on average. The GPC also shows, however, that in addition to the lamellar material, there exists a proportion of extended chain crystals in the powder, which may reside within fibrillar structures which have been observed by SEM. NMR measurements show that the amorphous phase in the powder is more constrained than that of the moulded material, and that the powder also has a fraction of monoclinic crystalline material associated with internal stress. Further NMR experiments on the highly crystalline residues from the nitric acid degradation experiments lead to an estimation of the spin–lattice relaxation time in a 100% crystalline PE of 2.5±0.1s in an external field of 4.7T.

Chain Mobility in Polymer Systems: On the Borderline between Solid and Melt. 3. Phase Transformations in Nascent Ultrahigh Molecular Weight Polyethylene Reactor Powder at Elevated Pressure As Revealed by in Situ Raman Spectroscopy

The phase transformations under elevated pressure have been followed in situ by X-ray and Raman spectroscopy for ultrahigh-molecular-weight polyethylene (UHMW−PE) reactor powder. Using the in situ X-ray as a reference for the Raman work, it has been shown that Raman spectroscopy is a convenient method to follow phase transformations in polyethylene under pressure. The distinction has been made between different solid phases in polyethylene (monoclinic, orthorhombic, and hexagonal phases) and the melt by studying the conformational changes occurring within the skeletal mode (1000−1200 cm-1) upon the phase transformations. The crystal field splitting (1415/1440 cm-1) in the −CH2− bending mode of the Raman spectrum of polyethylene, related to the orthorhombic unit cell, was confirmed to be sensitive to transformations between different crystal structures. By following the changes occurring in the methylene bending mode together with the changes within the C−C skeletal mode, it has been shown that the crystal...

Characterization of ultra high molecular weight polyethyelene nascent reactor powders by X-ray diffraction and solid state NMR

The nascent ultra-high molecular weight polyethylene (UHMWPE) reactor powders are prepared by polymerizing ethylene in the presence of soluble magnesium complexes under various reaction conditions. It is found that the polymerization process applied in the study provides a high yield of UHMWPE even at low polymerization temperature, and the reactor powders exhibit high melting temperatures (137–143°C) and heat of melting (180–210 J/g). X-ray diffraction (XRD) patterns and solid state 13C magic angle spinning (MAS) NMR spectra of the powders synthesized under various reaction conditions are examined and compared with those of a commercial UHMWPE powder. Our results reveal that a significant portion of the monoclinic phase is identified when the resin is polymerized at relatively low temperatures. Temperature dependent XRD and NMR experiments show that UHMWPE reactor powders synthesized in the study exhibit more drastic thermal behaviors than the commercial UHMWPE, and this difference becomes more pronounced with the reactor powders polymerized at low temperatures. The increased crystallinity and crystal size in newly synthesized UHMWPE powders at elevated temperatures (100–125°C) may significantly improve the processability of UHMWPE reactor powders into high strength tapes below their melting temperature.

Morphology and melting behavior of nascent ultra-high molecular weight polyethylene

The nascent morphology of UHMW PE exhibits high melting point, high crystallinity, and increased WAXS line breadth relative to samples formed by melt crystallization. Different empirical relationships between crystal size and melting point are observed for nascent and molded samples. This differentiation is removed following nitric acid treatment of the nascent flake. Solid-state annealing behavior is differentiated by several regimes. Regime I is characterized by increasing crystallite dimensions and crystallinity at low annealing temperatures. Regime II[a] and II[b] is identified by double melting in DSC scans of moldings and nascent flake, respectively. The double melting is due to partial melting with incomplete recrystallization. Regime II[a] of moldings is differentiated from Regime II[b] of flake by an increase in melting point of the higher melting endotherm. Within Regime II[b], the partial melting of the nascent structure is sensitive to the distribution of morphological stability. Regime III is initiated at annealing temperatures approaching the zero heating rate melting point, and shows melting kinetics by DSC or time-resolved WAXS using synchrotron x-ray radiation. The superheat, partially associated with Regime III behavior, is sensitive to morphological heterogeneity and annealing history. Morphological models are discussed which highlight the role of noncrystalline regions and melting kinetics on the melting behavior of nascent form crystallinity. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 495–517, 1998

Coextrusion drawing of reactor powder of ultrahigh molecular weight polyethylene

AbstractCertain nascent polymers have been shown to have unusual thermal and morphological properties that are irretrievably lost once the polymer has been melted or otherwise reduced to the isotropic state. We show further that nascent ultrahigh molecular weight polyethylene “reactor powder” exhibits a remarkable ductility when uniaxially drawn by coextrusion techniques after initial compaction in film form at 100°C. When drawn at a temperature of 110°C, draw ratios of 56 have been obtained, resulting in an enhanced tensile modulus of 58 GPa. Thermal analyses and dynamic mechanical measurements were also made towards understanding the initial and final morphologies.