Ultrahigh Molecular Weight Linear Polyethylene Research Articles

Ultra-high molecular weight polyethylene: preparation and applications

Ultra-high molecular weight polyethylene is a kind of popular engineering material because of its unique properties stemming from high molecular weights. Nowadays, the preparations and applications of this type of material are widely researched. This review mainly focuses on the preparation of ultra-high molecular weight polyethylene using three types of typical catalysts (heterogeneous Ziegler-Natta catalysts, Fujita’s catalysts and α-Diimine Nickel (II) catalysts) and applications in two significant areas (bulletproof membranes and lithium-ion batteries). Ziegler-Natta catalysts and Fujita’s catalysts favor the synthesis of linear ultra-high molecular weight polyethylene, but α-Diimine Nickel (II) catalysis favors β-hydride elimination which leads to branched products. Changes in steric, composition, activators, temperature and pressure will affect the tendency towards different mechanisms and influence structures and properties of final products.

A readily available neutral nickel catalyst for accessing linear ultrahigh molecular weight polyethylene in a living manner

It is of great importance and is highly desired in the olefin polymerization catalysis that a simple catalyst derived from cheap starting materials and short synthetic pathways exhibits superior catalytic property towards a polymerization reaction. In this contribution, starting from commercially available and cheap materials, by using a short and simple three-step reaction route we report a readily available neutral phenoxy-imine nickel catalyst, which significantly enables the formation of linear ultrahigh molecular weight polyethylene (~5 brs/1000C, Mn = 1531 kDa) in a living polymerization (PDI = 1.08–1.11). An enhancement of 77 times on the polymer molecular weight occurs compared to the classical 2,6-diisopropyl substituted phenoxy-imine nickel catalyst. This avoids a tedious synthetic procedure and the use of an excess aluminum reagent as the activator and the scavenger.

Polymerization of Low-Entangled Ultrahigh Molecular Weight Polyethylene: Analytical Model and Computer Simulations

We developed a theoretical model of linear ultrahigh molecular weight polyethylene (UHMWPE) homogeneous polymerization. We considered polymerization to be living and occurring in a poor solvent. We...

Ethylene homopolymerization and copolymerization with 1-hexene and 1-octene catalyzed by titanium(IV) dichloride TADDOLate complex activated with MAO

The synthesis and olefin polymerization behavior of a new TADDOL-based Ti(IV) complex, (4R,5R)-2,2-dimethyl-??,??,?????,?????-tetrakis[bis-(3,5-trifluoromethyl)phenyl]-1,3-dioxolane-4,5-dimethanolato-titanium(IV) dichloride, are described. Upon activation with MAO, this complex polymerized ethylene, producing ultra-high molecular weight linear polyethylene (UHMWPE) with activities up to 4500 kg mol (Ti)???1 [C2H4]???1 h???1 atm???1 and molecular weights up to 3.25????????106. The optimal temperature for UHMWPE synthesis was 50 ??C. This complex is also capable of copolymerizing ethylene with 1-hexene and 1-octene, giving high molecular weight copolymers with ??-olefin incorporation up to 7.8%. The copolymers, obtained with a different ratio of comonomers, are statistical, according to the analysis of the 13C NMR spectra. The reaction parameters that influenced the copolymerization behavior, such as comonomer concentration, reaction temperature and [Al]/[Ti] molar ratio, are examined in detail. Furthermore, high catalytic activities up to 12,531 kg mol(Ti)???1 [C2H4]???1 h???1 atm???1 were observed in copolymerization of ethylene and 1-hexene or 1-octene with the 2/MAO catalytic system. The obtained copolymers possess high molecular weights (Mw???=???1.4????????106???ethylene/1-hexene and 1.86????????106???ethylene/1-octene) with broad MWD (Mw/Mn???=???3.04???8.23) and high comonomer incorporation degrees (up to 6.2 mol% of 1-hexene and 7.8 mol% of 1-octene). Depending on the synthesis conditions, it is possible to form both a statistical copolymer and a block copolymer.

Highly thermally conductive and mechanically robust composite of linear ultrahigh molecular weight polyethylene and boron nitride via constructing nacre-like structure

With the rapid development of modern electronic, developing highly thermally conductive and mechanically strong composites for achieving efficient and reliable thermal management has become an urgent task. In this work, boron nitride/linear-ultrahigh molecular weight polyethylene (LUHMWPE) composites with strong mechanical performance and high thermal conductivity (23.03 W/(mK) were fabricated by solid-state extrusion (SSE) method. The thermal conductivity achieved in this work is the highest for bulk polymer so far. The effects of extrusion draw ratio (EDR) on the morphology, structure, and thermal properties were also investigated. With increase in EDR, the original isotropic structure gradually transforms into nacre-like structure due to the extensive elongational flow field during SSE, accompanied by the augment of thermal conduction performance and mechanical properties. We attribute the high thermal conductivity and mechanical properties to the construction of nacre-like structure, where the h-BN platelets and LUHMWPE molecular chains are highly oriented. Our bioinspired composites have potential applications in thermal management field for modern electronics.

High thermal conductivity of chain-aligned bulk linear ultra-high molecular weight polyethylene

It is difficult for bulk polymers to be used in the thermal management field because of their low thermal conductivity (TC). Considering that there have been few studies on the enhancement of TC for bulk polymers, we, in this work, used linear ultrahigh molecular weight polyethylene (LUHMWPE), which has a reduced number of branching, rather than conventional UHMWPE to successfully produce a high TC bulk polymer via utilizing solid state extrusion (SSE) to obtain a highly oriented structure. The high orientation of polymer backbone chains parallel to the extrusion direction and the increased crystallinity were responsible for the greatly improved TC. The oriented bulk LUHMWPE's thermal conductivity reached 4.7 W/mK, approximately 13 times that of its compression-molded sample, is also higher than the conventional UHMWPE prepared by SSE (3.0 W/mK). Moreover, it is found that the final thermal conduction performance of bulk LUHMWPE had a positive correlation with the extrusion draw ratio. The result of scanning electron microscopy shows that a number of nanofiberlike structures were formed during SSE, accounting for the super high tensile strength (120.4 MPa) of the bulks. The enhanced thermal conduction performance and high tensile strength make neat LUHMWPE a highly potential candidate to be used in electronic packaging areas.

Achievement of strictly linear ultra-high molecular weight polyethylene with narrow dispersity by dint of nitro-enhanced 2,6-bis(imino)pyridylchromium chloride complexes

The influence of reaction parameters and the electronic/steric effects of chromium complexes on the catalytic performance was investigated in detail.

Heterogeneous Distribution of Entanglements in a Nonequilibrium Polymer Melt of UHMWPE: Influence on Crystallization without and with Graphene Oxide

In the past, studies have been performed to follow chain dynamics in an equilibrium polymer melt using low molar mass polymers. Here we show that in linear ultrahigh molecular weight polyethylene entanglements formed during or after polymerization are influencing differently the overall chain topology of the polymer melt. When a disentangled UHMWPE sample is crystallized under isothermal conditions after melting, two endothermic peaks are observed. The high temperature peak is related to the melting of crystals obtained on crystallization from the disentangled domains of the heterogeneous (nonequilibrium) polymer melt, whereas the low melting temperature peak is related to the melting of crystals formed from entangled domains of the melt. On increasing the annealing time in melt, the enthalpy of the lower melting temperature peak increases at the expense of the high melting temperature peak due to the transformation of the disentangled nonequilibrium melt into the entangled equilibrium one. However, indep...

Raman study of uniaxial deformation of single-crystal mats of ultrahigh molecular weight linear polyethylene

We present for the first time a Raman spectroscopic study of the deformation process of solution-crystallized single-crystal mats of ultrahigh molecular weight linear polyethylene (UHMW PE). We study the deformed regions of the films, drawn only until the formation of the neck, and the films of much higher draw ratios, just before rupture starts. For comparison, we have also carried out Raman investigations of films produced by compression of UHMW PE powder. We have found that the uniaxial molecular orientation in the neck region of the single-crystal mat films develops more slowly as compared to the films, prepared by compression of the UHMW PE powder.

Appearance of Perfect Amorphous Linear Bulk Polyethylene under Applied Electric Field and the Analysis by Radial Distribution Function and Direct Tunneling Effect

Without melting flow, linear ultrahigh molecular weight polyethylene (UHMWPE) provided X-ray intensity curve from only amorphous halo at 129.0 °C (surface temperature, Ts arisen by Joule heat) lower than the conventionally known melting point 145.5 °C on applying electric field to UHMWPE-nickel-coated carbon fiber (NiCF) composite. Such surprising phenomenon was analyzed by simultaneous measurements of X-ray intensity, electric current, and Ts as a function of time. The calculated radial distribution function revealed the amorphous structure with disordered chain arrangement. The appearance of such amorphous phase was arisen by the phenomenon that the transferring electrons between overlapped adjacent NiCFs by tunneling effect struck together with X-ray photons and some of the transferring electron flown out from the gap to UHMWPE matrix collided against carbon atoms of UHMWPE. The impact by the collision caused disordering chain arrangement in crystal grains.

Isotactic-Specific Polymerization of Propene by aCs-Symmetric Zirconium(IV) Complex Bearing a Dianionic Tridentate [−NNN−] Amidomethylpyrrolidepyridine Ligand

The synthesis and characterization of a new tridentate amidomethylpyrrolidepyridine ligand (LigH2) and of the corresponding LigZr(NMe2)2 complex (1) are described. Characterization of 1 by single-crystal X-ray diffraction analysis showed a slightly distorted square-pyramidal geometry. Variable-temperature NMR analysis suggested that 1 adopts a Cs-symmetric structure in solution. Complex 1 in combination with AliBu2H and methylalumoxane afforded a highly active single-site catalyst for the polymerization of ethylene and propene, producing ultrahigh molecular weight linear polyethylene and isotactic polypropylene via an “enantiomorphic sites” mechanism of steric control.

Influence of Crystal Thickness and Topological Constraints on Chain Diffusion in Linear Polyethylene

(13) C solid-state exchange NMR is applied to study the influence of morphology on chain diffusion between crystalline and noncrystalline regions in ultrahigh molecular weight linear polyethylene (PE). Lamellar-doubling reduces the exchange rate by a factor of two indicating that the chain diffusion coefficient is largely independent of the lamellar thickness. This is discussed in terms of molecular processes in the crystallites leading to chain diffusion, confirming that the role of defects is minor compared to helical jumps of extended stems. Hindrance of the chain diffusion resulting from chain entanglements was only observed after the chains diffuse over long distances. Moreover, the role of the interphase between the noncrystalline and the crystalline regions on chain diffusion is discussed.

Restricted Segmental Mobility Can Facilitate Medium-Range Chain Diffusion: A NMR Study of Morphological Influence on Chain Dynamics of Polyethylene

The influence of the morphology on the local motion in the noncrystalline regions and chain diffusion between crystalline and noncrystalline regions is studied in ultrahigh molecular weight linear polyethylene: The behaviors of samples of the same material crystallized from the melt and from solution are compared. The geometrical restrictions of the conformational transitions are probed via anisotropic NMR interactions, i.e., 13C−1H dipole–dipole couplings and 13C chemical shift anisotropy. As they are averaged out under MAS, recoupling techniques are applied to yield sideband patterns or quasi-static NMR spectra, from which residual anisotropies are determined. Chain diffusion is probed by 13C exchange NMR. As expected, the local conformational transitions are more restricted in the solution crystallized sample compared with the melt crystallized one. Moreover, the motional narrowing observed by both techniques indicates that the local mobility in the noncrystalline regions of the solution crystallized s...

Strain-Induced Changes of Free Volume Measured by Positron Lifetime Spectroscopy in Ultrahigh Molecular Weight Polyethylene

Positron lifetime spectroscopy and wide-angle X-ray scattering have been applied to investigate the structural changes induced by tensile deformation in ultrahigh molecular weight linear polyethylene. The results reveal a correlation between the crystallinity, the positron annihilation characteristics, and the strain of the polymer. The probability of positronium formation increases and the crystallinity of the material decreases with increasing tensile strain. The size distribution of the free-volume holes, where positronium atoms form, is determined from their lifetime distribution. The positron lifetime measurements are interpreted in terms of a free-volume approach assuming that the free volume associated with each liquidlike cell of the amorphous phase is divided into free-volume holes whose size distribution is given by a normal frequency function. The cumulative distribution functions determined from the positron annihilation measurements agrees with those predicted by the free-volume model. This indicates that the positrons contributing to the lifetime component associated with orthopositronium annihilation really probe the free volume in the amorphous phase of the polymer. The experiments evidence free-volume changes induced by tensile strain which are unrecoverable after relieving the stress.

Effect of high energy radiation on the stress–relaxation of ultra‐high molecular weight linear polyethylene

AbstractStress–relaxation behavior was studied in ultra‐high and normal molecular weight linear polyethylenes (UHMWPE and NMWPE) as a function of radiation dose over the range 0–128 Mrad. Irradiation up to 16 Mrad raises the crystallinity in both types of PE, as demonstrated previously,1 and thus increases the initial modulus. Also, the initial modulus of NMWPE is higher than that of UHMWPE because the former has a higher crystallinity. Consequently, the initial stress at a constant imposed strain of 1% varies greatly between the two materials. To eliminate the effect of this initial difference on relaxed stress, the stress–relaxation data were normalized with respect to the initial stress and plotted as the fraction, retained stress after time t/initial stress. The normalized plots show no significant difference between NMWPE and UHMWPE in their stress–relaxation behavior. For both materials stress retention improves progressively with increasing radiation dose, the percentage improvement being greatest at long times (50% at 50 h and 64 Mrad, compared with 6% at 10−2 h). These results are interpreted to indicate that radiation crosslinking in the amorphous phase is independent of molecular weight and preferentially retards those molecular motions responsible for short relaxation times. The motions in question could involve molecular flow in the amorphous phase or “pull out” of tie molecules from the crystalline lamellae.

Pecularities of the thermomechanical behaviour of ultra-high molecular weight linear polyethylene and its blends with linear polyethylene of normal molecular weight

Ultra-high molecular weight polyethylene UHMWPE (Mw=4 · 106,Is=O g/ 10 min), high density polyethylene of normal molecular weight NMWPE (Is= 4.8 g/10 min) and their blends have been investigated by means of thermomechanical loading in constant and impulse regime. It has been established that after melting, NMWPE passes to a viscous-liquid state. After melting at 138 °C UHMWPE passes to a high-elastic state. The transition of UHMWPE to a viscous-liquid state takes place at temperatures higher than 180 °C and is accompanied by a high-elastic reversible deformation. The blends of UHMWPE with 10 and 20 mass % of NMWPE show a plateau on the thermomechanical curves, corresponding to a high-elastic state, in a shorter temperature range where the deformation is greater. The blends containing the higher percent of NMWPE show thermomechanical curves lacking such a plateau. All blends are characterized by a singular thermomechanically defined temperature of melting, which increases with increase of UHMWPE content. The existence of the high-elastic state in the curves of UHMWPE and its blends containing NMWPE less than 30 mass % above their melting temperatures is explained by the high degree of physical crosslinking of UHMWPE.

Radiation‐induced crystallinity changes in linear polyethylene: Influence of aging

AbstractA previous paper described an unusual crystallinity effect observed in ultrahigh molecular weight linear polyethylene (UHMW PE) and conventional high density polyethylene (HDPE). It was discovered that upon exposure to high energy radiation, these polymers experience a significant increase in the degree of crystallinity. The present paper describes another equally unexpected and surprising phenomenon observed in irradiated UHMW PE and HDPE. It was accidentally found that the irradiated polyethylenes exhibit an aging effect; their heat of fusion and hence their degree of crystallinity increases monotonically with the aging time (since initial irradiation) at ambient conditions. Surprisingly, the aging process in irradiated polyethylenes was observed to persist even after 31 months. The magnitude of the aging effect is a strong function of the initial molecular weight of the unirradiated polymers and the irradiation dose. The aging phenomenon could not be accelerated by thermal annealing. The exact mechanisms causing the aging phenomenon remain rather elusive at the present time, but possible reasons are explored.

Effect of high-energy radiation on the uniaxial tensile creep behaviour of ultra-high molecular weight linear polyethylene

The tensile creep (and other tensile) properties of ultra-high molecular weight polyethylene (UHMW PE) have been determined before and after electron beam irradiation and compared with similar results on normal molecular weight high-density polyethylene (NMW PE). In both polymers, irradiation increases the tensile modulus and the yield stress whilst reducing creep. The major effects occur over the first 20 MRad irradiation dose, though creep strain continues to diminish with dose in UHMW PE up to 64 MRad. Most of the effects can be attributed to crosslinking in the amorphous phase, though the rise in yield stress seems to require crosslinking in the crystalline phase, and the initial rise in modulus in UHMW PE seems to reflect a rise in crystallinity. Comparison with other polymers shows that the creep behaviour of UHMW PE remains relatively poor, even after irradiation. The improvements obtained may, however, be significant in applications where creep resistance is of secondary importance compared with, say, impact and wear resistance, in which UHMW PE excels.

Impact fatigue response of ultra-high molecular weight linear polyethylene

The impact fatigue response of ultra-high molecular weight linear polyethylene (UHMW LPE), in a special test, has been examined and the results are presented in this paper. In an attempt to understand the influence of high molecular weight on impact strength, identical measurements were made on a normal molecular weight linear polyethylene (NMW LPE). UHMW LPE is found to have a superior impact and impact fatigue behaviour to the NMW LPE. Almost all of the UHMW LPE materials perform equally well in the present impact fatigue test. However, one of the high bulk density UHMW LPE materials, resin G, performs quite poorly. Photomicrographs of the free surface of this material show that this may result from poor interparticle fusion during compression moulding.