Polyethylene Fractions Research Articles

Exciton self-trapping in bulk polyethylene

We studied the behaviour of an injected electron–hole pair in crystalline polyethylenetheoretically. Time-dependent adiabatic evolution by ab initio molecular dynamics simulationsshow that the pair will become self-trapped in the perfect crystal, with a trapping energy ofabout 0.38 eV, with formation of a pair of trans-gauche conformational defects, threeC2H4 units apart on the same chain. The electron is confined in the interchain pocket created by a local,120° rotation of the chain between the two defects, while the hole resides on the chain and ismuch less bound. Despite the large energy stored in the trapped excitation, there does notappear to be a direct non-radiative channel for electron–hole recombination. This suggeststhat intrinsic self-trapping of electron–hole pairs inside the ideal quasi-crystalline fractionof polyethylene might not be directly relevant for electrical damage in high-voltage cables.

Synthesis of polyethylene with bimodal molecular weight distribution by supported iron‐based catalyst

AbstractThrough immobilization of two iron‐based complexes, [((2,6‐MePh)N = C(Me))2C5H3N]FeCl2 (1) and [((2,6‐iPrPh)N = C(Me))2C5H3N]FeCl2 (2), on SiO2 pretreated with tetraethylaluminoxane (TEAO), two supported iron‐based catalysts, 1/TEAO/SiO2 (3) and 2/TEAO/SiO2 (4), were prepared. These two supported catalysts 3 and 4 could be used to catalyze ethylene polymerization with moderate polymerization activity and prepare linear high‐density polyethylene with bimodal molecular weight distribution (MWD). It was demonstrated that immobilization of catalyst could significantly improve molecular weight (MW) of high‐MW fraction of the resultant polyethylene, as well as maintain bimodal MWD of polyethylene produced by the corresponding homogeneous catalysts. Such bimodal MWD of polyethylene produced by supported iron‐based catalysts could be well tailored by varying polymerization conditions, such as ethylene pressure and molar ratio of Al to Fe. It has been proven that TEAO is an efficient activator for both homogeneous and heterogeneous iron‐based catalysts for producing polyethylene with bimodal MWD. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5662–5669, 2004

Trapping of excitons at chemical defects in polyethylene.

In a previous paper we studied an injected electron-hole pair in crystalline polyethylene (PE) and found that the exciton becomes weakly self-trapped in a narrow interchain pocket comprised between two gauche defects. Despite the large energy stored in the trapped excitation, there did not appear to be a direct nonradiative channel for electron-hole recombination. Actual polyethylene systems of practical use are, however, neither crystalline nor pure. To understand the fate of an electron-hole pair in the impure case, we studied by ab initio simulations the evolution of an exciton trapped on three common chemical defects found in polyethylene: a grafted carbonyl (C=O); an intrachain vinyl group (C=C); a grafted carboxyl (COOH). Ab initio simulations lead to predict three different outcomes: trapping, nonradiative recombination, and homolitic bond-breaking, respectively. This suggests that extrinsic self-trapping of electron-hole pairs over chemical defects inside the quasicrystalline fraction of PE could be relevant for electrical damage in high-voltage cables.

Isothermal Lamellar Thickening in Linear Polyethylene: Correlation between the Evolution of the Degree of Crystallinity and the Melting Temperature

The isothermal lamellar thickening behavior of a linear polyethylene fraction was investigated using differential scanning calorimetry. Experimental results indicate very similar evolutions for both the degree of crystallinity and the resulting apparent melting temperature with isothermal crystallization time. Deconvolution of the primary and secondary crystallization processes was carried out numerically using the classical logarithmic law for lamellar thickening. Reliability of the deconvolution process was investigated using artificial data generated using the Avrami kinetic law for the development of primary crystallinity. We show that the shift of the apparent melting temperature with time can be quantitatively accounted for by the intrinsic rates of primary crystallization and lamellar thickening. We also show that use of a single kinetic parameter for secondary crystallization is sufficient to account for the change in both the degree of crystallinity and the apparent melting temperature as a funct...

Isobornyl Methacrylate as a Reactive Solvent of Polyethylene

Solutions containing 15 wt, -% of a low-molar-mass polyethylene (PE) in isobornyl methacrylate (IBoMA), containing 0, 5 or 10 wt.-% of 1,4 butanediol dimethacrylate (BDDMA) as crosslinker, were polymerized using either benzoyl peroxide (BPO), at 80°C, or dicumyl peroxide (DCPO), with a thermal cycle attaining 150°C, as initiators. Phase separation of an amorphous PE-rich phase took place when carrying out the reaction at temperatures higher than the PE melting temperature. Partial crystallization of PE was observed when cooling to room temperature. Depending on the initial amount of BDDMA, the fraction of PE that was phase separated varied between 57 and 66% of the initial amount, with crystalline fractions in the range of 15 to 42 %. The use of IBoMA as a reactive solvent of PE has two main advantages over other reactive solvents reported in the literature: a) it has a very low vapor pressure, and b) its free-radical polymerization gives a polymer with a relatively high glass transition temperature.

ポリエチレン複合管のクリープ変形解析(OS2-1 複合材料システム,OS2 先端材料システム設計とメゾメカニックス)

Polyethylene pipes have become to be used widely as water or gas pipes since The Great Hanshin-Awaji Earthquake. Recently, it is examined to be used polyethylene pipes as large size pipes whose diameter is about one meter. However, the value of elastic modulus of polyethylene is so small that the thickness of the pipe need to be thick in order to obtain desired stiffness of the pipe. In this result, the problem that the inner diameter of the pipe is very small is occurred. To avoid this problem, polyethylene composite pipes is made up from three layers, the material of inner-outer layer of the pipe is polyethylene and the material of layer between them is polyethylene elongated sheet which has high value of elastic modulus about thirty three times that of polyethylene. In this study, the analysis of creep behavior of this composite pipe which subjected to pressure interior of the pipe will be performed, and the velocity of total strain of the pipe are derived by using constitutive equations of creep under triaxial stress fields which obtained from the theorem of Mises-Mises type. The change in the total strain of the pipe with time is numerical calculated by incremental approach, and the effect of the volume fraction of polyethylene elongated sheet on the total strain of the pipe will be investigated.

Rapid debinding of 316L stainless steel injection moulded component

Wax-based binder system is widely used but they suffer from long debinding time and a tendency to slump or distort during debinding. This has been a major obstacle for the economic process for metal injection moulding (MIM). For improving the debinding process, two-step debinding process has been introduced. Gas-atomised 316L stainless steel powder was injection moulded using two types of multi-component binder system comprising (1) a major fraction of paraffin wax and a minor fraction of polyethylene (PE) and stearic acid (SA) as a lubricant, (2) a major fraction of polyethylene glycol (PEG) and a minor fraction of polymethyl methacrylate (PMMA) binder system. Debinding was carried out in two steps; first, the moulded part is immersed in heptane or distilled water at 60°C to remove the major component of the binder and then heated to remove the remaining binder. The results show that no swelling or distortion was observed on the moulded specimens on both binder systems. Furthermore, the specimens had an adequate strength for handling even after solvent extraction. Large pore were formed from the surface to the interior of the debound part during solvent extraction, allowed easy escape of pyrolysis gases during thermal debinding. Thermal debinding with ramp heating at rates from 3 to 15°C/min was found to be successful.

Reversible melting of polyethylene extended-chain crystals detected by temperature-modulated calorimetry

The melting and crystallization of extended-chain crystals of polyethylene are analyzed with standard differential scanning calorimetry and temperature-modulated differential scanning calorimetry. For short-chain, flexible paraffins and polyethylene fractions up to 10 nm length, fully reversible melting was possible for extended-chain crystals, as is expected for small molecules in the presence of crystal nuclei. Up to 100 nm length, full eutectic separation occurs with decreasingly reversible melting. The higher-molar-mass polymers form solid solution crystals and retain a rapidly decreasing reversible component during their melting that decreases to zero about 1.5 K before the end of melting. An attempt is made to link this reversible melting to the known, detailed morphology and phase diagram of the analyzed sample that was pressure-crystallized to reach chain extension and practically complete crystallization. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2219–2227, 2002

Lamellar and Crystalline Layer Thickness of Single Crystals of Narrow Molecular Weight Fractions of Linear Polyethylene

The thermal dependence of the lamellar thickness (Lp) and the thickness of crystalline layers (Lcryst) of linear polyethylene single crystals has been revisited. LPE fractions with different average molecular weights were crystallized at the same temperatures to avoid the delicate problem of an arbitrary choice of their equilibrium dissolution temperature to calculate their degree of supercooling. This procedure allows direct check of the thicknesses of the crystalline layers dependence on the degree of supercooling. Indeed, the equilibrium melting and dissolution temperatures depend on the average molecular weight. Mats of linear polyethylene single crystals of various narrow molecular weight fractions were obtained from dilute p-xylene solutions. Their lamellar thickness was determined from the SAXS intensity curve. The crystalline layers thickness was calculated from the linear correlation function of the SAXS intensity curves and from the LAM mode frequency of the Raman spectrum. It depends as expected on the crystallization temperature. However, at a given crystallization temperature, both the thicknesses of the lamellae and of the crystalline layers are nearly independent of molecular weight and therefore also of the degree of supercooling estimated from a well-known semiempirical relationship.

Direct Evidence of Regimes I, II, and III in Linear Polyethylene Fractions As Revealed by Spherulite Growth Rates

Isothermal spherulite growth rates were measured over a sufficient range of undercoolings, ΔT, for a narrow linear polyethylene fraction M = 70 300 (70.3K), polydispersity 1.12, such that the fraction exhibited all three growth regimes as crystallized from the subcooled melt. The I−II transition occurred at ΔTI-II = 15.8 °C and the II−III transition at ΔTII-III = 23.8 °C. (Neither transition was fully abrupt.) The nucleation constants Kg and preexponential factors G0 that described the absolute growth rates for each regime were determined, thus quantifying key parameters for all three regimes for a single specimen measured in the same apparatus. The Kg's for 70.3K conformed to the predicted relationship Kg(III) ≅ Kg(I) = 2Kg(II). Theoretical relationships for the preexponential factors were employed using the observed G0's to investigate the nature of the transport of chain segments to the growth front. It was reconfirmed that this process was forced “near-ideal” reptation for an M ≅ 30K fraction. For M = 70.3K, it was found that the reptational transport mechanism in regimes II and III was perturbed and thereby slowed beyond that attributable to “near-ideal” forced reptation; the additional retardation was taken to be the result of labile chain attachments on a surface some distance from the site where the dangling chain was being drawn onto the substrate. In another test, the expression Sk/ao for the stem separation between primary surface nuclei in regime II was employed to calculate ΔTI-II and ΔTII-III. This was successful for both M ≅ 30K (near-ideal reptation) and M = 70.3K (perturbed reptation). In this test, earlier estimates of quantities of importance to nucleation theory, such as C0, nIII, and the substrate length L, were found to be either identical or only slightly modified. The treatment leads to satisfactory numerical estimates of the absolute substrate completion rate g and the nucleation rate i, and is consistent with the crystal morphology present in melt-crystallized PE, including the lenticular crystal → truncated lozenge transformation associated with the I → II regime transition. In general, this work provides significant additional support for the “three regime” concept in narrow PE fractions crystallized from the melt through a consideration of nucleation, regime, and reptation concepts.

Crystallization kinetics and melting behavior of metallocene short‐chain branched polyethylene fractions

AbstractMetallocene polyethylene (mPE) fractions are recognized as being more homogeneous with respect to short‐chain branch (SCB) distribution as compared with unfractionated mPEs. Differential scanning calorimetry and polarized optical microscopy (POM) were used to study the influences of SCB content on the crystallization kinetics, melting behavior, and crystal morphology of four butyl‐branched mPE fractions. The parent mPE of the studied fractions was also investigated for comparative purposes. mPE fractions showed a much simpler crystallization behavior as compared with their parent mPE during the cooling experiments. The Ozawa equation was successfully used to analyze the nonisothermal crystallization kinetics of the fractions. The Ozawa exponent n decreased from about 3.5 to 2 as the temperature declined for each fraction, indicating the crystal‐growth geometry changed from three‐dimensional to two‐dimensional. For isothermal crystallization, the fraction with a lesser SCB content exhibited a higher crystallization temperature (Tc) window. The results from the Avrami equation analysis showed the exponent n values were around 3 (with minor variation), which implied that the crystal‐growth geometry is pseudo‐three‐dimensional. Both of the activation energies for nonisothermal and isothermal crystallization were determined for each fraction with Kissinger and Arrhenius‐type equations, respectively. Double melting peaks were observed for both nonisothermally or isothermally crystallized specimens. The high‐melting peak was confirmed induced via the annealing effect during heating scans. The Hoffman–Weeks plot was inapplicable in obtaining the equilibrium melting temperature (Tm°) for each fraction. The relationship between Tc and Tm for the fractions is approximately Tm = Tc (°C) + 8.3. The POM results indicated that the crystals of parent or fractions formed under cooling conditions did not exhibit the typical spherulitic morphology as a result of the high SCB content. © 2002 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 325–337, 2002

Melting and Crystallization of Polyethylene of Different Molar Mass by Calorimetry

Crystallization and melting of n-paraffins of chain lengths up to C60H122 can be largely reversible, and practically no supercooling is seen in differential scanning calorimetry (DSC) and temperature-modulated DSC (TMDSC). To find the changes with chain length for this effect, polyethylene fractions of oligomers of mass 560, 1150, and 2150 Da and a polymer of mass 15 520 Da were analyzed. The mode of analysis was quasi-isothermal TMDSC with an amplitude of 0.5 K about a fixed series of temperatures. For the oligomer of 560 Da, a mainly reversing melting was seen, similar to the behavior of paraffins of the same molar mass. The oligomer of 2150 Da as well as the polymer shows an irreversible crystallization with a crystallization of more than 5.0 K below the melting temperature, and the oligomer of 1150 Da is intermediate. Typical for polymers, a small amount of reversing material remains in the melting range of the polymer. The 1150 and the 2150 Da oligomers grow to extended-chain crystals, and the polymer yields folded-chain crystals. This research reveals that there is little difference in reversibility between monodisperse paraffins and fractions with similar average mass. There is no major difference in supercooling behavior for extended- and folded-chain crystals. The critical chain length for which a substantial supercooling is observed is 12.5 nm. The three types of reversible latent heat observed in macromolecules are traced to the short-chain molecules.

Determination of molecular dimensions of linear polyethylene by light scattering

The molecular dimensions of linear polyethylene published in the literature were reevaluated in order to discuss the radii of gyration R G and their dependence on molar mass, M. The relationships R G= bM a and the exponents a of this equation in comparison with the unperturbed dimensions of linear polyethylene obtained from viscometric measurements allowed the discussion of the reliability of the data and the reasons for deviations. The exponent a determined by light scattering for linear polyethylene in good solvent, tetraline, showed molecular dissolution, whereas in 1-chloronaphthalene and trichlorobenzene, the exponent a is lower than expected for good solvents. Microgels and aggregates of molecules affect the exponents. Radii of gyration measured directly by light scattering at fractions of linear polyethylene in the theta solvent, diphenylmethane, indicate incomplete molecular dissolution and the presence of branched structures.

Preferential orientation of crystallites spatially confined in lamellar microdomains of polyethylene-block-[atactic poly(propylene)

Synchrotron small angle X-ray scattering (SAXS), wide angle X-ray scattering (WAXS), and transmission electron microscopy were carried out for an oriented polyethylene-block-[atactic poly(propylene)] with a molecular weight of 1.13×105 and a volume fraction of polyethylene of 0.5. Isothermal crystallization at 93°C did not destroy the pre-formed microdomain, however, with a higher crystallization temperature, the microdomain was more heavily deformed and more crystalline lamella grew. In WAXS profiles, preferential orientation of (020) reflection peak was observed, indicating that the crystalline lamella grew in parallel with the micro domain interface.

The Conformational Change with Temperature of End Group Sequences of Low Molecular Weight Polyethylene

A detailed analysis of the temperature dependent solid state carbon-13 spectrum of a low molecular weight polyethylene fraction, that was isothermally crystallized in extended chain forms is reported. The observed resonances were assigned to specific carbons and the 13C spin-lattice relaxation times, T1C, for the internal and α-methylene carbons as well as the end-group methyl carbons were determined. Based on the analysis, and an accompanying thermogram, obtained by differential scanning calorimetry, it was concluded that conformational disordering of the chain-end sequence takes place at temperatures well below the fusion range. This conclusion is in accord with the standard model that has been taken for an equilibrium crystallite.

N-Alkane Homogeneous Nucleation: Crossover to Polymer Behavior

Homogeneous nucleation of crystallizing n-alkanes (CH3−(CH2)n-2−CH3, 17 ≤ n ≤ 60) and some low-MW polyethylene (PE) fractions were studied using calorimetry and compared to reanalyzed PE data of Ross and Frolen. The undercooling and derived surface energy of the n-alkanes starts increasing for n as low as 25, far below the low-n limit for chain-folding. This behavior appears to extrapolate to the high undercooling exhibited by high-MW PE. The behavior is discussed in terms of a possible crossover between full-molecule nuclei of low-n and “bundle” nuclei of larger n. It is also related to the negentropic model, in that the surface energy increases when the “cilia” dangling from the bundle nucleus exceed a length where their entropic cost begins to significantly raise the surface energy. At low n, homogeneous nucleation occurs into the metastable rotator phase whose potential importance at high n is discussed.

A comparison of degree of properties enhancement produced by thermal annealing between polyethylene and calcium carbonate–polyethylene composites

The annealing of unfilled and filled composites was carried out to assess the effect on property changes in both types of materials. By employing DSC and tensile testing, it was found that increasing annealed temperature increases melting temperature, crystallinity, modulus and tensile strength, while yield strain remained constant throughout the test. The increase in thermal and mechanical properties was associated with the change in morphology and thickness of lamellae of polyethylene fraction in the materials. In general, the degree of change in properties was independent of filler content. However, the increase in tensile strength was found to depend upon filler incorporation. All compositions of filled polyethylene exhibited a similar degree of strength increase, but less than the increase seen in unfilled polyethylene. This is due to the premature failure of composites due to interfacial debonding in filled polyethylene.

Molecular weight dependence of melt crystallization behavior and crystal morphology of low molecular weight linear polyethylene fractions

Melt crystallization behavior and corresponding crystal morphology of five low molecular weight (3,900 ≤ MW ≤ 20,800) linear polyethylene (PE) fractions have been investigated. The overall crystallization data indicate that the lower molecular weight (MW) fraction possesses a higher crystallization rate at the same undercooling (ΔT). On the contrary, at the same crystallization temperature (Tc) the rate increases with MW. The Avrami exponent (n) varies from ca. 3 to 4 with decreasing ΔT for the fractions studied, which implies the nucleation process changes from athermal type to thermal type as Tc increases. For the low MW PE’s, the different crystal growth regimes (regime I and II) have been first time identified via linear crystal growth rate (G) measurements. The regime I/II transition temperatures are close to previously reported data, which were obtained through a different method. As reported for intermediate MW PE’s, the transitions occur at an almost constant ΔT of 17.5±1 °C for each fraction studied. Morphological study shows that single crystals could be formed isothermally at low ΔT’s. Typical banded spherulites and axialites, which are MW and ΔT dependent, are also observed. Orthorhombic structure is ascertained to be the dominant crystal structure that exists irrespective of MW and crystal growth regime.

Molecular structure of polyethylene produced with supported vanadium-magnesium catalyst

The molecular structure of polyethylene (PE) produced with supported titanium-magnesium (TMC) and vanadium-magnesium (VMC) catalysts has been studied by means of IR and 13C NMR spectroscopic methods. It has been shown that PE produced with VMC in the presence of hydrogen differs by a lower content of double bonds and higher branching compared with PE obtained with TMC. Furthermore, branching is observed both in the heptane-soluble low molecular weight fraction and in the high molecular weight fraction of PE. 13C NMR studies of the low molecular weight fraction of PE showed the presence of methyl branches. Intramolecular isomerization of the growing polymer chains involving hydrogen seems to be a possible reason for the formation of methyl branches.

The crystallization of blends of different types of polyethylene: The role of crystallization conditions

Cocrystallization in blends of linear and branched polyethylenes has been studied under both isothermal and slow-cooling crystallization conditions. Before the more common, polydisperse-type polyethylenes were studied and the results analysed, model systems were investigated in detail. The components used in the model binary blends were molecular weight fractions of linear polyethylene, hydrogenated poly(butadiene) as a model for the ethylene-1-alkene copolymers, and a three-arm star hydrogenated poly(butadiene) as a model for the long chain branched polyethylenes. It was found that a key factor in governing the extent of cocrystallization in these blends is the closeness of crystallization rates of each of the components. The extent of the cocrystallization thus diminishes with increasing concentration of the linear component in the blend. It is found that copolymer composition and molecular structure also have a strong influence on cocrystallization. The amount of cocrystallization is favoured at the lowest isothermal crystallization temperatures and is maximized under quenching conditions. The general features that influence cocrystallization, which have evolved from this study, are discussed.