Polyethylene Fractions Research Articles

On the interfacial energy of extended-chain crystals from polyethylene melt by revised Flory equations of fusion

To analyze extended-chain crystalline systems composed of linear polyethylene, Flory's conventional theory of fusion is reconsidered by introducing a new concept of crystallinity. When this new treatment is applied to a melting case of a low molecular weight polyethylene fraction (Mn = 5600) isothermally bulk crystallized, a certain result that very large lamellar thickness was caused by a very small increase in crystallization temperature can satisfactorily be explained by a significant change in interfacial free energy of the crystallite end. Further, it shows 14–17 kJ/mol as a nonequilibrium value range of interfacial free energy for highly crystalline polyethylene fractions of low molecular weight Mn ≦ 5600 by using the previous data presented by other workers. A similar result is also obtained on the Mn = 5600 fraction by analyzing from a standpoint of equilibrium crystallinity. In either case, the estimated range of interfacial free energy is consistent with the conventional range. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1293–1303, 1998

Morphology of polyethylene blended with thermotropic hydroxyethyl cellulose acetate

Morphology of low-density polyethylene (PE)/thermotropic hydroxyethyl cellulose acetate (HECA) blends, and the melting and crystallization behavior of PE in the blends were studied. The “sea island” morphology was observed in PE/HECA blends. The process of melting and crystallization of PE in blends was independent of the HECA fraction when the PE fraction was larger than 50 wt %. When the PE fraction is smaller than 20 wt %, however, multiple crystallization was observed in the low temperature region. HECA was incompatible with PE crystals in the blends, but partially compatible with the amorphous part of PE. HECA could exist between the PE lamellae in PE spherulites, and concentric ring morphology was observed in spherulites. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:1285–1291, 1998

Polysiloxanes: paradigm or paradox

Spherulitic growth rates and physical properties of polysiloxanes are well correlated for a wide range of molecular weights. Below the zero shear entanglement molecular weight, Mc, chain folding is probably the norm and fractions are brittle, but after Mc is traversed there is a significant decrease in crystallinity, increase in interfacial surface energy and change in lamellar morphology as polymer fractions go from brittle to tough. The chain folded crystallization model with reptation, as a chain folding facilitator, fails to account for this behavior. Appropriate property data for polyethylene and polyisoprene fractions also supports this thesis which now appears to be more of a paradigm than a paradox.

Kinetics of chlorosulfonation and OsO4 attack in the interlamellar regions of low and high density polyethylene studied by microhardness

The microhardness of low density polyethylene (LDPE) fractions and of high density polyethylene (HDPE) samples has been studied as a function of molecular weight in the range of about 2 × 104 up to 4 × 106. Details of the lamellar structure were determined by transmission electron microscopy (TEM). The observed decrease of hardness with increasing molecular weight is mainly due to the increase in thickness of the interlamellar layers (i.e. a decrease of crystallinity). After chemical treatment with chlorosulfonic acid and with OsO4 the samples show a drastic hardness increase. The kinetics of OsO4 treatment as revealed by microhardness has been examined. The hardness increase is explained in terms of the large reduction in molecular mobility of the amorphous, interlamellar layers. The strongest hardening effects arise in consequence of the preferred reaction of these defect layers with increasing molecular weight.

Melt crystalfization and crystal morphology of two low molecular weight linear polyethylene fractions

Melt-crystallization behavior and single-crystal morphology of two low molecular weight (LMW) linear polyethylene (PE) fractions of 3900 and 5800 have been investigated. Linear growth rates along the b axis (G b) of these fractions were measured via polarized light microscopy (PLM). The two fractions show a growth rate change at an undercooling of 17°C (at 117°C and 120°C, respectively, for these two fractions), which may be identified as the regime I/II transition. This transition does not correspond to a single-crystal morphological change from a truncated lozenge with curved (200) and (110) planes to a lenticular crystal as proposed previously. However, this morphological change can be observed at a temperature higher than the regime transition (at 122°C and 124°C), at which the cusps of the G b data can be observed for these two fractions. Based on our morphological study via PLM, transmission electron microscopy, electron diffraction, small-angle x-ray scattering (SAXS), and differential scanning calorimetry (DSC) experiments, it is found that within a 2°C temperature region, the G b change is accompanied by a sharp long period increase and a drastic change in single-crystal morphology from a truncated lozenge with curved (200) and (110) planes to a lenticular-shape crystal. The morphological change may result from a sudden increase in the G b coupled with a smaller change in the growth rate along the a axis with undercooling. This implies that, within this temperature region (2°C), the crystals may undergo substantial changes in the geometry of the (110) and (200) crystal growth fronts and chain folding behavior.

Thermolysis of polyethylene/polystyrene mixtures

The thermolysis of 60/40 mixtures of polyethylene (PE) and polystyrene (PS) was investigated at temperatures below 440°C. Liquid yield from the mixture, 84.1%, was comparable to yields obtained with the individual polymers. The yields of styrene monomer, 57.1%, and α-olefins, 27.7%, increased over those obtained when the polymers were processed individually. A significant interaction was observed between the polymers in which the addition of PS enhanced the rate of thermolysis of PE. It is proposed that this enhancement was due to the abstraction of hydrogen from the PE fraction by polystyryl radicals. The result of this effect is to increase the rate of volatile production from the PE and increase the solubility of the residue in chloroform. The data also support a mechanism for dimer production other than the generally accepted 1,3 transfer. © 1996 John Wiley & Sons, Inc.

Pressure Change of the Phase Diagram of the Binary Mixture of Low Molecular Weight Polyethylenes

Effects of pressure on the phase behaviors of the three polyethylene(PE)–polyethylene(PE) binary mixtures with different molecular weights (MW=10000, 2000, and 1000) were studied. High pressure differential thermal analysis (DTA) was performed at pressures up to 500 MPa. For the three binary mixtures, the melting temperature Tm of the higher molecular weight PE decreased with decreasing the weight fraction of the high molecular weight PE, while Tm of the PES with lower MW of 1000 and 2000 did not change with the weight fraction. At elevated pressure, the phase diagram resulted in an increase of the difference between Tm of the higher molecular weight PE and that of the lower molecular weight PE. The phase diagram of melting of PE 2000 and PE 1000 binary mixture was eutectic type at 0.1 MPa. The eutectic point did not change with pressure. Effects of mixing PE 1000 and PE 2000 on the appearance of the high pressure phase and the formation of the extended chain crystal of PE 10000 was studied as a function of the weight fraction. The molecular mechanism of the melting and crystallization process of PE in the binary mixture under high pressure was considered.

Electrostatic separation of polymer powders

Abstract Polyethylene and polyvinylchloride are the most abundant plastic materials in municipal waste. However, no technology is commercially available so far for an efficient recovery of these materials. We studied the separation of polyethylene (PE) from polyvinylchloride (PVC) powder particles in an electric field separator after tribocharging. Tribocharging of the particles took place in a copper-lined cyclone separator. The influence of electric field strength, relative humidity of the air and particle velocity on the separation efficiency was investigated. The average recovered content of the PE fractions was > 90% at a mass yield of > 60%. The average recovered content of PVC was > 40% with an average mass yield of > 30%.

Study of the Phase Structure of Linear Polyethylene by Means of Small-Angle X-ray Scattering and Raman Spectroscopy

A study of the phase structure of two linear polyethylene fractions has been carried out by Raman spectroscopy and from the analysis of the interface distribution function from the SAXS intensity data. Very good agreement is obtained for the core crystallite thickness, the liquid-like region, and the thickness of the interface from these two independent techniques. Moreover, the X-ray analysis distinguishes isothermally formed crystals from quenched crystals in cases where the Raman technique is not selective.

Wide-angle X-ray scattering studies on polyethylene at low temperatures

Linear polyethylene (Lupolen 6041 D) and a polyethylene fraction (PE 2000) were investigated by WAXS at temperatures 8 K<T<293 K. From the position and integral widths of the reflections the lattice constants (and expansion coefficients) and lattice distortions are determined. Lattice distortions are of the strain type, induced by inherent stresses. They increase with decreasing temperature up to ∼50 K and then to lower temperatures a decrease is observed. This is explained by the decrease of the thermal expansion coefficients. For PE 2000 for some reflections a minimum in the integral widths at T∼38 K occurs. The explanation of this effect is given by the occurence of a stress induced martensitic transformation (orthorhombic to monoclinic) of part of the material. The corresponding weak reflections are observed.

The attempted copolymerization of styrene and ethylene with monocyclopentadienyltitanium trichloride/methylaluminoxane catalyst. Effect of polymerization conditions

The copolymerization of styrene and ethylene was attempted in the presence of the catalytic system CpTiCl 3-methylaluminoxane. Polymerizations were carried out at different polymerization temperatures and Al Ti mole ratios. The structure of the polymer was investigated with FTIR, DSC, GPC, and 13C-NMR techniques. The polymerization product was composed of polyethylene and syndiotactic polystyrene. No ethylene-styrene copolymer was detected. The composition of the product was dependent on the polymerization conditions. The catalyst activity was highest at polymerization temperature 30 °C and Al Ti ratio 1500. Styrene concentration of the product was highest at polymerization temperature 50 °C and Al Ti ratio 1500. The melting temperature of the polyethylene fraction was dependent on the styrene content of the product, suggesting some interaction between the monomers in the polymerization reaction. The formation of separate polymer fractions may indicate a different polymerization mechanism for the two monomers, or the presence of more than one active species in the catalyst, one producing polyethylene and the other syndiotactic polystyrene.

Absence of phase separation effects in blends of linear polyethylene fractions of differing molecular weight

Blends of linear polyethylenes of differing molecular weights ( MWs) have been studied to try to determine whether there is any observable phase separation in the melt. Using four linear polyethylene fractions, of MWs between 2 × 10 6 and 2.5 × 10 3, blends were prepared and four blend systems investigated, the two high MW polymers being blended with each of the two lower MW materials in turn. The blends were studied by transmission electron microscopy and differential scanning calorimetry, techniques previously used to study blends of linear with branched polyethylenes. In these systems, for some compositions of blends of linear with branched polyethylene, quenched from some temperatures, phase separation on a scale of micrometres has been recorded. In contrast, no indications of phase separation were observed on examination of the blends of linear with linear polyethylenes. From this it follows that if there is any liquid-liquid phase separation in linear with linear polyethylene blends, taking place as a result of molecular weight differences, it cannot be detected by the methods used to detect liquid—liquid phase separation in linear with branched polyethylene blends; and, conversely, the phase separation observed in linear with branched polyethylene blend systems does not take place as a result of molecular weight difference alone.

New approach to the mechanism of oligomer crystallization

A new mechanism for crystal growth of oligomers has been proposed. In order to grow, the crystals must have well ordered facets. This has been taken into account by assuming that their thickness is larger than C/(T*–Tc), where C is an appropriate constant, T* a reference temperature and Tc the temperature of crystallization. Application of this relation to crystals which have thermodynamic size is justified. These crystals grow by crystallographic attachment of sufficiently long molecules in a process exactly comparable to the growth of a crystal of an alk-1-ene (known from the measurement of the absolute entropy of the crystals). From these premises, the growth rate of poly(ethylene oxide) lamellae is predicted with surprising success. The segregation of the molecules according to their molecular weight and the very sensitive effect of seeding is also predicted. Application of the model to the crystallization of polyethylene fractions is discussed.

Water treeing in binary linear polyethylene blends: the mechanical aspect

Water treeing tests were performed on low density polyethylene (LDPE) and four different binary blends of sharp linear polyethylene (LPE) fractions (M/sub w/=2500 and 76000), which were either quenched in air from the melt or isothermally crystallised at 123/spl deg/C. Although the morphology and initial mechanical properties of the materials tested were significantly different, the vented tree growth characteristics were similar for all of them. This is in disagreement with the electromechanical models of water treeing, which correlate water tree growth with the fracture toughness of the material. Time to breakdown distributions were also similar for both LDPE and the binary LPE blends, which indicates that, regardless of the initial material morphology and the actual structure of water trees, the length of water trees is one of the controlling factors in insulation failure. The visible light image of water trees in LPE blends did not disappear upon drying as it usually does in LDPE and crosslinked polyethylene insulation. >

Molecular, thermal, and morphological characterization of narrowly branched fractions of 1-octene linear low-density polyethylene. 3. Lamellar and spherulitic morphology

The semicrystalline lamellar morphology, the crystalline phase, and the spherulitic structure of 1-octene linear low-density polyethylene (LLDPE) fractions with a narrow short chain branching distribution were studied. The average short chain branching content of the fractions studied increases from 2.9 to 28.2 branches per 1000 carbon atoms while the molecular weight M w concomitantly decreases from 2.7×10 5 to 1.9×10 4 . Characteristic morphological parameters include the number-average lamellar thickness, the thickness of the crystalline core and of the transition layers, the periodicity of the lamellar stacks, the unit cell dimensions of the crystalline phase, and the average radius of the spherulites

Thermoreversible gelation of low-molecular-weight linear polyethylene fractions and normal hydrocarbons

The thermodynamic and morphological features of thermoreversible gels formed by low molecular weight fractions of linear polyethylene and by n-bydrocarbons formed as a result of crystallization from dilute and moderately dilute solutions have been investigated. The critical concentration for gel formation, c * , of the polyethylenes follows an unusual pattern with molecular weight. A discontinuity is observed at about M=4×10 3 followed by an initial decrease in c * as the chain length is decreased. With a further decrease in molecular weight, c * becomes invariant. These unusual results can be attributed to the formation of extended chain crystallites.

Molecular, thermal and morphological characterization of narrowly branched fractions of 1-octene linear low-density polyethylene: 1. Molecular and thermal characterization

A 1-octene linear low-density polyethylene (LLDPE) with a bimodal short-chain branching (SCB) distribution has been fractionated with respect to the SCB content using preparative temperature-rising elution fractionation (PTREF). The average SCB content of the fractions obtained ranges from 2.9 to 28.2 branches per 1000 C atoms. Their weight-average molecular weight decreases with increasing degree of branching, while the polydispersity remains rather broad. The fractions exhibit a broad single melting endotherm in contrast to the multiple melting endotherms of the unfractionated copolymer. Dynamic crystallization of the fractions from the melt results in two separate exotherms.

Investigations of the crystallization of polyethylene by means of simultaneous small‐angle and wide‐angle X‐ray scattering

AbstractThe isothermal crystallization of linear polyethylene fractions was studied by means of simultaneous measurements of wide‐angle X‐ray scattering (WAXS) and small‐angle X‐ray scattering (SAXS) employing synchrotron radiation. From WAXS the overall degree of crystallinity xc was determined. From SAXS, the scattering power Q was evaluated and compared with several different models for the crystallizing system. When the scattering arises solely from the supermolecular structures, such as spherulites, Q is proportional to xs·xcL·(1 − xcL), where xs is the fraction of material transformed into such structures and xcL is the degree of crystallinity within these structures. By comparing xc with Q it was possible to separate the primary and secondary crystallization and to show that secondary crystallization takes place within the lamellar stacks where the crystals become thicker and the amorphous layers become thinner. The process of recrystallization during annealing of crystalline samples was studied in the same way. Other models were treated in a similar manner.

Growth of polyethylene single crystals from the melt: change in lateral habit and regime I?II transition

Using the technique of extraction, single crystals have been obtained from polyethylene fractions isothermally crystallized from the melt at atmospheric pressure. It has been found that the lateral habit of single crystals changes in the vicinity of the transition temperature of growth regime (regime I–II): lenticular shape elongated in the direction of theb axis (type A) in the range of regime I and truncated lozenge with curved edges of {200} and {110} growth faces (type B) in that of regime II. The transition of lateral habit causes a drastic change in the width of {110} growth faces; {110} growth faces are well developed in type B crystals while they cannot be observed and must be very small in type-A crystals. It has been shown that the growth regime of the small {110} growth face of type-A crystals must be in regime I; hence the regime I–II transition can be explained as the result of this change in lateral habit (width of the {110} growth face).

Consolidation and compaction characteristics of a three-component particulate system

The consolidation and compaction characteristics of a three-component particulate system consisting of dibasic calcium phosphate dihydrate (DCP), microcrystalline cellulose (MCC) and polyethylene (PE) has been studied. The effects of compression speed on mean yield pressure, extent of particle rearrangement, crushing strength, elastic recovery and energy analysis were measured. It was found that the incorporation of up to 1% w/w of PE in a mixture composed of 25% DCP and 75% MCC did not significantly compromise tablet properties and it was possible to obtain intact compacts at a relatively low compaction force of 5.0 RN. This suggested that the 25% DCP/75% MCC mixture can suitably accommodate the incorporation of a very viscoelastic material without significantly affecting bond strength. The mean yield pressure of DCP-MCC-PE compacts surprisingly decreased with a progressive increase in concentration of PE. It is suggested that relatively greater fragmentation of DCP and more efficient filling of the pores occurs as the elastic PE absorbs more of the applied force. The crushing strength of the tablets decreased with increasing speed of compression, due probably to the elastic nature and extensive energy-absorption properties of PE reducing the overall net compaction energy available for particle-particle bonding. It was found that as the mass fraction of PE was increased, this produced a corresponding increase in the elastic recovery of the tablets, due again to the high elasticity of PE. The extent of particle rearrangement (Db) decreased with increasing concentration of PE; suggesting that adhesion between the particles correspondingly increased, most likely due to complex interactions of the three components.