Crystalline Polyethylene Research Articles

Study of C3H5+ ion deposition on polystyrene and polyethylene surfaces using molecular dynamics simulations

Molecular dynamics simulations of ion deposition processes are used to study the deposition of C3H5+ ions on crystalline polystyrene (PS) and polyethylene (PE) surfaces at energies of 50 and 25 eV. For each system, 80 trajectories are carried out on pristine surfaces and the incident angle in every case is normal to the surface. The forces are determined using the reactive empirical bond order method developed by Tersoff and parametrized for hydrocarbons by Brenner, coupled to long-range Lennard–Jones potentials. The simulations predict that the ions deposited at 50 eV either dissociate and stick to the surface or remain on the surface intact in 98% of the trajectories on PS, and in 89% of the trajectories on PE. At 25 eV, the ions are deposited intact in 70% of the trajectories on PS and dissociate in only 3%. No dissociation of the incident ions is predicted to occur on PE at 25 eV. Rather, the ions scatter away in 90% of the trajectories. Consequently, ion deposition on PE at 25 eV is predicted to be very inefficient for thin-film growth. Many more ions or major ion fragments (such as C2Hn and CH2) remain near the surface on PS than PE at 50 eV. Thus, in general, polyatomic ion deposition for thin film growth is more efficient on PS than PE, and deposition at 50 eV is more efficient than deposition at 25 eV.

Morphology, Mechanical Properties and Stress-relaxation of Biaxially Oriented Polyethylene Films Irradiated with β-radiation

The present study describes an influence of β-radiation on morphology, mechanical properties and stress-relaxation of biaxially oriented polyethylene films. The mechanical properties and stress-relaxation of irradiated crosslinked films were studied as a function of the irradiation dose applied. The relaxation time spectra of crosslinked films were observed to display additional relaxation times. Irradiation dose of 4 megarads (MR) was found to be optimal from the viewpoint of mechanical properties. The crystallization of crosslinked and noncrosslinked polyethylene was compared. It was shown that irradiation affects the final crystallinity of polyethylene. Lower crystallinity was observed due to lower mobility of crosslinked molecules.

Self-trapping vs. non-trapping of electrons and holes in organic insulators: polyethylene

We show, by electronic structure based molecular dynamics simulations, that an extra electron injected in crystalline polyethylene should fall spontaneously into a self-trapped state, a shallow donor with a large novel distortion pattern involving a pair of trans-gauche defects. Parallel calculations show instead that a hole will remain free and delocalized. We trace the difference of behavior to the intrachain nature of the hole, as opposed to the interchain one of the electron, and argue that applicability of this concept could be more general. Thus electrons (but not holes) should tend to self-trap in saturated organic insulators, but not for example in aromatic insulators, where both carriers are intrachain.

Determination of the crystallinity of polyethylene/α‐olefin copolymers by thermal analysis: Relationship of the heat of fusion of 100% polyethylene crystal and the density

AbstractThe heat of fusion measured with differential scanning calorimetry (DSC) is typically divided by a constant value of the heat of fusion of 100% polyethylene (PE) crystal (ΔH) for the estimation of the fraction crystallinity of PE copolymers, regardless of the density [i.e., the short‐chain branching (SCB) concentration]. In this work, values of ΔH of about 288 J/g were determined with a combined DSC and X‐ray diffraction (XRD) method for a series of PE copolymers containing SCB from 0 to 50 Br/1000 C (density = 0.965–0.865 g/cc). There was no systematic change in ΔH observed across this density range. This result supports the suitability of determining the fraction crystallinity of PE of any density by the simple division of the observed heat of fusion determined by DSC by a constant value of ΔH. This DSC method yielded values of PE crystallinity in good agreement with corresponding values determined by XRD for a series of PE copolymers. The determination of ΔH involved a small precision error for higher density (lower SCB) PEs, but the precision error increased for lower density (i.e., higher SCB) PEs. This was due to the difficulty in measuring the heat of fusion for lower density PEs, which exhibited low values of the heat of fusion and melted only slightly above room temperature, and due to the difficulty of measuring lower values of crystallinity by XRD. The crystal thickness measured by small‐angle X‐ray scattering for this series of PE copolymers decreased exponentially from about 280 to 6 Å. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1637–1643, 2002

Synthesis of model polycyclohexylene/polyethylene miktoarm star copolymers with three and four arms

AbstractThe synthesis of well‐defined 3‐ and 4‐miktoarm star copolymers of the A2B and A3B types is described, where A is 1,4‐polybutadiene and B is poly(1,3‐cyclohexadiene). The synthetic approach involves the reaction of poly(1,3‐cyclohexadienyl)lithium with an excess of methyltrichlorosilane or tetrachlorosilane followed, after the removal of excess silane, by a small excess of polybutadienyllithium. Characterization was carried out by size exclusion chromatography, low‐angle laser light scattering, laser differential refractometry, and NMR spectroscopy. The complete heterogeneous catalytic hydrogenation of the A2B and A3B miktoarm stars, with a calcium carbonate‐supported palladium catalyst, leads to the formation of A2B and A3B miktoarm stars with one amorphous polycyclohexylene arm with an extremely high glass‐transition temperature and two or three crystalline polyethylene arms. Differential scanning calorimetry was used to determine the glass‐transition temperature of the amorphous blocks of the starting and hydrogenated stars and the melting temperature of polyethylene. Solid‐state 13C NMR spectroscopy was performed to ensure the complete saturation of the polycyclohexadiene and polybutadiene arms. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2575–2582, 2002

Ionic Conductivity in Physical Networks of Polyethylene−Polyether−Polyethylene Triblock Copolymers

A self-assembling ABA triblock copolymer complexed with lithium bis(trifluorometliylsulfonyl)imide (LiTFSI) salt showed ion conductivities (sigma) approaching 10(-5) S/cm at 20 degreesC. The block copolymer had endblocks of polyethylene (PE) and midblocks of poly(ethylene oxide-co-propylene oxide) (PEOPO) and formed physical networks with PEOPO chains interconnected by crystalline PE domains. Thermal analysis showed a PEOPO glass transition at -68 degreesC and a PE melting point at 100 degreesC. When the melting region of the PE phase was reached, sigma of the electrolytes was found to increase sharply to attain the same level as the corresponding electrolyte based on the neat PEOPO precursor block. Furthermore, the level of sigma of the block copolymer electrolytes was found to be strongly dependent on the thermal history of the samples, and they showed a hysteresis behavior where sigma increased by a factor of approximate to3 after annealing above 110 degreesC. This may be explained by a reduction of the chain constraints of the PEOPO blocks in the physical network after annealing. The electrolytes behaved essentially like thermoplastics and were soft, tacky, self-supporting materials at room temperature. (Less)

Atomistic Simulation of the αc-Relaxation in Crystalline Polyethylene

The existence and mobility of conformational defects that could account for the αc-relaxation in crystalline polyethylene are studied by atomistic simulation. Candidate defects are identified by an exhaustive search of conformation space. Their mobility for translation along the chain in the crystal is estimated using the Transition State Theory. Among point defects, we find a large number of candidates for the αc-relaxation. This solution space is simplified by grouping solutions into families that yield a common minimum energy defect conformation. Dynamic pathways for defect propagation from one unit cell to the next vary substantially in energy barrier, ranging from 4 to 15 kcal/mol, in accord with experimental values of 5−22 kcal/mol. The highest mobilities (lowest energy barriers) were obtained for pathways that visit different defect conformations as they pass from one unit cell to the next. The most facile dynamic pathway for the αc-relaxation was found to involve the cooperative rotation of two to...

The measurement of the crystallinity of polymers by DSC

The procedures adopted and the inherent assumptions made in the measurement of crystallinity of polymers by differential scanning calorimetry (DSC) are reviewed. The inherent problem in all DSC measurements is concurrent recrystallisation and melting of the polymer sample on heating to the melting point and the variation of the enthalpies of crystallisation and melting, heat capacities and degree of crystallinity with temperature. A First Law procedure is suggested which involves heating the sample between two set temperatures, T 1 and T 2. T 1 is selected by the requirement that the degree of crystallinity of the sample should not change either with temperature or time, and be representative of the sample during its use. T 1 is taken to be ambient or just above the glass transition temperature. T 2 is taken to be just above the observed last trace of crystallinity. Integrating the observed specific heat difference between the sample and the completely amorphous material during these two temperature ranges determined the residual enthalpy of fusion at T 1. Problems are noted in the use of this procedure in that the specific heat of the liquid should not be arbitrarily chosen since this leads to systematic errors in the heat of crystallisation. The degrees of crystallinity of metallocene polyethylene (m-PE) and polyethylene terephthalate (PET) measured by this procedure have been compared with values measured by density, determined at room temperature.

Diffusion of topological solitons and dielectric αc relaxation in a polymeric crystal

A molecular dynamics simulation was performed to estimate the effective mass and the diffusion and friction coefficients of point defects in macromolecular chains of crystalline polyethylene. The results were compared with theoretical mass and kinetic coefficient predictions for topological solitons, with which these defects were identified. The results are used to discuss the soliton model of dielectric αc relaxation in weakly oxidized polyethylene.

Synthesis and ethylene polymerization catalysis of mono(cyclopentadienyl)lanthanide compounds with the pyrazinamide ligand

In this work the synthesis of mono(cyclopentadienyl)lanthanide compounds containing methanesulfonate anion and pyrazinamide ligand is presented. The compounds were characterized by elemental analyses, complexometric titration with EDTA, thermal analyses, and vibrational spectra in the infrared region. In a preliminary catalytic study these compounds were active in ethylene polymerization when MAO was used as cocatalyst producing low crystalline polyethylene.

Molecular Simulation of the Influence of Chemical Cross-Links on the Shear Strength of Carbon Nanotube−Polymer Interfaces

The influence of chemical cross-links between a single-walled fullerene nanotube and a polymer matrix on the matrix−nanotube shear strength has been studied using molecular dynamics simulations. A (10,10) nanotube embedded in either a crystalline or amorphous polyethylene matrix is used as a model for a nonbonded interface (in the absence of cross-links). The simulations predict that shear strengths and critical lengths required for load transfer can be enhanced and decreased, respectively, by over an order of magnitude with the formation of cross-links involving less than 1% of the nanotube carbon atoms. At this level of chemical functionalization, calculations also predict that there is a negligible change in tensile modulus for a (10,10) nanotube.

Preparation and evaluation of press-coated aminophylline tablet using crystalline cellulose and polyethylene glycol in the outer shell for timed-release dosage forms

In an attempt to achieve chronopharmacotherapy for asthma, press-coated tablets (250 mg), which contained aminophylline in the core tablet in the form of low-substituted hydroxypropylcellulose (L-HPC) and coated with crystalline cellulose (PH-102) and polyethylene glycol (PEG) at various molecular weights and mixing ratios in the amounts of PH-102 and PEG as the outer shell (press-coating material), were prepared (chronopharmaceutics). Their applicability as timed-release (delayed-release) tablets with a lag time of disintegration and a subsequent rapid drug release phase was investigated. Various types of press-coated tablets were prepared using a tableting machine, and their aminophylline dissolution profiles were evaluated by the JP paddle method. Tablets with the timed-release characteristics could be prepared, and the lag time of disintegration was prolonged as the molecular weight and the amount of PEG, for example PEG 500,000, in the outer shell were increased. The lag time of disintegration could be controlled by the above-mentioned method, however, the pH of the medium had no effect on disintegration of the tablet and dissolution behavior of theophylline. The press-coated tablet (core tablet:aminophylline 50 mg, L-HPC and PEG 6000; outer shell:PH-102:PEG = 8:2 200 mg) with the timed-release characteristics was administered orally to rabbits for an in vivo test. Theophylline was first detected in plasma more than 2 h after administration; thus, this tablet showed a timed-release characteristics in the gastrointestinal tract. The time (tmax) required to reach the maximum plasma theophylline concentration (Cmax) observed after administration of the press-coated tablet was significantly (p < 0.05) delayed compared with that observed after administration of aminophylline solution in the control experiment. However, there was no difference in Cmax and area under the plasma theophylline concentration-time curve (AUC0-->24) between the press-coated tablet and aminophylline solution. These results suggest that the press-coated aminophylline tablet (with the timed-release characteristic) offers a promising forms of theophylline chronotherapy for asthma.

Modeling of Screw Dislocation Dynamics in Crystalline Polyethylene

Recently [1], we studied the plastic deformation of crystalline polyethylene (PE) by molecular-dynamic modeling and showed that the major mechanism of initiation of this process in the presence of only edge dislocations is dissociation of dislocation dipoles formed by dislocations whose Burgers vectors are parallel to the axes of polymer chains. The contribution of screw dislocations to plastic deformation of PE has not been studied. As follows from experimental data [2], this contribution is significant. Indeed, the screw dislocations parallel to polymer chains in crystalline PE have a minimum line energy density; thus, they should dominate plastic deformation. The mechanisms of screw dislocation mobility in PE are still poorly understood. The Peierls stress for these dislocations was found by Peterson [3]; however, the moduli of elasticity used in [3] differed considerably from later values [4], which has cast some doubt on the validity of these findings. At least, we can state with assurance that the Peierls barrier for the [001] screw dislocations (we use the coordinate system with the z axis aligned with the polymer chains) is several orders of magnitude higher than the corresponding barrier for edge dislocations, because the motion of the [001] screw dislocations results in the displacement of polymer chains relative to one another rather than in their deformation. Hence, this process is dominated by the interchain interaction. The Young modulus in the [001] direction thereby has no effect on the mobility of the [001] screw dislocations, and, hence, the small parameter equal to the ratio of the shear modulus to the Young modulus is absent in the expressions describing the screw dislocation dynamics. As was shown in [5], the presence of this small parameter is responsible for the small height of the Peierls barrier for edge dislocations in PE. Therefore, plastic deformation associated with the motion of screw dislocations in polymer crystals differs considerably from plasticity caused by edge dislocation dynamics, because, for edge dislocations with the Burgers vector parallel to the axes of the polymer chains, the Peierls barrier is nine orders of magnitude lower than the shear modulus and can be neglected. For the [001] screw dislocations, competition between two mechanisms of initiation of plastic flow might be expected: overcoming of the Peierls barrier and dissociation of dislocation dipoles (i.e., dissociation of pairs of dislocations whose Burgers vectors are opposite in sign). Herein, we elucidated the role of these processes in the initiation of plastic deformation of crystalline PE and determined the yield stress of crystalline PE containing rectilinear [001] screw dislocations.

Kinetics of the Solid-State Chlorination of High-Density Polyethylene

The kinetics of the solid-state chlorination of polyethylene within the temperature range 293 to 353 K was studied using the generalized Avrami-Erofeev equation. The thermally initiated process for heterogeneous chlorination of polyethylene was observed to take place in two stages. The first stage was kinetically controlled, whereas in the second stage the diffusion control of the reaction predominated. The increase of temperature resulted in the reduction of the transition from the “mixed” diffusion–kinetic region of the rate-determining step to the one controlled entirely by diffusion. A temperature increase to over 345 K led to a process controlled by the diffusion phenomena only. A number of kinetic parameters were calculated for each stage such as the apparent rate constants and the corresponding activation energies, the pre-exponential factors in the Arrhenius equation, entropy changes, and steric factors. Based on the observed changes in both the crystallinity and porous structure of polyethylene, predominant chlorination of the amorphous regions in the surface layers of the polymer particles can be assumed at the initial stage. Following that, the reaction proceeded with the simultaneous chlorination of both the amorphous and crystalline regions into the depth of particles, regardless of the hampered diffusion of the gaseous reactant.

Ab initio calculation of the Young's modulus of α‐polyamides

AbstractIn earlier works the supermolecule model has been applied to the calculation of the Young's modulus of crystalline polyethylene and polyamide‐6. In the supermolecule model a crystalline polymer is represented as a single finite chain divided into a head, body, and tail part. The body contains a number of monomer units and is representative for a polymer chain. In this article, this model has been used to study the geometric properties and the elastic moduli of the α form of other polyamides: polyamide‐2 (or polyglycine), polyamide‐3, polyamide‐4, polyamide‐11, and polyamide‐6,6. All calculations have been performed with a linearly constrained body. The results have been compared to other theoretical and experimental results if available. © 2002 Wiley Periodicals, Inc.; DOI 10.1002/qua.2002;10121

Flow improvement of waxy oils mediated by self-aggregating partially crystallizable diblock copolymers

Precipitation and gelation of long chain paraffins from oil presents a challenge to the recovery and processing of waxy crude oils and fuel stocks. Diblock polymers consisting of a crystallizable polyethylene (PE) block and an amorphous poly(ethylenepropylene) (PEP) block self-assemble in oil to form expansive plate-like aggregates, consisting of a PE core within a PEP brush layer. In the presence of crystallizable paraffins the crystalline PE core can promote nucleation of solubilized long chain paraffins or may cocrystallize with the paraffin phase with the soluble PEP brush providing steric stabilization of the wax crystals. We examine the effect of PE-b-PEP additives of varying PEP brush length (5 and 11 K) on the yield stresses of straight chain paraffin gels (ranging in length from 24 to 36 carbons) in decane. PE-b-PEP addition at levels as low as 500 ppm produce reductions of wax gel yield stresses by factors of 3000. At higher PE-b-PEP addition levels gels can be formed with higher yield values than solutions without polymer. The location of the minimum in the yield stress with respect to polymer addition depends on the molecular weight of the paraffin and the PEP brush length. Microscopic crystal dimensions and mobility correlate with the observed rheological results. Potential underlying mechanisms for the observed efficiencies are discussed.

Homogeneous and heterogeneous alkyl-alkoxo-lanthanide type catalysts for polymerization and block-copolymerization of ethylene and methyl methacrylate

The combination of the neodymium tert-butoxide Nd 3(μ 3-OBu t )2(μ 2-OBu t ) 3(OBu t ) 4(THF) 2 ( 1), resulting from the metathetic exchange between NdCl 3 and Bu t ONa in THF, with 1 equiv. of a dialkylmagnesium reagent affords a moderately active cyclopentadienyl-free lanthanide catalyst for the pseudo-living polymerization of ethylene. Polymerizations performed under mild conditions (0 °C, 1 atm) in the presence of PhSiH 3 or H 2 as a transfer agent led to low molecular weight linear polyethylenes with activities in the range 5–20 kg PE (mol Nd) −1 atm −1 h −1. In the absence of such a co-reagent, highly linear and crystalline polyethylenes of high molecular weight are produced with ca. halved activity. Under those reaction conditions, a polymer-like precipitate ( 2) forms during the polymerization course. Most interestingly, solid 2 proved to be an equivalent catalyst to the initial in situ system for the slurry polymerization of ethylene. Solid 2, which is assumed to be the Nd-polyethylenyl intermediate, shows also significant ethylene polymerization activity under solid–gas conditions and enables the efficient preparation of ethylene-methyl methacrylate diblock copolymers.

Density functional calculations on the structure of crystalline polyethylene under high pressures

The geometrical structures of the crystalline polyethylene under several different external pressures up to 10 GPa are optimized by a pseudopotential plane wave density functional method. Both local density (LDA) and generalized gradient (GGA) approximations for exchange-correlation energy and potential are used. It is found that LDA heavily underestimate the geometry parameters under ambient pressure but GGA successfully correct them and get results in good agreements with the experimental geometry. The calculated GGA volume is about 94 Å3 in comparison with the x-ray scattering value of about 92 Å3 and the neutron scattering value of 88 Å3. The bulk and Young’s modulus are calculated by means of several different methods. The Young’s modulus along the chain ranges from about 350 to about 400 GPa which is in good agreement with the experimental results. But the bulk modulus is several times larger than those of experiments, indicating a different description of the interchain interactions by both LDA and GGA. The band structures are also calculated and their changes with the external pressure are discussed.

NUMERICAL MODELING OF MICROHARDNESS TESTS FOR POLYMER MATERIALS1*

A computational method in conjunction with indentation measurements is presented to determine stiffness properties of different polymeric materials. Experiments were performed using a Vickers-type indenter. Five different polymers were tested: polyethylene naphthalate (PEN), copolymer amorphous polyethylene terephthalate/polyethylene naphthalate (PET/PEN 70/30), copolymer crystalline polyethylene terephthalate/polyethylene naphthalate (PET/PEN 70/30), a random copolyester of polyethylene naphthalate/hydroxybenzoate (PEN/PHB 50/50), and a blend of isotactic polypropylene/ethylene co-propylene rubber (i-PP/EPR 85/15). Elastic properties are determined from the unloading curve of indentation. Identification of plastic properties of polymers are performed from the loading curve of indentation. A novel approach based on experimental design and finite-element analysis is discussed. Yield stress and tangent modulus are obtained by minimizing the deviation between the calculated and the experimental data. Results are verified by the finite-element method. Good agreement between the experimental and numerical load-indentation depth curves is observed. *Dedicated to Prof. Francisco J. Baltá Calleja on the occasion of his 65th birthday.

Studying the length of trans conformational sequences in polyethylene using Raman spectroscopy: a computational study

The long standing question of how long a trans sequence in a polyethylene needs to be in order to contribute to the all-trans Raman bands at 1060 and 1130cm−1 is addressed. Various hexadecane, C16H34, conformers were studied using ab initio type calculations revealing vibrational frequencies and intensities. We found that only trans sequences longer than 10 trans bonds reveal Raman intensity primarily selectively at the same frequencies as the C–C stretching modes arising from crystalline polyethylene. Since it is statistically highly unlikely that such long trans sequences occur in the amorphous phase, intensity arising from all-trans sequences not belonging to crystalline structure may only be expected as a consequence of strong non-equilibrium conditions (e.g. shear), or the presence of tie-molecules.