Polyethylene Molecules Research Articles

Reactivity of stressed molecules: calculation of the effect of chain deformation on scission for macromolecules and middle macroradicals

The effect of deformation ε of polyethylene molecule and radical on activation energy E a of the reaction of its monomolecular scission was calculated using MNDO and AM1 methods. Propane, octane, and decane molecules were used as models for PE chain. Reactions of molecule and radical scission were shown to be strongly accelerated by deformation, its activation energy, in contrast to previously studied reactions, being parabolic in chain deformation. The E a( ε) dependence for scission of molecules is shown to be linear only in the region of small (ε<3%) strains, for scission of macroradical such region is absent. Both semi-empirical methods give close results, but the AM1 method provides a better description for deformation of macroradicals. The proposed approach makes it possible to analytically describe calculated results, including the case of a reaction proceeding under the influence of constant force acting on a molecule. The possible cause for difference between the dependences of the local and integral rate constants on the load are considered.

NONDESTRUCTIVE CONTROL OF POLYETHYLENE BLANKET INSULATION BY MEANS OF LOCK-IN THERMOGRAPHY

It is well known that cross-linking of polyethylene molecules into three-dimensional networks improves material properties; in particular, it enhances impact strength, thermal performance and chemical resistance. A product of good characteristics can be obtained if the manufacturing process is adequate and all involved machines are working properly. In this context, infrared thermography as a remote imaging technique may be a valuable tool to monitor the polyethylene cross-linking processes. Experimental tests were carried out on specimens without treatment and on specimens treated with either chemical processes or electron beam irradiation. Our results prove that lock-in thermography can detect local non-uniformity of material characteristics due to either the extrusion or cross-linking processes, and material differences linked to the different compound.

Theory of transport of long polymer molecules through carbon nanotube channels.

A theory of transport of long chain polymer molecules through carbon nanotube (CNT) channels is developed using the Fokker-Planck equation and direct molecular dynamics simulations. The mean transport or translocation time tau is found to depend on the chemical potential energy, the entropy, and the diffusion coefficient. A power law dependence tau approximately N2 is found, where N is the number of monomers in a molecule. For 10(5)-unit long polyethylene molecules, tau is estimated to be approximately 1 micros. The diffusion coefficient of long polymer molecules inside CNTs, like that of short ones, is found to be a few orders of magnitude larger than in ordinary silicate based zeolite systems.

Phase structural analyses of polyethylene extrusion coatings on high‐density papers. I. Monoclinic crystallinity

AbstractIn the present work the morphology of high‐density polyethylene (HDPE) extrusion coating layer on high‐density paper (HDP) has been investigated. An uneven layer with a high content of crystallinity against the paper surface was discovered. The methods applied were solid‐state 13C NMR Spectroscopy and Atomic Force Microscopy. The highly crystalline layer was found to be mainly monoclinic crystallinity. The formation of the monoclinic crystallites was probably initiated by orientation of the polyethylene molecules by drawing, adhesion to the fibrous paper surface, and pressure. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 218–225, 2004

Computational method for analysis of polyethylene biodegradation

In a previous study concerning the biodegradation of polyethylene, we proposed a mathematical model based on two primary factors: the direct consumption or absorption of small molecules and the successive weight loss of large molecules due to β-oxidation. Our model is an initial value problem consisting of a differential equation whose independent variable is time. Its unknown variable represents the total weight of all the polyethylene molecules that belong to a molecular-weight class specified by a parameter. In this paper, we describe a numerical technique to introduce experimental results into analysis of our model. We first establish its mathematical foundation in order to guarantee its validity, by showing that the initial value problem associated with the differential equation has a unique solution. Our computational technique is based on a linear system of differential equations derived from the original problem. We introduce some numerical results to illustrate our technique as a practical application of the linear approximation. In particular, we show how to solve the inverse problem to determine the consumption rate and the β-oxidation rate numerically, and illustrate our numerical technique by analyzing the GPC patterns of polyethylene wax obtained before and after 5 weeks cultivation of a fungus, Aspergillus sp. AK-3. A numerical simulation based on these degradation rates confirms that the primary factors of the polyethylene biodegradation posed in modeling are indeed appropriate.

A numerical model to simulate the deformation and fracture of polyethylene fibres

A model was developed to simulate the mechanical behaviour of ideal polyethylene (PE) fibres made up of an ordered network of PE molecules of finite molecular weight completely extended and oriented along the fibre axis. The fibre was represented by a two-dimensional lattice, the nodes being linked by elastic rods which could carry normal and shear forces and stood for the intrachain covalent bonds and the interchain van der Waals forces. Fibre deformation was simulated incrementally, and the stresses on the bonds after each deformation step were computed with a numerical algorithm, which was also used to re-establish the mechanical equilibrium after bond rupture, that was introduced by the kinetic theory of fracture and a Monte Carlo lottery. Using appropriate values for the bond properties (stiffness, activation energy, etc), the effect of molecular weight, strain rate and temperature on the tensile behaviour and on the deformation micromechanisms of PE fibres was studied. The good qualitative agreement with the experimental data, regardless of the simplicity of the fibre model, indicated the excellent potential of these discrete models to simulate the deformation of polymeric fibres.

Elastic moduli of highly stretched tie molecules in solid polyethylene

The elastic properties of interlamellar bridges in semicrystalline polyethylene (PE) were estimated from the molecular-mechanics calculations on the assumption that the energy loading of a chain backbone represents the principal deformation mechanism. The calculations result in the force–length functions featuring abrupt discontinuities due to sequential annihilation of the defects by the conformational transitions. The correlation of the chain elastic moduli E with the concentration of defects in the chain and with the chain extension ratio x were established. The distribution functions ζ(E) of Young's moduli of interlamellar bridges in semicrystalline PE were calculated by using the literature data on the chain length distributions of tie molecules. The impact of the distribution function of moduli ζ(E) on the overall elastic response of solid PE materials was examined, particularly in cases of the stacked lamellae morphology involving so-called hard elastic PE.

Broadening of the fluorescence spectra of hydrocarbons in ethylene-vinyl acetate copolymers and the dynamics of the glass transition

The steady-state fluorescence of two fluorescent condensed aromatic hydrocarbons (anthracene and pyrene) with different fluorescence decay rates (a few nanoseconds and hundreds of nanoseconds, respectively) were employed to investigate the relaxation processes of several random ethylene-co-vinyl acetate copolymers (EVA). Copolymers with various comonomer compositions (EVA-9, EVA-18, EVA-25 and EVA-33) were studied. Data for the EVAs were compared with the same aromatic molecules in low-density polyethylene (LDPE), high-density polyethylene (HDPE) and poly(vinyl acetate) (PVAc) homopolymer models. The fluorescence rate constants were measured by single photon counting at room temperature. The wavelength dependence of the emission spectra (the edge-excitation red-shift (EERS)) were studied at room temperature and at 77 K and the data were interpreted based on the correlation of the differences of decay rates for both molecules and the time correlation of the polymer relaxation processes. The spectral broadenings (full-width at half-maximum (FWHM)) of the fluorescence spectra were studied from 20 to 400 K, which included temperature ranges above and below the glass transition, and these broadenings are compared.

Metal Powder Substrate-Assisted Laser Desorption/Ionization Mass Spectrometry for Polyethylene Analysis

Polyethylene is one of the most important industrial polymers and is also one of the most challenging polymers to be characterized by mass spectrometry. We have developed a substrate-assisted laser desorption/ionization (LDI) mass spectrometric method for polyethylene analysis. In this method, cobalt, copper, nickel, or iron metal powders are used as a sample substrate and silver nitrate is used as the cationization reagent. Using a conventional UV LDI time-of-flight mass spectrometer, intact oligomer ions having masses up to 5000 u can be detected. Cobalt is found to produce spectra with the highest signal-to-noise ratio and the lowest level of fragmentation. Cobalt powder size is shown to have some effect on the spectra produced. The best results are obtained with the use of cobalt powders with diameters ranging from 30 to 100 microm. Fragmentation cannot be totally eliminated, but the fragment ion peaks can be readily discerned from the intact polyethylene ions in the substrate-assisted LDI spectrum. Thus, the average molecular masses of low-mass polyethylene samples can be determined by using this method. A rapid heating model is used to account for the effectiveness of using the coarse metal powders to assist the analysis of intact polyethylene molecules by LDI.

Radiation Grafting of Polyethylene onto Conductive Carbon Black and Application as a Novel Gas Sensor

Radiation grafting of polyethylene (PE) onto carbon black was carried out by γ-ray irradiation of PE-adsorbed carbon black. The percentage of PE grafting and decomposition temperature of PE grafted onto the surface were determined by thermogravimetric analysis. At low irradiation temperature, the percentage of grafting was very small in spite of higher irradiation dose. On the contrary, at high temperatures near or above the melting point of PE, the grafting of PE onto the carbon black surface proceeded and the percentage of grafting exceeded 90% when the irradiation dose reached to 200 kGy. The results indicate that the adsorbed PE was completely grafted onto the carbon black surface. The decomposition temperature of the grafted PE on the surface was higher than both free (unadsorbed) PE and adsorbed PE on the carbon black surface, indicating that there is a covalent bond between the carbon black and PE molecule. Using the PE-grafted carbon black and PE, conductive PE/PE-grafted carbon black composite was prepared. Electric resistance suddenly increased in cyclohexane vapor over 104 times, and returned to initial resistance when transferred to air, indicating that the composite can be used as a novel gas sensor.

Mechanisms and kinetics of hydrogen yield from polymers by irradiation

Hydrogen yield from polyethylene by gamma-rays radiolysis was studied to understand the saturation phenomena of products with increase of dose in radiation resistance and also radiation processing of polymers. The hydrogen yield as a primary product was proportional with dose in the lower dose range and decreases gradually with dose above 10 kGy. The mechanism and kinetics were analyzed with a model of radiation protection effect by double bonds formed in polyethylene matrix during irradiation, that is, the double bond formed by primal irradiation reduces the hydrogen detachment from polyethylene molecules surrounding double bond by the following irradiation. By this analysis, one double bond covers about 800 methylene (–CH 2–) groups for the reduction of hydrogen detachment from polyethylene chains. When the 800 (–CH 2–) were concentrated in a sphere with a double bond at center, the radius was calculated to be 1.6 nm, where the radiation energy might be stabilized. G(H 2) was 4.0 for virgin polyethylene chains and was estimated to be 0.6–1 for the chains within the sphere.

Luminescence lifetimes of single molecules in disordered media

Linewidth measurements for single terrylene molecules in polyethylene at a temperature of 30 mK indicate that there is a distribution of lifetimes for the terrylene molecules with a relative standard deviation of ∼20%. An analysis of the linewidth–line area correlation shows that the variations arise from approximately equal radiative and nonradiative contributions. A simple model suggests that the distribution of radiative lifetimes in disordered media is a general effect caused by the same interactions responsible for inhomogeneous broadening. In addition to the transition frequency, the luminescence lifetime of a probe molecule can be used to study the nano-environment of the probe.

Spectral diffusion in polyethylene: Single-molecule studies performed between 30 mK and 1.8 K

Linewidth distributions for single terrylene molecules in polyethylene have been measured in the temperature range from 30 mK to 1.83 K. The temperature dependence of the average linewidth is linear over the full temperature range. Linewidth distributions were simulated using the tunneling two-level system model and compared to the data in order to extract the lifetime-limited linewidth distributions and the distributions of coupling strengths between the probe molecules and the two-level systems. Differences of the average lifetime and coupling strengths were observed for samples produced with different sample-preparation techniques.

Impregnation of Polyethylene (PE) with Styrene Using Supercritical CO2 as the Swelling Agent and Preparation of PE/Polystyrene Composites

Impregnation of low-density polyethylene (LDPE) with styrene using supercritical (SC) CO2 as the swelling agent was studied at 35 °C in a pressure range from 90 to 160 bar. The concentration of styrene in the fluid phase ranged from 0 to 1.668 mol/L. The soaking time varied from 4 to 36 h. In the presence of azobis(isobutyronitrile), the initiator, styrene polymerized to some extent in the soaking process, which results in an increase in the mass uptake. LDPE/polystyrene (PS) composites were prepared by the polymerization of styrene further in SC CO2-swollen LDPE substrates at higher temperature. Using this method, the composition of the composites can be controlled by the soaking time, pressure, and styrene concentration in the fluid phase. Some of the PS molecules in LDPE/PS blends entangle with the polyethylene molecules, which causes significant improvement in the impact strength, the tensile strength, and the elongation at the break.

Complementarity of universal calibration SEC and 13C NMR in determining the branching state of polyethylene

The quantitation of long-chain branching (LCB) and short-chain branching (SCB) in polyethylene (PE) was accomplished with a combination of carbon nuclear magnetic resonance (13C NMR) spectroscopy and size exclusion chromatography (SEC) with universal calibration. We demonstrate how the spectroscopic and chromatographic techniques can supplement each other, as neither is capable individually of completely describing the molecular architecture imparted by the various types of branching. The essential lack of impact of SCB on the hydrodynamic volume imposes a limit on SEC for determining this type of branching, whereas highly effective LCB in the PE molecule may not offer a statistically large enough amount of long chains for accurate determination by NMR. A variety of examples are given for PE, showcasing the advantages and shortcomings of each analytical method and their complementarity. Additionally, the importance of choosing an appropriate linear standard and viscosity shielding ratio (ϵ) for the Zimm–Stockmayer branching calculations employed for analyzing SEC data is emphasized with an examination of the effect on the results of using a branched standard and various ϵ values. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3120–3135, 2000

Zero–phonon lines of single molecules in polyethylene down to millikelvin temperatures

Linewidth distributions for single terrylene molecules in polyethylene have been measured in the temperature range from 30 mK to 1.83 K. The temperature dependence of the average linewidth is best described by a linear relationship over the full temperature range. At 30 mK, the linewidth distribution has a full-width at half-maximum of ∼18.6 MHz and an average linewidth of 42.8(6) MHz.

Effects of stress and sulfur dioxide on Spectra®

Polymers are often subjected to aggressive environments where mechanical and chemical mechanisms may act synergistically to degrade the polymers. This research focused on the effects of stress and sulfur dioxide (SO2) environments on ultrahigh molecular weight polyethylene (UHMWPE) fibers (Spectra®). Spectra® yarn specimens were exposed to stress, SO2, and/or near ultraviolet (UV) light environments. Degradation, a decrease in tensile properties, was measured by tensile testing specimens immediately after exposure to the stress/environment conditions. Creep lifetime degradation of Spectra® single fiber specimens was also measured in SO2 and/or near UV light environments. The Spectra® fibers were found to degrade due to the applied stress. This was evidenced by a decrease in the ultimate strength subsequent to an applied creep load and the short creep lifetimes. No chemical degradation was observed, nor were any mechanical–chemical synergisms observed due to the SO2 and/or near UV light environments. The creep lifetimes of the Spectra® fibers, however, increased slightly in environments of SO2 and/or near UV light. The degradation of the Spectra® fibers is consistent with their fibrillar morphology and was attributed to chain scission of the interfibrillar tie chains due to the applied creep load. Since no degradation of the Spectra® was observed in the SO2 and/or near UV light environments, it was concluded that it is relatively insensitive to environmental attack from SO2 and/or near UV light. The increase in the creep lifetimes of the Spectra® fibers suggests that the SO2 and/or near UV light does affect the Spectra® fibers. Based on this and the work of other researchers, it is hypothesized that the SO2 and/or near UV light are, to a limited extent, capable of crosslinking or branching linear polyethylene molecules. Such crosslinking or branching appears to be minimal, altering only the creep lifetimes and leaving the other tensile properties largely unaffected.

Microwave irradiation effect on diffusion of organic molecules in polymer

Microwave irradiation effects on diffusion of some small organic molecules in polyethylene, polyvinylchloride, and silicone rubber were investigated using general weight-gain method. To eliminate the development of hot spots and inhomogeneous temperature distribution that occur often during microwave heating, sorption/diffusion solutions were well mixed with a magnetic stirrer at atmospheric pressure. The deviations of the diffusion rates between microwave and conventional diffusions were shown to be below 2.1%. This result reveals that previously reported nonthermal effects of microwave to promote diffusion do not exist in the examined diffusant/polymer systems.

Solubilities of small molecules in polyethylene evaluated by a test-particle-insertion method

Free-energy related properties for penetrant sorption (water, methane, argon, oxygen, nitrogen, ethane, propane, and carbon dioxide) in amorphous polyethylene have been evaluated by a test-particle-insertion method with the excluded volume map sampling (EVMS). Two model systems, one a long chain model composed of 4 linear chains each having 1002 carbon atoms and another a short chain model composed of 167 molecules of C24H50, were compared in relation to the penetrant solubility at 298 K. The time-dependent molecular configurations were obtained by a molecular dynamics (MD) simulation under constant particle number, constant pressure, and constant temperature (NPT) conditions. The solubility coefficient obtained by the EVMS method was larger in the order, C3H8&amp;gt;C2H6&amp;gt;CO2&amp;gt;CH4&amp;gt;Ar&amp;gt;O2&amp;gt;N2, similar to the experimental results, and the absolute values were also in agreement with the experimental value. In the long chain host-molecules, reflecting the higher density, the solubility was about 0.85–0.90 times smaller for most penetrants tested, while ethane and propane showed higher solubility in contrast. Analysis of the averaged unoccupied volume fraction and their distribution in both systems revealed that big holes, in which a sphere with a diameter larger than 5.0 Å can be introduced, were formed in the long chain model irrespective of the denser structure. These findings resulted in higher solubility of a larger penetrant such as ethane and propane in the long chain matrix.

Solubility of Small Molecules and Their Mixtures in Polyethylene

The solubility of small molecules and their mixtures in polyethylene is studied using a recently proposed united atom force field.1 By comparing the simulation results with experimental data on solubility of small molecules in semicrystalline polyethylene, it is found that crystallinity can severely effect the solubility of a solute, especially if it is highly soluble in the polymer of interest. It is also observed that small molecules form aggregates within the polymer in the liquid phase.