Semicrystalline Low Density Polyethylene Research Articles

Advancing spin-coating technique for semi-crystalline low-density polyethylene thin films

Polymeric thin films are widely used for coating substrates or as freestanding materials. They are prepared using physical/chemical vapor deposition, dip coating, solution casting, or spin coating. Among these, spin-coated films offer uniform thickness and reproducible results. These films are typically made from amorphous polymers, which are soluble in solvents at ambient temperature, making it easier to coat the polymer solution on the substrate. Despite the advantages of this technique, it has hardly been used for semi-crystalline polymers. In this work, we report the synthesis of freestanding porous thin films using semi-crystalline polymers, specifically low-density polyethylene (LDPE), through the utilization of spin coating. The effect of polymer concentration and porosity on thickness and strength was investigated. The as-prepared thin film was characterized by XRD, XPS, FTIR, and SEM techniques. The increase in crystallinity (14 % change) of the thin film was attributed to the rearrangement and uniform alignment of the polymer chains, which further resulted in relatively higher tensile strength.

Modeling the visco-hyperelastic–viscoplastic behavior of photodegraded semi-crystalline low-density polyethylene films

In this work we propose to model the mechanical and fracture response of semi-crystalline low-density polyethylene (LDPE) films exposed to accelerated ultraviolet (UV) ageing using a physically based visco-hyperelastic–viscoplastic approach. UV irradiation induces an alteration of the chemical and structural properties of the semi-crystalline polymer, which affects significantly its mechanical behavior. In this work, pristine and oxidized low-density polyethylene films are characterized by conventional physico-chemical and mechanical techniques (FTIR spectroscopy, DSC, HT-GPC, and uniaxial tensile testing). Polyethylene exhibits an oxidation-induced strengthening for a low range of UV radiation doses and a cavitation-induced softening for higher UV radiation doses. A competing multi-scale phenomena induced by UV radiation are incorporated into the constitutive model to capture the macroscopically observed mechanical and fracture behavior. Namely, the model will incorporate at the nano scale, chain scissions and cross-linking and at the meso and macro scales, oxidation-induced cracking, chemi-crystallization, and mechanical damage. The model used the high-temperature gel permeation chromatography-measured molecular weight as degradation indicator. The model was able to capture accurately the evolution of the macroscopically observed mechanical and fracture behavior over a wide range of UV irradiation doses.

A new look at the semicrystalline polymer – liquid systems: Phase diagrams low-density polyethylene – n-alkanes

Experimental phase diagrams for the sealed systems low-density polyethylene (LDPE) – n-decane and low-density polyethylene – n-tridecane were plotted for the first time in a wide temperature-concentration range using an original optical method and gravimetry. Liquid-liquid phase separation of LDPE and n-decane with the lower critical solution temperature of 280 °C was detected. Two branches of the solubility curve were constructed, which describe the temperature dependence of the component ratios in the semicrystalline LDPE with absorbed n-decane and in the liquid n-decane containing extracted polymer. In the domain between the solubility curve and polymer full amorphization curve the system LDPE – alkane can be identified with a single-phase microheterogeneous gel with crystallites as physical crosslinks. Below the solubility curve the system is two-phase but the true equilibrium between the phases cannot be established without destroying polymer crystallites. An empirical approach based on analyzing the parameters of the polymer full amorphization curve was applied to find that n-decane is a slightly better solvent for LDPE than n-tridecane.

A new prediction method for organic liquids sorption into polymers

A new prediction method for sorption of low-molecular organic liquids (non-solvents) in polymers is presented. It was derived from the gravimetric data for sorption of linear and cyclic alkanes, aromatics and alcohols into various kinds of polymers (glassy polystyrene, rubbery polydimethylsiloxane, semicrystalline low-density polyethylene, ion exchange Nafion, and copolymeric poly(styrene–butadiene–styrene) in temperature range 5–50°C. Our method is based on a normalised function φQ(t) evaluated from series of the equilibrium mass swelling degrees Qm of a model liquid in one polymer at each temperature from the indicated temperature interval to the corresponding Qm at the highest temperature of this interval. A knowledge of the proper function φQ(t) for given polymer enables to predict a sorption of any other liquid with the same properties or interactions with given polymer in given temperature range just from a knowledge of a single experimental Qm value at any temperature from the indicated temperature interval. The steepest temperature dependent normalised function φQ(t) exhibited LDPE+liquid systems. Contrarily, PDMS+liquid systems were practically temperature independent. The thermodynamic analysis of the sorption process in studied polymer–liquid systems comprised evaluation of the appropriate values of the sorption enthalpy ΔsH and the sorption entropy ΔsS. The values of the resulting Gibbs energy of sorption ΔsG were negative in all cases and thus confirmed the spontaneity of liquids sorption in polymers over the whole temperature range.

Experimental study for starve‐fed single screw extrusion of thermoplastics

AbstractAn experimental study of the polymer behavior in a starve‐fed single screw extrusion is presented. Various polymeric materials, semicrystalline low density polyethylene (LDPE), polypropylene (PP), and (LDPE/PS) polyblends were investigated at various operating conditions. A “screw pulling‐out” technique was used to study polymer behavior along the screw. The solid conveying, melting position, the extent of starvation, and the fully filled regions were observed. Polymer samples were stripped off from the screw which was removed from the machine to investigate melting mechanism. It was seen that filling of the screw channel increases with the flow rate at a fixed screw speed, and decreases with the screw speed at a fixed flow rate. Contiguous solids melting mechanism was observed for flood fed extrusion, but it was not observed for starve‐fed extrusion. A new two‐stage physical model of polymer melting has been proposed with conductive mechanism for melting in the starve‐fed region and dispersed melting mechanism in the fully filled region. Melting action seems to be faster for starve feeding than for flood feeding, since the pellets are not compacted into a dense solid bed. It was observed that the pressure and power consumption considerably decrease with starvation. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers

Experimental study of melting of LDPE/PS polyblend in an intermeshing counter‐rotating twin screw extruder

AbstractAn experimental study is presented of the melting mechanism in a starve‐fed closely intermeshing counter‐rotating twin screw extruder of a modular Leistritz design. Various polymeric materials, semicrystalline low density polyethylene (LDPE), amorphous polystyrene (PS), and (LDPE/PS) polyblend were investigated at various operating conditions. A “screw pulling‐out” technique was used to investigate polymer behavior along the screw axis. In particular, the solid conveying, melting positions, the extent of starved character along the screw, and the fully filled regions were observed. Polymer samples were stripped off from each screw which was removed from the machine to investigate melting mechanism. Generally, it has been concluded that the melting mechanism revealed by White and Wilczyński for polyolefines has been proved for other polymeric materials under study. This mechanism consists of pellets being dragged into the calendering gap where they are melted due to calendering action. The molten polymer is expelled from the gap and pushes against the pellet bed which is continuously dragged into the gap. The composite modeling of an intermeshing counter‐rotating twin screw extrusion of polyblends has also been discussed. POLYM. ENG. SCI., 52:449–458, 2012. © 2011 Society of Plastics Engineers

Coupling effect during vapour permeation of organic mixtures through polymeric membranes

A mass transport of organic compounds (benzene, hexane, cyclohexane and methanol) and their binary mixtures at wide activity range through dense semicrystalline low density polyethylene membranes is presented. Single and binary vapour permeation measurements were performed at 25°C and at 35°C by the differential flow permeation apparatus method connected with GCMS device. Determined values of steady-state molar fluxes revealed the differences between single and binary permeation as well as the mutual coupling effect of second component in mixture on permeation flux of the first one.

Comparative study of sorption and permeation techniques for the determination of heptane and toluene transport in polyethylene membranes

A comprehensive study of the mass transport of two volatile organic compounds (VOCs) heptane and toluene and their binary mixtures through dense semicrystalline low density polyethylene membranes (LDPE) at 25 °C is presented. The use of the widest possible concentration range (VOCs activity of 0.001–1 at given temperature) and a set of four different experimental techniques enabled to reveal differences between permeation and sorption of studied vapours and to clarify the effect of feed and instantaneous VOCs concentration inside the LDPE membrane. The differential flow permeation method and fixed-volume pressure increase method were used for pure and binary vapour permeation while sorption measurements (from liquid and vapour phases) were performed gravimetrically and by the Vapour Phase Calibration technique (VPC). This work is the first and unique comparative study on a so wide range of vapour activities and a so vast number of different techniques, giving important insight into the transport phenomena themselves and on how results are affected by the specific technique used. Results are demonstrated and discussed in detail on the basis of a pair of penetrants with different physico-chemical properties and molecular shape (toluene vs. heptane). Information about differences between concentration dependent permeation and sorption, coupling effect, molecular clustering and polymer–solvent interactions is reported as well. The establishment of specific concentration profiles inside the LDPE membrane during permeation and sorption experiments evoke time-dependent structure changes of the LDPE matrix and such chain reorganisations consequently influence the transport parameters and they are also related to a certain form of high concentration-evoked molecular aggregation (clustering). X-Ray diffraction measurements excluded that the observed phenomena could be due to significant irreversible changes in the crystallinity of the material. Toluene and heptane favourably influence one another's transport and consequently both sorption and permeation experiments reveal a distinct coupling effect. Permeation experiments show a gradual increase of the integral diffusion with increasing vapour activity whereas VPC and gravimetric sorption analysis find a maximum around activity a = 0.7. This difference is due to the deviation from Fickian behaviour at high activity and to fundamental differences in the measurement principle.

Effects of Control Factors on Injection Foaming of Amorphous and Crystalline Resins

In this study the effects on cell structure of plastics foamed during injection molding due to differences in resin viscoelasticity were investigated. Amorphous polystyrene resin and semicrystalline low density polyethylene resin, with widely different viscoelasticity behaviors, were used as examples. Injection velocity, mold temperature, retention time and screw shape were selected as control factors that could affect cell nucleation and growth during injection molding. Mold temperature was found to strongly affect the resin flow and the resultant foam cell structure.

Monosubstituted derivatives of pyrene: Comparison of their spectral behaviour in solution and in polymer matrices

Spectral properties of monosubstituted derivatives of pyrene were compared in low viscosity solvents such as cyclohexane, chloroform, and methanol, in high viscosity solvents such as heptadecane and polybutene oil, in glassy polymer matrices above T g (atactic polypropylene) and below T g (polystyrene polymethyl methacrylate, polyvinylchloride), and in semi-crystalline low density polyethylene. The derivatives under study were: 1-pyrenylmethanol (I), benzyl-(1-pyrenylmethyl) ether (II), di(1-pyrenylmethyl) ether (III) and 4-(1-pyrenyl)butyl-(1-pyrenylmethyl) ether (IV). Absorption spectra of these probes were not influenced by environment. Emission spectra of III exhibited exclusively excimer emission in low viscosity solvents. Probe IV yielded mainly monomer emission. In viscous solvents and glassy polymer matrices above and below T g, as well as in semi-crystalline polyethylene, monomer emission was exclusively observed for all probes. Distinct differences were observed in emission decay curves. Probes derived from pyrene with a bulky substituent exhibited biexponential decays with a short component (∼10 ns) in viscose solvents and polymer matrices. In semi-crystalline polyethylene, the decay of monomer emission of all probes was monoexponential with a lifetime around 200 ns.

Orientation Texture and Gas Transport in Semicrystalline Block Copolymer Blends

Abstract The shear-induced morphologies produced by plane strain compression, using a channel die, were investigated in semicrystalline ethylene/ethylene-propylene (E/EP/E and E/EP) block copolymers blended with semicrystalline low density polyethylene and amorphous polyisoprene homopolymers. Two dimensional small-angle x-ray scattering (SAXS) was used to determine the lamellar orientation relative to the specimen boundaries. Using a cooling rate of 0.27 °C/s and a load of 9.2 MPa, a normal to the plane of shear microstructure orientation texture was produced in the blends. The E homopolymer chains co-crystallize within the confinement of the oriented microphase separated E block copolymer domains. The long period spacing of the oriented microphase separated E/EP block microstructure remained unchanged, while the spacing associated with the crystallization of the E chains increased with increasing weight percent of E homopolymer in the blend. Gas permeability coefficients for He and CO2 were measured for isotropic and oriented blends. The separation properties of these polymer systems was altered by changing the mechanism of gas transport. A selective solvent was used to develop a uniform porous structure in the oriented blend morphologies. The semicrystalline block copolymer blend structure oriented perpendicular to the direction and normal to the plane of shear presents the opportunity to create a high flux porous structure, while keeping the pores of nanometer dimensions, thus providing some selectivity and retaining mechanical strength.

The colloid-structure of semicrystalline low-density polyethylene

The length-distribution of stereoregular sequences in low-density polyethylene (LDPE) is determined by a computer-aided analysis of cp-melting curves within the framework of thermodynamics of eutectoid copolymers. Small-angle x-ray scattering patterns are then described by using the structure data derived. One of the results is that fluctuations of the mean electron-density are increasingly reduced with an increasing degree of crystallinity.

Comparison of water treeing in an amorphous and in a semicrystalline polymer

In this paper, we compare the growth and the structure of water trees in amorphous polystyrene and semicrystalline low-density polyethylene. The structure and the growth of the water trees observed in each of the polymers are very different. These different aspects are discussed in terms of mechanical fatigue, and the role played by the crystalline layers in the resistance of semicrystalline polymers to water treeing is pointed out.