Mechanical and transport properties of drawn low-density polyethylene

Quenched, quenched and annealed, and slowly cooled branched low-density polyethylene films were drawn at 25, 40, and 60°. The true draw ratio λ^L of the volume element was obtained and used to characterize the dependence on plastic deformation of the density, drawing stress, and work of plastic deformation, and the sorption and diffusion of methylene chloride. The effects observed are similar but less drastic than on linear high-density polyethylene. In particular, the transformation from the original lamellar to the final fibrous structure seems to be fastest for λ^L between 3 and 4. But the changes of vapor transport clearly indicate that the transformation is not yet complete even at the highest draw ratio λ^L = 6, just before the sample breaks. Annealing at 90°C of the drawn samples with free ends restores or even increases the transport properties beyond those of the undrawn sample without causing the fibrous structure to revert to the original lamellar structure.     

Mechanical and transport properties of drawn crosslinked low-density polyethylene

The values of drawing dependence of the density ρ, axial elastic modulus E, and maximum draw ratio λ of crosslinked low-density polyethylene (CLPE) rather similar to those obtained with un-crosslinked branched material of similarly low density. Very much the same applies to the equilibrium concentration of sorbed methylene chloride in the amorphous component and the zero-concentration diffusion coefficient D₀. The exponential concentration coefficient γD , however, even at the maximum draw ratio, shows no indication of the rapid increase so characteristic of the completed transformation from the lamellar to the fibrous structure. On the basis of this finding, one can understand the small deviations in the dependence of the mechanical properties between the crosslinked and uncrosslinked branched material. The segments between the crosslinks, much shorter than the free molecules, favor the formation of the interfibrillar tie molecules that limit the drawability of the sample. But since they cannot be extended to the same length as the free molecules, they contribute less to the total fraction of tie molecules per amorphous layer and hence yield a smaller axial elastic modulus.     

Thermal and flame retardant properties of novel intumescent flame retardant low-density polyethylene (LDPE) composites

A char-forming agent (CFA) and silica-gel-microencapsulated ammonium polyphosphate (MCAPP) were selected to form novel intumescent flame retardant system (IFRs), and then the influence of this novel IFRs on the thermal and flame retardant properties of low-density polyethylene (LDPE) were studied. The results of cone calorimetry show that the flame retardant properties of LDPE with 30?wt% novel IFR (CFA/MCAPP?=?1:3) improve remarkably. The heat release rate peak, total heat release (THR) decreases, respectively, from 1479.6 to 273.5?kW?m^?2 and from 108.0 to 80.5?MJ?m^?2. The LDPE composite with CFA/MCAPP?=?1:3 has the excellent water resistance, and it can still obtain a UL-94?V-0 rating after treated with water at 70?°C for 168?h.     

Time dependence of mechanical and transport properties of drawn and annealed linear polyethylene

Linear polyethylene both as drawn, or drawn and subsequently annealed with free ends, changes its length, density, crystallinity, elastic modulus, sorption, and diffusivity as the sample stands completely unrestrained at room temperature. Most of these changes occur during the first few hours. But they are important on a molecular scale since they suggest strongly that drawn, and drawn and annealed samples are far from equilibrium. As a consequence of the tendency of each mobile tie molecule in the amorphous conformation to retract and to crystallize, the specimen approaches but does not reach complete equilibrium. The transient seems to be caused by slow crystallization of tie molecules which creates crystalline bridges across the amorphous layers.     

Morphology and properties of low-density (branched) polyethylene

The different types of morphology that can be developed in a large number of low-density (branched) polyethylene whole polymers, as well as in a series of fractions, have been studied for two different extreme crystallization modes. Concomitantly, thermodynamic properties of the same samples have also been determined. After isothermal crystallization at elevated temperatures, spherulitic structures are found in all the whole polymer samples. On the other hand, after rapid crystallization a variety of different types of supermolecular structures are observed which are shown to depend systematically on the concentration of side-chain branches and the relative proportion of high molecular weight species in the sample. This temperature dependence of the morphological forms is opposite to that previously reported for linear polyethylene. The studies with the fractions show that the individual species are not the cause of this behavior; rather, the total composition is the important factor. The thermodynamic properties are also quite different from those of linear polyethylene in showing virtually no molecular weight dependence and being governed primarily by the concentration of short-chain branches. The degrees of crystallinity as determined from density and enthalpy of fusion measurements do not vary much with the two extreme crystallization conditions employed, are not sensitive to the morphology, and differ from one another, even when well-developed spherulites are formed. A major influence of the branching concentration on these properties is clearly indicated.     

High-energy C+ ion-irradiated low-density polyethylene (LDPE): Spectroscopic and morphological investigation

The optical absorption spectra of 3.6 MeV ¹²C ions irradiated low-density polyethylene (LDPE) show the gradual increase in the optical absorption and the shift from the near UV to the visible region with increase in ion fluence. The increase in absorption may be attributed to the generation of a conjugated system of bonds, which are also corroborated by FTIR. Scanning electron microscopic (SEM) studies reveal that the formation of linear network structure occurred at relatively lower ion fluence of 5×10¹³ ions/cm².     

Synergistic effects of zeolites on a novel intumescent flame-retardant low-density polyethylene (LDPE) system

A char forming agent and silica-gel microencapsulated APP were selected to form novel intumescent flame-retardant system (IFR) to prepare flame-retardant low-density polyethylene (LDPE) composites, and then the influence of zeolites on the thermal and flame-retardant properties of flame-retardant LDPE composites were studied. With the addition of 1 wt% zeolites to LDPE/IFR system, the LOI value increases from 29.0 to 34.0 %. The results of cone calorimetry show that the heat release rate peak and total heat release of the intumescent flame-retardant LDPE composite with 1 wt% zeolites decreases remarkably compared with that of without zeolites. The scanning electron microscopy indicates zeolites with suitable content can improve the quality of the char layer of flame-retardant LDPE composite which is more coherent and dense. The zeolites with the appropriate content can remarkably improve the flame-retardant properties of the LDPE composites.     

Shrinkage and retractive forces during the annealing of drawn low-density polyethylene

Shrinkage of unconstrained low-density polyethylene samples and the retractive stress of samples with ends fixed have been investigated as a function of the annealing time t_A and temperature T_A on material drawn at room temperature to draw ratios λ between 4 and 6. The shrinkage increases with t_A and T_A. The retractive stress on a sample annealed with ends fixed goes through a maximum as the sample is annealed and then drops to a limiting value which increases with T_A as long as T_A is at or below 80°C and rapidly decreases with higher T_A. The drop from the maximum to the limiting retractive stress, slow at lower T_A and rapid at higher T_A, seems to be a consequence of rapid pulling of chain segments out of crystal block in which interfibrillar tie molecules are anchored. This process is facilitated by the high T_A, which softens the crystal matrix. At constant end-to-end distance, the contour length of the tie molecules is irreversibly increased, and this causes a reduction in the contribution of the affected tie molecules to the overall retraction stress. Hence one finds a substantially higher retraction stress during first heating than during subsequent cooling and heating of the drawn sample.     

Chemical transformations and changes in free volume holes in high-energy proton irradiated low-density polyethylene (LDPE)

Low-density polyethylene (LDPE) was irradiated with 4 MeV protons at different ion fluences. Chemical modifications were studied by UV–vis and Fourier transformed infrared (FT-IR) spectroscopy. Further, the changes in free volume holes were studied using positron annihilation lifetime spectroscopy. The different UV–vis absorption spectra show the maximum absorption at around 280 nm for the lower fluences whereas at the highest fluence a red shift is observed. This indicates formation of chromophoric groups that have the maximum absorption in that region. The formation of vinylidine, trans-vinylene, end-vinyl group, unsaturated linkage and carbonyl group were detected from FT-IR spectra. Positron annihilation lifetime spectroscopy reveals that the free volume hole size and their relative fraction first decrease, attaining a minimum value and subsequently increases with increase in ion fluence. These changes in free volume holes have been well correlated with the observed chemical transformations.     

Transport properties of methylene chloride in drawn polyethylene as a function of the draw ratio

The sorption and diffusion constants of CH₂Cl₂ at room temperature in quenched polyethylene film drawn at 60°C to different draw ratios λ between 6 and 25 drop drastically between λ = 8 and λ = 9 and then remain nearly constant, dropping only slightly up to λ = 25. Also, the exponential dependence of diffusion constant on concentration of sorbent increases abruptly in the same draw interval and then remains constant at the higher draw ratios. The data may be explained well by a composite madel: a low-permeability fiber structure embedded in a high-permeability spherulitic matrix. As the draw ratio is increased, the initially spherulitic film is gradually transformed into the fiber structure with the transformation being completed between λ = 8 and λ = 9. During subsequent drawing to λ = 25 the mutual arrangement of microfibrils, the basic elements of the fiber structure, changes by longitudinal sliding. However, their transport properties remain nearly constant. The diffusion constant drops a little as a consequence of the increased fraction of tie molecules which reduces the number of unperturbed sorption sites.     

Rheo-optical near-infrared (NIR) spectroscopy study of low-density polyethylene (LDPE) in conjunction with projection two-dimensional (2D) correlation analysis

A rheo-optical characterization technique based on near-infrared (NIR) spectroscopy is developed specifically to probe the submolecular-level deformation caused during a mechanical test. An illustrative example of the mechanical deformation of low-density polyethylene (LDPE) is provided to show how it can be utilized. A set of NIR spectra of the polymer sample were collected by using an acousto-optic tunable filter (AOTF) NIR spectrometer coupled with a tensile testing machine as an excitation device. While the substantial level of variation of spectral intensity was readily captured during the mechanical deformation of the LDPE, main feature of the NIR spectra was overwhelmed by the contribution from the baseline change. Projection 2D correlation analysis was then applied to selectively extract the signal contribution from the baseline fluctuation. The 2D correlation spectra revealed the predominant extension of amorphous tie chains followed by the rotation of crystalline lamellae, which induce elastic and plastic deformation of the LDPE, respectively.     

Proton (3 MeV) and copper (120 MeV) ion irradiation effects in low-density polyethylene (LDPE)

Low-density polyethylene (LDPE) was irradiated with proton (3 MeV) and copper (120 MeV) ions to analyze the induced modifications with respect to optical and structural properties. In the present investigation, the fluence for proton irradiation was varied up to 2×10¹⁵ protons cm⁻², while that for copper beam was kept in the range of 1×10¹ to 1×10¹³ ions cm⁻² to study the swift heavy ion-induced modifications in LDPE. Ultraviolet–visible (UV–vis), FTIR and X-ray diffraction (XRD) techniques were utilized to study the induced changes. The analysis of UV–vis absorption studies reveals that there is decrease of optical energy gap up to 43% on proton irradiation (at 2×10¹⁵ ions cm⁻²), whereas the copper beam (at 1×10¹³ ions cm⁻²) leads to a decrease of 51%. FTIR analysis indicated the presence of unsaturations due to vinyl end groups in the irradiated sample. The formation of OH and CO groups has also been observed. XRD analysis revealed that the semi-crystalline LDPE losses its crystallinity on swift ion irradiation. It was found that the proton beam (2×10¹⁵ ions cm⁻²) decreased the crystallite size by 23% whereas this decrease is of 31% in case of the copper ion-irradiated LDPE at 1×10¹³ ions cm⁻².     

Characterization and properties of polyethylene blends II. Linear low-density with conventional low-density polyethylene

Melting and crystallization phenomena in blends of a linear low-density polyethylene (LLDPE) (ethylene butene-1 copolymer) with a conventional low-density (branched) polyethylene (LDPE) are explored with emphasis on composition by differential scanning calorimetry (DSC) and light scattering (LS). Two endotherms are evident in the DSC studies of the blends, which suggests the formation of separate crystals. Light-scattering studies indicate that the blend system is predominantly volume filled by the LLDPE component whereby the LDPE component crystallizes as a secondary process within the domain of the LLDPE spherulites. In contrast to those of the LLDPE/HDPE blends, the mechanical and optical relaxation behavior of the LLDPE/LDPE blends are dominated by the LLDPE component in the vicinities of γ and β regions, whereas the trend reverses at high temperature α regions. This observation is accounted for on the basis of the relative restrictions imposed by the deformation of spherulites (which are primarily made up of the LLDPE component) at different time scales.     

Characterization and properties of polyethylene blends I. Linear low-density polyethylene with high-density polyethylene

A blend system of linear low-density polyethylene (LLDPE) (ethylene butene-1 copolymer) with high-density (linear) polyethylene (HDPE) is investigated by differential scanning calorimetry (DSC), wide-angle x-ray diffraction (WAXD), small-angle x-ray scattering (SAXS), Raman longitudinal-acoustic-mode spectroscopy (LAM), and light scattering (LS). For slowly cooled or quenched samples, one single endotherm is evident in the DSC curve which depends on the composition. No separate peaks are observed in the WAXD, SAXS, Raman-LAM, and LS studies on the LLDPE/HDPE blends. This observation along with the fact that no peak broadening is observed suggests that these peaks are associated with the presence of a single component. In no case did we see double peaks or a broadened peak that might be associated with two closely spaced unresolved peaks. This suggests that segregation has not taken place at the structural levels of crystalline, lamellar, and spherulitic textures. A single-step drop in the scattered intensity (I_Hv) as a function of temperature is seen in the LS studies. It is therefore concluded that cocrystallization between the LLDPE and HDPE components occurs. The mechanical and optical α, β, and γ relaxations of these blends are explored by dynamic birefringence. The 50/50 blend displays the intermediate relaxation behavior between those of the components in all α, β, and γ regions. This observation is reminiscent of the characteristic of the typical miscible blends.     

Simplified approach to modelling the catalytic degradation of low-density polyethylene (LDPE) by applying catalyst-free LDPE-TG profiles and the Friedman method

The course of the thermogravimetric degradation of LDPE in the presence of different aluminosilicate catalysts was modelled by applying a differential isoconversional Friedman approach. An analysis of catalyst-free PE-TG profiles confirmed that the degradation profiles predicted by various reaction models overlap over the entire conversion range once the data are analysed using a differential isoconversional Friedman approach. The results demonstrate that the catalytic degradation of LDPE can be predicted by a correlation twin, i.e. the two specific functional relations between the activation energy, pre-exponential factor and conversion. The crucial step for ensuring good agreement between the predicted and the measured profiles is to extrapolate the discrete values of the activation energies and pre-exponential factors to the zero conversion. It turns out that linear extrapolation and interpolation from the discrete values outperforms regression functions based on various order polynomials, and that apparent deviations from the global trend at lower conversions are not a consequence of the misinterpretation of the experimental results but are an experimental fact. The assumption about the compensation effect between the pre-exponential factor and activation energy holds within the conversion range from 10 to 90%. However, it is generally unsuitable for modelling purposes due to the uncertain extrapolation of the kinetic parameters to the zero conversion.

     

Thermal, mechanical and electrical properties of copper powder filled low-density and linear low-density polyethylene composites

Low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE) with different copper contents were prepared by melt mixing. The copper powder particle distributions were found to be relatively uniform at both low and high copper contents. There was cluster formation of copper particles at higher Cu contents, as well as the formation of percolation paths of copper in the PE matrices. The DSC results show that Cu content has little influence on the melting temperatures of LDPE and LLDPE in these composites. From melting enthalpy results it seems as if copper particles act as nucleating agents, giving rise to increased crystallinities of the polyethylene. The thermal stability of the LDPE filled with Cu powder is better than that for the unfilled polymer. The LLDPE composites show better stability only at lower Cu contents. Generally, the composites show poorer mechanical properties (except Young's modulus) compared to the unfilled polymers. The thermal and electrical conductivities of the composites were higher than that of the pure polyethylene matrix for both the LDPE and LLDPE. From these results the percolation concentration was determined as 18.7 vol.% copper for both polymers.     

Morphological changes in annealed ultrahigh molecular weight polyethylene (UHMWPE) gel-films

X-ray diffraction of the UHMWPE gel-films has shown that high heating rate annealing of the films causes deformation of the crystallites. The apparent crystal sizes change with the annealing temperature (T_a). With T_a below 110°C, the dimensions of the crystallites in c-axis (L_c) increase associated with the decreasing in a-axis (L_a) and b-axis (L_b), but the changes are small because of the higher energy barrier. When T_a is raised to 138°C, L_c is nearly doubled its value at 60°C whereas L_b increases and L_a remains unchanged. IR spectral subtraction was applied to reveal the morphological changes of the gel-films upon annealing. It has been found that bands of the same type of conformation have different sensitivities subject to the thermal treatment at different annealing temperatures. The results can be understood by assuming the changing process being completed through chain segments sliding along the (020) sectors. © 1995 John Wiley & Sons, Inc.     

Morphology of gamma-irradiated,low-density,bulk polyethylene

Low-density bulk polyethylene was subjected to gamma radiation doses between 0.1 and 100 megarads. Surfaces produced by fracture at liquid nitrogen temperatures were studied using scanning electron microscopy. The unirradiated polyethylene and polymer irradiated to 0.1 megarad had spherical units with a diameter of about 0.15 micron lying on the fracture surface. The units lay along the edges of the lamellae and were formed by the fracture that was, therefore, not entirely brittle. Polyethylene irradiated between 1 megarad and 100 megarads exhibited spherical voids, with a diameter of about 0.1 micron in the fracture surface. The voids have been produced either by the evolution of gas in the polymer during irradiation, the most likely mechanism, or as a function of the fracture mechanism only.     

Dilute solution properties and phase separation of branched low-density polyethylene

Viscosity, light scattering, and precipitation temperature measurements on dilute solutions of high-density and low-density polyethylene fractions have been carried out and a theory by Flory for phase equilibrium of linear polymers has been extended to branched polymer. From the results, it is shown that the entropy parameter ψ, depends on branching; a method for the determination of long-chain branching in polymer fractions is proposed combining precipitation temperature and molecular weight measurements. The method has been applied to the evaluation of long-chain branching in low-density polyethylene.