The effects of high energy electron beam irradiation on the thermal and structural properties of low density polyethylene

Radiation is currently being exploited to modify polyethylene in order to improve properties for various applications such as hip replacements. This paper thoroughly examines the effects of high energy electron beam irradiation (10 MeV) on low density polyethylene (LDPE) material. ASTM (American Society for Testing and Materials) testing specimens were manufactured from LDPE and subjected to a broad range of doses ranging between 25 and 400 kGy at room temperature in an air atmosphere. Extensive characterisation techniques such as modulated differential scanning calorimetry (MDSC) and the Fourier transform infrared spectroscopy (FTIR) were conducted on the non-irradiated and irradiated samples. While considering the semicrystalline nature of LDPE during the MDSC experiment, the melting temperature (T_m) and the temperature crystallinity (T_c) were calculated. This revealed that the T_m and the T_c decreased in temperature as the irradiation dose increased. The FTIR analysis was implemented to evaluate the presence of polar species such as carbonyl groups and trans-vinylene double bond groups. The IR spectra illustrated that the concentration of characteristic bands for trans-vinylene bonds increased with increasing radiation dose indicating the formation of carbonyl bond groups. Furthermore, the results demonstrated an occurrence of oxidative degradation due to the formation of carbonyl groups at 1718 cm^?1.     

Flame surface modification of polyethylene sheets

High density polyethylene sheets 2 mm thick were flame treated to modify the surface properties. Sheets treated using a flame with air to gas (methane) ratio ∼ 10:1 at different distances between the inner cone tip of the flame and the polymer surface were investigated. Grafting of selected monomers as maleic anhydride, acrylamide and glycidyl methacrylate was attempted by flame treatment of sheets covered with a monomer layer. Good grafting results were obtained with acrylamide and maleic anhydride. The surface temperature-time dependence during the flame treatment was measured with a high resolution thermocouple. Scanning Electron Microscopy (SEM) allowed evidencing a modified thickness of about 120 μ. The chemical surface modification was studied by X ray Photoelectron Spectroscopy (XPS) and Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFT). The hydroxyl, carbonyl and carboxyl content was measured after derivatization with reagents containing an elemental tag to facilitate XPS analysis of surface functional groups. In comparison to the untreated polyethylene, wetting tension and contact angle of the flamed materials showed a strong variation. This variation was almost independent of the distance between the flame and the polymer surface. Adhesion between treated polyethylene and a polyurethane adhesive was determined using T-peel test measurements. High adhesion levels were found with flame treated polyethylene at 5 mm distance. XPS results indicate that when adhesion is high, the hydroxyl is in excess compared to the other measured functions, i.e. carbonyl and carboxyl species.     

Mechanical properties of blends of low density with linear low density polyethylene

Torque values and mechanical properties are reported for blends of low density with linear low density polyethylene (PE). The torque values show that incompatible mixtures in the molten state are formed when the torques of the pure polymers are almost equal. Semicompatible behaviour is shown in the other cases. The mechanical properties indicate that semicompatible mixtures are formed in the solid state. The mechanical properties of these blends are strongly influenced by the linear low density polyethylene only for contents exceeding 25%.     

Water-tree growth in low density polyethylene

In contrast to the degradation of polyolefinic insulations stressed by pure electrical, inhomogeneous fields of sufficient intensity (electrical treeing) the growth of water-trees (which require the presence of water in addition to the electrical load) has been found in some cases to be dependent on the morphology and the state of the resin. This state may be influenced by annealing.Water-treeing is accompanied by local accumulation of water requiring an enlargement of free volumes within the amorphous regions of the semicrystalline polymer. p]The results of our investigations may be interpreted by assuming annealing dependent variations of free volume size-distribution as well as annealing dependent resistance against deformations within the interior of the polymer which allow the growth of waterclusters.     

Chemical modification of fluorinated self-assembled monolayer surfaces by low energy reactive ion bombardment

Reactive collisions of low energy (<100-eV) mass-selected ions are used to chemically modify fluorinated self-assembled monolayer surfaces comprised of alkanethiolate chains CF₃(CF₂)₁₁(CH₂)₂S— bound to Au. Typical experiments were done by using 1-nA/cm² beams and submonolayer doses of reactant ions. Characterization of the modified surface was achieved by in situ chemical sputtering (60-eV Xe^+·) and by independent high mass resolution time-of-flight-secondary ionization mass spectrometry (TOF-SIMS) (15–25-keV, Ga⁺) experiments. Treatment with Si³⁵C1 ₄ ^+· produced a surface from which Xe⁺ sputtering liberated CF₂ ³⁵C1⁺ ions, which suggested Cl-for-F halogen exchange at the surface. Isotopic labeling studies that used Si³⁵Cl₂ ³⁷Cl ₂ ^+· ; and experiments with bromine-containing and iodine-containing projectiles, confirmed this reaction. High mass resolution TOF-SIMS spectra, as well as high spatial resolution images, provided further evidence as to the existence of halogen-exchanged species at the bombarded surface. Analogous Cl-for-F halogen substitution was observed in a model gas-phase reaction. The ion-surface reaction is suggested to proceed through an intermediate fluoronium ion in which the projectile is bonded to the target molecule. The most significant conclusion of the study is that selective chemical modification of monolayer surfaces can be achieved by using reactive ion beams, which lead to new covalent bonds at the surface and in the scattered ions.     

Modification of surface properties of high and low density polyethylene by Ar plasma discharge

Structural changes induced by Ar plasma discharge in low and high density polyethylene (LDPE and HDPE) were studied by different techniques. AFM and SEM methods were used to determine surface morphology, the changes in chemical structure were followed using FTIR and UV-vis spectroscopy. The content and the depth profile of incorporated oxygen was determined by RBS method. The degree of polymer ablation was determined gravimetrically. Standard goniometry was used to determine contact angle and to follow aging of plasma modified polymer. As a result of plasma treatment a lamellar structure or spherulites appear on the surface of HDPE and LDPE, respectively. Pronounced increase of the surface roughness is observed on HDPE contrary to LDPE. Plasma treatment for 400 s leads to the ablation of the surface layer of about 0.6 and 1 μm thick for LDPE and HDPE, respectively. Plasma treatment results in oxidation of the polymer surface layer which is more pronounced in HDPE. Concentration maximum of incorporated oxygen lies 25 nm beneath the sample surface in both polymer types. After exposure to plasma discharge carbonyl, carboxyl and amide groups were detected in the polymer surface layer together with CC bonds either in aromatic or in aliphatic structures. Immediately after the plasma treatment strong decline of the contact angle is observed, the decline being larger in HDPE. Later, in aged specimens the contact angle increases rapidly. The increase, which may be due to rearrangement of degraded structures, is stronger in the specimens exposed to plasma for longer times.     

Photochemical modification of polyethylene surface with aryl azides

It has been shown that the formation of a covalently grafted modifying layer takes place during the photolysis of polycrystalline layers of 2-azidoanthraquinone and 4-azidobenzoyl azide on the surface of polyethylene. Its thickness is determined by the amount of the azide applied, and phenyl isocyanate groups formed by the photolysis of 4-azidobenzoyl azide are prone to further functionalization of the modified surface with primary amines.     

Surface modification of low density polyethylene (LDPE) film by low pressure O2 plasma treatment

In this work, low pressure glow discharge O₂ plasma has been used to increase wettability in a LDPE film in order to improve adhesion properties and make it useful for technical applications. Surface energy values have been estimated using contact angle measurements for different exposure times and different test liquids. In addition, plasma-treated samples have been subjected to an aging process to determine the durability of the plasma treatment. Characterization of the surface changes due to the plasma treatment has been carried out by means of Fourier transformed infrared spectroscopy (FTIR) to determine the presence of polar species such as carbonyl, carboxyl and hydroxyl groups. In addition to this, atomic force microscopy (AFM) analysis has been used to evaluate changes in surface morphology and roughness. Furthermore, and considering the semicrystalline nature of the LDPE film, a calorimetric study using differential scanning calorimetry (DSC) has been carried out to determine changes in crystallinity and degradation temperatures induced by the plasma treatment. The results show that low pressure O₂ plasma improves wettability in LDPE films and no significant changes can be observed at longer exposure times. Nevertheless, we can observe that short exposure times to low pressure O₂ plasma promote the formation of some polar species on the exposed surface and longer exposure times cause slight abrasion on LDPE films as observed by the little increase in surface roughness.     

Viscoelastic properties of linear low density polyethylene melts

The importance of linear low density polyethylene (LLDPE) is being rapidly felt. The combination of favourable production economics and excellent product performance characteristics has enabled this new plastic to gain acceptance for a wide variety of applications. No studies on the rheological properties of LLDPE exist to date. The viscous and elastic properties of this new polymer has now been investigated and unified curves for viscosity and normal stress differences are given. The temperature dependence of the rheological properties of LLDPE has also been studied.     

Elastic moduli and structure of low density polyethylene

Summary Low density polyethylene film is drawn at room temperature four times the original length and subjected to thermal annealing at 60, 80, and 100 °C keeping the film length constant. Long spacing measured by SAXS increased with increasing temperature of annealing; the increase of the long spacing is presumed to be due to the decrease of the number of micelles through relaxation during the annealing. Simultaneous measurement of the changes of the long spacing and the film length by stretching is carried out and stress-extension curves are obtained. The values of the initial moduli of the long spacingE ₁ and the film lengthY are very near to each other. Elastic modulus of the crystal latticeE _c is known to be 235 GN/m² and that of the amorphous regionE _a is found to be 0.15 GN/m². When higher stress is applied than in the case of the initial modulus, the percentage of extension of film is much greater than that of the long spacing. The discrepancy is explained by the increase of the number of micelles through stress crystallization.Dedicated to Professor Dr. K. Ueberreiter on the occasion of his 70th birthday.     

Nonisothermal crystallization kinetics and melting behavior of bimodal medium density polyethylene/low density polyethylene blends

Nonisothermal crystallization kinetics and melting behavior of bimodal-medium-density- polyethylene (BMDPE) and the blends of BMDPE/LDPE were studied using differential scanning calorimetry (DSC) at various scanning rates. The Avrami analysis modified by Jeziorny and a method developed by Mo were employed to describe the nonisothermal crystallization process of BMDPE. The BMDPE DSC data were analyzed by the theory of Ozawa. Kinetic parameters such as the Avrami exponent (n), the kinetic crystallization rate constant (Z_c), the peak temperatures (T_p) and the half-time of crystallization (t_1/2) etc. were determined at various scanning rates. The appearance of double melting peaks and the double crystallization peaks in the heating and cooling DSC curves of BMDPE/LDPE blends indicated that the BMDPE and LDPE could crystallize respectively.     

Lamellar structure in melt crystallized low density polyethylene

The investigation of representative details in the lamellar microstructure of LDPE observed after controlled chlorosulfonation using both EM and SAXD is reported. For this purpose melt crystallized PE with two branch contents (=0.28 and 2.53 branches per 10²CH²) has been prepared using Kanig's technique over a range of temperatures and treatment times. During the first treatment hours the selective incorporation of electron-dense atoms at the lamellar surface produces a macroscopic weight increase, swelling of the sample and a concurrent decrease of the SAXS intensity. The main result, however, is that the thickness of the lamellar structure does not vary with treatment time. After long chlorosulfonation times a saturation of electron-dense atoms within the surface layer and a reduction in the lateral dimensions of the lamellae take place. Optimum conditions for revealing the representative morphology are such as to lead to a weight increase of 50% for PE with=0.28 of branches and only to an increase of 10% for material of branch content represented by an value of 2.53.On leave from Inst. Estructura de la Materia, Madrid-6. Spain     

Synthesis of ion exchange membranes from ozonized high density polyethylene

The synthesis and the characterization of graft copolymers prepared from ozonized high density polyethylene (HDPE) are described. The powder of HDPE was treated with ozone in well defined conditions and then copolymerized with monomers, such as, acrylic acid (AA), N,N-dimethylamino-2 ethylmethacrylate (MADAME) and vinyl phosphonic acid (VPA). Cationic exchange membranes were prepared from the grafted copolymers of AA and VPA and anionic exchange membrane from the grafted copolymer of MADAME. The obtained copolymers were characterized by the grafting rate, FTIR spectroscopy, scaning electronic microscopy, thickness, exchange capacity and electrical resistance.     

Conversion of low density polyethylene foams into auxetic metamaterials

Cellular polymers, such as polyethylene foams, are commonly used in the packaging industry. These materials have short service life and generate a high volume of waste after use. In order to valorize this waste and produce added-value applications, it is proposed to convert these materials into highly efficient energy absorption structures. This was done by modifying the original cellular morphology of the foams (spheroidal or polygonal) into a re-entrant structure to produce auxetic materials. This work presents an optimized process combining mechanical compression and solvent vapor evaporation-condensation leading to low density foams (77–200 kg/m³) having negative Poisson's ratios (NPR). Three series of recycled low density polyethylene (LDPE) foams with an initial density of 16, 21, and 36 kg/m³ were used to optimize the processing conditions in terms of treatment temperature, time, and pressure. From all the samples prepared, a minimum Poisson's ratio of −3.5 was obtained. To further characterize the samples, the final foam structure was analyzed to relate with mechanical properties and compare with conventional foams having positive Poisson's ratios. The results are discussed using tensile properties and energy dissipation which were shown to be highly improved for auxetic foams. Overall, the resulting foams can be used in several applications such as sport and military protection equipment.     

Carbon ion irradiation induced surface modification of polypropylene

Polypropylene was irradiated with ¹²C ions of 3.6 and 5.4 MeV energies in the fluence range of 5×10¹³–5×10¹⁴ ions/cm² using 3 MV tandem accelerator. Ion penetration was limited to a few microns and surface modifications were investigated by scanning electron microscopy. At the lowest ion fluence only blister formation of various sizes (1–6 μm) were observed, but at higher fluence (1×10¹⁴ ions/cm²) a three-dimensional network structure was found to form. A gradual degradation in the network structure was observed with further increase in the ion fluence. The dose dependence of the changes on surface morphology of polypropylene is discussed.     

Effect of chemical modification on structural and energy characteristics of the surface of polyethylene and polyvinyl chloride films

The possibility of controlling the surface energy and wettability of polyvinyl chloride and polyethylene films by chemical gas-phase modification was studied. The surface of the initial and chemically modified polymeric materials was examined by atomic force microscopy.     

Natural weathering test for films of various formulations of low density polyethylene (LDPE) and linear low density polyethylene (LLDPE)

Natural (outdoor) weathering test was performed to investigate the UV stability of thin films (0.06 mm) of linear low density polyethylene (LLDPE) and low density polyethylene (LDPE). The PE films were prepared from various formulations of LLDPE and LDPE resins. Some of these films contained a single high molecular mass HALS only, along with a primary antioxidant (i.e. Irganox 1010) and a secondary antioxidant (i.e. Irgafos 168 or Alkanox TNPP), while others contained HALS and UVA (i.e. Chimassorb 81 or Tinuvin P or Tinuvin 326) along with these antioxidants. The HALS used was either an oligomeric or a synergistic mixture of a high molecular mass (HMM) hindered amine stabilizer and co-additives. The UV stability was investigated by exposing the prepared films at 45° towards south in the direct sunshine up to 365 days. Fifty percent of tensile strength retention was determined for all these exposed films and it was found that the films containing a single HALS gained improved UV stability by about two to 12 fold over the pure films. On the other hand, films that contained a combination of HALS and UVA obtained further improved UV stability over the films containing a single HALS (both have antioxidants). Films containing a single HALS reached 50% TS retention within 205 days, whereas, films containing a combination of HALS and UVA reached 50% TS retention within 590 days, which is about three times further improvement in UV stability.     

Compatibilizing effect of isocyanate functional group on polyethylene terephthalate/low density polyethylene blends

To evaluate the compatibilizing effects of isocyanate (NCO) functional group on the polyethylene terephthalate/low density polyethylene (PET/LDPE) blends, LDPE grafted with 2-hydroxyethyl methacrylate-isophorone diisocyanate (LDPE-g-HI) was prepared and blended with PET. The chemical reaction occurred during the melt blending in the PET/LDPE-g-HI blends was confirmed by the result of IR spectra. In the light of the blend morphology, the dispersions of the PET/LDPE-g-HI blends were very fine over the PET/LDPE blends. DSC thermograms indicated that PET microdispersions produced by the slow cooling of the PET/LDPE-g-HI blends were largely amorphous, with low crystallinity, due to the chemical bonding. The tensile strengths of the PET/LDPE-g-HI blends were higher than those of the PET/LDPE blends having a poor adhesion. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 447–453, 1998     

Influence of crosslinking on physical properties of low density polyethylene

Influence of crosslinking on physical properties of low density polyethylene was studied.The results indicated that,at low degrees of crosslinking,the network hardly affects the crystallinity,elastic modulus(E) and yielding stress while it improves the tensile strength and strain at break simultaneously.Tensile strength reaches a maximum of about 24 MPa at 1.5 phr dicumyl peroxide(DCP) then decreases to a constant value of about 18 MPa due to decrease of crystallinity.E reaches its maximum at 0.5 phr DCP corresponding to gel fraction of about 75%without marked change in crystallinity.The crosslinked polyethylene exhibits two yielding processes,and both yielding stresses approximately linearly depend on crystallinity.