Effect of functionalized graphene on the physical properties of linear low density polyethylene nanocomposites

Dodecyl amine-modified graphene (DA-G)/linear low density polyethylene (LLDPE) nanocomposites were prepared through solution mixing. Field emission scanning electron microscopy analysis revealed homogeneous dispersions of graphene layers in the nanocomposites. X-ray diffraction analysis showed that the average crystallite size of the nanocomposites was increased. However, the % crystallinity was found to decrease due to the formation of a random interface. Dynamic mechanical analysis showed that the storage moduli of the nanocomposites were much higher than that of neat LLDPE. The nanocomposites were also more thermally stable than neat LLDPE. Isothermal thermogravimetry showed that homogeneously distributed graphene could act as a good inhibitor during thermal degradation of the nanocomposites. Differential scanning calorimetry showed that the crystallization temperature of the nanocomposites increased with increasing DA-G content. Thermomechanical analysis showed that the dimensional stability of the nanocomposites was significantly increased by the addition of the DA-G. The coefficients of thermal expansion decreased with increasing DA-G content. The oxygen and nitrogen permeability of the nanocomposites was lower than that of neat LLDPE.     

Effect of electron-irradiation on cross-link density and crystalline structure of low- and high-density polyethylene

Low- and high-density polyethylenes (LDPE and HDPE) were cross-linked in solid state by electron beam irradiation. Molar mass between cross-link joints, M_c, and cross-link density,ν, were calculated using rubber elasticity theory and hot set data. The results showed that the ν and creep modulus increased and creep strain and M_c decreased with increasing irradiation dose. As compared to HDPE, the LDPE had higher ν and lower M_c values at a similar irradiation dose. X-ray analysis and differential scanning calorimetry investigation of first heating cycle revealed no changes in crystalline structure of the irradiated samples. This was attributed to immobilization of radicals frozen in the crystalline phase. As a result of hindered mobility of the polymeric chains, these radicals were not able to cross-link the chains in the crystalline region. However, after melting of the crystals and during subsequent re-solidification process, different levels of crystallinity were developed depending on the applied irradiation dose. The irradiated samples with higher dose had lower crystallization and melting temperatures with reduced crystallinities. These confined crystallization behaviors, observed after a series of cooling and heating cycles, could be attributed to the decrease in the M_c values. The length of chain segment needed for usual crystallization by chain folding is decreased due to formation of cross-link joints and hence the crystallization process was hindered.     

Influence of cross-link density on rheological properties of temperature-sensitive microgel suspensions

The influence of the cross-link density on rheological properties of thermosensitive microgels was investigated. The temperature-sensitive hydrogel particles consisted of poly (N-isopropylacrylamide) (PNiPAM) chemically cross-linked with several different molar ratios of N,N′-methylenebisacrylamide. The variation of cross-link density leads to soft spheres that possess a different particle interaction potential and a different swelling ratio. With increasing temperature the microgel particles decrease in size and with it the effective volume fraction, which leads to strong changes in rheological properties. The relative zero-shear viscosity and the plateau modulus at different temperatures superpose to mastercurves when plotted versus the effective volume fraction. Up to an effective volume fraction of 0.5 the microgels behaved like hard spheres and the maximum volume fraction, as determined from the divergence of the zero-shear viscosity, was mainly dominated by the polydispersity of the spheres and not by the cross-link density. The plateau modulus, on the other hand, revealed soft-sphere behavior and the interaction potential became softer with decreasing cross-linker content. Received: 15 December 1999 Accepted: 15 February 2000     

Linear low density polyethylene: Microstructure and sealing properties correlation

The microstructures of four commercial linear low density polyethylenes (LLDPE) were evaluated and correlated with their sealing properties. Atomic Force Microscopy (AFM), Temperature Rising Elution Fractionation (TREF), Differential Scanning Calorimetry (DSC) and Crystallization Analysis Fractionation (CRYSTAF) experiments revealed that the comonomer distribution is one of the main factors that influence the sealing properties. The superior sealing performance showed by metallocene LLDPE samples in comparison to Ziegler-Natta LLDPE samples can be attributed to their well balanced chemical composition distribution.     

Recycling of crate material: Weatherability of stabilised recycled high density polyethylene

The uv stabilising effect of various stabilisers added to degraded, yellow-pigmented, high density polyethylene (HDPE) crate material originating from crates that have been in use for 10–13 years was investigated. For that purpose, outdoor and Xenotest-1200 exposure tests were carried out, and rates of oxygen absorption during photo-oxidation were determined. The uv stability of the degraded HDPE could be considerably improved—in particular, by the addition of a hindered amine type stabiliser (Tinuvin 770). The uv stability attainable is such that reuse of recycled crate material for the production of new crates is, after upgrading, a realistic possibility.A comparison of the results of outdoor and accelerated ageing showed that the acceleration factors strongly depend on the type of stabiliser applied. This can be attributed to the different mechanisms of stabilisation by which the various additives act.The uv stability of the recycled polymer appeared to be almost independent of the degree of degradation of the crates before recycling. This implies that it is not necessary to exclude from recycling heavily degraded crates which had failed due to brittle fracture.     

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.     

Natural weathering of low density polyethylene—III: Mechanical properties

Mechanical properties are presented for LDPE samples naturally weathered for various periods both in the absence and the presence of a light stabilizer system. Qualitative property/structure correlations are proposed, taking into account also previous structural results for the same samples.     

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%.     

Influence of the structure of linear density polyethylene on the rheological and mechanical properties of blends with low density polyethylene

Rheological and mechanical data on LLDPE/LDPE blends are presented in order to evaluate the influence of the structure of LLDPE on the properties of these blends. The influence of the comonomer type is negligible, while the molecular weight exerts an important effect on properties. For both rheological and mechanical properties, the blends made from LLDPE of large MFI seem the more interesting.     

Natural weathering of low density polyethylene: Part 2—Dielectric properties

Dielectric properties of naturally weathered low density polyethylenes have been measured as a function of temperature, frequency and weathering time. The results show that dielectric loss, dielectric constant and the β transition increase with the weathering time. However, the deterioration of the dielectric properties is strongly reduced by the presence of a uv stabilizer system.The changes in the dielectric properties can be qualitatively explained by the formation of polar groups and crosslinks.     

Mechanical properties of high‐density polyethylene and crosslinked high‐density polyethylene in crude oil and its components

The tensile and stress‐relaxation properties of an uncrosslinked and a loosely silane‐crosslinked high‐density polyethylene exposed to organic “crude‐oil” penetrants were assessed. The measurements were performed on penetrant‐saturated samples, surrounded by the organic liquid throughout the experiment. The penetrant solubilities in the two polymers were similar and in accordance with predicted values based on the solubility parameter method. The stiffness and strength of the swollen samples were significantly less than those of the dry samples, indicating a plasticization of the amorphous component. Raman spectroscopy on polyethylene exposed to deuterated n‐hexane revealed a penetrant‐induced partial melting/dissolution of the crystal surface and an intact crystal core component. The stress‐relaxation rates, within the time frame of the experiment (～1 s to 18 h), were approximately the same, independent of silane‐crosslinks and the presence of penetrants. This indicated that the mechanical α‐relaxation, which is the main relaxation process occurring in the measured time interval, was not affected by the penetrants. Consequently, its rate seemed to be independent of the crystal surface dissolution (decrease in the content of crystal‐core interface). The shape of the “log stress–log time” curves of the swollen samples was, however, different from that of the dry samples. This was most likely attributed to a time‐dependent saturation of penetrant to a higher level associated with the stretched state of the polymer sample. The silane crosslinks affected only the elongation at break, which was less than that of the uncrosslinked material. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 641–648, 2006     

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.     

Morphology and mechanical properties of blown films of a low‐density polyethylene/linear low‐density polyethylene blend

The morphologies of films blown from a low‐density polyethylene (LDPE), a linear low‐density polyethylene (LLDPE), and their blend have been characterized and compared using transmission electron microscopy, small‐angle X‐ray scattering, infrared dichroism, and thermal shrinkage techniques. The blending has a significant effect on film morphology. Under similar processing conditions, the LLDPE film has a relatively random crystal orientation. The film made from the LDPE/LLDPE blend possesses the highest degree of crystal orientation. However, the LDPE film has the greatest amorphous phase orientation. A mechanism is proposed to account for this unusual phenomenon. Cocrystallization between LDPE and LLDPE occurs in the blowing process of the LDPE and LLDPE blend. The structure–property relationship is also discussed. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 507–518, 2002; DOI 10.1002/polb.10115     

Vitrimers: directing chemical reactivity to control material properties

The development of more sustainable materials with a prolonged useful lifetime is a key requirement for a transition towards a more circular economy. However, polymer materials that are long-lasting and highly durable also tend to have a limited application potential for re-use. This is because such materials derive their durable properties from a high degree of chemical connectivity, resulting in rigid meshes or networks of polymer chains with a high intrinsic resistance to deformation. Once such polymers are fully synthesised, thermal (re)processing becomes hard (or impossible) to achieve without damaging the degree of chemical connectivity, and most recycling options quickly lead to a drop or even loss of material properties. In this context, both academic and industrial researchers have taken a keen interest in materials design that combines high degrees of chemical connectivity with an improved thermal (re)processability, mediated through a dynamic exchange reaction of covalent bonds. In particular vitrimer materials offer a promising concept because they completely maintain their degree of chemical connectivity at all times, yet can show a clear thermally driven plasticity and liquid behavior, enabled through rapid bond rearrangement reactions within the network. In the past decade, many suitable dynamic covalent chemistries were developed to create vitrimer materials, and are now applicable to a wide range of polymer matrices. The material properties of vitrimers, however, do not solely rely on the chemical structure of the polymer matrix, but also on the chemical reactivity of the dynamic bonds. Thus, chemical reactivity considerations become an integral part of material design, which has to take into account for example catalytic and cross-reactivity effects. This mini-review will aim to provide an overview of recent efforts aimed at understanding and controlling dynamic cross-linking reactions within vitrimers, and how directing this chemical reactivity can be used as a handle to steer material properties. Hence, it is shown how a focus on a fundamental chemical understanding can pave the way towards new sustainable materials and applications.

In this minireview, we survey recent advances in the development of vitrimer materials. Focus on how to chemically control their material properties is used to highlight challenges for boosting the potential of this emerging class of polymer materials.     

Flammability of radiation cross-linked low density polyethylene as an insulating material for wire and cable

Various formulations of low-density polyethylene blended with ethylene vinyl acetate were prepared to improve the flame retardancy for wire and cable applications. The prepared formulations were cross-linked by γ-rays to 50, 100, 150 and 200 kGy in the presence of trimethylolpropane triacrylate (TMPTA). The effect of thermal aging on mechanical properties of these formulations were investigated. In addition, the influence of various combinations of aluminum trihydroxide and zinc borate as flame retardant fillers on the flammability was explored. Limiting oxygen index (LOI) and average extent of burning were used to characterize the flammability of investigated formulations. An improved flame retardancy of low density polyethylene was achieved by various combinations of flame ratardant fillers and cross-linking by gamma radiation.     

Thermal properties of high density polyethylene composites with natural fibres: Coupling agent effect

High density polyethylene composites with curaua fibres were prepared using an intermeshing co-rotating extruder and two different coupling agents. The thermal stability of the components was studied by thermogravimetric and differential scanning analysis, as well as by the oxidation induction time. Maleic anhydride grafted polyethylene, used as coupling agent, affected the composite stability more markedly than did poly(ethylene-co-vinyl acetate). However, oxidation induction times were analogous for composites with and without coupling agents. Results also indicated that a higher fibre-matrix interaction precludes the crystallinity enhancement caused by the fibre.     

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.     

Rheological, mechanical and transport properties of blown films of high density polyethylene nanocomposites

In this work, high density polyethylene (HDPE) was mixed in a twin screw extruder with organophilic treated clay, Cloisite 20A, and a compatibility agent, a HDPE grafted with maleic anhydride (PEMA). The screw profile was changed from a less dispersive (Profile 1) to a high dispersive configuration (Profile 2). A masterbatch procedure was used to obtain a final organoclay concentration of 5 wt.%. Both profiles allowed the intercalation of the HDPE into the clay, increasing the clay’s gallery distance to 3.7 nm. However, the samples produced with Profile 2 (Nano 2 samples) were more elastic and had a more stable structure than the samples produced with Profile 1. Therefore, two kind of blown films of Nano 2 samples were made: FN1 and FN2. The last one was blown at a higher screw velocity than the FN1. Both films had an increase of 95% in the elastic modulus and a reduction of 60% and 45% in O₂ and water vapor permeability rates, respectively, compared to the film of pure HDPE. However, the FN2 structure was more unstable than the FN1 structure. It was concluded that both screw profiles gave the same level of HDPE intercalation in the clay; however, the more dispersive profile produced more time-stable and elastic structures. The increase in the elongation rate during the film blowing process produced also more time-stable morphologies; however, this higher orientation created matrix/filler interfacial defects.     

Morphology and tensile properties of glass bead filled low density polyethylene composites: MATERIAL PROPERTIES

The mechanical properties of glass bead filled low density polyethylene (LDPE) composites in tension have been investigated by using an Instron material testing machine. It is found that with increase of the glass bead weight fraction (φ) the tensile modulus (E_c) and the tensile yield stress (σ_yc) increase as a form of nonlinear function but contrary to the elongation strain at break; the correlation between E_c and φ accords with the logarithmic mixing rule and the relationship between σ_y and the volume fraction (φ_f) can be described by means of a second order equation; the effects of the glass bead diameter on the mechanical properties are not large; when φ and the bead size are suitable, the enhanced toughness effect of the filled-systems is more significant; the tensile strength of the glass bead filled system pretreated with a coupling agent are somewhat greater than those of the untreated system. In addition, the morphology of the samples is studied to explain the relationship between the micro-structure and the mechanical properties of the composites.     

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.