Morphology and crystallization of blends of linear low density polyethylene with a semiflexible liquid crystalline polymer

The morphology and the crystallization behavior of blends of linear low density polyethylene (LLDPE) with an experimental sample of a semiflexible liquid crystalline polymer (SBH 112 by Eniricerche, Italy) have been studied by differential scanning calorimetry (DSC), polarized optical microscopy (POM) and scanning electron microscopy (SEM). The blends possess a two-phase morphology, due to immiscibility of the two components. SEM observations show that dispersion of the minor SBH phase is favored at low (相似文献   

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.     

Hydroperoxide formation in irradiated polyethylene

Spectroscopic analysis for hydroperoxide in irradiated ultrahigh molecular weight polyethylene, on the basis of the formation of a nitrate derivative after exposure to dilute nitric oxide, is examined. Hydroperoxide is found to be an important intermediate in the oxidation of polyethylene and is believed to result from hydrogen abstraction reactions by peroxy radicals in a polyethylene matrix. During γ irradiation in air, the rates of bimolecular combination of peroxy radicals on the surface to form ketones or hydrogen abstraction to form hydroperoxides are similar. However, as a result of bimolecular combination, the concentration of peroxy radicals decreases. After irradiation and storage in ambient air, isolated peroxy radicals below the polymer surface induce a slow chain reaction leading to a long-term increase in hydroperoxides and carbonyls. Differences in hydroperoxide and oxygen content for samples irradiated in air or vacuum are primarily confined to or near the surface. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3309–3316, 1999     

Effects of charge trapping in gamma irradiated and accelerated aged low-density polyethylene

The effects of gamma irradiation in air and accelerated aging of low-density polyethylene (LDPE) were studied through dielectric loss (tan δ) analysis in the temperature range from 25 to 325 K and using thermally stimulated discharge current (TSDC) measurements. The radiation-induced oxidation was observed using IR spectroscopy. The influence of radiation dose and accelerated aging on the intensities and the positions of the γ and β dielectric relaxation maxima were correlated with maxima of TSDC measurements, and found to be strongly dependent upon the changes in the microstructure of the amorphous phase and on the surface of the crystallites induced by oxidation and crosslinking.     

Nonisothermal crystallization and melting behavior of mPE/LLDPE/LDPE ternary blends

Nonisothermal crystallization kinetics of ternary blends of the metallocence polyethylene (mPE), low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE) were studied using DSC at various scanning rates. The Ozawa theory and a method developed by Mo were employed to describe the nonisothermal crystallization process of the two selected ternary blends. The results speak that Mo method is successful in describing the nonisothermal crystallization process of mPE/LLDPE/LDPE ternary blends, while Ozawa theory is not accurate to interpret the whole process of nonisothermal crystallization. Each ternary blend in this study shows different crystallization and melting behavior due to its different mPE content. The crystallinity of the ternary blends rises with increasing mPE content, and mPE improve the crystallization of the blends at low temperature. The crystallization activation energy of the five ternary blends that had been calculated from Vyazovkin method was increased with mPE content, indicating that the more mPE in the blends, the easier the nucleus or microcrystallites form at the primary stage of nonisothermal crystallization. LLDPE and mPE may form mixed crystals due to none separated-peaks were observed around the main melting or crystallization peak when the ternary blends were heating or cooling. The fixed small content of LDPE made little influence on the main crystallization behavior of the ternary blends and the crystallization behavior was mainly determined by the content of mPE and LLDPE.     

Supercritical CO<Subscript>2</Subscript> assisted preparation of open-cell foams of linear low-density polyethylene and linear low-density polyethylene/carbon nanotube composites

The open-cell structure foams of linear low-density polyethylene (LLDPE) and linear low-density polyethylene (LLDPE)/multi-wall carbon nanotubes (MWCNTs) composites are prepared by using supercritical carbon dioxide (sc-CO₂) as a foaming agent. The effects of processing parameters (foaming temperature, saturation pressure, and depressurization rate) and the addition of MWCNTs on the evolution of cell opening are studied systematically. For LLDPE foaming, the foaming temperature and saturation pressure are two key factors for preparing open-cell foams. An increase in temperature and pressure promotes both the cell wall thinning and cell rupture, because a high temperature results in a decrease in the viscosity of the polymer, and a high pressure leads to a larger amount of cell nucleation. Moreover, for the given temperature and pressure, the high pressurization rate results in a high pressure gradient, favoring cell rupture. For LLDPE/MWCNTs foaming, the addition of MWCNTs not only promotes the cell heterogeneous nucleation, but also prevents the cell collapse during cell opening, which is critical to achieve the open-cell structures with small cell size and high cell density.     

Effects of thermal ageing on mechanical and thermal behaviors of linear low density polyethylene pipe

The mechanical and thermal behaviors of linear low density polyethylene (LLDPE) pipe with variation in thermal exposure time were studied. The prolongation of thermal exposure time leads to a progressive increase, until 6000 h, in tensile strength and a slight increase in hardness, while a proportional decrease in elongation at break. These results can be explained by the increase of crystallinity, followed by the increase of crosslinking density and the decrease in chain mobility due to thermal oxidation as the exposure time increases. The additional ageing to the antioxidant-depleted LLDPE pipe induces the formation of T2 endotherm, which leads to a negative effect in mechanical properties. Long-term hydrostatic pressure test result implies the existence of transition point from ductile to brittle fracture in terms of the thermal exposure time. Chemiluminescence (CL) and oxidation induction time (OIT) tests are employed to monitor the thermo-oxidative degradation of LLDPE pipe. The CL emission intensity increases with increasing with thermal exposure time. Furthermore, the OIT result suggests that after 6000 h of the thermal ageing, the depletion of antioxidant originally added in LLDPE pipe occurs. Fourier transform-infrared spectroscopy results show the increase of carbonyl (-CO) and hydroxyl (O-H) function groups on the surface of thermally exposed LLDPE pipe. This result suggests that the hydrocarbon groups locally undergo the oxidation on the LLDPE surface due to thermal degradation.     

Surface studies of ultra‐high molecular weight polyethylene irradiated with high‐energy pulsed electron beams in air

Ultra‐high molecular weight polyethylene (UHMWPE) was irradiated in air with high‐energy (9 MeV), pulsed electron beams to doses ranging from 2.5 to 100 Mrad and subsequently heat treated at 120°C for a time period of 120 min. Surface characterization of the target side of irradiated UHMWPE samples was carried out both before and after the heat treatment by means of attenuated total reflection Fourier‐transform infrared (FTIR/ATR) spectroscopy and microhardness measurement. The obtained results provided further evidence supporting our earlier observation (Tretinnikov, O. N.; Ogata, S.; Ikada, Y. Polymer 1998, 39, 6115) that thermal decomposition of hydroperoxides formed upon irradiation of UHMWPE with high‐energy, pulsed electron beams in air leads to surface crosslinking, and the subsequent surface hardening of the irradiated polymer. Importantly, we found that this phenomenon has the highest contribution to the surface hardness enhancement of the polymer when the radiation dose is in the range of 10–30 Mrad. In addition, we found that this irradiation and subsequent heat treatment of UHMWPE in air does not lead to formation of carbonyl‐containing products unless the radiation dose exceeds 20 Mrad. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1503–1512, 1999     

Rheological investigation of the influence of molecular structure on natural and accelerated UV degradation of linear low density polyethylene

Metallocene and Ziegler-Natta (ZN) linear low density polyethylenes (LLDPEs) of different branch types and contents as well as linear high density polyethylene (HDPE) were exposed to natural and accelerated weather conditions. The degree of UV degradation of exposed samples was measured by rheological techniques and results were compared with unexposed polymers. Dynamic shear measurements were performed in an ARES rheometer in the linear viscoelastic range. The degree of enhancement or reduction in viscosity and elasticity was used as a measure of the degree of cross-linking or chain scission, respectively. The degradation results of LLDPE suggest that both cross-linking and chain scission are taking place. Chain scission dominated the degradation at high levels of short chain branching (SCB) and long exposure times. The degradation mechanism of m-LLDPE and ZN-LLDPE is similar; however, m-LLDPE showed a higher degradation rate than ZN-LLDPE of similar M_w and average SCB. ZN-LLDPE was found to be more stable than a similar m-LLDPE. Comonomer type had little influence on degradation. Dynamic shear rheology was very useful in revealing the influence of different molecular parameters and it exposed the degradation mechanism.     

Structure and properties of compatibilized recycled poly(ethylene terephthalate)/linear low density polyethylene blends

Blends of recycled poly(ethylene terephthalate) (R-PET) and linear low density polyethylene (LLDPE) were compatibilized with poly(styrene-ethylene/butyldiene-styrene) (SEBS) and maleic anhydride-grafted poly(styrene-ethylene/butyldiene-styrene) (SEBS-g-MA). Effects of compatilizer were evaluated systematically by study of mechanical, thermal and morphology properties together with crystallization behavior of PET. Tensile properties of the blends were improved effectively by the addition of 10 wt% SEBS-g-MA, elongation at break and charpy impact strength were increased with the increasing content of compatilizer. SEBS-g-MA is more effectual on mechanical properties of R-PET/LLDPE blends than SEBS. DSC analysis illustrates crystallinities of PET and LLDPE were increased by compatilizer at annealing condition. WAXD and FT-IR spectra show that annealing influences crystallization behavior of PET. Different compatilizer content results in different morphology structure, in particular, higher SEBS-g-MA content can induce the formation of a salami microstructure.     

Effect of oscillatory shear on the mechanical properties and crystalline morphology of linear low density polyethylene

In this study,effects of oscillatory shear with different frequencies(0-2.5 Hz) and amplitudes(0-20 mm) on the mechanical properties and crystalline morphology of linear low density polyethylene(LLDPE) were investigated.It was found that the mechanical properties of LLDPE are improved because of the more perfect crystalline structure when LLDPE crystallizes under low-frequency and small-amplitude(0.2 Hz/4 mm) oscillatory shear.The mechanical properties can be further improved by increasing either the frequency or the amplitude of oscillatory shear.The Young's modulus and tensile strength of LLDPE are improved by 27% and 20%,respectively,when the frequency is increased to 2.5 Hz and the amplitude is maintained at 4 mm; while the Young's modulus and tensile strength are improved by 49% and 47%,respectively,when the amplitude is increased to 20 mm and the frequency is remained as 0.2 Hz.The crystallinity and microstructure of LLDPE under different oscillatory shear conditions were investigated by using differential scanning calorimetry,wide angle X-ray diffraction and scanning electron microscopy to shed light on the mechanism for the improvement of mechanical properties.     

Positron annihilation in irradiated and unirradiated polyethylenes

Positron lifetimes were measured for four kinds of polyethylene samples and were resolved into four components. The temperature dependence of the two longlived components was examined in detail. In agreement with other results, the longest lived component could be reasonably assigned to ortho-positronium located in amorphous regions. This component was shown to be sensitive to the defects in high-density polyethylene introducted in the course of its production. Both the intensity and the lifetime of the second longest lived component were structure insensitive, i.e., they did not change even on passing through the melting point. This component has been tentatively assigned to a positronium compound state. The effect of gamma-ray irradiation was also examined. Although the intensity (I₄) of the longest lived component was reduced by the irradiation, correlation between I₄ and the free radical concentration was poor, and the reduction in I₄ caused by the irradiation is considered to be due to structure change and not to chemical reasons.     

Radiation resistance and structural transitions in polymer-based composites irradiated by electrons

Mechanical relaxation in different types of electron-irradiated foiled laminates is experimentally studied. It is shown that the interaction of the polymer binder of composites with the metallic coating can appreciably affect radiation-induced alterations both in characteristics of structural transitions in a layer adjacent to metal coating and radiation resistance of a material as a whole.     

Prediction of the flow‐induced crystallization in high‐density polyethylene by a continuum model

In this work, the isothermal flow‐induced crystallization (FIC) of high‐density polyethylene (HDPE) under a simple shear flow was investigated. Two experimental modes, including steady shear and preshear treatment, were performed on the polymer melt. Based on the nonequilibrium thermodynamic theory, the FIC process of HDPE was predicted through the modification of a continuum FIC model. The theoretical predictions of the evolution of both the viscosity in steady shear flow and the complex modulus under preshear treatment were essentially related to the crystallinity of HDPE, in agreement with the experimental findings. Both experimental and predicted results showed that the applied flow field could accelerate the crystallization kinetics of HDPE significantly. However, the effect of the intensity of shear flow on the crystallization of HDPE was finite, showing a saturation phenomenon, namely, the accelerated degree of crystallization tending to level off when the shear rate was large enough. In additional, it was found that the predicted crystallinity of HDPE was very low in induction period either in steady shear flow or by preshear treatment. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 531–538, 2009     

Preparation of polypseudorotaxane composed of linear and cyclic polyethylene oxides and its application to solid polymer electrolyte

The mixture of linear polyethylene glycol with molecular weight of 20,000 (l-PEG_20K) and cyclic polyethylene glycol with molecular weight of 1,000 (c-PEG_1K) was ultrasonicated in acetonitrile. After evaporating the solvent, the residue was analyzed by DSC to show a remarkable decrease of crystallization temperature. Such a large crystallization suppression was not observed when linear polyethylene glycol with molecular weight of 1,000 (l-PEG_1K) was added instead of c-PEG_1K. Further, the mixture of cyclic polyethylene glycols (c-PEG_6K and c-PEG_1K) did not exhibit a significant crystallization suppression. These experimental results indicated that formation of polypseudorotaxane through ultrasonication-assisted ring penetration played an important role in the crystallization suppression. Ionic conductivities of the polypseudorotaxane-based polymer electrolytes prepared from polyethylene oxide with molecular weight of 600,000 (PEO_600K) and c-PEG_1K showed conductivity enhancement especially at low Li salt concentration.     

Sorption and transport of linear alkane hydrocarbons in biaxially oriented polyethylene terephthalate

Equilibrium sorption and uptake kinetics of n‐butane and n‐pentane in uniform, biaxially oriented, semicrystalline polyethylene terephthalate films were examined at 35 °C and for pressures ranging from 0 to approximately 76 cmHg. Sorption isotherms were well described by the dual‐mode sorption model. Sorption kinetics were described either by Fickian diffusion or a two‐stage model incorporating Fickian diffusion at short times and protracted polymer structural relaxation at long times. Diffusion coefficients increased with increasing penetrant concentration. n‐Butane solubility was lower than that of n‐pentane, consistent with the more condensable nature of n‐pentane. However, n‐butane diffusion coefficients were higher than those of n‐pentane. Infinite‐dilution, estimated amorphous phase diffusion and solubility coefficients were well correlated with penetrant critical volume and critical temperature, respectively. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 1160–1172, 2001     

Morphological,rheological, crystalline and mechanical properties of ethylene-vinyl acetate copolymer/linear low-density polyethylene/amphiphilic graphene oxide nanocomposites

Chemical modification of graphene oxide has become a popular method for imparting unique properties to extend its application. Here, we show a simple way to synthesize amphiphilic graphene oxide (AGO) by grafting quaternary ammonium salt onto GO sheets. The AGO sheets not only showed high thermal stability and good dispersion in many polar and non-polar solvents in comparison to GO sheets but also the chemical modification maintained the two-dimensional structure. As a result, the AGO sheets improve the interfacial interaction between ethylene-vinyl acetate copolymer (EVA) and linear low-density polyethylene (LLDPE). Because of the large size of AGO, the location of AGO is very dependent on the mixing strategy. The AGO was dispersed in the EVA phase when AGO was mixed first with EVA and then with LLDPE, whereas it was confined in the LLDPE phase when AGO was mixed first with LLDPE and then with EVA. AGO sheets were found at the interface of LLDPE and EVA when AGO, EVA, and LLDPE were mixed together, suggesting that AGO has a high interfacial interaction with both LLDPE and EVA. These high interfacial interactions lead to high tensile strength, Young's modulus, complex viscosity and crystallization temperature in comparison to the EVA/LLDPE blends without AGO sheets.     

Radiation-induced crosslinking of polyethylene in the presence of acetylene: A gel fraction,UV-visible,and ESR spectroscopy study

Electron-spin resonance (ESR) and altraviolet (UV) visible spectroscopic evidence has been found for the formation of diene, triene and tetraene, following the irradiation of polyethylene in the presence of acetylene. The polyenes are formed by a mechanism which is different from that observed under vacuum or with inert gas saturation. The sum of the G (polyene) values obtained by UV spectroscopy is almost half that of initial radical formation. It is concluded that polyene bridges, predominantly diene, form crosslinks between radical pairs. G (X) values determined from gel fraction data, using Saito-Kang-Dole theory, are found to be greatly in error and misrepresentative of crosslink changes. © 1993 John Wiley & Sons, Inc.     

The dependence of slow crack growth in a linear polyethylene on test temperature and morphology

The slow crack growth behavior of a linear polyethylene with different morphologies was studied by using three point bending with a single edge notched specimen at testing tem-peratures from 30 to 80°C. The morphology was varied by annealing the quenched material at temperatures from 86°C to 135°C. It was found that at test temperatures of 60°C or less, the changes in failure time with annealing temperature are very similar to the change in density with a maximum at 130°C. At testing temperatures above 60°C, the relationship of between failure time and annealing temperature is altered when the test is in the range of the α transition temperature. These results indicate that with respect to slow crack growth in the case of a homopolymer the strength of the crystals is relatively more important than the number of tie molecules. © 1993 John Wiley & Sons, Inc.