Numerical simulation of annealing of dose effects in radiation-induced conductivity of polymers

Numerical simulation of dose effects in radiation-induced conductivity in polymers and of roomtemperature annealing of these effects was performed in the formalism of the generalized Rose-Fowler-Vaisberg model, which allows for bipolar electron transport of charge carriers and for generation of radiationinduced traps. It was shown that neither the distinct dose effect nor its long annealing time can be explained unless the radiation-chemical aspect of irradiation is taken into account. It was found that satisfactory fit of theoretical prediction to published data for a number of polymers (PS, PET, LDPE, polyvinylcarbazole) can be achieved in the case of the reasonable choice of the parameter for the injection of radiation-induced traps (free radicals), although certain difficulties are met. This coincidence is attainable if the other parameters of the model are fixed and their values have been determined in independent experiments.     

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.     

Temperature variations of NMR second moments for drawn tapes based on polypropylene and polyethylene

Isotropic and drawn tapes prepared from isotactic polypropylene (PP), low density polyethylene (LDPE) and their blend PP/LDPE (70/30) have been studied by broad‐line nuclear magnetic resonance (NMR) in the temperature range from 120 up to 320 K. The glass‐transition temperatures, T_g, for studied samples have been determined from the temperature dependencies of the NMR second moments. It was found that the NMR spectra and their second moments are additive for isotropic blend in the whole temperature range, while a significant differences from addition rule appear for drawn PP/LDPE blend, when weighted average second moment is calculated by means of second moments of equally drawn homopolymers. It was found out that the LDPE component in the blend is drawn to a greater extent than PP component.     

Orientation of LDPE Crystals from Microscale to Nanoscale via Microlayer or Nanolayer Coextrusion

The microlayer or nanolayer coextrusion of hundreds or thousands of alternating low density polyethylene(LDPE)/polystyrene(PS) microlayers or nanolayers were used to study the orientation of LDPE crystals in the confined quasi-two-dimensional or two-dimensional space. The clear and continuous layer structures from microscale to nanoscale can be found in SEM images. The morphology evolution of LDPE crystals in the confined microlayer or nanolayer can be varied from 3D spherulites, 2D spherulites, stacked edge-on lamellar, to single edge-on lamellar. Due to the orientation of the LDPE crystals, the tensile strength of the films increases obviously when the layer thickness reduces to nanoscale. The 2D small angle X-ray scattering(SAXS) patterns can reflect the average degree of orientation of LDPE in the confined layers. The stacking of LDPE lamellae is suppressed in interlamination and oppositely in parallel to the extrusion direction. The specific orientation function f can be calculated from the patterns. The infrared dichroism further confirms the mutation of the orientation of LDPE crystals from microscale to nanoscale in the confined space.     

Orientation of LDPE crystals from microscale to nanoscale <Emphasis Type="Italic">via</Emphasis> microlayer or nanolayer coextrusion

The microlayer or nanolayer coextrusion of hundreds or thousands of alternating low density polyethylene (LDPE)/polystyrene (PS) microlayers or nanolayers were used to study the orientation of LDPE crystals in the confined quasi-two-dimensional or two-dimensional space. The clear and continuous layer structures from microscale to nanoscale can be found in SEM images. The morphology evolution of LDPE crystals in the confined microlayer or nanolayer can be varied from 3D spherulites, 2D spherulites, stacked edge-on lamellar, to single edge-on lamellar. Due to the orientation of the LDPE crystals, the tensile strength of the films increases obviously when the layer thickness reduces to nanoscale. The 2D small angle X-ray scattering (SAXS) patterns can reflect the average degree of orientation of LDPE in the confined layers. The stacking of LDPE lamellae is suppressed in interlamination and oppositely in parallel to the extrusion direction. The specific orientation function f can be calculated from the patterns. The infrared dichroism further confirms the mutation of the orientation of LDPE crystals from microscale to nanoscale in the confined space.     

Stability of the interphase interaction and changes in the microdestruction mechanisms in low density polyethylene on high energy irradiation

The effect of gamma irradiation on non-isothermally crystallized low density polyethylene (LDPE) has been studied by thermal-mechanical analysis (TMA) and transmission and scanning electron microscopy (TEM and SEM, respectively). Analysis of the TMA curves of the irradiated samples shows that the radiation-induced crosslinking of the macromolecular chains in the dose range 0–10 Mrad causes an increase, decrease and partial freezing of the mechanical microstresses located at the phase surfaces in the bulk of the polymer. These effects are explained in terms of the physical kinetics and macromolecular mobility. At temperatures above 120 °C., gamma-irradiated LDPE with doses above 5 Mrad develops a physical state corresponding to a high elastic state with thermotropic mesophases. Below 5 Mrad, controllable changes of the melting entropy and enthalpy of the crystalline regions and the free energy of the crystalline lamellae can be achieved. The TEM and SEM study shows an increase in the degree of co-operation of the sublevels of the superstructural molecular organization with the irradiation dose, even in areas that undergo plastic microdestruction.     

Effect of X-ray radiation on the structure of P (VDF/TrFE) copolymers

Structural changes in poly(vinylidene fluoride)-trifluoroethylene [P(VDF-TrFE)] copolymers caused by X-ray irradiation were investigated by molecular weight determination, EPR analysis, FTIR spectroscopy, gel content, DSC thermal analysis, X-ray diffraction, and piezoelectricity measurements. Samples exhibit radiation-induced conductivity (RIC) due to the formation of radical ions. These radicals are generated by bond cleavage, which could react, leading to structural changes such as oxidation, double bond formation, chain scission, and crosslinking. The increasing gel content with radiation dose indicated that crosslinkings of the polymer chains predominate. Irradiation on P(VDF-TrFE) caused the melting temperature, heat of fusion, and Curie temperature to decrease. These results are consistent with the partial destruction of crystalline domains. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1201–1205, 1997     

Invertase immobilization onto radiation-induced graft copolymerized polyethylene pellets

The graft copolymer poly(ethylene-g-acrylic acid) (LDPE-g-AA) was prepared by radiation-induced graft copolymerization of acrylic acid onto low density polyethylene (LDPE) pellets, and characterized by infrared photoacoustic spectroscopy and scanning electron microscopy (SEM). The presence of the grafted poly(acrylic acid) (PAA) was established. Invertase was immobilized onto the graft polymer and the thermodynamic parameters of the soluble and immobilized enzyme were determined. The Michaelis constant, K_m, and the maximum reaction velocity, V_max, were determined for the free and the immobilized invertase. The Michaelis constant, K_m was larger for the immobilized invertase than for the free enzyme, whereas V_max was smaller for the immobilized invertase. The thermal stability of the immobilized invertase was higher than that of the free enzyme.     

Pyrolysis of olive residue/low density polyethylene mixture: Part I Thermogravimetric kinetics

This paper demonstrates the thermal pyrolysis of olive residue, low density polyethylene (LDPE) and olive residue/LDPE mixture in an inert atmosphere of N2 using thermogravimetric analysis (TGA). Measurements were carried out in the temperature range 300K~973K at heating rates of 2K/min, 10K/min, 20K/min and 50K/min. Based on the results obtained, three temperature regimes were selected for studying the nonisothermal kinetics of olive residue/LDPE mixture. The first two were dominated by the olive residue pyrolysis, while the third was linked to the LDPE pyrolysis, which occurred at much higher temperatures. Discrepancies between the experimental and calculated TG/DTG profiles were considered as a measurement of the extent of interactions occurring on copyrolysis. The maximum degradation temperatures of each component in the mixture were higher than those the individual components; thus an increase in thermal stability was expected. The kinetic parameters associated with thermal degradation were determined using Friedman isoconversional method.     

A study of gamma-irradiated polyethylenes by temperature modulated differential scanning calorimetry

Various polyethylenes (PEs) and the effects of high-energy radiation on their structures were widely studied in the past using conventional Differential Scanning Calorimetry (DSC) measurements. In this work, we used the Temperature Modulated Differential Scanning Calorimetry (TMDSC) technique in order to obtain more information about the influence of the initial structural differences and gamma radiation on the evolution in structure and thermal properties of different polyethylenes. For this reason, low density polyethylene (LDPE), linear low density polyethylene (LLDPE) and high density polyethylene (HDPE) samples were exposed to gamma radiation, in air, to a wide range of absorbed doses (up to 2400 kGy). The separation of the total heat flow TMDSC signal into a reversing and non-reversing part enabled us to observe the low-temperature enthalpy relaxation (related to the existence of the “rigid amorphous phase”) and recrystallisation processes, as well as to follow their radiation-induced evolution and/or that of melting in a more revealing manner compared to the case of the conventional DSC. Consequently, our results indicate that TMDSC could improve the understanding of radiation-induced effects in polymers.     

A unique behavior of water drops induced by low-density polyethylene surface with a sharp wettability transition

A low-density polyethylene (LDPE) surface with a sharp wettability gradient and high hysteresis was prepared, on which a unique behavior of water drops was found. The water contact angle of one water drop on the less hydrophobic region was larger than that on the more hydrophobic end, which was much different from the general phenomenon. The unique behavior is believed to be induced by the high hysteresis of the LDPE surface and the sharp change in wettability. The driving and hysteresis forces acting on the water drops were calculated and analyzed in detail. The reasons resulting to such a unique phenomenon were further explained.     

Chemiluminescence study on the oxidation of several polyolefins—I. Thermal-induced degradation of additive-free polyolefins

Chemiluminescence (CL) has been applied to evaluate the oxidation susceptibility of various polyolefins: low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE) and isotactic polypropylene (i-PP). The intensity of CL emission in inert atmosphere could be related to the previous oxidation level. The thermal stability at 170 °C of the hydroperoxides in LDPE seems to be lower than that in LLDPE or HDPE. The kinetic parameters of the oxidation at 170 °C in oxygen, calculated from CL data, suggest the following stability order: HDPE > LLDPE > LDPEi-PP. The intensity of CL emission was related to the CH₃ content as evaluated by Fourier transform infra-red spectroscopy.     

低密度聚乙烯熔体的非线性依时性流变特性及表征

进行了一种低密度聚乙烯 (LDPE ,PE FSB 2 3D0 2 2 Q2 0 0 )熔体的触变环实验 ,实验结果表明该LDPE的粘弹力学特性具有复杂的非线性依时特性 ;进一步的应力增长实验表明在定常剪切速率 5s- 1 下应力会随着剪切时间的延长而明显下降 .文中用一个变型的Huang方程表征了实验得到的触变环 ,模型计算与各实验结果基本吻合 ;但同时注意到模型的参数不具有通用性 ,没有全面反映LDPE(Q2 0 0 )熔体的粘弹力学特性 ,此外 ,变型Huang方程也无法预测材料在剪切之后的应力松弛 .     

The effect of flame retardant-modified sepiolite nanofibers on thermal degradation and fire retardancy of low-density polyethylene

Flame retardant-modified sepiolite nanofiber (PSPHD-SEP) was fabricated through chemical grafting by introducing intumescent flame retardant oligomer (PSPHD) onto the surface of sepiolite fiber. Various sepiolite/low-density polyethylene (SEP/LDPE) composites have been prepared successfully via melt blending. The dispersion of various SEPs in LDPE matrix was observed by scanning electron microscope and transmission electron microscope. The thermal degradation behaviors of various SEP/LDPE composites with 3 mass% acid-modified sepiolite fiber (a-SEP) or PSPHD-SEP have been investigated employing thermogravimetric analysis/derivative thermogravimetry. The thermal degradation kinetics of neat LDPE, a-SEP/LDPE and PSPHD-SEP/LDPE systems was comparatively analyzed by means of Friedman and Flynn–Wall–Ozawa methods to further comprehend the effect of a-SEP and PSPHD-SEP on the thermal stability of LDPE. Due to the addition of PSPHD-SEP, the limiting oxygen index value of PSPHD-SEP/LDPE composite can reach 21.3%, and the UL-94V-2 rating is obtained. The cone calorimetry (CONE) tests showed that a reduced peak heat release rate can be achieved for PSPHD-SEP/LDPE composite accompanying with gas-phase fire retardant action.

     

低密度聚乙烯的接枝改性

低密度聚乙烯的接枝改性;低密度聚乙烯;甲基丙烯酸缩水甘油酯;非等温结晶Avrami方程     

Morphology studies and ac electrical property of low density polyethylene/octavinyl polyhedral oligomeric silsesquioxane composite dielectrics

This paper presents the results of morphological and ac electrical investigations on low density polyethylene (LDPE) composites with octavinyl polyhedral oligomeric silsesquioxane (POSS). It has been shown that at low loadings, the frequency dependence of dielectric constant and dielectric loss for the LDPE/POSS composites showed unusual behaviors when compared with conventional (micro-sized particulates) composites. The ac breakdown strength was measured and statistical analysis was applied to the results to determine the effects of POSS loadings on the dielectric strength of LDPE. The morphological characterization showed that the presence of POSS additives apparently altered the supermolecular structure of LDPE and resulted in more homogeneous morphology when compared with the neat LDPE. The structure-property relationship was discussed and it was concluded that the final dielectric properties of the composites were determined not only by the incorporation of POSS additives but also by the supermolecular structure of LDPE. Rheological analyses of LDPE/POSS composite were also performed and the results showed that the octavinyl-POSS had good compatibility with LDPE.     

The compatibilization effect of ethylene/styrene interpolymer on polystyrene/polyethylene blends

In this study, ethylene/styrene interpolymer (ESI) was used as compatibilizer for the blends of polystyrene (PS) and low‐density polyethylene (LDPE). The mechanical properties including impact, tensile properties, and morphology of the blends were investigated by means of uniaxial tension, instrumented falling‐weight impact measurements, and scanning electron microscopy. Impact measurements indicated that the impact strength of the blends increases slowly with LDPE content up to 40 wt %; thereafter, it increases sharply with increasing LDPE content. The impact energy of the LDPE‐rich blends exceeded that of pure LDPE, implying that the LDPE polymer can be further toughened by the incorporation of brittle PS minor phase in the presence of ESI. Tensile tests showed that the yield strength of the PS/LDPE/ESI blends decreases considerably with increasing LDPE content. However, the elongation at break of the blends tended to increase significantly with increasing LDPE content. The compatibilization efficiency of ESI and polystyrene‐hydrogenated butadiene‐polystyrene triblock copolymers (SEBS) for PS/LDPE 50/50 was further compared. Mechanical properties show that ESI is more effective to achieve a combination of LDPE toughness and PS rigidity than SEBS. The correlation between the impact property and morphology of the ESI‐compatibilized PS/LDPE blends is discussed. The excellent tensile ductility of the LDPE‐rich blends resulted from shield yielding of the matrix. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2136–2146, 2007     

Dilation of polyethylene by sorption of carbon dioxide

The dilation of low-density polyethylene accompanied by the sorption of CO₂ was measured by microscopy under pressures up to 50 atm at temperatures from 25 to 55°C. The dilatometry measurement, which is also applied to the determination of the thermal expansion coefficient, is directly performed by a cathetometer. The dilation of LDPE by sorbed CO₂ is linear with concentration. The buoyancy correction is described for the CO₂ sorption isotherms in LDPE. The partial molar volume of CO₂ in LDPE, calculated from the dilation and the sorption isotherms, is almost independent of temperature.     

电火花引发HEMA 接枝LDPE 薄膜及其血液相容性

采用接枝量、ATR－IR、SEM、与水接触角、溶血试验和再钙化时间等测试手段研究了电火花引发甲基丙烯酸β－羟乙酯(HEMA)表面接枝低密度聚乙烯(LAPE)薄膜的接枝聚合反应影响因素、表面结构和血液相容性。结果表明,电火花能有效引发HEMA在LDMA薄膜表面接枝聚合反应,随接枝聚合反应时间延长、单体浓度的增大。接枝量增大。随反应温度升高,接枝量增大到一最大值后,进一步升高反应温度,接枝量下降,最佳接枝聚合温度为60℃当在60℃单体φ=5％水溶液是反应2h时,经空气气氛和1.5kV电火花预处理72s和LDPE薄膜表面接枝量可达169ug/cm^2。接枝改性后LDPE薄膜与水的接触下降,亲水性增加,溶血程度减小,再钙化时间延长,血液相容性得到改善。     

A novel intumescent flame‐retardant LDPE system and its thermo‐oxidative degradation and flame‐retardant mechanisms

A carbonization agent, 3,9‐di (2‐hydroxyisopropyl)‐2,4,8,10‐tetraoxa‐3,9‐diphosphaspiro‐[5,5]‐undecane (SPEPO), was synthesized from pentaerythritol (PER), phosphorus trichloride, formic acid, and acetone as raw materials. The structure of SPEPO was characterized by FTIR and ¹H‐NMR. As a carbonization agent and an acid source, SPEPO can form a novel intumescent flame‐retardant (IFR) system for low density polyethylene (LDPE) together with ammonium polyphosphate (APP) and melamine phosphate (MP). The flame retardancy and thermal behavior of the IFR system for LDPE were investigated by limiting oxygen index (LOI), UL‐94 test, and thermogravimetric analysis (TGA). When the weight ratio of SPEPO, APP, and MP is 7:7:1 and their total loading level is 30%, the IFR‐LDPE presents the optimal flame retardancy (LOI value of 27.6 and UL‐94 V‐0 rating). However, SPEPO, APP, or MP can only show a very poor flame‐retardant performance when used alone. This indicates that there is a synergistic effect among SPEPO, APP, and MP. TGA results obtained in air demonstrate that SPEPO has an ability of char formation itself, and the char residue of SPEPO can reach 24 wt% at 700°C. The IFR can change the thermal degradation behavior of LDPE, enhance T_max of the decomposition peak of LDPE, and promote LDPE to form char based on the calculated and the experimental data of residues. According to the results of Py‐GC/MS in combination with FTIR of the char residues at different temperatures, a possible flame‐retardant mechanism has been proposed. Copyright © 2008 John Wiley & Sons, Ltd.