Dielectric loss of polyethylene below 4.2°K arising from proton tunneling

The dielectric loss of high-density polyethylene (HDPE: melt-crystallized films and single-crystal mats), low-density polyethylene (LDPE), and copolymers of ethylene and vinyl alcohol (PEVA) was measured at 1.5 to 4.2°K in the frequency range from 10 Hz to 10 kHz. Results for HDPE show dispersion curves almost corresponding to a single relaxation and are interpreted in terms of phonon-assisted tunneling of the protons of hydroxyl groups which are accidentally attached to tertiary carbons (carbons with a short branch). Dispersion curves for LDPE and PEVA are quite broad, indicating a wide distribution of relaxation times. The loss level of PEVA passes through a maximum at 7.5% vinyl alcohol, suggesting that interaction between a couple of neighboring hydroxyl groups depresses the loss. A potential calculation for the rotation of a hydroxyl group in the orthorhombic lattice of polyethylene yielded a double-minimum potential for the tunneling when the lattice is assumed to have a distortion which is acceptable from x-ray analysis. The potential quantitatively explains the observed values of relaxation time and relaxation strength of HDPE. The concentration of hydroxyl groups in HDPE, which varies among the samples, is estimated to be of the order of 10¹⁶ cm^?3.     

Dielectric Relaxations in Plasma-Polymerized Hydrocarbons and Fluorocarbons

The dielectric behavior of plasma-polymerized ethylene (PPE), ethylene/acetylene (PPEA), ethane/vinyl chloride (PPEVC) and tetrafluoroethylene (PPTFE) was studied over a frequency range of 10² to 10⁵ Hz between -150 and 100°C. After exposure to the atmosphere, each of the polymers exhibited a pronounced loss peak. This relaxation process was designated as the γ_pprocess, and was attributed to the local mode motion of several molecular segments. The average activation energy for all four polymers was found to be 13.8 kcal/mole. It was proposed that oxidation introduces carbonyl groups into the nonpolar polymer chain. The added polar carbonyl groups then act as tracers to render the molecular motions observable by dielectric measurements. The presence of these carbonyl groups was confirmed by concomitant IR spectroscopic determinations. Comparison of the experimental data with the Kirkwood-Froelich theory was found to be satisfactory.     

Dielectric study of post-irradiation effects in gamma-irradiated polyethylenes

The post-irradiation dielectric behaviour of different polyethylenes (PEs) has been studied by means of dielectric loss (tan δ) analysis over the wide temperature (25–325 K) and frequency (1 kHz–1 MHz) ranges. For this reason, low density polyethylene (LDPE), linear low density polyethylene (LLDPE) and high density polyethylene (HDPE) samples were previously gamma irradiated in air to absorbed dose of 300 kGy. The irradiated samples were divided into two groups, and for the first one annealing treatment which can substantially reduce the concentration of free radicals were employed. For the second group, e.g. samples stored in air at room temperature after irradiation, post-irradiation evolution in free radical concentration, dielectric relaxation spectra and carbonyl content was investigated as a function of storage time, up to 90 days. Dielectric relaxation behaviour is related to differences in the initial structures of PEs (such as branching, crystallinity, etc.) and to the radiation-induced effects; carbonyl groups that were introduced by irradiation and/or delayed (post-irradiation) oxidation were regarded as tracer groups. Electron spin resonance (ESR), differential scanning calorimetry (DSC), infrared (IR) spectroscopy and gel measurements were used to determine the changes in free radical concentration, crystal fraction, oxidation and degree of network formation, respectively.     

The bridged cyclopentadienyl indenyl (fluorenyl) zirconocene complexes for polyethylene macromonomers

The synthesis of long‐chain branched polyethylene includes the generation of vinyl‐terminated polyethylene macromonomers and the copolymerization of these macromonomers with ethylene. Four new bridged cyclopentadienyl indenyl (fluorenyl) zirconocene complexes 1a–b, 2a–b were prepared and showed high activities for ethylene homopolymerization upon the activation of methylaluminoxane. The steric bulk of bridged substituent has a profound effect on the catalytic activity as well as on the molecular weight of resulting polyethylene. Complex 1b showed the highest activity of up to 5.32 × 10⁶ g PE/(mol Zr h) for ethylene homopolymerization at 70 °C, which was higher than that of Cp₂ZrCl₂. The polyethylenes produced with complexes 1a–d/MAO are mostly vinyl‐terminated, possess low molecular weight and fit as macromonomers. The (p‐MePh)₂C‐bridged cyclopentadienyl indenyl zirconocene complex 1a could produce polyethylene macromonomer with selectivity for the vinyl‐terminal as high as 94.9%. Copyright © 2010 John Wiley & Sons, Ltd.     

Properties of poly(vinyl chloride) blends with polycarbonates and chlorinated polyethylene

The miscibility of polycarbonates derived from Bisphenol A or 2,5,2′,5′-tetramethyl-Bisphenol A with poly(vinyl chloride), chlorinated poly(vinyl chloride), and vinyl chloride-vinylidene chloride copolymers has been investigated. In miscible blends a shift of the position of the carbonyl absorption in the IR spectra indicates dipolar interactions between the polymers. The miscibility of chlorinated polyethylenes and reduced poly(vinyl chloride)s among each others demonstrates besides the importance of polar groups the influence of their distribution within the polymer chains for the compatibility of the polymers. The investigations on the miscibility have been carried out by differential scanning calorimetry, and by casting films with microscopical observation of the resulting structures.     

Properties of low-molecular-weight polyethylene treated with gas plasma

Polyethylene films were evaporated in gas plasmas of Ar, N₂, O₂, and H₂O. The deposits were analyzed by infrared (IR) spectroscopy to determine the concentration of characteristic functional groups. The deposit prepared in Ar-atmosphere had a rather high concentration of methyl group and many double bonds were produced in the film. The deposits prepared in Ar- and N₂-plasmas produced similar spectra, which showed twice the concentration of methyl group than the deposit in Ar-atmosphere and also contained a few carbonyl and hydroxyl groups. The film treated in O₂-plasma contained the largest amount of carbonyl group and the lowest number of double bonds among the plasma-treated deposits. Dielectric loss curves against temperature for the deposits treated in these plasmas showed a broad peak near 20°C. For O₂-plasma-treated film the loss tangent curves showed a sharp maximum. The activation energy for the relaxation of Ar-, O₂-, and H₂O-plasma-treated films had the same value of 50.6 kcal/mol. The observed relaxation in the films prepared in gas plasmas was considered due to the β process and was attributed to the motion of oxidized branched polyethylene.     

Intramolecular 9-membered hydrogen bonding of 2-arylmethylphenols having carbonyl groups at 2′-position

Thermodynamic parameters of nine-membered intramolecular hydrogen bonding between carbonyl groups and phenolic hydroxyl groups of 2-arylmethylphenols having methoxycarbonyl, dimethylcarbamoyl, and formyl groups were determined by variable temperature ¹H NMR studies and van't Hoff analysis. The enthalpy of the hydrogen bonding was related to the electron-withdrawing ability of the substituents on the phenol and the basicity of the carbonyl group. The entropy loss of the hydrogen bonding was dependent on the rotation freedom of the phenol group.     

Photodegradation of polyethylenes: Comparative effect of Fe and Ca-stearates as pro-oxidant additives

The effect of iron and calcium stearates on the degradation of polyethylene (LDPE and LLDPE), under natural and artificial exposure, has been studied. The activity of stearates has been evaluated by chemiluminescence and FTIR of polyethylenes. The analysis of the molecular weight changes and content of degradation products identified by GC-MS during ageing process confirmed their pro-degrading activity. Films containing stearates exhibited lower CL emission, and revealed the higher efficiency of Fe-Stearate compared to Ca-Stearate in decomposing hydroperoxides, leading to higher degradation during processing. The results were confirmed by TGA analysis, where the weight loss onset and T_max shifted to lower temperatures in polyethylenes with incorporation of Fe- and Ca-stearates compared to pure polymers.Polyethylenes were outdoor and accelerated exposed, and CL measured at different period of times. Chemiluminescence temperature-ramping tests under nitrogen showed the formation of a peroxide peak at lower temperature, and a significant increase in carbonyl index for PE containing stearates was found by FTIR. The results were supported by GC-MS, where the concentration of extracted products identified in the polyethylenes containing Fe-stearate was significant, and a much greater decrease in molecular weight was determined by GPC, which confirmed the development of degradation for polyethylenes with Fe-Stearates in comparison to pure or Ca-stearate polyethylenes.     

Dielectric relaxation study of gamma irradiated oriented low-density polyethylene

The influence of drawing, gamma irradiation and accelerated aging on the dielectric relaxation of low-density polyethylene has been studied using dielectric loss tangent measurements in the temperature range from 25 to 325 K and in the frequency range from 10³ to 10⁶ Hz. The intensity, position and activation energy of the γ- and β-dielectric relaxations were found to be strongly dependent upon the changes in the microstructure of the amorphous phase induced by uniaxial orientation, oxidation and crosslinking.     

Thermodynamics of crystalline linear high polymers. IV. The effect of ethyl,acetate, and hydroxyl side groups on the properties of polyethylene

Copolymers of ethylene with vinyl acetate, vinyl alcohol, and butene-1 have been investigated by differential thermal analysis. The method of fast heating is used to approximate a zero entropy production heating path. The activity of crystallizable units in the melt, the crystallinity, and the a-axis spacings are determined and compared with previous results for copolymers of ethylene and propylene and carbon monoxide. Carbonyl and hydroxyl groups form point defects, forming solutions in both the crystalline and amorphous regions. Methyl, ethyl, and acetate groups form large amorphous defects. The maximum melting point of polyethylene is calculated to be 142.6°C.     

Oxidative degradation products of irradiated polyethylene

Oxidative thermal degradation products of polyethylenes at various temperatures crosslinked with electron beams have been analyzed with gas chromatography and mass spectrometry techniques. Carbon monoxide and carbon dioxide are determined at a temperature range of 200–340°C, and the activation energies of the unirradiated and the irradiated polyethylene (at 100 Mrad) are 13.5 and 11.4 Kcal/mole, respectively. C₁ to C₈ hydrocarbons produced in air and in nitrogen are determined at temperatures from 400 to 450°C for the polyethylenes. The irradiated polyethylene produces less hydrocarbons in air than the unirradiated polyethylene, contrary to the fact that the crosslinked polymer evolves more hydrocarbons than the unirradiated polymer in a nitrogen atmosphere. Aldehydes and ketones are observed in the volatile oxidative degradation products, and these carbonyl compounds increase quantitatively with increase of temperature up to about 460°C. It is concluded that irradiated polyethylene is thermally more unstable in the absence of oxygen and more easily oxidable at low degradation temperatures in air than unirradiated polyethylene. Irradiated polyethylene, however, is more heat-stable than unirradiated polyethylene from the standpoint of the ignition process.     

Dielectric and rheo-optical properties of some ethylene-carbon monoxide copolymers

Studies were made on films of copolymers of ethylene with 0.5 and 1.0 mole-% carbon monoxide. The carbon monoxide appeared negligibly to affect the degree of crystallinity, melting point, morphology, and dynamic mechanical spectra. Infrared dichroism showed that the orientation of the carbonyl groups was comparable with that of the crystalline CH₂ groups and indicated that the carbonyl groups are at least partially within the crystals. This is confirmed by x-ray measurements which indicate an expansion of the a-axis spacing and by an appreciable increase in the height of the α dielectric loss peak which has been assigned to crystalline motion. This α loss peak moves to a lower temperature with increasing carbonyl content, while the γ dielectric loss peak moves to higher temperatures. Activation energies of 25, 35, and 15 kcal/mole for the α, β, and γ peaks, respectively, were independent of carbonyl content and comparable with values for oxidized polyethylene.     

Radiolytic unsaturation decay in polyethylene. Part I—general review and analysis with additional new work

Previously published mechanisms for radiolytic vinyl decay in polyethylene lead to first-order, second-order and modified first-order relations. Such equations are fitted using regression analysis to the results of unsaturation measurements on irradiated polyethylenes carried out by the present author (and colleagues) or made available to him by another. It is found that the modified first-order relation best represents the radiolytic decay of vinyl, vinylidene and added allyl unsaturation as triallylisocyanurate or triallylcyanurate in polyethylene at room temperature.     

Application of thiol‐ene click chemistry to preparation of functional polyethylene with high molecular weight and high polar group content: Influence of thiol structure and vinyl type on reactivity

A series of functional polyethylenes have been simply and efficiently synthesized via the combination of regioselective ethylene/5‐vinyl‐2‐norbornene (VNB) copolymerization using [PhNC(CF₃)CHCO(Ph)]₂TiCl₂ catalyst and following ultraviolet light initiated thiol‐ene click reaction. On treatment of ethylene/VNB copolymer with different thiols including mercaptoethanol, 1‐thioglycerol, methyl mercaptoacetate, methyl mercaptopropionate, 2‐mercaptoethylamine, mercaptoacetic acid, and mercaptopropanoic acid, various polar groups have been successfully introduced into the polyethylene. Except 2‐mercaptoethylamine, the functionalizations are quite efficient with the degree of functionalization higher than 94%, which is independent of thiol structure and double bond content. The content of polar group in functional polyethylene can be tuned in a wide range of 0–30 mol %. Gel permeation chromatography profiles indicate all functional polyethylenes that have very high molecular weights (160–336 kg/mol) with homogeneous formation. Besides, systematic investigation of the influence of vinyl type and thiol structure on reactivity has been also carried out. By treatment of mercaptoethanol with different copolymers (ethylene/VNB, ethylene/5‐ethylidene‐2‐norbornene, and ethylene/dicyclopentadiene copolymer), the order of vinyl reactivity can be summarized as terminal > internal > cyclic double bond. For different thiols, the reactivity has the sequence of SHCH₂COOH > SHCH₂COOCH₃ > SHCH₂CH₂COOH > SHCH₂CH₂COOCH₃ > SHCH₂CH(OH)CH₂OH > SHCH₂CH₂OH > SHCH₂CH₂NH₂, which is depended on the solubility and the electron‐withdrawing inductive effect of polar group. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Dynamic structure of poly(vinyl pyrrolidone)/ethyl alcohol mixtures studied by time domain reflectometry

Dielectric studies of poly(vinyl pyrrolidone)/ethyl alcohol (PVP–E) binary mixtures with concentration variations were carried out in the frequency range of 10 MHz to 10 GHz by time domain reflectometry at 15, 25, 35, and 45 °C. One relaxation process, corresponding to ethyl alcohol molecules in the poly(vinyl pyrrolidone) (PVP) matrix, was observed in this frequency range for all the mixtures. The static dielectric constant of the PVP–E mixtures decreased linearly with an increase in the weight fraction of PVP. The observed anomalous increase in the value of the relaxation time (τ) of these mixtures was interpreted by the consideration of the variation in the local structure of self‐associated ethyl alcohol molecules and also the PVP behavior as a geometric constraint for the rotational motion of ethyl alcohol molecules. Furthermore, the τ values of these mixtures were independent of the viscosity. The energy parameters for the dielectric relaxation process (the free energy, enthalpy, and entropy of activation for the dipolar orientation) were determined to confirm the transient behavior of the heterogeneous species due to the breaking and re‐forming of hydrogen bonds with the internal rotation of ? OH groups in the ordered structure of the PVP–E mixtures. On the basis of the evaluated dielectric parameters, the formation of supermolecular structure in the PVP–E mixtures in dynamic equilibrium was sketched and examined by the consideration of the hydrogen bonding between the terminal hydroxyl groups of self‐associated ethyl alcohol flexible chains and the carbonyl groups of monomer units of PVP coiled chains. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1134–1143, 2005     

Proton NMR studies of a polybenzimidazole in solution

Molecular aggregation of poly(4,4′-diphenyl ether-5,5′-bibenzimidazole)(PBI) in solution has been studied by high resolution proton NMR. PBI and model compounds have been synthesized, purified, and characterized. Proton resonances in the NMR spectrum of PBI are assigned by comparison with the proton resonances of the model compounds. Spectra are studied by total line-shape analysis, assuming each absorption curve to be Lorentzian. For PBI in N,N-dimethylacetamide (DMAc), the resonance due to the proton of a hydroxyl group formed by proton exchange between the imino group of PBI and the carbonyl group of DMAc is observed. The activation energy for the proton exchange, obtained from Arrhenius plots of the temperature dependence of the chemical shifts of the hydroxyl proton and the imino proton, was found to increase in the order corresponding to dissociation energy of the N? H···O?C hydrogen bond. The chemical shifts in the NMR spectra of PBI-DMAc solutions on the addition of LiCl are strongly dependent on the polymer-salt ratio; and thereby the coordination position of LiCl to PBI is tentatively identified, assuming a pseudocontact LiCl-induced shift. The dependence of the chemical shifts of protons in PBI on the dielectric constant of the solvent is demonstrated by using polar solvents of varying dielectric constant, such as N-methylpyrrolidone, dimethylsulfoxide, and formic acid. The viscosity of the PBI-DMAc solutions is reported at various temperatures and concentrations of LiCl. The results from viscometry are explicable in terms of the NMR observations.     

Degradation and stabilisation of poly(ethylene-stat-vinyl acetate): 1 - Spectroscopic and rheological examination of thermal and thermo-oxidative degradation mechanisms

The degradation of ethylene vinyl acetate (EVA) copolymers was compared with low density polyethylene (LDPE), poly(vinyl acetate) (PVAc) and poly(vinyl chloride) (PVC) using FTIR, UV-visible and fluorescence spectroscopy as well as thermal and rheological analyses. Thermal, thermo-oxidative and photo-oxidative studies were conducted. Thermo-oxidation below 180 °C shows more similarities between EVA and LDPE. The luminescence spectra of degraded EVA and LDPE were almost identical but very different to that of PVAc. UV-vis analysis showed that the polyenes present in aged PVC were unlikely to be the same species responsible for the observed colour formation in aged EVA. It is suggested that they are polyconjugated carbonyl products. Rheological analysis also showed the evolution of crosslinking reactions during thermo-oxidation. FTIR studies after thermal degradation in inert conditions 290 °C showed complete loss of the ester functionality and associated lactone formation along with some evidence for ketonic and unsaturated carbonyl groups. Degradation in air at 180 °C, however, revealed that loss of the ester group was not so marked, with PVAc exhibiting the greatest stability. This was in line with the induction time to onset of autocatalytic carbonyl growth at 180 °C; the latter showed an apparent exponential decrease with increasing vinyl acetate content up to 28% w/w. Fluorescence analysis produced trends that complemented those of carbonyl index; the time to decomposition of initial fluorescent α,β-unsaturated carbonyl species coincided with the time to onset of carbonyl growth. Furthermore, the rate of formation of the new fluorescent species produced in EVA, and LDPE was similar to that of carbonyl growth. These new fluorescent species are therefore likely to be di- or tri-carbonyl products.     

用差减红外光谱分峰法测定乙丙齐聚物中端羟基的微结构

合成了端羟基乙丙齐聚物。用差减红外光谱对端羟基进行了分析。扣除了溶剂中三氯甲烷的干扰,用RUN.CAPR程序进行分峰,以正戊醇-1、3-甲基丁醇-1和丁醇-2为模型化合物标定伯、仲羟基吸收峰位置,并进行蜂面积积分,用模型化合物回归处理后得到定量公式:A=15.8667×10~3Cp+0.25674,以此求得乙丙齐聚物中伯、仲羟基的官能度。     

Dielectric studies of photodegradation and photooxidation of polystyrene

The effects of vacuum photolysis at 254 nm and the short-(λ = 254 nm) and long-wave (λ > 300 nm) photooxidations on the dielectric constants (?′) and dielectric losses (?″) of polystyrene have been investigated at 25 ± 1°C. Dielectric constants generally increase on photodegradation, but more pronounced increases occur in the low-frequency region on vacuum photolysis. It is suggested that such increases are associated with Maxwell-Wagner-Sillars polarization. Increases in dielectric losses are observed in three main frequency regions: around 10² Hz, 10⁴ Hz, and at 3 × 10⁶ Hz. The lowest frequency loss which occurs in both the vacuum-irradiated and in the photooxidized samples is attributed to a combination of the effects of interfacial polarization and of the increased direct-current conductivity of the polymer which occurs as a result of these reactions. The 10⁴ Hz dispersion associated only with photooxidation is related to the presence of small, volatile, polar oxidation products, like ketones. The loss peak overlaps the intrinsic γ-relaxation of polystyrene, and it appears that the motions of small molecules, or of relaxing dipoles in these, are coupled to the phenyl group rotational vibrations. The high-frequency losses (3 × 10⁶ Hz) are ascribed to orientation polarization of carbonyl dipoles attached to chain ends, these compounds being produced as a result of hydroperoxide decompositions. A good correlation between the carbonyl dipole relaxation strength and carbonyl concentration is observed. It would appear that relaxation strength measurements could provide quantitative kinetic information for polystyrene oxidation.