Mechanistical studies on the electron-induced degradation of polymers: polyethylene, polytetrafluoroethylene, and polystyrene

Mechanisms of the electron-induced degradation of three polymers utilized in aerospace applications (polyethylene (PE), polytetrafluoroethylene (PTFE), and polystyrene (PS)) were examined over a temperature range of 10 K to 300 K at ultra high vacuum conditions (～10(-11) Torr). These processes simulate the interaction of secondary electrons generated in the track of galactic cosmic ray particles in the near-Earth space environment with polymer material. The chemical alterations at the macromolecular level were monitored on-line and in situ by Fourier-transform infrared spectroscopy and mass spectrometry. These data yielded important information on the temperature dependent kinetics on the formation of, for instance, trans-vinylene groups (-CH=CH-) in PE, benzene (C(6)H(6)) production in PS, fluorinated trans-vinylene (-CF=CF-) and terminal vinyl (-CF=CF(2)) groups in PTFE together with molecular hydrogen release in PE and PS. Additional data on the radiation-induced development of unsaturated, conjugated bonds were collected via UV-vis spectroscopy. Temperature dependent G-values for trans-vinylene formation (G(-CH=CH-) ≈ 25-2.5 × 10(-4) units (100 eV)(-1) from 10-300 K) and molecular hydrogen evolution (G(H(2)) ≈ 8-80 × 10(-5) molecules (100 eV)(-1) from 10-300 K) for irradiated PE were calculated to quantify the degree of polymer degradation following electron irradiation. These values are typically two to three orders of magnitude less than G-values previously published for the irradiation of polymers with energetic particles of higher mass.     

A study on degradation of germanium coating on Kapton used for spacecraft sunshield application

Sunshield membranes made of germanium‐coated black polyimide (GBP) or Kapton are often used on the reflector/transmitter antenna of satellites for thermal control applications. However, the germanium top layer is prone to degrade during ground storage and implementation. Hence, vacuum/inert gas‐sealed packaging is required for storing the membranes, followed by a staggered fabrication schedule as the shelf‐life of the GBP is identified as only ~6 months. In the present study, microstructural, thermo‐optical, and electrical properties along with X‐ray photoelectron spectroscopy (XPS) studies for evaluating oxidation states of the as‐received and degraded GBP films have been investigated thoroughly. The radio frequency (RF) loss behavior of both the films has also been studied for S band (2.5–3.5 GHz), Ku band (10.5–14.5 GHz), and Ka band (30–35 GHz). Copyright © 2015 John Wiley & Sons, Ltd.     

Small angle and quasi-elastic neutron scattering studies on polymethylmethacrylate latices in nonpolar media

Poly(methylmethacrylate) particles stabilised by poly(12-hydroxystearic acid) in a hydrocarbon medium provide, as a polymer latex, a useful system for investigating the properties of a sterically stabilised concentrated dispersion. In the present work these latices were investigated by small angle coherent neutron scattering and by quasielastic incoherent scattering. The former technique was used to probe the dimensions of the surface layer of poly(12-hydroxystearic acid) and to determine the diameter of the core particle of poly(methylmethacrylate) using dilute dispersions. In concentrated dispersions it was used to determine the structure factor. The latter was compared with that expected for an interacting system of hard spheres and reasonable agreement obtained. The quasielastic technique was used to probe the translational diffusion of the poly(12-hydroxystearic acid) molecules on the surface and the diffusion of the total particles in the concentrated systems.     

Theoretical studies on the degradation of ladder polymers

The random degradation of four- and six-membered ring ladder polymers was simulated on a digital computer by utilizing a Monte Carlo model. The degradation was compared with that of a single-chain linear polymer undergoing an identical degradation reaction. Significant differences in the change in molecular weight as a function of time were noted between the ladder polymer and the single chain polymer. Similar studies were conducted with imperfect ladder polymers which had occasional missing bonds in the ladder structure. These imperfections produced a marked drop in molecular weight at a given time compared with the perfect ladder polymer.     

聚酰亚胺侵蚀机理及防护效应的研究

在微波电离型原子氧(AO)源地面模拟设备中对空间材料聚酰亚胺(Kapton)及有机硅防护层进行原子氧剥蚀效应试验.用光电子能谱(XPS)、红外光谱(FTIR)和扫描电镜(SEM)对试验前后试样的表面形貌、质量及化学结构进行表征研究.AO对Kapton表现了较严重的侵蚀作用,原来平整的表面形貌变为毛毯状;而所施用的有机硅涂层的表面形貌则变化甚少,表明该涂层具有较明显的防护效果.     

Molecular aggregation and mechanical properties of Kapton H

Molecular aggregation of Kapton H (KH) was investigated by small-angle x-ray scattering (SAXS). Superstructure parameters were estimated using a one-dimensional model, taking into account that the SAXS from KH is anisotropic out of the film plane. The results show that KH has a two-phase structure with a volume ratio of the ordered to the less-ordered phase of about 1:1. The β dispersion in dynamic mechanical properties is reasonably ascribed to oscillations of p-phenylene groups in the main chain.     

Wavelength effect on the photoinduced reaction of polymethylmethacrylate

Polymethylmethacrylate (PMMA) containing benzophenone (BP) was photo-irradiated with monochromatic radiation of wavelength 260–360 nm using the Okazaki Large Spec-trograph (OLS). On irradiation of PMMA films containing BP in air, the sensitized main-chain scission and photocrosslinking of PMMA took place simultaneously. These reactions are dependent on irradiation wavelength. The threshold wavelength for both reactions is found to be ca. 380 nm. The number of main-chain scission and amount of gel increased with the increase of BP concentration in PMMA. Photosensitized main-chain scission favors the irradiation of radiation at ca. 280 nm and photocrosslinking takes place efficiently with the exposure of 340 nm radiation. A possible mechanism for photosensitized reaction is proposed. © 1995 John Wiley & Sons, Inc.     

Adsorption and electron-induced dissociation of ethanethiol on au(111)

Dissociation of ethanethiol and the formation of Au-adatom-diethylthiolate rows on the Au(111) surface were investigated using scanning tunneling microscopy (STM) at low temperature. Ethanethiol molecules physisorb on Au(111) at 120 K by sequentially occupation of the elbow site, the fcc domain before covering the whole surface with a semiliquid layer without long-range order. Scanning the physisorbed layer with a sample bias higher than +1.2 V leads to dissociation via cleaving the H-S bond. One of the dissociation products, ethylthiolate, forms a double-row structure with the rows aligned in one of the ?112?? directions. These double rows arise from the Au-adatom-dithiolate species: CH(3)CH(2)S-Au-SCH(2)CH(3).     

甲醛光催化降解反应机理的量子化学研究

分别采用B3LYP,MP2方法在6-311++G(2df,pd)水平研究了甲醛光催化降解反应的微观机理,找到了可能的反应通道,预测反应产物为HCOOH与H2O.并得到了各反应通道的反应物、中间体、过渡态和产物的优化构型、谐振频率.成功地解释了实验结论.从键长和能量的变化角度,讨论了化学反应过程中化学键的变化规律,整个反应通道中各势能面均较低,从理论角度分析该反应室温下能够进行,为空气中的甲醛降解反应的实验研究提供理论依据.     

Fourier transform infrared studies on the thermal degradation of polyacrylonitrile

Fourier transform infrared studies of the thermal degradation of polyacrylonitrile (PAN) conducted at 200°C in air and under reduced pressure are presented. The spectra are markedly superior to those published previously. A mechanism consistent with the infrared results obtained under reduced pressure is advanced, and assignments for the infrared bands occuring in the spectrum of both the reduced pressure and air degraded PAN are given.     

Adsorption and electron-induced polymerization of methyl methacrylate on Ru(1010)

The adsorption and electron irradiation of methyl methacrylate (MMA) on a Ru(1010) surface have been studied using x-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD), and low energy ion scattering. TPD analysis indicates that a monolayer of MMA chemisorbs and dissociates on the Ru(1010) surface. The reaction products observed upon heating include H(2), CO, CO(2), and a small amount of MMA. Physisorbed multilayers of MMA desorb at temperatures around 170 K. Electron irradiation of physisorbed MMA at 140 K leads to a modification of the MMA film: The XPS spectra show an increase in thermal stability of the film with retention of the MMA structure, and indicate that electron irradiation induces polymerization. An increase in the electron bombardment fluence induces a degradation of the formed polymerized species and leads to the accumulation of carbon on the Ru surface. These results are relevant to the accumulation of carbon on surfaces of Ru films that serve as capping layers on MoSi multilayer mirrors used in extreme ultraviolet lithography.     

Isothermal and non-isothermal pyrolysis kinetics of Kapton polyimide

The kinetics involved in the thermal decomposition of Kapton^® polyimide 100HN under nitrogen atmosphere were studied by applying various fitting techniques to the isothermal and non-isothermal gravimetric data. The correlation of the reaction mechanism fitting, the analytical model fitting and the isoconversional method to these data was examined in relation to the kinetic parameters and the kinetic predictions. The mechanisms for solid-state reactions fit the isothermal data very well but result in highly uncertain values for the kinetic parameters when applied to the non-isothermal data. Isoconversional methods show that the apparent activation energy depends on the extent of conversion but do not provide information for the reaction order and the pre-exponential factor. Three single heating-rate analytical models by Coats-Redfern, MacCallum-Tanner and van Krevelen were analysed using the non-isothermal data. A multi-heating rate model is proposed and its validity is compared to the single-heating rate models on the basis of kinetic predictions.     

Damage effect of keV proton irradiation on aluminized Kapton film

Damage effects of proton irradiation on aluminized Kapton films were investigated through ground simulation experiments, in which the energy of protons was 90 keV and the flux was 5.0×10¹¹ cm⁻² s⁻¹. Changes in surface morphology, optical properties, microhardness and microstructure of Kapton/Al were analyzed. The experimental results show that proton irradiation results in the increase of surface roughness and microhardness, and the degradation of the reflective property in the 500–1200 nm range. Raman spectra and XPS analysis demonstrate that changes in chemical structure occurred during irradiation, including the bond breaking of the part of the carbonyl and aromatic ether, ring opening reaction of cyclic imide, and the formation of some new bonds and structure.     

Spectroscopic studies on the initial stages of thermal degradation of polychloroprene

Thermal degradation of polychloroprene under nitrogen, especially at the initial stages, has been studied by using ¹H-NMR, ¹³C-NMR and FT–IR spectroscopy. A model polymer of low molecular weight (M _n = 6300) was prepared to avoid gelation during degradation. None of isomerized 1,2 unit has been found in the original polymer. Allylic rearrangement of 1,2 unit was the first-stage reaction, which was finished within 30 min at 150°C. The extent of HCl loss was proportional to the decrease of isomerized 1,2 unit. It has been suggested that the next-stage reaction is dehydrochlorination of the isomerized 1,2 unit. The presence of terminal vinyl group and the increased amount of olefinic proton were not found in the degraded polymer. The back-biting mechanism involving a six-membered cyclization process is proposed for the dehydrochlorination. The thermal racemization has been also found to take place in the 3,4 unit.     

Thermal and electron-induced decomposition of 2-butanol on Pt(111)

The adsorption, thermal evolution, and electron irradiation of 2-butanol on Pt(111) were investigated with reflection absorption infrared spectroscopy (RAIRS). A simulated vibrational spectrum of a single 2-butanol molecule was calculated using density functional theory to facilitate vibrational assignments. Exposures of 0.2 Langmuir (L) and lower result in both isolated 2-butanol molecules with minimal lateral interactions and hydrogen-bonded clusters. The thermal evolution following a 4.0 L exposure shows that the hydrogen-bonded multilayer desorbs around 170 K, leaving a 2-butanol monolayer where hydrogen bonding still exists. At 190 K, a new feature at 1699 cm(-1) is attributed to the formation of butanone. Irradiation with 750 or 100 eV electrons leads to 2-butanol desorption and partial conversion to butanone, as indicated by the appearance of a peak at 1709 cm(-1).     

Thermochemical and kinetic studies on the effects of cobalt salicylate on the oxidative degradation of polystyrene

The effect of cobalt salicylate on the oxidative degradation and ignition of polystyrene has been studied. It was found that cobalt salicylate sensitizes both the degradation and ignition of polystyrene by facilitating electron-transfer processes in the propagation step. From thermochemical and kinetic studies it was found that the cobalt ion, owing to its ability to exist in variable valence states, promotes electron transfer in the propagation step of polymer degradation, increasing the rate of propagation and consequently the overall rate. Using solid-phase thermal ignition theory, an attempt has been made to explain the sensitization of ignition by the cobalt ion.

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Zusammenfassung Die Wirkung von Kobaltsalicylat auf den oxydativen Abbau und die Entzündung von Polystyrol wurde untersucht. Es wurde festgestellt, daß Kobaltsalicylat sowohl den Abbau als auch die Entzündung des Polystyrols durch Erleichterung von Elektrontransferprozessen im Kettenfortpflanzungsschritt begünstigt. Thermochemische und kinetische Untersuchungen ergeben, daß das Kobaltion infolge seiner Fähigkeit, in mehreren Valenzstufen aufzutreten, den Elektronentransfer im Kettenfortpflanzungsschritt erleichtert wodurch die Geschwindigkeit der Kettenfortpflanzung und damit die Geschwindigkeit des Gesamtprozesses erhöht wird. Basierend auf der Theorie der thermischen Festphasenentzündung wird ein Versuch unternommen, Erleichterung der Entzündung durch Kobaltionen zu erklären.

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Thermal degradation studies on some metal hydrazinic complexes

The paper deals with the characterization of three hydrazinic complexes with Ni, Cu and Cr respectively, by means of non-isothermal thermal methods, TG, DTG and DTA, under nitrogen atmosphere in order to investigate the structure-thermostability-thermal degradation mechanism correlation. The thermal analysis made evident the degradation mechanisms characteristic of every sample in accordance with the chemical structure. The quantitative analysis by TG-DTG afforded the estimation of the metal amount in the complex on the basis of the resulting metallic oxide nature as well as of some aspects of the thermal degradation mechanism supported by mass spectral measurements. The melting points given by DTA and confirmed by the Boetius method and the initial temperatures of thermal degradation from TG-DTG-DTA afforded to ascertain the temperature range proper for using and storing the complexes under study which show potential practical applications as drugs.     

Dithiothreitol-based polyurethanes. Synthesis and degradation studies

The synthesis, characterization, and some properties of new copolyurethanes are described. These copolyurethanes were obtained by reaction of 1,4-di-S-benzyl-d,l-dithiothreitol (DTTSBn) and triethylene glycol (TEG) with 1,6-hexamethylene diisocyanate (HMDI). The copolymer compositions were studied by ¹H NMR, revealing that the content of the copolymer units is in good agreement with that of their corresponding feed. The PU(TEG-HMDI) homopolymer exhibited a high crystallinity, but the introduction of the DTTSBn diol led to a reduction in the crystallinity of the copolymers and an increase of the stiffness, with associated increases in the T_g values. In their TG curves, the copolymers exhibited a mixed trend of the related homopolymers; all of them were thermally stable, with degradation temperatures above 250 °C and with higher thermal resistance displayed by the polymers with higher TEG contents. The chemical and enzymatically catalyzed hydrolytic degradations of the macromolecules were tested. PU(TEG-HMDI) was the only polymer degraded under physiological conditions, but an increase of temperature markedly affected the degradation rates. Two proteolytic enzymes (papain and α-chymotrypsin) and two esterase enzymes (cholesterol esterase and lipase) were chosen to perform enzyme-mediated hydrolysis trials, the first reported use of pancreatic lipase for urethane-bond hydrolysis in polyurethanes.     

Effect of photostabilizers on the rate of photodegradation of polymethylmethacrylate

Study of the action of photostabilizers on the rate of macromolecule scission for PMMA films irradiated with 2537 nm light in air or in inert gas demonstrated that the effectiveness of photostabilizers is determined solely by their absorbing capacity. Photoconversion of PMMA is accelerated by addition of monomer or of some antioxidants. Simultaneous acceleration of the accumulation of free radicals, photodegradation and scission of CH₂ groups is also observed. The initiating additives possess a lower photochemical activity relative to PMMA. The acceleration is due to stronger light absorption by these additives, compared to the polymer.