Mechano-chemical degradation and stabilization of polymers

When polymers are subjected to sheer, macroalkyl radicals are formed which lead to accelerated oxidation. This may occur in the polymer melt during processing or under conditions of mechanical fatigue at lower temperatures (e.g., in cross-linked rubbers). Under these conditions, chain-breaking electron acceptor (CB-A) antioxidants are found to be relatively more effective than under conditions of thermal oxidation and the evidence suggests that the reduced and oxidized couples of some antioxidants, for example, phenol/phenoxyl or hydroxylamine/nitroxyl, can deactivate radicals catalytically.     

Thermal degradation studies of perchlorate-doped conductive polymers

The thermogravimetric behavior of polypyrrole (PPY), polybithiophene (PBT), and poly-aniline (PAN) perchlorate complexes and their corresponding base polymers have been studied. The PBT–perchlorate complex and base polymer decompose at significantly lower temperatures than do the PPY and PAN counterparts. All three perchlorate complexes retain half or more of their original conductivities with no changes in the doping levels after one cycle of heating to 150°C in air and cooling to room temperature. However, after heating at 150°C for 24 h, only the PAN–perchlorate complex shows no significant change in the doping level, although its conductivity decreases by more than two orders of magnitude. X-ray photoelectron spectroscopy analyses of the perchlorate complexes reveal the presence of at least two distinct chlorine species. The thermal decomposition of the perchlorate anions results in the formation of volatile chlorine species as well in chlorine covalently bonded to the polymer. The thermal decomposition of the PPY–perchlorate complex also results in the conversion of positively charged nitrogens to iminelike structures.     

Characterization of cured polysulfide polymers by thermal degradation: Pyrolysis-GC and thermogravimetric studies

Polymers prepared by curing thiol-terminated liquid polysulfide polymer [HS(RS₂)_nRSH, R = ? CH₂CH₂OCH₂OCH₂CH₂? ] with p-quinonedioxime, PbO₂, and MnO₂ were studied by pyrolysis-GC and thermogravimetry. Characteristic differences were observed in the composition of the pyrolyzates under flash pyrolysis at 420 and 470°C. The concentration of the cyclic monomer, 1,3-dioxa-6,7-dithionane, in the pyrolyzates was found to vary considerably from polymer to polymer. The change in mechanism with the extent of degradation and the corresponding overall activation energies were evaluated by thermogravimetry. The pyrolysis-GC and thermogravimetric data were shown to provide partial characterization of the substrates. Mn appears to be in a coordinated complex in the MnO₂-cured polymer.     

Thermal degradation of polymers. Part I. Thermogravimetric and differential thermal analysis studies of atactic poly-m-aminostyrene and related polymers

Thermogravimetric and differential thermal analysis have been employed to study the thermal degradation patterns of poly-m-aminostyrenes and poly-m-acetamidostyrenes of different molecular weights and also the corresponding 2:1 copolymers with styrene. The thermal degradation routes have also been compared and contrasted with that of polystyrene. Experiments have been carried out in static air and dynamic nitrogen, and the different behavior exhibited by these polymers has been reconciled in terms of their differing structures and different mechanisms for pyrolytic degradation.     

Microbial degradation of synthetic polymers

Successful processes for the microbial degradation of synthetic polymers began to emerge in 1978. Organisms capable of biodegradation of polyurethane polyester, polyethylene, polyethyleneglycol, polystyrene, polyvinylalcohol, polyacrylonitrile, nylon 3 and nylon 6 were identified. There was a maximum molecular weight for attack and in some cases only oligomers were susceptible. The polymers were converted to biomass or smaller molecules or polymers without certain side groups. The degradation processes should be capable of much improvement by accelerated evolution of the microbial agents in continuous cultures. Co-oxidation of the polymers with another substrate, for example methane, with methane-oxidising bacteria as agents is suggested as a possible degradative means. The insolubility of polymers is a serious barrier to microbial attack and constitutes a major problem for investigation. Ultimately bio-degradation should provide a mild means for the recycling of synthetic polymers with the advantage of greater control over the degree of degradation.     

Chemiluminescence apparatus and method for studying thermal oxidative stability of polymers

A chemiluminescence multi-sample apparatus and method are described for determining polymer stability by measuring the intensity of the light emitted during thermal oxidation. Depending on the nature of a material analyzed the experiments are performed either under O₂ atmosphere at a constant temperature or under N₂ atmosphere at a constant heating rate. In the former case applicable to polypropylene (PP) and acrylonitrile-butadiene-styrene copolymers (ABS), parameters such as induction time and oxidation rate can be evaluated. In the latter case applicable to nylon and polyethylene terephthalate (PET), the extent of oxidation in a certain temperature region can be evaluated by measuring the area under the intensity of light—temperature curve. Along with providing a great deal of knowledge on thermal oxidative stability, the chemiluminescence approach gives the additional information concerning polymer quality. The appearance of the low-temperature pulses on the chemiluminescence curve observed before the onset of autocatalytic oxidation is associated with the history and processing of the sample and with the natural aging of the polymer.     

Molecular and thermal studies of carbon fiber precursor polymers with low thermal‐oxidative stabilization characteristics

In this investigation, terpolymers, copolymers, and homopolymer of acrylonitrile with dimethylaminopropyl acrylamide (DMAPA), itaconic acid (IA) viz., poly(acrylonitrile‐ran‐3‐dimethylaminopropyl acrylamide‐ran‐itaconic acid) [P(AN‐DMAPP‐IA)], poly(acrylonitrile‐co‐3, dimethylaminopropyl acrylamide) [P(AN‐DMAPP)] were synthesized with varying amounts of comonomers using solution polymerization process. The chemical structure, composition, bonding network were determined employing infrared, 1H and, 13‐carbon nuclear magnetic resonance spectroscopic techniques. Molecular characteristics of as‐synthesized polymers such as different kinds of average molecular weights, molecular weight distribution were estimated applying solution viscometry and size exclusion chromatography. The influence of comonomers (DMPAA, IA) on the thermal stabilization characteristics of acrylonitrile terpolymers in comparison with copolymers and homopolymers of acrylonitrile were studied using differential scanning calorimetry (DSC), hyphenated thermal techniques (thermal gravimetry coupled with differential thermal analyzer).The DSC curves of P(AN‐DMAPP‐IA) exhibit a distinct broader bimodal peaks with thermal exotherm initiating at as low as 165 °C, and followed by two peaks with temperature difference of 42 °C, releasing the evolved heat at a release rate of 0.7–0.11 J g^?1s^?1over 10 min as compared to 1.2, 7.5 J g^?1s^?1 in 4.5, 2 min as observed in P(AN‐DMAPP), polyacrylonitrile, respectively. The thermal stability of P(AN‐DMAPP‐IA) and P(AN‐DMAPP), as evidenced by TGA‐DTA was found to be higher than PAN homopolymers. Specific heat capacity measurements confirmed the DSC results. Bulk densities of P(AN‐DMAPP‐IA) were in the range 0.31–0.35 g/cc. These results confirm the low‐temperature stabilization characteristics and suitability of P(AN‐DMAPP‐IA) as low cost carbon fiber precursor polymers. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46381.     

Light stabilization of polymers using opaque pigments

The ultraviolet light screening characteristics of monodisperse and polydisperse opaque spherical pigment particles are examined in the nonreflection limit. The screening capability for polydisperse particle size distributions is most uniquely described in terms of the area-average radius. The light screening is related to the pigment's area-average radius and volume fraction loading. Light intensity as a function of depth is stated for systems containing an opaque pigment and a soluble UV light absorber.     

含氯聚合物的热稳定和热降解

综述了部分含氯聚合物的热稳定性和热降解研究进展，着重介绍了氯化聚乙烯、聚偏氯乙烯、氯磺化聚乙烯、氯丁橡胶、氯化天然橡胶、氯化聚丙烯等受热脱氯化氢的过程和机理，针对上述含氯聚合物材料的热稳定和热降解，运用现代分析方法，如：差示扫描量热法、差热分析、热重分析法等进行了研究。     

聚氯乙烯的环境问题与大分子稳定化研究进展

从聚氯乙烯生产、加工和回收利用等环节入手,对聚氯乙烯带来的环境问题进行了探讨。就开发无污染稳定剂的问题提出了对稳定剂进行功能和使用阶段上细分化的新观点。通过对近年来国内外聚氯乙烯热降解和稳定化理论研究的追踪,着重介绍了极化子理论在聚氯乙烯稳定化原理研究中的应用,并对我国聚氯乙烯稳定剂的开发方向提出了一些新的思考。     

Synthesis, characterization, thermal degradation, and comparative chain dynamics studies of weak-link polysulfide polymers

Synthesis, spectroscopic and thermal characterization of two new classes of polysulfide polymers: poly[1-(phenoxymethyl) ethylene polysulfide] (PPMEP), and poly[1-(phenoxy) ethylene polysulfide] (PPEP) is presented. The direct pyrolysis mass spectrometry (DP-MS) technique, used to study the thermal degradation behavior of these polysulfide polymers, indicated that the polymers underwent degradation through the weak-links scission. The thermal stability of the polysulfide polymers decreased as the “rank” (number of sulfur atoms in the polysulfide linkage; n = 1, 2, 4) increased. The main-chain flexibility of these polysulfide polymers in terms of their ¹³C NMR spin-lattice relaxation time (T ₁ ) measurements on the backbone methine (-CH-) and methylene (-CH₂-) carbons are reported here for the first time. A comparative study of the solution chain dynamics indicated that it increased as “rank” of the polysulfide linkages decreased as well as by introducing side chain spacers such as, ether (-O-) or methyleneoxy (-CH₂O-) groups.     

Thermal degradation of some perfluoroalkylene-linked polymers

Poly (1,3-phenylenehexafluorotrimethylene) and perfluoroalkylene-linked polyanhydride, polycarbonate and polyester have been studied by thermogravimetry, pyrolysis–gas chromatography–mass spectrometery and fluoride ion analysis. The thermal stability is governed by the ease of formation of specific cyclic degradation products.     

The thermal degradation of perfluoro polymers

A number of perfluoro polymers were heated at a constant rate of temperature rise in an atmosphere of nitrogen or air, and the volatile degradation products were passed into an aqueous solution, where the F^? ions produced were monitored with a fluoride ion-selective electrode. Compounds giving rise to F^? ions were formed in both nitrogen and air, but in larger amounts in the latter atmosphere. For the formation of appreciable quantities of F^? ions, temperatures greater than 500°C were normally necessary.     

The thermal degradation of hydrofluoro polymers

The evolution of hydrogen fluoride (or other degradation products giving rise to Fions in aqueous solution) from a number of hydrofluoro polymers has been studied in nitrogen or air over a range of temperatures using a fluoride ion selective electrode to monitor the process. The results in an inert atmosphere illustrate the effect of chemical structure upon the relative ease of hydrogen fluoride elimination.     

Aspects of stabilization with phosphorous antioxidants in polymers

As plastics are being used in a variety of applications, demands on a greater level of processing stability are increasing. Phosphites are noteworthy as effective processing stabilizer and the performance of phosphite antioxidants can be correlated to the chemical structure of phosphites. Cyclic phosphite esters derived from 2, 2′ methylene bis-2, 4-di-tert-butylphenol and some commercially available phosphites were submitted to comparative studies. Decomposition of cumene hydroperoxide, melt flow of polypropylene and consumption of additives after multiple extrusions were investigated to understand the activity of phosphites as process stabilizers in polypropylene. This study suggests that phosphites play an important role in process stabilization when used in combination with sterically hindered phenols, and that the activity of phosphites may be predicted by their reactivity on hydroperoxide.     

Thermal degradation of liquid polysulfide polymers: Pyrolysis–GC–MS and thermogravimetric studies

The thermal decomposition of Thioped, a liquid polysulfide polymer based on dichloroethyl formal, was studied using pyrolysis–GC–MS. The nature and composition of the products of pyrolysis at 358 and 485°C are given. A striking feature of the product analysis is the presence of several disulfide compounds only at the higher decomposition temperature. The pyrolysis-GC of other liquid polysulfide polymers (LP-2, LP-32, LP-3, and LP-33) showed that molecular weight had only marginal effect on the product composition. Ionic and radical mechanisms were considered to account for the product formation. Isothermal and dynamic thermogravimetric studies were carried out to differentiate between the two mechanisms. The results were found to be in agreement with a free radical mechanism, with cleavage of the formal C? O linkage as the preferred mode of initiation. The overall activation energy for the decomposition was found to be 190 kJ mol^?1.     

Determination of degradation rate of acrylonitrile polymers

Low‐temperature degradations of acrylonitrile homopolymers and acrylonitrile/acrylamide copolymers in air were investigated by IR and thermogravimetry. The degradation rates of acrylonitrile polymers were determined by measurement of normalized absorbance of ? C?N band and effects of the degradation temperature, time, and comonomer on the rate of degradations were discussed. It was found that the polymers start to degrade as the temperature increased to 150°C. In the temperature range of 180–210°C, values of the rate of polymer degradation were maximal and the rate of degradation of acrylonitrile/acrylamide copolymers was higher compared with that of acrylonitrile homopolymers. At 150°C, acrylonitrile homopolymers had an induction period of about 0.5 h. The rate of degradation of acrylonitrile/acrylamide copolymers showed an obvious trend of increase along with acrylamide concentration, and changes of the rate became less prominent as the weight ratio of acrylamide/acrylonitrile went beyond 5/95. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1511–1514, 2006     

Comparative investigation on the thermal degradation and stabilization of carbon fiber precursors

Thermal degradation and stabilization of two kinds of polyacrylonitrile (PAN) fibers have been investigated by a combination of FT-IR, differential scanning calorimetry (DSC), modulated DSC, thermogravimetry (TG), thermal shrinkage behavior, in situ mass spectrometry (MS), and tensile property examinations. The two types of precursor fibers exhibit distinct properties after oxidative stabilization, but they can both make carbon fibers with equivalent mechanical properties. Compared with PAN/itaconic acid precursor fibers, the fibers containing acrylamide comonomers show a doublet appearance, broader exothermic peak, lower threshold degradation temperature, and more amount of heat evolved in DSC thermogram, which is favorable to obtain uniform microstructures in oxidative stabilization process. The two types of samples produce different ring structures in the thermal degradation and stabilization process, as evidenced by results from tensile test, TG–MS and thermal shrinkage behavior analyses. In addition, the molecular rearrangement or melting of ordered structures accompanying with nitrile polymerization was also detected from modulated DSC.