DTA studies on the thermal oxidation and crosslinking reactions of carboxyl-terminated polybutadiene

Studies on the thermal oxidation of carboxyl-terminated polybutadiene in the presence of antioxidants have been carried out by dynamic DTA. Bis-thioacetylacetonato nickel(II) compounds are found to be effective in inhibiting the air oxidation reaction in the polymer. The crosslinking reaction of the polymer through the double bonds present in the polymer molecule is desensitized by the antioxidants and the effect is more with N-phenyl-1-naphthylamine. An exothermic peak formed at 270°C in the presence of tris(2-methylaziridinyl-1) phosphine oxide has been identified as the curing reaction. The infrared spectra of CTPB in the presence of MAPO at various temperatures confirm the various stages of reaction.     

The structure and stability of oxidised polybutadiene

Polybutadiene has been oxidised at 0–20°C for periods from one day up to several months and also at elevated temperatures. Microstructural changes in the polymer due to oxidation have been followed by ir and uv spectroscopy. Oxidised samples of the polymer have been degraded in nitrogen or in vacuum under programmed heating conditions by TG, DSC and thermal volatilisation analysis (TVA). In TVA degradations, non-condensable gaseous products have been studied by adsorption TVA, condensable volatile products have been separated by subambient TVA for identification, cold ring fraction materials have been examined spectroscopically and the ir spectrum of the polymer residue, after degradation to 440°C, has also been obtained. From this considerable amount of data it has been possible to propose structures present in oxidised polybutadienes and to suggest some degradative processes involved in the subsequent thermal degradation. Oxidation leads to a considerable lowering of the threshold temperature for the main decomposition process; in samples oxidised at low temperatures, an additional early stage of degradation, commencing near 100°C, is found, due to peroxide decomposition.     

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.     

Study of the curing of an acetylene-terminated polyphenylene resin with thermal expansion measurements

The curing of an acetylene-terminated polyphenylene resin (Hercules H-Resin) was followed by thermal expansion measurements. This approach proved useful in optimizing the curing conditions of the resin. Curing the polymer in air led to the formation of carboxyl groups, whereas curing under nitrogen did not. The thermal expansion coefficient is a minimum (32 ppm deg^?1) for a cure cycle of 250°C for 30 min, followed by 350°C for 30 min. Heating at temperatures above 350°C led to degradation of the crosslinked polymer and an increase in the thermal expansion coefficient.     

The thermal degradation of poly(diethyl fumarate)

The kinetics and mechanism of the thermal degradation of poly(diethyl fumarate) (PDEF) were studied by thermogravimetry, as well as by analysis of the thermolysis volatiles and polymer residue. The characteristic mass loss temperatures were determined, as were the overall thermal degradation activation energies of three PDEF samples of varying molar mass. Ethylene and ethanol were present in the thermolysis volatiles at degradation temperatures below 300 °C, while diethyl fumarate was also evidenced at higher degradation temperatures. The amount of monomer increased with increasing degradation temperature. The dependence of the molar mass of the residual polymer on the degradation time and temperature was established and the number of main-chain scissions per monomer unit, s/P₀, calculated. A thermal degradation mechanism including de-esterification and random main-chain scission is proposed. The thermal degradation of PDEF was compared to the thermolysis of poly(ethyl methacrylate) (PEMA), poly(diethyl itaconate) (PDEI) and poly(ethyl acrylate) (PEA).     

Phthalocyanine polymers. V. Novel type of thermally stable polyimides derived from metal phthalocyanine tetraamines and pyromellitic dianhydride

New poly(metal phthalocyanine) pyromellitimides of copper, cobalt, nickel, and zinc were synthesized and elemental, IR, TGA, and inherent viscosity measurement studies were done to characterize these polymers. They showed exceptional thermal and thermo-oxidative stability with char yields at 800°C ranging from 75 to 87% in a nitrogen atmosphere. The ratio of the polymer decomposition temperatures in air and nitrogen (PDT) (air)/PDT (N₂) varied from 0.91 to 0.97, which indicated that the onset of degradation of these polymers is greatly affected by the presence of an oxidizing atmosphere. Isothermal studies were done to evaluate the long-term thermal stability of these polymers.     

Degradation study on the thermally stable nickel phthalocyanine sheet polymer

Poly (nickel phthalocyanine) which has exceptional thermal stability is synthesized. Knowledge of modes of degradation of this polymer is found to be necessary for its high temperature applications. This polymer showed very high thermal stability with maximum polymer decomposition temperatures (PDT_max) of 500 °C in air and 890 °C in N₂, with char yield 93% at 800 °C. Because of its excellent thermal stability, degradation study with MS as well as GC–MS techniques were found to be very difficult. The present publication deals with MS and GC–MS studies of nickel phthalocyanine sheet polymer at high temperatures ranging from 700 to 1000 °C. Tentative mechanisms are proposed for its modes of fragmentations and based on GC–MS studies, the most probable degradation products are identified.     

Isothermal thermogravimetric studies of molten diacetate terminated polyoxymethylene in nitrogen and air

Thermogravimetric studies of diacetate terminated polyoxymethylene in its molten state were made in nitrogen and air atmosphere. They also allow the determination of changes in the thermal stability of the polymer in the course of decomposition. Oxygen has an accelerating effect on the decomposition of the polyoxymethylene. The beginning of the weight loss in air occurs at lower temperatures and its rate increases. The activation energies of the degradation in air and in nitrogen are not very different as compared to the preexponential factors and reaction orders whose values vary considerably.     

Crystallization during polymerization of poly-p-xylylene. III. Crystal structure and molecular orientation as a function of temperature

Polymerization of p-xylylene was carried out from the gas phase with monomer produced by the pyrolysis of [2,2]-p-cyclophane. The crystalline form and preferred orientation of as-polymerized polymer deposited at various temperatures (?196 to 80°C) were investigated by x-ray diffraction methods. The melting behavior and other thermal transitions were studied by DSC. At 80°C the polymer film deposit is a mixture of the α and β forms, while between 60 and 0°C the deposit is of the α form. At lower temperature the polymer deposit is mainly of the β form, which shows diffuse reflections. At liquid nitrogen temperature it is of the β form with sharp reflections, contaminated with a small amount of oligomer. It was also found that at low temperatures, fibrillar crystals grow from the substrate in a direction 45° against the gas flow, and at even lower temperature, well-oriented filmlike crystals grow perpendicular to the substrate surface.     

Photo- and thermal curing of tri-functional methacrylate with degradable property

Tri-functional methacrylate (MA3) having thermally degradable tertiary ester linkages was synthesized by a convergent approach. When the MA3 film containing a thermal initiator and a photoacid generator was heated under nitrogen atmosphere, it became insoluble. The thermally cured MA3 became soluble after irradiated and followed by baking at 120 °C. Acid catalyzed decomposition of the tertiary ester linkages of the MA3 moiety occurred at lower temperatures than non-catalyzed one. A mechanism for the photo- or thermal curing and photoassisted-thermal degradation was studied by FT-IR, ¹H NMR, TGA, MS, and SEC analyses.     

Exactly alternating silarylene–siloxane polymers. VII. Thermal stability and degradation behavior of p-silphenylene–siloxane polymers with methyl,vinyl, hydrido,and/or fluoroalkyl side groups

The thermal stability and degradation behavior of a series of nine different exactly alternating silphenylene-siloxane polymers which contained methyl, vinyl, hydrido, 3,3,3-trifluoropropyl, and tridecafluoro-1,1,2,2-tetrahydrooctyl side groups, or their combinations, were investigated by dynamic and isothermal gravimetric analyses in air and in nitrogen. Two distinctly different mechanisms were observed in these atmospheres: a complex multi-step weight loss process in air and a single-step process in nitrogen. In nitrogen all polymers produced black, insoluble, highly stable degradation residues which were characterized by high carbon content. In contrast, in air the nonfluorine containing polymers degraded to pure silica, while the fluoroalkyl substituted polymers may have formed fluorosilicates of unspecified structures. There appears to be no significant molecular weight effect on the thermal stability of these polymers, at least not above an M _w value of about 35,000. Isothermal investigations indicate that 300°C in air and 350°C in nitrogen may be possible upper use temperatures for the methylvinyl substituted, exactly alternating silphenylene–siloxane polymers for extended periods of time. A strong thermostabilizing effect by vinyl side groups on the degradation behavior of these polymers was established. The extent of stabilization depends on the content of vinyl units, but it can already be clearly seen at the 5 mol % vinyl level, and it increases exponentially with increasing vinyl concentration. In contrast to this behavior, by comparison with the parent all-methyl substituted, exactly alternating silphenylene–siloxane polymers, the hydrido and fluroalkyl side groups reduce overall polymer thermal stability in terms of the degradation onset temperature, the temperature for 50% weight loss, and the amount of degradation residue. The presence of these groups also extends the later stages of the degradation processes to higher temperatures. Based on these and previous results, an order of stability is proposed as a function of the type of the substituent side groups for the thermal degradation of these polymers.     

Thermal degradation of organo-soluble polyimides

The thermal degradation behavior of two organo-soluble polyimides was investigated by high resolution pyrolysis-gas chromatography/mass spectrometry. The pyrolyzates of the polymers at various temperatures were identified and characterized quantitatively. The relationship between the polymer structure and pyrolyzate distribution was discussed. The kinetic parameters of the thermal degradation were calculated based on thermogravimetric measurements. Finally, the thermal degradation mechanism for the polymers was suggested.     

Inhibited degradation of nylon 66 in presence of nitrogen dioxide,ozone, air,and near—ultraviolet radiation

Degradation of nylon 66 films of different morphologies was studied in the presence of nitrogen dioxide, ozone, oxygen, and near-ultraviolet radiation (λ > 2900 Å). Films cast from formic acid solution showed normal random degradation, whereas films cast from benzyl alcohol solutions and dried at elevated temperatures under nitrogen showed very strongly inhibited random degradation. This inhibition may be due to protection of peptide groups by hydrogen bonding with benzaldehyde or benzoic acid or even to their chemical reactions at elevated temperatures. Oxygen was not rigorously excluded during preparation of the films. Degradation of nylon 66 films cast from formic acid solutions at room temperature containing benzaldehyde or benzoic acid, respectively, is also inhibited. The energy of activation for inhibited degradation in presence of nitrogen dioxide is relatively small, indicating that the process is either controlled by diffusion of polymer radicals from medium cages or by diffusion of gases into the polymer. The degradation kinetics can be expressed by “weak”-link random degradation. The weak links are in the present case unprotected peptide groups. The functional relationship between chain scission rate constants and NO₂ pressure is linear.     

Application of pyrolysis-capillary gas chromatography with NPD detection in thermal degradation of polyphosphazenes study

Polyphosphazenes represent a unique class of polymers with a backbone composed of alternating phosphorous and nitrogen atoms. The thermal behaviour and decomposition of a variety of polyphosphazenes depends on the type of side groups present. Especially those that bear aryloxy side groups, possess a high temperature stability as well as excellent flame resistance. Pyrolysis-capillary gas chromatography has been used in a study of three polyphosphazene samples for thermal stability characterisation. Degradation products were detected with three single detectors for flame ionisation (FID), nitrogen-phosphorous sensitivity (NPD) and mass spectrometry (MSD) at different pyrolysis temperatures ranging from 300°C up to 800°C. The NPD responses for phosphorous or nitrogen fragments of polyphosphazenes have been used for the construction of degradation product schemes and the examination of the thermal stability of the polyphosphazene’s backbone. Partial identification of the degradation products present in the gaseous phase was achieved by MSD. The polyphosphazenes thermal degradation conversion rates were at a maximum at 450–500°C. At various pyrolysis temperatures, the calculated N/P peak area ratio is a function of the degree of polyphosphazene-N=P-chain degradation, and reflective of the nitrogen — phosphorous detector sensitivity. NPD proved to be suitable tool for characterization of polyphospazene thermal stability.     

Poly(ethylene sulfide). II. Thermal degradation and stabilization

High molecular weight poly(ethylene sulfide) undergoes severe thermal degradation at the high temperatures (220–260°C) required for processing in injection-molding equipment. Thermal degradation of the polymer is accompanied by gas evolution and a decrease in melt viscosity. Stabilization of poly(ethylene sulfide) can be effectively accomplished by addition of small concentrations of certain 1,2-polyamines, preferably together with certain zinc salts as coadditives. Use of this stabilizer system inhibits thermal degradation to a remarkable extent, making it possible to mold the polymer at these high temperatures and obtain excellent physical and mechanical properties. Investigation of the thermal degradation process was carried out. The rate at which gases evolved from unstabilized poly(ethylene sulfide) resins of various molecular weights and preparative histories and from model compounds of the same organic backbone structure was measured at temperatures ranging from 220 to 260°C. Rate of gas evolution from the resins, irrespective of chain length or preparation, was found to be constant at 230°C. The evolved gases, analyzed by infrared spectroscopy and gas chromatography, contained ethylene. Nearly identical apparent activation energies were found for the gas evolution reaction from the resin and model compounds. The ΔE^* values were in good agreement with ΔE^* determined by other techniques, 58 ± 2 kcal/mole. This is about the energy requirement expected for the homolytic cleavage of a carbon–sulfur bond of the type present in a poly(ethylene sulfide) structure. The rate and analytical data indicate that the degradative mechanism at processing (molding) temperatures is primarily due to the organic structure of the polymer. A mechanism of thermal stabilization is proposed in which the polyamine and zinc salt, in presence of molten polymer at processing temperatures, form a two-centered electron transfer complex, capable of reacting with both radicals of the homolytically cleaved bond, “healing” the scission, so to speak.     

Enhanced thermal properties of highly monodispersed ZnO nanoparticle/poly(styrene-<Emphasis Type="Italic">co</Emphasis>-acrylonitrile) nanocomposite

Chemical based approach for the synthesis of poly(styrene-co-acrylonitrile)/ZnO nanocomposites with different ZnO nanoparticle content by in situ emulsion polymerization is discussed. A significant increase in the thermal degradation temperature, melting and crystallization temperatures in all nanocomposite is observed. Increasing ZnO loading to polymer matrix enhances the flame retardant ability of polymer matrix with an appreciable increase in thermal degradation temperature of pristine polymer.     

Physical properties of films made of copoly(ether-imide)s with long poly(ethylene oxide) segments

Polyimides having long poly(ethylene oxide), PEO, moieties in the main chain have been synthesized by a classical two-steps polycondensation method with good yield and high molecular weight. In contrast with previous works on this topic, essentially full conversion of the polyamic acid to polyimide was attained by heating at relatively low temperatures (around 160 °C).These copolyimides undergo an increase of phase separation between the PEO part and the polyimide one after a thermal annealing. This phase separation increases gas separation properties of membranes made up of these copolymers. An exhaustive study of polymer properties as a function of the thermal treatment has been carried out in order to figure out the origin of this behavior. The analysis performed included TGA, DSC, SAXS and mechanical testing.The polymers studied in this paper have medium thermal stability. In fact, degradation of the polyether chain under nitrogen takes place at temperatures above 300 °C. However, their thermal stabilities were much lower under oxidant atmosphere.     

Thermal degradation of cellulose in nitrogen

The thermal degradation of four different forms of cellulose in nitrogen has been studied by using a thermobalance. In TG experiments a total weight loss at 900°C was 80% in the cases of film and pulp samples and 83% for two powder forms. The results for the isothermal degradation of the four samples at 270°C are plotted as degree of degradation α against reduced time t/t_0.5 and compared with the master plots of Sharp, Brindley, and Achar. The experimental data fit most closely the plot for the Avrami-Erofeev equation in the form kt = {–ln (1–α)}^1/n where n = 2. An activation energy of 144 kJ/mole has been found for the degradation of one of the celluloses from the results of isothermal runs at six different temperatures. It is postulated here that the thermal degradation occurs by random nucleation and nucleus growth in the cellulose fibrils so as to yield a carbon whose microporous structure is a replica of the pore system in the parent cellulose.     

Thermal behavior of poly-p-xylylene-m-carborane

Thermal analysis, infrared spectroscopy, and gel-permeation chromatography studies were undertaken to determine the behavior of poly(p-xylylene-m-carborane) at elevated temperature. Results show that the polymer softened at about 200°C, probably because of polymorphism. Chlorine atoms from chain ends also ruptured at this temperature. This initiated subsequent hydrogen abstraction and thermal oxidation reactions that resulted in the decomposition of the polymer. The process of degradation closely parallels the thermal oxidation of polybenzyl and other polymers with readily activated methylene groups. The volatile products that formed at 300 and 400°C were produced because of the cleavage of methylene groups and their oxidation products. Larger polymer segments containing phenylene and m-carborane groups were evolved at higher temperatures. Some crosslinking occurred when the polymer was heated in air at temperatures above 200°C. The degree of polydispersity of the polymer fraction that remained soluble in organic solvents increased with corresponding increase of temperature.     

Mechanistic study of thermal behavior of poly(vinyl acetate) blended with aluminum tribromide: an investigation aided by IR and Py–GC–MS techniques

Thermal behavior of poly(vinyl acetate) in the presence of aluminum tribromide (thin film cast from common solvent) is studied by thermoanalytical, IR, and Py–GC–MS techniques. Pyrolytical characterization reveals two-step degradation for neat polymer whereas blends pyrolyze in three stages. The detailed examination of blends was performed to identify the compounds formed. Moreover, the mixed film sample was heated at 200, 300, and 400 °C and residues were analyzed by IR spectroscopy in order to follow the progress of degradation. Acetic acid does not appear as major product of the blends’ degradation due to the formation of acetyl bromide, development of polyene backbone at high temperatures with intact acetate units, conversion of acetate to formate under the influence of additive, etc. The effective flame retardance of additive for polymer is noteworthy. Various schemes have been outlined to show the probable degradation mechanism.