Analysis of degradation products by thermal volatilization analysis at subambient temperatures

In the subambient thermal volatilization analysis (TVA) technique, degradation products initially at ?196°C are allowed to warm up to ambient temperature in a controlled manner under vacuum conditions, and volatilization from the sample tube to a trap at ?196°C is monitored by means of a Pirani gauge. The technique is discussed in relation to earlier TVA work in which volatilization from a heated polymer sample was followed. Design and operation of a subambient TVA system are described, and examples of the application of the technique to the study of the degradation products of seven polymers are considered.     

The influence of tacticity on the thermal degradation of PVC

Based on experimental results on the kinetic zip length of polyene formation and on the polyene distribution in degraded PVC it is deduced, using quantitative model conceptions, that both the configuration and the conformation of the PVC macromolecule may influence the initiation, propagation and termination of thermal dehydrochlorination. In this way it is possible to explain the low ‘basic’ stability of PVC as being determined by the tacticity.     

Cross-linking and gel formation in the thermal degradation of PVC

Cross-linking in the thermal degradation of PVC was investigated by measuring the dependence of number-average molecular weight, gel fraction, molecular weight distribution and intrinsic viscosity on degradation time. A linear relationship was found between the cross-linking density and reaction time. Only one fifth of the polyene sequences formed participate in the cross-linking process.

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Evidence of random chain scissions at low temperatures in the thermal degradation of anionic polystyrenes

In the thermal degradation of some anionic polystyrene samples chain scission was found to occur in the temperature range 180–220°C. The resulting polymer molecules thereafter remain stable up to about 280°C, above which strong reductions occur in the degree of polymerization. It is shown that the low temperature degradation is due to the presence of a small fraction of weak links, which could be tentatively attributed to some oxidation of the polymers.     

Evolution of HCN by thermal oxidative degradation from nylon 66 at high temperatures including burning

Work on the evolution of HCN from nylon 66 was extended to temperatures from ca. 300 to 695°C. Below ca. 300°C the evolution of HCN is governed by chemical decomposition, and above 300°C the evolution is controlled by diffusion. Above 530°C oxidation of HCN becomes noticeable and ignition occurs at 590°C. The rate constants for all of the temperature ranges and for the oxidation of HCN are given in terms of Arrhenius equations. The activation energy for the oxidation of HCN before ignition (590°C) is reached is 47 kcal/mole, and beyond this point, the oxidation is controlled by diffusion. The rate constants increase linearly with oxygen concentration as long as HCN oxidation is negligible; however, they pass through maxima if HCN oxidation is appreciable (some HCN is evolved even in the absence of oxygen). A new flash degradation apparatus has been constructed for these high-temperature ranges and a degradation mechanism has been proposed which is in satisfactory agreement with the experimental results.     

Initiated oxidation of polyenes formed in the thermal degradation of PVC

Polyenes formed in the thermal degradation of PVC are readily oxidized in the liquid phase in the presence of a radical initiator. In pure oxygen at a constant rate of initiation, different length polyenes are consumed by a first-order reaction: the rate of consumption is proportional to the polyene length and to the square root of initiator concentration. Applying the relationships given by the theory of chain reactions, we determined the ratio of the rate constant of chain propagation calculated for one double bond k₂ to the square root of the chain-termination rate constant k₄. The value obtained, which is relatively high in comparison to other unsaturated hydrocarbons, reflects very well the high reactivity of polyenes of the degraded PVC sample towards oxidation. Intramolecular chain propagation steps are also likely to play a role in the oxidation of polyenes.     

Cyclopentadienylation of PVC: Characterization and thermal and thermooxidative degradation studies

PVC has been cyclopentadienylated by two conventional basic, LiCp and NaCp, and a new acidic, Me₂CpAl, cyclopentadienylating agent. PVCs treated with basic cyclopentadienylating agents undergo severe random dehydrochlorination and exhibit a significant decrease in thermal and thermooxidative stability. In contrast, according to ozonization and degradation experiments, Me₂CpAl does not cause dehydrochlorination during cyclopentadienylation. The thermal stability of PVC treated with relatively high concentrations of Me₂CpAl and Me₃Al at 25°C markedly increases due to substitution of labile chlorines in PVC with methyl groups. Initial thermal dehydrochlorination behavior of virgin PVC and samples treated with Me₂CpAl at ?30°C are similar. In contrast, thermooxidative stability decreases on Me₂CpAl treatment at ?30°C; this is attributed to ease of oxidation of pendant cyclopentadienyl groups; that is, the formation of peroxy radicals that may initiate dehydrochlorination by attacking unchanged repeat units in PVC. Acceleration of thermal dehydrochlorination disappears and the length of polyene sequences is reduced on Me₂CpAl and Me₃Al treatment. These observations are attributed to differences in rates of protonation-deprotonation; that is, rates of reinitation of zipping of treated and untreated PVCs during thermal degration. The effect of traces of aluminum residues on degradation of modified PVCs, however, cannot be neglected.     

Calibration of conductance cells at various temperatures

Precise conductance measurements on aqueous potassium chloride solutions at 0, 10, 18, and 25°C have been made under various conditions over a concentration range 10^?4?2 mole-dm^?3, yielding the conductance equations $$\begin{gathered} 25^\circ C:\Lambda = 149.873 - 95.01\sqrt c + 38.48c log c + 183.1c - 176.4c^{3/2} \hfill \\ 18^\circ C:\Lambda = 129.497 - 80.38\sqrt c + 32.87c log c + 154.3c - 143.0c^{3/2} \hfill \\ 10^\circ C:\Lambda = 107.359 - 64.98\sqrt c + 27.07c log c + 125.4c - 110.3c^{3/2} \hfill \\ 0^\circ C:\Lambda = 81.700 - 47.80\sqrt c + 20.60c log c + 93.8c - 79.3c^{3/2} \hfill \\ \end{gathered} $$ which are proposed for calibration of conductance cells.     

Hydrolytic polycondensation of methylphenyldichlorosilane at various temperatures

Products of hydrolytic polycondensation of methylphenyldichlorosilane at −60 and +20°C were studied by gel permeation chromatography.     

Morphology and mechanical properties of ultrahigh molecular weight polypropylene prepared by gelation/crystallization at various temperatures

Films of ultrahigh molecular weight (5.4×10⁶) polypropylene were produced by gelation/crystallization at various temperatures from dilute decalin solutions according to the method of Smith and Lemstra. The temperatures chosen were 20°, 30°, 50°, and 60°C. With increasing the temperature, the long period and crystallinity of the resultant gel film increased. By contrast, when the films were stretched up to 50 } 60 times, the increases in Young's modulus and crystallinity become more significant, as the temperature of the gelation/crystallization became lower. This interesting phenomenon is thought to be due to the dependence of the number of entanglements on the temperatures concerning gelation/crystallization and evaporation of solvent from the gel to form a film.     

Influence of the tacticity on thermal degradation of PVC. IV. Degradation in solution in comparison with degradation at solid state

Four fractions of PVC were obtained by successive extractions with appropriate solvents from PVC samples prepared in bulk at 60, 40, 0, and ?50°C. Then they were carefully characterized, especially as far as stereoregularity is concerned. The samples were submitted to thermal degradation both at solid state and as solutions in dioctyl phthalate. The type of the polyene sequence distribution in degraded samples was determined by both the UV-visible spectroscopy and the number of chain scissions resulting from the ozonization reaction. The degradation rate was proved to rise together with the syndiotactic sequence content in agreement with results in Parts I–III, obtained only with degradation at solid state. On the other hand, The content of the long polyenes relative to that of the short polyenes in the degraded polymers was found to be higher, and the number of chain scissions through ozonization reaction lower, when the syndiotactic sequence content is high. These results enable us to conclude that the actual mechanism of the PVC thermal degradation depends markedly on the tacticity distribution regardless of whether the degradation occurs either at solid state or in solution.     

Synthesis and characterization of ZnO nanowires by thermal oxidation of Zn thin films at various temperatures

In this research high-quality zinc oxide (ZnO) nanowires have been synthesized by thermal oxidation of metallic Zn thin films. Metallic Zn films with thicknesses of 250 nm have been deposited on a glass substrate by the PVD technique. The deposited zinc thin films were oxidized in air at various temperatures ranging between 450 °C to 650 °C. Surface morphology, structural and optical properties of the ZnO nanowires were examined by scanning electron microscope (SEM), X-ray diffraction (XRD), energy dispersive X-ray (EDX) and photoluminescence (PL) measurements. XRD analysis demonstrated that the ZnO nanowires has a wurtzite structure with orientation of (002), and the nanowires prepared at 600 °C has a better crystalline quality than samples prepared at other temperatures. SEM results indicate that by increasing the oxidation temperature, the dimensions of the ZnO nanowires increase. The optimum temperature for synthesizing high density, ZnO nanowires was determined to be 600 °C. EDX results revealed that only Zn and O are present in the samples, indicating a pure ZnO composition. The PL spectra of as-synthesized nanowires exhibited a strong UV emission and a relatively weak green emission.     

Formation of 2-chlorobenzylidenemalononitrile (CS riot control agent) thermal degradation products at elevated temperatures

2-Chlorobenzylidenemalononitrile (CS riot control agent) has been shown to produce a number of thermal degradation products when dispersed at high temperature. We hypothesized that these CS-derived compounds are formed by energy input from heating during the dispersion process. Here we identified organic CS-derived compounds formed from purified CS subjected to temperatures ranging from 300 to 900 degrees C in an inert atmosphere with analysis of tube furnace effluent by gas chromatography and mass spectrometry. We conclude that the production of many CS-derived compounds previously observed during high-temperature dispersion is likely to be heat related.     

HCN evolution from thermal and oxidative degradation of poly(diphenyl methane pyromellitimide) at high temperatures

HCN evolution from thermal and oxidative degradation of poly(diphenyl methane pyromellitimide) has been investigated over a range of temperatures from 500 to 1000°C; rate constants and Arrhenius equations have been determined. Kinetics and mechanisms have been proposed and quantitatively evaluated. They account well for the experimental results. The rate determining steps are C? N scission for thermal degradation and H abstraction from the methylene bridge by O₂ for oxidative degradation, respectively. At high temperatures, oxidation and thermal decomposition of the evolved HCN take place on its passage through the hot zone of the furnace in the highest range of temperatures (800–1000°C). Additional HCN is produced (>800°C) from the char obtained during thermal and oxidative degradation.     

The thermal reaction of HNCO at moderate temperatures

The thermal reaction of HNCO has been studied in a static cell at temperatures between 873 and 1220 K and a constant pressure of 800 torr under highly diluted conditions. The reaction was measurable above 1000 K by FTIR spectrometry. The products detected include CO, CO₂, HCN, NH₃, and the unreacted HNCO. In this moderate temperature regime, the rates of product formation and HNCO decay cannot be accounted for by a previously established high-temperature mechanism, assuming HNCO → NH + CO (1) as the initiation process. Instead, a new bimolecular reaction, 2HNCO → CO₂ + HNCNH (2), has been invoked to interpret the disappearance of HNCO as well as the formation of various products, most importantly CO₂. The concentration profiles of all measured species can be quantitatively modeled, throughout the temperature range analyzed, by varying k₂ using a modified mechanism. The kinetically modeled values of k₂ can be effectively represented by This result agrees closely with that computed with the conventional transition-state theory using the TST parameters predicted by the BAC-MP4 method: The bimolecular reaction takes place via a stable 4-membered ring intermediate which is isoelectronic with diketene; viz.      

Study of the miscibility and the thermal degradation of PVC/PMMA blends

The aim of this paper is to study the miscibility and the thermal degradation of PVC/PMMA blends. For that purpose, blends of variable compositions from 0 to 100 wt% were prepared with and without plasticizer. Their physico-chemical characterization was carried out by differential scanning calorimetric analysis (DSC) and Fourier transform infrared spectroscopy (FTIR). Their thermal degradation under nitrogen at 185°C was studied and the HCl evolved from PVC was measured by the pH method. Degraded samples were characterized, after purification, by FTIR and UV-visible spectroscopy. The DSC analysis showed polymer miscibility up to 60 wt% of PMMA. This miscibility is due to a specific interaction of hydrogen bonding type between carbonyl groups (C=O) of PMMA and hydrogen from (CHCl) groups of PVC as evidenced by FTIR analysis. On the other hand, it was found that PMMA exerted a stabilizing effect on the thermal degradation of PVC by reducing the zip dehydrochlorination and by leading to the formation of short polyenes.     

Analysis of nitromethane thermal decomposition at low temperatures

The thermal decomposition of nitromethane (NM) over the temperature range from 580 to 700 K at pressures of 4 Torr to 40 atm was analyzed. On the basis of literature data, with the use of theoretical transitional curves of the modified Kassel integral, the rate constants k of NM decomposition at the upper pressure limit were determined. The values thus obtained are in good agreement with the results of extrapolation of the high-temperature (1000–1400 K) k _{1, ∞} values to lower temperatures. The reasons for which the NM decomposition rate constants differ by two orders of magnitude at low temperatures are considered. A general expression for the NM decomposition rate constant at the upper pressure limit over the 580–1400 K temperature range was determined: k _{1, ∞} = (1.8 ± 0.7) × 10¹⁶ exp((?58.5 ± 2)/R _T ) s^?1. These data disprove the hypothesis that a nitro-nitrite rearrangement takes place during the NM decomposition at low temperatures.     

Ion-solvent interactions in aqueous solutions at various temperatures

The temperature variation of the limiting partial molar volumes of a number of electrolytes in aqueous solution has been examined in terms of solvent electrostriction. The Desnoyers, Verrall, and Conway theory has been modified and extended to cover the temperature range 0–100°C. It has been shown that electrostriction effects alone cannot account for the observed maxima in the V° –T plots for various electrolytes. It is concluded that solvent structural changes over this temperature range may well be important.