NMR relaxation study of thermal degradation in cured PMR polyimides

We have studied cross-linking and thermal degradation of high-performance first-and second-generation PMR-15 polyimides, both thermoset and thermoplastic versions, by performing nonspectroscopic NMR solid echo T^*₂ relaxation measurements at temperatures up to 430°C using probes built for this purpose. We employ signal averaging and automated decomposition of the relaxation decays into two Gaussian components, the slower of which gradually appears above 300°C. Tracking the molecular mobility spectrum in terms of the relative intensity of the components and their relaxation times as temperature is cycled, we detect essentially no irreversible effects below the glass transition, measure permanent mobility reductions attributable to completion of cure, and find that exposure to temperatures above 380°C on the order of 1 h is required for substantial thermal degradation to occur. These results are closely supported by thermal and mechanical measurements on parallel specimens. Second-generation PMR resins appear to have higher microscopic rigidity and reduced viscous fraction at high temperatures. ©1995 John Wiley & Sons, Inc.     

Kinetics of thermal degradation of wood biomass

Pyrolysis kinetics of a hardwood representative, beech (Fagus sylvatica), was investigated by two different kinetic approaches: model-free isoconversional method and model-fitting method. The model-free isoconversional method was used for the determination of apparent kinetic parameters, i.e. the activation energy and pre-exponential factor. The model fitting method was used for the optimization of kinetic parameters of the reaction pathways of three selected reaction mechanisms: one-step, two-step, and three-step one. In both approaches, thermo-gravimetric data were used at five heating rates: 2°C min^?1, 5°C min^?1, 10°C min^?1, 15°C min^?1 and 20°C min^?1. As the most suitable mechanism, the three-step mechanism containing the intermediate degradation step was chosen. This selection was supported by experimental results from the ¹³C NMR analysis of solid residues prepared at the key temperatures within the range of 230–500°C. The progress of mass fraction values of each component in this mechanism was simulated. Conclusions from the simulation were confronted with experimental results from the ¹³C NMR.     

Thermal degradation of polyimides—II: Mechanism of carbon dioxide formation during thermal degradation

Thermal degradation of polyimides was investigated using a selected polymer and a simple low molecular model viz. phthalimide. Based on the analysis of the degradation products, a mechanism for CO₂ formation was proposed; it could be adapted to explain the thermal degradation processes for polyimides.     

Kinetics of thermal degradation and thermal oxidative degradation of poly(p-dioxanone)

The kinetics of the thermal degradation and thermal oxidative degradation of poly(p-dioxanone) (PPDO) were investigated by thermogravimetric analysis. Kissinger method, Friedman method, Flynn-Wall-Ozawa method and Coats-Redfern method have been used to determine the activation energies of PPDO degradation. The results showed that the thermal stability of PPDO in pure nitrogen is higher than that in air atmosphere. The analyses of the solid-state processes mechanism of PPDO by Coats-Redfern method and Criado et al. method showed: the thermal degradation process of PPDO goes to a mechanism involving random nucleation with one nucleus on the individual particle (F₁ mechanism); otherwise, the thermal oxidative degradation process of PPDO is corresponding to a nucleation and growth mechanism (A₂ mechanism).     

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.     

Kinetics of thermal degradation of poly(-caprolactone)

The thermal degradation/modification dynamics of poly(-caprolactone) (PCL) was investigated in a thermogravimetric analyzer under non-isothermal and isothermal conditions. The time evolution of the molecular weight distribution during degradation was studied using gel permeation chromatography. Experimental molecular weight evolution and weight loss profile were modeled using continuous distribution kinetics. The degradation exhibited distinctly different behavior under non-isothermal and isothermal heating. Under non-isothermal heating, the mass of the polymer remained constant at initial stages with rapid degradation at longer times. The Friedman and Chang methods of analysis showed a 3-fold change (from 18 to 55–62 kcal mol⁻¹) in the activation energy from low temperatures to high temperatures during degradation. This suggested the governing mechanism changes during degradation and was explained using two parallel mechanisms (random chain scission and specific chain end scission) without invoking the sequential reaction mechanisms. Under isothermal heating, the polymer degraded by pure unzipping of specific products from the chain end.     

Investigation of thermal and thermooxidative degradation of some polyimides containing oxyphenylene groups in the main chain

Data for thermogravimetric analyses of some polymides synthesized from diamines and dianhydrides containing various amounts of oxyphenylene groups have been used to show the effect of the chemical structure of polyimide on its thermooxidative stability. Two polyimides were taken as examples demonstrating the possibilites of combining the methods of thermal analysis (TGA, DTA and TVA) for studying the transformations of polyamic acids and polyimides over the temperature range 20–800°.     

Kinetics of thermal degradation of poly(vinyl chloride)

Extensively studied thermal degradation of polyvinyl chloride (PVC) occurs with formation of free hydrogen chloride and conjugated double bonds absorbing light in visible region. Thermogravimetric monitoring of PVC blends degradation kinetics by the loss of HCl is often complicated by evaporation and degradation of plasticizers and additives. Spectroscopic PVC degradation kinetics monitoring by absorbance of forming conjugated polyenes is specific and should not be affected by plasticizers loss. The kinetics of isothermal degradation monitored by thermal gravimetric analysis in real time was compared with batch data obtained by UV/Visible absorption spectroscopy. Effects of plasticizer on kinetics of polyene formation were examined. Thermal degradation of PVC films plasticized with di-(2-ethylhexyl) phthalate (DEHP) and 1,2,4-benzenedicarboxylic acid, tri-(3-ethylhexyl) ester (TOTM) was monitored by conjugated double bonds light absorption at 350 nm at 160, 180, and 200 °C. Plasticizer-free PVC powder degradation kinetics and that of plasticized films were also obtained thermogravimetrically at temperatures ranging from 160 to 220 °C. Plasticizer-free PVC powder degradation and spectroscopically monitored degradation of plasticized PVC films occurred with the same apparent activation energy of ≈150 kJ mol⁻¹. No difference in degradation kinetics of films plasticized with DEHP and TOTM was detected.     

阻燃共聚酯/粘土复合物热降解动力学研究

用插层共聚方法合成了含磷共聚酯／粘土复合物。用热重(TG)方法考察热降解动力学。通过在空气中以不同的升温速率升温至设定温度,用Kissinger法和Hymn-Wall-Ozawa法对数据进行处理。结果表明,粘土组分含量较高的反应活化能较大,热稳定性较好。     

Kinetics of thermal and acidic degradation of poly-1,3-dioxolane

The kinetics and parameters of thermal and acidic degradation of poly-1,3-dioxolane were investigated in order to elucidate the mechanism of degradation and to obtain information on the nature of active centers. Both homolytic and heterolytic breaking of a macromolecule were shown to be random and occur at the acetal bond. Thermal degradation was found to proceed in two stages, depending on temperature and involving active centers of a different nature, i.e., macroions and macroradicals. The rate-determining step of thermal degradation appears to be one involving the radical component, similarly to thermal degradation of olefin polyoxides. Acidic degradation occurs solely by the depolymerization mechanism, as in the case of polyaldehydes. It was concluded that the degradation mechanism depends not only on the chain structure and the thermodynamic properties of the system, but also on the nature of active centers.     

Effect of structure on the thermal stability of polyimides

Polyimides with different proportions of m-phenylene and p-phenylene (or p,p′-biphenylene) were prepared by polymerizing different molar ratios of m-phenylene diamine and p-phenylene diamine (or p,p′-diaminobiphenyl) with pyromellitic dianhydride in dimethylformamide at 0°C. Chemical cyclodehydration of polyamic acids resulted in the corresponding polyimides. Polymers were characterized by infrared (IR), viscosity, and density measurements. Viscosity and density of polymers decreased with an increase on m-phenylene groups in the backbone. The thermal and thermooxidative stabilities were investigated by dynamic thermogravimetry. Stability decreased when m-phenylene groups were introduced in the backbone.     

Kinetics study of thermal oxidative degradation of ABS containing flame retardant components

The thermal oxidative degradation kinetics of pure acrylonitrile–butadiene–styrene (ABS) and the flame-retarded ABS materials with intumescent flame retardant (IFR) were investigated using Kissinger, Flynn–Wall–Ozawa, and Horowitz–Metzger methods. The results showed that the degradation of all samples included two stages, the activation energy at the first stage decreased by the incorporation of these flame retardant components, while increased at the second stage. The activation energy order of the flame-retarded ABS samples at stage 2 illustrates the relationship between the composition of IFRs and their flame retardancy, FR materials with appropriate acid agent/char former ratio has higher activation energy and better flame retardancy.     

Kinetics of thermal degradation in non-isothermal conditions of some phosphorus-containing polyesters and polyesterimides

The thermal decomposition behavior of some phosphorus-containing polyesters and a polyesterimide was studied using thermogravimetric analysis in air at several heating rates between 5 °C/min and 20 °C/min. The results of this study, realized for polymers with phosphorus linkage as pendant group, were compared with the behavior of some polymers having the same backbone structure, with phosphorus in the main chain, respectively, without phosphorus. The kinetic processing of data was carried out using the Coats-Redfern, Reich-Levi, Flynn-Wall-Ozawa and Kissinger methods.     

Synthesis and thermal properties of aryl-substituted rod-like polyimides

This article describes the synthesis and thermal characterization of novel aryl-substituted rod-like homopolyimides. Synthetic aspects of monomer syntheses and one-step polymer synthesis in m-cresol are presented. Polyimides with rod-like chain structure are based on monophenylated pyromellitic dianhydride (MPPMDA), diphenylated pyromellitic dianhydride (DPPMDA), and as rod-like diamine units on phenylated para-phenylene diamine and 1,1′-binaphthyl-4,4′-diamine. As partial flexible units, which have the possibility to be converted into an extended chain conformation, commercially available 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and 3,4′-oxydianiline (3,4′-ODA) were used. The polyimides were investigated with respect to solution properties and thermal behavior. © 1993 John Wiley & Sons, Inc.     

Effect of thermal history on the deformation and failure of polyimides

Thermal‐processing structure‐property relationships for polyetherimide (PEI), poly(4,4′‐oxydiphenylene pyromellitimide) (POPPI), and phenylethynyl‐terminated imide (PETI‐5) composite matrices are reported from a fundamental perspective. For thermoplastic PEI, deformation and failure depend primarily on free volume as evidenced by moisture‐absorption, mechanical‐property, and mass‐density changes as a function of annealing. The deformation of POPPI can be divided into the following three regimes as a function of annealing temperature: (1) physical aging‐induced glassy state free‐volume decreases, (2) thermally activated microvoid collapse, and (3) chemical degradation. In the case of PETI‐5, macroscopic defects, free volume, and polymer morphology control deformation. The effects of residual crystallinity on deformation are reported, and it is shown that mechanical toughness can be significantly decreased upon annealing below the glass‐transition temperature. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2947–2959, 2001     

Evaluation of the thermal oxidative stability of polyimides via TGA techniques

Thermogravimetric analysis (TGA) has been used for many years to evaluate polymer thermal stability. The objective of this study is to determine if weight-loss curves from TGA and isothermal TGA (IGA) can be used to determine degradation activation energies and thus rank the thermal stability (TS) and thermooxidative stability (TOS) for selected polyimides. Two high-temperature stable addition-cured polyimides and two aromatic condensation polyimides, all four containing fluorinated connecting linkages in the dianhydride monomers, were compared. Three TGA kinetic methods (Coats/Redfern, Ingraham/Marier, Horowitz/Metzger) were used to determine the activation energy for decomposition in air. The results were then used to rank polyimide stability compared to more traditional rankings based on long-term isothermal air aging weight-loss (IWL) studies and thermal decomposition temperatures (T_d) from TGA data. Use of TGA coupled to a Fourier transform infrared (TGA–FTIR) spectrophotometer allowed for the simultaneous identification and relative quantification of evolved decomposition products (CO₂, CO, ArNCO, and CHF₃) of the four polyimides degraded in air or nitrogen. Isothermal TGA–FTIR (IGA–FTIR) was also done in air to determine the relative rate of product evolution at a constant temperature. Activation energies using TGA and IGA data were determined and then compared with IWL values for the degradation of the polyimide to examine for correlations of real-life thermal oxidative aging to accelerated aging techniques. The Coats/Redfern method and T_d were found to best reproduce stability rankings of those from long-term, high-temperature IWL studies. Together, they may provide a time-saving technique to evaluate polyimide thermal oxidative stability. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3943–3956, 1999     

Reaction mechanism and products of the thermal conversion of hydroxy-containing polyimides

Thermal treatments of hydroxyl-containing polyimide materials at high temperatures are believed to lead to unique structural and chemical changes to the polymer backbone and side groups. These rearrangements when applied to specific polymeric membranes improve both their permeability and selectivity over conventional materials and this development has been attributed to the formation of nanoporous cavities in the treated products. However, the mechanisms of the possible chemical reactions that take place and the structures of the end-products have been poorly understood. With validated evidence from experiments and virtual screening of different precursors and possible products by molecular simulation, we have determined that hydroxy-containing polyimides can be thermally rearranged into poly(biphenylene bisimide) polymers under designed conditions. The proposed reaction mechanism is novel and calls into question the previously reported polybenzoxazole mechanism of thermal rearrangement. The new results indicate that the exact conditions of synthesis and thermal treatment of the starting polyimides will be critical to the properties of the membranes produced.     

Kinetics of thermal degradation applied to starches from different botanical origins by non-isothermal procedures

Amylose content, crystallinity, morphology and the kinetic of thermal degradation to starches from different botanical origins are described based on XRD, SEM, DSC and TG/DTG curves. Applying the non-isothermal isoconversional Wall-Flynn-Ozawa method on the TG/DTG curves average activation energy (0.10 ≤ α ≤ 0.70) E = 144.1 ± 9.8, 171.6 ± 14.6, 158.3 ± 7.4 and 159.4 ± 15 kJ mol⁻¹ could be obtained for thermal degradation of corn, rice, potato and cassava starches, respectively. From E values and the generalized time θ, the Sesták-Berggren (SB) in which f(α) = α^m(1 − α)ⁿ seems to be most suitable kinetic model in describing physicogeometrically the thermal degradation for the samples regardless of its botanical origins. The determination of the kinetic exponents m and n allows to obtain the pre-exponential factor (0.2 ≤ α ≤ 0.8) ln A = 8.8, 10.4, 9.2 and 8.9 min⁻¹ for corn, rice, potato and cassava starches, respectively. There were not significant differences between values of the kinetic triplet of the starches, indicating that, despite structural differences, these had little influence on the thermal degradation process of the starches.