The regression of isothermal thermogravimetric data to evaluate degradation Ea values of polymers: A comparison with literature methods and an evaluation of lifetime predictions reliability. Part II

The kinetics of the isothermal degradation in static air atmosphere of four well known polymers, polyethylene (PE), polystyrene (PS), polycarbonate (PC) and poly(methyl methacrylate) (PMMA) was studied by both a long-term (more than three years) experiment at relatively low temperature (423 K) and a set of short-term experiments at higher temperatures. The activation energy (E_a) values of degradation were determined by both the MacCallum and Wilkinson literature methods, and were compared with those obtained through a new very simple method we set up, based on the direct regression of TG mass loss data. About two years ago we published the results concerning PE and PS because their mass losses during long-term experiments were sufficiently high. The long-term degradation experiments were continued until now and in this second part we report the results concerning PC and PMMA. The degradation E_a values calculated from short-term experimental data through the three different methods were in good agreement with each other for both PC and PMMA, thus confirming the general applicability of our simple method for the determination of E_a. The experimental data at lower temperature of PC were not in agreement with those at higher temperatures, thus confirming the low reliability of the kinetic parameters (and then of lifetime predictions) at low temperature determined by experiments at higher temperatures. Partially disagreeing results were obtained for PMMA, which were discussed and interpreted.     

End-life Prediction of Commercial PLA Used for Food Packaging through Short Term TGA Experiments： Real Chance or Low Reliability？

A long-term(about nine months) isothermal degradation experiment of two different commercial polylactide(PLA) samples used for food packaging was carried out at a relatively low temperature(423 K). Thermooxidative degradations of the same polymers were carried out in a thermogravimetric(TG) analyser, at higher temperatures(453 K ≤ T ≤ 523 K), under isothermal heating conditions. The obtained set of experimental TG data was used to determine the apparent activation energy(Ea) of degradation through two isothermal kinetic methods. The results from long-term experiment evidenced considerable mass loss for both PLA samples in the investigated period, but the experimental data were not in agreement with those from the short-term degradations at higher temperatures, thus suggesting a different degradation kinetics, and, then a low reliability of the lifetime predictions for polymers in service or degradation forecasts for the end of their life based on experiments at higher temperatures.     

Determination of degradation apparent activation energy values of polymers

The isothermal degradation of three aromatic polyetherketones was studied in an inert environment at various temperatures in the range 683-803 K. In the first degradation stage (mass loss D￡20%) a linear increase of D as a function of heating time (t) was observed and the corresponding kinetic D=D _o+bt equations at various temperatures were directly drawn by smoothing the experimental TG data. The b values, which represent the mass loss rates during degradation, increased as a function of temperature according to Arrhenius-type equations, from which degradation E _a values were determined, which appear in agreement with those from literature methods. Some differences observed were discussed and interpreted. This revised version was published online in July 2006 with corrections to the Cover Date.     

呋喃西林水溶液在光和热作用下的稳定性

The influence of both light and heat on the stability of nitrofurazone aqueous solution was studied. Results show that in either heating experiments or the exposure to light at high temperatures, the degradation rate obeyed zero-order kinetics. The total rate constant ktotal caused by both light and heat can be divided into two parts: ktotal =kdark klight, where kdark and klight are the degradation rate constants caused by heat and light, respectively. The klight can be expressed as klight=Alight*exp(-Ea,light/RT)*E, where E is the illuminance of light, and Alight and Ea,light both are experimental constants. The values of these kinetic parameters were determined based on the experiments in the dark and upon exposure to three different light sources. Results show that the values of Alight and Ea, light varied with the light source. To save time, labor, and drugs, exponential heating experiments were employed and compared with the isothermal experiments. Results indicated that kinetic parameters obtained by exponential heating experiments are comparable to those obtained by isothermal experiments either in the dark or upon exposure to light.     

Determination of the Thermal Degradation Kinetic Parameters of Carbon Fibre Reinforced Epoxy Using TG

In this work, a kinetic study on the thermal degradation of carbon fibre reinforced epoxy is presented. The degradation is investigated by means of dynamic thermogravimetric analysis (TG) in air and inert atmosphere at heating rates from 0.5 to 20°C min⁻¹ . Curves obtained by TG in air are quite different from those obtained in nitrogen. A three-step loss is observed during dynamic TG in air while mass loss proceeded as a two step process in nitrogen at fast heating rate. To elucidate this difference, a kinetic analysis is carried on. A kinetic model described by the Kissinger method or by the Ozawa method gives the kinetic parameters of the composite decomposition. Apparent activation energy calculated by Kissinger method in oxidative atmosphere for each step is between 40–50 kJ mol⁻¹ upper than E _a calculated in inert atmosphere. The thermo-oxidative degradation illustrated by Ozawa method shows a stable apparent activation energy (E _a ≈130 kJ mol⁻¹ ) even though the thermal degradation in nitrogen flow presents a maximum E _a for 15% mass loss (E _a ≈60 kJ mol⁻¹ ). This revised version was published online in July 2006 with corrections to the Cover Date.     

Kinetic study of the thermal degradation of PS/MMT nanocomposites preparedwith imidazolium surfactants

Styrene and montmorillonite organically modified with imidazolium surfactants (MMT) at various alkyl chain lengths (C₁₂, C₁₆ and C₁₈) were used to prepare the corresponding PS/MMT/C₁₂, PS/MMT/C₁₆ and PS/MMT/C₁₈ nanocomposites by in situ polymerization. XRD and TEM analyses evidenced the formation of both intercalated and exfoliated structures. The glass transition temperatures (T _g) of nanocomposites, as well as that of neat PS, were obtained by DSC measurements. The thermal degradations were carried out in the scanning mode, in both inert and oxidative environments, and the initial temperatures of decomposition (T _i) and the apparent activation energies of degradation (E _a) were determined. Due to an oxidative degradation mechanism, the T _i and E _a values in air atmosphere were lower than those under nitrogen. The results indicated that nanocomposites are more thermally stable than polystyrene, and suggested an increasing degree of exfoliation as a function of alkyl chain length of surfactant, associated with enhancing thermal stability.     

Thermal analysis of soil-buried oxo-biodegradable polyethylene based blends

Low-density polyethylene (LDPE) blended with poly(3-hydroxybutyrate) (PHB) and additivated with pro-oxidant were soil buried for 180 days and characterized using thermogravimetry (TG) and differential scanning calorimetry (DSC). TG data showed that both onset and maximum rate degradation temperatures decreased as a function of biodegradation time. Apparent activation energies (E _a) using the Broido integral method decreased with the burial time increasing. PE crystallinity degree values increased in general up to 2 months of biodegradation. At the end of the soil burial (SB) test these values decreased principally for samples that were previously thermo-oxidized in an oven.     

Chemical reactions occurring in the thermal treatment of PC/PMMA blends

The chemical reactions occurring in the thermal treatment of bisphenol-A polycarbonate (PC) and poly(methyl methacrylate) (PMMA) blends have been investigated by nuclear magnetic resonance (NMR), mass spectrometry (MS), size exclusion chromatography (SEC), and thermogravimetry (TG). Our results suggest that in the melt-mixing of PC/PMMA blends, at 230°C, no exchange reactions occur and that only the depolymerization reaction of PMMA has been observed. In the presence of an ester-exchange catalyst (SnOBu₂), an exchange reaction was found to occur at 230°C, but no trace of PC/PMMA graft copolymer has been observed. Instead, an exchange reaction between the monomer methyl methacrylate (MMA), generated in the unzipping of PMMA chains, and the carbonate groups of PC has been suggested. This is due to the diffusion of MMA at the interface or even into the PC domains, where it can react with PC producing low molar mass PC oligomers bearing methacrylate and methyl carbonate chain ends and leaving the undecomposed PMMA chains unaffected. The TG curves of PC/PMMA blends prepared by mechanical mixing and by casting from THF show two separated degradation steps corresponding to that of homopolymers. This behavior is different from that of a transparent film of PC/PMMA blend, obtained by solvent casting from DCB/CHCl₃, which shows a single degradation step indicating that the degradation rate of PC is increased by the presence of PMMA in the blend. The thermal degradation products obtained by DPMS of this blend consist of methyl methacrylate (MMA), cyclic carbonates arising from the degradation of PMMA and PC, respectively, and a series of open chain bisphenol-A carbonate oligomers with methacrylate and methyl carbonate terminal groups. The presence of the latter compounds suggests a thermally activated exchange reaction occurring above 300°C between MMA and PC. The presence of bisphenol-A carbonate oligomers bearing methyl ether end groups, generated by a thermally activated decarboxylation of the methyl carbonate end groups of PC, has also been observed among the pyrolysis products. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1873–1884, 1998     

Synthesis and kinetic analysis of isothermal and non-isothermal decomposition of ammonium dinitramide prills

Ammonium dinitramide (ADN) prills were prepared by emulsion crystallization and characterized by optical microscopic, thermogravimetric (TG) and differential scanning calorimetric (DSC) techniques. The isothermal and non-isothermal decomposition kinetics of ADN prills were studied by TG. The differential isoconversional method of Friedman (FR) and integral isoconversional method of Vyazovkin were used to investigate the dependence of activation energy (E _a) with conversion (α) and the results were compared with literature data. The dependence of activation energy was also derived from isothermal data. A strong dependence of E _a with α is observed for the ADN prills. All the methods showed an initial increase in E _a up to α=∼0.2 and later decreases over the rest of conversion. The apparent E _a values of FR method are higher than that of Vyazovkin method up to α=∼0.45. The calculated mean E _a values by FR, Vyazovkin and standard isoconversional method for α between 0.05 and 0.95 were 211.0, 203.9 and 156.9 kJ mol⁻¹, respectively.     

TG studies of a composite solid rocket propellant based on HTPB-binder

Thermal decomposition kinetics of solid rocket propellants based on hydroxyl-terminated polybutadiene-HTPB binder was studied by applying the Arrhenius and Flynn-Wall-Ozawa's methods. The thermal decomposition data of the propellant samples were analyzed by thermogravimetric analysis (TG/DTG) at different heating rates in the temperature range of 300-1200 K. TG curves showed that the thermal degradation occurred in three main stages regardless of the plasticizer (DOA) raw material, the partial HTPB/IPDI binder and the total ammonium perchlorate decompositions. The kinetic parameters E _a (activation energy) and A (pre-exponential factor) and the compensation parameter (S _p) were determined. The apparent activation energies obtained from different methods showed a very good agreement. This revised version was published online in August 2006 with corrections to the Cover Date.     

Degradation of polymer blends—Part XI. Blends of polystyrene with polyisoprene

The degradation behaviour of polystyrene and cis-1,4-polyisoprene when both are present in the same film as a 1:1 blend has been compared with that when the polymers are degraded separately. Degradations have been studied under programmed heating conditions using TG, TVA, DTA and DSC and also under isothermal conditions at 340 and 360°C. Volatile products of degradation have been studied and separated by sub-ambient TVA and also identified by spectroscopic methods. The volatile products from the blend are the same as those from the constituent polymers. Volatile production occurs less readily for each polymer than when it is degraded alone. Stabilisation of PS is especially marked and under isothermal conditions at the above temperatures, PS does not evolve volatiles until PI degradation is completed. Chain scission in PS, prior to volatilisation, is increased, however, in the presence of PI. It is concluded that the increased scission results from attack on PS by PI radicals of short chain length and that the stabilisation effect on the PS is due to an inhibiting action of dipentene evolved by the PI. Both these reactions follow diffusion of mobile species of rather low volatility from the PI phase into the PS phase.     

Thermal and rheological behaviours of some random aromatic amino-ended polyethersulfone/polyetherethersulfone copolymers

The thermal and rheological characterizations of seven random, low molecular weight (M_n ≅ 9500 g mol⁻¹), H₂N-ended polyethersulfone/polyetherethersulfone (PES/PEES) copolymers, at various PES/PEES ratios, were performed. The glass transition temperatures (T_g) were determined by DSC. Degradations were carried out in a thermobalance, under flowing nitrogen, in dynamic heating conditions from 35 °C to 650 °C. The initial decomposition temperatures (T_i) and the half decomposition temperatures (T_1/2) were directly determined by TG curves, while the apparent activation energies of degradation (E_a) were obtained by the Kissinger method. In addition, the complex viscosities (η^∗) of the molten polymers were determined in experimental conditions of linear viscoelasticity. T_g, E_a and η^∗ values increased linearly with PES% content, while T_i and T_1/2 values showed opposite behaviour. In every case both PES and PEES homopolymers felt outside linearity. The results obtained are discussed and interpreted, and compared with those of corresponding Cl-ended copolymers previously studied.     

Surface Study of Polymers by Static Secondary Ion Mass Spectrometry Using a Magnetic-Sector Mass Spectrometer

Results from static SIMS analysis of six thermoplastic polymers — polytetrafluoroethylene (PTFE), polyethylene (PE), polymethyl methacrylate (PMMA), polyethylene terephthalate (PET), polystyrene (PS) and polycarbonate (PC) — using a magnetic-sector SIMS instrument and O₂⁺ primary beam are presented. For PTFE as a representative sample, the charging effect is reduced only with a metal grid when analyzing positive secondary ions. When negative secondary ions are analyzed, excessive charges are self-compensated with a normal-incidence electron gun. Positive-ion spectra collected agree with spectra obtained using either a quadrupole or time-of-flight SIMS instrument and noble-gas ion beams. The agreement is objectively demonstrated by means of the capability to compare spectra in the NIST/EPA/MSDC mass spectral database. The merits of the use of high-mass resolution, of which magnetic-sector SIMS is inherently capable, to provide analytical information about the molecular species native to the sample are demonstrated in distinguishing three ambiguous peaks with nominal mass ratios m/z = 27, 39 and 59 from PMMA.     

Effects of concentration of nonionic surfactant and molecular weight of polymers on the morphology of anisotropic polystyrene/poly(methyl methacrylate) composite particles prepared by solvent evaporation method

The effects of the concentration of polyoxyethylene octylphenyl ether (OP-10) as a nonionic surfactant and the molecular weight of polymers (polystyrene (PS) and poly(methyl methacrylate) (PMMA)) on the morphology of anisotropic PS/PMMA composite particles were investigated. In the case of polymers with lower molecular weight (M _w ≈ 6.0 × 10⁴ g/mol), the PS/PMMA composite particles have dimple, via acorn, to hemispherical shapes along with the increase of the OP-10 concentration. On the other hand, when the polymers have higher molecular weight (M _w ≈ 3.3 × 10⁵ g/mol), the morphology of PS/PMMA composite particles changed from dimple, via hemispherical, to snowman-like structure while the concentration of OP-10 was increased. Furthermore, thermodynamic analysis was first simply made by spreading coefficients, and the results indicated that both the concentration of OP-10 aqueous solution and the molecular weight of polymers were very important to the final morphology of anisotropic composite particles.     

Model of sorption of simple molecules in polymers

A model of simple molecule sorption in polymers is proposed which embraces both the glassy and rubbery regions, and incorporates the successful dual-mode model below the glass-transition temperature. Hole filling is shown to be an important sorption mechanism both above and below T_g, although saturation effects do not occur in the rubbery polymer. The model interprets the “dual-mode” Langmuir and Henry's law parameters at the molecular level, and a simple statistical mechanical analysis allows estimation of the parameter values, as well as specifying certain interrelationships between the parameters. Applications of the model to gas solubility data in five polymers are considered [polyethylene (PE), poly(ethylene terephthalate) (PET), polystyrene (PS), polymethacrylate (PMA), poly(vinyl acetate) (PVAc)] and semiquantitative agreement is obtained for PE, PET, and to a lesser extent, PS. For PMA and PVAc, the agreement is qualitative only.     

The relation between relaxed enthalpy and volume during physical aging of amorphous polymers and selenium

This paper reports physical aging results for PMMA, PMMA/PEO blends, PS, PC, PVC and PET (semicrystalline). Also included in this study is amorphous selenium. Temperature down-jumps from equilibrium above T_g to a temperature below T_g were carried out. Relaxed enthalpy, Δh and volume contraction, Δv, were measured. From the aging records, the constant ratio Δh/Δv = K_a was evaluated. For the polymeric samples K_a values of about 2 GPa were observed, thus similar to the inverse value of the isothermal compressibility close to T_g. Similarly for Se the K_a value obtained from Δh and Δv was in fair agreement with its isothermal compressibility.     

Micromechanical fast quasi‐static detection of α and β relaxations with nanograms of polymer

Micromechanical string resonators are used as a highly sensitive tool for the detection of glass transition (T_g or α relaxation) and sub‐T_g (β relaxation) temperatures of polystyrene (PS) and poly (methyl methacrylate) (PMMA). The characterization technique allows for a fast detection of mechanical relaxations of polymers with only few nanograms of sample in a quasi‐static condition. The polymers are spray coated on one side of silicon nitride (SiN) microstrings. These are pre‐stressed suspended structures clamped on both ends to a silicon frame. The resonance frequency of the microstrings is then monitored as a function of increasing temperature. α and β relaxations in the polymer affect the net static tensile stress of the microstring and result in measureable local frequency slope maxima. T_g of PS and PMMA is detected at 91 ±2°C and 114 ±2°C, respectively. The results match well with the glass transition values of 93.6°C and 114.5°C obtained from differential scanning calorimetry of PS and PMMA, respectively. The β relaxation temperatures are detected at 30 ± 2°C and 33 ± 2°C for PS and PMMA which is in accordance with values reported in literature. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1035–1039     

Intimate blending of binary polymer systems from their common cyclodextrin inclusion compounds

A procedure for the formation of intimate blends of three binary polymer systems polycarbonate (PC)/poly(methyl methacrylate) (PMMA), PC/poly(vinyl acetate) (PVAc) and PMMA/PVAc is described. PC/PMMA, PC/PVAc, and PMMA/PVAc pairs were included in γ‐cyclodextrin (γ‐CD) channels and were then simultaneously coalesced from their common γ‐CD inclusion compounds (ICs) to obtain intimately mixed blends. The formation of ICs between polymer pairs and γ‐CD were confirmed by wide‐angle X‐ray diffraction (WAXD), fourier transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC). It was observed [solution ¹H nuclear magnetic resonance (NMR)] that the ratios of polymers in coalesced PC/PMMA and PC/PVAc binary blends are significantly different than the starting ratios, and PC was found to be preferentially included in γ‐CD channels when compared with PMMA or PVAc. Physical mixtures of polymer pairs were also prepared by coprecipitation and solution casting methods for comparison. DSC, solid‐state ¹H NMR, thermogravimetric analysis (TGA), and direct insertion probe pyrolysis mass spectrometry (DIP‐MS) data indicated that the PC/PMMA, PC/PVAc, and PMMA/PVAc binary polymer blends were homogeneously mixed when they were coalesced from their ICs. A single, common glass transition temperature (T_g) recorded by DSC heating scans strongly suggested the presence of a homogeneous amorphous phase in the coalesced binary polymer blends, which is retained after thermal cycling to 270 °C. The physical mixture samples showed two distinct T_gs and ¹H T_1ρ values for the polymer components, which indicated phase‐separated blends with domain sizes above 5 nm, while the coalesced blends exhibited uniform ¹H spin‐lattice relaxation values, indicating intimate blending in the coalesced samples. The TGA results of coalesced and physical binary blends of PC/PMMA and PC/PVAc reveal that in the presence of PC, the thermal stability of both PMMA and PVAc increases. Yet, the presence of PMMA and PVAc decreases the thermal stability of PC itself. DIP‐MS observations suggested that the degradation mechanisms of the polymers changed in the coalesced blends, which was attributed to the presence of molecular interactions between the well‐mixed polymer components in the coalesced samples. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2578–2593, 2005     

Thermal decomposition kinetics of some transition metal coordination polymers of fumaroyl bis (paramethoxyphenylcarbamide) using DTG/DTA techniques

In the present work, thermal degradation behaviors of the Zn (II), Cd(II), and Hg(II) coordination polymers of fumaroyl bis (paramethoxyphenylcarbamide) (fbpmpc) have been investigated by using thermogravimetric (TG) analysis, differential thermal analysis (DTA) and derivative thermogravimetry (DTG) analysis under non-isothermal conditions in nitrogen atmosphere at multiple heating rates. TG–DTA study noteworthy inferred the presence of lattice water in outer sphere of all the polymers. The decomposition was carried out in three-four well-separated stages where involved the loss of water molecules in the first step followed by organic ligand. Furthermore, the kinetics and thermodynamic stabilities of multi-steps thermal degradation were evaluated. The activation energy (E_a), order of reaction (n), Arrhenius factor (A), enthalpy change (ΔH), entropy change (ΔS) and free energy change (ΔG) of coordination polymers were obtained by using the Coats–Redfern (CR) method. Ultimately, based on initial, half and final decomposition temperature, and kinetics parameters values the orders of thermal stability were estimated.     

Thermal cure kinetics of epoxidized linseed oil with anhydride hardener

The thermal cure kinetics of an epoxidized linseed oil with methyl nadic anhydride as curing agent and 1-methyl imidazole as catalyst was studied by differential scanning calorimetry (DSC). The curing process was evaluated by non-isothermal DSC measurements; three iso-conversional methods for kinetic analysis of the original thermo-chemical data were applied to calculate the changes in apparent activation energy in dependence of conversion during the cross-linking reaction. All three iso-conversional methods provided consistent activation energy versus time profiles for the complex curing process. The accuracy and predictive power of the kinetic methods were evaluated by isothermal DSC measurements performed at temperatures above the glass transition temperature of the completely cured mixture (T _g∞ ). It was found that the predictions obtained from the iso-conversional method by Vyazovkin yielded the best agreement with the experimental values. The corresponding activation energy (E _a) regime showed an increase in E _a at the beginning of the curing which was followed by a continuous decrease as the cross-linking proceeded. This decrease in E _a is explained by a diffusion controlled reaction kinetics which is caused by two phenomena, gelation and vitrification. Gelation during curing of the epoxidized linseed/methyl nadic anhydride system was characterized by rheological measurements using a plate/plate rheometer and vitrification of the system was confirmed experimentally by detecting a significant decrease in complex heat capacity using alternating differential scanning calorimetry (ADSC) measurements.