Analysis of nanomaterials and nanocomposites by thermoanalytical methods

Thermochimica Acta 675 (2019) 140–163

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Analysis of nanomaterials and nanocomposites by thermoanalytical methods Kinga Pielichowska , Katarzyna Nowicka ⁎

T

AGH University of Science and Technology, Faculty of Materials Science and Ceramics, Department of Biomaterials and Composites, Al. Mickiewicza 30, 30-059 Kraków, Poland

ARTICLE INFO

ABSTRACT

Keywords: Thermogravimetry TG-FTIR TG-MS TG-GC/MS Nanomaterials Nanocomposites

In this work the thermoanalytical methods used for the characterization of nanomaterials and nanocomposites are presented. They include thermogravimetry coupled with infrared spectroscopy (TG-FTIR), mass spectrometry (TG-MS) and gas chromathography (TG-GC/MS) which are used to ascertain the influence of nanoparticles on the thermal stability and degradation mechanism of polymer nanocomposites. Other hyphenated techniques pyrolysis coupled with gas chromatography and/or mass spectrometry (Py-GC, Py-GC/MS), as well as a relatively new technique based on a combination of scanning thermal microscopy and localized thermal analysis known as micro-thermal analysis, can provide valuable data concerning preparation, characterization and decomposition behavior of a wide range of nanomaterials and nanocomposites. Thermoanalytical methods help to determine the thermal properties of nanoparticles and to understand the influence of nanomaterials on polymer phase transitions, thermally induced chemical reactions, as well as thermal transport properties in advanced polymer composites.

1. Introduction Nanotechnology has a deep impact on science, economy and society in the early 21st century. Science and engineering developments in this new discipline are thought to add to the progress in electronics, medicine, energy and the environment, biotechnology and information technology. The thermal characterization of nanomaterials during their processing, applications and recycling, is a crucial issue especially for the inorganic, carbon and polymeric materials. Thermoanalytical and thermophysical testing methods have been successfully applied for the investigation of thermal behaviour such as phase transitions, thermally induced chemical reactions and decompositions, gas adsorption and desorption studies, and thermal transport properties in various areas of materials science [1].

Thermal analysis (TA) is a well-established group of techniques for obtaining qualitative and quantitative information about the effects of heat treatments on materials of all kinds, including new chemical compounds, polymers, ceramics, alloys, composites, nanocomposites, foods and medicines [2]. Thermal analysis techniques in which physical property of a substance is measured as a function of the temperature when the sample is subjected to a controlled temperature program, are widely used for testing the materials’ thermal properties, but there is no information on the qualitative aspects of the evolved gases during materials thermal degradation [3]. Thermogravimetry (TG) is a well-known technique used for the determination of the weight loss characteristics of the investigated samples during heating and associated reaction kinetics [4]. Thermal events causing a mass loss of the material are desorption, outgassing,

Abbreviations: ABS, acrylonitrile-butadiene-styrene terpolymer; AFM, atomic force microscopy; APP, ammonium polyphosphate; BA, boehmite alumina; CNCs, cellulose nanocrystals; CNF, carbon nanofibre; CNTs, carbon nanotubes; DSC, differential scianning calorimetry; DTA, differntial thermal analysis; EGA, evolved gas analysis; EP, epoxy resin; FS, fumed silica; HDPE, high density polyethylene; HIPS, high impact polystyrene; HNBR, hydrogenated nitrile-butadiene rubber; LDPE, low density polyethylene; LS, layered silicate; MMA, methyl methacrylate; MMT, montmorillonite; MTDSC, modulated temperature differential scianning calorimetry; MWCNTs, multi-walled carbon nanotubes; OMMT, organically modified montmorillonite; PA6, polyamide 6; PANI, polyaniline; PCL, poly(ε-caprolactone); PET, poly(ethylene terephthalate); PBSA, poly(butylene succinate-co-butyleneadipate); PBSu, poly(butylene succinate); PBT, poly(butylene terephthalate); PC, polycarbonate; PHBV, poly(3-hydroxybutyrate-co-3-hydroxyvalerate); PLLA, poly(L-lactide); PMMA, poly(methyl methacrylate); POM, polyoxymethylene; POSS, polyhedral oligomeric silsesquioxanes; PP, polypropylene; PS, polystyrene; PU, polyurethanes; PVA, poly(vinyl alcohol); PVB, poly(vinyl butyral); PVC, poly(vinyl chloride); PVP, poly(vinyl pyrrolidone); Py-GC, pyrolysis coupled with gas chromatography; Py-GC/MS, pyrolysis coupled with gas chromatography and mass spectrometry; SBS, styrene-butadiene-styrene triblock copolymer; SPU, segmented polyurethane; SWCNTs, single-walled carbon nanotubes; TA, thermal analysis; TEM, transmission electron microscopy; TG, thermogravimetry ⁎ Corresponding author. E-mail address: [email protected] (K. Pielichowska). https://doi.org/10.1016/j.tca.2019.03.014 Received 10 December 2018; Received in revised form 11 March 2019; Accepted 12 March 2019 Available online 15 March 2019 0040-6031/ © 2019 Elsevier B.V. All rights reserved.

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dehydration, foaming, vaporization, sublimation, decomposition, oxidation, reduction and chemical reactions, while mass gain can be connected to adsorption or absorption, oxidation and chemical reactions [5]. TG is a common technique in the characterization of materials. In different type of materials, loss of impurities during heating or thermal degradation are observed as weight losses with respect to time or temperature. However, no chemical information about the gases evolved on heating can be obtained from TG alone. The capability of TG for materials characterization is greatly increased if other techniques are coupled with thermogravimetry in order to identify either the residue or the products evolved during the experiment [6]. If the gaseous degradation products are simultaneously analysed using infrared (IR) and mass spectroscopic (MS) techniques (evolved gas analysis (EGA)), then it is possible to identify the compounds evolved and determine the temperature range over which they are released [7]. The IUPAC Compendium of Chemical Terminology defines the EGA as a “technique in which the nature and/or amount of volatile product(s) released by a substance subjected to a controlled temperature program is (are) determined” [8]. Both simultaneous and sequential techniques have been developed for identification of these gases and volatiles and they are known as evolved gas analysis methods [9–13]. Thermogravimetric analyzers, connected to conventional gas analytical instruments, like gas chromatograph [14], infra-red spectrophotometer [15] and mass spectrometer [16–19], have been widely used equipment for EGA studies relevant to solid-gas transformations [20,21]. Numerous research works analyze the effect of nanoparticles on the thermal stability of different materials, and less papers devoted to the decomposition mechanisms [22–31]. The evolution of new products of degradation in nanomaterials was observed by thermogravimetry coupled with mass spectrometry (TG-MS), gas chromatography/MS (TGGC/MS) or infrared spectroscopy (TG-FTIR) [32–35]. Using these techniques it was possible to e.g. postulate the mechanism of thermal stability improvement of polymeric nanocomposites. based on different polymers in terms of radical stability [36–38].

these is the problem of transfer of high boiling compounds. It was recognized in the past that high boiling compounds can travel very slowly through the transfer line, causing misleading gas evolution profiles and possible cross contamination of subsequent samples [42,45]. TG-FTIR interface has to meet some requirements such as representative gas sampling; minimized dilution effects and low decomposition, short response time, high sensitivity, high resolution and corrosion resistance [46]. During TG-FTIR measurement, the gas flowing from the thermobalance moves through the transfer line to the infrared spectrometer’s gas cell; importantly, the transfer line should not change the pressure of the gas which is the same both in the thermobalance and in the spectrometer – Fig. 2. These conditions are usually met with transfer lines having an inner diameter of ca. 2 mm. The gas transfer time depends on the gas-flow rate, and it is in the range of few seconds with laminar flow profile. The results of the FTIR spectroscopy are displayed as absorbance or transmittance spectra, i.e. the wavelengths (wavenumbers) of the radiation absorbed by a molecule or functional groups of the molecule are shown as peaks in the range of wavenumbers covered by the instrument, usually 4000−400 cm−1 for the middle infrared range. The requirements on the resolution of FTIR spectrometer depend on the application, and the resolution of 4 cm−1 is sufficient in most cases [47]. 2.2. TG-MS TG-MS hyphenated technique has proven particularly useful due to its advantages that include (i) specificity – an ability to distinguish the actual molecule masses comprising a sample of the material evolved, and (ii) high sensitivity [18]. Mass spectrometry is an alternative to infrared spectroscopy, although the interface design is complicated by the requirement to operate the mass spectrometer under high vacuum. Various splitter designs [48] have been developed in order to reduce the transfer line pressure down to a level suitable for injection into the mass spectrometer. Again, the transfer lines are to be heated to prevent condensation of less volatile products [6]. In a TG-MS system, the gaseous products generated by volatilisation, sublimation or chemical reaction, are flushed out of the furnace chamber with a purge gas. The evolved gases are introduced in the MS detector (MSD) through a coupling system, which acts both as an MSD inlet and a pressure-reduction system [49]. Typical TG-MS system configuration is presented in Fig. 3. The coupling system between TG and MS should meet several criteria such as rapid evolved-gas transfer from the sample pan in the TG analyser to MSD, no degradation or condensation of evolved gases, no interferences on the specifications of the TG and the MSD, no air insertion during TG furnace opening, simplicity in construction, easy system decoupling and cleaning, low cost, and continuous monitoring and versatility with respect to TG and/or MSD [3,47]. In addition, if the system is used for EGA, then high sensitivity and repeatable flow conditions are necessary [49]. The coupling interface between a thermobalance and a quadrupole mass spectrometer needs to ensure (i) efficient transfer of a representative part of the evolved gases from the thermobalance, and (ii) pressure reduction – Fig. 4. The advantage of high sensitivity of the mass spectrometers can be taken without restriction, and this leads to detection limits in the parts per billion (ppb) ranges. The only disadvantage is the need to disconnect the QMS when loading the samples into the thermobalance and the time-consuming evacuation procedure with each sample measurement. As the majority of TG experiments is performed in a gas flow with a system open to the surrounding at atmospheric pressure, the coupling TG-MS interface shall contain a pressure-reduction system [47]. The most widely used interface for coupling TG with MS is a double stage pressure reduction system, in which the first pressure reduction step is realized using a capillary and an orifice, while the second step is

2. Thermoanalytical methods 2.1. TG-FTIR The potential advantages of interfacing an FTIR spectrophotometer to a TG unit were originally shown in the early 1980s [39–41]. The coupling of these two techniques provides synergistic information that are not available from either technique alone. As a method of evolved gas analysis, FTIR is sensitive, fast, and can detect every type of gas molecule except homonuclear diatomics [42]. FTIR analyses after different time of degradation made it possible to gain an insight towards identification of the characteristic absorption bands [23]. To obtain the IR spectra of the volatiles evolved during the programmed analysis, the thermo-analytical instrument is coupled with a FTIR spectrometer by means of a heated transfer line (to prevent the condensation of less volatile products). The released vapors or gases are transferred to the heated gas cell of the FTIR instrument, and the temperatures of the cell and of the transfer line can be independently selected [10]. Schematic diagram of the typical TG-FTIR system is presented in Fig. 1. The TG-FTIR usually work at normal pressure, although some of them can operate under vacuum. The core of the coupling is the transfer line, which consists of a heated tube or pipe that connects the TG apparatus to the heated infrared gas cell. The proper transfer line ensures that the whole gas stream from the thermobalance flows through the detector and not just a fraction of it. The main advantages of the transfer line are the simple handling, its flexible application and the relative low cost [44]. Nevertheless, there are certain limitations in the technique that have not been completely solved. The most important of 141

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Fig. 1. Schematic diagram of the TG-FTIR system. Reprinted from [43] with permission from Elsevier.

Fig. 2. Coupling of TG and FTIR with a transfer line. Reprinted from [47] with permission from Elsevier.

achieved by an orifice or a more advanced Skimmer system [50] – Fig. 5. The Skimmer coupling system combines good gas flow conditions for complete gas transfer with homogeneous heating of first and second pressure reduction steps to prevent condensation [51]. Usually, TG-MS couplings include quadrupole mass spectrometers with electron impact ionisation (EI). Although MS is a very powerful analytical technique for elemental and molecular analysis, the currently commercially available TG-MS systems with EI ionisation cannot provide a realistic representation of complex organic vapours evolved due to the large degree of fragmentation of organic molecules upon the standard electron impact ionisation with electrons of 70 eV kinetic energy. First thermal desorption and pyrolysis studies [52,53] as well as TG studies using laser or deuterium lamp-based soft ionisation mass spectrometry as detecting method [54] revealed that meaningful information on the molecular signature of the thermal decomposition processes can be achieved [55]. A prototype of a thermal analyser coupled with a single photon ionization orthogonal acceleration time-of-flight mass spectrometer (TG/DSC-SPI-oaTOFMS) has been designed by Zimmermann et al. [55,56] – Fig. 6. In this system, a very soft ionisation could be achieved. In Fig. 6b mass spectra of heptadecane obtained using the TG-QMS is presented. The mass spectrometer was operated in SPI (Fig. 6b, bottom) and EI (Fig. 6b, top) modes. Fig. 6b clearly demonstrates that heptadecane, a rather fragile long-chain alkane, could be detected as molecular peak solely by SPI-MS while the EI-MS is dominated by fragmented peaks [56]. SPI-MS is capable of recording the molecular organic signature of the evolved organic gases from the pyrolysis of large-scale produced polymers such as polyethylene (PE), polystyrene (PS) or poly(vinyl chloride) (PVC). This includes e.g. dimers or oligomeric products which have not been detected by the conventional TG mass spectrometric methods using electron impact ionisation (EI).

2.3. TG-GC/MS The TG-GC/MS hyphenated system enables direct measurement of mass changes vs temperature and the separation/identification of volatile products by gas chromatography and mass spectroscopy detector [57]. Although TG-EGA techniques display various advantages and can be applied to identify degradation products of a whole set of materials, including polymers and low-molecular-weight compounds, they have two distinct disadvantages: in hyphenated analyses the presence of components at very low concentrations may be masked by higher concentration components, and also the simultaneous evolution of more than one compound may make identification very difficult. By incorporating the separation capability of gas chromatography (GC) in the system, the components of the mixture can be separated. This can be achieved by trapping the gases evolved over the duration of the TG run and performing post-run analysis using gas chromatography-mass spectrometry (GC–MS) [7]. Once the identity of individual components is established, the interpretation of real time results can be viewed with greater confidence [7]. 2.4. Analytical pyrolysis Pyrolysis is an analytical technique in which large molecules are degraded into smaller volatiles species using only thermal energy in an inert atmosphere. Pyrolysis, combined with analytical methods, such as gas chromatography and/or mass spectrometry (Py–GC/MS) has become a quick, convenient and powerful tool for characterising polymeric materials [58]. Pyrolysis systems generally use one of three heating techniques: the heated filament, Curie-Point and furnace [59]. Microfurnaces provide a constantly heated, isothermal pyrolysis zone into which samples are introduced by a liquid syringe, solid plunger syringe or in a little cup [59,60]. Curie-point pyrolyzers [61] apply the sample to a piece of ferromagnetic metal which is inserted into the pyrolyzer, then heated rapidly through induction of current using a 142

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Fig. 3. (a) TG/MS system configuration: [1] thermogravimetric analyser; [2] coupling system; and [3] mass spectrometric detector. (b) TG/MS coupling system consisting of a TG/MS interface and a deactivated capillary with a thermomantle (transfer line). Reprinted from [49] with permission from Elsevier.

high frequency coil. Filament pyrolyzers [62] use a piece of resistive metal (frequently platinum) with a wide temperature range and circuitry capable of causing the filament to heat to a programmed temperature at a controllable rate [59]. Pyrolyzer is interfaced with the analytical column of the GC via the injection port. A flow of inert gas (usually nitrogen or helium) flushes the pyrolyzates into the column,

where components are separated [58]. There are different types of detectors used in conjunction with pyrolysis–GC such as mass spectrometer (MS), Quadrupole MS (QMS), selected ion monitoring (SIM), time-of-flight mass spectrometry (TOF-MS), isotope-ratio mass spectrometry (IRMS), differential mobility spectrometry (DMS), ion mobility spectrometry (IMS), flame ionization detection (FID) and atomic 143

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Fig. 4. Combination of thermogravimetry and mass spectrometry by a coupling interface. Reprinted from [47] with permission from Elsevier.

Fig. 5. Gas flow and pressure conditions in a two stage Skimmer coupling system. Reprinted from [51] with permission from Elsevier.

Fig. 6. (a) Schematic representation of the TGEBEL-SPI-QMS prototype. (b) TG quadrupole mass spectra of heptadecane recorded with the developed TG-QMS instruments using the conventional electron impact ionisation (EI, 70 eV; hard ionisation, top) single photon ionisation (Ar-EBEL, 9.8 eV: soft ionisation, bottom). Reprinted from [56] with permission from Springer.

emission detection (AED) [58]. An example of laser pyrolysis GC/TOFMS system is presented in Fig. 7. Generally, the major advantage of pyrolysis compared with other techniques is the simple sample preparation, but in some cases time consuming pre-treatments like hydrolysis or dissolution step and/or a more complex derivatization process of the sample are required to make it amenable for Py–GC/MS [58,64]. Analytical pyrolysis has already been used in a range of applications and it offers great potential for the future analysis of nanomaterials and nanocomposites. 2.5. Micro-thermal analysis Micro-thermal analysis combines the imaging possibilities of atomic force microscopy with the ability to characterise, with high spatial resolution, the materials thermal behaviour at micro- and nanoscale [65]. The conventional AFM tip was replaced by a miniature heater/thermometer which enables a surface to be visualised according to its response to the input of heat [66] – Fig. 8. It is possible to combine AFM with DSC, MTDSC, TG and TMA. In order to provide full chemical characterization, a method of catching the gases that are evolved when the thermal probe is used to pyrolyze the surface has been developed. The evolved gases can then be thermally desorbed and passed into a GC/MS system for separation and

Fig. 7. Schematic of the laser pyrolysis fast GC/TOF-MS system. Reprinted from [63] with permission from Elsevier. 144

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3.1. Inorganic materials and ceramics Mayne et al. [131] investigated the thermal behaviour of two SiCN nanopowders (C/N = 0.87 and C/N = O-22 under two different atmospheres (He, N2) by means of thermogravimetry coupled with mass spectrometry. Authors found the same thermal behavior for the two SiCN nanopowders, in both helium and nitrogen in the 25–1200 °C range- very little mass loss occurs because of the evolution of adsorbed water, residual synthesis gases, as well as hydrogen, methane and carbon monoxide. However, above 1200 °C under helium atmosphere, the significant mass loss associated with the release of nitrogen and oxide species, was attributed to the decomposition process, whereas under nitrogen the mass gain resulted from the nitriding process. In another work, TG-FTIR/MS were used to examine the effect of chemical variation (alkyl chain length, number of alkyl groups, and unsaturation) of the organic modifiers on the thermal stability and degradation products of the organically modified montmorillonite (OMMT) [69]. Researchers revealed that the release of organic compounds from organically modified LS occurs in several stages which may indicate complex decomposition mechanism. Surprisingly, the chain length of exchange cations has almost no influence on the decomposition temperature of OMMT. Thermal evolution of the structure of a Mg–Al–CO3 layered double hydroxide (LDH) under an inert atmosphere was investigated by Yang and co-workers using in situ FTIR, DTA, TG-MS and HTXRD techniques [70]. Based on the thermoanalytical results, the authors proposed the thermal evolution scheme of the structure of a Mg–Al–CO3 – Fig. 10. In the range 70–190 °C, loosely held interlayer water was lost, and there are two different co-existing crystal phases of Mg–Al–CO3 LDH present (phase I and II). Next, in the range 190–280 °C, the OHe groups bonded with Al3+ begin to disappear at 190 °C, and this process ends at 280 °C – phase I is transformed into phase II. Above 280 °C, rupture of the OH − groups linked with Mg2+ starts and this process continues up to 405 °C; degradation of the LDH structure is also observed in the same region. At the end, in the range 405–580 °C, CO32− loss occurs and material becomes an amorphous metastable mixed solid oxide solution. Through the pyrolysis of crosslinked organically-modified alkoxy silane precursors, silicon oxycarbide (SiOxCy) can be formed, and TGMS data help to suggest the pyrolysis mechanism [75]. Due to the diversity of structure of the used precursors and their latent functional groups involved in the heating processes, it seems that the SiCxOy ceramic residues are the result of a series of reactions. The major reaction pathways were postulated as the loss of –CH3 groups in the

Fig. 8. Thermal probe (schematic). Reprinted from [66] with permission from Elsevier.

identification [67]. In this technique the tube comes to a fine point which is placed immediately adjacent to the heated tip using a micromanipulator. As the tip is heated a syringe is used to draw gas through the tube that is then located in a thermal desorption unit for analysis of the trapped volatiles by e.g. GC–MS. Total ion chromatogram and MS spectrum of polystyrene are shown in Fig. 9. By combining atomic force microscopy with thermoanalytical methods a versatile hyphenated technique was obtained that may be quite useful for characterization of nanomaterials and polymer nanocomposites. 3. Application of thermoanalytical methods for characterization of nanomaterials and nanocomposites The thermoanalytical techniques were shown to offer a large potential for the characterization of the nanosized materials and composites, both in fundamental and applied research. They are used for e.g. monitoring of the annealing processes of powdered precursors for nanomaterials, studying the thermal and thermooxidative behavior of nanoparticles and nanomaterials, including determination of the kinetic parameters. In Table 1 examples of thermoanalytical methods applications for nanomaterials and polymer nanocomposites were presented.

Fig. 9. Total ion chromatogram (top) from the pyrolysis. The peak at retention time 3.66 min is due to bleed from the GC column. The peak at 4.47 min is identified as styrene monomer (mass spectrum shown beneath) arising from the thermal degradation of polymer. Reprinted from [66] with permission from Elsevier. 145

146

Carbon-based nanomaterials

Chemically modified SWCNTs

MWCNTs

MWCNTs

MWCNTs

Nanostructured carbonaceous material Carbon nanofiber

Hafnium alkoxide-modified polysilazane Amorphous carbon nitride

Au and Ag nanoparticles

Copper chromite

Pt/C

AgNCO

ZnO2

ZnO ZnO

APP and aluminium hydroxide with porous kaolinite ZnO

Ammonium perchlorate over CoC2O4 LiBH4/activated carbon (AC) Ammonia borane/mesoporous carbon (AB/CMK) Silicon oxycarbide

Mg–Al–CO3 Indium and gallium doped ZnO

Heat treatment process Nickel nitrate hexahydrate impregnated carbon fiber mat in conditions replicated those used prior to chemical vapor deposition Thermal decomposition of oxidized MWCNTs Thermal decomposition of oxidized MWCNTs Thermal decomposition of oxidized MWCNTs Confirmation of the covalent attachment of long-chain groups to the side walls SWCNTs

Decomposition/nitridition Thermal stability and degradation products Thermal evolution of the structure Study of the preparation process with thermal-purification procedure Decomposition Hydrogen storage properties Dehydrogenation of AB and AB/CMK Reaction pathways of conversion of polysiloxane precursors to oxycarbide ceramics Suppression of methane/air explosion Investigation properties of Zno nanocrystals prepared by radiation method Thermal properties of Zno powder Decomposition of zinc diketonates in acetonitrile to zinc oxide nanoparticles Conversion of zinc peroxide to zinc oxide Thermal decomposition Preparation of NdCro3 nanoparticles Oleylamine desorption From Pt/C catalysts and carbon support degradation Thermal decomposition of copper ethylamine chromate Thermal decomposition study of HAuCl4·3H2O and AgNO3 Polymer-to-ceramic transformation Decomposition Product of C6N7Cl3

Inorganic nanomaterials

SiCN Organically modified layered silicate (OLS)

Investigated phenomenon

Material

Table 1 Examples of thermoanalytical techniques applications in nanomaterials and nanocomposites.

TG-FTIR

TG-MS

TG-MS

TG-FTIR

TG-MS DSC/TG-MS

TG-MS TG-MS

TG/DTA-FTIR

TG-MS

DTA/TG/MS DSC/TG-MS STA/MS

TG-MS

–

argon

argon

ratio of oxygen and nitrogen 1:4

[93]

[92]

[91]

[90]

[88] [89]

[86] [87]

[85]

[84]

[81] [82] [83]

[80]

[78] [79]

[77]

[76]

[72] [73] [74] [75]

[70] [71]

[68] [69]

Ref.

(continued on next page)

nitrogen nitrogen to 750 °C, next the gas was switched to 5% hydrogen in nitrogen

argon air, argon

air

nitrogen

argon argon argon

–

air

–

TG-MS TG-MS TG-IR

nitrogen

argon argon – helium

argon air or argon

helium/ nitrogen nitrogen

Atmosphere

TG-MS

DSC/TG-MS TG/DSC/MS TG-MS TG-MS

TG-MS TG-FTIR and TG-MS

TG-MS TG-FTIR/MS

Thermoanalytical method

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nitrogen nitrogen air, helium air nitrogen or air

TG-FTIR TG-FTIR TG-MS Py-GC-MS Py-GC-MS TG-MS TG-FTIR Py/GC-MS Py-GC/MS Py-GC/MS, TG-FTIR Py-GC-MS TG-MS TG-FTIR TG-FTIR TG-FTIR Py-GC/MS Py-GC/MS TG-FTIR TG–FTIR–GC/MS TG-MS TG-FTIR TG-FTIR TG-FTIR TG-FTIR/QMS TG/DTA-MS TG-FTIR

Thermal degradation Thermal degradation Thermooxidative degradation Thermal degradation Products of thermal degradation Thermal degradation mechanism Decomposition Degradation mechanism and kinetics Thermal degradation mechanism Thermal stability Thermal degradation Thermal degradation Thermal degradation mechanism Thermal degradation Thermal degradation Thermal degradation Thermal decomposition Thermal degradation Polyhedral oligomeric silsesquioxane Flame-retardant performance and mechanisms Thermal degradation Thermal degradation Thermal and thermo-oxidative decomposition Thermal and thermooxidation degradation Thermal degradation kinetics and scissoring mechanism

147

PE/reduced graphite oxide

EP/graphene oxide

TG-GC/MS

TG-FTIR

Py-GC/MS TG-FTIR

nitrogen

–

Argon nitrogen

nitrogen nitrogen air, helium helium air nitrogen helium helium helium helium nitrogen air nitrogen helium helium nitrogen nitrogen

nitrogen and air nitrogen nitrogen air air or nitrogen nitrogen

– air

nitrogen nitrogen or synthetic air

TG-FTIR TG-FTIR TG-FTIR TG-FTIR TG-FTIR TG-FTIR

Polyurethanes/lignin PU/MMT Silanized PU PA6/clay PA6/clay/flame retardant PS, ABS, PMMA, PP and PE nanocomposites with clay PMMA/MMT SBS/MWCNTs PMMA/TiO2 PMMA/ZnO/OMMT PVC/metallic oxides/OMMT PVA/silica PET/silica PS/brush structure clays PS/Fe-MMT Poly(o-methylaniline)/maghnite clay HNBR clay Silicone rubber/silica nanocomposite PBT/metal oxides/aluminium phosphinate PLLA/layered double hydroxides Polyesters/metallic oxides Polyester/cellulose nanocrystals Poly(propylene sebacate) nanocomposites with FS or MWCNTs or MMT PC filled with solid bisphenol A bis(diphenyl phosphate) and MMT PC/ABS/POSS Polycarbonate nanocomposite based on diphenylphosphine oxide-containing POSS PU/POSS nanocomposite PVP/ammonium metatungstate nanofibers EP/layered silicates

PE/organo-layered silicate PP/Mg-Al layered double hydroxides

TG-FTIR, TG-GC/MS TG-FTIR Py/GC-MS

Thermal decomposition of HDPE Thermal degradation and stabilization effects Thermal degradation mechanism Thermal-oxidative degradation and photo-oxidative degradation Thermal degradation Thermal degradation Thermal degradation mechanism Thermal degradation Thermal degradation Thermal degradation

HDPE/montmorillonite LLDPE/MWCNTS

Atmosphere

Polymer nanocomposites

Thermoanalytical method

Investigated phenomenon

Material

Table 1 (continued)

[130]

[129]

[123–126] [127] [128]

[121] [122]

[120]

[34] [103,104] [105] [106] [107] [108] [109] [110–112] [30] [113] [114] [115] [31] [116] [117] [118] [119]

[98] [99] [29] [100] [101] [102]

[96] [97]

[94] [95]

Ref.

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Fig. 10. The thermal evolution of Mg–Al−CO3 LDH as a function of temperature. Reprinted from [70] with permission from Elsevier.

oxidized to CO2. The acetic acid IR pattern is very much in line with the databank reference spectrum, this confirming that some of the acetate coming from the zinc precursor is transformed into acetic acid and electrostatically coordinated within the material structure. Results of the TG-FTIR analysis are coherent with the TG-MS data which indicate the presence of methoxyethanol traces and small amounts of water. Generation of methoxyethanol acetate was confirmed by GC-MS analysis of the surnatant solution after the inorganic nanoparticles had been removed. Authors assumed that the hydrolysis of zinc acetate is the first step of the overall reaction – the esterification of acetate with methoxyethanol could lead to the release of hydroxyl groups linked to the zinc species whose condensation results in ZnO and water formation, as showed in Fig. 13. Nanoscale ZnO powder has been synthesized at low temperature via reaction of zinc acetate dehydrate (Zn(CH3COO)2⋅2H2O) and sodium hydroxide (NaOH) by Al-Kahlout [78] and investigated using i.a. DTA/ TG/MS. Detailed analysis of the evolved gases, recorded simultaneously up to 500 °C by MS and IR spectroscopy, show the evolution of H2O and CO2. The evolved water comes from the dehydration of the organic ligands and the loss of structural water while the evolution of CO2 results from the burning of carbon. Thermal decomposition process of ammonium perchlorate (AP), catalyzed in situ by CoC2O4, was investigated by Zongxue and coworkers who applied TG-MS to detect the volatile products [72]. Authors revealed that CoC2O4 shows an intensive catalytic effect on the thermal decomposition of AP, and the main products of degradation are H2O, NH3, O2, HCl, Cl2, NO, N2O and NO2. Lithium-catalyzed dehydrogenation of ammonia borane (AB) within mesoporous carbon (CMK-3) framework for chemical hydrogen storage was investigated by Li et al. [74]. Analysis of the hydrogen desorption profiles of AB and AB/mesoporous carbon measured by thermogravimetry combined with mass spectroscopy showed that the neat AB starts to decompose at above 100 °C and runs via a two-step process. Compared with the neat AB, the decomposition of AB/mesoporous carbon nanostructures occurs below 75 °C with only one-step for the release of gaseous substances. A simplified mechanism of AB decomposition on CMK-3 is presented in Fig. 14. BeH hydridic hydrogen in AB as the electron donor can associate with the surface−OH groups in mesoporous carbon to form surface eOeH⋯ H–BH2NH3 structures, thereby destabilizing the network of AB. This explains the enhancement of dehydrogenation, intensive NH3

Fig. 11. Reactivity of functional groups during the conversion of polymeric-gel into oxycarbide ceramic. Reprinted from [75] with permission from Elsevier.

pyrolysis process by nucleophilic attack of uncondensed Si–OH groups (50 °C < T < 650 °C), and as homolytic cleavage initiated by the radicals issued from the hemolytic cleavage of Si–H bonds (600 °C < T < 750 °C). In the absence of Si–H in the structure of the precursor, it follows that the homolytic cleavage of SieCH3 occurs at temperatures above 750 °C – Fig. 11. This information can be useful to design the preceramic polymer thermal treatment protocol yielding ceramic residue with desired set of properties for a particular application. Pure ZnO is an n-type semiconductor, and it has been extensively studied for a long time because of its use in a wide range of applications, e.g. in electronic industry [132]. The effects of indium and gallium doping on structural, electrical and optical properties of ZnO nanoparticles, prepared by hydrothermal reaction in ethanol and methoxyethanol, were studied by Cimitan et al. using among others TGFTIR technique – Fig. 12. By analyzing the TG profiles and FTIR spectra of In-doped ZnO synthesized in methoxyethanol authors attributed the two founded TG steps to methoxyethanol, acetic acid and CO2 formation. In the range of 140–230 °C organic residue is progressively desorbed from the surface while, at a higher temperature, the carbonaceous phase is mainly 148

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Fig. 12. TG-FTIR curves for sample IZO-M6. Reprinted from [71] with permission from Elsevier.

release, as well as borazine suppression when heating AB inside mesoporous carbon [74]. Schmidt et al. [81] investigated the thermal behavior of AgNCO (silver isocyanate) using a thermoanalytical method - DTA/TG-MS, and they observed the specific decomposition of AgNCO leading to a hybrid composite consisting of silver (nano)particles distributed in a polymeric matrix. Abdelkader et al. [133] synthesized via a sol-gel method Cassiterite (tin oxide, SnO2) nanoparticles. TG was used to clarify the appropriate heat treatment temperature for the tin complex, and TG-MS was applied to analyze and identify gas products evolved during the heat treatment. Form TG curves three well defined regions of weight loss can be detected. The first mass loss was identified as the dehydration of two H2O molecules (m/z 16, 17, 18) and the formation of insoluble SnC2O4 crystals during heating from 25 to 80 °C which remain stable up to 250 °C. At 330 °C SnC2O4 transforms completely into SnO2. During heating from 250 to 330 °C the organic ligand (m/z 29 C2H5 or CHO) and other residues (m/z 12, 15, 44 from carbon, CH3- and carbon dioxide, respectively) are oxidized. The chemical reaction ends up at about 380 °C, resulting in the formation of SnO2 powder. ZnO nanoparticles preparation route, including zinc hydroxide nitrate (ZnHN) or zinc hydroxide carbonate (ZnHC) precipitation, thermal treatment and nano-milling was elaborated by Japić et al. [134]. Thermal decomposition of ZnHN and ZnHC was investigated by TG-DTA–MS method. The first mass loss occured in the 30–110 °C range and was attributed to the sample drying. The decomposition up to 200 °C was associated with the precursor dehydration, while increasing temperature from 200 to 300 °C gradually reduced the nitrate group content and led to ZnO formation.

3.2. Carbon nanomaterials Carbons with controlled pore shapes, sizes and distributions, socalled nanostructured carbons, have been extensively studied as they are widely used for catalysis supports, separation, gas storage, doublelayer capacitors and electrodes [88,135–138]. Different methods of preparation have been elaborated, for instance reaction-induced phase separation of miscible blends of phenolic resin/PMMA which yields carbonaceous materials having continuous nanopores [88]. TG/MS was used to characterize these materials giving an insight into the differences in structural changes related to the types of phenolic resins. Thornton and Walker [89] investigated by TG-MS carbon deposition kinetics and the carbon nanostructures formed during the chemical vapor deposition of ethane and synthetic natural gas, with and without added hydrogen, over a nickel catalyst, supported on three-dimensional (3D) carbon fiber preforms – Fig. 15. During heating to 500 °C a mass loss of 33% was observed caused by splitting off the water and decomposition of the nickel nitrate hexahydrate giving rise to an increase in the MS signal identified as water vapor, nitrogen dioxide, and oxygen. Next, in the temperature range from 500 to 750 °C in nitrogen, there was a sharp weight loss of 4.8% observed at 600 °C due to the formation of carbon dioxide during reduction of nickel oxide to nickel by the carbon fibers. Finally, hydrogen was introduced at 750 °C, and the gases evolved were hydrocarbons, water vapor, and oxygen. This was accounted to possible gas phase reactions due to contamination of the gas supply, the catalytic gasification of the carbon fibers by hydrogen, oxygen or water vapor interacting with the metallic nickel, and/or the release of oxygenated species, which were adsorbed on the surface of the carbon fibers. Among carbon nanotubes (CNTs), synthesized first by Iijima in 1991 [139], numerous classes of materials have emerged, including singleFig. 13. Proposed mechanism for doped ZnO nanoparticles formation. Reprinted from [71] with permission from Elsevier.

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Fig. 14. The mechanism of hydrogen release of AB/CMK-3 nanocomposite. Reprinted from [74] with permission from Wiley.

walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs), commercial carbon nanotubes (CBTs), complex MWCNTs with aluminum oxide (MWCNTs/Al2O3), and modified multi-walled carbon nanotubes (mMWCNTs) [90]. Hence, Chou and co-workers [90] analyzed MWCNTs and mMWCNTs by TG-FTIR pointing out that for the same heating rate the mMWCNTs have less thermal hazard than MWCNTs with higher activation energy. In the next work, multi-walled carbon nanotubes have been functionalized by a dielectric barrier discharge plasma in the air and compared to those functionalized in HNO3. Quantitative analyses of gases evolved during the temperature programmed desorption of the functionalized nanotubes were performed using Fourier transform infrared spectroscopy and gas chromatography. Authors show that the extent of functionalization increases with increasing the discharge power as well as compared to the acid treatment, plasma functionalization offers the advantages of much shorter treatment time [140]. In the work by Samori et al. [91] the conditions for oxidizing multiwalled carbon nanotubes to assure a narrow length distribution have been optimized. Moreover, authors reported on the combination of different methods (i.a. TG-MS), to determine the number of functional groups generated during strong acid treatment and a further amidation reaction. They found a good correlation using the colorimetric Kaiser test, thermogravimetric analysis and potentiometric argentometric titration. Brković et al. [141] performed covalent sidewall functionalization of MWCNTs using two approaches, direct and indirect cycloaddition through diethyl malonate, based on the Bingel reaction, to enhance the electrical properties and lower sheet resistance. The covalently attached organic functionality at MWCNTs surface was desorbed and analyzed using TG-MS method to collect molecular ions, fragments which could result from the decomposition of various oxygen- and nitrogen- containing groups, CO2 (m/z 44), CHN (m/z 27), acetylium (m/z 43) and ketene ions (m/z 42). Magnetically recyclable Ni/graphene (Ni/G) nanocomposites were synthesized via an in situ reduction growth process for selective reduction of nitroarenes into corresponding azoxybenzene by Pahalagedara and co-workers [142]. TG analysis results indicated that when these materials are heated under air, a small weight loss was observed below 200 °C, and next weight gain at about 400 °C. According to TG-MS data, in CO2 evolution profile, Ni nanomaterial shows a single peak between 200 and 300 °C. On the other hand, Ni/G nanocomposite shows two peaks, between 200–300 °C and 350–550 °C. Yin et al. [143] investigated the thermodynamic state and kinetic process of low-temperature deoxygenation reaction of graphene oxide (GO) by using DSC, TG-MS and XPS. They found that the thermal reduction reaction of GO was exothermic with degassing of CO2, CO and

H2O [144]. TG-MS method was also useful for investigation of graphite hydrogenation/deuteration with potassium intercalation. It has been found that kind of graphite, the potassium concentration in the intercalation compound as well as the type of the solvent strongly influence the course of hydrogenation/deuteration. In another work the authors synthesized graphene nanosheets by taking advantage of the confined space between the layers of an inorganic host material. By using TG-MS losses of water (detected as H2O+ at m/z = 18), dodecyl sulfonate (detected as SO2+ at m/z = 64) and MMA (detected as C5H8O2+ at m/ z = 100) were found to occur on heating between 300 and 400 °C. Based on TG-MS and Raman spectroscopy results, the mechanism of formation of the graphene nanosheets was postulated: first, carbonization of MMA to form amorphous carbon in the two-dimensional galleries of the rigid host-layered double hydroxide at ˜250 °C; second, decomposition of rigid host-layered double hydroxide to periclase at ˜440 °C, and third, formation of graphene nanosheets from amorphous carbon layers via graphitization at higher temperatures (˜900 °C) [145]. Vecera et al. [146] described the quantitative discharging of reduced graphite forms, e.g. surface-deposited graphenides, with benzonitrile (PhCN) that results in their conversion to graphene. For product characterization TG-MS was applied - no weight loss after heating up to 550 °C under nitrogen atmosphere was found for the reoxidized graphitic material. Discharging of GIC with PhCN leads to formation of graphene structure – Fig. 16. 3.3. Polymer nanocomposites During the last two decades, a new class of filler-reinforced thermoplastics has been extensively investigated using fillers in nanometer size range, preferably less than 100 nm. The advantage of nanofillers is that they are dispersed within the polymer matrix exploiting unique interactions between the combined materials. These polymer/nanoparticle systems are referred as nanocomposites and have been the rapidly growing field of investigation for developing the materials. Numerous research efforts were devoted to determination of the structure–property relationships and their improvement [147], as there is a well-justified hope that polymer-nanocomposites can overcome the limitations of traditional micro-composites [22]. The ability of TG-FTIR and TG-MS techniques to control simultaneously the mass loss of polymeric materials and the composition of gaseous degradation products can be widely applied in material research field, e.g. for the evaluation of hazardous materials decomposition pathways [9,10,94,98,148,149]. 3.3.1. Polymer/clay nanocomposites Pramoda et al. [100] investigated the thermal degradation of Fig. 15. TG-MS data for the carbon fiber mat impregnated with nickel nitrate hexahydrate: (a) Showing the TG data and how the weight percent of the impregnated carbon mat changes with temperature; (b) Showing the corresponding MS data N2 swiched to 5% volume fraction of H2 in N2 at 750 °C. Reprinted from [89] with permission from Elsevier.

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Fig. 16. Oxidative side-reactions of graphenides and the inhibition by PhCN treatment. (a) The sample is divided into two parts (GA and GB). In contrast to GA, GB was treated with PhCN before exposure to ambient conditions. The dotted line indicates pristine graphite (GP) being dispersed in THF and PhCN as reference. (b) Statistical Raman spectroscopy mean spectra of 10,000 measured pixels (100 × 100 mm). (c) TG mass loss profiles of GA, GB and the reference GP. (d) Correlating MS traces for m/z 2 and 18 indicating hydrogenation and hydroxylation for GA and vanishing ion current detection in the case of the PhCN treated sample GB compared with the reference GP. Reprinted from [146] with permission from Springer Nature Publishing AG.

polyamide 6 (PA6) and PA6-clay nanocomposites prepared by melt compounding using thermogravimetric analysis coupled to Fourier transform infrared spectroscopy. The thermal decomposition of PA6 and its clay nanocomposites takes place first with the evolution of the cyclic monomer (caprolactam), followed by the other volatile gases like CO2 and NH3. Based on the IR spectral analysis, the presence of oligomeric products with nitrile and vinyl chain ends is postulated. In another work, the thermal decomposition of organophosphorus fireretardant and/or organonanoclay was examined employing TG-FTIR, to give an insight into their fundamental behaviour and interaction in polymer nanocomposites. The addition of organophosphorus fire-retardant and/or organonanoclay in PA6 seems to have a synergistic effect on the thermal decomposition of PA6 [150]. Su and Wilkie [102] studied the thermal degradation of clay nanocomposites based on the main thermoplastic polymers, such as polyethylene, polypropylene, polystyrene, poly(methyl methacrylate) and acrylonitrile-butadiene-styrene terpolymer, using TG-FTIR. The nanocomposites that have been studied included immiscible, intercalated and exfoliated systems and the composition of the evolved gases was qualitatively similar to those of the virgin polymer. The thermal degradation of PMMA and its nanocomposites has been studied by Costache and co-workers to determine if the presence of clays (anionic and cationic) or CNTs has an influence on the degradation pathway [34]. Thermal degradation has been studied by cone calorimetry and TG, and the products of degradation have been analysed with FTIR and GC–MS. Authors did not found significant differences in the degradation products of the polymer and its nanocomposites, but revealed that the degradation of the nanocomposites occurs at higher temperatures. Py-GC/MS and TG-FTIR were used to compare degradation of PS and a PS/brush structure clay nanocomposites with by Chen et al. [111]. They revealed that the degradation mechanism of the PS/clay with brush structure nanocomposites is different to that of unmodified PS. In PS/clay composites unusually high yield of α-methylstyrene was observed that was attributed to intensification of intermolecular radical transfer reactions that increase the probability of interchain reactions. In a next study the same research group found that those PS/clay nanostructured composites are more stable to oxidation as oxidation

products accumulate in nanocomposite because of hindered diffusion due to barrier effect [112]. Further studies revealed that nanoconfinement of PS chains leads to significant changes of their dynamics, physicochemical behaviour and thermal stability. In the latter phenomena, larger values of activation energy of the thermal degradation processes were found for PS/clay nanocomposites compared to pristine PS. Authors postulated that the radicals formed during polymer chain scission are nanoconfined, permitting a variety of bimolecular reactions and barrier effect to occur after the nanoconfinement, but it is not the factor which prevents mass transport and offers thermal protection at the early stages of PS/layered silicate nanocomposites’ decomposition [110]. Kong et al. [30] prepared high impact PS (HIPS)/Fe-MMT nanocomposites via melting intercalation, and they found that the thermal stability of HIPS/Fe-MMT nanocomposites increased as compared with pristine HIPS. Based on the Py-GC/MS results, authors found that the HIPS/Fe-MMT nanocomposites showed an unusually high yield of αmethylstyrene that indicated intensification of intermolecular radical transfer reactions. They concluded that the unique properties of the nanocomposites result from the strong interactions between the silicate structural layers and the polymeric chains and Fe may constitute an important site of radical trapping. A Py-GC/MS and TG study on organomodified-layered silicate intermediates used for an in situ preparation of PE nanocomposites was performed by Bertini et al. [96]. The combination of pyrolysis and thermal decomposition data allowed to describe the evolution of the organoclay structure after the reactive pretreatment steps with zirconocene or bis(imino)pyridine iron precatalyst and alkylaluminoxane cocatalyst. Authors suggested that the identification of some typical decomposition products coming from the metallic complex precursor may be considered an evidence of the formation of heterogeneous organoclay-immobilized catalyst. Cervantes and co-workers [99] prepared segmented polyurethane (SPU)/clay nanocomposites with three types of clay - Cloisite™ Na+ (unmodified MMT), Cloisite™ 15 A and Cloisite™ 30B, which are organically modified clays, and studied their decomposition pathways by using TG-FTIR technique. The results proved that the thermal degradation of unfilled SPU and the 4, 6 and 10 wt.% clay containing nanocomposites (hand-mixed) occurred in two stages: (i) the 151

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Fig. 17. Suggested mechanism for thermal degradation of SPU. Reprinted from [99] with permission from Elsevier.

Fig. 18. Suggested mechanism of thermal degradation of SPU/Cloisite™ 30B nanocomposite prepared by in situ polymerization. Reprinted from [99] with permission from Elsevier.

degradation of hard segments, and (ii) the degradation of soft segments (Fig. 17). It has been found that the thermal stability of the investigated nanocomposites was not improved by increasing nanoclay concentration except for SPU/Cloisite® 15 A nanocomposites where a 40 °C increase in Tonset was reported. For nanocomposites prepared by an in situ polymerization, higher thermal stability than for the corresponding hand mixed nanocomposites was observed. FTIR spectra of the evolved gases from thermal degradation of SPU/MMT nanocomposites were qualitatively similar to the unfilled polymer, although nanocomposites containing Cloisite™ 30B emitted compounds containing isocyanate groups. Moreover, SPU/Cloisite™ 30B nanocomposites prepared by in situ polymerization, exhibited a different degradation path which included the presence of carbon dioxide in the second degradation stage – Fig. 18. Authors postulated that the metallic species within the aluminosilicate potentially catalyze reactions between carbonaceous residue and oxygen in the layered structure of the MMT yielding CO2 [151]. This hypothesis is supported by the findings of Takeichi et al. [152] who suggested that the layered silicates make the path longer for escaping the thermally decomposed volatiles, but some amounts of the thermally decomposed volatiles are captured by OMMT. The thermal degradation of hydrogenated nitrile-butadiene rubber (HNBR)/clay and HNBR/clay/CNTs nanocomposites was investigated by TG alone and TG-FTIR, and the kinetic analysis was performed using Kissinger, Flynn–Wall–Ozawa and Friedman methods [114]. As it can be seen in Fig. 19 clay and CNTs do not considerably change the reaction mechanism of the thermal degradation.

The main thermal degradation products of HNBR are olefins with unsaturated end groups and acetonitrile, and the clay-CNTs mixed filler network was found to reduce the diffusion rate of degradation products. Kong et al. [153] investigated PVC/OMMT nanocomposites prepared by a melt intercalation method and the degradation mechanisms were investigated by Py-GC/MS. The presence of Fe-OMMT decreases the amount of aromatic products (i.e., benzene, toluene, ethylbenzene, naphthalene, etc.) formed during PVC decomposition. Importantly, no traces of styrene, chlorobenzene, and methylnaphthalene were detected in the volatile gases. Moreover, the total amount of gas products released from PVC/Fe-OMMT during decomposition was much lower than that from virgin PVC, indicating that Fe-OMMT can be considered as a useful smoke suppressant for PVC. The pyrolysis and flammability of phosphonium-modified layered silicate epoxy resin nanocomposites (EP/LS) with two flame retardants, melamine borate (MB) and APP was described by Schartel et al. [128]. Authors based on the TG-FTIR results proposed a decomposition model displayed in Fig. 20. Decomposition of EP is initiated by the release of butandioic acid, then a subsequent release of CO2, CO, CH4 and phenolic/carbonylic species takes place. Next, the decarboxylation reaction of the dicarbonic acid or fragmentation of CH4, as well as the formation of volatile, saturated decomposition products are accompanied by the production of small amounts of carbonaceous char residue. LS does not significant influence the decomposition of EP. Adding melamine borate results in an aminolysis reaction (left), observed through the release of amide species and isocyanic acid to the gas phase, and incorporation of APP also leads to an aminolysis reaction. In contrast to MB, APP is 152

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unchanged after the addition of bisphenol A bis(diphenyl phosphate) and OMMT. Nevertheless, the carbonate linkage could be stabilized by bisphenol A bis(diphenyl phosphate) combined with OMMT as revealed by TG-FTIR data. Papadopoulos et al. [26] obtained and characterized poly(propylene 2,5-furan dicarboxylate) (PPF) nanocomposites with aluminosilicate clays Cloisite®-Na (MMT), Cloisite®-20 A (MMT 20 A), and halloysite nanotubes (HNT), and then performed detailed Py/GC–MS studies on the influence of the nanoclays on the thermal properties and degradation mechanism of PPF. Two degradation mechanisms were postulated: the heterolytic (β-scission) and the homolytic (acyl-oxygen and alkyloxygen scission) – Fig. 21. Moreover, the β-scission was the dominant degradation mechanism and some effect of nanoadditives on degradation mechanism were observed. In the chromatogram of nanocomposites after pyrolysis at 360 °C additional peaks with retention time (Rt) 13.90, 14.08, and 14.25 min were found and they have been identified as characteristic for 5-((allyloxy)carbonyl)furan-2-carboxylic acid. Moreover, an increase in the intensity of the peaks with Rt ≈ 20.25 and 24.50 min from 2-(3-((furan-2-carbonyl)oxy)propyl)5-methylfuran-2,5-dicarboxylate and molecule that results from random radical disproportionation were observed, respectively. 3.3.2. Polymer/inorganic oxides nanocomposites Zinc oxide (ZnO) is an interesting electro-optical material due to its structural properties [155]. Hybrid materials consisting of zinc oxide and polymers exhibit the merits of advantageous properties of zinc oxide and easy processing/flexibility of polymers. Soluble polymers with good mechanical properties, that are able to form stable films, can be used in large-area flat panel displays and in light-emitting diodes (PLED) [156]. In a work by Chen et al. [156] hybrid ZnO/polymer (PS or PMMA) films were obtained by focused pulsed laser ablation. By using TG-FTIR it has been found that ZnO-containing hybrid films have higher thermal stability than pure polymers because of strong interactions among ZnO nanoparticles and macrochains. PMMA/ZnO and PMMA/OMMT nanocomposites prepared by melt blending were investigated by Laachachi et al. [106]. The presented PyGC/MS results showed formation of larger amounts of methanol and methacrylic acid, and a decrease in the formation of propanoic acid methyl ester during the decomposition of PMMA/ZnO nanocomposites while significant reduction in the quantity of all degradation products of PMMA was observed with increasing of OMMT concentration in the matrix, confirming its role of diffusion barrier. Nanocomposites of PVC, metallic oxides (copper, molybdenum, and zinc), and OMMT were prepared in a melt-blending or intercalation-inthe-molten state process and investigated toward thermal properties by Rodolfo and co-workers [107]. The TG-MS results indicated significant reduction in benzene formation, as well as enhancements in carbonaceous char residue formation resulting from the presence of metals in the PVC/OMMT composites. In another work, Peng et al. [157] investigated the thermooxidative degradation of poly(vinyl alcohol)/silica (PVA/SiO2) nanocomposites prepared with self-assembly monolayer technique. The results show that the thermooxidative stability of PVA/SiO2 nanocomposites has been greatly improved compared to the pure PVA, although the decomposition pathways followed by TG/MS seem to remain rather unaltered. In the next work of this group [108] authors investigated the thermal degradation mechanism of PVA/SiO2 nanocomposites prepared with self-assembly and solution-compounding techniques. Authors found that the introduction of SiO2 nanoparticles leads to a remarkable change in the degradation mechanism. The degradation products identified by TG-FTIR and Py-GC/MS suggests that the first degradation step of the nanocomposites includes elimination reactions of H2O and residual acetate groups, as well as some chain-scission reactions. In the second decomposition step chain-scission and cyclization reactions

Fig. 19. Three-dimension FTIR spectra corresponding to gases involved in the thermal degradation of HNBR composites (a) HNBR/clay (100/5); (b) HNBR/ clay/CNTs (100/5/0,4). Reprinted from [114] with permission from Elsevier.

incorporated in the EP network and induces the release of water (right). J. Liu et al. [154] investigated by TG-FTIR epoxy resin modified with SnO2 nanowires decorated by MnO2 nanosheets, to enhance the flame retardancy and inhibite the smoke production during combustion. Relative intensity of CO2 bands at 2360 cm− [1] for EP/2 wt.% SnO2-MnO2 increased in comparison to that of unmodified EP, suggesting that the combustible gases may be diluted by the non-combustible CO2 in the gas phase; as a result, the EP thermal degradation process is delayed. Generally, with incorporation of SnO2-MnO2, the intensity of gas emission is lower. Thermal degradation and flame retardancy mechanisms of polycarbonate (PC) filled with solid bisphenol A bis(diphenyl phosphate) and MMT were comprehensively cross-examined by cone calorimetry, TG-FTIR and TG-MS in the work by Feng and co-workers [120]. Authors revealed that under an inert atmosphere, the main pyrolysis products evolved during the thermal degradation of polycarbonate were carbon dioxide, methane, carbonates, benzene and its derivatives, as well as phenol and its derivatives. These degradation products remained almost 153

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Fig. 20. Major decomposition pathway of EP (in the middle), EP/MB (left) and EP/APP (right); observed decomposition products in gray. Reprinted from [128] with permission from Wiley.

Fig. 21. Main degradation mechanisms of PPF. Reprinted form [26] with permission from MDPI.

dominate, and continual elimination of residual acetate groups takes place. Laachachi et al. [105] investigated the thermal properties of TiO2 filled PMMA by TG, DSC, Py–GC/MS. TiO2 nanoparticles significantly improve the thermal stability of PMMA – this effect was attributed, at

least partially, to the limited mobility of polymer chains evidenced by the increase in the glass transition temperature with the increase of TiO2 amount. On the basis of the obtained results thermal degradation mechanism of PMMA/TiO2 composites has been proposed – Fig. 22.

Fig. 22. Scheme of the degradation mechanism of PMMA-TiO2 composites. Reprinted from [105] with permission from Elsevier. 154

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Fig. 23. Decomposition model of PBT (in the middle), with AlPI (left) and metal oxides (right). Demonstrated decomposition products in gray. Reprinted form [31] with permission from Elsevier.

The flame retardancy of poly(butylene terephthalate) (PBT) containing aluminum diethylphosphinate (AlPi) and/or nanometric metal oxides such as TiO2 or Al2O3 was investigated by Gallo et al. [31] by means of TA, TG-FTIR, flammability tests (LOI, UL 94, cone calorimetry), and chemical analysis of residue (ATR-FTIR). As a result, active flame retardancy mechanisms were postulated – Fig. 23. Authors revealed that AlPi acts mainly in the gas phase through the release of diethylphosphic acid, which provides flame inhibition. Part of AlPi remains in the solid phase and reacts with the PBT to form phosphinate-terephthalate salts that decompose to aluminum phosphate at higher temperatures. Next, the metal oxides interact with the PBT and promote the formation of additional stable carbonaceous char in the condensed phase. Qi and co-workers [158] synthesized polyaniline/NiO (PANI/NiO) composites by an in situ polymerization in the presence of HCl and investigated thermal stability of the composites using TG-MS. The obtained thermo-analytical profiles show that the main products for oxidative degradation of both PANI and PANI/NiO composite were H2O, CO2, NO and NO2. Results of TG analysis revealed that with increasing of NiO contents in PANI/NiO composites, thermal stability of PANI/NiO

composites first increases, and when the NiO content is higher than 66.2 wt. %, it decreases. The influence of nanometric alumina oxide and submicron alumina trihydrate on the thermal stability and fire behaviour of an unsaturated polyester resin (UPR) has been investigated by Tibiletti et al. [117]. The thermal degradation behaviour of the composites was studied using TG and Py-GC/MS. Py-GC/MS experiments show that the main degradation products of the unmodified UPR are phthalic anhydride and styrene. The addition of fillers does not change the degradation products of the UPR, thus excluding any chemical effect that could explain their influence on the fire behaviour. In a work of Chrissafis and co-workers [119] poly(propylene sebacate) (PPSeb) nanocomposites with fumed silica nanoparticles (SiO2), MWCNTs or MMT were obtained via in situ polymerization; the thermal decomposition pathways were investigated using TG, TG-FTIR and TGGC/MS. The recorded spectra of volatile products from PPSeb and its nanocomposites (2-propenal, 2-propenol, 3-hydroxypropanal, 1,3-propanediol, sebacic acid, as well as its allyl and diallyl compounds) were almost identical, indicating that the decomposition products with similar chemical structure were produced for all composite materials. 155

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3.3.3. Polymer/carbon-based nanocomposites Carbon nanomaterials are increasingly applied in polymer composites as they provide excellent mechanical properties and high thermal conductivity. For instance, Bocchini et al. [95] studied MWCNTs)/UPR nanocomposites in order to understand the stabilization mechanism. TG-FTIR and Py/GC–MS results demonstrated that MWCNTs presence slightly delays thermal volatilization (15–20 °C) without modification of the thermal degradation mechanism. However, the thermal oxidative degradation was delayed by about 100 °C independently from MWCNTs concentration in the range used (0.5–3.0 wt.%). Authors postulated that the stabilization is due to the formation of a thin protective film of MWCNTS/carbon char generated on the surface of the nanocomposites. The thermal degradation behavior of styrene-butadiene-styrene triblock copolymer (SBS) and SBS/MWCNTs composites, prepared by solution processing and melt mixing, was investigated by Lu and coworkers [103]. Researchers found that degradation activation energy of SBS/MWCNTs composite prepared by melt mixing was higher than that by solution processing, which was attributed to the good dispersion of MWCNTs in SBS and the interactions between MWCNTs and SBS. Moreover, the evolved gas analysis during thermal degradation under nitrogen atmosphere revealed the presence of aliphatic and aromatic CeH absorption bands, indicating that the SBS thermal degradation proceeds by a random chain scission mechanism. In the study by Tang et al. [159], hybrid carbon nanofibre (CNF) papers containing clay, polyhedral oligomeric silsesquioxanes (POSS) or APP were prepared and incorporated on the composite laminates through injection moulding process. Based on the on-line combined results of TG and IR techniques it can be concluded that the resin and APP decomposed early, resulting in the volatilization of aromatic and carbonyl moieties, and formation of vinyl compounds and phosphocarbonaceous complex – Fig. 24. The Si-O moieties were released at a higher temperature (above 440 °C) due to POSS decomposition which usually starts with splitting off organic functionalizations [160]. Expanded graphitic materials were prepared by Ramos et al. [161] from two different precursors: micrometric synthetic graphite and graphitized CNF, and tested as anodes for sodium-ion batteries. Using TG-MS it has been found that materials preparation involves the oxidation of the precursors followed by partial thermal reduction. A weight loss of ca. 33%, which is associated with the release of oxygen functional groups in graphite oxides, mainly as H2O, CO2 and CO, was detected in the range 140–300 °C with temperature of maximum weight loss rate at 190–200 °C. TG-FTIR and Py-GC/MS were used to demonstrate that the addition of fullerenes (C60) increases the onset decomposition temperature of high density polyethylene (HDPE) by about 10 °C with more heavy compounds (more than 34 carbon atoms) in nitrogen by trapping carbon centered radicals and it is quite independent of concentration. In air, the thermal stability of HDPE is also remarkably improved, especially at high C60 content due to the radical trapping effect of C60, which inhibits hydrogen abstraction with transforming the radicals into non-radical species and suppresses the chain scission. It has been also observed that, with high C60 concentration, it traps alkyl peroxide radicals and alkyl radicals to inhibit the hydrogen abstraction to suppress the chain scission [162]. Poly(ε-caprolactone) (PCL) was reinforced with amino-functionalized multi-walled carbon naono-tubes (f-MWCNTs) by an in situ ring opening polymerization [163]. Based on the results from TG and Py–GC/MS it was concluded that the thermal stability of the matrix is not enhanced by f-MWCNTs and random chain scission via cis-elimination, intramolecular transesterification and unzipping reactions dominate the decomposition of PCL. Moreover, the highest amount of filler was found to catalyze decomposition reactions without interfering with the detected mechanisms. In the next work they investigated thermal and decomposition behavior of PCL nanocomposites with clay-supported CNT hybrids using

Fig. 24. FTIR spectra of volatilized products at different temperature during thermal degradation: (a) CNF-5/Clay/APP-9 nanopaper infused resin; (b) CNF5/POSS-3/APP-7 nanopaper infused resin. Reprinted from [159] with permission from Elsevier.

TG and Py-GC/MS. It has been found thet hydroxyl groups on the surfaces of clays and CNTs accelerated degradation rates of PCL. The main degradation pathways are intramolecular transesterification reactions, and/or unzipping reactions from the α-carboxylic acid chainends. At higher temperature cis-elimination also occurred [25]. For poly(butylene naphthalate)/MWCNTs nanocomposites it was observed that the thermal stability was higher for pristine polymer than for nanocomposites. Based on Py-GC/MS results it was also postulated that the main degradation pathway of poly(butylene naphthalate) and its nanocomposites is heterolytic β-scission [24]. Tarani et al. [164] studied degradation behavior of HDPE/graphene nanocomposites using Py-GC/MS. They found that main decomposition products of these nanostructured materials were linear hydrocarbons, dienes and alkenes up to 32 carbon atoms in the main chain and that the degradation mechanism involved chain scission reactions, followed by β-scission propagation reactions, radical reactions and, finally, the termination stage. Similar results were observed for polypropylene/ graphene/glass-fibre nanocomposites where main degradation products were also alkenes [27]. Roumeli et al. [165] investigated the thermal degradation behavior 156

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Fig. 25. Decomposition scheme of PEX. (a) intramolecular hydrogen transfer, (b) condensation of silanol groups and (c) free radical formation via scission of CeC bonds. Reprinted from [166] with permission from Elsevier.

of PP nanocomposites with MWCNTs and nanodiamonds (NDs) using TG and Py-GC/MS. The obtained results revealed that the well dispersed MWCNTs cause nanoconfinement in the polymer chains and enhances thermal stability of polymer matrix while the spherical NDs also cause the same effect enhancing the thermal stability of PP but in a less efficient manner. In another work of this research group, the effect of MWCNTs on the thermal degradation behavior of crosslinked high density polyethylene (PEX) was investigated; it was found that the presence of MWCNTs greatly affects the thermal properties of PEX [166]. The results of thermogravimetric analysis and Py-GC/MS reveal a drastic modification of the initial decomposition reactions of PEX due to the presence of MWCNTs, while the rest of the reaction remains essentially the same. Based on obtained results two possible decomposition mechanisms for

PEX were suggested – Fig. 25. The same authors also investigated the degradation behavior of syndiotactic PS - based nanocomposites containing MWCNTs and NDs. Py-GC/MS results indicated that the main degradation products are styrene monomers, dimers and trimers. Depending on the temperature of pyrolization different intensities of decomposition fragments evolution were observed. It has been observed that the decomposition temperature and insertion of nanoparticles (NDs or MWCNTs) influence on the intensity of the recorded fragments evolution, however, there was no change of the degradation mechanism of sPS [167]. 3.3.4. Polymer/other nanofiller nanocomposites The decomposition of poly(ethylene terephthalate) (PET)/silica nanocomposites was investigated using TG and Py-GC/MS by Zheng 157

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Fig. 26. Thermogravimetry (TG) and differential thermogravimetry (DTG) curves in He for PoMea created into Magh-H (a) and Magh-Cu (b) and evolved gases of the polymer created into Magh-H (c) and Magh-Cu (d). Reprinted from [113] with permission from Elsevier.

and co-workers [109]. They found the influence of the filler on the activation energies of the decomposition process and on the residual carbon content – both parameters increase with an increase of silica nanoparticles concentration. One of the stabilization mechanisms postulated was the adsorption of the decomposed products on the surface of silica. The influence of the nano-dispersed boehmite (AlOOH) on PC/ABS blends has been investigated using TG, TG-FTIR and cone calorimetry with different external heat fluxes in the work by Pawlowski and Schartel [168]. Authors postulated possible flame retardancy mechanisms explaining the role of AlOOH in decreasing the peak heat release rate. Salavagione et al. [113] synthesized and characterized poly(o-methylaniline)(PoMea)/maghnite (Magh) nanocomposites, and they found remarkable differences in the properties of the polymers that have been observed by TG-MS and FTIR. Differences between both extracted polymers can be observed from the weight loss profiles and MS spectra (Fig. 26). Thus, the weight loss of the PoMea extracted from Magh-Cu nanocomposites occurs in two well-defined processes at about 200 and 400 °C (Fig. 6a), whereas in the case of PoMea from Magh-H the weight is lost in a more continuous way and the process at 200 °C cannot be distinguished. These differences in the decomposition of the polymers suggest a different chain structure, i.e. the polymer from Magh-H shows a higher degree of branching than that from Magh-Cu. Etienne and co-workers [169] aimed at improving PVC thermal stability by incorporation of poly(vinyl butyral) (PVB) and calcium carbonate nanoparticles. Data obtained from thermogravimetry coupled with mass and infrared spectroscopy helped to elucidate the stabilizing mechanism - both PVB and CaCO3 nanoparticles acted as HCl scavengers and afforded a significant delay of both the onset degradation temperature and HCl release. Pielichowska investigated the thermal degradation of polyoxymethylene/hydroxyapatite (POM/HAp) nanocomposites by TG, TGMS and TG-FTIR. It was found that incorporation of HAp leads to a

significant decrease of POM (both homopolymer and copolymer) thermal stability [170,171], and different degradation mechanism in POM/HAp materials are operating as compared to pristine POM – introduction of HAp promotes degradation pathway leading mainly to formaldehyde emission, almost without other decomposition products [28,172]. The thermal degradation behavior of biodegradable poly(L-lactide) (PLLA)/layered double hydroxide (P-LDH) nanocomposites was explored using TG and Py-GC/MS in an inert atmosphere by Chiang et al. [116]. The identification of the thermal degradation products by PyGC/MS evidently shows that introducing P-LDH into PLLA leads to a remarkable change during the thermal degradation process – Fig. 27. The main reaction path of neat PLLA was through inter- and intratransesterification to form lactides and oligomers, whereby the main gaseous products obtained from PLLA/P-LDH nanocomposites were lactides, regardless of the degradation temperature. These results suggest that the thermal degradation behavior of PLLA/P-LDH nanocomposites is governed by the favored formation of lactide by the unzipping depolymerization reaction, which is catalyzed by Mg and Al components in P-LDH [116]. Szilagyi et al. [127] prepared PVP and AMT (ammonium metatungstate) (NH4)6[H2W12O40]⋅nH2O composite nanofibers by an electrospinning process from aqueous solutions of PVP and AMT. According to TG-MS results, the composite fibers thermally decomposed and oxidized in the air in four steps between 25 and 570 °C. Based on these results, the PVP/AMT fibers were annealed in a furnace between 500 and 600 °C and pure, high quality, 250 nm thick and tens of micrometer long m-WO3 nanofibers were obtained. The polymer-to-ceramic transformation of a hafnium alkoxidemodified polysilazane was investigated via TG-MS, nuclear magnetic resonance (MAS NMR) and transmission electron microscopy (TEM) by Ionescu and co-workers [86]. Authors revealed that the hafnium-containing polysilazane transforms at 700 °C into an amorphous single phase ceramic, i.e. SiHfCNO. Upon pyrolysis at higher temperatures (ca. 900 °C), amorphous hafnium nanoparticles begin to precipitate 158

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Fig. 27. A: Thermal degradation mechanism of PLLA; B: scheme of the possible thermal mechanism of PLLA/P-LDH nanocomposite. Reprinted from [116]. With permission from Elsevier.

throughout the SiCN(O) matrix as revealed by NMR and TEM. By adding well-dispersed cellulose nanocrystals (CNCs) into bacterial poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) matrix “green nanocomposites” were prepared by Yu and co-workers [118]. To understand the mechanism of the enhancement in thermal stability of PHBV nanocomposites, their thermal degradation processes were investigated by TG-FTIR – Fig. 28. Authors observed similar infrared bands for neat PHBV and the nanocomposites, indicating formation of almost the same decomposition products. The results imply that the main decomposition pathway of PHBV do not seem to be affected by introducing CNCs, but the temperature for producing the detectable volatile products increases from 246 °C for neat PHBV to a maximum of 299 °C for the nanocomposite with 10 wt.% CNCs. Lichtenstein and Lavoine investigated the thermal degradation mechanism of nanocellulose using TG-MS [173]. They observed that cellulose nanofibers (CelNF) undergo degradation at a lower temperatures than the initial pulps. In the first step a dehydration, followed by a breakdown of the glucosidic structure and aromatization, occur both for the cellulose pulp and cellulose nanofibers. The 2,2,6,6-Tetramethyl-1piperidinyloxy (TEMPO)-mediated oxidation lead to a decrease in the

thermal stability of CNF. It was observed that MS signals recorded during the cellulose nanofibers and TeCelNF COOH degradation occurred generally at similar temperatures in contrast to the degradation of TeCelNF COONa. The signals from water and carbon dioxide were found during all three degradation steps, whereas the signals with m/z > 35 amu were only recorded in the second and third degradation step - it was concludes that in the first degradation step of TeCelNF COONa evaporation of water and carbon dioxide occurs, whereby dehydration and decarboxylation of the sodium carboxylate groups are involved in the degradation mechanisms of the TeCelNF COONa cellulose chains. The flame retardancy mechanisms of a polyhedral oligomeric silsesquioxane containing 9,10- dihydro-9-oxa-10-phosphaphenanthrene10-oxide (DOPO-POSS) in PC/ABS blends were discussed by Zhang et al. [121]. Researchers found on the basis of TG, FTIR, and XPS data significant changes in the decomposition of PC/ABS with 10 wt% DOPO-POSS compared with unmodified PC/ABS. It was postulated that the enhancement of the thermal-oxidative stability of PC/ABS with DOPO-POSS is attributed to the interaction between DOPO-POSS and PC/ABS at elevated temperatures. Vuorinen et al. [174] investigated the thermal degradation behavior 159

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decomposition pathway of PBSu is mainly via β-hydrogen scission and leads to the formation of CO, CO2, H2O, 1,3-butadiene, tetrahydrofuran, succinic acid and its anhydride, allyl- and diallyl compounds. In lower extent decomposition with α-hydrogen scission, leads to production of aldehydes. Additionally, it was revealed that for PBSu/SiNT nanocomposites the evolving compounds were the same and appeared in the same order as in neat PBSu, suggesting that SiNTs do not change the decomposition mechanism of PBSu, while the addition of SrHNRs has a minor effect on PBSu decomposition mechanism and some differences in the retention times of evolved products were observed. Chrissafis et al. [177] studied thermal behavior of PS based nanocomposites containing Cu-nanofibers and Ag-nanoparticles. Py-GC/MS studies show that even though the nanocomposite materials were more stable than PS, the decomposition mechanism remains the same. 4. Conclusions Thermoanalytical methods like TG-MS, TG-FTIR, TG-GC/MS, analytical pyrolysis or miro-thermal analysis have been extensively used to determine the composition, reactivity and degradation mechanisms of different nanomaterials and nanocomposites at the whole life-cycle preparation, characterization and recycling. The coupling of thermal analysis with spectroscopic/chromatographic methods provides synergistic information that is not available from either technique alone. As a method of evolved gas analysis, FTIR or MS are sensitive, fast, and can yield ‘finger print’ information on most of the compounds evolved during the thermal treatment. However, the interface has to meet some requirements such as representative gas sampling; minimized dilution effects and low decomposition, short response time, high sensitivity, high resolution and corrosion resistance. For instance, various splitter designs have been developed so far in order to reduce the transfer line pressure down to a level suitable for injection into the mass spectrometer. Moreover, the transfer lines are to be heated to prevent condensation of less volatile products. Acknowledgments The authors are grateful to the Polish National Science Centre for financial support under the contract No. UMO-2016/21/B/ST8/00449.

Fig. 28. (a) TG/FT-IR stack plots for neat PHBV and (b) the nanocomposites CNCs. Reprinted from [118] with permission from Elsevier.

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