Application of Simultaneous and Coupled Thermal Analysis Techniques in Studies on the Melting Process, Course of Pyrolysis and Oxidative Decomposition of Fused Triazinylacetohydrazides

Abstract

The effect of the structure of promising antioxidant agents with prospective medical use, i.e., unsubstituted and para-substituted annelated triazinylacetic acid hydrazides, on their melting points, thermal stabilities, pyrolysis and oxidative decomposition stages and the type of volatiles emitted under heating with the use of DSC and TG/DTG/FTIR/QMS methods was evaluated and discussed. The melting point of the investigated compounds increased with an enhanced number of electrons (directly correlated with their molecular weight). Melting enthalpy values were determined and presented for all the studied compounds. The pyrolysis and oxidative decomposition processes of the analysed molecules consisted of several poorly separated stages, which indicated a multi-step course of the decomposition reactions. It was found that the thermal stability of the tested compounds depended on the type of substituent at the para position of the phenyl moiety or its absence. In both atmospheres used (air and helium), the thermal stability increased in relation to R as follows: -CH3 ≤ -OCH3 < -H < -OC2H5. In an inert atmosphere, it was higher by approx. 8–18 °C than in an oxidative atmosphere. The pyrolysis was connected with the emission of NH3, HCN, HNCO, HCONH2, HCHO, CO2, CO and H2O in the case of all the tested compounds, regardless of the substituent attached. In the case of the derivative containing the para-CH3 group, para-toluidine was an additional emitted aromatic product. In turn, emissions of aniline and alcohol (methanol or ethanol) for compounds with the para-OCH3 and para-OC2H5 groups, respectively, were confirmed. In oxidative conditions, the release of NH3, NO, HCN, HNCO, HCONH2, CO2, H2O and cyanogen (for all the compounds) and para-toluidine (for the para-CH3 derivative), aniline (for para-OCH3, para-OC2H5 and unsubstituted derivatives) and acetaldehyde (for the para-OC2H5 derivative) were clearly observed. No alcohol emissions were recorded for either compound containing the para-OCH3- or para-OC2H5-substitututed phenyl ring. These results confirmed that the pyrolysis and oxidative decomposition of the investigated annelated triazinylacetohydrazides occurred according to the radical mechanism. Moreover, in the presence of oxygen, the reactions of volatiles and residues with oxygen (oxidation) and the combustion process additionally proceeded.

Thermal Characterization of Crosslinked Polymeric Microspheres Bearing Thiol Groups Studied by TG/FTIR/DSC under Non-Oxidative Conditions

“…Based on the 3D FTIR spectrum ( Figure 3 ) and the spectra extracted at maximum degradation temperatures ( Figure 4 A), it can be concluded that the depolymerization occurs at the beginning of poly(GMA- co -1,4DMB) microsphere decay and is followed by isomerization in the gas phase. The FTIR spectrum of gases gathered at 269 °C reveals the following bands: bands at 3059 cm −1 associated with vibrations of the methylene group in the oxirane ring [ 39 ]; bands at 1737 cm −1 from the vibrations of the C=O group in esters [ 40 ]; bands at 1650 cm −1 associated with vibrations of the C=C bond of the methacrylate group; bands at 1158 cm −1 related to vibrations of the C-O-C group in esters; bands at 939 cm −1 derived from vibrations of the =CH 2 group; bands at 842 cm −1 due to vibrations of the oxirane ring [ 41 ]. These bands are characteristic of GMA monomer [ 42 ] and their presence testifies that depolymerization takes place at the beginning of the decomposition.…”

“…In the FTIR spectrum of the gaseous poly(GMA- co -TRIM) decomposition products collected at the first emission maximum value at 255 °C ( Figure 7 A), bands characteristic of polyGMA decomposition by depolymerization can be observed. The absorption bands at 849 cm −1 associated with oxirane ring vibrations [ 41 ], those at 1160 cm −1 from C-O-C vibrations in the ester, those at 1744 cm −1 from vibrations of the C=O group and those at 3057 cm −1 related to the methylene group vibrations in the oxirane ring indicate the emission of GMA monomers [ 39 , 42 ]. For the TRIM-crosslinked copolymer, the bands characteristic of acrolein are not as intense as in the case of the copolymer with 1,4DMB, but bands in the spectrum region of 2700–2800 cm −1 can still be observed ( Figure 5 ) [ 16 ].…”

“…Since an overall endothermic peak is observed in the DSC curve in the temperature region of the first degradation step, it can be assumed that the possible isomerization of oxirane groups is not as intense as in the case of the 1,4DMB-crosslinked copolymer and methacrolein is probably released due to the polymer network breakdown. In addition, at higher decomposition temperatures, mainly bands characteristic of water (3500–3600 and 1500–1600 cm −1 ), carbon dioxide (2360 cm −1 ) and carbon monoxide (2180 and 670 cm −1 ) formed by decarboxylation and dehydration processes of the created solid products of pyrolysis are observed [ 39 , 40 , 46 ]. Moreover, in order to follow the changes in the chemical structure of the copolymer during heating, it was heated to 245, 370 and 510 °C in a helium atmosphere, and then FTIR/ATR spectra of the preheated samples were recorded.…”

“…In the spectrum of gaseous products of poly(GMA- co -1,4DMB)-SH pyrolysis recorded at the first maximum of emission at 355 °C ( Figure 13 ), the following absorption bands are visible: bands above 3500 cm −1 (the -OH stretching vibrations), at 3080 cm −1 (from the stretching vibrations of the =C-H) [ 39 ], in the region of 2980–2890 cm −1 (the C-H stretching vibrations), at 2816–2747 cm −1 (the aldehyde C-H stretching vibrations) [ 49 ], at 2360–2326 cm −1 (CO 2 ), at 2173–2114 cm −1 (CO) [ 39 , 40 , 46 ], at 2072–2047 cm −1 (the C=O vibration in COS) [ 50 , 51 ], at 1794–1700 cm −1 (the C=O stretching vibrations) [ 40 ], at 1651 cm −1 (the C=C stretching vibrations), at 1503 cm −1 (the aromatic C=C stretching vibrations), at 1457 and 1376–1342 cm −1 (the C-H deformation vibrations), at 986 and 943 cm −1 (the =C-H out-of-plane deformation vibrations) and 896 cm −1 (the out-of-plane C-H vibrations of 1,4 substitute aromatic ring) [ 41 ]. The maximum carbon dioxide emission is observed at 370 °C, whereas in the spectra collected at 383 °C and 483 °C, the intensity of absorption bands derived from organic groups is smaller than at 355 °C.…”

“…From the 3D-FTIR diagram of the gaseous pyrolysis products of poly(GMA-co-TRIM)-SH (Figure 9), the volatile spectra that evolved at the emission maxima were extracted. The spectrum collected at 353 • C (Figure 10A) shows absorption bands above 3500 cm −1 (the -OH stretching vibrations), at 3100 cm −1 (from the stretching vibrations of the =C-H) [39], in the range of 2970-2866 cm −1 (the C-H stretching vibrations), at 2806-2774 cm −1 (the aldehyde C-H stretching vibrations) [49], at 2360-2326 cm −1 (indicating CO 2 presence), at 2173-2114 cm −1 (for CO) [39,40,46], at 2072-2047 cm −1 (representing the C=O vibration in carbonyl sulfide (COS) [50,51]), at 1790-1713 cm −1 (for the C=O stretching vibrations) [40], at 1651 cm −1 (for the C=C stretching vibrations), at 1457 and 1376-1342 cm −1 (for the C-H deformation vibrations), at 1131 cm −1 (for the C-O deformation vibrations) and at 943 cm −1 (for the =C-H out-of-plane deformation vibrations) [41]. Considering the 3D FTIR spectrum, it is evident that the maximum CO 2 emission occurs between the first and second steps of copolymer degradation around 403 • C. In the spectrum from T max3 , only the absorption bands derived from water (3600-3500 and 1500-1600 cm −1 ) and CO 2 (2360-2326 and 671 cm −1 ) can be observed [49].…”