Concetto Puglisi

Evolution of aromatics in the thermal degradation of poly(vinyl chloride): A mechanistic study

Abstract Various aspects of the thermal decomposition, flammability and smoke evolution from PVC have been investigated in an attempt to account for the mechanism of evolution of aromatics from this polymer. The most relevant experimental observations for the understanding of the mechanism of thermal decomposition of PVC were obtained by the method of direct pyrolysis in the mass spectrometer. A number of facts that had partly escaped previous workers was revealed: that in each of the two degradation steps of PVC a family of aromatic hydrocarbons was formed with different structures; that the two families of aromatics are suppressed at considerably different rates by the addition of metal oxides. The selective suppression of aromatics is related to the fact that the two families of aromatics are evolved from linear and from crosslinked polyene chains, respectively. Another conclusion is that the intramolecular cyclization processes in linear and crosslinked polyenes are satisfactorily explained by the cyclization of polyene radicals, whereas mechanisms based solely on Diels-Alder processes meet with several difficulties. The close analogy found with the thermal degradation of polyacetylene supports these conclusions. Another model system is provided by PVC 2 and PVF 2 . These vinylidene polymers do not form alkyl-aromatics, only unsubstituted aromatics are evolved and the thermal degradation occurs in a single stage.

Application of size exclusion chromatography matrix-assisted laser desorption/ionization time-of-flight to the determination of molecular masses in polydisperse polymers

The determination of molecular mass (MM) data for polydisperse polymers by size exclusion chromatography matrix assisted laser desorption/ionization time-of-flight (SEC/MALDI-TOF) involves the fractionation of samples through an analytical SEC. Selected fractions are then analysed by MALDI-TOF and the mass spectra of these nearly monodisperse samples allow the determination of Mn and Mw averages. To test the reliability of the molecular mass estimates by the SEC/MALDI-TOF method, a sample of polymethylmethacrylate (PMMA), two samples of polydimethylsiloxane (PDMS), and four samples of copolyesters, all polydisperse, were analysed. The results show that the molecular mass values of PMMA fractions obtained by MALDI-TOF are coincident with those obtained using the SEC calibration plots obtained with anionic PMMA standards. In the case of the two polydimethylsiloxanes (PDMS1 and PDMS2: linear and cyclic, respectively), two slightly differing SEC calibration plots were obtained, reflecting the different structures of the polymer chains of the two samples. The SEC traces of four copolyesters were obtained in tetrahydrofuran and CHCl3. Data on MM, MM distribution solvent effects and copolymer composition are reported.

Characterization of end groups in nylon 6 by MALDI‐TOF mass spectrometry

Four samples of Ny6, each terminated by different end groups, i.e., diamino terminated, monoamino terminated (monocapped), dicarboxyl terminated, and amino-carboxyl terminated, were synthesized and analyzed by MALDI-TOF Mass Spectrometry, in order to accurately characterize their structure by direct identification of mass resolved chains. A self-calibrating method for the MALDI-TOF mass spectra of polymeric samples was used in order to distinguish the end groups existing in the four samples of Ny6. The MALDI-TOF spectra showed the presence of protonated, sodiated, and potassiated ions that were assigned to Ny6 chains containing the expecteted end groups. Furthermore, the MALDI-TOF spectra made possible the simultaneous detection of the cyclic oligomers of Ny6 present in these samples, thus achieving the full structural characterization of the molecular species present in these polyamides.

Chemical reactions which occur in the thermal treatment of polycarbonate/polyethyleneterephthalate blends, investigated by direct pyrolysis mass spectrometry

Abstract The chemical reactions which occur in the thermal treatment of polycarbonate/poly(ethyleneterephthalate) (PC/PET) blends have been investigated by gradual heating (10°C/min) using thermogravimetry and direct pyrolysis into the mass spectrometer. Exchange reactions occur even in the temperature range below 300°C, but the transesterification equilibrium is affected by the evolution of thermal degradation products. Ethylene carbonate was detected in the first decomposition stage (280–380°C), and is evolved together with a series of cyclic compounds containing units of PC and PET in varying ratios. The overall thermal reaction evolves towards the formation of the most thermally stable polymer, i.e. a totally aromatic polyester (polymer III, Table 1), which was found to be the end product of the thermal processes occurring in the system investigated. Several thermal decomposition products, originating from the PC/PET blends in the range 250–600°C, were identified. The compounds detected have masses sufficiently high to be structurally significant, since they contain at least one copolymer repeating unit. The reactions which occur during the thermal treatment of the PC/PET blend are discussed in detail.

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.

Thermal degradation mechanisms of polyetherimide investigated by direct pyrolysis mass spectrometry

SUMMARY: The thermal degradation mechanisms of poly[2,2-bis(3,4-dicarboxyphenoxy)phenylpropane-2phenylenediimide] (PEI) have been investigated by thermogravimetry (TG) and by direct pyrolysis mass spectrometry (DPMS). TG data show that PEI has a main decomposition step centred at about 510 C followed by a less marked step in the 600 ‐ 650 C temperature range and leaving about 60% of charred residue at 800 C. The total ion curve (TIC) of a purified PEI sample, obtained by DPMS, closely reproduces the two maxima appearing in the derivative TG (DTG) curve, whereas the TIC curve of a crude PEI sample shows two less pronounced maxima in the temperature range of 250 ‐ 450 C due to low molar mass compounds, which volatilize undecomposed in the high vacuum of the MS. The structure of the pyrolysis compounds obtained in the first thermal degradation step of a purified PEI sample suggest that they are mainly formed by the scission of: i) the isopropylidene bridge of bisphenol A; ii) the oxygen-phthalimide bond; iii) the phenyl-phthalimide bond, which are apparently the weakest bonds of PEI. Extensive hydrogen transfer reactions and subsequent condensation reactions may account for the high amount of char residue. The pyrolysis compounds obtained in the second degradation step (620 C) are mainly constituted of CO2, benzene, aniline, benzonintrile, phenylenediamine, and dibenzonitrile, which may be generated by further thermal degradation reactions of pyrolysis compounds containing N H phthalimide as end groups. Another degradation processes which may account for CO2 formation is the hydrolysis of the imide moiety to form poly(amic acid) units which produce an aromatic amide structure by decarboxylation. The pyrolysis of an aromatic polyamide (NOMEX) was then studied for comparison. The structure of the pyrolysis products detected by the DPMS analysis of both polymers allowed a detailed schematization of the thermal degradation pathways involved in the degradation of PEI and on the reactions leading to the formation of the charred residue.

Matrix-assisted laser desorption/ionisation time-of-flight characterisation of biodegradable aliphatic copolyesters

Matrix-assisted laser desorption/ionisation time-of-flight (MALDI-TOF) and the off-line size exclusion chromatography matrix-assisted laser desorption/ionisation (SEC/MALDI) method has been applied to the structural characterisation and the molar mass (MM) determination of a series of biodegradable copolyesters synthesised by high temperature melt polycondensation reaction, and of two commercial copolyesters with the trade name Bionolle. The MALDI-TOF spectra of these copolymers showed the presence of cyclic oligomers in the lower mass region, in accord with expectations from polycondensation kinetics, and the presence of all linear species expected from their method of synthesis. The presence of unexpected linear species with olefin and carboxyl as end groups suggested the occurrence of undesirable thermal degradation processes during the melt polycondensation reaction. The absolute average molar masses obtained by the SEC-MALDI method turned out to be lower, by a factor of about two for succinate/adipate copolymers, and by a factor of three for succinate/sebacate copolymers, with respect to those computed by using polystyrene standards in SEC. Furthermore, the MALDI-TOF spectra of SEC fractions allowed not only the detection of linear and cyclic oligomers contained in these samples, but also the simultaneous determination of the average molar mass of both cyclic and linear oligomers. Due to the smaller hydrodynamic volume of cyclic chains with respect to linear ones, the ratio (M( cycle)/M( linear))( Ve) at a fixed elution volume was found to be 1.25, in good agreement with the theoretical value of 1.24. Copyright 2000 John Wiley & Sons, Ltd.

Further studies on the thermal decomposition processes in polycarbonates

Abstract The thermal decomposition mechanisms of poly(trimethylene carbonate), poly(neopentylene carbonate), poly(2-phenyltrimethylene carbonate) and poly(p-xylylene carbonate) have been investigated by EI/CI direct pyrolysis mass spectrometry and by desorption chemical ionization (DCI) mass spectrometry. The results indicate that all four polycarbonates decompose by an intramolecular exchange, an ionic process that produces cyclic carbonates as pyrolysis products. Other thermal processes (Reactions 1c-li, Scheme 1) also occur during the pyrolysis, and are all explained on the basis of well-established ionic reactions (Scheme 1).

Thermal degradation products of poly(styrenesulfides) investigated by direct pyrolysis—mass spectrometry and flash pyrolysis—gas chromatography/mass spectrometry

The thermal degradation products of two sulfur polymers, poly(styrenedisulfide) (PSD) and poly(styrenetetrasulfide) (PST), were investigated in parallel by direct pyrolysis-mass spectrometry (DPMS) and by flash pyrolysis-GC/MS (Py-GC/MS). The time-scale of the two pyrolysis techniques is quite different, and therefore they were able to detect significantly different products in the pyrolysis of PSD and PST because of the thermal lability of sulfur-containing compounds. However, the results obtained are not contradictory, and satisfactory mechanisms for the thermal degradation of PSD and PST have been derived from the overall evidence available. Pyrolysis compounds containing sulfur, styrene, and a number of cyclic styrene sulfides and diphenyldithianes have been observed by DPMS. However, in flash pyrolysis-GC/MS, styrene, sulfur, only one cyclic styrene sulfide, and two isomers of diphenylthiophene have been detected. These thiophene derivatives were indeed absent among the compounds obtained by DPMS because they were the terminal (most thermally stable) species arising from further decomposition of the cyclic styrene sulfides formed in the primary thermal degradation processes of PSD and PST.

Thermal Oxidation Products of Nylon 6 Determined by MALDI‐TOF Mass Spectrometry

Matrix-assisted laser desorption ionisation (MALDI) mass spectrometry was used, in an attempt to find firm evidence for the structure of the species produced in the thermal oxidative degradation of Nylon 6 (Ny6), at 250°C in air. The MALDI spectra of the products showed the presence of polymer chains containing aldehydes, amides, methyl and N-formamide terminal groups. The aldehydes undergo further oxidation to produce carboxylic end groups. The formation of azomethines, from the further reaction of aldehydes with amino-terminated Ny6 chains, is also supported by the appearance of specific peaks in the MALDI spectra.