Primary thermal degradation processes of poly(ether/ketone) and poly(ether ketone)/poly(ether-sulfone) copolymers investigated by direct pyrolysis-mass spectrometry

The mechanisms of thermal degradation of poly(ether-ketone) (PEK) and four poly(ether-ketone)/poly(ether-sulfone) copolymers (PEK/PES) have been investigated by direct pyrolysis-mass spectrometry (DPMS). Several families of pyrolysis compounds with H, OH and CHO end-groups have been identified in the pyrolysis mass spectra of PEK. All these pyrolysis compounds can arise from degradation mechanisms involving cleavages of the bridged groups (diphenyl ether and dibenzophenone units). Our data show that the main degradation products of PEK are aldehydes, most likely formed by an intramolecular thermal cleavage of benzophenone units. Compounds containing dibenzofuran units have also been observed in the DCI mass spectrum of PEK. The thermal decomposition of a low molecular weight PEK sample occurs in two stages with the maxima of decomposition at 390°C and 490°C, respectively. This fact indicates the occurrence of an end-group initiated thermal decomposition in the early degradation stage which is not present in the case of the high molecular weight PEK sample. The pyrolysis of PEK does not produce compounds containing biphenyl units, indicating that extrusion of carbonyls or recominbation processes are not involved. The thermal degradation compounds of the PEK/PES copolymers originate from the thermal cleavage of the bridge groups (diphenyl ether, benzophenone and diphenyl sulfone). The pyrolysis mass spectra of 1:1 (alt.), 1:1 (random), 3:1 and 1:3 PEK/PES copolymers appear essentially identical (apart for obvious differences in peak intensities), indicating that the molecular rearrangements (SO₂ extrusion, transesterification, cleavage of bridges) which occur at higher temperatures and/or in the pyrolysis processes are able to randomize the distribution of comonomer units originally present in the copolymers. Differences in peak intensities have been found to reflect almost quantitatively the molar composition of the copolymers.     

Sequence distribution of poly(ether-sulfone)/poly(ether-ketone) copolymers by mass spectrometry and 13C NMR

The sequence distributions of four poly(ether-sulfone)/poly(ether-ketone) (PES/PEK) copolymer samples were determined by means of fast atom bombardment mass spectrometry (FAB-MS) and by ¹³C NMR spectroscopy. The controlled partial degradation of the copolymer chains to produce oligomers suitable of FAB-MS analysis was performed with sodium methoxide in dimethyl sulfoxide solution at 130°C, and the experimental conditions were optimized. The sequential arrangements of ether-sulfone/ether-ketone units present in these materials were estimated by a best-fit minimization method using the MACO4 computer program which compares the experimental FAB-MS spectral intensities with theoretical intensities. Random PES/PEK copolymer samples showed quite the same sequence arrangements as expected from monomer feed-ratios used in the syntheses. Instead, a PES/PEK copolymer sample expected to be exactly alternating (from the synthesis procedure) showed 44% of random sequences owing to the transetherification reaction which occurred in the synthesis. The results of sequential analysis obtained from ¹³C NMR data compare well with the data obtained by the FAB-MS analysis.     

Thermal degradation mechanisms of polyetherimide investigated by direct pyrolysis mass spectrometry

The thermal degradation mechanisms of poly[2,2′-bis(3,4-dicarboxyphenoxy)phenylpropane-2-phenylenediimide] ( 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.     

The curing reaction of poly(ether-sulfone)-modified epoxy resin

One of the useful methods to improve the toughness of epoxy resin is by mixing the resin with poly(ether-sulfone) (PES). In the present work, two hydroxyl-terminated PESs with molecular weights M _n = 28 600 and 4 200 were used for blending with epoxy resin. The curing reaction of diglycidyl ether/bisphenol-A (DGEBA) with 4,4′-diaminodiphenyl sulfone (DDS) in the presence of hydroxyl-terminated PES was studied by means of differential scanning calorimetry (DSC) and gel-permeation chromatography (GPC). For the DGEBA-DDS-PES system with a stoichiometric ratio of epoxy and amino groups the DSC experimental results showed that at a fixed molecular weight of PES the curing reaction rate decreases with increasing PES concentration. At a fixed PES concentration (in the range of between 0 and 20 wt.-% of PES), the DGEBA-DDS system modified with hydroxyl-terminated PES with lower molecular weight had a faster curing reaction rate at low conversion and a slower curing reaction rate at high conversion. The GPC results showed the evidence of etherification between low-molecular-weight PES with epoxy resin. However, very little etherification of high-molecular-weight hydroxyl-terminated PES with epoxy resin was found. Based on the experimental results, a curing reaction mechanism of DGEBA with DDS in the presence of hydroxyl-terminated PES is proposed.     

Acoustic properties of poly(phenylene sulfide) crystallized in the presence of sorbed nitrous oxide

Poly(phenylene sulfide) annealed thermally and under the influence of nitrous oxide pressure is investigated using the acoustic vibrating reed technique. The acoustic parameters velocity of sound, c, and loss tangent, tan δ, are calculated. The discussion concerning the structural changes of the polymer during the applied annealing procedures is based on the temperature dependence of these parameters and the transition temperatures T_G and T' determined therefrom.     

On the kinetics and mechanism of thermal degradation of poly(p-methylstyrene)

The thermal behavior of polydisperse poly(p-methylstyrene) (PPMS) samples with M?_w's of 99200 and 356000 g/mol was investigated under oxygen-free conditions at temperatures between 220 and 345°C. The reaction products were analysed by gel permeation chromatography (GPC), gas chromatography (GC) and swelling measurements. In the initial stage of the reaction crosslinking occurs; at higher temperatures the gel decomposes to polymers with a low degree of polymerization and to volatile products with molecular weights in the range up to trimers. A radical chain mechanism is proposed for degradation which explains the experimental results. On this basis a model is evaluated for gel formation, which is discussed in detail. The results of the present investigation are compared with the thermal degradation of polystyrene (PS) which is chemically similar but does not crosslink.     

Thermal degradation of polyurethanes investigated by direct pyrolysis in the mass spectrometer

Direct pyrolysis in the mass spectrometer yielded evidence on the mechanisms of thermal decomposition of N-monosubstituted and N-disubstituted polyurethanes. Our results indicate that N-monosubstituted polyurethanes undergo quantitatively a depolycondensation process, producing diisocyanate and diol. In contrast, disubstituted polyurethanes decompose to give selectively secondary amine, olefine and CO₂.     

Synthesis and functionalization of bromomethylated poly(phenylene sulfide)

A soluble poly(phenylene sulfide) (PPS) derivative was synthesized by oxidative polymerization of bis(3,5-dimethylphenyl) disulfide. The obtained poly(thio-2,6-dimethyl-1,4-phenylene) ( I ) (M_w = 22 900, M_n = 8 400) was brominated by the reaction with N-bromosuccinimide (NBS). The bromomethyl substituents on PPS are reactive and were converted quantitatively to hydroxymethyl groups by hydrolysis in NMP at 100°C. The reaction of the bromomethylated PPS with tertiary amines gave the quaternized polymer, which is stable up to 145°C in the solid state and in refluxing methanol.     

A study of the thermal degradation of poly(mono-n-alkyl itaconates)

The thermal degradation of the first six members of poly(mono-n-alkyl itaconates) by thermogravimetry (TG), differential scanning calorimetry (DSC), Fourier Transform infrared (FTIR) and FTIR evolved gas analysis (FTIR-EGA) was studied. The degradation mechanism involves cyclic anhydride formation followed by crosslinking due to the linear anhydride formation. In both processes water and/or alcohol evolution was detected. Also, decarboxylation processes take place on all polymers at high temperatures ( > 180°C). A mechanism of the degradation process is proposed.     

On the stereoisomerization in thermal degradation of isotactic poly(propylene)

The terminal microstructure of the hydrogenated α,ω-diisopropenyloligo(propylene) prepared by thermal degradation of isotactic poly(propylene) at 370°C was determined by means of ¹³C NMR spectroscopy, and therefrom the stereoisomerization reaction of the reacting polymer segment is discussed in relation to polymer dynamics. With respect to the asterisked carbons of the propyl end group CH₃CH₂^*CH₂ (n-Pr) and of the isopropyl end group (^*CH₃)₂CH (i-Pr) formed by hydrogenation, the mole fractions of triads (m meso, r racemo) F_mm = 0.64, F_mr = 0.09, F_rr = 0.12 of n-Pr-^*CH₂ differ from the microtacticity of the main chain of telechelic oligomers, and the mole fraction of i-Pr-^*CH₃ is as follows: F(δ_tζ_t) + F(δ_eζ_e) = 0.74 (m); F(δ_tζ_e) + F(δ_eζ_t) = 0.26 (r). These results clearly show that the stereoisomerization occurs only at the asymmetric carbon near the end group of the telechelic oligomer. The value of the mole fraction of i-Pr-^*CH₃ is consistent with F_m = 0.73 (F_mm + F_mr) and F_r = 0.27 (F_rr + F_rm) of n-Pr-^*CH₂ at the outer position of the main chain rather than with F_m = 0.79 (F_mm + F_rm) and F_r = 0.21 (F_rr + F_mr) at the inner position. These results could be consistently traced by a simulation based on a reaction model including stepwise intramolecular hydrogen abstraction of the tertiary on-chain macroradical near the center of the main chain before the successive β-scission. This stepwise back-biting is estimated to occur once (20%) and twice (80%) via a quasi six-membered cyclic structure in a transition state, and occurs in a very limited part of the molecular chain.     

Synthesis of mainly linear poly(dichlorophenylene oxide)s by thermal polymerization in solution

Thermal decomposition of bis(4-bromo-2,6-dichlorophenoxo)-(N,N,N',N'-tetramethylethy-lenediamiene)copper(II) complex in toluene, bis(4-bromo-2,6-dichlorophenoxo)(N,N-dimethyl-formamide)copper(II) complex in N,N-dimethylformamide and bis(4-bromo-2,6-dichlorophenoxo)(dimethyl sulfoxide)copper(II) complex in dimethyl sulfoxide was achieved at 70°C. 4-Bromo-2,6-dichlorophenol derivatives prefer 1,4-addition to 1,2-addition, leading to highly linear polymers irrespective of the substituted ligands. The complexes were characterized by IR spectroscopy and C, H, N elemental analysis. The characterization of the synthesized polymers were achieved by ¹H NMR, ¹³C NMR and IR spectroscopy. The highest polymer yield was obtained from the decomposition of bis(4-bromo-2,6-dichlorophenoxo)(N,N,N',N'-tetramethyl-ethylenediamine)copper(II) complex.     

Morphology,crystallization and thermal behaviour of poly(ethylene oxide)/poly(vinyl acetate) blends

Blends of poly(ethylene oxide) (PEO) and poly(vinyl acetate) (PVAc) show a unique value of the glass transition temperature, intermediate between that of plain polymers. The addition of PVAc to PEO causes a depression in both the spherulite growth rate (G) and the overall kinetic rate constant (K_n). Such depression is larger at higher undercooling and, at a given crystallization temperature, it increases with the content of PVAc. The experimental G and K_n data were analyzed by means of latest kinetic theories in order to determine the influence of composition on the process of surface secondary nucleation. The melt behaviour of PEO/PVAc blends cannot be explained only in terms of diluent effects due to the compatibility of the components in the melt. Especially, at lower undercooling it is likely that annealing and morphological effects must also be taken into account. The morphology of thin films of blends, isothermally crystallized from melt, suggests that an amorphous mixed phase (PEO + PVAc) is formed in interlamellar regions. It was found that plain PEO crystals grow according to a regime I process of surface secondary nucleation while in the case of blends the crystals of PEO grow via regime II mechanism.     

Polyene formation and crosslinking during the thermal degradation of poly(vinyl chloride)

Polyvinyl chloride (PVC) was degraded at processing temperatures in an inert atmosphere. The samples were analyzed by means of a size exclusion chromatograph equipped with a differential refractometer and a photo-diode array UV-visible detector. This enabled us to record online the presence of polyenes at different positions along the molecular weight distribution of the degraded polymer. Polyenes proved to be concentrated on the low molecular weight side of the distribution and, more specifically, on the high molecular weight side, which had been broadened upon degradation. The results indicate that low molecular weight PVC degrades relatively fast and that PVC molecules containing polyenes crosslink easily.     

Reverse thermo-responsive poly(ethylene oxide) and poly(propylene oxide) multiblock copolymers

Aiming at developing new reverse thermo-responsive polymers, poly(ethylene oxide)-poly(propylene oxide) multiblock copolymers were synthesized by covalently binding the two components using carbonyl chloride and diacyl chlorides as the coupling molecules. The appropriate selection of the various components allowed the generation of systems displaying much enhanced rheological properties. For example, 15 wt% aqueous solutions of an alternating poly(ether-carbonate) comprising PEO6000 and PPO3000 segments, achieved a viscosity of 140,000 Pas, while the commercially available Pluronic F127 displayed 5,000 Pas only. Furthermore, the structure of the chain extender played a key role in determining the sol-gel transition. While poly(ether-ester)s containing therephtaloyl (para) and isophtaloyl (metha) coupling units failed to gel at any concentration, a 15 wt% aqueous solution of the polymer chain-extended with phtaloyl chloride (ortho) gelled at 43 degrees C. The water solutions were also studied by dynamic light scattering and a clear influence of the PEO/PPO ratio on the aggregate size was observed. By incorporating short aliphatic oligoesters into the backbone, prior to the chain extension stage, reverse thermal gelation-displaying biodegradable poly(ether-ester-carbonate)s, were generated.     

Enzymatic degradation of poly(ether urethane) and poly(carbonate urethane) by cholesterol esterase

This study examined the effect of cholesterol esterase (CE) on the degradation of commercial poly(ether urethane) (PEU) and poly(carbonate urethane) (PCU). Unstrained PEU and PCU films were incubated in 400 U/mL CE solution or a buffer control for 36 days. The study used a concentration of cholesterol esterase that was considerably higher than the estimated physiological level in order to accelerate degradation. However, characterization of treated polyurethane films with SEM, attenuated total reflectance Fourier transform infrared (ATR-FTIR) and GPC analysis revealed only a small loss in surface soft segment content. Comparison with implanted PEU and PCU films led to the conclusion that any effect of enzymatic hydrolysis was confined to the immediate surface, and the magnitude of the effect was too small to contribute significantly to in vivo degradation. The study confirmed that oxidation, rather than enzymatic hydrolysis, is the primary mechanism responsible for the observed biodegradation of PEU and PCU. The oxidative H(2)O(2)/CoCl(2) treatment continues to accurately predict the long-term biostability of polyurethanes.     

Polyene sequence distribution in modified poly(vinyl chloride) after thermal degradation

The products resulting from the reaction of PVC with sodium benzenethiolate were degraded to 0,3% at 180°C in the solid state and at 160°C in solution in trichlorobenzene. The polyene distribution of the polymers after degradation was studied by both UV-visible and resonance Raman spectroscopies, as a function of the degree of substitution. The results show that there are two types of behaviour: that of the PVC sample prior to the substitution reaction together with the samples modified up to a definite degree of substitution which depends on the starting isotactic content, and that of samples with higher degrees of substitution. The former group exhibits not only a steady improvement in thermal stability but also a preferential formation of polyenes of 7 – 9 double bonds whose concentration decreases with increasing degree of substitution. Conversely, for the second group of samples the thermal stability decreases with the degree of substitution and no specific absorption bands are observed. On the basis of earlier work on the selective substitution of the isotactic GTTG and heterotactic TTTG triads during the first stage of the reaction, the present results show that the bands at 393, 416, and 437 nm are related to specific polyenes which result from initiation by the above quoted conformations in PVC, a conclusion for which confirmatory evidence was obtained by resonance Raman spectroscopic examination of the samples. There is, therefore, clear evidence for the occurrence of two distinct degradation mechanisms, one involving initiation by the unstable triad conformations and the other via random initiation at stable and normal structures. To this may be added the initiation by defect structures, which have been extensively documented in the literature.     

In situ accelerated degradation of polyoxyethylene/poly(epsilon-caprolactone) multiblock copolymer by moderate thermal treatment

Alternating amphiphilic multiblock copolymers, consisting of polyoxyethylene (POE) and poly(epsilon-caprolactone) (PCL) of various lengths, were synthesized by a polycondensation reaction between dicarboxylated PEG and dihydroxyl PCL. The polymer formed a physical hydrogel by PCL crystallization. For in vitro hydrolysis in phosphate-buffered saline solution, the change of molecular weight depended on the composing block length of POE. The polymer with longer POE showed a faster decline in molecular weight. The mass remaining at the end of two weeks at 25 degrees C was more than 95 w%. However, when the swollen hydrogels were exposed to temperatures slightly above PCL melting point for 30 min, the degradation rate was accelerated and the mass remaining dropped to less than 10 wt% in one week. In vivo degradation after hydrogel implantation, the polymer degraded as under in vitro. However, the implant irradiated with infrared (IR) accelerated its degradation similar to a treatment with elevated temperature.     

Thermal resistance of crosslinked and substituted poly(phenylene sulfide)s

Crosslinked and substituted poly(phenylene sulfide)s, prepared by reacting p-chlorosubstituted benzenes with sulfur, were investigated by TG in air. Using the difference method and an assumed first-order calculation, the reaction order, activation energy and frequency factor were determined. The pyrolyses were carried out under nitrogen and in the presence of 5% oxygen. The IR spectra and the sulfur contents of the polymers and pyrolysis products were determined. The highest activation energies were found for unsubstituted polymers. Linear poly(phenylene sulfide) has the best thermal stability, but also the other polymers show rather good stabilities up to 400–500°C. The IR spectra and sulfur-contents of the pyrolysis products suggest stable, condensed, and sulfur containing aromatic intermediates formed during the degradations. The obtained polymers can be manufactured over large temperature ranges by common techniques.     

Gpc study of thermo-oxidative degradation of poly(2,6-dimethyl-1,4-phenylene oxide)

The method of gel permeation chromatography was used to study the structural changes of poly(2,6-dimethyl-1,4-phenylene oxide) in the course of thermooxidative degradation. During the degradation two principal processes occur: random scission and crosslinking. The latter process leads to the formation of insoluble fractions. The reaction rates and activation energies of both processes have been determined. The mechanism of degradation as well as the influence of some additives is discussed on the basis of experimental data.     

Synthesis of a polyethylene-graft-polystyrene copolymer and its compatibilization for linear low density polyethylene/poly(phenylene oxide) blends

Based on unsteady diffusionkinetics, polyethylene(PE)-graft-polystyrene (PS) copolymers were designed and synthesized with a heterogeneous high yield titanium-based catalyst by copolymerization of ethylene with a PS-macromonomer using 1-hexene as a short main agent to promote the incorporation of the PS-macromonomer. The presence of 1-hexene facilitated the diffusion of the PS-macromonomer, giving rise to the significantly increased incorporation of the PS-macromonomer. Compatibilization of blends of linear low density polyethylene (LLDPE)/poly(phenylene oxide) (PPO) with the PE-g-PS copolymer were investigated using scanning electron microscopy (SEM) and dynamic mechanical analysis (DMA).