Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
Spin-Trapping Analysis and Characterization of Thermal Degradation of Thermoplastic Poly(ether−ester) Elastomer Masayo Sono, Kenji Kinashi, Wataru Sakai,* and Naoto Tsutsumi Faculty of Materials Science & Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan S Supporting Information *
ABSTRACT: We investigated the thermal degradation of poly(butylene terephthalate)-co-poly(ethylene oxide) (PBTco-PEO), a thermoplastic poly(ether−ester) elastomer (TPEE), through proton nuclear magnetic resonance (NMR) spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), gel permeation chromatography (GPC), and spin-trapping electron spin resonance (ESR) analysis to detect radical intermediates present during degradation. Three kinds of PBT-co-PEO with different component ratios of PBT/ PEO were synthesized. The thermal degradation process of PBT-co-PEO was divided into four weight-loss stages. In the first stage from room temperature to 120 °C, we confirmed the production of two radical intermediates, O−•CH−CH2 and •CH2−, by the spin-trapping method. This suggested that the initial decomposition occurred at the OCH2−CH2O bond in the PEO units. Upon annealing at 120 °C, PBT-co-PEO showed a small degree of random degradation, while higher PBT contents induced gelation and the production of the characteristic oligomer PBT−PEO−PBT. The gelation was attributed to cross-linking between two O−•CH−CH2 moieties of PBT. In the second stage from 120 to 340 °C, a large thermo-oxidative degradation of the PEO segment occurred, accompanied by an increase in the radical amount of spin adducts. In the third stage, the thermooxidative degradation of PBT units was observed. The radical intermediates were thus shown to be a primary factor of the thermal degradation characteristics of TPEE.
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INTRODUCTION
Electron spin resonance (ESR) spectroscopy is an important method for studying radicals because it offers highly sensitive and specific detection of radical species.2,3 In ESR spectroscopy, the spin-trapping method has been extensively used to detect and identify short-lived radicals in chemistry, biology, and medical fields.4−9 The short-lived radical species reacts with the spin-trapping reagent to form a stable radical, typically a nitroxide-type radical. This radical species, known as a spin adduct, is much more stable than the original free radical and is easily observable using typical ESR equipment. In most cases, the ESR spectrum of the spin adduct exhibits hyperfine structure (hfs) from spin interactions with the atom that the number of the nuclear spin is not zero, such as 14N (I = 1) and 1 H (I = 1/2) nuclei, around the nitroxide group. The splitting of the spectral peaks of hfs can be used to determine the molecular structure of the original radical R•. The spin-trapping method has mainly been applied to study the radical reactions of polymer materials in solution systems. Danilczuk et al. studied the radical reactions of protonexchange membranes in fuel cells (FCs) by direct ESR and spin-trapping techniques.10−13 They performed an experiment of an in situ FC inserted in the resonator of the ESR
Because polymer materials are utilized widely in human society, polymer degradation and stability are of great interest.1 Polymer degradation occurs through certain radical reactions. In thermal degradation, homolysis is the equivalent dissociation of chain bonds by kinetic energy in which molecular chains begin to move by thermal energy. β-Scission occurs at the main chain after hydrogen abstraction. Main-chain scission proceeds more actively under an oxygen atmosphere in chain reactions that produce oxyl radicals. The chemical reaction mechanisms of degradation for various polymer materials have been extensively studied in the past. However, they typically use ultraviolet−visible (UV−vis) and Fourier-transform infrared (FT-IR) spectroscopies and nuclear magnetic resonance (NMR) spectroscopy to investigate the molecular structures of products. Mass spectroscopy (MS), gas chromatography (GC) MS, and matrix assisted laser desorption ionization− time-of-flight (MALDI-TOF) MS are also applied to thermal degradation analysis of polymer materials. However, these analytical methods cannot detect and analyze radical species. Therefore, the proposed degradation mechanisms and radical species were analogueically determined based only on the reaction products identified after degradation. No direct information regarding the radical intermediates formed during degradation is available using these techniques. © XXXX American Chemical Society
Received: December 18, 2017 Revised: January 12, 2018
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DOI: 10.1021/acs.macromol.7b02654 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
softens and flows like a thermoplastic material, enabling processing by conventional molding. Many researchers have discussed TPEE degradation.18−24 Botelho et al. studied the thermo-oxidative degradation of TPEE and model compounds at 140 °C. The degradation processes were characterized based on oxygen uptake and CO/ CO2 formation by analyzing the gas phase from gas chromatography as well as by FT-IR, gas chromatography− mass spectroscopy (GC-MS), and NMR analysis. The oxidation of the PBT-co-PEO copolymer to form hydroperoxide occurred on the carbon atom in the α-position relative to the ether oxygen atom.18 However, the radical intermediates present during the degradation of TPEE have not yet been reported. Chen et al. studied the thermo-oxidative degradation of PEO in an aqueous solution by GC-MS, GC-flame ionization detection (FID), and FT-IR as well as the spin-trapping method with DMPO to determine the hydroperoxide concentration during degradation.25,26 They detected •OH and PEO-C• radicals even in the relatively low temperature range of 40−92 °C.26 In this study, we focused on the PBT-co-PEO TPEE and evaluated the degradation mechanism of polymer materials overall, based on both the detection of radical intermediates generated by heating and the investigation of changes in physical properties. We synthesized three kinds of PBT-co-PEO with different ratios of rigid and flexible segments and investigated their character of thermal degradation. We applied the spin-trapping method to detect and analyze the radical intermediates formed during the thermal degradation of PBTco-PEO to confirm the degradation mechanism. In addition, changes in thermal weight loss, molecular weight distribution, and dynamic viscoelastic properties were measured by thermogravimetric analysis (TGA), gel permeation chromatography (GPC), and dynamic mechanical analysis (DMA), respectively, and the degradation mechanism of PBT-co-PEO was discussed from various perspectives.
spectrometer, which permits the observation of reaction processes at the anode and cathode sides separately. The formation of HO• and HOO• radicals, as well as H• and D• atoms, was monitored using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), a well-known spin-trapping reagent, in aqueous solution. They also observed radical fragments derived from the degradation of the proton-exchange membrane and some lowmolecular-weight model compounds and detected carboncentered radical adducts as well. They reported that HO• caused membrane degradation for the in situ FC. Schlick et al. demonstrated that CeO2 nanoparticles mitigated the chemical degradation process of FC membranes by measuring the scavenging of the hydroxyl radicals using the spin-trapping method.14 Tabata et al. detected the radicals generated by γirradiation in a poly(methyl methacrylate) (PMMA)/benzene solution using tri-tert-butylnitrosobenzene (TTBNB) as a spintrapping reagent. They reported an ESR spectrum consisting of two spin-adducts derived from •COOCH3 and •CH3. The former •COOCH3 was formed by side-chain elimination following homolysis of the main chain, and the latter •CH3 was formed by •COOCH3 decomposition. They proved the assignment of •CH3 using deuterated PMMA, the ester of which contained deuterated CD3.15 On the other hand, some studies using spin-trapping methods in the solid state have also been reported.16 Our research also focuses on the degradation reaction in solid polymer materials that produce radical intermediates during the reaction. In a recent study, we applied the spin-trapping method to detect and assign the short-lived radical intermediates formed during the degradation of poly(butylene terephthalate) (PBT).17 To confirm the degradation mechanism of PBT in detail, we used low-molecular-weight model compounds as well as their deuterated analogues. The assignment of the radical structure via the spin-trapping technique is usually based on the analysis of hfs of the ESR spectra. However, we cannot obtain comprehensive structural information, such as the molecular mass of the spin adduct, using only hfs, nor can we distinguish between alternative structures predicted to have similar hfs. High-performance liquid chromatography (HPLC) electrospray ionization mass spectroscopy (ESI-MS) was also used in order to determine the molecular structures of the spin adducts. We concluded that the primary thermal degradation of PBT began by hydrogen abstraction at the α carbon, −O−CH2−, on the PBT chain and that main-chain decomposition proceeded by subsequent βscission at the ester linkage. We also revealed that the primary degradation process occurred even at 110 °C with very low concentrations of radicals of ∼10−5 mol/L.17 Recently, thermoplastic poly(ether−ester) elastomers (TPEEs), one type of thermoplastic elastomers (TPEs), have attracted attention because of their characteristics and versatile applicability. Because they can be used in wider temperature ranges with excellent durability compared to other TPEs, TPEEs are widely used in industrial fields, such as automotive parts, and daily necessities. The TPEE studied here contains two parts: a soft segment composed of flexible poly(ethylene oxide) (PEO) units with a low glass transition temperature and entropic elasticity and a hard PBT units composed of rigid crystalline block with high melting temperature to prevent plastic deformation. The rigid crystalline PBT segments behave as physical cross-linking points for elasticity. At higher temperatures, the dissociation of physical bonds in the TPEE occurs by melting of the crystalline region; the copolymer then
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EXPERIMENTAL SECTION
Materials. Figure 1 shows the molecular structures of the selected TPEE of poly(butylene terephthalate)-co-poly(ethylene oxide) (PBT-
Figure 1. Materials used in this paper: (a) poly(butylene terephthalate)-co-poly(ethylene oxide) (PBT-co-PEO) and (b) spintrapping reagent, 2,4,6-tri-tert-butylnitrosobenzene (TTBNB).
co-PEO) and the spin-trapping reagent of 2,4,6-tri-tert-butylnitrosobenzene (TTBNB). Dimethyl terephthalate (DMT) was recrystallized using acetone, and 1,4-butanediol (BDO) was distilled before use, respectively. Poly(ethylene glycol) (PEG) with the molecular weight of 1000 g/mol, titanium(IV) isopropoxide (>97.0%) as catalyst, diethoxyethane (DEE) as a low-molecular-weight compound, TTBNB, and methanol were used as received without further purification. All materials above were purchased from Wako Pure Chemical Industries, Ltd. Poly(butylene terephthalate) (PBT) was purchased from SigmaAldrich Co. and purified by reprecipitation two times using 1,1,1,3,3,3B
DOI: 10.1021/acs.macromol.7b02654 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules hexafluoroisopropanol (HFIP) and methanol. HFIP was purchased from Fluorochem Ltd. and used as received. Synthesis of PBT-co-PEO Copolymers. PBT-co-PEO copolymers with varying PBT/PEO weight ratios were prepared by polycondensation from DMT, BDO, and PEG in two steps: the transesterification of DMT by BDO to form bis(4-hydroxybutylene) terephthalate (BHBT) and the subsequent melt-polycondensation of BHBT with PEG. Both steps were carried out in the presence of titanium(IV) isopropoxide (1.25 wt % relative to DMT) as a catalyst. The first step was performed in a 500 mL four-necked separable flask equipped with a mechanical stirrer, magnetic stirrer seal (Mighty MAGshiel, Nakamura Scientific Instruments Industry Co., Ltd.), nitrogen inlet, thermometer, and cold trap for collecting byproducts. The reactor was first charged with DMT and BDO at the desired ratios and the catalyst. The molar ratio of the diester (DMT) and diol (BDO) was 1.0:1.5. The transesterification reaction was carried out under a constant flow of nitrogen at 100 mL/min and 160−165 °C for 2 h. During the reaction, methanol was distilled and collected as a byproduct. After the first step, the PEG was added to the reactor. The nitrogen inlet was replaced with a vacuum pump and a cold trap of acetone with dry ice for collecting the byproducts. Then, the reaction temperature was gradually increased to 250 °C for the second step. The reactor was gradually evacuated to a final pressure
