- Email: [email protected]
FTIR analysis
Polymer Testing 75 (2019) 220–228
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Material Behaviour
Thermal degradation study of some poly(arylene ether nitrile)s by TG/MS/ FTIR analysis
T
Gabriela Lisaa,*, Corneliu Hamciucb, Elena Hamciucb, Nita Tudorachib Gheorghe Asachi Technical University of Iasi, Faculty of Chemical Engineering and Environmental Protection “Cristofor Simionescu”, Department of Chemical Engineering, 73 Prof.dr.doc. D. Mangeron Street, 700050, Iasi, Romania b “Petru Poni” Institute of Macromolecular Chemistry, 41A Aleea Gr. Ghica Voda, 700487, Iasi, Romania a
A R T I C LE I N FO
A B S T R A C T
Keywords: Aromatic polyethers Benzonitrile groups Thermal stability Mechanism of thermal decomposition TG/MS/FTIR analysis
A study on thermal and thermo-oxidative stability, i.e. the degradation mechanism for a series of poly(arylene ether nitrile)s was performed by applying simultaneous mass spectrometry and Fourier transform infrared spectroscopy of gas products from a thermogravimetric analyzer into two working atmospheres, air and helium. The polymers were prepared by nucleophilic aromatic substitution of 2,6-dichlorobenzonitrile with phenolphthalein, 2,2-bis(4-hydroxyphenyl)propane and 2,5-bis(4-hydroxyphenyl)-1,3,4-oxadiazole. A poly(arylene ether nitrile) was prepared by cyclopolycondensation reaction, in two steps, starting from 4,4′-oxydiphthalic anhydride and 2,6-bis(3-aminophenoxy)benzonitrile. The influence of benzonitrile units, isopropylidene linkages, phthalide groups, oxadiazole or imide rings on the thermal degradation behavior was discussed in detail. The onset mechanism of the thermal degradation process in the two working atmospheres was similar. The major identified products in the decomposition gases were carbon dioxide, water, carbon monoxide, benzene, phenol and benzonitrile. In addition, nitrogen monoxide was observed in the case of polymers containing oxadiazole or imide rings.
1. Introduction Poly(arylene ether nitrile)s (PAENs) represent a class of thermoplastic polymers characterized by good thermal and thermo-oxidative stability, excellent mechanical and electrical properties, chemical and radiation resistance [1–5]. The presence of highly resistant eCN groups, connected to the aromatic rings, maintains a high thermal stability. In addition, these groups represent potential sites for crosslinking reactions that allow the thermoplastic polymer to be converted into a thermoset with improved solvent resistance and enhanced thermal stability [6]. Due to the high polarity of eCN groups, the PAENs exhibit higher dielectric constant compared to the corresponding aromatic polyethers without these groups [7,8]. Therefore, PAENs are candidates frequently used as dielectric materials or as polymer matrix for the preparation of high-dielectric permittivity composites [9–12] used for advanced applications, including high energy density film capacitors, gate dielectric and electroactive materials. The requirements for such polymer matrix are high dielectric constant and low dielectric loss over large interval of temperature and frequency, high thermal stability and easy processability. Many structures of PAENs have been prepared and reported in the
*
literature. The polymers were mainly synthesized by aromatic nucleophilic substitution of dichloro- or difluorobenzonitrile with aromatic dioles such as hydroquinone or 1,3-dihydroxybenzene, in the presence of potassium carbonate, at high temperature. However, these compounds exhibit relative low glass transition temperature, high crystalinity and low solubility in organic solvents. A method to improve the properties of these polymers is the introduction into their structure of voluminous units, such as phthalide groups [13,14]. These units increase the macromolecular rigidity of PAENs by limiting chain mobility and decrease interfacial interactions due to reduced macromolecule packaging and reduced crystallinity. Thus, phenolphthalein based PAENs have improved characteristics in terms of glass transition temperature, thermal stability and solubility in organic solvents [7,15,16]. Another method to improve the thermal properties of PAENs refers to the introduction of different heterocycles, such as 1,3,4-oxadiazole or imide rings, in their macromolecular chains. For example aromatic polyimides containing eCN groups and ether linkages [17], especially the polymer derived from 4,4′-oxydiphthalic anhydride and 2,6-bis(3aminophenoxy)benzonitrile, are promising materials with piezoelectric properties that can be used at high temperatures [18–20]. These polymers exhibit also high thermal stability and high dielectric constant
Corresponding author. Tel.: +40 232 278683; fax: +40 232 271311. E-mail addresses: [email protected], [email protected] (G. Lisa).
https://doi.org/10.1016/j.polymertesting.2019.02.012 Received 25 July 2018; Received in revised form 14 December 2018; Accepted 11 February 2019 Available online 14 February 2019 0142-9418/ © 2019 Elsevier Ltd. All rights reserved.
Polymer Testing 75 (2019) 220–228
G. Lisa, et al.
being thus suitable as polymer matrices for the preparation of composite materials with special electric and piezoelectric properties. Previously, we studied the thermal and thermo-oxidative degradation of some aromatic polyethers containing phenylquinoxaline or 1,3,4-oxadiazole rings, aromatic polyetherimides, and polyimides with dimethylsiloxane segments by using simultaneous mass spectrometry and Fourier transform infrared spectrometry of off-gases from a thermogravimetric analyzer (TG/MS/FTIR technique) [21–23]. Establishing the mechanism of initiation of thermal degradation allows the determination of the decomposition products, thus it is possible to evaluate the toxicity of the products resulting from the decomposition of the respective polymers. Herein, a study on thermal behavior of some poly(arylene ether nitrile)s by means of coupled analyzes TG/MS/FTIR is presented. The influence of chemical structure (the presence in the polymer unit structure of benzonitrile groups, isopropylidene linkages, phthalide groups, oxadiazole or imide rings) on thermal stability and degradation mechanism is discussed in detail. According to our knowledge, TG/MS/ FTIR technique is used for the first time to evaluate the thermal degradation mechanism of polymers containing phthalide groups. PAENs are potential candidates frequently used as dielectric materials or as polymer matrix for the preparation of high-dielectric permittivity composites used for advanced applications, including high energy density film capacitors, gate dielectric and electroactive materials. The requirements for such polymer matrix are high dielectric constant and low dielectric loss over large interval of temperature and frequency, high thermal stability and easy processability. The importance of our study arises from the fact that by establishing the way in which the initiation of degradation occurs, we obtained information that could contribute to the development of new PAENs with high thermal stability both in the air and in helium.
Fig. 1. Chemical structures of the polymers.
ranged between 8.1 and 15.6 mg and the applied heating rate was 10 °C⋅min−1 in the temperature range 25–670 °C in air or 25–680 °C in helium with a flow of 50 mL⋅min−1. For constructive reasons, the oven of the device used for the thermogravimetric analysis operates until reaches 700 °C, after that it stops heating for emergency (protection) reasons, the temperature starts to fall, and the sampler no longer reaches 700 °C. The differences of temperature that arise (670 °C in the air and 680 °C in He) are caused by the differences of density between the two gases and the different thermal conductivity through which heat is transmitted from the oven wall to the sampler thermocouple. The oven thermocouple is fitted into the shell, and the transmission of heat is performed through convection through the two gases (air or helium) and for this reason a delay may appear. The thermogravimetric analyzer was calibrated for temperature and sensitivity using the melting points of the standard metals (Hg, In, Sn, Bi, Zn) [23]. The gases released during thermal decomposition processes are transferred by two isothermal transition lines to FTIR and mass spectrometer. The gases are introduced in TGA-IR external modulus of FTIR spectrometer, and FTIR spectra are recorded from 600 to 4000 cm−1 with a resolution of 4 cm−1. The transfer gases line to mass spectrometer is manufactured from quartz. The mass spectra were recorded under the electron impact ionization energy of 70 eV. The acquisition of data was recorded with Aeolos® 7.0 software, in spectrum scanning (SCAN) mode in the range of m/z = 1–300, measuring time was ca. 0.5 s for one channel, resulting in time/cycle of approximately 150 s [26].
2. Experimental 2.1. Materials 2,6-Dichlorobenzonitrile, phenolphthalein, 2,2-bis(4-hydroxyphenyl)propane, 4,4′-oxydiphthalic anhydride were provided from Aldrich and used as received. 2,5-Bis(4-hydroxyphenyl)-1,3,4-oxadiazole and 2,6-bis(3-aminophenoxy)benzonitrile were prepared as described in the literature [24,25]. 2.2. Preparation of the polymers The polymers PAEN-1, PAEN-2 and PAEN-3 were prepared by nucleophilic aromatic substitution of 2,6-dichlorobenzonitrile with phenolphthalein, 2,2-bis(4-hydroxyphenyl)propane and 2,5-bis(4-hydroxyphenyl)-1,3,4-oxadiazole, respectively [16]. PAEN-4 was prepared in two steps. In the first step a precursor poly(amic acid) was formed by reacting stoichiometric amounts of 4,4′-oxydiphthalic anhydride and 2,6-bis(3-aminophenoxy)benzonitrile, in N-methyl-2-pyrrolidone as solvent. In the second step the poly(amic acid) was imidized in solution at 180 °C for 6 h, under nitrogen [25]. The structures of the studied polymers are shown in Fig. 1.
3. Results and discussions 3.1. FTIR spectroscopy of the polymers The structures of polymers were identified by infrared spectroscopy. In the FTIR spectra of all polymers characteristic absorption bands appeared at 3046–3084 (aromatic CeH), around 2236 (C^N groups), 1600 and 1500 (C]C aromatic), 1250 cm−1 (aromatic CeOeC). PAEN1 showed a characteristic absorption band at 1784 cm−1 (C]O of phthalide group) while PAEN-2 exhibited characteristic bands at 2970 and 2875 cm−1 (isopropylidene groups). In the case of PAEN-3 characteristic bands appeared at 1018 and 965 cm−1 (=CeOeC = of oxadiazole rings). PAEN-4 showed characteristic bands for imide rings at 1772 and 1727 cm−1 (asymmetrical and symmetrical C]O stretching vibration of imide ring), 1384 (CeN of imide ring) and 740 cm−1 (C]O of imide ring, bending vibrations) (Figs. S1–S4 in Supplementary data).
2.3. Methods The infrared spectra were recorded on a FTIR Bruker Vertex 70 spectrometer at frequencies ranging from 4000 to 500 cm−1 (resolution 2 cm−1, 32 scans). Samples were grounded with KBr and pressed into pellets. The coupled technique TG/MS/FTIR was applied using a device that is composed of a thermogravimetric analysis device types STA 449 F1 Jupiter (Netzsch, Germany), coupled with a spectrophotometer type Vertex-70 FTIR (Bruker, Germany) and a mass spectrometer QMS model 403 C Aëolos (Netzsch, Germany). The analyzed samples mass 221
Polymer Testing 75 (2019) 220–228
G. Lisa, et al.
Fig. 2. TG and DTG curves of the polymers in air (a) and in helium (b).
3.2. Thermal stability
3.3. TG/MS/FTIR studies
The thermal stability of polymers was evaluated by thermogravimetric analysis in two working atmospheres: air and helium. The thermogravimetric (TG) curves and the derivative thermogravimetric (DTG) curves (Fig. 2) were recorded at 10 °C·min−1. The main thermogravimetric characteristics are shown in Table 1. The results reveal that the thermal decomposition of polymers in air takes place in two stages. The TG curves also reveal the fact that degradation in the final stage in air for PAEN-1, PAEN-3 and PAEN-4 is incomplete. The quantity of residue at 670 °C in this working atmosphere varies between 3 and 33%. The degradation in helium atmosphere is characterized by a single decomposition stage, at the end of which a residual quantity of 32–64% is obtained. The comparative analysis of the main thermogravimetric characteristics (Tonset, Tpeak, T5% and T10%) obtained during the first decomposition stage, in the two working atmospheres, shows very close values, suggesting a similar degradation mechanism. The thermal stability of polymers can be analyzed taking into account the following criteria: Tonset and Tpeak from the first stage of thermal decomposition, T5% and T10%. Taking these criteria into consideration, the decreasing series of thermal stability for the analyzed polymers is as follows: PAEN-4 > PAEN-1 > PAEN-2 > PAEN-3 in air or helium. The best thermal stability can be found for the polyether that contains benzonitrile groups and imide rings in its chemical structure.
3.3.1. Poly(arylene ether nitrile) containing phthalide groups, PAEN-1 The TG/MS/FTIR coupled technique applied to polymers in two working atmospheres, air and helium, made possible to obtain information about the degradation mechanism, identification of the main degradation products, and estimation of the impact of polymers on the environment in the case of thermal decomposition. The MS spectra of PAEN-1 in air reveal that the thermal decomposition of this polymer (Scheme 1) starts at the phthalide groups and ether linkages [17] and continues to the benzonitrile groups. At temperatures above 560 °C, the increase of the ionic fragment quantities of m/z = 44 (CO2+) (Fig. 3) and m/z = 18 (H2O+) (Fig. S5 in Supplementary data) released from the thermal decomposition process suggests that a thermal oxidation process takes place. The evolution of the ionic current for ionic fragments m/z = 44 (CO2+), m/z = 78 (C6H6+) and m/z = 94 (C6H5OH+), presented in Figs. 3 and 4 and S6 in Supplementary data, confirms the proposed thermal degradation mechanism for PAEN-1, in both air and helium atmospheres. The MS spectra of PAEN-1 reveal the presence of ionic fragments m/ z = 39 (C3H3+), m/z = 50 (C4H2+), m/z = 51 (C4H3+), m/z = 52 (C4H4+) and m/z = 65 (C5H5+). These are clear indications of the presence of benzene and phenol in the gases resulting from the decomposition of this polymer [27]. The proposed degradation
Table 1 The main thermogravimetric parameters for aromatic polyethers containing benzonitrile units. Sample
Atmosphere
Degradation stage
Tonset (°C)
T5% (°C)
T10% (°C)
Tpeak (°C)
Tendset (°C)
W (%)
R (%)
PAEN-1
air
501
483 454
498 469
PAEN-3
helium air
459 438
476 465
PAEN-4
helium air
423 519
458 547
523
548
508 648 508 496 590 501 495 657 495 553 590 556
524 – 548 522 662 538 533 – 591 585 – 643
22.79 62.68 36.21 41.47 55.39 58.72 30.37 53.25 44.76 22.32 45.40 36.05
14.53
helium air
467 567 468 452 542 455 433 571 438 505 585 511
486
PAEN-2
I II I I II I I II I I II I
helium
63.79 3.14 41.28 16.38 55.24 32.28 63.95
Tonset - the temperature at which the thermal decomposition begins; Tpeak - the temperature at which the degradation rate is maximum; T5%, T10% - the temperature corresponding to 5 and 10% mass loss; W - mass losses in each stage, R-residue at 670 °C in air and 680 °C in helium. 222
Polymer Testing 75 (2019) 220–228
G. Lisa, et al.
Scheme 1. The proposed thermal degradation mechanism for PAEN-1.
associated with phenol presence (m/z = 94) [29], and at 1373 cm−1, associated with benzonitrile presence (m/z = 103) [30]. The ionic current intensity in MS spectra is low in the case of these fragments (Fig. S6 in Supplementary data). The FTIR spectra of residues were also analyzed in order to obtain additional information about the degradation mechanism of PAEN-1. The spectra of PAEN-1 heated in air up to 550 and 670 °C were recorded. The spectrum of PAEN-1 heated up to 550 °C (Fig. S1 in Supplementary data) showed absorption band at 2228 cm−1 suggesting
mechanism is also supported by FTIR spectra obtained by analyzing the gases resulting from thermal decomposition of PAEN-1 (Fig. 5). The bands that characterize water in the range of 3700–3400 cm−1, CO2 in the range of 2400–2200 and 740-600 cm−1, CO in the range of 2200–2000 cm−1 and benzene in the range of 1600–1400 cm−1 can be clearly seen [28]. The presence of CO is also confirmed by the evolution of the ionic current for m/z = 28 (CO+), as shown in Fig. S7 in Supplementary data. FTIR spectra shown in Fig. 5 for PAEN-1 present smaller peaks with maximum intensity at 746 and 1250 cm−1,
Fig. 3. Ionic current variation with temperature for the fragments m/z = 44 in air (a) and helium (b). 223
Polymer Testing 75 (2019) 220–228
G. Lisa, et al.
Fig. 4. Ionic current variation with temperature for the fragments m/z = 78 in air (a) and helium (b).
Fig. 5. FTIR spectra of the volatile products evolved during thermal decomposition of aromatic polyethers containing benzonitrile units in air (a) and helium (b).
that eCN groups were thermally stable up to this temperature. The characteristic C]O band of the phthalide unit was not present, thus demonstrating that decomposition of these units occurred up to 550 °C. Another band appeared at 1709 cm−1, probable due to the presence of some carboxylic groups. A strong band was observed at 1600 cm−1 due to the high content of aromatic compounds in the char. Also, the characteristic band of aromatic ether bonds at 1240 cm−1 was not observed, thus demonstrating the decomposition of these bonds up to 550 °C. In the FTIR spectrum of PAEN-1 heated in air up to 670 °C (Fig. S1 in Supplementary data) a strong band at 1635 cm−1 (due to the aromatic characteristic of the char) can be observed. The band of eCN groups at 2230 cm−1 disappeared completely. The FTIR spectra of PAEN-1 at 680 °C in helium (Fig. S1 in Supplementary data) exhibited similar absorption bands. In addition, a slight peak at 2223 cm−1 can be observed due to the presence of a small amount of eCN groups.
current corresponding to the fragment m/z = 15 (CH3+) (Fig. S5 in Supplementary data), starting at a temperature of about 450 °C, both in air and helium, should be mentioned. The FTIR spectra presented in Fig. 5 show peaks in the 3100–2910 and 1450-1360 cm−1 regions, which are the characteristic bands for eCH3 groups [31,32]. Thermal decomposition probably continues with β and γ scissions (Scheme 2). The intense peak for ionic fragment m/z = 94 (C6H5OH+) (Fig. S6), correlated with peaks of FTIR spectrum of the gases with a maximum intensity at 746 cm−1and 1250 cm−1 (Fig. 5), can be associated with the presence of phenol [29]. The presence of substituted benzene ring in the gases resulting from the decomposition of PAEN-2 is supported by the existence of a strong peak in the mass spectrum of the fragment m/z = 91 (C7H7+) (Fig. S8 in Supplementary data). This is correlated with the observations made in the FTIR spectra of Fig. 5, which exhibit characteristic benzene bands in the range 1600–1400 cm−1 and characteristic eCH2 bands in the range 2910–2720 cm−1 [33]. Also, the presence of the ionic fragment m/z = 119 (HOC6H4CN+) (Fig. S8 in Supplementary data) in the mass spectra as well as the occurrence of peaks in the range 1400–1100 cm−1 of the FTIR spectra presented in Fig. 5, confirm the proposed degradation mechanism (Scheme 2). The FTIR spectra of the residue obtained at different temperatures were also analyzed in order to obtain additional information on the degradation mechanism. In the FTIR spectra of PAEN-2 heated up to 530 °C (Fig. S2 in Supplementary data) an increase of the absorption band at 1600 cm−1 due to the formation of aromatic compounds is
3.4. Poly(arylene ether nitrile) containing isopropylidene groups, PAEN-2 The main thermogravimetric characteristics presented in Table 1, as well as in the MS and FTIR spectra of PAEN-2, demonstrate that the onset mechanism is not influenced by the atmosphere in which thermal decomposition takes place. The MS spectra of PAEN-2 show that thermal decomposition starts at the isopropylidene and aromatic ether linkages [17] by the α and β scissions (Scheme 2). In support of this statement, an increase in the intensity of the ionic 224
Polymer Testing 75 (2019) 220–228
G. Lisa, et al.
Scheme 2. The proposed thermal degradation mechanism for PAEN-2.
Fig. 6. Ionic current variation with temperature for the fragments m/z = 103 in air (a) and helium (b).
observed. The band at 2230 cm−1 characteristic of the eCN groups decreases in intensity, also the bands characteristic of the isopropylidene groups are considerably reduced in intensity, suggesting that the degradation of these groups occurred up to 530 °C. The band at 1240 cm−1, characteristic for aromatic ether bonds, disappeared completely. In the FTIR spectra of PAEN-2 heated in air up to 670 °C (Fig. S2 in Supplementary data) the characteristic band of the eCN groups has completely disappeared. Based on these observations and taking into account those set forth in a previous study on the degradation mechanism of some polyimides, m/z = 103 (Fig. 6) can also be associated
with the presence of the ionic fragment (C6H5CH = CH2+) resulting from the decomposition of the bisphenol A group [22]. The TG curves shown in Fig. 2 reveal that the smallest residue was obtained from PAEN-2 in both working atmospheres. Compared to other samples, PAEN-2 shows a higher ionic current intensity for fragments m/z = 44 (CO2+) (Fig. 3) and m/z = 18 (H2O+) (Fig. S5 in Supporting Information), if the working atmosphere is air. This polymer presents a significant increase of the ionic current intensity in the case of ionic fragments m/z = 78 (C6H6+) (Fig. 4) and m/z = 103 (C6H5CH = CH2+) or (C6H5CN+) (Fig. 6), if the degradation takes
225
Polymer Testing 75 (2019) 220–228
G. Lisa, et al.
Scheme 3. The proposed thermal degradation mechanism for PAEN-3.
characteristic of the aromatic ether linkages completely disappeared, while the bands characteristic to oxadiazole rings were very weak, which showed that the decomposition of ether linkages and oxadiazole rings occurred. FTIR spectra of the polymer heated in air up to 670 °C (Fig. S3 in Supplementary data) shows a low absorption value at 2220 cm−1 due to the presence of a reduced content of eCN groups. A strong band appeared at 1614 cm−1 due to the presence of aromatic compounds in the PAEN-3 char.
place in helium. 3.5. Poly(arylene ether nitrile) containing 1,3,4-oxadiazole rings, PAEN-3 The main thermogravimetric characteristics (Table 1) reveal that the lowest thermal stability occurs in the presence of oxadiazole rings in the polymer structure. Similar results were obtained in previous studies when the thermal stability of some aromatic polyethers [21], polyetherimides [22] or polyimide-polydimethylsiloxane copolymers [23] was studied, and it was established that the presence of oxadiazole rings slightly decreases the thermal stability. The MS and FTIR spectra obtained from gas analysis resulting from the thermal decomposition of this polymer indicate that the degradation process is likely to start with α and β scissions (Scheme 3). In that way, the proposed degradation mechanism is supported by the identification of ionic fragments m/z = 128 (NCC6H4CN+) (Fig. S9 in Supplementary data), m/z = 94 (C6H5OH+) (Fig. S6 in Supplementary data), m/z = 78 (C6H6+) (Fig. 4) and m/z = 103 (C6H5CN+) (Fig. 6). The FTIR spectra shown in Fig. 5 confirm the proposed degradation mechanism, highlighting small peaks with a maximum intensity at 746 cm−1 and 1250 cm−1, which are characteristic for phenol. Also, the characteristic benzene bands appear in the range 1600–1400 cm−1 and a small peak at 1373 cm−1 confirms the presence of benzonitrile groups. In the case of this polymer there were recorded variations in ionic current intensity of the fragments m/ z = 42 (NCO+) and m/z = 30 (NO+) [21] as shown in Fig. S9 in Supplementary data. FTIR spectra obtained for residues were also analyzed after heating PAEN-3 at different temperatures. It was found that the results were consistent with those previously described in relation to the proposed decomposition mechanism. In the case of FTIR spectra of PAEN-3 heated up to 550 °C (Fig. S3 in Supplementary data), it can be seen that the characteristic band at 2228 cm−1 is still present, suggesting that the eCN groups did not decompose. The band
3.6. Poly(arylene ether nitrile) containing imide rings, PAEN-4 The best thermal stability of the analyzed polymers was determined in the case of PAEN-4, which contains imide rings. The MS and FTIR spectra obtained from gas analysis resulting from the thermal decomposition of this polymer indicate that the degradation process is likely to start with α scission and continue with β scission (Scheme 4). Regardless of the working atmosphere, the following ionic fragments were identified: m/z = 44 (CO2+) (Fig. 3), m/z = 28 (CO+) (Fig. S7 in Supplementary data), m/z = 30 (NO+), m/z = 42 (NCO+), m/ z = 93 (C6H5NH2+) (Fig. S10 in Supplementary data), and m/z = 103 (C6H5CN+) (Fig. 6), which in the literature [22,23,34–36] are related to thermal decomposition of imide rings. An increase in the ionic current intensity was identified for ionic fragments m/z = 78 (C6H6+) (Fig. 4) and m/z = 94 (C6H5OH+) (Fig. S6 in Supplementary data). The peaks obtained for these fragments are more intense when the degradation takes place in helium. The FTIR spectra recorded by analyzing the gases that result from the thermal decomposition of PAEN-4 are shown in Fig. 5 for temperatures of 552 °C and 598 °C if degradation occurs in air and 565 °C and 640 °C if it occurs in helium. It can be seen small peaks with a maximum intensity at 746 and 1250 cm−1 characteristic for phenol, benzene characteristic bands in the range 1600–1400 cm−1 and a weak peak at 1373 cm−1 which confirms the presence of benzonitrile. Water characteristic bands in the range 226
Polymer Testing 75 (2019) 220–228
G. Lisa, et al.
Scheme 4. The proposed thermal degradation mechanism for PAEN-4.
3700–3400 cm−1, CO2 characteristic bands in the ranges 24002200 cm−1 and 740-600 cm−1 and CO characteristic bands in the range 2200–2000 cm−1 are also observed. In the case of this polymer, there were analyzed the residues obtained at different temperatures and the results were consistent with those described above in relation to the degradation mechanism. From the FTIR spectrum of PAEN-4 heated up to 580 °C (Fig. S4 in Supplementary data), the presence of characteristic bands for imide rings can be observed, thus demonstrating that at this temperature they have not yet been decomposed. A band appeared at 1870 cm−1 (may be anhydride groups). The characteristic band for ether linkages at 1240 cm−1 was not observed. The FTIR spectrum of the sample heated up to 600 °C (Fig. S4 in Supplementary data) showed that the characteristic bands of imide rings diminished considerably, but the band of eCN groups was still present. In the case of FTIR spectrum heated up to 670 °C in air atmosphere (Fig. S4 in Supplementary data), a strong band at 1590 cm−1 and another band with a lower intensity at 1847 cm−1 can be seen. The FTIR spectrum of PAEN-4 heated in helium up to 680 °C (Fig. S4 in Supplementary data) showed an absorption band at 2360 cm−1 and a very low band at 2220 cm−1, probably due to the presence of some eCN groups.
groups and imide rings decompose at a temperature above 550 °C. The major degradation products identified in the decomposition gases are: carbon dioxide, water, carbon monoxide, benzene, phenol, benzonitrile. In addition, nitrogen monoxide is identified in the case of PAEN- 3 and PAEN-4. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.polymertesting.2019.02.012. References [1] V. Lakshmana, A. Saxena, K.N. Ninan, Poly(arylene ether nitriles), J. Macromol. Sci. Polym. Rev. 42 (2002) 513–540. [2] X.B. Liu, R.H. Du, L.L. Hao, S. Wang, G.P. Cao, H. Jiang, Synthesis, characterization and reheological property of biphenyl-based polyarylene ether nitrile copolymers, Express Polym. Lett. 1 (2007) 499–505. [3] H. Tang, Z. Pu, X. Huang, J. Wei, X. Liu, Z. Lin, Novel blue-emitting carboxylfunctionalized poly(arylene ether nitrile)s with excellent thermal and mechanical properties, Polym. Chem. 5 (2014) 3673–3679. [4] A. Saxena, R. Sadhana, V.L. Rao, M. Kanakavel, K.N. Ninan, Synthesis and properties of polyarylene ether nitrile copolymers, Polym. Bull. 50 (2003) 219–226. [5] A. Saxena, R. Sadhana, V.L. Rao, P.V. Ravindran, K.N. Ninan, Synthesis and properties of poly(ether nitrile sulfone). Copolymers with pendant methyl groups, J. Appl. Polym. Sci. 97 (2005) 1987–1994. [6] N. Mushtaq, G. Chen, L.R. Sidra, Y. Liu, X. Fang, Synthesis and crosslinking study of isomeric poly(thioether ether imide)s containning pendant nitrile and terminal phthalonitrile groups, Polym. Chem. 7 (2016) 7427–7435. [7] H. Tang, J. Yang, J. Zhong, R. Zhao, X. Liu, Synthesis and dielectric properties of polyarylene ether nitriles with high thermal stability and high mechanical strength, Mater. Lett. 65 (2011) 2758–2761. [8] D.H. Wang, B.A. Kurish, I. Treufeld, l. Zhu, L.S. Tan, Synthesis and characterization of high nnitrile content polyimides as dielectric films for electrical energy storage, J. Polym. Sci., Part A: Polym. Chem. 53 (2015) 422–436. [9] Y. Zhan, Y. Fan, Y. Pan, H. Li, Y. He, Construction of advanced poly(arylene ether nitrile)/multi-walled carbon nanotubes nanocomposites by controlling the precise interface, J. Mater. Sci. 51 (2016) 2090–2100. [10] C. Li, A. Tang, Y. Zou, X. Liu, Preparation and dielectric properties of polyarylene ether nitriles/TiO2 nanocomposite films, Mater. Lett. 59 (2005) 59–63. [11] E. Hamciuc, C. Hamciuc, I. Bacosca, M. Cristea, L. Okrasa, Thermal and electrical properties of nitrile-containing polyimide/BaTiO3 composite films, Polym. Compos. 32 (2011) 846–855. [12] J. Zhong, H. Tang, Y. Chen, X. Liu, The preparation, mechanical and dielectric properties of PEN/HBCuPc hybrid films, J. Mater. Sci. Mater. Electron. 21 (2010) 1244–1248. [13] C. Hamciuc, E. Hamciuc, A.M. Ipate, L. Okrasa, Copoly(1,3,4-oxadiazole-ether)s
4. Conclusions A study on the thermal degradation of some poly(arylene ether nitrile)s by TG/MS/FTIR technique was performed. The thermogravimetric and derivative thermogravimetric analyses determined the following decreasing series of thermal stability of polymers in both air and helium atmospheres: PAEN-4 > PAEN-1 > PAEN-2 > PAEN-3. The obtained results have also shown that the introduction of 1,3,4-oxadiazole rings into the poly(arylene ether nitrile) structure leads to a decrease in thermal stability. By applying the TG/MS/FTIR technique, it was established that the thermal decomposition onset mechanism of the analyzed polymers is not influenced by the atmosphere in which the degradation occurs: air or helium. Thus, the thermal decomposition starts at oxadiazole rings (for PAEN-3), isopropylidene and aromatic ether linkages (for PAEN-2), phthalide groups and ether linkages (for PAEN-1) and ether linkages (for PAEN-4). The FTIR spectra recorded for residues obtained at different temperatures indicate that the nitrile 227
Polymer Testing 75 (2019) 220–228
G. Lisa, et al.
[14]
[15] [16]
[17]
[18] [19] [20]
[21]
[22]
[23]
[24] [25]
containing phthalide groups and thin films made therefrom, Polymer 49 (2008) 681–690. X. Xiong, L. Zhou, R. Ren, S. Liu, P. Chen, The thermal decomposition behavior and kinetics of epoxy resins cured with a novel phthalide-containing aromatic diamine, Polym. Test. 68 (2018) 46–52. C. Li, Y. Gu, X. Liu, Synthesis and properties of phenolphthalein-based polyarylene ether nitrile copolymers, Mater. Lett. 60 (2006) 137–141. H. Tang, J. Yang, J. Zhong, R. Zhao, X. Liu, Synthesis of high glass transition temperature fluorescent polyarylene ether nitrile copolymers, Mater. Lett. 65 (2011) 1703–1706. P. Patel, T.R. Hull, R.W. McCabe, D. Flath, J. Grasmeder, M. Percy, Mechanism of thermal decomposition of poly(ether ether ketone) (PEEK) from a review of decomposition studies, Polym. Degrad. Stabil. 95 (2010) 709–718. I. Bacosca, E. Hamciuc, M. Bruma, Effect of nitrile groups and of flexible linkages on the properties of poly(ether imide) films, Soft Mater. 11 (2013) 465–475. C. Hamciuc, E. Hamciuc, M. Ignat, G. Zarnescu, Aromatic poly(ether imide)s containing nitrile groups, High Perform. Polym. 21 (2009) 205–218. E. Hamciuc, C. Hamciuc, M. Cazacu, M. Ignat, G. Zarnescu, Polyimide-polydimethylsiloxane copolymers containing nitrile groups, Eur. Polym. J. 45 (2009) 182–190. G. Lisa, A.M. Ipate, C. Hamciuc, N. Tudorache, Thermal and thermo-oxidative degradation of some aromatic polyethers containing phenylquinoxaline and/or 1,3,4oxadiazole rings, J. Anal. Appl. Pyrol. 112 (2015) 37–47. G. Lisa, C. Hamciuc, E. Hamciuc, N. Tudorachi, Thermal and thermo-oxidative stability and probable degradation mechanism of some polyetherimides, J. Anal. Appl. Pyrol. 118 (2016) 114–154. C. Hamciuc, G. Lisa, E. Hamciuc, N. Tudorachi, Thermal decomposition study of some polyimide-polydimethylsiloxane copolymers, J. Anal. Appl. Pyrol. 129 (2018) 204–214. A.E. Siegrist, E. Maeder, N. Dunnenberger, 2,5-Diaryl-1,3,4- Oxadiazoles, Swiss Pat 383985. E. Hamciuc, M. Bruma, C. Hamciuc, R. Lungu, Aromatic polyimides containing
polar nitrile groups, Rev. Roum. Chem. 51 (2006) 765–771. [26] G. Lisa, D. Apreutesei Wilson, D. Scutaru, N. Tudorachi, N. Hurduc, Investigation of thermal degradation of some ferrocene liquid crystals, Thermochim. Acta 507–508 (2010) 49–59. [27] R.M. Silverstein, F.X. Webster, D.J. Kiemle, Spectrometric Identification of Organic Compounds, seventh ed., John Wiley and Sons, NewYork, NY, 2005. [28] G. Binghua, L. Chen, J. Yu, J. Zhu, G. Wang, Z. Hu, The pyrolysis behaviors of ternary copolyimide derived from aromatic dianhydride and aromatic diisocyanates, J. Appl. Polym. Sci. 131 (2014) 0165 (1-11). [29] X. Wang, Y. Hu, L. Song, W. Xing, H. Lu, P. Lv, G. Jie, Flame retardancy and thermal degradation mechanism of epoxy resin composites based on a DOPO substituted organophosphorus oligomer, Polymer 51 (2010) 2435–2445. [30] S.M. Alshehri, T. Ahamad, Thermal degradation and evolved gas analysis of N,N′bis(2-hydroxyethyl) linseed amide (BHLA) during pyrolysis and combustion, J. Therm. Anal. Calorim. 114 (2013) 1029–1037. [31] M. Worzakowska, TG/DSC/FTIR/QMS studies on the oxidative decomposition of terpene acrylate homopolymers, J. Therm. Anal. Calorim. 127 (2017) 2025–2035. [32] M.A. Corres, M. Zubitur, M. Cortazar, A. Múgica, Thermal and thermo-oxidative degradation of poly(hydroxy ether of bisphenol-A) studied by TGA/FTIR and TGA/ MS, J. Anal. Appl. Pyrol. 92 (2011) 407–416. [33] A.C. Sprânceană, M. Darie, S. Ciauşu, N. Tudorachi, G. Lisă, Comparative analysis of thermal stability of building insulation materials, Environ. Eng. Manag. J. 16 (2017) 2831–2842. [34] B. Guo, L. Chen, J. Yu, J. Zhu, G. Wang, Z. Hu, The pyrolysis behaviors of ternary copolyimide derived from aromatic dianhydride and aromatic diisocyanates, J. Appl. Polym. Sci. 131 (2014) 40163. [35] W. Xie, W.P. Pan, K.C. Chuang, Thermal degradation study of polymerization of monomeric reactants (PMR) polyimides, J. Therm. Anal. Calorim. 64 (2001) 477–485. [36] L.H. Perng, Thermal decomposition characteristics of poly(ether imide) by TG/MS, J. Polym. Res. 7 (2000) 185–193.
228
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Material Behaviour
Thermal degradation study of some poly(arylene ether nitrile)s by TG/MS/ FTIR analysis
T
Gabriela Lisaa,*, Corneliu Hamciucb, Elena Hamciucb, Nita Tudorachib Gheorghe Asachi Technical University of Iasi, Faculty of Chemical Engineering and Environmental Protection “Cristofor Simionescu”, Department of Chemical Engineering, 73 Prof.dr.doc. D. Mangeron Street, 700050, Iasi, Romania b “Petru Poni” Institute of Macromolecular Chemistry, 41A Aleea Gr. Ghica Voda, 700487, Iasi, Romania a
A R T I C LE I N FO
A B S T R A C T
Keywords: Aromatic polyethers Benzonitrile groups Thermal stability Mechanism of thermal decomposition TG/MS/FTIR analysis
A study on thermal and thermo-oxidative stability, i.e. the degradation mechanism for a series of poly(arylene ether nitrile)s was performed by applying simultaneous mass spectrometry and Fourier transform infrared spectroscopy of gas products from a thermogravimetric analyzer into two working atmospheres, air and helium. The polymers were prepared by nucleophilic aromatic substitution of 2,6-dichlorobenzonitrile with phenolphthalein, 2,2-bis(4-hydroxyphenyl)propane and 2,5-bis(4-hydroxyphenyl)-1,3,4-oxadiazole. A poly(arylene ether nitrile) was prepared by cyclopolycondensation reaction, in two steps, starting from 4,4′-oxydiphthalic anhydride and 2,6-bis(3-aminophenoxy)benzonitrile. The influence of benzonitrile units, isopropylidene linkages, phthalide groups, oxadiazole or imide rings on the thermal degradation behavior was discussed in detail. The onset mechanism of the thermal degradation process in the two working atmospheres was similar. The major identified products in the decomposition gases were carbon dioxide, water, carbon monoxide, benzene, phenol and benzonitrile. In addition, nitrogen monoxide was observed in the case of polymers containing oxadiazole or imide rings.
1. Introduction Poly(arylene ether nitrile)s (PAENs) represent a class of thermoplastic polymers characterized by good thermal and thermo-oxidative stability, excellent mechanical and electrical properties, chemical and radiation resistance [1–5]. The presence of highly resistant eCN groups, connected to the aromatic rings, maintains a high thermal stability. In addition, these groups represent potential sites for crosslinking reactions that allow the thermoplastic polymer to be converted into a thermoset with improved solvent resistance and enhanced thermal stability [6]. Due to the high polarity of eCN groups, the PAENs exhibit higher dielectric constant compared to the corresponding aromatic polyethers without these groups [7,8]. Therefore, PAENs are candidates frequently used as dielectric materials or as polymer matrix for the preparation of high-dielectric permittivity composites [9–12] used for advanced applications, including high energy density film capacitors, gate dielectric and electroactive materials. The requirements for such polymer matrix are high dielectric constant and low dielectric loss over large interval of temperature and frequency, high thermal stability and easy processability. Many structures of PAENs have been prepared and reported in the
*
literature. The polymers were mainly synthesized by aromatic nucleophilic substitution of dichloro- or difluorobenzonitrile with aromatic dioles such as hydroquinone or 1,3-dihydroxybenzene, in the presence of potassium carbonate, at high temperature. However, these compounds exhibit relative low glass transition temperature, high crystalinity and low solubility in organic solvents. A method to improve the properties of these polymers is the introduction into their structure of voluminous units, such as phthalide groups [13,14]. These units increase the macromolecular rigidity of PAENs by limiting chain mobility and decrease interfacial interactions due to reduced macromolecule packaging and reduced crystallinity. Thus, phenolphthalein based PAENs have improved characteristics in terms of glass transition temperature, thermal stability and solubility in organic solvents [7,15,16]. Another method to improve the thermal properties of PAENs refers to the introduction of different heterocycles, such as 1,3,4-oxadiazole or imide rings, in their macromolecular chains. For example aromatic polyimides containing eCN groups and ether linkages [17], especially the polymer derived from 4,4′-oxydiphthalic anhydride and 2,6-bis(3aminophenoxy)benzonitrile, are promising materials with piezoelectric properties that can be used at high temperatures [18–20]. These polymers exhibit also high thermal stability and high dielectric constant
Corresponding author. Tel.: +40 232 278683; fax: +40 232 271311. E-mail addresses: [email protected], [email protected] (G. Lisa).
https://doi.org/10.1016/j.polymertesting.2019.02.012 Received 25 July 2018; Received in revised form 14 December 2018; Accepted 11 February 2019 Available online 14 February 2019 0142-9418/ © 2019 Elsevier Ltd. All rights reserved.
Polymer Testing 75 (2019) 220–228
G. Lisa, et al.
being thus suitable as polymer matrices for the preparation of composite materials with special electric and piezoelectric properties. Previously, we studied the thermal and thermo-oxidative degradation of some aromatic polyethers containing phenylquinoxaline or 1,3,4-oxadiazole rings, aromatic polyetherimides, and polyimides with dimethylsiloxane segments by using simultaneous mass spectrometry and Fourier transform infrared spectrometry of off-gases from a thermogravimetric analyzer (TG/MS/FTIR technique) [21–23]. Establishing the mechanism of initiation of thermal degradation allows the determination of the decomposition products, thus it is possible to evaluate the toxicity of the products resulting from the decomposition of the respective polymers. Herein, a study on thermal behavior of some poly(arylene ether nitrile)s by means of coupled analyzes TG/MS/FTIR is presented. The influence of chemical structure (the presence in the polymer unit structure of benzonitrile groups, isopropylidene linkages, phthalide groups, oxadiazole or imide rings) on thermal stability and degradation mechanism is discussed in detail. According to our knowledge, TG/MS/ FTIR technique is used for the first time to evaluate the thermal degradation mechanism of polymers containing phthalide groups. PAENs are potential candidates frequently used as dielectric materials or as polymer matrix for the preparation of high-dielectric permittivity composites used for advanced applications, including high energy density film capacitors, gate dielectric and electroactive materials. The requirements for such polymer matrix are high dielectric constant and low dielectric loss over large interval of temperature and frequency, high thermal stability and easy processability. The importance of our study arises from the fact that by establishing the way in which the initiation of degradation occurs, we obtained information that could contribute to the development of new PAENs with high thermal stability both in the air and in helium.
Fig. 1. Chemical structures of the polymers.
ranged between 8.1 and 15.6 mg and the applied heating rate was 10 °C⋅min−1 in the temperature range 25–670 °C in air or 25–680 °C in helium with a flow of 50 mL⋅min−1. For constructive reasons, the oven of the device used for the thermogravimetric analysis operates until reaches 700 °C, after that it stops heating for emergency (protection) reasons, the temperature starts to fall, and the sampler no longer reaches 700 °C. The differences of temperature that arise (670 °C in the air and 680 °C in He) are caused by the differences of density between the two gases and the different thermal conductivity through which heat is transmitted from the oven wall to the sampler thermocouple. The oven thermocouple is fitted into the shell, and the transmission of heat is performed through convection through the two gases (air or helium) and for this reason a delay may appear. The thermogravimetric analyzer was calibrated for temperature and sensitivity using the melting points of the standard metals (Hg, In, Sn, Bi, Zn) [23]. The gases released during thermal decomposition processes are transferred by two isothermal transition lines to FTIR and mass spectrometer. The gases are introduced in TGA-IR external modulus of FTIR spectrometer, and FTIR spectra are recorded from 600 to 4000 cm−1 with a resolution of 4 cm−1. The transfer gases line to mass spectrometer is manufactured from quartz. The mass spectra were recorded under the electron impact ionization energy of 70 eV. The acquisition of data was recorded with Aeolos® 7.0 software, in spectrum scanning (SCAN) mode in the range of m/z = 1–300, measuring time was ca. 0.5 s for one channel, resulting in time/cycle of approximately 150 s [26].
2. Experimental 2.1. Materials 2,6-Dichlorobenzonitrile, phenolphthalein, 2,2-bis(4-hydroxyphenyl)propane, 4,4′-oxydiphthalic anhydride were provided from Aldrich and used as received. 2,5-Bis(4-hydroxyphenyl)-1,3,4-oxadiazole and 2,6-bis(3-aminophenoxy)benzonitrile were prepared as described in the literature [24,25]. 2.2. Preparation of the polymers The polymers PAEN-1, PAEN-2 and PAEN-3 were prepared by nucleophilic aromatic substitution of 2,6-dichlorobenzonitrile with phenolphthalein, 2,2-bis(4-hydroxyphenyl)propane and 2,5-bis(4-hydroxyphenyl)-1,3,4-oxadiazole, respectively [16]. PAEN-4 was prepared in two steps. In the first step a precursor poly(amic acid) was formed by reacting stoichiometric amounts of 4,4′-oxydiphthalic anhydride and 2,6-bis(3-aminophenoxy)benzonitrile, in N-methyl-2-pyrrolidone as solvent. In the second step the poly(amic acid) was imidized in solution at 180 °C for 6 h, under nitrogen [25]. The structures of the studied polymers are shown in Fig. 1.
3. Results and discussions 3.1. FTIR spectroscopy of the polymers The structures of polymers were identified by infrared spectroscopy. In the FTIR spectra of all polymers characteristic absorption bands appeared at 3046–3084 (aromatic CeH), around 2236 (C^N groups), 1600 and 1500 (C]C aromatic), 1250 cm−1 (aromatic CeOeC). PAEN1 showed a characteristic absorption band at 1784 cm−1 (C]O of phthalide group) while PAEN-2 exhibited characteristic bands at 2970 and 2875 cm−1 (isopropylidene groups). In the case of PAEN-3 characteristic bands appeared at 1018 and 965 cm−1 (=CeOeC = of oxadiazole rings). PAEN-4 showed characteristic bands for imide rings at 1772 and 1727 cm−1 (asymmetrical and symmetrical C]O stretching vibration of imide ring), 1384 (CeN of imide ring) and 740 cm−1 (C]O of imide ring, bending vibrations) (Figs. S1–S4 in Supplementary data).
2.3. Methods The infrared spectra were recorded on a FTIR Bruker Vertex 70 spectrometer at frequencies ranging from 4000 to 500 cm−1 (resolution 2 cm−1, 32 scans). Samples were grounded with KBr and pressed into pellets. The coupled technique TG/MS/FTIR was applied using a device that is composed of a thermogravimetric analysis device types STA 449 F1 Jupiter (Netzsch, Germany), coupled with a spectrophotometer type Vertex-70 FTIR (Bruker, Germany) and a mass spectrometer QMS model 403 C Aëolos (Netzsch, Germany). The analyzed samples mass 221
Polymer Testing 75 (2019) 220–228
G. Lisa, et al.
Fig. 2. TG and DTG curves of the polymers in air (a) and in helium (b).
3.2. Thermal stability
3.3. TG/MS/FTIR studies
The thermal stability of polymers was evaluated by thermogravimetric analysis in two working atmospheres: air and helium. The thermogravimetric (TG) curves and the derivative thermogravimetric (DTG) curves (Fig. 2) were recorded at 10 °C·min−1. The main thermogravimetric characteristics are shown in Table 1. The results reveal that the thermal decomposition of polymers in air takes place in two stages. The TG curves also reveal the fact that degradation in the final stage in air for PAEN-1, PAEN-3 and PAEN-4 is incomplete. The quantity of residue at 670 °C in this working atmosphere varies between 3 and 33%. The degradation in helium atmosphere is characterized by a single decomposition stage, at the end of which a residual quantity of 32–64% is obtained. The comparative analysis of the main thermogravimetric characteristics (Tonset, Tpeak, T5% and T10%) obtained during the first decomposition stage, in the two working atmospheres, shows very close values, suggesting a similar degradation mechanism. The thermal stability of polymers can be analyzed taking into account the following criteria: Tonset and Tpeak from the first stage of thermal decomposition, T5% and T10%. Taking these criteria into consideration, the decreasing series of thermal stability for the analyzed polymers is as follows: PAEN-4 > PAEN-1 > PAEN-2 > PAEN-3 in air or helium. The best thermal stability can be found for the polyether that contains benzonitrile groups and imide rings in its chemical structure.
3.3.1. Poly(arylene ether nitrile) containing phthalide groups, PAEN-1 The TG/MS/FTIR coupled technique applied to polymers in two working atmospheres, air and helium, made possible to obtain information about the degradation mechanism, identification of the main degradation products, and estimation of the impact of polymers on the environment in the case of thermal decomposition. The MS spectra of PAEN-1 in air reveal that the thermal decomposition of this polymer (Scheme 1) starts at the phthalide groups and ether linkages [17] and continues to the benzonitrile groups. At temperatures above 560 °C, the increase of the ionic fragment quantities of m/z = 44 (CO2+) (Fig. 3) and m/z = 18 (H2O+) (Fig. S5 in Supplementary data) released from the thermal decomposition process suggests that a thermal oxidation process takes place. The evolution of the ionic current for ionic fragments m/z = 44 (CO2+), m/z = 78 (C6H6+) and m/z = 94 (C6H5OH+), presented in Figs. 3 and 4 and S6 in Supplementary data, confirms the proposed thermal degradation mechanism for PAEN-1, in both air and helium atmospheres. The MS spectra of PAEN-1 reveal the presence of ionic fragments m/ z = 39 (C3H3+), m/z = 50 (C4H2+), m/z = 51 (C4H3+), m/z = 52 (C4H4+) and m/z = 65 (C5H5+). These are clear indications of the presence of benzene and phenol in the gases resulting from the decomposition of this polymer [27]. The proposed degradation
Table 1 The main thermogravimetric parameters for aromatic polyethers containing benzonitrile units. Sample
Atmosphere
Degradation stage
Tonset (°C)
T5% (°C)
T10% (°C)
Tpeak (°C)
Tendset (°C)
W (%)
R (%)
PAEN-1
air
501
483 454
498 469
PAEN-3
helium air
459 438
476 465
PAEN-4
helium air
423 519
458 547
523
548
508 648 508 496 590 501 495 657 495 553 590 556
524 – 548 522 662 538 533 – 591 585 – 643
22.79 62.68 36.21 41.47 55.39 58.72 30.37 53.25 44.76 22.32 45.40 36.05
14.53
helium air
467 567 468 452 542 455 433 571 438 505 585 511
486
PAEN-2
I II I I II I I II I I II I
helium
63.79 3.14 41.28 16.38 55.24 32.28 63.95
Tonset - the temperature at which the thermal decomposition begins; Tpeak - the temperature at which the degradation rate is maximum; T5%, T10% - the temperature corresponding to 5 and 10% mass loss; W - mass losses in each stage, R-residue at 670 °C in air and 680 °C in helium. 222
Polymer Testing 75 (2019) 220–228
G. Lisa, et al.
Scheme 1. The proposed thermal degradation mechanism for PAEN-1.
associated with phenol presence (m/z = 94) [29], and at 1373 cm−1, associated with benzonitrile presence (m/z = 103) [30]. The ionic current intensity in MS spectra is low in the case of these fragments (Fig. S6 in Supplementary data). The FTIR spectra of residues were also analyzed in order to obtain additional information about the degradation mechanism of PAEN-1. The spectra of PAEN-1 heated in air up to 550 and 670 °C were recorded. The spectrum of PAEN-1 heated up to 550 °C (Fig. S1 in Supplementary data) showed absorption band at 2228 cm−1 suggesting
mechanism is also supported by FTIR spectra obtained by analyzing the gases resulting from thermal decomposition of PAEN-1 (Fig. 5). The bands that characterize water in the range of 3700–3400 cm−1, CO2 in the range of 2400–2200 and 740-600 cm−1, CO in the range of 2200–2000 cm−1 and benzene in the range of 1600–1400 cm−1 can be clearly seen [28]. The presence of CO is also confirmed by the evolution of the ionic current for m/z = 28 (CO+), as shown in Fig. S7 in Supplementary data. FTIR spectra shown in Fig. 5 for PAEN-1 present smaller peaks with maximum intensity at 746 and 1250 cm−1,
Fig. 3. Ionic current variation with temperature for the fragments m/z = 44 in air (a) and helium (b). 223
Polymer Testing 75 (2019) 220–228
G. Lisa, et al.
Fig. 4. Ionic current variation with temperature for the fragments m/z = 78 in air (a) and helium (b).
Fig. 5. FTIR spectra of the volatile products evolved during thermal decomposition of aromatic polyethers containing benzonitrile units in air (a) and helium (b).
that eCN groups were thermally stable up to this temperature. The characteristic C]O band of the phthalide unit was not present, thus demonstrating that decomposition of these units occurred up to 550 °C. Another band appeared at 1709 cm−1, probable due to the presence of some carboxylic groups. A strong band was observed at 1600 cm−1 due to the high content of aromatic compounds in the char. Also, the characteristic band of aromatic ether bonds at 1240 cm−1 was not observed, thus demonstrating the decomposition of these bonds up to 550 °C. In the FTIR spectrum of PAEN-1 heated in air up to 670 °C (Fig. S1 in Supplementary data) a strong band at 1635 cm−1 (due to the aromatic characteristic of the char) can be observed. The band of eCN groups at 2230 cm−1 disappeared completely. The FTIR spectra of PAEN-1 at 680 °C in helium (Fig. S1 in Supplementary data) exhibited similar absorption bands. In addition, a slight peak at 2223 cm−1 can be observed due to the presence of a small amount of eCN groups.
current corresponding to the fragment m/z = 15 (CH3+) (Fig. S5 in Supplementary data), starting at a temperature of about 450 °C, both in air and helium, should be mentioned. The FTIR spectra presented in Fig. 5 show peaks in the 3100–2910 and 1450-1360 cm−1 regions, which are the characteristic bands for eCH3 groups [31,32]. Thermal decomposition probably continues with β and γ scissions (Scheme 2). The intense peak for ionic fragment m/z = 94 (C6H5OH+) (Fig. S6), correlated with peaks of FTIR spectrum of the gases with a maximum intensity at 746 cm−1and 1250 cm−1 (Fig. 5), can be associated with the presence of phenol [29]. The presence of substituted benzene ring in the gases resulting from the decomposition of PAEN-2 is supported by the existence of a strong peak in the mass spectrum of the fragment m/z = 91 (C7H7+) (Fig. S8 in Supplementary data). This is correlated with the observations made in the FTIR spectra of Fig. 5, which exhibit characteristic benzene bands in the range 1600–1400 cm−1 and characteristic eCH2 bands in the range 2910–2720 cm−1 [33]. Also, the presence of the ionic fragment m/z = 119 (HOC6H4CN+) (Fig. S8 in Supplementary data) in the mass spectra as well as the occurrence of peaks in the range 1400–1100 cm−1 of the FTIR spectra presented in Fig. 5, confirm the proposed degradation mechanism (Scheme 2). The FTIR spectra of the residue obtained at different temperatures were also analyzed in order to obtain additional information on the degradation mechanism. In the FTIR spectra of PAEN-2 heated up to 530 °C (Fig. S2 in Supplementary data) an increase of the absorption band at 1600 cm−1 due to the formation of aromatic compounds is
3.4. Poly(arylene ether nitrile) containing isopropylidene groups, PAEN-2 The main thermogravimetric characteristics presented in Table 1, as well as in the MS and FTIR spectra of PAEN-2, demonstrate that the onset mechanism is not influenced by the atmosphere in which thermal decomposition takes place. The MS spectra of PAEN-2 show that thermal decomposition starts at the isopropylidene and aromatic ether linkages [17] by the α and β scissions (Scheme 2). In support of this statement, an increase in the intensity of the ionic 224
Polymer Testing 75 (2019) 220–228
G. Lisa, et al.
Scheme 2. The proposed thermal degradation mechanism for PAEN-2.
Fig. 6. Ionic current variation with temperature for the fragments m/z = 103 in air (a) and helium (b).
observed. The band at 2230 cm−1 characteristic of the eCN groups decreases in intensity, also the bands characteristic of the isopropylidene groups are considerably reduced in intensity, suggesting that the degradation of these groups occurred up to 530 °C. The band at 1240 cm−1, characteristic for aromatic ether bonds, disappeared completely. In the FTIR spectra of PAEN-2 heated in air up to 670 °C (Fig. S2 in Supplementary data) the characteristic band of the eCN groups has completely disappeared. Based on these observations and taking into account those set forth in a previous study on the degradation mechanism of some polyimides, m/z = 103 (Fig. 6) can also be associated
with the presence of the ionic fragment (C6H5CH = CH2+) resulting from the decomposition of the bisphenol A group [22]. The TG curves shown in Fig. 2 reveal that the smallest residue was obtained from PAEN-2 in both working atmospheres. Compared to other samples, PAEN-2 shows a higher ionic current intensity for fragments m/z = 44 (CO2+) (Fig. 3) and m/z = 18 (H2O+) (Fig. S5 in Supporting Information), if the working atmosphere is air. This polymer presents a significant increase of the ionic current intensity in the case of ionic fragments m/z = 78 (C6H6+) (Fig. 4) and m/z = 103 (C6H5CH = CH2+) or (C6H5CN+) (Fig. 6), if the degradation takes
225
Polymer Testing 75 (2019) 220–228
G. Lisa, et al.
Scheme 3. The proposed thermal degradation mechanism for PAEN-3.
characteristic of the aromatic ether linkages completely disappeared, while the bands characteristic to oxadiazole rings were very weak, which showed that the decomposition of ether linkages and oxadiazole rings occurred. FTIR spectra of the polymer heated in air up to 670 °C (Fig. S3 in Supplementary data) shows a low absorption value at 2220 cm−1 due to the presence of a reduced content of eCN groups. A strong band appeared at 1614 cm−1 due to the presence of aromatic compounds in the PAEN-3 char.
place in helium. 3.5. Poly(arylene ether nitrile) containing 1,3,4-oxadiazole rings, PAEN-3 The main thermogravimetric characteristics (Table 1) reveal that the lowest thermal stability occurs in the presence of oxadiazole rings in the polymer structure. Similar results were obtained in previous studies when the thermal stability of some aromatic polyethers [21], polyetherimides [22] or polyimide-polydimethylsiloxane copolymers [23] was studied, and it was established that the presence of oxadiazole rings slightly decreases the thermal stability. The MS and FTIR spectra obtained from gas analysis resulting from the thermal decomposition of this polymer indicate that the degradation process is likely to start with α and β scissions (Scheme 3). In that way, the proposed degradation mechanism is supported by the identification of ionic fragments m/z = 128 (NCC6H4CN+) (Fig. S9 in Supplementary data), m/z = 94 (C6H5OH+) (Fig. S6 in Supplementary data), m/z = 78 (C6H6+) (Fig. 4) and m/z = 103 (C6H5CN+) (Fig. 6). The FTIR spectra shown in Fig. 5 confirm the proposed degradation mechanism, highlighting small peaks with a maximum intensity at 746 cm−1 and 1250 cm−1, which are characteristic for phenol. Also, the characteristic benzene bands appear in the range 1600–1400 cm−1 and a small peak at 1373 cm−1 confirms the presence of benzonitrile groups. In the case of this polymer there were recorded variations in ionic current intensity of the fragments m/ z = 42 (NCO+) and m/z = 30 (NO+) [21] as shown in Fig. S9 in Supplementary data. FTIR spectra obtained for residues were also analyzed after heating PAEN-3 at different temperatures. It was found that the results were consistent with those previously described in relation to the proposed decomposition mechanism. In the case of FTIR spectra of PAEN-3 heated up to 550 °C (Fig. S3 in Supplementary data), it can be seen that the characteristic band at 2228 cm−1 is still present, suggesting that the eCN groups did not decompose. The band
3.6. Poly(arylene ether nitrile) containing imide rings, PAEN-4 The best thermal stability of the analyzed polymers was determined in the case of PAEN-4, which contains imide rings. The MS and FTIR spectra obtained from gas analysis resulting from the thermal decomposition of this polymer indicate that the degradation process is likely to start with α scission and continue with β scission (Scheme 4). Regardless of the working atmosphere, the following ionic fragments were identified: m/z = 44 (CO2+) (Fig. 3), m/z = 28 (CO+) (Fig. S7 in Supplementary data), m/z = 30 (NO+), m/z = 42 (NCO+), m/ z = 93 (C6H5NH2+) (Fig. S10 in Supplementary data), and m/z = 103 (C6H5CN+) (Fig. 6), which in the literature [22,23,34–36] are related to thermal decomposition of imide rings. An increase in the ionic current intensity was identified for ionic fragments m/z = 78 (C6H6+) (Fig. 4) and m/z = 94 (C6H5OH+) (Fig. S6 in Supplementary data). The peaks obtained for these fragments are more intense when the degradation takes place in helium. The FTIR spectra recorded by analyzing the gases that result from the thermal decomposition of PAEN-4 are shown in Fig. 5 for temperatures of 552 °C and 598 °C if degradation occurs in air and 565 °C and 640 °C if it occurs in helium. It can be seen small peaks with a maximum intensity at 746 and 1250 cm−1 characteristic for phenol, benzene characteristic bands in the range 1600–1400 cm−1 and a weak peak at 1373 cm−1 which confirms the presence of benzonitrile. Water characteristic bands in the range 226
Polymer Testing 75 (2019) 220–228
G. Lisa, et al.
Scheme 4. The proposed thermal degradation mechanism for PAEN-4.
3700–3400 cm−1, CO2 characteristic bands in the ranges 24002200 cm−1 and 740-600 cm−1 and CO characteristic bands in the range 2200–2000 cm−1 are also observed. In the case of this polymer, there were analyzed the residues obtained at different temperatures and the results were consistent with those described above in relation to the degradation mechanism. From the FTIR spectrum of PAEN-4 heated up to 580 °C (Fig. S4 in Supplementary data), the presence of characteristic bands for imide rings can be observed, thus demonstrating that at this temperature they have not yet been decomposed. A band appeared at 1870 cm−1 (may be anhydride groups). The characteristic band for ether linkages at 1240 cm−1 was not observed. The FTIR spectrum of the sample heated up to 600 °C (Fig. S4 in Supplementary data) showed that the characteristic bands of imide rings diminished considerably, but the band of eCN groups was still present. In the case of FTIR spectrum heated up to 670 °C in air atmosphere (Fig. S4 in Supplementary data), a strong band at 1590 cm−1 and another band with a lower intensity at 1847 cm−1 can be seen. The FTIR spectrum of PAEN-4 heated in helium up to 680 °C (Fig. S4 in Supplementary data) showed an absorption band at 2360 cm−1 and a very low band at 2220 cm−1, probably due to the presence of some eCN groups.
groups and imide rings decompose at a temperature above 550 °C. The major degradation products identified in the decomposition gases are: carbon dioxide, water, carbon monoxide, benzene, phenol, benzonitrile. In addition, nitrogen monoxide is identified in the case of PAEN- 3 and PAEN-4. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.polymertesting.2019.02.012. References [1] V. Lakshmana, A. Saxena, K.N. Ninan, Poly(arylene ether nitriles), J. Macromol. Sci. Polym. Rev. 42 (2002) 513–540. [2] X.B. Liu, R.H. Du, L.L. Hao, S. Wang, G.P. Cao, H. Jiang, Synthesis, characterization and reheological property of biphenyl-based polyarylene ether nitrile copolymers, Express Polym. Lett. 1 (2007) 499–505. [3] H. Tang, Z. Pu, X. Huang, J. Wei, X. Liu, Z. Lin, Novel blue-emitting carboxylfunctionalized poly(arylene ether nitrile)s with excellent thermal and mechanical properties, Polym. Chem. 5 (2014) 3673–3679. [4] A. Saxena, R. Sadhana, V.L. Rao, M. Kanakavel, K.N. Ninan, Synthesis and properties of polyarylene ether nitrile copolymers, Polym. Bull. 50 (2003) 219–226. [5] A. Saxena, R. Sadhana, V.L. Rao, P.V. Ravindran, K.N. Ninan, Synthesis and properties of poly(ether nitrile sulfone). Copolymers with pendant methyl groups, J. Appl. Polym. Sci. 97 (2005) 1987–1994. [6] N. Mushtaq, G. Chen, L.R. Sidra, Y. Liu, X. Fang, Synthesis and crosslinking study of isomeric poly(thioether ether imide)s containning pendant nitrile and terminal phthalonitrile groups, Polym. Chem. 7 (2016) 7427–7435. [7] H. Tang, J. Yang, J. Zhong, R. Zhao, X. Liu, Synthesis and dielectric properties of polyarylene ether nitriles with high thermal stability and high mechanical strength, Mater. Lett. 65 (2011) 2758–2761. [8] D.H. Wang, B.A. Kurish, I. Treufeld, l. Zhu, L.S. Tan, Synthesis and characterization of high nnitrile content polyimides as dielectric films for electrical energy storage, J. Polym. Sci., Part A: Polym. Chem. 53 (2015) 422–436. [9] Y. Zhan, Y. Fan, Y. Pan, H. Li, Y. He, Construction of advanced poly(arylene ether nitrile)/multi-walled carbon nanotubes nanocomposites by controlling the precise interface, J. Mater. Sci. 51 (2016) 2090–2100. [10] C. Li, A. Tang, Y. Zou, X. Liu, Preparation and dielectric properties of polyarylene ether nitriles/TiO2 nanocomposite films, Mater. Lett. 59 (2005) 59–63. [11] E. Hamciuc, C. Hamciuc, I. Bacosca, M. Cristea, L. Okrasa, Thermal and electrical properties of nitrile-containing polyimide/BaTiO3 composite films, Polym. Compos. 32 (2011) 846–855. [12] J. Zhong, H. Tang, Y. Chen, X. Liu, The preparation, mechanical and dielectric properties of PEN/HBCuPc hybrid films, J. Mater. Sci. Mater. Electron. 21 (2010) 1244–1248. [13] C. Hamciuc, E. Hamciuc, A.M. Ipate, L. Okrasa, Copoly(1,3,4-oxadiazole-ether)s
4. Conclusions A study on the thermal degradation of some poly(arylene ether nitrile)s by TG/MS/FTIR technique was performed. The thermogravimetric and derivative thermogravimetric analyses determined the following decreasing series of thermal stability of polymers in both air and helium atmospheres: PAEN-4 > PAEN-1 > PAEN-2 > PAEN-3. The obtained results have also shown that the introduction of 1,3,4-oxadiazole rings into the poly(arylene ether nitrile) structure leads to a decrease in thermal stability. By applying the TG/MS/FTIR technique, it was established that the thermal decomposition onset mechanism of the analyzed polymers is not influenced by the atmosphere in which the degradation occurs: air or helium. Thus, the thermal decomposition starts at oxadiazole rings (for PAEN-3), isopropylidene and aromatic ether linkages (for PAEN-2), phthalide groups and ether linkages (for PAEN-1) and ether linkages (for PAEN-4). The FTIR spectra recorded for residues obtained at different temperatures indicate that the nitrile 227
Polymer Testing 75 (2019) 220–228
G. Lisa, et al.
[14]
[15] [16]
[17]
[18] [19] [20]
[21]
[22]
[23]
[24] [25]
containing phthalide groups and thin films made therefrom, Polymer 49 (2008) 681–690. X. Xiong, L. Zhou, R. Ren, S. Liu, P. Chen, The thermal decomposition behavior and kinetics of epoxy resins cured with a novel phthalide-containing aromatic diamine, Polym. Test. 68 (2018) 46–52. C. Li, Y. Gu, X. Liu, Synthesis and properties of phenolphthalein-based polyarylene ether nitrile copolymers, Mater. Lett. 60 (2006) 137–141. H. Tang, J. Yang, J. Zhong, R. Zhao, X. Liu, Synthesis of high glass transition temperature fluorescent polyarylene ether nitrile copolymers, Mater. Lett. 65 (2011) 1703–1706. P. Patel, T.R. Hull, R.W. McCabe, D. Flath, J. Grasmeder, M. Percy, Mechanism of thermal decomposition of poly(ether ether ketone) (PEEK) from a review of decomposition studies, Polym. Degrad. Stabil. 95 (2010) 709–718. I. Bacosca, E. Hamciuc, M. Bruma, Effect of nitrile groups and of flexible linkages on the properties of poly(ether imide) films, Soft Mater. 11 (2013) 465–475. C. Hamciuc, E. Hamciuc, M. Ignat, G. Zarnescu, Aromatic poly(ether imide)s containing nitrile groups, High Perform. Polym. 21 (2009) 205–218. E. Hamciuc, C. Hamciuc, M. Cazacu, M. Ignat, G. Zarnescu, Polyimide-polydimethylsiloxane copolymers containing nitrile groups, Eur. Polym. J. 45 (2009) 182–190. G. Lisa, A.M. Ipate, C. Hamciuc, N. Tudorache, Thermal and thermo-oxidative degradation of some aromatic polyethers containing phenylquinoxaline and/or 1,3,4oxadiazole rings, J. Anal. Appl. Pyrol. 112 (2015) 37–47. G. Lisa, C. Hamciuc, E. Hamciuc, N. Tudorachi, Thermal and thermo-oxidative stability and probable degradation mechanism of some polyetherimides, J. Anal. Appl. Pyrol. 118 (2016) 114–154. C. Hamciuc, G. Lisa, E. Hamciuc, N. Tudorachi, Thermal decomposition study of some polyimide-polydimethylsiloxane copolymers, J. Anal. Appl. Pyrol. 129 (2018) 204–214. A.E. Siegrist, E. Maeder, N. Dunnenberger, 2,5-Diaryl-1,3,4- Oxadiazoles, Swiss Pat 383985. E. Hamciuc, M. Bruma, C. Hamciuc, R. Lungu, Aromatic polyimides containing
polar nitrile groups, Rev. Roum. Chem. 51 (2006) 765–771. [26] G. Lisa, D. Apreutesei Wilson, D. Scutaru, N. Tudorachi, N. Hurduc, Investigation of thermal degradation of some ferrocene liquid crystals, Thermochim. Acta 507–508 (2010) 49–59. [27] R.M. Silverstein, F.X. Webster, D.J. Kiemle, Spectrometric Identification of Organic Compounds, seventh ed., John Wiley and Sons, NewYork, NY, 2005. [28] G. Binghua, L. Chen, J. Yu, J. Zhu, G. Wang, Z. Hu, The pyrolysis behaviors of ternary copolyimide derived from aromatic dianhydride and aromatic diisocyanates, J. Appl. Polym. Sci. 131 (2014) 0165 (1-11). [29] X. Wang, Y. Hu, L. Song, W. Xing, H. Lu, P. Lv, G. Jie, Flame retardancy and thermal degradation mechanism of epoxy resin composites based on a DOPO substituted organophosphorus oligomer, Polymer 51 (2010) 2435–2445. [30] S.M. Alshehri, T. Ahamad, Thermal degradation and evolved gas analysis of N,N′bis(2-hydroxyethyl) linseed amide (BHLA) during pyrolysis and combustion, J. Therm. Anal. Calorim. 114 (2013) 1029–1037. [31] M. Worzakowska, TG/DSC/FTIR/QMS studies on the oxidative decomposition of terpene acrylate homopolymers, J. Therm. Anal. Calorim. 127 (2017) 2025–2035. [32] M.A. Corres, M. Zubitur, M. Cortazar, A. Múgica, Thermal and thermo-oxidative degradation of poly(hydroxy ether of bisphenol-A) studied by TGA/FTIR and TGA/ MS, J. Anal. Appl. Pyrol. 92 (2011) 407–416. [33] A.C. Sprânceană, M. Darie, S. Ciauşu, N. Tudorachi, G. Lisă, Comparative analysis of thermal stability of building insulation materials, Environ. Eng. Manag. J. 16 (2017) 2831–2842. [34] B. Guo, L. Chen, J. Yu, J. Zhu, G. Wang, Z. Hu, The pyrolysis behaviors of ternary copolyimide derived from aromatic dianhydride and aromatic diisocyanates, J. Appl. Polym. Sci. 131 (2014) 40163. [35] W. Xie, W.P. Pan, K.C. Chuang, Thermal degradation study of polymerization of monomeric reactants (PMR) polyimides, J. Therm. Anal. Calorim. 64 (2001) 477–485. [36] L.H. Perng, Thermal decomposition characteristics of poly(ether imide) by TG/MS, J. Polym. Res. 7 (2000) 185–193.
228