Effects of Thioether Content on the Solubility and Thermal Properties

Oct 22, 2013 - College of Chemistry, Sichuan University, Chengdu 610064, P. R. China. § State Key Laboratory of Polymer Materials Engineering, Sichua...
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Effects of Thioether Content on the Solubility and Thermal Properties of Aromatic Polyesters Gang Zhang,† Xiu-jing Xing,‡ Dong-sheng Li,† Xiao-jun Wang,*,† and Jie Yang*,†,§ †

Institute of Materials Science & Technology, Analytical & Testing Center, Sichuan University, Chengdu 610064, P. R. China College of Chemistry, Sichuan University, Chengdu 610064, P. R. China § State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, P. R. China ‡

S Supporting Information *

ABSTRACT: Two kinds of aromatic polyesters containing thioether units had been prepared through the reaction of 4,4′thiodibenzoyl chloride (T-DC) (or 4,4′-bis(4-chloroformylphenylthio)benzene (BPBDC)) and 1,1-bis(4-hydroxyphenyl)-1phenylethane (BHPPE) by the method of interfacial polycondensation. These polyesters showed good solubility, and could afford tough films with tensile strengths of 103.6−108.3 MPa. The glass transition temperature (Tg) of these polyesters ranged from 189.8 to 235.6 °C and initial degradation temperatures (Td) were 450−454 °C. The activation energies of degradation were in range of 156.6−160.3 KJ/mol. The limiting oxygen indexes (LOIs) of these polyesters ranged from 37 to 39, and UL-94 V-0 rating could achieve via this approach. The thermal degradation kinetics and thermal pyrolysis mechanism of these polyesters was studied by thermogravimetric analysis and Py-GC/MS analysis, respectively.

1. INTRODUCTION Polyesters are well-known as high-performance engineering thermoplastics for their good thermal stability, chemical resistance, and good mechanical properties.1,2 They can be produced by melt processing to form different kinds of molded components, coatings, membranes, or polymer fibers. With increasingly stringent standards, applications of polyesters are limited in some fields such as high temperature and humidity condition because of their poor dimensional stability and thermal properties. To improve the thermal properties of polyesters, it acts as an efficient approach to synthesize fully aromatic polyesters in the past several years.3,4 As we know, the fully aromatic polyesters need high processing temperature or have poor solubility in organic solvents because of their extended rigid structures and low melting entropies, which greatly limited their development and application. Thus, improvement of the processability of aromatic polyester is necessary. To overcome these shortcomings, researchers have developed various approaches to obtain thermally stable and organic-soluble fully aromatic polyesters. These approaches included the introduction of bulky pendant groups,5−10 nonsymmetric groups,11−15 hyperbranched structure,16 trifluoromethyl groups,17−19 functional groups,20 and flexible chains21 into the polymer backbone and so on. It is known that the thioether bond is a flexible linkage. It can be incorporated into the polymer backbone to improve the resin’s processability, such as: poly(phenylene sulfide),22 poly(arylene sulfide sulfone) 23−25 and poly(arylene sulfide amide),26,27 They all have excellent processability and mechanical, thermal, and antioxidant properties. In this work, BHPPE was used as monomer because its good thermal stability and bulky pendent group. BPBDC and TDC containing thioether unit was prepared. The two kinds of diacid chloride were conducted to react with BHPPE to form thioether containing aromatic polyesters. The effects that © 2013 American Chemical Society

introduction of thioether unit into the polymer backbone have on the solubility, thermal property, and flame retardancy of these aromatic polyesters is discussed. Meantime, the polymers’ thermal decomposition behavior was studied in detail.

2. EXPERIMENTAL SECTION 2.1. Materials. Acetophenone, phenol, benzyltriethylammonium chloride (BTEAC) (AR, Chengdu, Kelong Chemical Industry Company), mercaptoacetic acid (99%, Aladin Reagent Company), sodium hydroxide (NaOH), dimethyl sulfoxide (DMSO) (AR, SiChuan ChengDu ChangLian Chemical Reagent Company), sodium sulfide (Na2S·xH2O, Na2S% ≈ 60%) (Nafine Chemical Industry Group Co., Ltd.), dichloromethane (AR, Chengdu, Kelong Chemical Industry Company) was purified by reduced pressure distillation. 4-fluorobenzoic acid (4-FBA) was made in our laboratory, the other reagents and solvents were commercially obtained. 2.2. Monomer Synthesis. 2.2.1. 1,1-bis(4-Hydroxyphenyl)-1-phenylethane (BHPPE) (Shown as Scheme 1). BHPPE was prepared with a similar method reported earlier.17 Here we conducted the nucleophilic reactions with H3PO4 as solvent and mercaptoacetic acid as catalyst under 60 °C. Yield: 203.3 g, 70.1%. Elemental anal. Found (%): C, 82.32 (82.73); H, 6.36 (6.25) (data in brackets are calculated). 1H NMR (600 MHz, DMSOd6/TMS, ppm): 2.010 (s, 3H, H1), 6.633−6.655 (d, 4H, H2), 6.789−6.811 (d, 4H, H3), 7.001−7.020 (d, 2H, H4), 7.166− 7.269 (m, 3H, H5−H6), 9.245 (s, 2H, H7). 13C NMR (600 MHz, DMSO-d6/TMS, ppm): 30.285 (Ca), 50.610 (Cb), Received: Revised: Accepted: Published: 16577

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Scheme 1. Synthesis Routes of BHPPE

Scheme 2. Synthesis Routes of PE-(a-b) and PE-R

polyester of TPC and BHPPE with interfacial polymerization method) to compare its properties with PE-(a-b). PE-b: yield 57.7 g, 90.7%. Elemental anal. Found (%): C, 75.67 (75.45); H, 4.46 (4.43) (data in brackets are calculated). FT-IR (casting film, cm−1): 3056 (C−H, aromatic ring), 2977, 2852 (−CH3), 1735 (−CO−), 1592, 1503 (CC aromatic ring), 1071 (-S-). 1H NMR (600 MHz, CDCl3, ppm): 2.201 (s, 3H, H1), 7.086−7.108 (m, 6H, H2), 7.145−7.167 (d, 4H, H3), 7.216−7.234 (d, 4H, H4), 7.262−7.281 (d, 2H, H5), 7.337− 7.358 (m, 2H, H6), 7.379−7.399 (d, 4H, H7), 7.425−7.448 (d, 4H, H8), 8.078−8.021 (d, 4H, H9). 2.4. Characterizations. The inherent viscosities of PE-(ab) were obtained in CHCl3 at 30 ± 0.1 °C with 0.500 g of polymer dissolving in 100 mL of CHCl3, using a Cannon− Ubbelodhe viscometer. The resulting values were obtained by Solomon-Ciuta equation (eq 1) as follows

114.443 (Cc), 125.563 (Cd), 127.662 (Ce), 128.183 (Cf), 129.184 (Cg), 139.468 (Ch), 149.854 (Ci), 155.161 (Cj). 2.2.2. 4,4′-Thiodibenzoyl Chloride (TDC) and 4,4′-Bis(4chloroformylphenylthio)benzene (BPBDC). TDC and BPBDC were synthesized by the reaction of thionyl chloride (SOCl2) and diacid with pyridine as catalyst at 46 °C in darkness according to our previous report.26 The crude product recrystallized from dichloromethane (or petroleum ether) was washed with petroleum ether and dried under vacuum at 56 °C to yield product. 2.3. Polymer Synthesis (shown in Scheme 2). A typical polymerization was performed as shown in Scheme 2. BHPPE (29 g, 0.1 mol), NaOH (8 g, 0.2 mol), BTEAC (1 g, 0.005 mol) and 200 mL of deionized water were added into a 1000 mL three-necked flask equipped with a mechanical stirrer and thermometer. Then the solution of TDC (31 g, 0.1 mol) (dissolved in 300 mL dichloromethane) was added dropwise within 2 h. The mixture was vigorous stirred at 30 °C for another 6 h. Next, the reaction solution was poured into ethanol with vigorous stirring to precipitate a white crude product, which was washed with hot water and ethanol several times, and then pulverized to powder and extracted with water and ethanol again. After drying under a vacuum at 100 °C for 12 h, 48.7 g (92.4%) of PE-a was obtained. Elemental anal. Found (%): C, 77.21 (77.25); H, 4.69 (4.58) (data in brackets are calculated). FT-IR (casting film, cm−1): 3065 (C−H, aromatic ring), 2981, 2868 (−CH3), 1735 (−CO−), 1590, 1503 (CC aromatic ring), 1072 (-S-). 1H NMR (600 MHz, CDCl3, ppm): 2.212 (s, 3H, H1), 7.109− 7.141 (m, 6H, H2−H3), 7.162−7.177 (d, 4H, H4), 7.218−7.245 (m, 1H, H5), 7.262−7.289 (m, 1H, H6), 7.465−7.479 (d, 4H, H7), 8.133−8.147 (d, 4H, H8). PE-b were prepared following a similar procedure as that of PE-a. We synthesized the polyester by terephthaloyl chloride (TPC, without thioether groups) and BHPPE (PE-R: the

ηinh =

2(ηsp − ln ηr ) C

(1)

Where η r = η/η0, ηsp = η/η0 − 1. The number-average molecular weights (Mn) and weightaverage molecular weight (Mw) were obtained via GPC performed with a Waters 1515 performance liquid chromatography pump. Tetrahydrofuran (THF) was used as an eluent at a flow rate of 1.0 mL/min at 20 °C. Polystyrene standards were used for calibration. The samples PE-(a-b) were measured with an elemental analyzer (EURO EA-3000). FT-IR spectroscopic measurements were performed on a Nexus670 FT-IR instrument. Nuclear magnetic resonance (NMR) spectra were obtained on a Bruker-600 NMR spectrometer in deuterated chloroform or DMSO. X-ray diffraction (XRD) (Philips X’pert Pro MPD) was used to study the polymers’ aggregation structure. The samples were subsequently dried at 100 °C in a vacuum oven for 12 h, then heated at 250 °C for 4 h. 16578

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An Instron Corporation 4302 instrument with a load cell of 10 kg was used to study the stress−strain behavior of the polyesters film samples. Measurements were performed at room temperature with film specimens (0.4 cm wide, 5 cm long, and about 0.06 mm thick), and an average of at least three replicas was used. Dynamic mechanical analysis (DMA) was performed on a TA-Q800 apparatus operating in tensile mode at a frequency of 1 Hz in the temperature ranging from 50 to 300 °C with a heating rate of 5 °C/min. Differential scanning calorimetry (DSC) was performed on a Netzsch DSC 200 PC thermal analysis instrument. The heating rate for the DSC measurements was 10 °C/min. Thermogravimetric analysis (TGA) was performed on a TGA Q500 V6.4 Build 193 thermal analysis instrument under a nitrogen atmosphere with a heating rate of 5, 10, 20, and 40 °C/min. Thermal degradation mechanism: The Pyrolysis/Gas Chromatography/Mass (Py/ GC-MS) spectrometry was conducted to investigated the thermal degradation mechanism of resultant polymers with different pyrolysis temperature of 450 and 600 °C. The samples were pyrolyzed successively at the required temperatures with heating rate of 4 °C/s under nitrogen atmosphere and kept at each temperature for 60 s. The UL-94 vertical test was performed according to the testing procedure of FMVSS 302/ZSO 3975 with a test specimen bar of 127 mm in length, 12.7 mm in width and about 1.27 mm in thickness. In the test, the polymer specimen was subjected to two 10 s ignitions. After the first ignition, the flame was removed and the time for the polymer to self-extinguish (t1) was recorded. Cotton ignition was noted when polymer dripping occurred during the test. After cooling, the second ignition was performed on the same sample and the selfextinguishing time (t2) and dripping characteristics were recorded. The flame test was performed on five specimens. If the average t1 plus t2 is less than 10 s without any dripping, the polymer is considered to be a V-0 material. If t1 plus t2 is in the range of 10−30 s without any dripping, the polymer is considered to be a V-1 material.11 The LOI was determined with an Atlas limiting oxygen index chamber. The solubility of these polyesters in various solvents was tested at room temperature and the boiling point of the solvent.

Figure 1. FT-IR spectra of monomer BHPPE and PE-(a-b).

Figure 2. 1H NMR spectrum of PE-a.

3:2:4:4:1:2:4:4 in theory, but the chemical shift of H2 and H3 were largely overlapped so that they can not be easily separated. The peak near 2.212 ppm in Figure 2 was attributed to the bond of C−H (−CH3). The aromatic protons were observed at 7.109−8.147 ppm. The ratio of corresponding integral curves was H1:(H2− H3):H4:H5:H6:H7:H8 = 3:6:4:1:2:4:4. In combination with the FT-IR results, these seven groups of peaks were ascribed to H1, H2, H3, H4, H5, H6, H7, and H8. The structure of PE-a was also characterized by 13C NMR as shown in Figure 4. The elemental analysis showed that the experimental results were similar to the calculated results. Above all, it indicates that the polyesters are successfully synthesized as shown in Scheme 2. The structures of PE-b were also confirmed by FT-IR, elemental analysis, 1H NMR (Figure 3) and 13C NMR (Figure 4). 3.3. Inherent Viscosity (ηinh) and Molecular Weights of PE-(a-b), PE-R, and U-100. The inherent viscosities and apparent molecular weights of PE-(a-b), PE-R and U-100 measured by GPC are summarized in the Supporting Information, Table 1, the ηinh values were 0.98, 0.87, 0.38, and 1.14 dL/g, repectively. PE-(a-b) showed Mn values in the range of 4.3−5.1 × 104 and Mw values in the range of 0.97−1.04 × 105, respectively. They were close to commercial aromatic polyesters U-100 and higher than PE-R. The higher molecular weight of PE-(a-b) was ascribed to their better solubility in dichloromethane than that of PE-R (PE-R precipitated from the solvent during the polymerization). The polydispersity indices (PDIs) of PE-(a-b) were 2.04 and 2.26, respectively. 3.4. Aggregate Structure of the PE-(a-b) and PE-R. Xray diffraction patterns of PE-(a-b) and PE-R samples that had

3. RESULTS AND DISCUSSION 3.1. Synthesis of Polyesters (PE-(a-b)) and PE-R. The polycondensation reaction of the diacid chloride and BHPPE was conducted by an interfacial polymerization method using BTEAC as the phase transfer catalyst at 30 °C. The polymerization temperature in this procedure could not exceed 40 °C, otherwise the dichloromethane would be evaporated, in which case it was difficult to dissolve the resultant polymer completely. The evaporation of solvent may lead to obtain low molecular weight resin. 3.2. FT-IR and 1H NMR Spectrum of PE-(a-b). The FTIR spectra (Figure 1) of PE-(a-b) shows the characteristic absorptions of ester group (−COO-) around 1735 cm−1 and the -S- group around 1080 cm−1. In comparison with the monomer (BHPPE), the absorption of −OH (near 3300 cm−1) disappeared. This finding suggests that the reaction of hydroxy and carboxyl chloride occurred. Additionally, the characteristic absorptions of aromatic rings near 1590 and 1480 cm−1 and −CH3 near 2970 and 2860 cm−1 were observed. The structure of PE-a was identified by 1H NMR method (Figure 2). Seven groups of peaks appeared in this spectrum. It should have eight group peaks and the ratio of corresponding integral curves was 16579

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amorphous nature of PE-(a-b) and PE-R. That ascribed to the existence of large bulky pendent group. The locomotion of molecular chain was limited by the large pendent group so it could not arrange in order. Compared with PE-(a-b), PE-R showed weak diffraction peaks, where indicates there is some partially ordered structure in PE-R. 3.5. Tensile Properties. The average tensile strengths of the samples of casting film PE-(a-b) and PE-R are given in the Supporting Information, Table 2. The average tensile strengths and the Young’s modulus values of PE-(a-b) and PE-R were 13.5−108.3 MPa and 0.4−2.6 GPa, respectively. It was found that PE-(a-b) had much higher tensile strength than PE-R. That was mainly attributed to the low molecular weight of PE-R. The elongation at break of PE-(a-b) was in the range of 13.2− 17.3%. PE-(a-b) behaved as ductile materials with moderate elongations at break and good tensile strengths as similar as U100. 3.6. Dynamic Mechanical Analysis. DMA was used to characterize PE-(a-b). As shown in Figure 6, the transition

Figure 3. 1H NMR spectrum of PE-b.

Figure 6. Storage modulus and loss modulus curves of PE-(a-b).

behavior could be observed obviously; they are defined as α relaxation. It is well-known that the glass transition temperature (Tg) of polymer can be determined by α relaxation, as it is usually related to the segment movements in the noncrystalline area. The glass temperature of PE-(a-b) was found to be 239.9 and 190.8 °C (PE-R was brittle, it broke off during the testing procedure, and its Tg data could not be obtained.), respectively. They are nearly the same as those obtained by the DSC method. The slight differences are mainly attributed to the different responses of the polymers to these two measurements. PE-a was found to have higher Tg value (239.9 °C) in this series because its lower content of flexible thioether linkage than that of PE-b. As listed in the Supporting Information, Table 2, these polyesters exhibited high storage modulus about 2.2−2.5 and 1.3−1.8 GPa at room temperature and 190 °C, respectively. That indicates these polyesters have good mechanical properties. 3.7. Thermal Properties of PE-(a-b), PE-R, and U-100. The results of DSC and TGA of PE-(a-b) and PE-R are displayed in Figures 7 and 8, respectively. As shown in Figure 7, the Tg values of PE-(a-b) and PE-R were 235.6 °C, 189.8 and 211.2 °C, respectively (see the Supporting Information, Table 1). The DSC curves of PE-(a-b) had no endothermic melting peak. Thus, DSC measurement also reveals that the amorphous nature of PE-(a-b). It suggests that the result of DSC is consistent with that of X-ray diffraction. In particular, PE-a had

Figure 4. 13C NMR spectra of PE-(a-b).

been annealed at 250 °C are shown in Figure 5. PE-(a-b) and PE-R showed no obvious crystalline peaks. That suggests the

Figure 5. XRD patterns of PE-(a-b) and PE-R. 16580

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rate were analysized. This method can be used to determine the activation energy of solid-state reactions from plots of the logarithm of the heating rate versus the inverse of the temperature at the maximum reaction rate in constant heating rate experiments,29 which does not need a precise knowledge of the reaction mechanism, using the following eq 2 ⎛ β ⎞ ln⎜ 2 ⎟ = ⎝ Tmax ⎠

{ln ARE + ln[n(1 − α

} − RTE

n−1 ] max )

max

(2)

where β is the heating rate, Tmax is the temperature corresponding to the inflection point of the thermal degradation curves at the maximum reaction rate, A is the pre-exponential factor, αmax is the maximum conversion, and n is the reaction order. The activation energy E can be calculated from the slope from the plot of ln(β/T2max) versus 1000/Tmax and fitting to a straight line. The thermal degradation curves obtained at different heating rate: 5, 10, 20, 40 and °C/min are shown in Figure 9. The values of initial decomposition

Figure 7. DSC curves of PE-(a-b) and PE-R at a heating rate of 10 °C/min in N2.

Figure 8. TGA curves of PE-(a-b) and PE-R at a heating rate of 10 °C/min in N2.

Figure 9. TGA curves of PE-a at a heating rate of 5, 10, 20, and 40 °C/ min in N2.

higher Tg than that of PE-b, PE-R, and U-100. It was almost 40 °C higher than that of U-100 and PE-b. The main reason is that the existence of high content of flexible thioether (-S-) linkage in the backbone of PE-b. During the experiment, an interesting phenomenon was discovered. Although the flexible unit (-S-) was incorporated into the main chain of PE-a, it had higher glass transition temperature than U-100 and PE-R (The similar phenomenon was observed in our earlier reports28). The exact reason of this phenomenon needs to further study. As shown in Figure 8, the initial degradation temperatures of PE-(a-b) and in PE-R nitrogen (T5%‑nitrogen) were 450, 454, and 390 °C, respectively. PE-(a-b) showed higher thermal stability than commercial product U-100. The char yield of PE-(a-b) at 800 °C in nitrogen was about 40%, whereas PE-R remained about 10% (see the Supporting Information, Table 1). The DSC and TGA result indicate that the introduction of flexible thioether (-S-) is beneficial for their thermal properties. The main reason was that the flexible thioether can improve the solubility of these polyesters. It was then beneficial for synthesis of highmolecular-weight resins. 3.8. Thermal Degradation Kinetics. The thermal decomposition behavior of these polyesters was studied with two kinds of methods: (1) differential method (Kissinger method): It is the basis of the most powerful methods for the determination of kinetic parameters. The changes in thermogravimetic data, which caused by variation of heating

temperature (Ti) and the temperature of the maximum rate (Tmax) are shown in the Supporting Information, Table 3. Figure 10 shows the curves of Kissinger method applied to experimental data at different heating rates. The activation energy (as shown in the Supporting Information, Table 4) of

Figure 10. Kissinger method applied to experimental data at a heating rate of 5, 10, 20, and 40 °C/min of PE-(a-b). 16581

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Figure 11. −log β vs 1000/T plots of PE-(a-b) at various conversions (5, 10, 20, 30, 40, and 50%).

Figure 12. Thermal decomposition pathway for PE-(a-b).

ing to 30% conversion was found to be very close to the value obtained using Kissinger’s method. The correlation coefficient was very close to 1, it suggests that the regression equation is fitting linear good. PE-b containing high content thioether linkage was found to have lower activation energy than that of PE-a. The reason for that may be attributed to that PE-b have a lower molecular weight and larger polydispersity indices or larger content of thioether than that of PE-a. 3.9. Thermal Decomposition Mechanism. To investigate the thermal decomposition mechanism of these polyesters, we studied the decomposition product pyrolysized at 450 and 600 °C in detail with Py/GC-MS. From the Supporting Information, Table 6, the initial pyrolysis stage of PE-(a-b) (at 450 °C) was found to contain six kinds of decomposition compounds such as: benzene, toluene, phenol, 4-methylphenol, diphenylmethane, and aromatic diacid (PE-a: 4, 4′-thiobenzoic acid, PE-b: 4,4′-Bis(4-carboxyphenylthio)benzene). This pyrolysis mechanism was mainly attributed to the main chain degradation and further degradation of diphenol units. The number of species of degradation product of PE-(ab) was as much as 10 and 15 when the pyrolysis temperature was increased to 600 °C. The aromatic diacid and diphenol decomposed into much smaller pieces with increase of pyrolysis temperature. The decomposition products contained some compounds with

the decomposition of PE-(a-b) was calculated from a straight line fit of a plot of ln(β/T2max) versus 1000/Tmax. The value obtained from Figure 10 for the activation energy (E) of PE-(ab) was 160.3 and 156.6 kJ/mol, respectively. The reaction order (n) can be calculated by the peak value of TGA secondary derivative curves. Then we can obtain the pre-exponential factor (ln A) value from eq 2. (2) Integral method (Flynn-Wall-Ozawa):30,31 ⎡ AE ⎤ E log β = log⎢ ⎥ − 2.315 − 0.4567 RT ⎣ g (α )R ⎦

(3)

where β is the heating rate, E is the activation energy, R is the ideal gas constant and g(α) is the conversion dependent term. It is used to determine the activation energy for given values of conversion. The activation energy for different conversion values can be calculated from a −logβ versus 1000/T plot. From the Doyle approximation, conversions values of 5, 10, 20, 30, 40, and 50% were used. The results of the Flynn−Wall− Ozawa analysis are given in Figure 11, which shows that the best fitting straight lines are nearly parallel, indicating a constant activation energy range of different conversions and confirming the validity of the approach used. Activation energies and correlation coefficient (R) corresponding to the different conversions are listed in the Supporting Information, Table 5. From these values, the activation energy correspond16582

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sulfur compared with the first pyrolysis step such as: hydrogen sulfide, benzenethiol, diphenyl sulfide, diphenyldisulfide, and so on. In addition, the pyrolysis compounds of PE-b appeared 1,3benzenedithiol, 1,4-benzenedithiol, and 4-(phenylthio)benzenthiol. Then 4-(phenylthio)benzenthiol can further isomerize and cyclize into 3-(phenylthio)benzenthiol and dibenzothiophene under high temperature as discribed in Figure 12. 3.10. Flame-Retardancy. The LOI values of PE-(a-b) and U-100 are summarized in Supporting Information, Table 7, they were 37, 39, and 37, respectively. It indicates that the resultant polyesters have almost same LOI values with commercial polyesters such as U-100. And PE-b was found to have a larger LOI value than PE-a. It suggests that the presence of -S- in the polymer chain can efficiently increase the resin’s LOI values. The Supporting Information, Table 7, also lists the UL-94 data for PE-(a-b) and U-100. The flame-retardancy of PE-(a-b) can reach V-0 grade, whereas U-100 can only reach V2 grade. It also indicates that the introduction of thioether unit into the polyester main chain is beneficial for its flameretardancy. The main reason is that the combustion of PE-(a-b) releases some nonflammable gas such as hydrogen sulfide and other aromatic compounds containing sulfur. The samples may then be segregated from air. 3.11. Solubility. The solubilities of PE-(a-b), PE-R, and U100 are summarized in the Supportign Information, Table 8. Compared with PE-R, the resultant polyesters (PE-(a-b)) showed a relatively good solubility. The solubility of PE-(a-b) was almost the same as U-100. They were found to be soluble in THF, CHCl3, CH2Cl2, NMP, and concentrated sulfuric acid. Their good solubility is attributed to the loose packed structures. The introduction of bulky pendent phenyl and thioether in the repeating unit play an important role to increase free volume in the polymer chains and decrease packing density so as to allow much solvent to come in (the density of PE-(a-b) was 1.18 and 1.17 g/cm3, respectively. It was slightly lower than PE-R (1.20 g/cm3)). Thus, these aromatic polyesters can be processed into products by cast, spin-, or dip-coating for their excellent solubility.

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ASSOCIATED CONTENT

S Supporting Information *

Tables S1−S8 as mentioned in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax: 86-28-8541-2866. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by research grants from the Youth Fund of Sichuan University (Grant 2012SCU11009). REFERENCES

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4. CONCLUSIONS Two kinds of easily soluble aromatic polyesters were synthesized from 1,1-bis(4-hydroxyphenyl)-1-phenylethane and TDC (or BPBDC). The introduction of bulky pendent phenyl and thioether unit increases free volume in the polymer chains and decreased packing density. Thus, the resultant polyesters showed good solubility, and were readily soluble in a wide range of organic solvents. Moreover, the bulky pendent phenyls and thioether unit do good for preserving the thermal stability of the polyesters, the glass transition temperatures of PE-(a-b) ranged from 189 to 235 °C, and the initial decomposition temperatures in nitrogen are all above 450 °C and char yields at 800 °C are about 40%. Also the aromatic polyesters containing thioether unit were found to be inherent flame retardant. Their flame retardancy can reach V-0 (UL 94). From the thermal degradation kinetics study, the thermal decomposition activation energy of PE-(a-b) was found to be about 160 KJ/mol, and the thermal degradation procedure could be divided into two steps associated with the pyrolysis of aromatic diphenol and diacid. 16583

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