Thermal transition behaviors, solubility, and mechanical properties of wholly aromatic para-, meta-poly(ether-amide)s: effect on numbers of para-aryl ether linkages

Abstract

In order to make clear how the numbers of para-aryl ether linkages affect the solubility, thermal transition behavior and mechanical properties of para-(p-PEAs), meta-poly(ether-amide)s (m-PEAs), a series of para-substituted aryl ether diamine monomers with 2, 3, 4 or 5 ether linkages were synthesized, and then reacted with terephthalic acid and isophthalic acid via the Yamazaki–Higashi phosphorylation method to prepare wholly aromatic PEAs. The PEAs with intrinsic viscosity values in the range of 0.81–1.16 dL g⁻¹ were obtained and characterized by FT-IR, NMR, and elemental analysis. Although the T_gs and T_ms of PEAs decrease with the increasing numbers of aryl ether linkages, their T_gs and T_ms still are above 180 °C and 320 °C, respectively. p-PEAs have a high crystallinity making it difficult to dissolve them in common organic solvents even when 5 aryl ether linkages are introduced. Conversely, except for m-PEA5 with a lower polarity, the other m-PEAs show good solubility due to their low crystallinity. Besides, the decomposition temperatures at 5% weight loss of all PEAs exceed 430 °C and their residual yields at 700 °C are above 50% in N₂, illustrating that they have excellent thermal stability. It is found that by casting m-PEAs DMAc solution in a dish, the transparent films with high Young's modulus (2.6–3.5 GPa) can be successfully obtained.

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Introduction

Wholly aromatic polyamides are synthetic polyamides in which at least 85% of the amide groups are connected directly to two aromatic rings, and they are well known as high-performance materials due to their superior thermal, mechanical, chemical properties and flame resistance.^1–4 The best known commercial aromatic polyamides are poly(p-phenylene terephthalamide) (PPPT) and poly(m-phenylene isophthalamide) (PMPI) synthesized by a low-temperature solution method with aromatic diacid dichlorides and diamines. Both of them can be transformed upon solution by wet spinning into flame-resistant, cut-resistant, and high tensile strength synthetic fibers as advantageous replacements for metals or ceramics in the aerospace and armament industry.^5,6 The wholly aromatic structure endow the stiff rod-like polymer backbones with a high cohesive energy and a high crystallization tendency due to their very favorable intermolecular hydrogen bonds. However, it makes them difficult to dissolve in common organic solvents. Besides this, often high processing temperature should be used. Both of these limit their wide application.

Many efforts have been made to improve the solubility and processability of aromatic polyamides via structural modification of diacids or diamines in aromatic backbone without a dramatic loss in the chemical, thermal, and mechanical properties.⁷ The key point is diminishing the cohesive energy of polymers through lowering the inter-chain interactions. Many chemical modifications such as the introduction of flexible linkages,^8–11 pendant substituents,^12–17 asymmetric moieties,^18–20 and copolymerization with small amounts of a third monomer have been reported.^21,22 In these methods, introducing para-aryl ether bonds as flexible linkages into the backbone of aromatic polyamides is an easy and popular approach, and the obtained polyamides show increasing solubility and processability.^23–33 Unfortunately, it was found that only incorporating para-aryl ether bonds into the aromatic polyamides' main-chain, even 3 ether bonds was introduced, these polyamides still was insoluble in organic solvents such as NMP or DMAc if no lithium chloride was added.²⁶ How about more para-aryl ether linkages? If 4 or 5 para-aryl ether bonds was introduced, how about the solubility of these obtained aromatic polyamides? Moreover, some literatures^7,9,10,20,26 show that when diamines react with isophthalic acid (IPA), the acquired aromatic polyamides present better solubility in polar organic solvents. If we incorporating more para-aryl ether bonds into meta-aromatic polyamides' main-chain, what effect it will bring?

In order to address the above issues, four kinds of diamines (2-ODA, 3-ODA, 4-ODA, 5-ODA) with the number of para-aryl ether linkages from 2 to 5 were synthesized firstly, and then they polymerized with terephthalic acid TPA, or isophthalic acid IPA, respectively, to synthesize two series of aromatic poly(ether-amide)s (PEAs) (p-PEAs and m-PEAs). A comparative study is performed to investigate the influence of para-aryl ether linkages numbers on the thermal stability, solubility, mechanical properties of the obtained p-PEAs and m-PEAs. Via this investigation, the relationships between the numbers of para-aryl ether linkages and the properties of p-PEAs and m-PEAs should be revealed.

Experimental

Materials

Terephthalic acid (TPA) was supplied by Jinan Chemical Fiber Co., Ltd. (Jinan, China). Isophthalic acid (IPA), hydroquinone, K₂CO₃ and triphenyl phosphite (TPP) were purchased from Chendu Kelong Chemical Co., Ltd. 4,4′-Dihydroxydiphenyl ether was purchased from Wuxi Yangshi Chemical Co., Ltd. 4,4′-Diaminodiphenyl ether (ODA), 4-chloronitrobenzene, p-fluoroacetophenone, 3-chloroperbenzoic acid, 5% Pd/C and lithium chloride were purchased from Aladdin Industrial Co., Ltd. N-Methyl-2-pyrrolidinone (NMP), N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), and pyridine were dehydrated with 4 Å molecular sieves prior to use. All other materials were used as received.

Monomer synthesis

1,4-Bis(4-nitrophenoxy)benzene (2-NO₂). 5.5 g (0.05 mol) hydroquinone (1), 14.2 g (0.09 mol) 4-chloronitrobenzene, 13.8 g (0.10 mol) K₂CO₃ and 150 mL N,N-dimethylformamide (DMF) were added in a 250 mL three-necked flask equipped with a nitrogen inlet. This mixture was stirred and heated at 150 °C for 20 h. After the dark reaction mixture cooled into room temperature, the mixture was poured into 300 mL water and the precipitated yellow solid product was collected by filtration, washed with water and dried in a vacuum oven. Yield 14.6 g (92%); mp: 238–240 °C (onset to the peak top temperature) according to differential scanning calorimetry (DSC) at a scan rate of 10 °C min⁻¹, lit.³⁴ 230–240 °C. IR (KBr): ν_max 1483 cm⁻¹ (Ar C–H stretch), 1341 cm⁻¹ (–NO₂ stretch), 1234, 1186, 1105, 1007 cm⁻¹ (Ar-O–C stretch), 872, 848 cm⁻¹ (Ar 1,4-para substituted). ¹H NMR (DMSO-d6) δ (ppm) 8.35–8.21 (m, 4H), 7.34 (s, 4H), 7.26–7.16 (m, 4H).

1,4-Bis(4-aminophenoxy)benzene (2-ODA). 5.0 g 2-NO₂, 0.25 g 5% Pd/C and 60 mL ethanol were added in a 250 mL three-necked flask and heated in 50 °C for 0.5 h. Then 10 mL hydrazine monohydrate was added dropwise into the mixture in 30 min with a constant pressure funnel. After complete addition, the mixture was heated at reflux temperature for 5 h and the hot solution was filtered to remove the Pd/C catalyst. The filtrate was collected and the product was obtained by cooling from ethanol as light yellow crystals. Yield 3.6 g (90%); mp: 176–179 °C, lit.³⁵ 170–180 °C. IR (KBr): ν_max 3398, 3313, 3220 cm⁻¹ (–NH₂ stretch), 1497 cm⁻¹ (Ar C–H stretch), 1216, 1109, 1077 cm⁻¹ (Ar-O–C stretch), 844, 821 cm⁻¹ (Ar 1,4-para substituted). ¹H NMR (DMSO-d6) δ (ppm) 6.82 (s, 4H, H_d), 6.79–6.67 (m, 4H, H_c), 6.66–6.48 (m, 4H, H_b), 4.98 (s, 4H, H_a). ¹³C NMR (DMSO-d6) δ (ppm) 153.86 (C_e), 147.03 (C_d), 145.45 (C_a), 120.70 (C_b), 118.52 (C_f), 115.36 (C_c). Elem. anal. calcd: C, 73.97%; H, 5.48%; N, 9.59%; O, 10.96%. Found: C, 73.99%; H, 5.44%; N, 9.47; O, 11.10%.

Bis[4-(4-nitrophenoxy)phenyl]ether (3-NO₂). Its synthetic procedure is similar to that of 2-NO₂ by using 4,4′-dihydroxydiphenyl (2) ether instead of hydroquinone (1). Yellow solid was obtained by filtration, washed with water and dried in a vacuum oven. Yield 18.0 g (90%); mp: 148–149 °C, lit.³⁶ 148–152 °C. IR (KBr): ν_max 1485 cm⁻¹ (Ar C–H stretch), 1342 cm⁻¹ (–NO₂ stretch), 1224, 1187, 1108 cm⁻¹ (Ar-O–C stretch), 862, 843 cm⁻¹ (Ar 1,4-para substituted). ¹H NMR (DMSO-d6) δ (ppm) 8.32–8.20 (m, 4H), 7.32–7.22 (m, 4H), 7.22–7.11 (m, 8H).

Bis[4-(4-aminophenoxy)phenyl]ether (3-ODA). Its synthetic procedure is also similar to that of 2-ODA. Off-white crystals were obtained from the cooled filtrate. Yield 3.7 g (85%); mp: 125 °C, lit.³⁶ 126–128 °C. IR (KBr): ν_max 3380, 3313 cm⁻¹ (–NH₂ stretch), 1501 cm⁻¹ (Ar C–H stretch), 1237, 1191, 1098 cm⁻¹ (Ar-O–C stretch), 869, 827 cm⁻¹ (Ar 1,4-para substituted). ¹H NMR (DMSO-d6) δ (ppm) 7.00–6.90 (m, 4H, H_e), 6.90–6.82 (m, 4H, H_d), 6.82–6.70 (m, 4H, H_c), 6.67–6.51 (m, 4H, H_b), 5.09 (s, 4H, H_a). ¹³C NMR (DMSO-d6) δ (ppm) 154.84 (C_h), 152.19 (C_e), 146.78 (C_d), 145.45 (C_a), 120.92 (C_b), 120.06 (C_f), 118.52 (C_g), 115.47 (C_c). Elem. anal. calcd: C, 75.00%; H, 5.21%; N, 7.29%; O, 12.50%. Found: C, 74.82%; H, 5.02%; N, 7.13; O, 13.03%.

1,4-Bis(p-acetylphenoxy)benzene (2-ketone). 5.5 g (0.05 mol) hydroquinone, 13.8 g (0.10 mol) p-fluoroacetophenone, 13.8 g (0.10 mol) K₂CO₃ and 150 mL N,N-dimethylacetamide (DMAc) were added in a 250 mL three-necked flask equipped with a nitrogen inlet. This mixture was heated at reflux temperature for 5 h. After cooling, the mixture was poured into 300 mL water and the precipitated brown solid product was collected by filtration, recrystallized and dried in a vacuum oven. Yield 16.5 g (95%); mp: 180 °C, lit.³⁷ 179–181 °C. IR (KBr): ν_max 1672 cm⁻¹ (C=O stretch), 1497 cm⁻¹ (Ar C–H stretch), 1233, 1190, 1110, 1015 cm⁻¹ (Ar-O–C stretch), 873, 842 cm⁻¹ (Ar 1,4-para substituted). ¹H NMR (CDCl₃) δ (ppm) 8.10–7.86 (m, 4H), 7.12 (s, 4H), 7.09–6.98 (m, 4H), 2.60 (s, 6H).

(1,4-Phenylenebis(oxy))bis(4,1-phenylene)diacetate (2-ester). 13.8 g (0.04 mol) 2-ketone and 20.3 g (0.10 mol) 85% m-CPBA was dissolved in 100 mL CHCl₃. This mixture was stirred under reflux for 5 h and white solid was removed by filtration. The filtrate was washed with NaHSO₃ (100 mL), NaHCO₃ (2 × 100 mL), and water (100 mL). The solvent was removed by distillation, and the crude product was recrystallized from isopropanol. Light yellow crystals were obtained, washed and dried in a vacuum oven. Yield 12.4 g (82%); mp: 113 °C, lit.³⁷ 110–113 °C. IR (KBr): ν_max 1754 cm⁻¹ (C=O stretch), 1489 cm⁻¹ (Ar C–H stretch), 1220, 1186, 1100, 1012 cm⁻¹ (Ar-O–C stretch), 848, 819 cm⁻¹ (Ar 1,4-para substituted). ¹H NMR (CDCl₃) δ (ppm) 7.19–6.84 (m, 12H), 2.32 (s, 6H).

1,4-Bis(p-hydroxyphenoxy)benzene (3). 7.6 g (0.02 mol) 2-ester was dissolved in 100 mL methanol and 0.5 M KOH/methanol solution 10 mL was added dropwise into the mixture. This mixture was stirred under reflux for 1 h. The solvent was removed by distillation, and the crude product was treated with a 1 M HCl solution. White solid was obtained by filtration, washed and dried in a vacuum oven. Yield 4.7 g (80%); mp: 193 °C, lit.³⁷ 190.5–192 °C. IR (KBr): ν_max 3285 cm⁻¹ (–OH stretch), 1502 cm⁻¹ (Ar C–H stretch), 1217, 1098, 1009 cm⁻¹ (Ar-O–C stretch), 874, 830 cm⁻¹ (Ar 1,4-para substituted). ¹H NMR (DMSO-d6) δ (ppm) 9.30 (s, 2H), 6.88 (d, J = 4.7 Hz, 4H), 6.87–6.82 (m, 4H), 6.78–6.73 (m, 4H).

Bis[p-(p-acetylphenoxy)phenyl]ether (3-ketone). Its synthetic procedure is similar to that of 2-ketone by using 4,4′-dihydroxydiphenyl (2) ether instead of hydroquinone (1). Beige solid product was obtained, recrystallized and dried in a vacuum oven. Yield 19.9 g (90%); mp: 175 °C, lit.³⁷ 179–180 °C. IR (KBr): ν_max 1678 cm⁻¹ (C=O stretch), 1500 cm⁻¹ (Ar C–H stretch), 1243, 1104, 1009 cm⁻¹ (Ar-O–C stretch), 868, 826 cm⁻¹ (Ar 1,4-para substituted). ¹H NMR (CDCl₃) δ (ppm) 8.05–7.88 (m, 4H), 7.09 (s, 4H), 7.05–7.00 (m, 8H), 2.59 (s, 6H).

[(Oxybis(4,1-phenylene)]is(oxy))bis(4,1-phenylene)diace-tate (3-ester). Its synthetic procedure is similar to that of 2-ester. Light yellow crystals were obtained, washed and dried in a vacuum oven. Yield 14.1 g (75%); mp: 149 °C, lit.³⁷ 153–154 °C. IR (KBr): ν_max 1760 cm⁻¹ (C=O stretch), 1500 cm⁻¹ (Ar C–H stretch), 1224, 1193, 1100, 1013 cm⁻¹ (Ar-O–C stretch), 827 cm⁻¹ (Ar 1,4-para substituted). ¹H NMR (CDCl₃) δ (ppm) 7.24–6.85 (m, 16H), 2.32 (s, 6H).

4,4′-Bis(p-hydroxyphenoxy)diphenyl ether (4). Its synthetic procedure is similar to that of (3). White solid was obtained by filtration, washed and dried in a vacuum oven. Yield 6.6 g (85%); mp: 214 °C, lit.³⁷ 214–215 °C. IR (KBr): ν_max 3321 cm⁻¹ (–OH stretch), 1500 cm⁻¹ (Ar C–H stretch), 1217, 1098, 1007 cm⁻¹ (Ar-O–C stretch), 828 cm⁻¹ (Ar 1,4-para substituted). ¹H NMR (DMSO-d6) δ (ppm) 9.38 (s, 2H), 7.02–6.94 (m, 4H), 6.94–6.83 (m, 8H), 6.83–6.74 (m, 4H).

1,4-Bis[4-(4-nitrophenoxy)phenoxy]benzene (4-NO₂). Its synthetic procedure is similar to that of 2-NO₂. Yellow solid was obtained by filtration, washed with water and dried in a vacuum oven. Yield 22.9 g (95%); mp: 140 °C. IR (KBr): ν_max 1485 cm⁻¹ (Ar C–H stretch), 1342 cm⁻¹ (–NO₂ stretch), 1218, 1191, 1107 cm⁻¹ (Ar-O–C stretch), 864, 840 cm⁻¹ (Ar 1,4-para substituted). ¹H NMR (DMSO-d6) δ (ppm) 8.31–8.21 (m, 4H), 7.32–7.19 (m, 4H), 7.19–7.08 (m, 12H).

1,4-Bis[4-(4-aminophenoxy)phenoxy]benzene (4-ODA). 5.0 g 4-NO₂, 0.25 g 5% Pd/C and 100 mL DMF were added in a in a 250 mL three-necked flask. Then 10 mL hydrazine monohydrate was added dropwise into the mixture in 30 min with a constant pressure funnel. After complete addition, the mixture was heated at reflux temperature for 5 h and the hot solution was filtered to remove the Pd/C catalyst. The filtrate was collected and the product was obtained by cooling from DMF as yellow crystals. Yield 3.3 g (75%); mp: 172 °C. IR (KBr): ν_max 3381, 3314 cm⁻¹ (–NH₂ stretch), 1498 cm⁻¹ (Ar C–H stretch), 1223, 1192, 1097, 1008 cm⁻¹ (Ar-O–C stretch), 827 cm⁻¹ (Ar 1,4-para substituted). ¹H NMR (DMSO-d6) δ (ppm) (dd, J = 6.9 Hz, 8H, H_e–f), 6.92–6.82 (m, 4H, H_d), 6.80–6.71 (m, 4H, H_c), 6.64–6.52 (m, 4H, H_b), 4.97 (s, 4H, H_a). ¹³C NMR (DMSO-d6) δ (ppm) 155.11 (C_i), 153.23 (C_h), 151.80 (C_e), 146.52 (C_d), 145.81 (C_a), 121.00 (C_b), 120.38 (C_f), 119.98 (C_j), 118.47 (C_g), 115.31 (C_c). Elem. anal. calcd: C, 75.63%; H, 5.04%; N, 5.88%; O, 13.45%. Found: C, 75.50%; H, 5.15%; N, 5.18; O, 14.17%.

4,4′-Oxybis[(4-(4-nitrophenoxy)phenoxy)]benzene (5-NO₂). Its synthetic procedure is similar to that of 4-NO₂. Yellow solid was obtained by filtration, washed with water and dried in a vacuum oven. Yield 27.1 g (95%); mp: 164–165 °C. IR (KBr): ν_max 1484 cm⁻¹ (Ar C–H stretch), 1343 cm⁻¹ (–NO₂ stretch), 1214, 1191, 1106, 1007 cm⁻¹ (Ar-O–C stretch), 857, 822 cm⁻¹ (Ar 1,4-para substituted). ¹H NMR (DMSO-d6) δ (ppm) 8.32–8.20 (m, 4H), 7.29–7.19 (m, 4H), 7.19–7.06 (m, 16H).

4,4′-Oxybis[(4-(4-aminophenoxy)phenoxy)]benzene (5-ODA). Its synthetic procedure is similar to that of 4-ODA. Yellow crystals were obtained from cooling DMF by filtration, washed with water and dried in a vacuum oven. Yield 3.5 g (77%); mp: 185 °C. IR (KBr): ν_max 3381, 3314 cm⁻¹ (–NH₂ stretch), 1499 cm⁻¹ (Ar C–H stretch), 1225, 1194, 1099, 1008 cm⁻¹ (Ar-O–C stretch), 826 cm⁻¹ (Ar 1,4-para substituted). ¹H NMR (DMSO-d6) δ (ppm) 7.16–6.92 (m, 12H, H_e–g), 6.92–6.81 (m, 4H, H_d), 6.82–6.70 (m, 4H, H_c), 6.67–6.48 (m, 4H, H_b), 4.97 (s, 4H, H_a). ¹³C NMR (DMSO-d6) δ (ppm) 155.18 (C_l), 153.49 (C_i), 152.89 (C_h), 151.75 (C_e), 146.54 (C_d), 145.83 (C_a), 121.02 (C_b), 120.46 (C_f), 120.34 (C_j), 119.99 (C_k), 118.49 (C_g), 115.33 (C_c). Elem. anal. calcd: C, 76.06%; H, 4.93%; N, 4.93%; O, 14.08%. Found: C, 75.58%; H, 5.14%; N, 4.78; O, 14.50%.

Polymer synthesis

Two series of PEAs were prepared by the direct polycondensation reaction of diacid with the various synthesized aromatic diamines (ODA, 2-ODA, 3-ODA, 4-ODA, 5-ODA). The polymers based on terephthalic acid were named polyterephthalamide (p-PEA), and based on isophthalic acid were named polyisophthalamide (m-PEA). A typical synthetic procedure for p-PEA1 is shown as follows.

A three-necked flask equipped with a mechanical stirrer and a nitrogen inlet was charged with 1.66 g (0.01 mol) TPA, 2.00 g (0.01 mol) ODA, 3.00 g LiCl, 6.83 g (0.022 mol) TPP, 10 mL pyridine and 60 mL NMP. The mixture was heated at 100 °C for 6 h with stirring. After cooling, this solution was poured slowly into 300 mL stirring methanol and the fiber-like precipitates were collected by filtration. Finally, the precipitates were washed with water and dried in a vacuum oven. Yield 3.3 g (95%); IR (KBr): ν_max 3431, 3041 cm⁻¹ (–NH stretch), 1656 cm⁻¹ (C=O stretch), 1207, 1099 cm⁻¹ (Ar-O–C stretch), 829 cm⁻¹ (Ar 1,4-para substituted). ¹H NMR (DMSO-d6/LiCl) δ (ppm) 10.47 (s, 2H, NH), 8.11 (s, 4H, H_a), 7.82 (d, J = 9.1 Hz, 4H, H_b), 7.06 (d, J = 5.3, 4H, H_c).

Polymer membrane preparation

A solution of m-PEAs was prepared by dissolving 0.50 g polymer in 10 mL DMAc. For p-PEAs, 5 wt% lithium chloride was added into the DMAc to obtain homogeneous solution. The solution was cast onto a Petri dish and placed in an oven (75 °C) overnight to evaporate the solvent. The obtained polymer membranes with the thickness of about 30–50 μm were used for tensile tests, thermal analysis, and transparency detection.

Characterization

The chemical structures of the products were confirmed by NMR and FTIR spectroscopy. The spectra were recorded using a Bruker AVANCE AVII-400 NMR instruments with tetramethylsilane as the internal reference. Fourier transform infrared spectra were collected using a Nicolet 6700 Spectrometer using KBr pellets.

The intrinsic viscosities of the PEAs were determined by an Ubbelohde viscometer with a concentration of 0.5 g dL⁻¹ at 30 °C in concentrated sulfuric acid.

Thermal transition temperatures were determined using a TA Q200 differential scanning calorimeter (DSC) under nitrogen atmosphere in a flow of 50 mL min⁻¹, using a heating and cooling rate of 10 °C min⁻¹. The modulated differential scanning calorimeter (MDSC) analyses were carried out under nitrogen atmosphere using amplitude of temperature modulation of ±1 °C and a period modulation of 60 s.

Wide-angle X-ray diffraction (WAXD) patterns were recorded on a Philips X'Pert X-ray diffractometer, using Ni-filtered copper Kα radiation and scattering angle (2θ) rang of 10–45°.

The solubility of PEAs was examined in various organic solvents such as CHCl₃, THF, DMF, DMSO, NMP, and so on at room temperature with a concentration of 5 g dL⁻¹.

Thermogravimetric studies (TGA) were conducted with a NETZSCH 209 F1 under nitrogen or air atmosphere at a heating rate of 10 °C min⁻¹.

Tensile properties were performed using Universal Testing Machine (SANS CMT4104, SHENZHEN SANS Testing Machine Co., Ltd), according to GB/T 1040.3-2006 at a speed of 5 mm min⁻¹. The thickness of films was tested by a Vernier caliper.

The transmittance of various PEAs membranes were analyzed by HITACHI U-1900 spectrophotometer (VARIAN, USA).

Results and discussion

Monomer synthesis

The synthesis routes for para-substituted aryl ether diamines are shown in Scheme 1. 2-ODA and 3-ODA were prepared through two steps (I and II) using conventional chlorine displacement followed by the Pd/C catalytic reduction of the obtained dinitro intermediate to the desired diamine monomer. 4-ODA and 5-ODA were prepared by three steps (III, IV and V)³⁷ to obtain the para-substituted bisphenols intermediates (3, 4) and then DMF was used as a solvent in II* step instead of ethanol. The ¹H NMR and ¹³C NMR spectra of aryl ether diamines are shown in Fig. 1. Their spectra are quite similar to each other because of their structural similarity. For all diamines, the chemical shifts of amino group proton are around 5.0 ppm, and the chemical shifts of phenyl group proton are observed in the range of 6.5–7.0 ppm. Besides, the chemical shifts of phenyl group carbon are distributed into two parts in the range of 115.0–125.0 ppm and 145.0–155.0 ppm due to the double substitution. The element analysis data are almost as same as the calculated ones. All the above results demonstrate that the target products have been synthesized successfully.

Scheme 1 Synthesis route of para-substituted aryl ether diamines.

Fig. 1 ¹H NMR and ¹³C NMR spectra of para-substituted aryl ether diamines.

Polymer synthesis

Two series aromatic PEAs were prepared via direct polycondensation of diacid and various synthesized aromatic diamines in NMP according to Yamazaki–Higashi method (shown in Scheme 2). Clear and homogeneous polymer solutions can be obtained in the presence of a pinch of lithium chloride in the solution. After pouring into methanol with stirring, the fiber-like precipitates were obtained with the intrinsic viscosity values in the range of 0.81–1.16 dL g⁻¹ (shown in Table 1). The typical ¹H NMR spectra of p-PEA1, p-PEA4, m-PEA1 and m-PEA4 are shown in Fig. 2. For all PEAs, the chemical shifts of amide group proton are around 10.5 ppm and the chemical shifts of phenyl ether group proton are divided into two parts in the range of 7.0–7.2 ppm and 7.7–7.9 ppm. For m-PEAs, the chemical shifts of phenyl group of diacid proton (H_a, H_b, H_c) are distributed into three parts in the range of 7.6–7.7 ppm, 8.1–8.2 ppm and 8.4–8.5 ppm. For p-PEAs, their chemical shifts of phenyl group of diacid proton (H_a) are only in the range of 8.1–8.2 ppm. The FTIR spectra of PEAs are given in the ESI (shown in Fig. S1†). Not surprisingly, their spectra are quite similar. The broad absorption peak around 3300 cm⁻¹ are assigned to the N–H stretching of amide group and the absorptions near 1650 cm⁻¹ and 1220 cm⁻¹ are assigned to C=O and C–O–C vibrations respectively. Both NMR and FT-IR results illustrate that via Yamazaki–Higashi method these two series aromatic PEAs can be obtained as expected.

Scheme 2 Synthesis procedures for p-PEAs and m-PEAs.

Table 1 DSC data and intrinsic viscosity of PEAs

Sample	[η]^a (dL g^−1)	Tg/°C	Tm/°C	Sample	[η]^a (dL g^−1)	Tg/°C	Tm/°C
a Measured in H2SO4 (0.5 g dL^−1) at 30 °C.b Decomposes before melting.c Measured by MDSC.
p-PEA1	0.86	—	>500^b	m-PEA1	1.09	266^c	423
p-PEA2	1.03	—	480	m-PEA2	1.16	234^c	399
p-PEA3	1.06	—	443	m-PEA3	0.81	214^c	365
p-PEA4	0.83	—	440	m-PEA4	0.91	194	356
p-PEA5	0.85	—	428	m-PEA5	0.87	186	328

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Fig. 2 ¹H NMR spectra of p-PEA1, p-PEA4, m-PEA1 and m-PEA4.

DSC and WAXD analysis

The glass transition temperature and melting behavior of PEAs were evaluated by DSC. Typical DSC curves of p-PEA1, p-PEA4, m-PEA1 and m-PEA4 are shown in Fig. 3a and b, and the obtained data are summarized in Table 1. Except for p-PEA1, all PEAs have a melting endothermic peak on the first heating scans before 500 °C. The melting temperatures of p-PEAs are higher than that of m-PEAs because of stronger inter-chain interactions and chain packing. Particularly, p-PEA4 has a shoulder at lower temperature in the T_m value because the incomplete crystal melts on lower temperature at first. m-PEA4 shows two peaks in the T_m value because it has two kinds of crystal forms demonstrated by its WAXD pattern (shown in Fig. 4). Their T_ms and T_gs values dependence of the numbers of aryl ether linkages are presented in Fig. 3c and d. From the figure we can see that with the increasing the numbers of aryl ether linkages, their T_m is decreased from 480 °C to 428 °C for p-PEAs, and 423 °C to 328 °C for m-PEAs, respectively. Simultaneously, the T_g of m-PEAs decreases from 266 °C to 186 °C because of the more flexible backbone. For p-PEAs, no discernible glass transitions can be found in the second heating scans as well as m-PEA1, m-PEA2 and m-PEA3. We use MDSC to investigate their reversible heat flow, and found p-PEAs still have no glass transitions. But for m-PEA1, m-PEA2, and m-PEA3, a clear reversible heat flow can be observed in their MDSC curves (shown in Fig. S2†), and their T_g is decreased from 266 to 214 °C, respectively (shown in Table 1).

Fig. 3 DSC thermograms of PEA1 and PEA4 for: (a) the first heating scans; (b) the second heating scans at 10 °C min⁻¹ in N₂; (c) T_ms and (d) T_gs values dependence of the aryl ether linkages.

Fig. 4 WAXD patterns of PEAs used as prepared without annealing treatment.

The crystal structures of PEAs are observed with WAXD (shown in Fig. 4). It can be seen that the p-PEA1 shows a strong characteristic diffraction peak at 2θ value of 20.7° revealing a high degree of crystallization. With the increase of aryl ether linkages, p-PEA2 and p-PEA3 show similar diffraction patterns as p-PEA1. But for p-PEA4 and p-PEA5, another diffraction peak at 2θ value of 19.4° appears, illustrating that a new crystal is formed by introducing more aryl ether linkages into p-PEA. In addition, the diffraction peak intensities of p-PEA4 and p-PEA5 increase and peaks become sharp as well. A new peak at 2θ value of 28° becomes more and more obvious along with the appearance of the second peak in p-PEA4 and p-PEA5. It may be the new crystal form's characteristic diffraction peak. All of these results reveal that the introduction of aryl ether linkages cannot change the crystalline nature of p-PEAs, and this will eventually affect their solubility, which will be discussed in the later part.

For m-PEAs, m-PEA1 shows two characteristic diffraction peaks at 2θ value of 19.3° and 20.2°. A small peak at 2θ value of 28° is obvious. With the increase of aryl ether linkages, m-PEA2 and m-PEA3 show broad peaks indicating they are amorphous in nature. While m-PEA4 and m-PEA5 show similar diffraction patterns as m-PEA1 besides their peak intensity is low, illustrating they have low crystallinity. Such a decrease of crystallinity can be ascribed to both meta-phenyl orientation and aryl ether linkages.

Polymer solubility

The solubility behavior of PEAs is tested qualitatively, and the results are summarized in Table 2. It can be seen that the m-PEAs exhibit better solubility than p-PEAs in some non-proton polar organic solvents such as NMP, DMF, DMSO, et al. The poor solubility of p-PEAs is attributed to their high crystallinity, which has been demonstrated by WAXD. A pinch of LiCl was added in DMSO to promote the p-PEAs dissolution. All p-PEAs were able to dissolve in the DMSO/LiCl solvent system. With the increasing numbers of the aryl ether linkages, PEAs present different variation trend, i.e. p-PEAs show no significant change in their solubility, while m-PEA5 becomes more difficult to dissolve, which is significantly different from other m-PEAs. In the DMSO/LiCl solvent system, p-PEAs showed better solubility with dissolving time reduced. As we know for m-PEAs, the meta-phenyl structure destroys the chains packing and decreases their crystallinity making these polymers have better solubility. Therefore, with the increase of aryl ether linkages, m-PEAs should show better solubility. Unusually, m-PEA5 shows the worse solubility than others m-PEAs. This illustrates that the solubility of m-PEAs is not only determined by its chemical and physical structure, but also affected by the interactions of polymers and solvents.

Table 2 Solubility of PEAs^a

Sample	NMP	DMAc	DMF	DMSO	H2SO4	THF	DMSO*
a + + +: easily soluble in 2 h; + +: soluble overnight; +h: soluble while heating; —: insoluble. All samples were tested using 0.05 g of polymer in 1 mL solvent at room temperature. * DMSO with a pinch of LiCl.
p-PEA1	—	—	—	—	+ + +	—	+h
p-PEA2	—	—	—	—	+ + +	—	+h
p-PEA3	—	—	—	—	+ + +	—	+ +
p-PEA4	—	—	—	—	+ + +	—	+ +
p-PEA5	—	—	—	—	+ + +	—	+ +
m-PEA1	+ + +	+ + +	+ + +	+ + +	+ + +	—	+ + +
m-PEA2	+ + +	+ + +	+ + +	+ + +	+ + +	—	+ + +
m-PEA3	+ + +	+ + +	+ + +	+ + +	+ + +	—	+ + +
m-PEA4	+ + +	+ + +	+ + +	+ + +	+ + +	—	+ + +
m-PEA5	+ +	+ +	+h	+h	+ + +	—	+ +

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As we know that the solubility parameter is one method to assess the interactions of polymers and solvents. Polymers will dissolve in the solvents whose solubility parameters are close to their own and the basic principle is “like dissolves like”. The term solubility parameter was first used by Hildebrand and developed by Hansen.³⁸ The solubility parameters can be calculated based on the group contribution methods, and two common methods have been reported by Hoy and Van Krevelen.³⁹ In these methods the solubility parameters are divided into three parts: δ_d, δ_p, δ_h, where δ_d is correlated with dispersion forces, δ_p is correlated with polar forces and δ_h is correlated with hydrogen bonding. In this study, the solubility parameter of m-PEAs was calculated based on Van Krevelen method (shown in ESI†), and the detailed calculated δ values are listed in Table 3. It can be seen that the solubility parameters δ, δ_d, δ_h of m-PEAs are almost the same with increasing numbers of aryl ether linkages, while the solubility parameter δ_p value decreases from 3.7 to 2.1. This means that m-PEA5 has the minimum polarity, which doesn't match the polarity of solvents very well. So it becomes more difficult to dissolve in these polar solvents even it has lower crystallinity.

Table 3 The calculated δ values of m-PEAs via Van Krevelen method

δ (J cm^−3)^1/2	m-PEA1	m-PEA2	m-PEA3	m-PEA4	m-PEA5
δd	14.6	14.6	14.7	14.7	14.7
δp	3.7	3.0	2.6	2.3	2.1
δh	6.3	6.2	6.1	6.1	6.0
δ	16.3	16.1	16.1	16.1	16.0

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Thermal properties

The initial decomposition temperatures of 5% weight loss (T_5%), the corresponding temperatures of maximum decomposition rate (T_max) and the residual yields at 700 °C of PEAs were investigated by TGA at a heating rate of 10 °C min⁻¹. In general, all PEAs show excellent thermal stabilities in both air and nitrogen atmosphere and the detailed data are summarized in Table 4. The typical TGA curves of p-PEA1, p-PEA4, m-PEA1 and m-PEA4 are shown in Fig. 5. As Table 4 shown, the T_5% of obtained PEAs are higher than 430 °C and their residue yields in nitrogen atmosphere are more than 50% at 700 °C, which is ascribed to their high aromatic content. Overall, p-PEAs exhibit better thermal stabilities, and its T_5% is higher than that of m-PEAs. In the pyrolysis research for Kevlar® and Nomex®, the initial decomposition temperature of aromatic polyamides is correlated with the hydrolysis of amide bonds because of the hydrogen bonds with crystal water.⁴⁰ The p-PEAs have high crystallinity than m-PEAs as shown in the WAXD patterns, therefore, it is more difficult for p-PEAs forming hydrogen bond with crystal water than m-PEAs which make the T_5% of p-PEAs higher. However, with the increasing numbers of aryl ether linkages, the T_5%s of m-PEA4 and m-PEA5 are higher than that of p-PEA4 and p-PEA5. For p-PEA4 and p-PEA5, the molecular chain becomes more flexible with the decease of crystallinity. It is easier to form hydrogen bonds with crystal water. Therefore, the T_5%s of p-PEA4 and p-PEA5 decrease. For m-PEA4 and m-PEA5, the increase of crystallinity makes their T_5%s higher than other m-PEAs. With a certain number of aryl ether linkages, such as 4 or 5, the T_5%s of m-PEAs can be higher than that of p-PEAs.

Table 4 TGA and DTG data of PEAs in N₂ and air atmosphere

Sample	N2			Air
	T5%/°C	Tmax/°C	Residual yield at 700 °C/%	T5%/°C	Tmax/°C	Residual yield at 700 °C/%
p-PEA1	498	533	68	498	579	0
p-PEA2	487	556	65	468	571	2
p-PEA3	464	551	62	440	548	0
p-PEA4	455	536	59	438	573	5
p-PEA5	445	536	56	432	538	3
m-PEA1	431	558	70	396	547	2
m-PEA2	433	569	65	407	562	2
m-PEA3	448	564	64	414	555	0
m-PEA4	463	527	55	427	571	2
m-PEA5	465	529	52	441	543	2

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Fig. 5 TGA curves of PEA1 and PEA4 both in (a) N₂ and (b) air atmosphere.

The T_maxs of PEAs are all around 530–580 °C correlated with the decomposition and rearrangement reaction of aryl ether structures. There is no any obvious trend with the increase of aryl ether linkages. The residual yield diminishes in both series because the more aryl ether linkages are induced, the harder to form a stable rearrangement product at a high temperature. To make it more clear, other measurements such as Py-GC-MS should be used, which will be conducted later. Actually, the residual yield is nearly the same when they have the same number of aryl ether linkages. The similar structure and the same number of benzene rings make it little difference between each other.

Mechanical and optical property

The mechanical properties of PEAs are listed in Table 5. Both the cast films exhibit the mechanical property of rigid materials, in which m-PEAs films are transparent and flexible with the tensile strengths, Young's modulus and elongations at break, in the ranges of 108–150 MPa, 2.6–3.5 GPa, and 3.2–6.6%. Because lithium chloride is used as solubility promoter, and it also can act as a plasticizer, p-PEA4 and p-PEA5 films present lower tensile strengths and high elongation at break values. For m-PEAs, with the increase of aryl ether linkages, the tensile strengths and modulus decrease gradually. Except for the brittle film of m-PEA5, the more aryl ether linkages are introduced, the weaker the rigidity of the polymers is. These resulted in the decease of tensile strength and modulus.

Table 5 Mechanical and optical properties of PEAs films^a

Samples	Tensile strength/MPa	Elongation at break/%	Tensile modulus/GPa	Film quality	Transmittance at 550 nm/%
a —: no reliable measured data. No data of p-PEA1 because of its film was not produced with poor solubility.
p-PEA2	—	—	—	Brittle	—
p-PEA3	—	—	—	Brittle	—
p-PEA4	87.3	5.8	0.8	Flexible	22.5
p-PEA5	82.0	5.4	0.7	Flexible	30.8
m-PEA1	150.2	3.2	3.5	Flexible	44.3
m-PEA2	134.3	4.4	3.0	Flexible	38.4
m-PEA3	136.5	3.4	2.8	Flexible	61.5
m-PEA4	108.0	4.5	2.6	Flexible	35.2
m-PEA5	70.5	6.6	0.6	Brittle	24.8

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The transmittance in visible region of m-PEAs films were tested by UV-vis spectra. The typical curves of m-PEA1, m-PEA2, m-PEA3 are shown as Fig. 6. The film of m-PEA3 has a transmittance with a value over 60% at 550 nm which can be considered transparent.¹⁰ However, the films of m-PEA1 and m-PEA2 are opaque with the values under 60% as shown in Table 5. Unfortunately, it is impossible to draw any hard conclusions from these results since the transmittance of polymers depends on a variety of factors, including the molecular structure, crystallization, and the presence of residual solvents.

Fig. 6 UV-vis spectrum in transmittance mode of m-PEA1, m-PEA2 and m-PEA3.

Conclusions

A series of aromatic diamines with different numbers of aryl ether linkages had been designed and synthesized. These “flexible” diamines were polymerized with TPA and IPA via Yamazaki–Higashi method to obtain p-PEAs and m-PEAs. The m-PEAs exhibit better solubility than p-PEAs in common non-proton polar organic solvents such as NMP, DMF, DMAc, DMSO for the meta orientation of phenyl diacid in main chain. With the increase of aryl ether linkages, p-PEAs show no significant change in their solubility, while m-PEA5 becomes more difficult to dissolve in polar solvents. The smaller polarity of m-PEA5 makes its solubility in polar solvents different from others. As determined by TGA and DSC, all PEAs show excellent thermal stabilities (T_5% above 430 °C, residual yield above 50%, T_g ∼ 186–266 °C, T_m ∼ 328–500 °C). With the increasing the numbers of aryl ether linkages, their T_ms are decreased from 480 to 428 °C for p-PEAs, and 423 to 328 °C for m-PEAs, respectively. Simultaneously, the T_g of m-PEAs decreases from 266 to 186 °C because of the more flexible backbone. The T_5%s of m-PEA4 and m-PEA5 are higher than that of p-PEA4 and p-PEA5 for the different changes of crystallinity. The residual yield diminishes in both series because of the less stable rearrangement product at a high temperature. Besides, due to the incorporation of aryl ether linkages, the transparent films of p-PEA4, p-PEA5 and m-PEAs can be easily made by casting. Furthermore, although some aryl ether linkages have been introduced, these films still present high tensile strength and tensile modulus. For m-PEAs, with the increase of aryl ether linkages, the tensile strengths and modulus decrease gradually along with the decrease of the rigidity of polymers. The film of m-PEA3 has a transmittance with a value over 60% at 550 nm which can be considered transparent. However, the films of m-PEA1 and m-PEA2 are opaque.

A system study is performed to investigate the influence of para-aryl ether linkages numbers on the properties of the PEAs. We found at the first time the aryl ether linkages have different effects on the properties of p-PEAs and m-PEAs, which will guide people for further molecular design.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21634006, 51421061), Program for Changjiang Scholars and Innovative Research Team in University of Ministry of Education of China (IRT1026), and SKLPME.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra11163g

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