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

Article abstract of DOI:10.1039/c6ra11163g

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.

Full text of DOI:10.1039/c6ra11163g

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PAPER
Thermal transition behaviors, solubility, and
mechanical properties of wholly aromatic para-,
meta-poly(ether-amide)s: effect on numbers of
para-aryl ether linkages†
Cite this: RSC Adv., 2016, 6, 84284
*
*
Wen-Qiang Zhang, Xiu-Li Wang, Gui-Cheng Liu, Li Chen and Yu-Zhong Wang
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 gꢀ1 were
obtained and characterized by FT-IR, NMR, and elemental analysis. Although the Tgs and Tms of PEAs
decrease with the increasing numbers of aryl ether linkages, their Tgs and Tms 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.
Received 29th April 2016
Accepted 30th August 2016
DOI: 10.1039/c6ra11163g
www.rsc.org/advances
difficult to dissolve in common organic solvents. Besides this,
oen high processing temperature should be used. Both of
Introduction
these limit their wide application.
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 ame resistance.1–4 The best known commercial
aromatic polyamides are poly(p-phenylene terephthalamide)
(PPPT) and poly(m-phenylene isophthalamide) (PMPI) synthe-
sized by a low-temperature solution method with aromatic
diacid dichlorides and diamines. Both of them can be trans-
formed upon solution by wet spinning into ame-resistant, cut-
resistant, and high tensile strength synthetic bers as advan-
tageous 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
Many efforts have been made to improve the solubility and
processability of aromatic polyamides via structural modica-
tion 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 modications such as the introduction of exible
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 exible 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 insol-
uble 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 literatures7,9,10,20,26 show that when diamines react with
isophthalic acid (IPA), the acquired aromatic polyamides
Center for Degradable and Flame-Retardant Polymeric Materials, College of
Chemistry, State Key Laboratory of Polymer Materials Engineering, National
Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), Sichuan
University, Chengdu 610064, China. E-mail: xiuliwang1@163.com; yzwang@scu.
edu.cn; Fax: +86-28-85410755; Tel: +86-28-85410755
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra11163g
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present better solubility in polar organic solvents. If we incor- from ethanol as light yellow crystals. Yield 3.6 g (90%); mp: 176–
porating more para-aryl ether bonds into meta-aromatic poly- 179 C, lit.35 170–180 C. IR (KBr): nmax 3398, 3313, 3220 cmꢀ1
amides' main-chain, what effect it will bring?
(–NH2 stretch), 1497 cmꢀ1 (Ar C–H stretch), 1216, 1109, 1077
ꢁ
ꢁ
In order to address the above issues, four kinds of diamines (2- cmꢀ1 (Ar-O–C stretch), 844, 821 cmꢀ1 (Ar 1,4-para substituted).
ODA, 3-ODA, 4-ODA, 5-ODA) with the number of para-aryl ether 1H NMR (DMSO-d6) d (ppm) 6.82 (s, 4H, Hd), 6.79–6.67 (m, 4H,
linkages from 2 to 5 were synthesized rstly, and then they Hc), 6.66–6.48 (m, 4H, Hb), 4.98 (s, 4H, Ha). 13C NMR (DMSO-d6)
polymerized with terephthalic acid TPA, or isophthalic acid IPA, d (ppm) 153.86 (Ce), 147.03 (Cd), 145.45 (Ca), 120.70 (Cb), 118.52
respectively, to synthesize two series of aromatic poly(ether- (Cf), 115.36 (Cc). Elem. anal. calcd: C, 73.97%; H, 5.48%; N,
amide)s (PEAs) (p-PEAs and m-PEAs). A comparative study is 9.59%; O, 10.96%. Found: C, 73.99%; H, 5.44%; N, 9.47; O,
performed to investigate the inuence of para-aryl ether linkages 11.10%.
numbers on the thermal stability, solubility, mechanical prop-
Bis[4-(4-nitrophenoxy)phenyl]ether (3-NO2). Its synthetic
erties of the obtained p-PEAs and m-PEAs. Via this investigation, procedure is similar to that of 2-NO2 by using 4,40-dihydrox-
the relationships between the numbers of para-aryl ether linkages ydiphenyl (2) ether instead of hydroquinone (1). Yellow solid
and the properties of p-PEAs and m-PEAs should be revealed.
was obtained by ltration, 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): nmax 1485 cmꢀ1 (Ar C–H stretch), 1342 cmꢀ1
ꢁ
Experimental
Materials
(–NO2 stretch), 1224, 1187, 1108 cmꢀ1 (Ar-O–C stretch), 862, 843
cmꢀ1 (Ar 1,4-para substituted). 1H NMR (DMSO-d6) d (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 ltrate. Yield 3.7 g (85%); mp:
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. 4,40-Dihydroxydiphenyl
ether was purchased from Wuxi Yangshi Chemical Co., Ltd. 4,40-
Diaminodiphenyl ether (ODA), 4-chloronitrobenzene, p-uo-
roacetophenone, 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
125 C, lit.36 126–128 C. IR (KBr): nmax 3380, 3313 cmꢀ1 (–NH2
ꢁ
ꢁ
stretch), 1501 cmꢀ1 (Ar C–H stretch), 1237, 1191, 1098 cmꢀ1 (Ar-
1
O–C stretch), 869, 827 cmꢀ1 (Ar 1,4-para substituted). H NMR
(DMSO-d6) d (ppm) 7.00–6.90 (m, 4H, He), 6.90–6.82 (m, 4H,
Hd), 6.82–6.70 (m, 4H, Hc), 6.67–6.51 (m, 4H, Hb), 5.09 (s, 4H,
Ha). 13C NMR (DMSO-d6) d (ppm) 154.84 (Ch), 152.19 (Ce),
146.78 (Cd), 145.45 (Ca), 120.92 (Cb), 120.06 (Cf), 118.52 (Cg),
115.47 (Cc). 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-uoroacetophenone, 13.8 g
ꢀ
with 4 A 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) (0.10 mol) K2CO3 and 150 mL N,N-dimethylacetamide (DMAc)
hydroquinone (1), 14.2 g (0.09 mol) 4-chloronitrobenzene, 13.8 were added in a 250 mL three-necked ask equipped with
g (0.10 mol) K2CO3 and 150 mL N,N-dimethylformamide (DMF) a nitrogen inlet. This mixture was heated at reux temperature
were added in a 250 mL three-necked ask equipped with for 5 h. Aer cooling, the mixture was poured into 300 mL water
ꢁ
a nitrogen inlet. This mixture was stirred and heated at 150 C and the precipitated brown solid product was collected by
for 20 h. Aer the dark reaction mixture cooled into room ltration, recrystallized and dried in a vacuum oven. Yield 16.5 g
temperature, the mixture was poured into 300 mL water and the (95%); mp: 180 C, lit.37 179–181 C. IR (KBr): nmax 1672 cmꢀ1
precipitated yellow solid product was collected by ltration, (C]O stretch), 1497 cmꢀ1 (Ar C–H stretch), 1233, 1190, 1110,
washed with water and dried in a vacuum oven. Yield 14.6 g 1015 cmꢀ1 (Ar-O–C stretch), 873, 842 cmꢀ1 (Ar 1,4-para
ꢁ
ꢁ
1
(92%); mp: 238–240 ꢁC (onset to the peak top temperature) substituted). H NMR (CDCl3) d (ppm) 8.10–7.86 (m, 4H), 7.12
according to differential scanning calorimetry (DSC) at a scan (s, 4H), 7.09–6.98 (m, 4H), 2.60 (s, 6H).
ꢀ1
rate of 10 C min , lit.34 230–240 ꢁC. IR (KBr): nmax 1483 cmꢀ1
(1,4-Phenylenebis(oxy))bis(4,1-phenylene)diacetate (2-ester).
ꢁ
(Ar C–H stretch), 1341 cmꢀ1 (–NO2 stretch), 1234, 1186, 1105, 13.8 g (0.04 mol) 2-ketone and 20.3 g (0.10 mol) 85% m-CPBA
1007 cmꢀ1 (Ar-O–C stretch), 872, 848 cmꢀ1 (Ar 1,4-para was dissolved in 100 mL CHCl3. This mixture was stirred under
substituted). 1H NMR (DMSO-d6) d (ppm) 8.35–8.21 (m, 4H), reux for 5 h and white solid was removed by ltration. The
7.34 (s, 4H), 7.26–7.16 (m, 4H).
ltrate was washed with NaHSO3 (100 mL), NaHCO3 (2 ꢂ 100
1,4-Bis(4-aminophenoxy)benzene (2-ODA). 5.0 g 2-NO2, 0.25 mL), and water (100 mL). The solvent was removed by distilla-
g 5% Pd/C and 60 mL ethanol were added in a 250 mL three- tion, and the crude product was recrystallized from iso-
necked ask and heated in 50 ꢁC for 0.5 h. Then 10 mL propanol. Light yellow crystals were obtained, washed and dried
hydrazine monohydrate was added dropwise into the mixture in in a vacuum oven. Yield 12.4 g (82%); mp: 113 ꢁC, lit.37 110–113
30 min with a constant pressure funnel. Aer complete addi- ꢁC. IR (KBr): nmax 1754 cmꢀ1 (C]O stretch), 1489 cmꢀ1 (Ar C–H
tion, the mixture was heated at reux temperature for 5 h and stretch), 1220, 1186, 1100, 1012 cmꢀ1 (Ar-O–C stretch), 848, 819
the hot solution was ltered to remove the Pd/C catalyst. The cmꢀ1 (Ar 1,4-para substituted). H NMR (CDCl3) d (ppm) 7.19–
1
ltrate was collected and the product was obtained by cooling 6.84 (m, 12H), 2.32 (s, 6H).
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1
substituted). H NMR (DMSO-d6) d (ppm) (dd, J ¼ 6.9 Hz, 8H,
e–f), 6.92–6.82 (m, 4H, Hd), 6.80–6.71 (m, 4H, Hc), 6.64–6.52
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 reux 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
ltration, washed and dried in a vacuum oven. Yield 4.7 g (80%);
H
(m, 4H, Hb), 4.97 (s, 4H, Ha). 13C NMR (DMSO-d6) d (ppm)
155.11 (Ci), 153.23 (Ch), 151.80 (Ce), 146.52 (Cd), 145.81 (Ca),
121.00 (Cb), 120.38 (Cf), 119.98 (Cj), 118.47 (Cg), 115.31 (Cc).
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%.
4,40-Oxybis[(4-(4-nitrophenoxy)phenoxy)]benzene (5-NO2).
Its synthetic procedure is similar to that of 4-NO2. Yellow solid
was obtained by ltration, washed with water and dried in
mp: 193 C, lit.37 190.5–192 C. IR (KBr): nmax 3285 cmꢀ1 (–OH
ꢁ
ꢁ
stretch), 1502 cmꢀ1 (Ar C–H stretch), 1217, 1098, 1009 cmꢀ1 (Ar-
1
O–C stretch), 874, 830 cmꢀ1 (Ar 1,4-para substituted). H NMR
ꢁ
a vacuum oven. Yield 27.1 g (95%); mp: 164–165 C. IR (KBr):
(DMSO-d6) d (ppm) 9.30 (s, 2H), 6.88 (d, J ¼ 4.7 Hz, 4H), 6.87–
nmax 1484 cmꢀ1 (Ar C–H stretch), 1343 cmꢀ1 (–NO2 stretch),
1214, 1191, 1106, 1007 cmꢀ1 (Ar-O–C stretch), 857, 822 cmꢀ1 (Ar
1,4-para substituted). 1H NMR (DMSO-d6) d (ppm) 8.32–8.20 (m,
4H), 7.29–7.19 (m, 4H), 7.19–7.06 (m, 16H).
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 4,40-dihydrox-
ydiphenyl (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):
nmax 1678 cmꢀ1 (C]O stretch), 1500 cmꢀ1 (Ar C–H stretch),
1243, 1104, 1009 cmꢀ1 (Ar-O–C stretch), 868, 826 cmꢀ1 (Ar 1,4-
4,40-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 ltration, washed
with water and dried in a vacuum oven. Yield 3.5 g (77%); mp:
185 ꢁC. IR (KBr): nmax 3381, 3314 cmꢀ1 (–NH2 stretch), 1499
cmꢀ1 (Ar C–H stretch), 1225, 1194, 1099, 1008 cmꢀ1 (Ar-O–C
stretch), 826 cmꢀ1 (Ar 1,4-para substituted). 1H NMR (DMSO-d6)
d (ppm) 7.16–6.92 (m, 12H, He–g), 6.92–6.81 (m, 4H, Hd), 6.82–
6.70 (m, 4H, Hc), 6.67–6.48 (m, 4H, Hb), 4.97 (s, 4H, Ha). 13C
NMR (DMSO-d6) d (ppm) 155.18 (Cl), 153.49 (Ci), 152.89 (Ch),
151.75 (Ce), 146.54 (Cd), 145.83 (Ca), 121.02 (Cb), 120.46 (Cf),
120.34 (Cj), 119.99 (Ck), 118.49 (Cg), 115.33 (Cc). 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%.
1
para substituted). H NMR (CDCl3) d (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): nmax 1760 cmꢀ1 (C]O stretch), 1500 cmꢀ1 (Ar C–H
stretch), 1224, 1193, 1100, 1013 cmꢀ1 (Ar-O–C stretch), 827 cmꢀ1
(Ar 1,4-para substituted). 1H NMR (CDCl3) d (ppm) 7.24–6.85 (m,
16H), 2.32 (s, 6H).
4,40-Bis(p-hydroxyphenoxy)diphenyl ether (4). Its synthetic
procedure is similar to that of (3). White solid was obtained by
ltration, washed and dried in a vacuum oven. Yield 6.6 g (85%);
mp: 214 ꢁC, lit.37 214–215 ꢁC. IR (KBr): nmax 3321 cmꢀ1 (–OH
stretch), 1500 cmꢀ1 (Ar C–H stretch), 1217, 1098, 1007 cmꢀ1 (Ar-
O–C stretch), 828 cmꢀ1 (Ar 1,4-para substituted). 1H NMR
(DMSO-d6) d (ppm) 9.38 (s, 2H), 7.02–6.94 (m, 4H), 6.94–6.83
(m, 8H), 6.83–6.74 (m, 4H).
Polymer synthesis
Two series of PEAs were prepared by the direct poly-
condensation 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 poly-
terephthalamide (p-PEA), and based on isophthalic acid were
named polyisophthalamide (m-PEA). A typical synthetic proce-
dure for p-PEA1 is shown as follows.
A three-necked ask 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. Aer cooling, this solution was poured slowly
into 300 mL stirring methanol and the ber-like precipitates
were collected by ltration. Finally, the precipitates were
washed with water and dried in a vacuum oven. Yield 3.3 g
(95%); IR (KBr): nmax 3431, 3041 cmꢀ1 (–NH stretch), 1656 cmꢀ1
(C]O stretch), 1207, 1099 cmꢀ1 (Ar-O–C stretch), 829 cmꢀ1 (Ar
1,4-para substituted). 1H NMR (DMSO-d6/LiCl) d (ppm) 10.47 (s,
2H, NH), 8.11 (s, 4H, Ha), 7.82 (d, J ¼ 9.1 Hz, 4H, Hb), 7.06 (d, J ¼
5.3, 4H, Hc).
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 ltration, washed with water and dried in a vacuum
oven. Yield 22.9 g (95%); mp: 140 C. IR (KBr): nmax 1485 cmꢀ1
ꢁ
(Ar C–H stretch), 1342 cmꢀ1 (–NO2 stretch), 1218, 1191, 1107
cmꢀ1 (Ar-O–C stretch), 864, 840 cmꢀ1 (Ar 1,4-para substituted).
1H NMR (DMSO-d6) d (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 ask. Then 10 mL hydrazine monohydrate was
added dropwise into the mixture in 30 min with a constant
pressure funnel. Aer complete addition, the mixture was
heated at reux temperature for 5 h and the hot solution was
ltered to remove the Pd/C catalyst. The ltrate was collected
and the product was obtained by cooling from DMF as yellow
crystals. Yield 3.3 g (75%); mp: 172 ꢁC. IR (KBr): nmax 3381, 3314
cmꢀ1 (–NH2 stretch), 1498 cmꢀ1 (Ar C–H stretch), 1223, 1192,
1097, 1008 cmꢀ1 (Ar-O–C stretch), 827 cmꢀ1 (Ar 1,4-para
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
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ꢁ
was cast onto a Petri dish and placed in an oven (75 C) over- Co., Ltd), according to GB/T 1040.3-2006 at a speed of 5 mm
night to evaporate the solvent. The obtained polymer minꢀ1. The thickness of lms was tested by a Vernier caliper.
membranes with the thickness of about 30–50 mm were used for
tensile tests, thermal analysis, and transparency detection.
The transmittance of various PEAs membranes were
analyzed by HITACHI U-1900 spectrophotometer (VARIAN,
USA).
Characterization
Results and discussion
Monomer synthesis
The chemical structures of the products were conrmed by
NMR and FTIR spectroscopy. The spectra were recorded using
a Bruker AVANCE AVII-400 NMR instruments with tetrame-
thylsilane 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 dLꢀ1 at 30
ꢁC in concentrated sulfuric acid.
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 mono-
mer. 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 shis of amino group proton are
around 5.0 ppm, and the chemical shis of phenyl group proton
are observed in the range of 6.5–7.0 ppm. Besides, the chemical
shis 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.
Thermal transition temperatures were determined using
a TA Q200 differential scanning calorimeter (DSC) under
nitrogen atmosphere in a ow of 50 mL minꢀ1, using a heating
and cooling rate of 10 ꢁC minꢀ1. The modulated differential
scanning calorimeter (MDSC) analyses were carried out under
nitrogen atmosphere using amplitude of temperature modula-
ꢁ
tion of ꢃ1 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-ltered copper
Ka radiation and scattering angle (2q) rang of 10–45ꢁ.
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 dLꢀ1
.
Thermogravimetric studies (TGA) were conducted with
a NETZSCH 209 F1 under nitrogen or air atmosphere at a heat-
Polymer synthesis
ꢀ1
ꢁ
ing rate of 10 C min
.
Two series aromatic PEAs were prepared via direct poly-
Tensile properties were performed using Universal Testing condensation of diacid and various synthesized aromatic
Machine (SANS CMT4104, SHENZHEN SANS Testing Machine diamines in NMP according to Yamazaki–Higashi method
Scheme 1 Synthesis route of para-substituted aryl ether diamines.
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Fig. 1 1H NMR and 13C NMR spectra of para-substituted aryl ether diamines.
(shown in Scheme 2). Clear and homogeneous polymer solu- PEAs, the chemical shis of phenyl group of diacid proton (Ha,
tions can be obtained in the presence of a pinch of lithium Hb, Hc) are distributed into three parts in the range of 7.6–7.7
chloride in the solution. Aer pouring into methanol with ppm, 8.1–8.2 ppm and 8.4–8.5 ppm. For p-PEAs, their chemical
stirring, the ber-like precipitates were obtained with the shis of phenyl group of diacid proton (Ha) are only in the range
intrinsic viscosity values in the range of 0.81–1.16 dL gꢀ1 (shown of 8.1–8.2 ppm. The FTIR spectra of PEAs are given in the ESI
1
in Table 1). The typical H NMR spectra of p-PEA1, p-PEA4, m- (shown in Fig. S1†). Not surprisingly, their spectra are quite
PEA1 and m-PEA4 are shown in Fig. 2. For all PEAs, the chemical similar. The broad absorption peak around 3300 cmꢀ1 are
shis of amide group proton are around 10.5 ppm and the assigned to the N–H stretching of amide group and the
chemical shis of phenyl ether group proton are divided into absorptions near 1650 cmꢀ1 and 1220 cmꢀ1 are assigned to
two parts in the range of 7.0–7.2 ppm and 7.7–7.9 ppm. For m- C]O and C–O–C vibrations respectively. Both NMR and FT-IR
Scheme 2 Synthesis procedures for p-PEAs and m-PEAs.
Table 1 DSC data and intrinsic viscosity of PEAs
Sample
[h]a (dL gꢀ1
)
Tg/ꢁC
Tm/ꢁC
Sample
[h]a (dL gꢀ1
)
Tg/ꢁC
Tm/ꢁC
p-PEA1
p-PEA2
p-PEA3
p-PEA4
p-PEA5
0.86
1.03
1.06
0.83
0.85
—
—
—
—
—
>500b
480
443
440
428
m-PEA1
m-PEA2
m-PEA3
m-PEA4
m-PEA5
1.09
1.16
0.81
0.91
0.87
266c
234c
214c
194
423
399
365
356
328
186
a
Measured in H2SO4 (0.5 g dLꢀ1) at 30 ꢁC. b Decomposes before melting. c Measured by MDSC.
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ꢁ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 Tm value because the incomplete crystal melts on lower
temperature at rst. m-PEA4 shows two peaks in the Tm value
because it has two kinds of crystal forms demonstrated by its
WAXD pattern (shown in Fig. 4). Their Tms and Tgs values
dependence of the numbers of aryl ether linkages are presented
in Fig. 3c and d. From the gure we can see that with the
increasing the numbers of aryl ether linkages, their Tm is
decreased from 480 ꢁC to 428 ꢁC for p-PEAs, and 423 ꢁC to 328 ꢁC
for m-PEAs, respectively. Simultaneously, the Tg of m-PEAs
decreases from 266 ꢁC to 186 ꢁC because of the more exible
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 ow,
and found p-PEAs still have no glass transitions. But for m-PEA1,
m-PEA2, and m-PEA3, a clear reversible heat ow can be observed
in their MDSC curves (shown in Fig. S2†), and their Tg is
decreased from 266 to 214 ꢁC, respectively (shown in Table 1).
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 2q value of 20.7ꢁ 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 2q value of 19.4ꢁ 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
Fig. 2 1H NMR spectra of p-PEA1, p-PEA4, m-PEA1 and m-PEA4.
results illustrate that via Yamazaki–Higashi method these two
series aromatic PEAs can be obtained as expected.
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 rst heating scans before 500
Fig. 3 DSC thermograms of PEA1 and PEA4 for: (a) the first heating scans; (b) the second heating scans at 10 ꢁC minꢀ1 in N2; (c) Tms and (d) Tgs
values dependence of the aryl ether linkages.
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Fig. 4 WAXD patterns of PEAs used as prepared without annealing treatment.
increase and peaks become sharp as well. A new peak at 2q value poor solubility of p-PEAs is attributed to their high crystallinity,
of 28ꢁ becomes more and more obvious along with the which has been demonstrated by WAXD. A pinch of LiCl was
appearance of the second peak in p-PEA4 and p-PEA5. It may be added in DMSO to promote the p-PEAs dissolution. All p-PEAs
the new crystal form's characteristic diffraction peak. All of were able to dissolve in the DMSO/LiCl solvent system. With the
these results reveal that the introduction of aryl ether linkages increasing numbers of the aryl ether linkages, PEAs present
cannot change the crystalline nature of p-PEAs, and this will different variation trend, i.e. p-PEAs show no signicant change
eventually affect their solubility, which will be discussed in the in their solubility, while m-PEA5 becomes more difficult to
later part.
dissolve, which is signicantly different from other m-PEAs. In
For m-PEAs, m-PEA1 shows two characteristic diffraction the DMSO/LiCl solvent system, p-PEAs showed better solubility
peaks at 2q value of 19.3ꢁ and 20.2ꢁ. A small peak at 2q value of with dissolving time reduced. As we know for m-PEAs, the meta-
28ꢁ is obvious. With the increase of aryl ether linkages, m-PEA2 phenyl structure destroys the chains packing and decreases
and m-PEA3 show broad peaks indicating they are amorphous their crystallinity making these polymers have better solubility.
in nature. While m-PEA4 and m-PEA5 show similar diffraction Therefore, with the increase of aryl ether linkages, m-PEAs
patterns as m-PEA1 besides their peak intensity is low, illus- should show better solubility. Unusually, m-PEA5 shows the
trating they have low crystallinity. Such a decrease of crystal- worse solubility than others m-PEAs. This illustrates that the
linity can be ascribed to both meta-phenyl orientation and aryl solubility of m-PEAs is not only determined by its chemical and
ether linkages.
physical structure, but also affected by the interactions of
polymers and solvents.
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 rst 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 Kreve-
len.39 In these methods the solubility parameters are divided
into three parts: dd, dp, dh, where dd is correlated with dispersion
forces, dp is correlated with polar forces and dh 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 d values are listed in Table 3.
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
Table 2 Solubility of PEAsa
Sample
NMP DMAc DMF DMSO H2SO4 THF DMSO*
p-PEA1
p-PEA2
p-PEA3
p-PEA4
p-PEA5
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
+ + +
+ + +
+ + +
+ + +
+ + +
+ + +
+ + +
+ + +
+ + +
+ + +
—
—
—
—
—
—
—
—
—
—
+h
+h
+ +
+ +
+ +
m-PEA1 + + + + + +
m-PEA2 + + + + + +
m-PEA3 + + + + + +
m-PEA4 + + + + + +
+ + + + + +
+ + + + + +
+ + + + + +
+ + + + + +
+ + +
+ + +
+ + +
+ + +
+ +
Table 3 The calculated d values of m-PEAs via Van Krevelen method
d (J cmꢀ3 1/2
)
m-PEA1
m-PEA2
m-PEA3
m-PEA4
m-PEA5
m-PEA5 + +
+ +
+h
+h
dd
dp
dh
d
14.6
3.7
6.3
14.6
3.0
6.2
14.7
2.6
6.1
14.7
2.3
6.1
14.7
2.1
6.0
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.
16.3
16.1
16.1
16.1
16.0
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Table 4 TGA and DTG data of PEAs in N2 and air atmosphere
N2
Air
Residual yield
Residual yield
at 700 ꢁC/%
Sample
T5%/ꢁC
Tmax/ꢁC
at 700 ꢁC/%
T5%/ꢁC
Tmax/ꢁC
p-PEA1
p-PEA2
p-PEA3
p-PEA4
p-PEA5
m-PEA1
m-PEA2
m-PEA3
m-PEA4
m-PEA5
498
487
464
455
445
431
433
448
463
465
533
556
551
536
536
558
569
564
527
529
68
65
62
59
56
70
65
64
55
52
498
468
440
438
432
396
407
414
427
441
579
571
548
573
538
547
562
555
571
543
0
2
0
5
3
2
2
0
2
2
It can be seen that the solubility parameters d, dd, dh of m-PEAs crystallinity than m-PEAs as shown in the WAXD patterns,
are almost the same with increasing numbers of aryl ether therefore, it is more difficult for p-PEAs forming hydrogen bond
linkages, while the solubility parameter dp value decreases from with crystal water than m-PEAs which make the T5% of p-PEAs
3.7 to 2.1. This means that m-PEA5 has the minimum polarity, higher. However, with the increasing numbers of aryl ether
which doesn't match the polarity of solvents very well. So it linkages, the T5%s of m-PEA4 and m-PEA5 are higher than that
becomes more difficult to dissolve in these polar solvents even it of p-PEA4 and p-PEA5. For p-PEA4 and p-PEA5, the molecular
has lower crystallinity.
chain becomes more exible with the decease of crystallinity. It
is easier to form hydrogen bonds with crystal water. Therefore,
the T5%s of p-PEA4 and p-PEA5 decrease. For m-PEA4 and m-
PEA5, the increase of crystallinity makes their T5%s higher than
other m-PEAs. With a certain number of aryl ether linkages,
such as 4 or 5, the T5%s of m-PEAs can be higher than that of p-
PEAs.
The Tmaxs 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.
Thermal properties
The initial decomposition temperatures of 5% weight loss
(T5%), the corresponding temperatures of maximum decompo-
sition rate (Tmax) and the residual yields at 700 ꢁC of PEAs were
investigated by TGA at a heating rate of 10 ꢁC minꢀ1. 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 T5% of ob-
ꢁ
tained 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 T5% 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
Fig. 5 TGA curves of PEA1 and PEA4 both in (a) N2 and (b) air atmosphere.
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Table 5 Mechanical and optical properties of PEAs filmsa
Tensile
strength/MPa
Elongation at
break/%
Tensile
modulus/GPa
Film
quality
Transmittance
at 550 nm/%
Samples
p-PEA2
p-PEA3
p-PEA4
p-PEA5
m-PEA1
m-PEA2
m-PEA3
m-PEA4
m-PEA5
—
—
—
—
—
—
Brittle
Brittle
—
—
87.3
82.0
150.2
134.3
136.5
108.0
70.5
5.8
5.4
3.2
4.4
3.4
4.5
6.6
0.8
0.7
3.5
3.0
2.8
2.6
0.6
Flexible
Flexible
Flexible
Flexible
Flexible
Flexible
Brittle
22.5
30.8
44.3
38.4
61.5
35.2
24.8
a
—: no reliable measured data. No data of p-PEA1 because of its lm was not produced with poor solubility.
Mechanical and optical property
Conclusions
The mechanical properties of PEAs are listed in Table 5. Both
A series of aromatic diamines with different numbers of aryl
ether linkages had been designed and synthesized. These
“exible” 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 signicant 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 (T5% above
430 ꢁC, residual yield above 50%, Tg ꢄ 186–266 ꢁC, Tm ꢄ 328–
500 ꢁC). With the increasing the numbers of aryl ether
the cast lms exhibit the mechanical property of rigid materials,
in which m-PEAs lms are transparent and exible 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 lms 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
lm 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.
The transmittance in visible region of m-PEAs lms were
tested by UV-vis spectra. The typical curves of m-PEA1, m-PEA2,
m-PEA3 are shown as Fig. 6. The lm of m-PEA3 has a trans-
mittance with a value over 60% at 550 nm which can be
considered transparent.10 However, the lms 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, crys-
tallization, and the presence of residual solvents.
ꢁ
linkages, their Tms are decreased from 480 to 428 C for p-
PEAs, and 423 to 328 ꢁC for m-PEAs, respectively. Simulta-
neously, the Tg of m-PEAs decreases from 266 to 186 ꢁC
because of the more exible backbone. The T5%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 rear-
rangement product at a high temperature. Besides, due to
the incorporation of aryl ether linkages, the transparent
lms of p-PEA4, p-PEA5 and m-PEAs can be easily made by
casting. Furthermore, although some aryl ether linkages
have been introduced, these lms 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 lm of m-PEA3 has a transmittance with
a value over 60% at 550 nm which can be considered trans-
parent. However, the lms of m-PEA1 and m-PEA2 are
opaque.
A system study is performed to investigate the inuence of
para-aryl ether linkages numbers on the properties of the PEAs.
We found at the rst 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.
Fig. 6 UV-vis spectrum in transmittance mode of m-PEA1, m-PEA2
and m-PEA3.
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18 G.-S. Liou, Y.-K. Fang and H.-J. Yen, J. Polym. Res., 2007, 14,
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Acknowledgements
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Science Foundation of China (21634006, 51421061), Program
for Changjiang Scholars and Innovative Research Team in
University of Ministry of Education of China (IRT1026), and
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