Synthesis and properties of novel polyimide fibers containing phosphorus groups in the main chain

Article abstract of DOI:10.1039/c6ra02344d

A series of polyamic acid copolymers (co-PAAs) containing phosphorous groups in the main chain were synthesized using different ratios of two diamines, i.e., bis(3-aminophenyl)methyl phosphine oxide (DAMPO) and 4,4′-oxydianiline (ODA), with 3,3′,4,4′-biphenyltetracarboxylic dianhydride (s-BPDA) by polycondensation in N,N′-dimethyacetamide (DMAc). The co-PAA solutions were spun into fibers by a dry-jet wet spinning process, and then polyimide copolymer (co-PI) fibers were obtained by thermal imidization. ATR-FTIR spectra and elemental analysis confirmed the chemical structure of PAA and PI fibers. The as-prepared PI fibers have smooth and dense surface as well as uniform diameter. Compared with the blank PI-0, the T_g values of co-PI fibers increased considerably with the increase in DAMPO content. TGA results indicated that the co-PI fibers possessed good thermal stability up to 510 °C and a residual char yield of up to 61% at 850 °C. All co-PI fibers exhibited excellent elongation, and their tensile strength and modulus can reach 0.9 GPa and 14.97 GPa when the molar ratio of DAMPO/ODA was 6/4 and the draw ratio was 3.0. The relationship between microstructure and mechanical property is also discussed.

Full text of DOI:10.1039/c6ra02344d

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Synthesis and Properties of Novel Polyimide Fibers Containing
Phosphorus Groups in the Main Chain
Yong Zhao,a,b Tao Feng,c Guomin Li,a Fangfang Liu,a Xuemin Dai,a Zhixin Dong,a* Xuepeng
Qiua*
a Polymer Composites Engineering Laboratory, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, P. R. China
b University of Chinese Academy of Sciences, Beijing 100049, P. R. China
c Academy of Space Electronic Information Technology
* Correspondence to: Xuepeng Qiu and Zhixin Dong (Tel: +86-0431-85262095; E-mail:
xp_q@ciac.ac.cn, zxdong@ciac.ac.cn )
ABSTRACT
A series of polyamic acid copolymers (co-PAAs) containing phosphorous groups in the main
chain were synthesized using different ratios of two diamines, i.e.,
bis(3-aminophenyl)methyl phosphine oxide (DAMPO) and 4,4′-oxydianiline (ODA), with
3,3′,4,4′-biphenyltetracarboxylic dianhydride (s-BPDA) by polycondensation in
N,N′-dimethyacetamide (DMAc). The co-PAA solutions were spun into fibers by a dry-jet wet
spinning process, and then polyimide copolymer (co-PI) fibers were obtained by thermal
imidization. ATR-FTIR spectra and elemental analysis confirmed the chemical structure of
PAA and PI fibers. The as-prepared PI fibers have smooth and dense surface as well as
uniform diameter. Compared with the blank PI-0, the Tg values of co-PI fibers increased
considerably with the increase in DAMPO content. TGA results indicated that the co-PI fibers
possessed good thermal stability up to 510 °C and a residual char yield of up to 61% at
850 °C. All co-PI fibers exhibited excellent elongation, and their tensile strength and modulus
can reach 0.9 GPa and 14.97 GPa when the molar ratio of DAMPO/ODA was 6/4 and the
draw ratio was 3.0. The relationship between microstructure and mechanical property is
also discussed.
KEYWORDS Polyimide fibers; Phosphorus monomer; Bis(3-aminophenyl)methyl phosphine
oxide
INTRODUCTION
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Many types of high-performance fibers, ranging from polymeric to carbon, boron, and
ceramic fibers, are commercially available. However, each type of fiber has certain
additional advantages. For example, in oxygen-free environments, carbon fibers can retain
their strength at extremely high temperature, whereas polymeric fibers are much lighter
than carbon fibers.
Among high-performance fibers, aromatic polyimide (PI) fibers are known for their excellent
resistance to heat, thermal stability, chemical and solvent resistance, and radiation stability.
Owing to these outstanding properties, aromatic PI fibers are widely used in manufacturing,
microelectronic, engineering, and aerospace industries. Since the earliest work on PI fibers
was reported by Irwin1, a few decades later, the preparation of high-performance PI fibers
was mainly carried out by Japanese2-10 and Soviet researchers11-14. Until recent years,
commercial PI fibers include not only P84® fibers (originally introduced in a patent by the
Upjohn company, developed by Lenzing AG, and now offered by Inspec Fibers/Degussa), but
also Yilun® and ASPITM fibers produced by Hipolyking and Aoshen in China. However, other
kinds of PI fibers are still in the laboratory stage.
Two methods are usually employed for the fabrication of PI fibers. First, PI solution is
directly used to spin PI fibers into a coagulation bath. The resulting PI fibers produced by a
one-step method usually possess a high tensile strength and initial modulus15-17. Cheng17 et
al. reported a kind of PI fiber, which was synthesized by 3,3',4,4'-biphenyltetracarboxylic
dianhydride (s-BPDA) and 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (PFMB), had a
tensile strength of 3.3 GPa and an initial modulus of 130 GPa. However, the solubility of PI
solutions and the toxicity of solvents hinder the large-scale production for the one-step
method18-20. Second, a typical two-step technique is primarily adopted in the preparation of
PI fibers21-23. A precursor polymer solution, such as polyamic acid (PAA) or poly (amic ester)
(PAE), is spun and followed by thermal or chemical imidization. Using the two-step method
to fabricate PI fibers can overcome the solubility problem of PI solutions. However, the
formation of voids from volatilization of small molecules by thermal imidization reduces the
mechanical properties of PI fibers. Park and Farris reported21 that pyromellitic
dianhydride/oxidianiline (ODA) fibers had tensile strength of 0.4 GPa and initial modulus of
5.2 GPa, respectively. Clair24 reported that the tensile strength and initial modulus of
3,3’,4,4’-benzophenonetetracarboxylic dianhydride/ODA fibers are 0.19 GPa and 3.6 GPa,
respectively. Recently, Liu25 and Wu22 reported that controlling the aromatic rigid units (AAQ
monomer), hydrogen bonding26, 27 (between AAQ and cyclic imide units), and draw ratios can
enhance the tensile strength and modulus to as high as 2.8 GPa and 115 GPa, respectively.
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PI fibers are commonly applied in high-temperature resistant filters and fireproof materials.
However, in some special applications, such as spaceships, space suits, and satellite ropes,
commercial PI fibers cannot meet the needs of flame retardancy and high-temperature
resistance. Meanwhile, phosphorus materials exhibit good flame retardancy and thermal
stability. Here, phosphorous-containing monomer, i.e., bis(m-aminophenyl)methyl
phosphine oxide (DAMPO), was synthesized and a series of co-PI fibers were prepared by
adjusting the percentage of DAMPO content through a two-step approach. The
high-temperature resistance and mechanical properties of as-prepared PI fibers were
discussed.
EXPERIMENTAL
Chemicals and Materials
4,4′-Oxydianiline (4,4′-ODA, >99.5%) and 3,3′,4,4′-biphenyltetracarboxylic dianhydride
(s-BPDA) were purchased from Shanghai Research Institute of Synthetic Resins, and s-BPDA
was dried in vacuum at 260 °C overnight prior to use. Triphenylphosphine,
N-methlypyrrolidinone (NMP), N,N-dimethylformamide (DMF), Tetrahydrofuran (THF),
Trifluoroacetic acid (TFA), LiCl, CCl4, dimethyl sulfoxide (DMSO) and EtOH were purchased
from Shanghai Darui Fine Chemical Co., Ltd. without further purification.
N,N′-dimethyacetamide (DMAc, analytical purity of ≥99.5%) was purchased from Tianjin Fine
Chemical Co., Ltd. and used as received. All other chemicals were obtained and without
further purification.
Synthesis of bis(m-aminophenyl) methyl phosphine oxide (DAMPO)
DAMPO was synthesized according to a modified method28-30. The synthetic route is
illustrated in SCHEME 1 and the details are as follows.
SCHEME 1. Synthetic route of bis(m-aminophenyl) methyl phosphine oxide (DAMPO)
Synthesis of methyl triphenylphosphonium iodide (MTI) 131-33
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CH3I (7.5 mol, 1064 g) was slowly added into a chilled solution of 7.5 mol Ph3P (1967.18 g) in
6 L of toluene, which was reacted in a 10 L round bottom flask in the ice bath. The mixed
solution was stirred for more than 6 h at 0 °C, and then filtered. The residue was washed
with fresh toluene and dried in the oven. White powder (Ph3PCH3I, MTI) was obtained with
1
more than 98 % yield. FT-IR (KBr): 2920, 2869 cm−1 (–CH3). H NMR (400 MHz, DMSO-d6) δ
(ppm) 7.92 (m, 3H), 7.88–7.73 (m, 12H), 3.24 (d, J=14.5 Hz, 3H). 31P NMR (162 MHz,
DMSO-d6) δ (ppm) 22.30. 13C NMR (101 MHz, DMSO-d6) δ (ppm) 134.71, 133.19, 133.08,
130.05, 129.93, 120.18, 119.31, 7.75, and 7.20. Theoretical calculation for elements of
C19H18IP was C: 56.46 %, H: 4.49 %, P: 7.66 %. Found elemental contents were C: 56.89 %, H:
4.48 %, P: 7.60%.
Synthesis of methyl biphenylphosphine oxide (MDPO) 234
MTI 1, (4.37 mol, 1767g) was transferred into a 10 L round bottom flask equipped with a
mechanical stirrer, a reflux condenser, and a nitrogen inlet. Sodium hydroxide (64.29 mol,
2571.43 g) and water (6250 ml) were then added into the flask. The mixture was stirred and
heated to reflux at about 100 °C for ~6 h, which produced a cloudy suspension. A clear layer
was formed (benzene layer) on the top of the reaction mixture. The reaction was monitored
by TLC (dichloromethane: methanol= 9:1). The mixture was extracted with dichloromethane,
washed for several times with water, dried with magnesium sulfate, and then the solvent
was removed. The mixture was finally dried in a vacuum oven for 24 h. The product was
1
obtained as a white solid with 95% yield, m.p. 114–116 °C. FT-IR (KBr): 1172 cm−1 (P=O). H
NMR (400 MHz, DMSO-d6) δ (ppm) 7.83–7.73 (m, 4H), 7.58–7.44 (m, 6H), 2.04 (d, J=13.4 Hz,
3H).31P NMR (162 MHz, DMSO-d6) δ (ppm) 27.27. 13C NMR (101 MHz, DMSO-d6) δ (ppm)
135.62, 134.64, 131.39, 130.19, 130.10, 128.58, 128.46, 16.25, 15.52. Theoretical calculation
for elements of C13H13OP was C: 72.21 %, H: 6.06 %, P: 14.33 %. Found elemental contents
were C: 72.35 %, H: 5.93 %, P: 14.41 %.
Synthesis of bis(m-nitrophenyl)methyl phosphine oxide (DNMPO) 328
Methyl biphenylphosphine oxide (MDPO) 2 (2 mol, 432.44 g) was charged into a 10 L round
bottom three-neck flask equipped with a dripping funnel and a mechanical stirrer. The
concentrated sulfuric acid (2000 ml) was added into the flask carefully and the system was
allowed to cool to 0 °C in the ice bath with stirring until the solid dissolved. The mixture of
concentrated sulfuric acid (2000 ml) and fuming nitric acid (4000 ml) was added dropwise
with the dripping funnel. The reaction was stirred for 4 h at 0–5 °C and maintained further
for 24 h at room temperature. The reaction was monitored by TLC (dichloromethane:
methanol = 9:1). The reaction mixture was poured into the ice water, and then the
precipitated solid was extracted with dichloromethane, washed with aqueous sodium
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bicarbonate solution until neutral pH was reached, and then the dichloromethane organic
layer was dried with magnesium sulfate. The solvent was removed and the obtained product
recrystallized with absolute glycol ether to obtain light yellow solid with 93% yield, m.p.
1
245–248 °C. FT-IR (KBr): 1530, 1351 cm−1 (-NO2) (as shown in FIG. S1). H NMR (400 MHz,
DMSO-d6) δ (ppm) 8.72-8.52 (m, 2H), 8.40 (dd, J=8.2, 2.3 Hz, 2H), 8.29 (dd, J=11.0, 7.6 Hz,
2H), 7.84 (td, J=7.9, 2.8 Hz, 2H), 2.32 (d, J=14.1 Hz, 3H) (as shown in FIG. S2 blue). 31P NMR
(162 MHz, DMSO-d6) δ (ppm) 26.62. 13C NMR (101 MHz, DMSO-d6) δ (ppm) 147.92, 147.79,
136.99, 136.67, 136.57, 136.01, 130.75, 130.63, 126.62, 125.01, 124.90, 15.59, and 14.86 (as
shown in FIG. S2 black). Theoretical calculation for elements of C13H13OP was C13H11N2O5P
was C: 50.99 %, H: 3.62 %, N: 9.15 %, P: 10.12 %. Found elemental contents were C: 51.15 %,
H: 3.71 %, N: 9.16 %, P: 10.09 %. (The spectra of FTIR, H1 and C13 NMR have been supported
in supplementary information.)
Synthesis of bis(m-aminophenyl) methyl phosphine oxide (DAMPO) 428
DNMPO (1.385 mol, 424 g) and ethanol (3000 ml) were charged into a 5 L round bottom
three-neck flask fitted with a dripping funnel, a mechanical stirrer, a reflux condenser, a
nitrogen inlet, and an oil bath. The 10% Pd/C was added before heating the mixture.
Hydrazine (8.308 mol, 415.9 g) was dropped into the solution carefully using a dripping
funnel when the mixture was heated to 90 °C. The black mixture was then refluxed rapidly at
90 °C. The reaction was monitored by TLC. Finally, the black mixture was filtered using
Buchner funnel via diatomite to afford ethanol solution. The solvent was removed, and the
obtained product was recrystallized by absolute ethanol with 96% yield, m. p. 155–156 °C.
FT-IR (KBr): 3367 and 3220 cm−1 (N-H) (as shown in FIG. S1). 1H NMR (400 MHz, DMSO-d6) δ
(ppm) 7.12 (td, J=7.8, 3.7 Hz, 1H), 6.96-6.87 (m, 1H), 6.80 (dd, J=11.4, 7.4 Hz, 1H), 6.73-6.65
(m, 1H), 5.35 (s, 2H), 1.82 (d, J=13.1 Hz, 1H) (as shown in FIG. S3 blue ). 31P NMR (162 MHz,
DMSO-d6) δ (ppm) 28.17. 13C NMR (101 MHz, DMSO-d6) δ (ppm) 148.79, 148.65, 136.16,
135.18, 129.10, 128.97, 117.10, 117.00, 116.46, 115.19, 115.08, 16.37, and 15.65 (as shown
in FIG. S3 black). Theoretical calculation for elements of C13H15N2OP was C: 63.41 %, H:
6.14 %, N: 11.38 %, P: 12.58 %. Found elemental contents were C: 63.61 %, H: 6.18 %, N:
11.48 %, P: 12.61 %. (The spectra of FTIR, H1 and C13 NMR have been supported in
supplementary information.)
Preparation of PAA solutions
A series of PAAs solutions with different phosphorus contents were successfully synthesized
by adjusting molar ratios of DAMPO/ODA. For example, in the reaction with co-PAA at a
molar ratio of 2/8, 192.23 g (0.96 mol) ODA and 59.10 g (0.24 mol) DAMPO were charged
into a flask equipped a stirrer, a nitrogen inlet, and a thermometer. The mixture was stirred
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in DAMc of 3735 mL. The s-BPDA (353.06 g, 1.20 mol) was quickly added into the mixture
when the solid of diamines was completely dissolved (15% solid content, w/w). The isotropic
solution was obtained with stirring for another 24 h at room temperature. Homo-PAA
(DAMPO/ODA=0/10) and co-PAAs (DAMPO/ODA=1/9 to 6/4) were prepared. We found that
gel particles were formed when the molar ratio of DAMPO/ODA was higher than 6/4, which
was harmful for extrusion of fibers from spinnerets. The proportions of the PAA solutions
and the corresponding inherent viscosities were listed in TABLE 2. The polymerization route
of the PAA solutions with phenyl phosphine oxide (PAA-PPO) was shown in SCHEME 2.
SCHEME 2. Synthetic route of PAA-PPO solutions (m/n = 0/10 to 6/4).
Preparation of co-PAA fibers and co-PI fibers
The above co-PAA solutions were spun through a dry-jet wet spinning method to form
co-PAA fibers. Then, the as-prepared co-PAA fibers were subjected to thermal imidization to
produce co-PI fibers, as shown in SCHEME 3.
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SCHEME 3. Two-step procedure for preparing co-PI fibers.
The PAA-PPO solution was filtered and degassed at room temperature prior to use. The
process of preparation of fibers for spinning was shown in Figure 1. Fibers were spun (3) via
the dry-jet wet spinning method with 15 % solid content into the coagulation bath (4) which
was consisted of water and DMAc in the specified proportion. The solidifying filament
entered into a second bath (6), and then into a heating tube (8 and 9), finally collected on
the winder (10). The collected fibers were called co-PAA fibers. The co-PAA fibers dried in
the oven for about 12 h, and then drew in a heating tube to produce co-PI fibers.
DRY-JET WET SPINNING PROCSS
1
2
5
7
3
8
9
10
4
6
1. air compressor; 2. polyamic acid solution; 3. spinning spinneret; 4. coagulation bath; 5
and 7. rollers; 6. washing bath; 8 and 9. drying-tube; 10. winder.
Figure 1. Fabrication flowchart of PAA fibers.
Characterization
Melting point was determined on a Thomas Hoover capillary melting point apparatus. Glass
transition temperature (Tg) was determined by differential scanning calorimetry (DSC) with
TA Q100 at a heating rate of 5 °C/min. The inherent viscosity (ηinh) of PAA solution was
measured with an Ubbelohde viscometer at a concentration of 0.5 dL/g in DMAc at 30 °C.
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Fourier transform infrared (FTIR) spectrum was recorded on a VERTEX 70 spectrometer with
the scanning wavenumber in the range of 400–4000 cm-1. The measurement of mechanical
properties was carried out on XQ-1 instrument with ASTM standard (D3379-75, edition 1987)
at a cross-head speed of 20 mm/min. More than 10 monofilaments for one sample were
tested and the average data were used to characterize the mechanical property of the
sample. TA-Q50 instrument was employed to analyze the thermal stability of the fibers at a
heating rate of 10 °C/min under nitrogen atmosphere. Dynamic mechanical analysis (DMA)
was performed on the Rheometric scientific DMTA-V in 1 Hz at a heating rate of 10 °C/min in
the range of 50–400 °C. The surface morphology of PI fibers was determined by XL-30 and
scanning electron microscope (SEM). X-ray scattering (SAXS) profiles were obtained by
Nanostar-u (Bruker AXS INC). The wavelength was 0.154 nm. A CCD X-ray detector (HI-STAR)
was used at a distance of 1062 nm from the sample. A software package FIT2D processed
the SAXS 2D images.
RESULTS AND DISCUSSION
Structure characterization of PAA and PI fibers containing-phosphorus groups
The FTIR spectra of PAA fibers was shown in Figure 2. The peaks at 3300 and 2948–2848
cm-1 were related to the absorptions of COOH and NH2, CH3 groups, respectively.
Additionally, Peaks at 1712, 1652, and 1533 cm-1 corresponded to the absorptions of
C=O(COOH), C=O(CONH) and C-NH, respectively. Peaks at 788–684 cm-1 corresponded to the
absorption of P-C in PPO groups. Furthermore, Peaks of CH3 and P-C became stronger with
increasing DAMPO content, as shown in Figure 2.
Figure 2. FTIR spectra of PAA fibers.
However, in FTIR spectra of PI fibers (Figure 3), a broad band at 3137–3000 cm-1
+
corresponded to NH2 group. The band at 2948–2848 cm-1 was related to the absorption of
CH3 groups. Peaks at 1774 and 1722 cm-1 belonged to the C=O group of asymmetric and
symmetric stretching vibrations of the aromatic imide ring, respectively. The peak at 1360
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cm-1 was attributed to the C-N stretching vibration of imide ring. The peak at 734 cm-1
implied the appearance of imide ring bending. The peaks at 1712, 1652, and 1533 cm-1 from
PAA fibers cannot be found in Figure 3. The characteristic peaks of PI fibers emerged with
1774, 1722, 1360, and 734 cm-1. The band of CH3 group at 2948–2848 cm-1 increased in
intensity with increasing DAMPO content. The abovementioned results indicated that PAA
fibers were successfully converted to PI fibers. The FTIR spectra were similar to those
reported in literature35.
Figure 3. FTIR spectra of PI fibers.
The elemental analysis results of PI fibers were presented in TABLE 1. With increasing
DAMPO content, carbon content decreased, whereas both hydrogen and nitrogen contents
increased. As shown in TABLE 1, the C, H and N elemental contents for PI-0 were in good
agreement with the theoretical values. With increase of DAMPO content, the C and N
elemental contents decreased. For example, for PI-2 and PI-6, their C contents were found
to be 72.02% and 69.37%, only slightly lower than the theoretical values (error less than 0.5%
and 1.8%, respectively), the N contents were 5.94% and 5.56% (error less than 0.8% and
3.5%, respectively). It was noticed that the PI-6 exhibited bigger errors in C and N contents.
Possibly, the difference between the experimental and theoretical values was due to
incomplete burning for these containing-phosphor polyimides. Faghili29 has reported the
results of elemental analysis of poly(amide-imide)s containing phosphine oxide. The similar
results showed that the experimental values in C and N contents were lower than
theoretical values of the containing-phosphr poly(amide-imide)s. Although experimental
errors affected the results to some extent, the trend of C, H, and N elemental contents
decreased with containing-phosphor monomers like those of theoretical values.
TABLE 1. Elemental analysis of PI fibers.
Theoretical value (%)
H
Experimental value (%)
H N
No.
C
N
C
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PI-0
PI-1
PI-2
PI-3
PI-4
PI-5
PI-6
73.35
72.88
72.42
71.97
71.53
71.09
70.67
3.08
3.11
3.15
3.18
3.21
3.25
3.28
6.11
73.25
72.34
72.02
71.39
70.56
69.31
69.37
3.20
3.14
3.19
3.23
3.26
3.30
3.21
6.09
6.05
5.99
5.93
5.87
5.82
5.76
5.99
5.94
5.81
5.75
5.59
5.56
Inherent viscosity of PAA solutions
For the PAA copolymer solutions, the molar ratios of DAMPO/ODA varied from 0/10 to 6/4.
Their inherent viscosities, as listed in TABLE 2, were as following: 1.77, 1.60, 1.59, 1.42, 1.40,
1.43, and 1.27 dL/g, corresponding to molar ratios of 1/10, 1/9, 2/8, 3/7, 4/6, 5/5, and 6/4,
respectively. The value of viscosity decreased with increasing of DAMPO content. The
polymerization procedure of PAA is a nucleophilic reaction, in which diamine is the
nucleophilic reagent. The reaction rate increases with increase of the charge density of the
amino nitrogen36. DAMPO and ODA have different structures. The oxygen atom in ODA is the
charge donor that results in increasing charge density of amino nitrogen. Thus, the reactivity
of ODA is higher. In the case of DAMPO, the electron-withdrawing group (P=O) reduces the
charge density of the amino nitrogen. Subsequently, the nucleophilic attack of DAMPO is
reduced. Thus, increasing DAMPO content will decrease the viscosity of the PAA solutions to
some extent.
TABLE 2. Inherent viscosities of PAA solutions and mechanical properties of PI fibers.
Mechanical Properties
Diamine
Polymer
No.
ratio
Draft a/Draw b
ratio
ηinh
Strength
Modulus
(GPa)
Elongation
(%)
(m:n)
(dl/g) (GPa)
PI-0
PI-1
PI-2
PI-3
PI-4
PI-5
PI-6
PI-6’
PI-6’’
0:10
1:9
2:8
3:7
4:6
5:5
6:4
6:4
6:4
3.8 / 1.6
3.8 / 1.6
3.8 / 1.6
3.8 /1.6
3.8 / 1.6
3.8 / 1.6
3.8 / 1.6
3.8 / 2.0
3.8 / 3.0
1.77
1.60
1.59
1.42
1.40
1.43
1.27
1.27
1.27
1.02±0.05 16.94±1.11 13.23±0.63
0.91±0.04 16.95±0.96 15.06±1.52
0.82±0.06 17.38±1.56 16.94±0.83
0.63±0.03 15.77±0.85 20.85±1.63
0.59±0.03 12.71±0.89 29.12±2.26
0.49±0.06 10.82±1.26 51.84±2.61
0.43±0.03 8.88±0.81
0.57±0.05 9.81±1.47
61.64±3.74
21.10±1.96
0.90±0.04 14.97±1.42 18.29±1.21
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a
b
The ratio of the take-up speed to the extrusion speed. The ratio of after-drawing speed to
before-drawing speed passing the heat tube.
Surface morphology of PI fibers
The surface morphology of PI fibers (a–g) was observed by SEM, as shown in Figure 4. All the
fibers exhibited smooth surfaces, and the diameters were uniform. The average diameter
was about 14 μm. The dense surfaces of all fibers came from the quick dual-diffusion
process in the coagulation bath and the uniform diameters with process controlling.
Figure 4. Surface morphologies of fibers (a-g corresponding to PI-0 to PI-6).
Glass transition temperatures of PI fibers
The Tg of PI fibers was measured by both DSC and DMA. The DSC curves were shown in
Figure 5. Their Tgs were 261, 265, 273, 279, 290, 295 and 297 °C, which corresponded to
molar ratios of DAMPO/ODA 1/10, 1/9, 2/8, 3/7, 4/6, 5/5, and 6/4, respectively. Definitely,
the Tgs of co-PI fibers were higher than the blank one (PI-0) and increasing DAMPO content
lead to an obvious increasing of Tgs. It was considered that higher Tg of co-PI fibers was
related to the rigid structure of DAMPO (without ether group in main chain) than ODA,
thereby requiring more energy (higher temperature) to move the chain segments. The
monomer, DAMPO, contains more rigid structure than ODA. Hence, increasing of DAMPO
content will result in higher Tgs of copolymer. Furthermore, the Tgs of PI fibers
(DAMPO/ODA 6/4) with different draw ratios (1.6, 2.0, and 3.0) were also studied in Figure 5
(b). Results indicated that increasing the draw ratio has no influence on glass transition
temperature.
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Figure 5. DSC spectra of PI fibers with DAMPO ratios (a) and draw ratios (b).
Tgs were also obtained using DMA, as presented in Figure 6 (a, b, c, d). In the range of 250–
310 °C, the α relaxation of PI fibers was observed and corresponded to the glass transition
temperature. The Tgs of PI-0, PI-1, PI-2, PI-3, PI-4, PI-5, and PI-6 were 260, 264, 271, 276, 285,
293 and 296 °C, respectively, indicating that the introduction of DAMPO efficiently improved
the glass transition temperature. The rigid structure of DAMPO leaded to higher Tg.
Meanwhile, the α relaxation of PI fibers nearly did not change with draw ratio (1.6, 2.0, and
3.0) of PI-6 fiber, as shown in Figure 6 (b and d). The abovementioned results were
consistent with those of DSC.
Figure 6. tan δ (a, b) and storage modulus (c,d) as a function of temperature.
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Thermal stability of PI fibers
The thermal stability of the PI fibers was characterized by thermogravimetric analysis (TGA),
and their TGA and derivative curves were shown in Figure 7. Nearly no weight loss was
observed from room temperature to 500 °C. In Figure 7 (b), PI-0 fiber only exhibited the
highest decomposition rate at 600 °C. However, the co-PI fibers showed two quicker
decomposition rates: the first one at 522 °C was associated with the decomposition of P-C
bond, whereas the second one at 600 °C was related to the decomposition of PI backbone.
With increasing DAMPO content, the decomposition started earlier because of easier
degradation of P-C bond. However, the phosphorous element had a positive effect on the
flame retardant. The temperatures of 5% and 10% weight loss (T5% and T10%, respectively)
were listed in TABLE 3. The T5%s decreased from 559 °C to 514 °C, and T10%s also decreased
o
from 583 °C to 523 °C along with DAMPO content. The residues at 850 C were 58.9, 64.4,
66.7, 63.0, 62.0, 61.3, and 61.9 % corresponding to DAMPO from 0 % to 60 %, which
indicated that introduction of DAMPO facilitated more char yield, i.e., better flame
retardancy.
Figure 7. TGA and their derivative curves of co-PI fibers.
TABLE 3. TGA data of PI fibers.
a
b
Ratio of diamine
DAMPO/ODA
T5%
T10%
(oC)
Residue
Sample
(oC)
at 850 oC (wt %)
PI-0
PI-1
PI-2
PI-3
PI-4
PI-5
0 / 10
1 / 9
2 / 8
3 / 7
4 / 6
5 / 5
559
569
583
592
568
535
533
527
58.9
64.4
66.7
63.0
62.0
61.3
534
515
521
515
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PI-6
6 / 4
514
523
61.9
*a Corresponding temperature at 5% weight loss, b Corresponding temperature at 10% weight loss.
Mechanical properties of PI fibers
For polymeric fibers, the mechanical property is considered to be a significant parameter,
particularly in engineering applications. Figure 8 and Figure 9 showed the variation of
mechanical properties with DAMPO content and draw ratio, and the details were listed in
TABLE 2. From PI-0 to PI-6, the tensile strengths were 1.02, 0.91, 0.82, 0.63, 0.59, 0.49, 0.43
GPa and the tensile modulus were 16.94, 16.95, 17.38, 15.77, 12.71, 10.82, 8.88 GPa.
Elongations increased obviously from 13.23 % to 61.64 %. Both strength and modulus
decreased with DAMPO content. The abovementioned results indicated that introduction of
DAMPO efficiently improved the elongation of co-PI fibers. In particular, the elongation of
PI-6 was nearly five times higher than that of PI-0. However, the draw ratio also influenced
the mechanical property. In Figure 9, PI-6’ and PI-6’’ had higher draw ratios of 2.0 and 3.0
comparing with PI-6 (draw ratio=1.6). Fibers with higher draw ratio exhibited higher strength,
modulus, and lower elongation. The abovementioned results showed that 60% DAMPO co-PI
fiber can also achieve adequate mechanical property like PI-0 fiber merely by increasing the
draw ratio.
Figure 8. Variation of mechanical properties with DAMPO content for PI fibers.
Figure 9. Variation of mechanical properties with draw ratio of co-PI fibers.
Molecular chains packing
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For aromatic PI fibers, drawing can lead to higher orientation of molecular chains along the
axial direction. During the heat drawing process, polymer chains rearranged themselves to
form an improved molecular alignment. In Figure 10 a, a series of broad peaks in XRD
patterns in the range of 15°–30° can be observed and with increase of DAMPO content, the
shape of XRD pattern was similar, thereby implying typical amorphous polymer chains in the
equator direction. In Figure 10 b, the resulting fibers PI-6, PI-6’, and PI-6’’ (with different
draw ratios) were also considered amorphous state and increasing the draw ratios doesn’t
seem to influence the condensed state. Possibly, the molecular chain rearrangement during
drawing locally forms small-ordered structure and higher orientation, which ultimately
contribute to increased tensile strength and modulus.
Figure 10. XRD patterns of co-PI fibers with equator direction. (a) different DAMPO contents;
(b) different draw ratios.
Micromorphology
During dry-jet wet-spinning process, the coagulation of fibers is a dual diffusion process,
namely, solvent diffuses into the bath, and the coagulant diffuses into the fibers. However,
the inner and external forces cannot always reach equilibrium because of different
concentration gradients for the solvent and coagulation, leading to defects like micro-voids
inside the fibers. Some researchers37-41 have suggested that the elongated streak in the SAXS
patterns was the scattering by long thin voids and parallel to the fiber axis. In this study, the
SAXS patterns of PI-6 with different draw ratios are presented in Figure 11. As a common
feature, it can be clearly seen that the scattering patterns exhibit a sharp and elongated
streak in intensity because of the needle-shaped micro-voids and aligned parallel to the fiber
direction. When the draw ratios increase, the elongated shapes become sharper, thereby
illustrating the presence of morphological changes in the fibers.
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Figure 11. 2D SAXS patterns of PI-6 fiber with draw ratio of 1.6 (a), 2.0 (b) and 3.0 (c).
The average length and the misorientation of micro-voids from the fiber axis are calculated
based on the method of Ruland42. Zhang found that the azimuthal scans of the streak at
different scattering vectors can be described by the Lorentzian/Cauchy- type functions43. The
micro-voids length (L) and the misorientation width (BΦ) can be calculated from the
following equation:
2
π
Bobs
=
+ BΦ ......(Cauchy − Canchy)
Lq
where Bobs is the full width at half maximum of the azimuthal profile from the streak fitted
with a Lorentzian function. q (q = 4
angle. λ is the X-ray wavelength.
π sinθ/λ) is the scattering vector. 2θ is the scattering
Figure 12 shows the Ruland plot of PI-6 with different draw ratios and related results are
presented in TABLE 4. With increasing draw ratio from 1.6 to 2.0 and 3.0, the length of the
micro-void L decreases from 167 nm to 162 nm and 153 nm, respectively. Consequently, the
micro-void misorientation (BΦ) decreases greatly with draw ratios, from 12.9° in draw ratio
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of 1.6 to 7.7° in draw ratio of 3.0. Figure 13 illustrates the micro-void in PI fibers. From the
abovementioned analysis, increasing the draw ratio increases the orientation of amorphous
state, which contributes to increased strength and modulus. Thus, increasing draw ratios is
an efficient approach to improve the mechanical properties of PI fibers.
Figure 12. Ruland plot of PI-6 with different draw ratios, (a) 1.6, (b) 2.0, (c) 3.0.
TABLE 4. The micro-voids parameters of PI fibers.
BΦ
Draw
ratio
L
(nm
)
(
rad
)
(o)
1.6
2.0
3.0
167
162
153
0.225
0.221
0.134
12.9
12.7
7.7
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Figure 13. Schematic diagram of the micro-void in the orientated fiber.
Solubility of fibers
The solubility of PI fibers in nine solvents (NMP, DMAc, DMF, THF, TFA, DMF/LiCl, CCl4,
DMSO, and EtOH) was investigated through 0.001 g of PI fibers in 20 mL of different solvents.
As shown in TABLE 5, all the fibers showed high resistance to these solvents, including polar
solvents such as EtOH, NMP, DMF, non-polar solvents such as CCl4. Among the fibers, only
PI-4 and PI-5 partially dissolved in TFA and only PI-6 sample can fully dissolve in TFA. All
above indicate that the polyimide fibers we have made have a high resistance to solvent and
the incorporation of DAMPO can improve solubility for polymers to some extent.
TABLE 5. Solubility of the PI fibers
Solventsa
Diamine
ratio
Polymer
No.
(
m:n
0:10
1:9
2:8
3:7
4:6
5:5
6:4
)
NMP DMAc DMF THF TFA DMF/LiCl CCl4 DMSO EtOH
PI-0
PI-1
PI-2
PI-3
PI-4
PI-5
PI-6
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
+-
+-
++
a ++, wholly soluble; +-partially soluble; --, not soluble; NMP, xxx;
CONCLUSIONS
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A series of PAA copolymer solutions containing phosphorous groups in main chain were
synthesized and then spun into PAA fibers via the dry-jet wet spinning processing. The PI
fibers were obtained by thermal imidization of PAA fibers. The main results are summarized
as follows. (i) The Tgs of PI fibers increase from 268 °C to 295 °C by DSC and from 262 °C to
297 °C by DMA with increasing phosphorous monomer (DAMPO). (ii) TGA results indicate a
good thermal stability of up to 510 °C. The residual char amounts at 850 °C are in the range
of 61 %-67 % for all co-PI fibers, and these amounts are higher than that of PI fiber without
phosphorous moieties. (iii) The co-PI fiber with DAMPO/ODA = 6/4 and draw ratio of 3.0
exhibits a tensile strength and an initial modulus of 0.90 GPa and 14.97 GPa, respectively,
which means the co-PI fibers can also reach adequate mechanical property like PI-0 fiber
only by increasing the draw ratio. (iv) All fibers are amorphous state, and increased strength
and modulus with draw ratio are attributed to the increased orientation of amorphous
regions in fibers. (v) All fibers we have made have a high resistance to solvent. In summary,
the co-PI fibers exhibit good thermal stability and mechanical properties with the
incorporation of phosphorous monomer in the main chain.
AKNOWLEDGEMENTS
The authors thank the National Basic Research Program of China (973 program, Key Project:
2014CB643604) and the National Natural Science Foundation of China (Grants 51373164) for
the support.
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Graphical abstracts
Different diamines ratios synthesized co-polyimide (co-PI) fibers and these fibers had good
thermal properties and mechanical performances with increased DAMPO contents.
1 / 1