chapter 2: synthesis and characterization of...
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38
CHAPTER 2: SYNTHESIS AND CHARACTERIZATION OF PROCESSABLE
AROMATIC POLYIMIDES
2.1 INTRODUCTION
Aromatic polyimides121,122
are well known for their excellent thermal, mechanical and
electrical properties and their outstanding chemical properties.123
These properties make
them useful in many high technological fields as high performance polymeric materials.
These materials are widely used in electrical, electronics, automotive and aerospace
industries.124
However, their applications are limited due to processing difficulties like
insolubility in common organic solvents and their extremely high softening or melting
temperature, which are partly due to the rigidity and strong inter chain interaction.125,126
Therefore, significant efforts have been made to improve the processability without
decreasing thermal stability. The most efficient method is incorporation of flexible
groups into the chain backbone which reduces the chain stiffness or introduction of bulky
pendant groups into the polymer backbone, which helps in the separation of polymer
chains and hinder the molecular packing or synthesis of polyimides such as
poly(etherimide)s,127
poly(esterimide)s67
and poly(amideimide)s.128
The objective of this work was to investigate the effects of introducing flexible
groups (-O-,-SO2-, and -C=O), and isopropylidene groups in the chain backbone in
polyimides. Two novel aromatic diamine monomers containing flexible groups (-O-, -
SO2-, and -C=O) and isopropylidene groups were synthesized. A series of polyimides
were synthesized from these new aromatic diamines and commercially available aromatic
dianhydrides. The flexible groups (-O-,-SO2-, and -C=O) and isopropylidene groups in
39
the chain will give flexibility and reduce the chain stiffnes and its effects on the
properties of the polyimides were investigated.
Two novel aromatic diamine monomers, bis-4,4’[(4-aminophenyl-2,2-
isopropylidene phenyloxy)]diphenyl sulfone and bis-4,4’[(4-aminophenyl-2,2-
isopropylidene phenyloxy)]benzophenones were synthesized and characterized by IR
and 1H-NMR spectroscopy. A series of processable polyimides were prepared from these
new diamines and dianhydrides. The structure of polyimides was characterized by IR and
1H-NMR spectroscopy. The polyimides were characterized by X-ray diffraction,
thermogravimetry, differential scanning calorimetery, gel permeation chromatography,
solution viscosity and solubility studies. The polyimides were studied for possible
application as electrical insulations materials for high temperature electrical applications.
2.2 EXPERIMENTAL
2.2.1 Materials
The compound 2-(4-aminophenyl)-2’-(4-hydroxyphenyl)propane was prepared in the
laboratory from bisphenol-A and aniline hydrochloride and recrystallised before use. The
compounds aniline, bisphenol-A, 4,4’-dichlorodiphenyl sulfone, 4,4’-difluorobenzo
phenone, pyromellitic dianhydride (PMDA), benzophenone tetracarboxylic dianhydride
(BTDA) and bisphenol-A dianhydride (BPADA) were purchased from Sigma Aldrich
Chemicals. N-methyl 2- pyrrolidone was dried with molecular sieves (4Ao) before use.
All the reagents used were of analytical grade.
40
2.2.2 Measurements
FT-IR spectra were obtained from Perkin Elmer Spectrum One and 1H-NMR spectra
were recorded on a Bruker 300 MHz instrument. X-ray diffractograms were obtained on
PANalytical-model: X’per PRO using CuKά radiation. Thermogravimetric and
differential scanning calorimetric analysis were performed on TA Instruments Model
SDT Q600 at a heating rate of 10 0C / min in nitrogen atmosphere. Gel permeation
chromatography measurements were carried out with JASCO CO-1560 intelligent
column thermostat. Inherent viscosities were determined at a concentration of 0.5 g /dL
in DMF. Dielectric constant and impedance measurements were carried out with HIOKI
LCR HiTester 3532-50 at a frequency of 10 MHz. The water absorption capacity of the
polyimides was measured by ASTM D570-81 procedure. The solubility of polyimides
was determined at a 5 wt % concentration in various solvents at room temperature or on
heating.
2.2.3 Synthesis of 2-(4-aminophenyl)-2-(4-hydroxyphenyl)propane
The starting compound 2-(4-aminophenyl)-2-(4-hydroxyphenyl) propane (III) was
synthesized as per the following procedure. Aniline hydrochloride (I) (25.9 g, 0.2 mol)
and bisphenol-A (II) (50.2 g, 0.22 mol) were taken in a flask and heated at 180 0C for 30
minutes under nitrogen atmosphere. The reaction mixture was poured into water and the
water solution was shaken up with ethyl acetate to remove phenolic impurities. The water
solution was neutralized with aqueous NaHCO3 solution until pH became 8, whereupon
the crude product precipitated. The product 2-(4-aminophenyl)-2-(4-hydroxyphenyl)
propane (III) was collected by filtration, dried and recrystalised from ethyl acetate, Yield:
60 %, Melting point: 183 0C.
41
2.2.4 Synthesis of diamine monomer
The diamines bis-4,4’[(4-aminophenyl-2,2-isopropylidene phenyloxy)]diphenyl sulfone
and bis-4,4’[(4-aminophenyl-2,2-isopropylidene phenyloxy)]benzophenone were
synthesized by the nucleophilic substitution reaction of 2-(4-aminophenyl)-2- (4-
hydroxyphenyl)propane with the corresponding 4,4’-dichlorodiphenyl sulfone/ 4,4’-
difluorobenzophenone in NMP.129,130
2.2.4.1 Synthesis of bis-4, 4’ [(4-aminophenyl-2,2-isopropylidene phenyloxy)]
diphenyl sulfone
The diamine monomer bis-4,4’[(4-aminophenyl-2,2-isopropylidene phenyloxy)] diphenyl
sulfone was prepared by the reaction of 2-(4-aminophenyl)-2-(4-hydroxy phenyl)propane
(III) with 4,4’-dichlorodiphenyl sulfone (IV). A typical procedure adopted was as
follows: 4,4’dichlorodiphenyl sulfone (5.74 g, 0.02 mol) was dissolved in 25 mL of dry
NMP and 15 mL of toluene in a three- necked flask. To this 2-(4-aminophenyl) -2-(4-
hydroxyphenyl)propane (9.09 g, 0.04 mol) and K2CO3 (6.91 g, 0.05 mol) were added.
The reaction mixture was heated to 140 0C for 6 h at nitrogen atmosphere with
continuous stirring. The water formed was removed azeotropically using Dean-Stark trap.
The reaction temperature was raised to 165 0C, toluene was removed and the reaction was
continued for 20 h. The reaction mixture was cooled and poured into water. The
precipitate formed was filtered, washed with 5 % NaOH solution and water. The diamine
bis-4,4’[(4-aminophenyl-2,2-isopropylidene phenyloxy)]diphenyl sulfone (V) collected
was dried in vacuum oven at 60 0C. Colour: Brown, Yield: 92 %, Melting point:118-120
0C, IR(KBr): 3446 cm
-1 (N-H, NH2), 3058 cm
-1 (C-H,Ar), 2962 cm
-1 (C-H, ali), 1586 cm
-
1(C-N), 1242 cm
-1(C-O) and 1151 cm
-1( S=O, sulfone),
1H-NMR (300 MHz, DMSO-
42
d6,ppm): δ 1.50 (s, 12 H,-CH3), δ 4.88 (s,4H, -NH2 ), δ 6.46-6.49 (d, 4H,Ar), δ 6.87-6.90
(d, 4H, Ar), δ 6.99-7.03 (d,4H,Ar), δ 7.06-7.09 (d, 4H, Ar), δ 7.24-7.26 (d,4H,Ar) and δ
7.88-7.92(d, 4H, Ar).
2.2.4.2 Synthesis of bis-4,4’[(4-aminophenyl-2,2-isopropylidene phenyloxy)]
benzophenone
The diamine bis-4,4’[(4-aminophenyl-2,2-isopropylidene phenyloxy)]benzophenone
(VII) was prepared by the same procedure as adopted for bis-4,4’[(4-aminophenyl-2, 2-
isopropylidene phenyloxy)]diphenyl sulfone (V) using 4,4’-difluorobenzophenone (VI)
instead of 4,4’-dichlorodiphenyl sulfone. Colour: Light brown, Yield: 90 %, Melting
point: 115-116 0C, IR (K Br) : 3446 cm
-1 (N-H, NH2), 3044 cm
-1 (C-H, Ar), 2963 cm
-1
(C-H, ali),1652 cm-1
(C=O, keto), 1595 cm-1
(C -N) and 1240 cm-1
(C-O).1H-NMR (300
MHz, DMSO-d6, ppm ) : δ 1.58 (s,12 H, -CH3), δ 4.88 (s, 4H, -NH2), δ 6.46-6.50 (d,4H,
Ar), δ 6.88-6.91 (d,H,Ar), δ 7.03-7.04 (d,4H,Ar), δ 7.04-7.06 (d,4H,Ar), δ 7.25-7.28
(d,4H,Ar) and δ 7.74-7.77 (d,4H,Ar).
2.2.5 Synthesis of polyimide
Polyimides (PI-1 to PI-6) were synthesized by polycondensation of newly synthesized
diamine monomers V and VII with corresponding commercially available aromatic
dianhydrides through high temperature solution imidization of polyamic acid.31-33
2.2.5.1 Synthesis of polyimide PI-1
To a stirred solution of bis-4,4’[(4-aminophenyl-2,2-isopropylidene phenyloxy)] diphenyl
sulfone (V) (3.01g, 0.0045 mol) in NMP (20 mL), PMDA (0.992 g, 0.0045 mol) was
added under nitrogen atmosphere and kept stirred at room temperature for 12 h. To this,
43
toluene (20 mL) was added and the resulting mixture was heated at 160 0C for 16 h, while
removing water azeotropically using Dean-Stark trap. After removing toluene the
reaction mixture was cooled, poured into water, and the precipitated polyimide PI-1was
filtered, washed with water and dried. The polyimide PI-1 was obtained as a brown
colour material, Yield: 88 %, IR (KBr):1776 cm-1
and 1726 cm-1
(C=O, imide) 720 cm-1
(imide ring) and 1148 cm-1
(S=O, sulfone), 1H-NMR (300 MHz, DMSO-d6, ppm ):
δ 1.64 (s, 12H, -CH3), δ 6.67-6.73 (s, b,10H, Ar), δ 7.58-7.67 (s, b,10H,Ar), δ 8.08-8.17
(s, b, 4H,Ar) and δ 8.31 (s, 2H,Ar).
2.2.5.2 Synthesis of polyimide PI-2
Polyimide PI-2 was synthesized from bis-4,4’[(4-aminophenyl-2,2-isopropylidene
phenyloxy)]diphenyl sulfone (V) and BTDA using the same procedure as adopted for PI-
1. The polyimide PI-2 was obtained as a brown coloured material, Yield: 86 %, IR (KBr):
1779 cm-1
and 1723 cm-1
(C=O, imide), 720 cm-1
(imide ring) and 1148 cm-1
(S=O,
sulfone), 1H-NMR (300 MHz, DMSO-d6, ppm ): δ 1.66 (s,12H,-CH3), δ 6.98-7.05 (s, b,
10H,Ar), δ 7.57-7.71(s,b,10H, Ar), δ 7.85-7.89 (s,b,6H,Ar), and δ 8.13-8.22 (s,b, 4H,Ar).
2.2.5.3 Synthesis of polyimide PI-3
Polyimide PI-3 was synthesized from bis-4,4’[(4-aminophenyl-2,2-isopropylidene
phenyloxy)]diphenyl sulfone (V) and BPADA using the same procedure as adopted for
PI-1. The polyimide PI-3 was obtained as a light brown coloured material, Yield: 80 %,
IR (KBr):1776 cm-1
and 1716 cm-1
(C=O, imide), 719 cm-1
(imide ring) and 1149 cm-1
(S=O, sulfone), 1H-NMR (300 MHz, DMSO-d6, ppm): δ 1.64 (s, 18H, -CH3 ), δ 7.06-7.10
(s, b, 14H, Ar), δ 7.51-7.55 (s, b, 18H, Ar) and δ 7.89-7.88 (s, b, 6H, Ar).
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2.2.5.4 Synthesis of polyimide PI-4
Polyimide PI-4 was synthesized from bis-4,4’[(4-aminophenyl-2,2-isopropylidene
phenyloxy)]benzophenone (VII) and PMDA using the same procedure as adopted for
PI-1. The polyimide PI-4 was obtained as a brown coloured material, Yield: 86 %, IR
(KBr):1775 cm-1
and 1725 cm-1
(C=O, imide), 721 cm-1
(imide ring) and 1651cm-1
(C=O,
keto).
2.2.5.5 Synthesis of polyimide PI-5
Polyimide PI-5 was synthesized from bis-4,4’[(4-aminophenyl-2,2-isopropylidene
phenyloxy)]benzophenone (VII) and BTDA using the same procedure as adopted for
PI-1. The polyimide PI-5 was obtained as a brown coloured material, Yield : 81 %, IR
(KBr): 1779 cm-1
and 1723 cm-1
(C=O, imide), 721(imide ring) and 1652 cm-1
(C=O,
keto), 1H-NMR (300 MHz, DMSO-d6,ppm): δ 1.66 (s,12H,-CH3), δ 7.33-7.40 (s,b,10H,
Ar), δ 7.53-7.60 (s, b, 10H, Ar), δ 7.73-7.84 (s, b, 4H, Ar) and δ 7.96-8.21 (s,b, 6H,Ar).
2.2.5.6 Synthesis of polyimide PI-6
Polyimide PI-6 was prepared from bis-4,4’[(4-aminophenyl-2,2-isopropylidene
phenyloxy)]benzophenone (VII) and BPADA using the same procedure as used for PI-1.
The polyimide PI-6 was obtained as a light brown coloured material, Yield: 79 % , IR
(KBr): 1777 cm-1
and 1716 cm-1
(C=O, imide), 721 cm-1
(imide ring) and 1651 cm-1
(C=O,
keto), 1H-NMR (300 MHz, DMSO-d6,ppm ): δ 1.68 (s,18H,-CH3 ), δ 7.05-7.15 (s,b,14H,
Ar), δ 7.33-7.55 (s, b,18H,Ar), δ 7.91-7.92 (s,b,4H,Ar), and δ 8.02 (s,2H,Ar).
45
2.3 RESULT AND DISCUSSION
2.3.1 Synthesis of 2-(4-aminophenyl)-2-(4-hydroxyphenyl)propane
The compound 2-(4-aminophenyl)-2-(4-hydroxyphenyl)propane (III) was prepared from
aniline hydrochloride (I) and bisphenol-A (II) under nitrogen atmosphere at 180 0C
(Scheme 2.1).
NH2HCl
+ HO
CH3
CH3
OHN2
HO
CH3
CH3
NH2
I II III
Scheme 2.1: Synthesis of 2-(4-aminophenyl)-2-(4-hydroxyphenyl)propane
2.3.2 Synthesis of diamine monomer containing flexible groups and isopropylidene
groups
The diamine monomers bis-4,4’[(4-aminophenyl-2,2-isopropylidene phenyloxy)]
diphenylsulfone (V) and bis-4,4’[(4-aminophenyl-2,2-isopropylidene phenyloxy)]
benzophenone (VII) were prepared by the aromatic nucleophilic substitution reaction of
2-(4-aminophenyl)-2-(4-hydroxyphenyl)propane (III) with the corresponding
4,4’dichlorodiphenyl sulfone (IV) or 4,4’-difluorobenzophenone (VI) in the presence of
K2CO3 at nitrogen atmosphere in NMP (Scheme 2.2).
46
Cl S Cl
O
O
HO
CH3
CH3
NH2+ 2
O
CH3
CH3
NH2O S
O
O
H2N
CH3
CH3
K2CO3
IV III
F C F HO
CH3
CH3
NH2+ 2
O
CH3
CH3
NH2O C
O
H2N
CH3
CH3
K2CO3
VI III
O
VII
V
Scheme 2.2: Synthesis of diamine monomers
2.3.2.1 FT-IR spectroscopic analysis of diamine monomers
The IR spectra of monomers bis-4,4’[(4-aminophenyl-2,2-isopropylidene
phenyloxy)]diphenyl sulfone (V) and bis-4,4’[(4-aminophenyl-2,2-isopropylidene
phenyloxy)]benzophenone (VII) are given in Figure 2.1.
47
Figure 2.1: IR spectra of diamine monomers
The IR spectrum of bis-4,4’[(4-aminophenyl-2,2-isopropylidene
phenyloxy)]diphenyl sulfone (V) showed a characteristic absorption at 3446 cm-1
(N-H
stretching, aromatic amine), 3058 cm-1
(C-H stretching, aromatic), 2962 cm-1
(C-H
stretching, aliphatic), 1586 cm-1
(C-N stretching), 1242 cm-1
(C-O stretching ) and 1151
cm-1
(S=O, sulfone). The IR spectrum of monomer bis-4,4’[(4-aminophenyl-2,2-
isopropylidene phenyloxy)]benzophenone (VII) showed a characteristic absorption at
3446 cm-1
(N-H stretching, aromatic amine), 3044 cm-1
(C-H stretching, aromatic), 2963
48
cm-1
(C-H stretching, aliphatic), 1595 cm-1
(C-N stretching), 1240 cm-1
(C-O stretching )
and 1652 cm-1
(C=O, ketone).
2.3.2.2 1H-NMR spectroscopic analysis of diamine monomers
1H-NMR spectrum of monomer bis-4,4’[(4-aminophenyl-2,2-isopropylidene
phenyloxy)]diphenyl sulfone (V) is given in Figure 2.2.
Figure 2.2: 1H-NMR spectrum of bis-4,4’[(4-aminophenyl-2,2-isopropylidene
phenyloxy)]diphenyl sulfone
The four aromatic protons ortho to the sulfone group appeared as a doublet at
7.88-7.92 δ ppm and the four aromatic protons meta to the sulfone group appeared as a
49
doublet at 6.87-6.90 δ ppm. The four aromatic protons ortho to the ether group and meta
to the isopropylidene group appeared as a doublet at 6.46-6.49 δ ppm. The four protons
meta to the ether group and ortho to the isopropylidene group appeared as a doublet at
7.06-7.09 δ ppm. The twelve aliphatic protons appeared at 1.50 δ ppm as a singlet. The
four aromatic protons meta to the amino group appeared as a doublet at 7.24-7.26 δ ppm.
The four protons ortho to the amino group appeared as a doublet at 6.99-7.03 δ ppm. The
four aromatic amino protons appeared as a singlet at 4.88 δ ppm. The protons designated
in the Figure 2.2 as “a” appeared in the down field at 7.88-7.92 δ ppm which was due to
the electron withdrawing –SO2- group. The peaks at 2.5 δ ppm and at 3.4 δ ppm are due
to DMSO and water in DMSO.
The 1H-NMR spectrum of monomer bis-4,4’[(4-aminophenyl-2,2-isopropylidene
phenyloxy)]benzophenone (VII) is given in Figure 2.3. The four aromatic protons ortho
to the keto group appeared as a doublet at 7.74-7.77 δ ppm and the four aromatic protons
meta to the keto group appeared as a doublet at 6.88-6.91 δ ppm. The four aromatic
protons ortho to the ether groups and meta to the isopropylidene group appeared as a
doublet at 6.46-6.50 δ ppm. The four protons meta to the ether group and ortho to the
isopropylidene group appeared as a doublet at 7.04-7.06 δ ppm. The twelve aliphatic
protons appeared at 1.58 δ ppm as a singlet. The four aromatic protons meta to the amino
group appeared as a doublet at 7.25-7.28 δ ppm. The four protons ortho to the amino
group appeared as a doublet at 7.03-7.04 δ ppm. The aromatic amino group appeared as a
singlet at 4.88 δ ppm. The protons designated in the Figure 2.3 as “a” appeared in the
down field at 7.74-7.77 δ ppm which is due to the electron withdrawing –C=O group.
The peaks at 2.5 δ ppm and at 3.4 δ ppm are due to DMSO and water in DMSO.
50
Figure 2.3: 1H-NMR spectrum of bis-4,4’[(4-aminophenyl-2,2-isopropylidene
phenyloxy)]benzophenone
The IR and 1H-NMR analysis of diamines V and VII are in accordance with the
proposed structures of diamines and the spectral data are summarized in Table 2.1.
51
Table 2.1: Physical characteristics and spectral data of diamine monomers
a Ar = aromatic, Ali = aliphatic, s = singlet, d = doublet
Diamine
monomer
Yield
%
Melting
point 0C
IR (KBr), cm-1
1H-NMR in DMSO-d6
δ ppm a
V
VII
93
90
118-120
115-116
3446(N-H,NH2),3058
(C-H,Ar),2962(C-H,Ali),
1586(C-N),1242(C-O),
1151 ( S=O, sulfone)
3446(N-H,NH2),3044
(C-H,Ar),2963(C-H,Ali)
1595(C -N), 1240(C-O)
1652 (C=O, keto),
1.50 (s, 12 H, -CH3),
4.88 (s, 4H, -NH2 ),
6.46-6.49(d, 4H,Ar),
6.87-6.90(d, 4H, Ar),
6.99-7.03(d, 4H,Ar),
7.06-7.09(d, 4H, Ar),
7.24-7.26(d, 4H, Ar),
7.88-7.92 (d, 4H, Ar)
1.58 (s, 12 H, -CH3),
4.88 (s, 4H, -NH2),
6.46-6.50 (d, 4H, Ar),
6.88-6.91 (d, H,Ar),
7.03-7.04(d, 4H, Ar),
7.04-7.06(d, 4H, Ar),
7.25-7.28(d, 4H, Ar)
7.74-7.77 (d, 4H, Ar)
52
2.3.3 Synthesis of polyimide through high temperature solution imidization
Polyimides (PI-1 to PI-6) were synthesized by polycondensation of diamine monomers
with corresponding aromatic dianhydrides in two-step method (Scheme 2.3).
O
CH3
CH3
NH2O XH2N
CH3
CH3
+
O
CH3
CH3
O X
CH3
CH3
X=
O
O
CH3
CH3
O C
O
S
O
O
PI - 1 to PI - 6
N
Ar =
V or VII
ArO O
O O
O O
ArN
O O
OO
n
Scheme 2.3: Synthesis of polyimides from new diamines
In the first step a soluble polyamic acid was prepared at room temperature. In the
next step complete cyclization of the intermediate polyamic acid was achieved by high
53
temperature solution imidization. The water formed was removed by toluene-water
azeotropic distillation at 160 0C for 16 h.
The inherent viscosities of the polyimides were measured in DMF at 30 0C and
were in the range 0.39-0.55 dL/g, indicating formation of polymers of moderate
molecular weight. The weight average molecular weights (Mw) determined by gel
permeation chromatography based on polystyrene standards in THF are presented in
Table 2.2.
Table 2.2: Preparation of polyimides
Polyimide
Code
Diamine
Dianhydride
Yield
%
η inh a
dL /g
GPC b
Mw(g/mol)
PI-1
PI-2
PI-3
PI-4
PI-5
PI-6
V
V
V
VII
VII
VII
PMDA
BTDA
BPADA
PMDA
BTDA
BPADA
88
86
80
86
81
79
0.55*
0.48 *
0.40*
0.50Ψ
0.43*
0.39*
53 363
43 754
42 518
`
a Measured with 0.5 g / dL at 30
0C , * = in DMF, Ψ = in H2SO4
b
Mw determined based on polystyrene standards in THF
54
2.3.3.1 FT-IR spectroscopic analysis
Representative IR spectra of polyimides PI-1, PI-3 and PI-5 are given in Figure 2.4.
Figure 2.4: IR spectra of polyimides PI-1, PI-3 and PI-5
The IR spectrum of PI-1 showed absorption at 1776 cm-1
(imide C=O symmetric
stretching), 1726 cm-1
(imide C=O asymmetric stretching), 1366 cm-1
(imide C-N
55
stretching), and 720 cm-1
(imide ring) associated with imide structure, indicating the
formation of imide rings. The disappearance of strong absorption corresponding to amino
group at 3446 cm-1
that was present in the monomer bis-4,4’[(4-aminophenyl-2,2-
isopropylidene phenyloxy)]diphenyl sulfone (V) and absence of peak at 1650 cm-1
corresponding to amic acid indicated complete imidization. The IR spectral data of
polyimides PI-1 to PI-6 showed the formation of imide ring between the diamines and the
aromatic dianhydrides.
2.3.3.2 1H-NMR spectroscopic analysis
1H-NMR spectra of polyimides PI-2 is given in Figure 2.5. The
1H-NMR spectrum of
polyimides PI-2 showed absorption at 1.66 (s,12H,-CH3) δ ppm, 6.98-7.05 (s,b,10H,Ar)
δ ppm, 7.57-7.71 (s,b,10H,Ar) δ ppm, 7.85-7.89 (s,b,6H,Ar) δ ppm, and 8.13-8.22
(s,b,4H,Ar) δ ppm. The disappearance of high field signal that was present in the
monomer bis-4,4’[(4-aminophenyl-2,2-isopropylidene phenyloxy)]diphenyl sulfone (V)
at 4.88 δ ppm corresponding to the amino group and the appearance of farthest downfield
signal in the range 8.13-8.22 δ ppm due to electron withdrawing imide group indicated
the formation of imide ring between the diamine bis-4, 4’[(4-aminophenyl-2,2-
isopropylidene phenyloxy)]diphenyl sulfone (V) and BTDA
56
Figure 2.5: 1H-NMR spectrum of polyimides PI-2
The 1H-NMR spectrum of polyimides PI-6 is given in Figure 2.6 showed absorption at
1.68 (s,18H, -CH3 ) δ ppm, 7.05-7.15(s, b 14H,Ar) δ ppm, 7.33-7.55 (s, b,18H,Ar) δ ppm,
7.91-7.92 (s,b,4H,Ar) δ ppm, 8.02 (s,2H,Ar) δ ppm. The disappearance of high field
signal that was present in the monomer bis-4,4’[(4-aminophenyl-2,2-isopropylidene
phenyloxy)]benzophenone (VII) at 4.88 δ ppm corresponding to the amino group and the
appearance of farthest downfield signal at 8.02 δ ppm and in the range 7.91-7.92 δ ppm
due to electron withdrawing imide group indicated the formation of imide ring between
the diamine bis-4,4’[(4-aminophenyl-2,2-isopropylidene phenyloxy)]benzophenone VII
57
and BPADA. The observed 1H-NMR spectral data were fully consistent with the
proposed chemical structure of the polyimides. The IR and 1H-NMR spectral
assignment of polyimides (PI-1 to PI-6) are given in Table 2.3.
Figure 2.6: 1H-NMR spectrums of polyimides PI-6
58
Table 2.3: Spectral data of polyimides
a Ar = aromatic, Ali = aliphatic, s = singlet, d = doublet
Polyimide
code
IR (KBr), cm-1
1H-NMR in DMSO-d6
δ ppm
a
PI -1
PI-2
PI-3
PI-4
PI-5
PI- 6
1776, 1726,720 (imide)
1148 (S=O, sulfone)
1779,1723, 719 (imide)
1148 (S=O, sulfone)
1776, 1716,719 (imide)
1149 (S=O, sulfone)
1775, 1725,721 (imide)
1651(C=O, keto)
1779,1723, 721 (imide)
1652(C=O, keto)
1777, 1716. 721(imide)
1651(C=O, keto)
1.64(s, 12H, -CH3), 6.67-6.73(s, b, 10H,Ar),
7.58-7.67(s,b,10H,Ar),8.08 -8.17(s,b,4H,Ar),
8.31(s,2H,Ar)
1.66 (s, 12H, -CH3), 6.98-7.05 (s, b, 10H,Ar),
7.57-7.71(s,b,10H,Ar),7.85-7.89(s,b,6H,Ar),
8.13-8.22(s,b, 4H,Ar)
1.64 (s, 18H, -CH3 ), 7.06-7.10 (s, b, 14H,Ar)
7.51-7.55 (s, b, 18H,Ar),
7.89-7.88 (s, b, 6H,Ar)
1.66(s, 12H, -CH3), 7.33-7.40(s, b, 10H,Ar),
7.53-7.60(s, b,10H,Ar),7.73-7.84(s,b,4H,Ar),
7.96-8.21(s,b, 6H,Ar)
1.68(s, 18H, -CH3 ),7.05-7.15(s, b, 14H,Ar),
7.33-7.55(s, b, 18H,Ar),7.91-7.92(s,b,4H,Ar),
8.02(s,2H,Ar)
59
2.3.4 Properties of polyimides
2.3.4.1 Solubility characteristics of polyimides
Polyimides were tested for solubility at 5 wt % concentration in different organic solvent.
The solubility characteristics of the polyimides are shown in Table 2.4. Except PI-1 &
PI-4 all the polyimides were soluble in DMF, DMAc, NMP, DMSO, H2SO4 and pyridine
at room temperature. All the polyimides were soluble in m-cresol on heating. Polyimides
derived from diamine monomer bis-4,4’[(4-aminophenyl-2,2-isopropylidene
phenyloxy)]diphenyl sulfone (V) showed higher solubility due to the presence of -SO2 -
group in addition to the ether groups in the chain backbone when compared to the
polyimides derived from monomer bis-4,4’[(4-aminophenyl-2,2-isopropylidene
phenyloxy)]benzophenone (VII).
.
Table 2.4: Solubility of polyimides
Polyimide
code
NMP
DMSO
DMF
DMAc Pyridine THF m-Cresol Acetone
Conc.
H2SO4
PI-1
PI-2
PI-3
PI-4
PI-5
PI-6
+
+
+
+ -
+
+
+ -
+
+
-
+
+
+ -
+
+
-
+
+
-
+
+
-
+ -
+
+ -
+ -
+ -
+ -
+ -
+ -
-
-
-
-
-
-
+
+
+
+
+
+
+ = Soluble at room temperature, + - = Soluble on heating, - = Insoluble
60
Polyimides derived from BPADA showed improved solubility in common
organic solvents due to the presence of more number of ether and isopropylidene groups
in the polyimide chain. The solubility of the polyimides decreased in the following order
BPADA-V, BPADA-VII, BTDA-V > BTDA-VII > PMDA-V > PMDA-VII. Among all,
polyimide PMDA-VII showed lower solubility due to the presence of rigid backbone
based on PMDA imides ring. The improved solubility of the polyimides may be
explained by the fact that the incorporation of flexible linkages and isopropylidene
groups into the polyimide backbone decrease the inter chain interaction, leading to
amorphous morphology which in turn increase solubility. Thus the solubility of
polyimides was governed by the structure of both diamines and dianhydrides.
2.3.4.2 Thermal properties of polyimides
The thermal properties of the polyimides were evaluated by TG and DSC in nitrogen
atmosphere at a heating rate of 10 0C / min. The thermal analysis data are summarized in
Table 2.5. The temperature at which 5 % and 10 % weight loss occurred in the ranges
290–400 0C and 378 - 465
0C respectively. Polyimides PI-4 exhibited high thermal
stability due to the presence of rigid backbone based on PMDA imides ring. Similarly the
polyimides PI-3 showed comparatively less thermal stability due to the presence of
isopropylidene groups in the backbone based on BPADA ring.
61
Table 2.5: Thermal properties of polyimides
Polyimide
N2 a
Tg0C
Texob 0
C T50C T10
0C T20
0C T30
0C
PI-1
PI-2
PI-3
PI-4
PI-5
PI-6
345
329
290
400
298
296
465
410
378
450
413
395
509
482
470
504
509
469
557
517
530
526
530
526
252
243
238
---
230
---
655
596
650
588
661
576
a Temperature at 5 %, 10 %, 20 % or 30 % weight loss in N2 atmosphere
b Temperature at which maximum exothermic peak observed
The DSC thermogram of these polyimides showed a broad exothermal peak due
to the degradation of the polyimides in the temperature range 576-661 0C. Endotherms
corresponding to the crystalline melt temperature (Tm) were not observed for any of the
polyimides, indicating that these polyimides were amorphous. The glass transition
temperature was observed in the temperature range 230 - 252 0C. The Tg value was high
for PI-1 because of rigid backbone based on PMDA. The Tg value decreased with
increasing numbers of flexible linkages and isopropylidene groups in the polyimide
chain. This was due to decrease in intermolecular interaction of chain, which increase the
flexibility of the chain and decrease in Tg value. TG curves are given in Figure 2.7. TG
curves indicated that these polyimides undergo rapid degradation around 405–4910C.
Thermal analysis indicated that these polyimides were thermally stable.
62
Figure 2.7: TG curves of polyimides (PI-1 to PI-6)
2.3.4.3 X-ray diffraction studies of polyimides
The crystallinity of the polyimides was examined by X-ray diffraction studies. The X-ray
diffraction patterns are given in Figure 2.8. The X-ray diffraction patterns were broad
with no well defined peaks which indicated that all these polyimides were amorphous.
This was due to the presence of flexible linkages like ether, sulfonyl and isopropylidene
groups in the polyimide chains that disturb the chain-to-chain interactions leading to an
amorphous morphology. The amorphous nature of these polyimides was well reflected in
their solubility characteristics. The solubility behavior was in agreement with the result of
X-ray diffraction studies.
63
Figure 2.8: X-ray diffractograms of polyimides (PI-1 to PI-6)
2.4 APPLICATIONS OF POLYIMIDES-HIGH TEMPERATURE INSULATION
The list of polyimides applications is unending and it still keeps growing with the
increasing demand of growing technologies. Polyimides are used in the form of films,
fibres, foams, plastics and adhesives. Polyimides films are used as insulation materials.
At present, annual production of polyimide films in the world amounts to 1000 ton. The
first place is occupied by PM film based on pyromellitic dianhydride and
diaminodiphenyl ether. Polyimides have promising dielectric constants and the film
exhibits high dielectric stability at elevated temperature. Polyimides films are used in
64
insulation of electrochemical items, cables, generators, electric motors, and other units
and parts operating at elevated temperatures as well as system operating at lower
temperature, which considerably extends their service life and ensures reliable protection
in the case of emergency overheating.
2.4.1 Film
The high temperature electrical insulation of polyimide film gains growing importance
due to the demand from various industrial and domestic insulation applications. Newer
polymeric materials are being investigated to meet demand of various types. Synthetic
efforts have been focused for improving the solubility of polyimides in organic solvents
for easy fabrication of polyimides into film without sacrificing the thermal stability and
insulation characteristics. The polyimides PI-1 to PI-6 reported in this chapter were
studied for possible application as high temperature electrical insulations.
2.4.2 Electrical properties of polyimides
The dielectric constant is an important parameter for selecting electrical insulation
material. The dielectric constant and impedance of the polyimides were determined at a
frequency of 10 MHz. The results are presented in Table 2.6. The dielectric constants of
polyimides were in the range 2.85 - 3.83. The impedance values of polyimides were in
the range 142 – 84 M Ohm. The polyimides have excellent electrical insulation character.
65
Table 2.6: Electrical properties of polyimides
Polyimides Dielectric
constant (є)
Impedance( Z)
M Ohm
PI -1
PI-2
PI-3
PI-4
PI-5
PI-6
3.63
2.98
2.85
3.83
3.25
3.05
104
135
142
84
120
127
2.4.3 Water absorbing capacity of polyimides
The determination of water absorption capacity is important because it can adversely
affect the mechanical and dielectric properties. The measurement of water absorbing
capacity of the polyimide was carried out following ASTM D570-81 procedure. The
polyimides film was placed in a vacuum oven at 80 0C till the film attained a constant
weight and then immediately weighed out to the nearest 0.001 g to get the initial weight
(W0). The film was then completely soaked in a container of deionized water maintained
at 25 0C. After 24 hours the film was removed from water and then quickly placed
between sheets of paper to remove the excess water and the film was weighed
immediately. The film was again immersed in water. After another 24 h soaking period,
the film was removed, dried and weighed for any weight gain. The procedure was
repeated till the film almost attains a constant weight. The total soaking time was 168 h
66
and the sample was weighed at a regular 24 hours time interval to get the final weight
(Wt).The percentage of increase in weight of the sample was calculated to the nearest
0.01 %. The amount of water absorption by polyimides under saturated condition for 168
h was in the range 1.0 –1.78 %. This low values may be due to the presence of water
repelling isopropylidene groups in the polyimides chain.
The polyimides were soluble in organic solvents due to the presence of ether and
isopropylidene groups and have good film forming characteristics. The polyimides were
thermally stable and have dielectric constant in the range 2.85 - 3.83. Polyimides films
can be used in insulation of electrical items, cables, generators, electric motors, and other
units and parts operating at elevated temperatures. Since the polyimide film has water
absorption capacity within the range of 1.0 - 1.78 %, the mechanical and dielectric
properties will not be affected by moisture. So, the film can also be used as insulation
materials for system operating at lower temperatures.
2.5 CONCLUSIONS
1 Two novel aromatic diamine monomers were synthesized in high yield A series of
processable aromatic polyimides were prepared through high temperature solution
imidization.
2 The inherent viscosities were in the range 0.39 - 0.55 dL / g, the weight average
molecular weights determined by GPC was in the range 42518 - 53363 g / mol,
indicating formation of polymers of moderate molecular weight.
3 X-ray diffraction reavealed that these polyimides were amorphous due to the
presence of flexible linkages and isopropylidene groups in the polymer chain. The
67
amorphous character was well reflected in the solubility, polyimides
exhibited high solubility in common organic solvents.
4 All the polyimides exhibited good thermal stability. The Tg values were observed
in the temperature range 230 - 252 0C. The Tg value was found to decrease with
increase of flexible linkages and isopropylidene groups in the polyimide chain.
5 The polyimides have dielectric constant in the range 2.85 - 3.83. They have
electrical insulation character. Polyimides films can be used in insulation of
electrical items operating at elevated temperatures. Since the polyimide film has
water absorption capacity within 1.0 - 1.78 %, the mechanical and dielectric
properties will not be affected by moisture.
6 Thus, these aromatic polyimides can be considered as promising processable high
temperature polymeric materials.