chapter 2: synthesis and characterization of...

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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).


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


(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).



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



(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).

44



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


(imide ring) and 1651cm-1

(C=O,

keto).



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).


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,

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.

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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.

chapter 2: synthesis and characterization of...

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