CHAPTER 2: SYNTHESIS AND CHARACTERIZATION OF PROCESSABLE AROMATIC POLYIMIDES

Transcription

1 38 CHAPTER 2: SYNTHESIS AND CHARACTERIZATIN F PRCESSABLE ARMATIC PLYIMIDES 2.1 INTRDUCTIN Aromatic polyimides 121,122 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)s 67 and poly(amideimide)s. 128 The objective of this work was to investigate the effects of introducing flexible groups (--,-S 2 -, and -C=), and isopropylidene groups in the chain backbone in polyimides. Two novel aromatic diamine monomers containing flexible groups (--, - S 2 -, and -C=) and isopropylidene groups were synthesized. A series of polyimides were synthesized from these new aromatic diamines and commercially available aromatic dianhydrides. The flexible groups (--,-S 2 -, and -C=) and isopropylidene groups in

2 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 1 H-NMR spectroscopy. A series of processable polyimides were prepared from these new diamines and dianhydrides. The structure of polyimides was characterized by IR and 1 H-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 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 (4A o ) before use. All the reagents used were of analytical grade.

3 Measurements FT-IR spectra were obtained from Perkin Elmer Spectrum ne and 1 H-NMR spectra were recorded on a Bruker 300 MHz instrument. X-ray diffractograms were obtained on PANalytical-model: X per PR using CuKά radiation. Thermogravimetric and differential scanning calorimetric analysis were performed on TA Instruments Model SDT Q600 at a heating rate of 10 0 C / min in nitrogen atmosphere. Gel permeation chromatography measurements were carried out with JASC C-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 HIKI LCR HiTester at a frequency of 10 MHz. The water absorption capacity of the polyimides was measured by ASTM D procedure. The solubility of polyimides was determined at a 5 wt % concentration in various solvents at room temperature or on heating 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 C 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 NaHC 3 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: C.

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, 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 K 2 C 3 (6.91 g, 0.05 mol) were added. The reaction mixture was heated to C 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 C, 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 % NaH 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 0 C. Colour: Brown, Yield: 92 %, Melting point: C, IR(KBr): 3446 cm -1 (N-H, NH 2 ), 3058 cm -1 (C-H,Ar), 2962 cm -1 (C-H, ali), 1586 cm - 1 (C-N), 1242 cm -1 (C-) and 1151 cm -1 ( S=, sulfone), 1 H-NMR (300 MHz, DMS-

5 42 d 6,ppm): δ 1.50 (s, 12 H,- ), δ 4.88 (s,4h, -NH 2 ), δ (d, 4H,Ar), δ (d, 4H, Ar), δ (d,4h,ar), δ (d, 4H, Ar), δ (d,4h,ar) and δ (d, 4H, Ar) 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: C, IR (K Br) : 3446 cm -1 (N-H, NH 2 ), 3044 cm -1 (C-H, Ar), 2963 cm -1 (C-H, ali),1652 cm -1 (C=, keto), 1595 cm -1 (C -N) and 1240 cm -1 (C-). 1 H-NMR (300 MHz, DMS-d 6, ppm ) : δ 1.58 (s,12 H, - ), δ 4.88 (s, 4H, -NH 2 ), δ (d,4h, Ar), δ (d,h,ar), δ (d,4h,ar), δ (d,4h,ar), δ (d,4h,ar) and δ (d,4h,ar) 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 Synthesis of polyimide PI-1 To a stirred solution of bis-4,4 [(4-aminophenyl-2,2-isopropylidene phenyloxy)] diphenyl sulfone (V) (3.01g, mol) in NMP (20 ml), PMDA (0.992 g, mol) was added under nitrogen atmosphere and kept stirred at room temperature for 12 h. To this,

6 43 toluene (20 ml) was added and the resulting mixture was heated at C 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=, imide) 720 cm -1 (imide ring) and 1148 cm -1 (S=, sulfone), 1 H-NMR (300 MHz, DMS-d 6, ppm ): δ 1.64 (s, 12H, - ), δ (s, b,10h, Ar), δ (s, b,10h,ar), δ (s, b, 4H,Ar) and δ 8.31 (s, 2H,Ar) 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=, imide), 720 cm -1 (imide ring) and 1148 cm -1 (S=, sulfone), 1 H-NMR (300 MHz, DMS-d 6, ppm ): δ 1.66 (s,12h,- ), δ (s, b, 10H,Ar), δ (s,b,10H, Ar), δ (s,b,6h,ar), and δ (s,b, 4H,Ar) 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=, imide), 719 cm -1 (imide ring) and 1149 cm -1 (S=, sulfone), 1 H-NMR (300 MHz, DMS-d 6, ppm): δ 1.64 (s, 18H, - ), δ (s, b, 14H, Ar), δ (s, b, 18H, Ar) and δ (s, b, 6H, Ar).

7 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=, imide), 721 cm -1 (imide ring) and 1651cm -1 (C=, keto) 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=, imide), 721(imide ring) and 1652 cm -1 (C=, keto), 1 H-NMR (300 MHz, DMS-d 6,ppm): δ 1.66 (s,12h,- ), δ (s,b,10h, Ar), δ (s, b, 10H, Ar), δ (s, b, 4H, Ar) and δ (s,b, 6H,Ar) 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=, imide), 721 cm -1 (imide ring) and 1651 cm -1 (C=, keto), 1 H-NMR (300 MHz, DMS-d 6,ppm ): δ 1.68 (s,18h,- ), δ (s,b,14h, Ar), δ (s, b,18h,ar), δ (s,b,4h,ar), and δ 8.02 (s,2h,ar).

8 RESULT AND DISCUSSIN 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 C (Scheme 2.1). NH 2 HCl H H N 2 H NH 2 I II III Scheme 2.1: Synthesis of 2-(4-aminophenyl)-2-(4-hydroxyphenyl)propane 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 K 2 C 3 at nitrogen atmosphere in NMP (Scheme 2.2).

9 46 Cl S Cl IV 2 K 2 C 3 H NH 2 III H 2 N S NH 2 V F C F 2 H VI NH 2 III K 2 C 3 H 2 N C VII NH 2 Scheme 2.2: Synthesis of diamine monomers 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.

10 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- stretching ) and 1151 cm -1 (S=, 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

11 48 cm -1 (C-H stretching, aliphatic), 1595 cm -1 (C-N stretching), 1240 cm -1 (C- stretching ) and 1652 cm -1 (C=, ketone) H-NMR spectroscopic analysis of diamine monomers 1 H-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: 1 H-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 δ ppm and the four aromatic protons meta to the sulfone group appeared as a

12 49 doublet at δ ppm. The four aromatic protons ortho to the ether group and meta to the isopropylidene group appeared as a doublet at δ ppm. The four protons meta to the ether group and ortho to the isopropylidene group appeared as a doublet at 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 δ ppm. The four protons ortho to the amino group appeared as a doublet at 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 δ ppm which was due to the electron withdrawing S 2 - group. The peaks at 2.5 δ ppm and at 3.4 δ ppm are due to DMS and water in DMS. The 1 H-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 δ ppm and the four aromatic protons meta to the keto group appeared as a doublet at δ ppm. The four aromatic protons ortho to the ether groups and meta to the isopropylidene group appeared as a doublet at δ ppm. The four protons meta to the ether group and ortho to the isopropylidene group appeared as a doublet at 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 δ ppm. The four protons ortho to the amino group appeared as a doublet at 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 δ ppm which is due to the electron withdrawing C= group. The peaks at 2.5 δ ppm and at 3.4 δ ppm are due to DMS and water in DMS.

13 50 Figure 2.3: 1 H-NMR spectrum of bis-4,4 [(4-aminophenyl-2,2-isopropylidene phenyloxy)]benzophenone The IR and 1 H-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.

14 51 Table 2.1: Physical characteristics and spectral data of diamine monomers Diamine monomer Yield % Melting point 0 C IR (KBr), cm -1 1 H-NMR in DMS-d 6 δ ppm a V (N-H,NH 2 ), (s, 12 H, - ), (C-H,Ar),2962(C-H,Ali), 4.88 (s, 4H, -NH 2 ), 1586(C-N),1242(C-), (d, 4H,Ar), 1151 ( S=, sulfone) (d, 4H, Ar), (d, 4H,Ar), (d, 4H, Ar), (d, 4H, Ar), (d, 4H, Ar) VII (N-H,NH 2 ), (s, 12 H, - ), (C-H,Ar),2963(C-H,Ali) 4.88 (s, 4H, -NH 2 ), 1595(C -N), 1240(C-) (d, 4H, Ar), 1652 (C=, keto), (d, H,Ar), (d, 4H, Ar), (d, 4H, Ar), (d, 4H, Ar) (d, 4H, Ar) a Ar = aromatic, Ali = aliphatic, s = singlet, d = doublet

15 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). Ar H 2 N V or VII X NH 2 N Ar N PI - 1 to PI - 6 X n Ar = X= S C 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

16 53 temperature solution imidization. The water formed was removed by toluene-water azeotropic distillation at C for 16 h. The inherent viscosities of the polyimides were measured in DMF at 30 0 C and were in the range dl/g, indicating formation of polymers of moderate molecular weight. The weight average molecular weights (M w ) 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 M w (g/mol) PI-1 V PMDA * PI-2 V BTDA * PI-3 V BPADA * PI-4 VII PMDA Ψ PI-5 VII BTDA * PI-6 VII BPADA * ` a Measured with 0.5 g / dl at 30 0 C, * = in DMF, Ψ = in H 2 S 4 b M w determined based on polystyrene standards in THF

17 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= symmetric stretching), 1726 cm -1 (imide C= asymmetric stretching), 1366 cm -1 (imide C-N

18 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 H-NMR spectroscopic analysis 1 H-NMR spectra of polyimides PI-2 is given in Figure 2.5. The 1 H-NMR spectrum of polyimides PI-2 showed absorption at 1.66 (s,12h,- ) δ ppm, (s,b,10h,ar) δ ppm, (s,b,10h,ar) δ ppm, (s,b,6h,ar) δ ppm, and (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 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

19 56 Figure 2.5: 1 H-NMR spectrum of polyimides PI-2 The 1 H-NMR spectrum of polyimides PI-6 is given in Figure 2.6 showed absorption at 1.68 (s,18h, - ) δ ppm, (s, b 14H,Ar) δ ppm, (s, b,18h,ar) δ ppm, (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 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

20 57 and BPADA. The observed 1 H-NMR spectral data were fully consistent with the proposed chemical structure of the polyimides. The IR and 1 H-NMR spectral assignment of polyimides (PI-1 to PI-6) are given in Table 2.3. Figure 2.6: 1 H-NMR spectrums of polyimides PI-6

21 58 Table 2.3: Spectral data of polyimides Polyimide code IR (KBr), cm -1 1 H-NMR in DMS-d 6 δ ppm a PI -1 PI-2 PI-3 PI-4 PI-5 PI , 1726,720 (imide) 1148 (S=, sulfone) 1779,1723, 719 (imide) 1148 (S=, sulfone) 1776, 1716,719 (imide) 1149 (S=, sulfone) 1775, 1725,721 (imide) 1651(C=, keto) 1779,1723, 721 (imide) 1652(C=, keto) 1777, (imide) 1651(C=, keto) 1.64(s, 12H, - ), (s, b, 10H,Ar), (s,b,10H,Ar), (s,b,4H,Ar), 8.31(s,2H,Ar) 1.66 (s, 12H, - ), (s, b, 10H,Ar), (s,b,10H,Ar), (s,b,6H,Ar), (s,b, 4H,Ar) 1.64 (s, 18H, - ), (s, b, 14H,Ar) (s, b, 18H,Ar), (s, b, 6H,Ar) 1.66(s, 12H, - ), (s, b, 10H,Ar), (s, b,10h,ar), (s,b,4h,ar), (s,b, 6H,Ar) 1.68(s, 18H, - ), (s, b, 14H,Ar), (s, b, 18H,Ar), (s,b,4H,Ar), 8.02(s,2H,Ar) a Ar = aromatic, Ali = aliphatic, s = singlet, d = doublet

22 Properties of polyimides 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, DMS, H 2 S 4 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 -S 2 - 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 DMS DMF DMAc Pyridine THF m-cresol Acetone Conc. H 2 S 4 PI PI PI PI PI PI = Soluble at room temperature, - = Soluble on heating, - = Insoluble

23 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 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 0 C / min. The thermal analysis data are summarized in Table 2.5. The temperature at which 5 % and 10 % weight loss occurred in the ranges C and C 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.

24 61 Table 2.5: Thermal properties of polyimides Polyimide N 2 a T 5 0 C T 10 0 C T 20 0 C T 30 0 C T g 0 C Texo b 0 C PI PI PI PI PI PI a Temperature at 5 %, 10 %, 20 % or 30 % weight loss in N 2 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 C. 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 C. 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 C. Thermal analysis indicated that these polyimides were thermally stable.

25 62 Figure 2.7: TG curves of polyimides (PI-1 to PI-6) 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.

26 63 Figure 2.8: X-ray diffractograms of polyimides (PI-1 to PI-6) 2.4 APPLICATINS F PLYIMIDES-HIGH TEMPERATURE INSULATIN 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

27 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 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 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 The impedance values of polyimides were in the range M hm. The polyimides have excellent electrical insulation character.

28 65 Table 2.6: Electrical properties of polyimides Polyimides PI -1 PI-2 PI-3 PI-4 PI-5 PI-6 Dielectric constant (є) Impedance( Z) M hm 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 D procedure. The polyimides film was placed in a vacuum oven at 80 0 C till the film attained a constant weight and then immediately weighed out to the nearest g to get the initial weight (W 0 ). The film was then completely soaked in a container of deionized water maintained at 25 0 C. 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

29 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 %. 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 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 %, 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 CNCLUSINS 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 dl / g, the weight average molecular weights determined by GPC was in the range 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

30 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 C. 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 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 %, 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.