Indian Journal of Textile Research

Vol. 10, December 1985, Pp. 171-174

Synthesis and Characterization of Polyarylenepyromellitimidesand Mechanical and Thermal Properties of Their Fibres

B S RAO and INDRA K VARMA

Centre for Materials Science & Technology, Indian Institute of Technology, New Delhi 110016, India

/' Received 8 April 1985; accepted 3 June 1985Polypyromellitimides based on 4, 4'-diaminobenzanilide (DAB), 2, 5-bis(p-aminophenyl)-I, 3, 4-oxadiazole (ODZ), 4, 4'­

diaminodiphenyl ether (E) and bis(3-aminophenyl)methylphosphine oxide (DAP) were synthesized and characterized by IRspectroscopy. Fibres were spun from a DMF solution of the corresponding polyamide-acid by wet spinning andcyclodehydrated to polyimide by chemical means. The 'as-spun' fibres had low tenacity and modulus. Fibres based on DABand ODZ diamines had higher modulus than those based on E. A decrease in the mechanical properties of fibres was observed

by heat-ageing at 200°C for several hours or on UV irradiatio~

Keywords: Fibre properties, Poly(arylenepyromellitimide) fibres, Polyimide fibres, Polypyromellitimides, Wet spinning

Several poly(arylenepyromellitimide) fibres with goodmechanical properties and elevated temperaturestability have been reported in the literaturet -3. Fibresbased on the well-known polypyromellitimide of 4,4'­diaminodiphenyl ether (PE) have been reported inearly 1960's.

PE

These fibres were prepared in two steps. First thepolyamic-acid fibres were obtained by dry- or wet­spinning. The fibres were cyclodehydrated topolyimide fibres by thermal (progressive heating up to300°C) or chemical means (acetic anhydride-pyridinemixture). During this conversion the fibres areextremely weak and higher strength is developed bystretching at high temperatures.

The properties of polyimide fibres can be altered bychanging the structure of the aromatic dianhydride orthe diamine used in the preparation of the polymer.Some modified polyimide fibres such as Kermel4-6(Rhone-Poulene) and polyimide "2080" (Up Johnfhave been commercialized in recent years. Kermelfibres are based on relatively cheap trimelliticanhydride and 4,4'-diisocyanatodiphenylmethane.

Direct spinning of polyimide from a-chlorophenol

has been reported recently by Ube Industries, Japans.

These fibres are based on biphthalic anhydride andcertain bridged diamines. Koton et al. have alsodescribed polyimide fibres with high modulus andtenacity9,10. These polyimides were based on adianhydride containing an ether group.

It would be of interest to study the effect of thestructure of diamine on thermal and mechanical

properties of polypyromellitimide fibres. This paperdeals with the synthesis and characterization offollowing poly(arylenepyromellitimide).

Fibres were prepared by the wet-spinning ofcorresponding poly(amide-acid) and subsequentchemical or thermal cyclization. The effect of

incorporation oflow percentages of phosphorus in thePE backbone was studied by preparing a copolymer.

(PE-DAP)

Experimental Procedure4, 4'-Diaminobenzanilide, bis(3-aminophenyl)

methylphosphine oxide, and 2, 5-bis(p-aminophenyl)­1, 3, 4-oxadiazole were prepared in the laboratory

171

INDIAN J. TEXT. RES., VOL. 10, DECEMBER 1985

Table I-Synthesis of PolyamicAcids:Conditions andResults of Polymerization

Oxadiazole ring is also an electron withdrawing groupby resonance.

. ,

'r

[II]dllg

1.181.961.67

1.330.292

DAP

Quanlilies of starling materials, gSample

PMDA DAB/E/ODZ

P-DAB 1.09 1.135PE 1.09 1.000P-ODZ 1.09 1.260PE-DAp· 2.181.600

'Copolymer of PMDA, E and DAP

N-~,\ ~ .®~N-1'JH2N-@-<O~ NH2~H2N~O~NH2

Incorporation ofDAP (strong electron withdrawingP =0 group) in the backbone of PE reduced theintrinsic viscosity. Phosphine oxide substituent is a - Igroup and since it is present at m-position, it will alsoreduce the reactivity of amino groups in DAP. Theabove diamine will react with PMDA with differentrates and hence the molecular weight of polymer couldalso be different.

The polypyromellitimides were characterized by IRspectroscopy (Fig. 1). Characteristic imide absorptionbands were observed at 1770-1780 cm -1 (imide I),1375 em -1 (imide 11), 1120em -1 (imide III), and720 em -I (imide IV) in the cyclodehydrated samples.The imide I band is attributed to the stretchingvibrations of weakly coupled carbonyl groups. Theimide II, III and IV bands have been assigned to axial,transverse and out-of-plane vibrations of the cyclicimide structures. In addition to these absorptions thepolyimide P-ODZ showed characteristic oxadiazolepeaks at 970-1020 and 1500em -1. Along withcharacteristic imide peaks at 1720, 1780 otherabsorptions arising because of P = 0 (at 1175± 15cm -I) and P-C6Hs (at 1425cm -I) were alsoobserved in the IR spectra of PE-DAP polyimide.

The relative thermal stability of polyimides wasdetermined by dynamic thermogravimetry. The

(E), the ether oxygen is a + R substituent and hence thisamine is expected to be more reactive than DAB whereelectron withdrawing amide substituent is present atp­position.

according Ito the methods reported earlier11 -14.PMDA (BDH) was purified by vacuum sublimationand DMF (E Merck) was distilled under reducedpressure.

Polyamic acid solutions in DMF were prepared bycondensation polymerization as described earlier 1 5 •

Polyamic acids were characterized by intrinsicviscosity determined in DMF by Ubbelohdesuspension level viscometer at 30°C. The density ofpolyimidelsamples in film form was determined by thesuspension method by using xylene and carbontetrachloridel6. The IR spectra of polyimides wererecorded using a Perkin Elmer 580-B grating infraredspectrophotometer. Thermal analysis was carried outusing a Stanton Red Croft TG-750 thermogravimetricanalyzer, a heating rate of JOoC/min being used.

The fibres were spun from polyamic acids with thehelp of aJ bench-scale wet-spinning equipment. Thepolyamic lacid fibres were vacuum-dried at 50°C andkept in a cyclization solution consisting of aceticanhydride-pyridine mixture (I: I) for 36 h at roomtemperature. After the completion of cyclode­hydration, fibres were dipped in dioxane to remove thecyclization solution and dried in vacuum oven at100°C. The mechanicaJ properties of these fibres weredetermined by using an Instron tensile tester, a gaugelength of 3 cm with a crosshead speed of 5 cm/minbeing used.

Thermal ageing of polyimide fibres was done bykeeping ~hem in a circulating air oven for periods of 6and 12h at 200°C and at 275°C. The tensile strength ofthese fibres was determined at room temperature. Theeffect of UV radiation on the mechanical properties ofthe fibres was determined by mounting polyimidefibres on a paper frame and exposing them to UV light(200-360, nm) for 40 h. For this purpose a UVphotochemical reactor consisting of medium pressuremercury lamp (125 W Philips) with a quartz filter wasused.

Results and DiscussionThe oonditions and the results of condensation

polymerization are given in Table 1.The polyamic acidPE had an intrinsic viscosity of 1.96, whereas P-ODZand P-DAB polyamic acids had intrinsic viscosities of1.67 and 1.18 respectively. The observed difference inthe intrinsic viscosity of polyamide acid may be due toa decrease in the DP of the polymers. Since thepolycondensation reaction was carried for the sameduration, a decrease in DP may be explained in termsof different rates of the reaction. An amine which ismore basic in character is expected to react at a fasterrate with PMDA, thus leading to higher DP and [/1].The basic nature of the amine depends on the inductiveand resonance effects. In 4, 4'-diaminodiphenyl ether

172

RAO and VARMA: POLYARYLENEPYROMELLITIMIDES AND PROPERTIES OF THEIR FIBRES

Fig. I-IR spectrum of polypyromellitimide (P-DAB)

polyamic acid solution was treated with aceticanhydride-pyridine-benzene mixture in the ratio of

1:1:1 by volume. DMF solutions of polyamic acidswere added to the cyclization solution along withstirring and kept for 24 h. Polyimides precipitated outduring cyclization were filtered and dried at loooeunder reduced pressure. The thermogravimetric tracesof these polymers in air atmosphere are given in Fig. L.The figure shows that in air, a significant weight lossstarts above 500°C. The initial decomposition of thesepolyimides is well above 500°C. However ~ 5%weight loss was observed in PE-DAP above lOOoe,which may be due to loss of water. The weight lossobserved above 2000e may be attributed to theremoval of residual DMF from the polymer. It mayalso be due to loss of water, indicating incompletecyclodehydration.

IBOO 1600 1400 120() 1000WAVE NJMBER, enil

BOO

Polyamic acid fibres were spun from DMF solutions

using DMF-water (30:70 v/v) as a coagulant. The jetstretch (ratio of takeup speed to extrusion speed of thepolymer) was varied between 4 and 20 during wetspinning. The dope concentrations of the spinningsolutions ofP-DAB, P-ODZ and PE were 15, 12.5 and

10% respectively. The polyamic acid fibres thusobtained were cyclodehydrated in pyridine-aceticanhydride for 36 h. The tensile properties of thepolyimide fibres are tabulated in Table 2.

Reduction in jet stretch ratio resulted in a decreasein tenacity but an increase in denier and per centextension. This behaviour is quite expected because theoriented fibre will have better tensile properties. Theamide-imide fibre P-DAB seems to be better than P­aDZ.

Incorporation of DAP into the backbone of PE bycopolymerization greatly reduced the tenacities. Theinitial modulus ofP-DAB and P-ODZ was higher thanthat of PE.

The undrawn polyimide fibre ofPE-DAP which wasprepared by chemical cyclodehydration frompolyamic acid had a tenacity of 0.72 g/d whereas thesame polyimide fibre prepared by thermal cyclizationfrom polyamic acid had tenacity of 0.38 g/d C[able 3).Heat ageing of these fibres resulted in reduction oftenacities. The chemically cyclodehydrated polyimidePE-DAP when heated at 275°e for 6 h had residual

tenacity of 0.13 and elongation of 3%. A similartreatment of the thermally prepared polyimide fibrereduced the tenacity to 0.21 and elongation to 8%.Thermally cyclized polyimide showed better retentionof tenacities than chemically cyclodehydrated

P-ODZ

PE-DAP

Table 2-Mechanical Properties of 'as-spun'Polyimide Fibres

Elon- Tenaci~yInitial

gationg/dmodulus

/;,g/d

25.0

1.0436.0

25.0

1.2634.0

72.51.3543.0

43.01.5580.0

70.0

0.8237.8

18.3

0.9340.242.5

1.0044.4

95.0

1.2515.665.0

1.728.755.0

1.5323.5

32.0

0.72

22.00.38

4 18410

5015

3020

II

6

74

10

5415

27

8

6412

43

15

34

4

9

12"24

Jet- Denierstretch

P-DAB

Sample

PE

• P-DDZ •A P-DAB

o PE-DAP

100 200

70

30

10

90

80

20

100

••..60I­or12

~ 50

~Vi 40LUQ:

Fig. 2- Thermogravimetric traces of polypyromellitimides in air

"Cyclizatioll was done by heating fibres to 250·C in air

173

INDIAN J. TEXT. RES., VOL. 10, DECEMBER 1985

Table t-Changes in Tenacity of Polyimide Fibresafter Irradiation to UV Light for 40 h

polyimide. The decrease in tenacity of the polyimide

fibre at high temperature may be due to polyimide

embrittleplent arising from cross-linking rather thanfrom reduction in molecular weight.

Photodegradation of polyimide fibre by UV lightresulted in a decrease of tenacities (Table 4). As

polyimides absorb strongly in the UV between 200 and

450 nm, these polymers deteriorate. Elongation seems

to be more sensitive to the photolytic process of

deterioration than the tensile strength. All the fibres

broke without elongation. 84% of original tenacity wasretained in irradiated PE fibres while in PE-DAP, P-

Table,3-Effect of Heating on the Tenacity ofPE-DAP FibresSample

Tempera-DurationTenacityElonga-No.

I ture hgldtion°c

%

Ia

200 00.72322

200 60.2663

200120.1674

275 60.133

5b

200 00.38226

200 60.36187

200120.3498

275 60.218

·Cyclization by chemical means (Samples 1-4) bCyclization by thermal means (Samples 5-8).--'¥"

aDZ and P-DAB retention in tenacities was 31,46 and

59% respectively.

These investigations, preliminary at this stage, thus

reveal that the mechanical properties of as-spun

polyimide fibres are dependent on the backbonestructure.

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Ph.D. thesis, Indian Institute of Technology, Delhi, 1978.

% Retention

of tenacity59

46

84

31

P-DABP-ODZPE

PE-DAP

Sample

174