Thermal, oxidative and radiation stability of polyimidesII. Polyimides based on bismaleimidohexane and

bismaleimidodiphenylsulphone with different diamines

Milena Marinovic-Cincovica,*, Dragan Babica, Renata Jovanovica,Katarina Popov-Pergalb, Miroslav Pergalc

aThe Vinca Institute of Nuclear Sciences, PO Box 522, 11001 Belgrade, YugoslaviabFaculty of Forestry Science, University of Belgrade, YU-11000 Belgrade, Yugoslavia

cFaculty of Sciences, University of Novi Sad, YU-21000 Novi Sad, Yugoslavia

Received 21 February 2003; accepted 12 March 2003

Abstract

Different polyimide resins were synthesized using 1,6-bis-maleimidohexane and the following compounds with terminal amino

groups: 4,40-diaminodiphenylmethane, 4,40-diaminodiphenylsulphone, 1,4-bis-(3-aminopropyl)-piperazine and 4,40NN0-bismaleimi-dodiphenylsulphone and 1,2-diaminoehtane and 1,4-bis-(3-aminopropyl)-piperazine. The thermal and thermooxidative behaviourof the polyimide resins was studied by thermogravimetric measurements (TG) in oxygen and nitrogen. Polyimide resins have been

irradiated and their radiation stability evaluated on the basis of thermal and thermooxidative behaviour of irradiated samples.# 2003 Elsevier Ltd. All rights reserved.

Keywords: Polyimide resins; Thermal and thermooxidative behaviour; Radiation stability

1. Introduction

Polyimides are used extensively in situations requiringhigh-performance plastic materials, where other engi-neering materials do not function, because of theirunique combination of superior mechanical, electrical,chemical and thermal properties [1]. This work is part ofa continuing study of the relations between polyimidestructure and their thermo-oxidative-radiation stability.The polyimide resins were obtained by the interaction

between 1,6-bis-maleimidohexane (BMH) and the fol-lowing compounds with terminal amino groups:4,40-diaminodiphenylmethane (BMH-DADPhM),4,40-diaminodiphenylsulphone (BMH-DADPhS), 1,4 -bis - (3 - aminopropyl) - piperazine (BMH - PAP) and4,40NN0-bisbismaleimidodiphenylsulphone (BMDPhS)and 1,2-diaminoehtane (BMDPhS-DAE), and 1,4-bis-(3-aminopropyl)-piperazine (BMDPhS-PAP). The ther-mal and thermooxidative stability of the polyimideresins was studied by thermogravimetric measurements

(TG) in oxygen and nitrogen. Samples of the polyimideresins have also been irradiated in air to a total absor-bed dose of 500 kGy.On the basis of some characteristic substructures pre-

sent in both components of each examined resin, it canbe assumed that several reactions take place. In princi-ple, the amino group can be added to the double bondin the maleimide ring or to the double bond in the chainobtained by the opening of the maleimide ring, when asecondary amide group is formed. Aminolysis of the mal-eimide ring can occur with the formation of two amidegroups, one by the attack of the amino group from thediamine and the other by the opening of the maleimidering or with the formation of a secondary amide groupcaused by the opening of maleimide ring. The assumedmain reaction types are given in our previous papers [2–6].

2. Experimental

2.1. Synthesis of 1,6-bis-maleimidohexane

A mixture of 5 g (0.016 mol) 1,6-bis-maleamidohex-ane, 29.93 ml (0.21 mol) acetanhydride and 4.146 g (0.05

0141-3910/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S0141-3910(03)00121-6

Polymer Degradation and Stability 81 (2003) 387–392

www.elsevier.com/locate/polydegstab

* Corresponding author. Tel.: +38-11-453-986; fax: +38-11-3-

0100.

E-mail address: milena@vin.bg.ac.yu (M. Marinovic-Cincovic).

mol) sodium acetate was heated under reflux for 2 h to120–125 �C. A clear solution formed. After cooling toroom temperature a 5% solution of sodium bicarbonatewas added to bring the pH to 7. A brown suspensionwas obtained, which was recrystallized from ethanol–water mixture. The yield was 1.65 g (37.3%), m.p.105 �C (decomp.; Fig. 1a).

2.2. Synthesis of 4,40-N,N0-bis-maleimidodiphenylsulfone

A mixture of 4.44 g (0.01 mol) 4,40-N,N0-bis-mal-eamidodiphenylsulfone, 6.83 ml (0.062 mol) acetanhy-dride and 0.657 g (0.008 mol) sodium acetate was heatedunder reflux to 110–120 �C. After 30 min a clear solu-tion formed and the heating was stopped. At roomtemperature a 5% solution of sodium bicarbonate wasadded to bring the pH to 7. The resulting solid suspen-sion was recrystallized from ethanol–water mixture. Theyield was 1.55 g (38%), m.p. 105 �C (decomp.; Fig. 1a).

2.3. Formation of polyimides

1. 1,6-N,N0-bis-maleimidohexane was melted at

174 �C and then diamine added with stirring.After homogenisation the temperature was ele-vated to 220 �C and the mixture heated at thistemperature for 10 h. The obtained polyimidewas chopped up.

2. 1,4-N,N0-Bis-maleimidodiphenylsulfone was

melted at 160 �C and then diamine added withstirring. After homogenization the temperaturewas elevated to 180 �C and the mixture is heated

at this temperature for 10 h. The obtained poly-imide was chopped up.

2.4. Irradiation

Irradiations were performed in air in the Co-60industrial radiation sterilization unit at the Vinca Insti-tute of Nuclear Sciences with a dose rate of 10 kGyh�1

and total absorbed dose of 500 kGy.

2.5. Thermogravimetric measurements

Thermogravimetric (TG) measurements were per-formed on a Perkin-Elmer TGS-2 instrument. Poly-imide samples (mass was about 5 mg) were heated from30 to 800 �C at a heating rate of 10 �C min�1 in oxygenor nitrogen atmosphere with a gas flow of 40 ml min�1.

3. Results and discussion

In this work the following subjects are discussed: ther-mal stability of studied polyimide (PI) samples based onthe TG measurements in nitrogen, thermooxidative stabi-lity of PI samples based on the TG measurements in oxy-gen, radiation stability of PI studied through the change inthermal and thermooxidative behaviour after irradiation.The analyses were based on:

� dynamic TG curve (weight loss as a function oftemperature), the shape of the curve and espe-cially its initial part

Fig. 1. (a) Model of 1,6-bis-maleimidohexane and 4,40-N,N0-bis-maleimidodiphenylsulfone molecule. (b) Structural formulae of compounds used

for polyimide synthesis.

388 M. Marinovic-Cincovic et al. / Polymer Degradation and Stability 81 (2003) 387–392

� the temperature values for certain weight loss,namely 0.5, 10 and 30% (the temperature for0.5% weight loss we considered as the initialdecomposition temperature—IDT)

� the rate of weight loss change as a function oftemperature (first derivative of dynamic TGcurve), i.e. its initial and overall shape.

It could be roughly generalized that the TG decom-position of all PI samples consists of two main phases.The first corresponds to partial degradation of thesample and the second is the result of complete thermaldecomposition. The samples could be characterized bythe position of decomposition peaks and their size (inthe first derivative curve), the difference being the resultof their different influence of the structure and differ-ences in degradation process [7].

3.1. Thermal stability

The data for unirradiated samples thermally treatedin nitrogen atmosphere show that in the initial stage ofdegradation according to IDT (Table 1) and DTGcurves shape (Fig. 2) the BMH-DADPhS sample hasthe best stability (IDT =296 �C) and the BMH-DADPhM sample (IDT =229 �C) the worst.The BMH-DADPhM sample degrades slowly over

the whole temperature range in several phases of whichthe one at 479 �C is most pronounced (Fig. 2, Table 2).This sample has no residue like other samples and it

Table 1

Temperature values for selected weight loss (0.5, 10 and 30%)

Samples

T0.5%(�C)

T10%(�C)

T30%(�C)

Samples:unirradiated/gas:oxygen

BMH-DADPhM

241 390 479

BMH-DADPhS

303 392 453

BMH-PAP

248 328 418

BMDPhS-DAE

261 364 405

BMDPhS-PAP

241 301 374

Samples:irradiated/gas:oxygen

BMH-DADPhM

227 353 437

BMH-DADPhS

288 382 456

BMH-PAP

295 338 396

BMDPhS-DAE

219 348 419

BMDPhS-PAP

234 325 410

Samples:unirradiated/gas:nitrogen

BMH-DADPhM

229 334 472

BMH-DADPhS

296 373 412

BMH-PAP

248 330 385

BMDPhS-DAE

234 345 389

BMDPhS-PAP

236 327 360

Samples:irradiated/gas:nitrogen

BMH-DADPhM

222 364 462

BMH-DADPhS

280 364 439

BMH-PAP

229 339 428

BMDPhS-DAE

244 361 403

BMDPhS-PAP

240 318 353

Fig. 2. TG curves and their first derivatives for unirradiated samples

in nitrogen atmosphere.

Table 2

Temperature maximum values of DTG curves (Tmax1,Tmax2) and resi-

due weight

Samples

Tmax1(�C) Tmax2(

�C)

m(%)

Samples:unirradiated/gas:oxygen

BMH-DADPhM

390 531 –

BMH-DADPhS

431 527 –

BMH-PAP

374 458 –

BMDPhS-DAE

391 548 –

BMDPhS-PAP

352 528 –

Samples:irradiated/gas:oxygen

BMH-DADPhM

370 509 –

BMH-DADPhS

413 522 –

BMH-PAP

384 449 –

BMDPhS-DAE

373,420 515 –

BMDPhS-PAP

337 510 –

Samples:unirradiated/gas:nitrogen

BMH-DADPhM

479 650 –

BMH-DADPhS

407 486 18.8

BMH-PAP

358 479 17.2

BMDPhS-DAE

408 – 24.8

BMDPhS-PAP

344 472 –

Samples:irradiated/gas:nitrogen

BMH-DADPhM

– 548 –

BMH-DADPhS

381 473 –

BMH-PAP

380 465 –

BMDPhS-DAE

404 577 –

BMDPhS-PAP

345 378 –

M. Marinovic-Cincovic et al. / Polymer Degradation and Stability 81 (2003) 387–392 389

degrades completely through the last broad degradationpeak at 650 �C (Fig. 2).Degradation of other samples roughly consists of two

main phases (two peaks in DTG curves) with residues athigh temperatures. The exception is sample BMDPHS-DAE, which has one phase (Fig. 2). The temperaturescorresponding to those phases (Table 2) indicate thatBMH-DADPhS has the best stability for both phases.For unirradiated samples treated in nitrogen there is a

plateau above 500 �C for samples BMH-PAP,BMDPhS-DAE, and BMDPhS-PAP. For samplesBMDPhS-DAE and BMDPhS-PAP it corresponds todiphenyl structure of bismaleimide while for sampleBMH-PAP it is due to degradation and additionalcrosslinking. Samples BMH-DADPhM and BMH-DADPhS do not have pronounced plateaus but theirdegradation rate above 500 �C is slower. This suggeststhat the influence of diphenyl structure in diamine is notso significant as in bismaleimide.

3.2. Thermooxidative stability

In the initial stage of degradation (IDT,TG curves;Table 1 T10%; Fig. 4) sample BMH-DADPhS shows thebest stability (IDT 303 �C), followed by BMH-DADPhM and BMDPhS-DAE (IDT=241 and 261 �Crespectively) and finally BMDPhS-PAP and BMH-PAP(IDT=241 and 248 �C respectively). At the laterstage the order of stability slightly changes (T30%) andbecomes BMH-DADPhM > BMH-DADPhS > BMH-PAP > BMDPhS-DAE > BMDPhS-PAP.The initial decrease of sample mass is slower in oxygen

than in nitrogen atmosphere due to oxidation reactions.As a result of this the TG curves in oxygen are shifted tohigher temperatures and they are much closer to eachother than in the case of thermal degradation in nitrogen.TG curves in the further stage of degradation at

higher temperatures are even closer to each other, indi-cating that products left after the oxidation in first stageare similar for all samples. Degradation is complete andthere is no residue above 600 �C.Formation of oxidized species delays degradation by

simple increasing of mass after oxidation and/or bygenerating more stable structures [8]. This delay is thesmallest in the case of sample BMH-DADPhM andsample BMH-PAP which could be explained by theirstructure. Methylene group is most susceptible to theoxidation in the propane chain of 1,4-bis-(3-aminopro-pyl)-piperazine.For unirradiated samples treated in oxygen there is no

plateau. The slowing of decomposition by oxidation isthe biggest for samples BMDPhS-PAP and BMDPhS-DAE (Fig. 4) indicating again that the mechanism ofdecomposition of diphenyl in bismaleimide and diamineis somewhat different probably due to differences instructure elements and system stiffness [9,10].

3.3. Radiation stability

General observations about the thermal and thermo-oxidative behaviour of irradiated polyimides were pre-sented in our previous paper and the comments madethere hold here as well [2]. The absorbed radiation doseused here was increased to 500 kGy as the effects of 100kGy were minimal. The effect of 500 kGy in nitrogenseams to be the following:

1. negligible effect in the range of initial degradation

temperatures;

2. slight slowing of the degradation as temperature

rises resulting in an increase of the plateau fromabout below 40% mass for unirradiated samplesto about 20% for irradiated samples; and

3. beyond the plateau degradation proceeds much

faster and there is no residue mass in irradiatedsamples.

The order of stability in nitrogen of irradiated sam-ples in the initial stage of degradation is: BMH-DADPhS is more stable than BMDPhS-DAE and at theend BMH-DADPhM, BMH-PAP and BMDPhS-PAP(Table 1, T0.5%; Fig. 3). During degradation BMDPhS-PAP is far more unstable than other samples namelyBMDPhS-DAE, BMH-PAP BMH-DADPhS and

Fig. 3. TG curves and their first derivatives for irradiated samples in

nitrogen atmosphere.

390 M. Marinovic-Cincovic et al. / Polymer Degradation and Stability 81 (2003) 387–392

BMH-DADPhM which is most stable in the tempera-ture midrange (400–500 �C; Fig. 3).The order of stability in oxygen of irradiated samples

in the initial stage of degradation is close to that innitrogen: the most stable is BMH-DADPhS, thenBMH-DADPhM, BMDPhS-DAE, BMH-PAP, andfinally BMDPhS-PAP. In the further phase of degrada-tion sample BMH-DADPhS is the best and BMH-PAPand BMDPhS-PAP the worst. There is no slowing ofdegradation after irradiation in oxygen as was seen innitrogen. On the other hand the thermogravimetriccurves are even closer to each other for irradiated sam-ples indicating that there is less difference in samplesafter irradiation. The degradation occurs in multiplepeaks. It is interesting that except for sample BMH-PAP the samples have the last peak at about the sametemperature 510–520 �C for both irradiated and uni-rradiated samples. This peak obviously corresponds todecomposition of diphenylene structure which is com-mon to all of the samples in question. Initial slowing byoxidation in irradiated samples (compared to thermaldegradation of irradiated samples in nitrogen) is lesspronounced than in unirradiated samples (Fig. 4 and 5).This effect is the biggest for sample BMDPhS-PAP aswas found for unirradiated samples. There is a sig-nificant delay in first part of thermal decomposition ofirradiated samples compared to unirradiated especiallyin the range 400–600 �C for samples BMH-DADPhS,

BMDPhS-DAE and BMDPhS-PAP and in the range of300–620 �C for sample BMH-PAP. Only for sampleBMH-DADPhM is the difference between irradiatedand unirradiated samples negligible.Thermooxidative decomposition of irradiated samples

compared to unirradiated is about the same in thewhole temperature range for sample BMH-DADPhS,faster from the beginning for BMH-DADPhM andBMH-PAP and slightly slower in the first part (up to400 �C) and faster in the second part (above 400 �C) forsamples BMDPhS-DAE and BMDPhS-PAP.

4. Conclusions

The thermal degradation of all PI samples consists oftwo or more phases as the result of complexity ofdegradation.The influence of diphenyl structure on thermal and

thermooxidative stability of unirradiated samples is notso great in diamine as it is in bismaleimide. In irradiatedsamples degradation of diphenyl structure is indepen-dent of its origin (diamine or bismaelimide) and itoccurs in the temperature region of 510–520 �C.Compared to our previous work (applied dose of 100

kGy) it can be concluded that higher radiation dose of500 kGy could still contribute to better stability of somepolyimides by additional cross-linking [11–12]. The

Fig. 4. TG curves and their first derivatives for unirradiated samples

in oxygen atmosphere.

Fig. 5. TG curves and their first derivatives for irradiated samples in

oxygen atmosphere.

M. Marinovic-Cincovic et al. / Polymer Degradation and Stability 81 (2003) 387–392 391

effect of deterioration of thermal and thermooxidativestability induced by radiation degradation at radiationdoses in the materials examined was still poor.

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