A comparative study of the effects of UV- andg-radiation on copolymers of acrylonitrile/butadieneFrancisco Cardona,1 David JT Hill,1 Peter J Pomery1* and Andrew K Whittaker2

1Polymer Materials and Radiation Group, Department of Chemistry, University of Queensland, QLD 4072, Australia2Centre for Magnetic Resonance, Department of Chemistry, University of Queensland, QLD 4072, Australia

Abstract: The radiolysis of nitrile rubbers with different acrylonitrile/butadiene composition and the

homopolymers, poly(butadiene) (PBD) and poly(acrylonitrile) (PAN) has been investigated and com-

pared with the photolysis of the same polymers. A signi®cantly different mechanism of degradation

was found for the two types of radiation. The results obtained by ESR, FTIR and measurements of

soluble fractions of irradiated samples, indicated that the acrylonitrile units of the nitrile rubbers are

more sensitive units to g-radiation, with the effects of irradiation increasing with the acrylonitrile

content. The reactions observed were consumption of double bonds, crosslinking, and cyclization with

the formation of conjugated double bonds. No chain-scission reactions were detected. In contrast to

g-irradiation, the effects of photolysis were centred at the butadiene units, and increases in the

acrylonitrile content resulted in a proportional decrease in the sensitivity of the copolymers.

Crosslinking and chain scission were identi®ed as the main effects of photolysis of NBR rubbers.

# 1999 Society of Chemical Industry

Keywords: nitrile rubbers; copolymers; radiolysis; photolysis

INTRODUCTIONAcrylonitrile/butadiene copolymers (NBR) are impor-

tant commercial materials displaying outstanding

resistance towards oils and aromatic solvents com-

bined with good ageing characteristics and high

abrasion resistance.1 Typical applications include use

as moulded goods of all types, rubberized cloth,

closed-cell sponge and adhesives. A signi®cant num-

ber of papers have been published on the photo-

oxidative degradation of NBR rubbers and the

homopolymers PAN and PBD.1±4 Thermal oxida-

tion,5,6 gamma radiation,7±9 X-ray radiation,10,11 and

more recently laser-induced photo-fragmentation

studies,12,13 have also been the subject of several

publications. However, the photolysis of nitrile

rubbers and homopolymers has not been investigated,

and signi®cant questions regarding the mechanism of

the g-radiolysis remain unanswered, such as the effect

of copolymer composition on the radiation sensitivity.

In this paper we have investigated and compared the

stability of PAN, PBD and NBR rubbers with different

AN/BD composition, against g and UV radiation

under vacuum. Mechanisms of degradation have been

proposed to account for the observed radical and

stable products.

EXPERIMENTALThree nitrile rubber samples, NBR-16 (Chemigom

N926), NBR-33 Chemigom N436) and NBR-46

(Chemigom N206), were obtained from the Austra-

lian Synthetic Rubber Co Ltd. The rubbers were

puri®ed by precipitation from chloroform into metha-

nol and dried for 24h in a vacuum oven at 30°C.

Samples of poly(butadiene) (PBD) and poly(acrylo-

nitrile) (PAN, 100000g molÿ1) were acquired from

Aldrich Chemical Co. The PBD sample had a

molecular weight (Mw) of 400000g molÿ1 and the

structure of the double bonds was 55% 1,4-trans, 36%

1,4-cis and 9% 1,2-vinyl, as determined by 13C NMR

spectroscopy.

The composition and microstructure of the NBR

samples were characterized using 1H and 13C NMR

spectra recorded on an AC 200-F spectrometer

operating at 50.2Hz for carbon. The results of this

analysis are given in Table 1.

Gel permeation chromatography (GPC) measure-

ments of molecular weight distributions were obtained

using a Waters Associates high performance liquid

chromatograph (HPLC) ®tted with ®ve ultrastyragel

columns of varying pore sizes (106, 105, 104, 103 and

102A). Automated signal detection was achieved using

Polymer International Polym Int 48:985±992 (1999)

* Correspondence to: Peter J Pomery, Department of Chemistry, University of Queensland, St Lucia, QLD 4072, AustraliaE-mail: pomery@chemistry.uq.edu.auContract/grant sponsor: Australian Research CouncilContract/grant sponsor: Australian Institute of Nuclear Science and Engineering(Received 15 January 1999; accepted 27 March 1999)

# 1999 Society of Chemical Industry. Polym Int 0959±8103/99/$17.50 985

a Waters 401 differential refractometer interfaced to

an IBM-PC. Molecular weights are quoted as poly-

styrene (PSTY) equivalents.

HPLC grade THF was used as the elutant for all

nitrile rubbers. Data points were collected at 1s

intervals for a total of 60min with an eluent ¯ow rate

of 1ml minÿ1 and a column pressure of 100lbf inÿ2.

The temperature was kept at 22±23°C for all measure-

ments. Molecular weight values of 1.02�105,

1.9�103 and 9.6�104 were obtained for NBR-16,

ÿ33 and ÿ46, respectively.

Gamma irradiation was carried out at 77K (for ESR

studies) and 300K with 60Co g-rays in an AECL

Gammacell 200 unit at the University of Queensland.

The dose rate was 1.25 and 1.38kGy hÿ1 at 77K and

300K, respectively.

FTIR spectra of the nitrile rubber were obtained

using a 2000 FTIR Perkin Elmer spectrometer. The

IR spectra of the g-irradiated samples were obtained

on pressed KBr tablets. Infrared spectra of the samples

after UV irradiation were recorded from samples

prepared by casting ®lms (about 50mm thickness)

over KBr tablets from solutions (5% w/v) in chloro-

form (N,N-dimethylformamide for PAN).

For UV irradiation, the ®lms deposited on KBr

tablets were placed inside quartz tubes. The tubes

were ¯ushed with nitrogen gas for 45min, and then

sealed off. The quartz tubes were held in a metal

sample holder inside a quartz beaker and surrounded

by water, to keep the temperature constant at 298K

during the irradiation. The ®lms were exposed to UV

radiation 40cm away from an Oriel high power

mercury/xenon lamp with an output intensity of

9.1mWcmÿ2, using a ®lter which removed wave-

lengths below 250nm. After irradiation the ®lms were

left in the quartz tubes for about 6.0h at room

temperature before opening, to allow the full decay

of the radical products.

For electron spin resonance (ESR) studies, the

samples were exposed to UV radiation in vacuo and at

77K inside the cavity of the ESR spectrometer, from

an un®ltered Oriel high power (1000W) mercury/

xenon lamp. The high energy UV wavelength cut-off

was 250nm. The ESR spectra were recorded on a

Bruker ER-200D ESR spectrometer interfaced to an

IBM compatible personal computer. The spectra were

obtained in the ®rst derivative form and then doubly

integrated to obtain the area of the absorption peak.

The concentration of radicals was determined by

comparison of the area under the absorption peak with

that of a standard provided by Varian (1% pitch in a

potassium chloride matrix). The concentration of the

radicals in the standard was 3.0�1015 spins cmÿ1.

After evaluating the saturation of the ESR signal, the

spectra were recorded with a microwave power of

20mW (which represents an attenuation of 40dB),

and modulation amplitude of 2.0G. Computer simu-

lations of the spectra were obtained using the

SIMOPR software written by Garrett.14

To determine the soluble fractions after irradiation,

samples were placed in 200-mesh stainless steel

baskets and then in a Soxhlet apparatus, and extracted

with re¯uxing chloroform (for nitrile rubbers and

polybutadiene) and N,N-dimethylformamide (for

polyacrylonitrile), for more than 48h. They were then

dried under reduced pressure at room temperature for

24h and weighed. This procedure was repeated until

constant weight was obtained. The gel fraction of the

polymers was determined from the weight of the

insoluble portion.

RESULTS AND DISCUSSIONESR spectroscopyUV irradiation

The ESR spectra of nitrile rubbers 1,4-PBD and PAN,

UV-irradiated in vacuo at 77K and recorded at 77K,

are shown in Fig 1. The spectrum of irradiated

polybutadiene, has been attributed by Carstensen15

as being mainly due to the allyl radicals (I) formed by

chain scission of the CÐC bond midway between the

butadiene units.

(I)

Until now, there have been no reports of the

successful simulation of the ESR spectra of polybuta-

diene and nitrile rubbers after UV irradiation at 77K

in vacuo. In this work computer simulations were

obtained from combinations of different amounts of

the radicals identi®ed in the ESR signal, until the best

®t with the experimental spectrum was obtained. The

experimental ESR signal of 1,4-PBD and the compu-

ter simulation are shown in Fig 1. The simulation

comprises allyl radicals of type (I) (60%), allyl radicals

of type (II) (28%) and a singlet attributed to polyenyl

radicals (III) (12%). The details of the hyper®ne

coupling constants and line-widths used in the

simulation are listed in Table 2, and are consistent

with literature values.15,16

(II) (III)

The ESR spectrum of UV-irradiated PAN shown in

Fig 1 is a singlet of width 1.4mT due to polyimine

radicals (IV). The polyimide radical has previously

Table 1. Compositions, diad fractions and double bond structures (%) of NBRrubbers determined by 1H and 13C NMR spectroscopy

Monomer unit NBR-16 NBR-33 NBR-46

Acrylonitrile AB diad 16.5 33.0 42.0

AA diad 0.0 0.0 4.0

Total 16.5 33.0 46.0

Butadiene 1,4-trans 62.4 54.0 49.0

1,4-cis 8.6 6.0 4.5

1,2-BD 12.5 7.0 0.5

Total 83.5 67.0 54.0

986 Polym Int 48:985±992 (1999)

F Cardona et al

been observed in UV- and g-irradiated PAN.17

(IV)

The ESR spectra of UV-irradiated rubbers are

shown in Fig 1. It can be seen that with increasing

acrylonitrile content the spectra start to resemble the

spectrum of polyacrylonitrile. Furthermore, increases

in acrylonitrile content also resulted in a clear decrease

in the size of the ESR signal; that is, the amount of

radicals formed in nitrile rubbers during irradiation

decreases with the acrylonitrile content. Photo-bleach-

ing of the nitrile rubbers with light of wavelength above

590nm or 475nm for 30min did not change the shape

or the intensity of the spectra, indicating that no

radical anions were present.

The computer simulation of the ESR signal of NBR-

46 is also shown in Fig 1. The simulation was obtained

by combination of radicals (I) (25%), radicals (II)

(45%) and a singlet (30%). The singlet is due to a

combination of polyenyl radicals (III) and polyimine

radicals (IV). It is not possible to determine the rela-

tive contribution of these two radicals to the spectrum;

however, for this material containing 46mol% AN

units, the maximum proportion of radicals derived

from the AN units (ie the polyimine radicals (IV)) is

30%. It follows therefore that the BD units are more

sensitive to degradation by UV light than the AN units.

This is consistent with the qualitative observation of a

decreased yield of radicals at higher AN contents. In

addition the reduced yield of polyimine radicals may

result from the breaking up of runs of AN units in the

random copolymers.

Finally it should be noted that the relative amount of

the radical (I) due to chain scission is lower in NBR46

than PBD. It is expected therefore that the yield of

chain scission will decrease with increasing AN

content, a result which is con®med by measurements

of soluble fractions of irradiated samples, as discussed

below.

g-Irradiation

The ESR spectrum of 1,4-PBD g-irradiated in vacuo

and recorded at 77K is shown in Fig 2. The spectrum,

obtained immediately after irradiation, consists of a

broad singlet and a superimposed septet, and is similar

to previously reported spectra.18,19 The septet was

assigned to allyl radical (II), and the singlet to the

polyenyl radical (III). On warming the sample to

temperatures near 180K, which is close to the glass

transition temperature, the intensity of the spectrum

decayed to about 40% of the intensity of the spectrum

at lower temperatures.

The computer simulation of the ESR signal of 1,4-

PBD g-irradiated at 77K (Fig 2) was obtained by

combination of the allyl radical (60%) and a singlet

(40%). The line-width of the singlet was 2.3mT,

which is considerably larger than that observed above

in the UV radiolysis of PDB. This apparently indicates

a lower degree of conjugation in the polyenyl radical

formed by g-radiolysis.

The ESR spectrum of PAN g-irradiated in vacuo

and recorded at 77K is shown in Fig 2. A computer

simulation of the spectrum is also included. The

hyper®ne structure of the ESR signal is associated with

the radicals resulting from hydrogen abstraction from

the methylene and methine groups, located on the

backbone of the polyacrylonitrile chain, as suggested

by other workers.20,21 Lang20 identi®ed the different

radicals which give rise to the hyper®ne structure of the

Figure 1. ESR spectra of samples after UV-radiolysis under vacuum at77K: (a) PBD; (b) PBD (simulation), (c) NBR16; (d) NBR33, (e) NBR48,(f) NBR48 (simulation) and (g) PAN.

Table 2. Hyperfine coupling constants and line-widths used in the simulationof ESR spectra

Radical aH (a) (mT) aH (b) (mT) DHpp (mT) Refs

I 1.4 0.4 (1), 1.3 (2) 1.2 15,16

II 1.5 1.67 (1), 1.3 (2) 1.4 15,16

III ± ± 1.2 15,16

IV ± ± 1.4 (UV) 21

2.3 (g)V 4.0 1.6 1.5 21, 31

VI 4.0 1.6 1.5 21, 31

VII 2.5 2.4 (H), 0.35 (N) 1.5 21, 32

VIII ± 3.3 1.5 21

Polym Int 48:985±992 (1999) 987

UV- and g-radiation of acrylonitrile/butadiene copolymers

ESR spectrum of PAN irradiated at 77K, including

the presence of the radical responsible for a singlet at

the centre of the ESR spectrum. The relative propor-

tion of each radical present obtain by computer

simulation of the signal recorded at 77K and by using

spectral subtraction techniques during the annealing

experiment, are given in Table 3. Photo-bleaching

experiments con®rmed that the contribution to the

spectra from radical anions was less than 3%.

The ESR spectra of NBR-46, 33, 16, g-irradiated in

vacuo at 77K and recorded at 77K, are also shown in

Fig 2. The spectrum of NBR-16 is similar to the

spectrum of polybutadiene, and so appears to be

composed of radicals (II) and (III). With increasing

acrylonitrile content of the rubber, ®ne structure

appears in the spectra, which start to resemble that

of polyacrylonitrile. The complex nature of the

spectra, and the obvious possible contribution of a

large number of different radicals to poorly resolved

spectra, precluded computer simulation.

The G values for formation of radicals at 77K were

determined from plots of the radical concentration as a

function of dose. The values are listed in Table 4 and

plotted in Fig 3 as a function of polymer composition.

The G value is a linear function of composition, as

expected for copolymers which do not contain strongly

protective groups. As is seen from Fig 2, the spectra of

the copolymers show features assignable to both types

of monomer unit.

Infrared spectroscopyUV radiation

The effect of UV radiation on the FTIR spectra of

nitrile rubbers is shown in Fig 4. The absorption bands

due to double bonds in the main chains

ÐRÐCH=CHÐRÐ (cis and trans at 740cmÿ1 and

Figure 2. ESR spectra of samples after g-radiolysis under vacuum at 77K;(a) PBD, (b) PBD (simulation), (c) NBR16, (d) NBR33, (e) NBR48, (f) PANand (g) PAN (simulation).

Table 3. Relative proportions of free radicals in the ESR spectrum of PANg-irradiated and recorded at 77K. The values were obtained by simulation,and by subtraction of spectra at different temperatures during warming(experimental)

Relative proportion (%)

Radical Simulated Experimental

IV Singlet: polyimine 38.0 32.0

V 38.0 40.0

VI 16.0 10.0

VII 3.0 4.0

VIII 5.0 14.0

Table 4. G values for g-radiation induced events in PBD, PAN and NBRrubbers

G value PBD NBR16 NBR33 NBR46 PAN

G(R) 0.8 1.18 1.43 1.72 2.97

G(db) 32.6 39.4 43.5 48.3 ±

G(S)/G(X) 0 0 0 0 0

G(X) 3.7 5.6 4.1 9.2 0.72

Figure 3. G(radical) for copolymers of BD and AN as a function ofcomposition.

988 Polym Int 48:985±992 (1999)

F Cardona et al

970cmÿ1, respectively), double bonds in 1,2-vinyl

groups ÐCH=CH2 (920cmÿ1) and nitrile groups

ÐCN (2235cmÿ1) were reduced in intensity on

irradiation, whereas a new broad band appeared at

3220cmÿ1. The new band is assigned to the stretching

vibrations of =NH and ÐNH2 groups. A small band

centred at 1375cmÿ1 appeared after about 24h

exposure in NBR-16 and NBR-33 rubbers, and is

assigned to the symmetrical deformations of the

methyl group. All these bands attained a constant

intensity after about 100h of radiation.

The changes in peak intensity with increasing dose,

measured with the 2840cmÿ1 band (due to the

stretching vibration of the methylene group), as

internal standard, are shown in Fig 5(a±d). These

plots show that the ole®nic groups in the butadiene

units undergo reaction to a greater extent than the

nitrile groups in the acrylonitrile units, and that rate of

these processes and of the formation of methyl units

decreases with increasing amounts of acrylonitrile in

the rubber. The peak due to methyl groups was not

detected in irradiated NBR-46 rubber. The high

reactivity of polybutadiene units to UV radiation is

due to the lower bond energy of the CÐH and CÐC

bonds at the allylic position,22 and the subsequent

greater stability of the allyl radicals as con®rmed by

ESR.

g-Radiation

The IR spectra of nitrile rubber samples (NBR-33),

before and after g-irradiation to different doses are

shown in Fig 6. The main effects after irradiation are

decreases in peaks due to the ole®nic groups, 1,4-cis(740cmÿ1), 1,4-trans (967cmÿ1) and 1,2-vinyl

(910cmÿ1) and nitrile groups ÐCN (2235cmÿ1).

The absorption band at 910cmÿ1 disappears at the

highest rate during exposure to g-radiation, as has been

reported previously for PBD.8 A new band appears at

Figure 4. FTIR spectra of NBR33; (a) unirradiated, (b) UV-irradiated for48h, and (c) UV-irradiated for 120h.

Figure 5. Changes in the relative absorbance of peaks in the FTIR spectra during UV-irradiation as a function of time; (a) CN groups (2250cmÿ1); (b) 1,2-vinylgroups (910cmÿ1); (c) 1,4-trans double bonds (967cmÿ1); (d) methyl groups (1375cmÿ1).

Polym Int 48:985±992 (1999) 989

UV- and g-radiation of acrylonitrile/butadiene copolymers

1640±1670cmÿ1, which has been assigned to the

formation of conjugated and cyclic structures.10 The

changes in the relative intensity of the various

absorption bands (Fig 7) were measured taking the

1450cmÿ1 band (CÐH deformation of CH2 groups)

as reference peak, as suggested by Degteva and Pak.23

No Ð(C=N)Ð groups (2185cmÿ1) were observed,

as has been reported for thermal degradation of nitrile

rubber. Infrared spectra also do not give any evidence

for isomerization of the double bonds, because the

majority of the double bonds in the original copoly-

mers are the more stable trans bonds.

The G values for consumption of double bonds G(d

b) up to 1.6MGy, obtained from the IR analysis of the

irradiated polymers, were calculated according to

O'Donnell and Sangster24 and are listed in Table 4.

Measurements of soluble fractions

g-Radiation

The values of G(X) and G(S) were determined from

measurement of the soluble fractions of polymers

which have partially gelled. Charlesby and Pinner25

showed that the soluble fraction s, of a polymer is

related to the radiation dose D in kGy, by eqn (1):

s� s1=2 � G�S�2G�X� �

9:6� 106

MwG�X�D �1�

The Charlesby±Pinner plots for the range of materials

studied is shown in Fig 8. There is a steady decrease in

the fraction of soluble polymer with doses above the

gel dose (Dg� 8.0kGy), while at higher doses a

straight line was obtained. For all of the polymers,

the intercept of the Charlesby±Pinner plot was zero;

hence the yield of main-chain scission reactions G(S)

was negligible. This conclusion is consistent with the

absence of radicals arising from chain scission in the

ESR spectra, as discussed above. The values of G(X),

obtained from the slope of the linear section of the

plots, are listed in Table 4. Previous workers have

reported values of G(X) for PBD27±30 ranging from 3.2

to 5.9, determined by measurements of soluble

fractions and swelling ratios. There is a large un-

certainty in the values of G(X) reported above, largely

due to possible errors in the value of Mw measured by

GPC. However, it appears that the values of G(X) for

PAN and PBD are lower than in the nitrile rubbers,

indicating that in the homopolymers the extent of

intramolecular crosslinking compared with inter-

molecular crosslinking may be greater than in the

copolymers.

UV-irradiation

The corresponding Charlesby±Pinner plots for the ®ve

Figure 6. FTIR spectra of NBR33; (a) unirradiated; (b) g-irradiated to1.6MGy; (c) g-irradiated to 2.4MGy.

Figure 7. Changes in the relative absorbance of peaks in the FTIR spectraduring UV irradiation as a function of dose; CN groups (2250cmÿ1);(b) 1,2-vinyl groups (910cmÿ1); (c) 1,4-trans double bonds (967cmÿ1);(d) crosslinked and cyclic structures (1375cmÿ1).

Figure 8. Charlesby–Pinner plots for g-irradiated PBD, PAN and nitrilerubbers.

990 Polym Int 48:985±992 (1999)

F Cardona et al

UV-irradiated samples are shown in Fig 9. The data

are plotted as functions of the square root of time,

rather than absorbed dose: hence absolute values of

G(X) could not be calculated from the slopes of these

plots. However, the values of the G(S)/G(X) ratio were

calculated from the intercept at zero irradiation time,

and are plotted in Fig 10. The decreasing ratio of

scission to crosslinking with increasing acrylonitrile

content suggests that chain scission occurs at the BD

units.

CONCLUSIONSThe fundamental differences between the mechanisms

of degradation of nitrile rubbers by g- and UV-

radiolysis can be brie¯y summarized as follows:

. The total yield of radical products decreases with

AN content during UV radiolysis, but increases

on g-radiolysis.. On UV radiolysis radicals derived from the PBD

dominate the ESR spectra: however, for g-radiolysis the spectra are composed of radicals

derived from both monomer units.. The ®nal yield of products mirrors the observa-

tion of radical yields, ie for UV the rate of

formation of products decreases with increasing

AN content, but increases on g-radiolysis.. On UV photolysis a signi®cant yield of chain

scission is observed, whereas for g-radiolysis no

scission occurs. The yield of chain scission on UV

radiolysis decreases with increasing AN content.

The above statements re¯ect the different mechan-

isms of deposition of energy into the polymers on

radiolysis, and the mechanism of energy transfer. On

g-radiolysis the initial processes occurring are ioniza-

tion of both BD and AN groups, followed by decay to

free radical intermediates. The radical species most

probably formed are those due to loss of H atoms,

which can escape the surrounding cage of neighbour-

ing chains. These radicals are likely to have consider-

able kinetic energy resulting from the very high energy

of the incident radiation. The highly mobile H atoms

may then abstract other H atoms, resulting in stable

polymer radicals situated at the allylic position. The

high concentration of H atoms also initiates the

formation of conjugated polyimine radicals.

On UV radiolysis the main chromophores at the

wavelengths used are the BD double bonds. The

energy absorbed directly by these groups is dissipated

by cleavage of both the allylic CÐH and CÐC bonds.

The AN groups have a UV absorption spectrum which

tails off above 220nm, and so do not participate

directly in the primary degradation processes.

ACKNOWLEDGEMENTSThe authors would like to acknowledge the Australian

Research Council and the Australian Institute of

Nuclear Science and Engineering for ®nancial support

of this work.

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