Opto-chemical response of CR-39 and polystyrene to swift heavy ion irradiation

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Nuclear Instruments and Methods in Physics Research B 255 (2007) 350–356

NIMBBeam Interactions

with Materials & Atoms

Opto-chemical response of CR-39 and polystyrene to swift heavyion irradiation

Lakhwant Singh *, Kawaljeet Singh Samra, Ravinder Singh

Department of Physics, Guru Nanak Dev University, Amritsar, 143 005 Punjab, India

Received 13 May 2006; received in revised form 16 November 2006Available online 22 December 2006

Abstract

The samples of CR-39 and polystyrene (PS) polymers have been irradiated with 64Cu9+ (120 MeV) and 12C5+ (70 MeV) ion beamshaving fluence ranging from 1 · 1011 to 1 · 1013 ions/cm�2. UV spectra of irradiated samples reveal that the optical band gap decreasesfrom 5.50 to 2.75 eV in CR-39 and from 4.36 to 1.73 eV in PS. The correlation between optical band gap and the number of carbonatoms in a cluster with modified Tauc’s equation has been discussed in case of CR-39. FTIR spectra reveal that there is the formationof hydroxyl, alkene, alkyne and carboxylic groups in the Cu-ion irradiated PS. In CR-39, changes in the intensity of the bands on irra-diation relative to pristine samples without appearance of any new band have been observed and discussed.� 2006 Elsevier B.V. All rights reserved.

PACS: 61.41.+e; 71.20.Rv; 78.66.Qn; 78.70.-g; 81.05.Lg

Keywords: CR-39; Polystyrene; Ion fluence; Optical band gap; UV–Vis and FTIR techniques

1. Introduction

The importance of polymers has increased very rapidlyduring the last few decades because of their low cost, easyprocessability and low weight. The use of ion beams tomodify polymer properties, opened a wide area of researchand utilization in various fields like industry, agriculture,ecology [1], sensorics [2,3], microelectronics [4] and nano-technology [5–7]. When a swift heavy ion enters a polymertarget, it loses most of its energy in exciting and ionizingthe atoms along its trajectory. The high value of energydeposited in a very small cylindrical region around theion path, called latent track, leads to dramatic modifica-tions in the target material [8–10]. The chemical changescaused by ionizing radiations involve the main-chain scis-sion, cross-linking, creation of free radicals and formationof saturated and unsaturated groups with stimulated evolu-tion of gasses [11].

0168-583X/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.nimb.2006.11.129

* Corresponding author. Tel.: +91 183 2450926; fax: +91 183 2258820.E-mail address: [email protected] (L. Singh).

The formation of carbonaceous clusters in polymerfilms with ion irradiation has been investigated extensively[12–14]. The optical studies of the so called blackening ofpolymer films can be correlated with the release of carbonatom from the polymer chain during irradiation conse-quently forming graphite like precipitation or clusters [13].The previous studies [15] reveal that in the case of high-energy heavy ion impact, the cluster formation sets in atminimum transferred electronic energy density of around10�3 eV/A3. However, the threshold energy density for lowenergy heavy ion impact is 10�1 eV/A3. Zeigler et al. [16]suggested highly energetic heavy ions form small clustersalong single ion tracks. The cluster size does not exceedthe track diameter (10 nm). However, in the case of overlap-ping of light low energy particles, the cluster size is not lim-ited by the track diameter and giant clusters are formed,exceeding 100 nm in size. For example, in the case ofPMMA irradiated with 35 MeV Li with a fluence of1 · 1012 ions cm�2, large fractional structures are produced,whereas 2 GeV of Au with a fluence of 5 · 1010 ions cm�2

in PMMA produces small clusters 5–6 nm in size [16].

mailto:[email protected]

L. Singh et al. / Nucl. Instr. and Meth. in Phys. Res. B 255 (2007) 350–356 351

Amorphous carbon or graphite is known to consist ofcompact clusters of fused sixfold rings (M). The numberof such rings in a cluster may be obtained from the opticalband gap, Eg, with Robertson’s relation [17].

In the present work, polystyrene and CR-39 polymersamples were irradiated with 64Cu9+ (120 MeV) and12C5+ (70 MeV) ion beams to investigate their responsetowards swift heavy ion irradiation with respect to opticaland chemical properties. In order to obtain the informationabout the number of rings (M) in a cluster in PS and thenumber of carbon atoms per conjugation length (N) inCR-39, UV–Vis spectroscopic technique has been used.

PS is one of the most stable polymers with respect toradiation and very large doses are required to produceany noticeable change [18]. The chemical structure ofmonomer of polystyrene is given below.

Table 1Irradiation conditions

Sr. No. Sam

1 PS-2 PS-3 PS-4 PS-5 PS-6 PS-7 PS-8 PS-9 PS-

10 PS-11 PS-12 CR13 CR14 CR15 CR16 CR17 CR18 CR19 CR

PS contains a large number of benzene rings, which areknown to have ‘‘protective’’ action in many radiation-chemical processes. However, heavy ion irradiation of suchpolymers induces drastic changes in their original structure[19,20]. The monomer of CR-39 is represented as follows:

for polystyrene and CR-39

ple Ion, energy and charge state Fl

0 Cu, 120 MeV, +9 01 Cu, 120 MeV, +9 1 ·2 Cu, 120 MeV, +9 5 ·3 Cu, 120 MeV, +9 1 ·4 Cu, 120 MeV, +9 5 ·5 Cu, 120 MeV, +9 1 ·6 C, 70 MeV, +5 1 ·7 C, 70 MeV, +5 5 ·8 C, 70 MeV, +5 1 ·9 C, 70 MeV, +5 5 ·10 C, 70 MeV, +5 1 ·-0 Cu, 120 MeV, +9 0-1 Cu, 120 MeV, +9 5 ·-2 Cu, 120 MeV, +9 5 ·-3 C, 70 MeV, +5 1 ·-4 C, 70 MeV, +5 5 ·-5 C, 70 MeV, +5 1 ·-6 C, 70 MeV, +5 5 ·-7 C, 70 MeV, +5 1 ·

In polymer form, there exists a three-dimensional net-work of polyallyl chains cross-linked by diethylene glycoldicarbonate linkages. The branching point in this net isthe tertiary carbon in the polyallyl chain [21].

2. Experimental procedures

The specimens of polystyrene (125 lm) and CR-39(250 lm) in the form of flat polished thin films wereobtained from Goodfellow Ltd. (England) and PershoreMoulding Ltd. (England) respectively. These were usedas-received form without any further treatment in the sizeof 1 cm · 1 cm. The samples were mounted on the slidingladder and irradiated with 64Cu9+ (120 MeV ) and 12C5+

(70 MeV) ion beams using 15UD pelletron facility in thegeneral purpose scattering chamber (GPSC) under highvacuum of 10�6 Torr at Inter-University Accelerator Cen-tre, New Delhi. The electronic stopping power of 64Cu9+

(120 MeV) ion in PS and CR-39 is 530.4 and 637.3 eV/Arespectively, while for 12C5+ (70 MeV) ion in the same tar-gets is 26.9 and 33.4 eV/A respectively [22]. The ion beamfluence was varied from 1 · 1011 to 1 · 1013 ions cm�2.Corresponding to the fluences used, the transferred energydensities, obtained from the dose-power relation, rangedfrom 10�4 to 10�1 eV/A3. In order to expose the whole tar-get area, the beam was scanned in the x–y plane. The beamcurrent was kept low to suppress thermal decompositionand was monitored intermittently with a Faraday cup.Other implant conditions are summarized in Table 1. Thesamples were analyzed with UV–Vis spectroscopy usingHitachi Modal U-3000 spectrophotometer in the range200–800 nm to observe the variation in optical band gapand Urbach’s energy. The chemical-changes were studiedusing Nexus 870 FTIR Spectrometer in the range 4000–500 cm�1.

uence (ions cm�2) Current (pnA) Comment

0 Transparent1011 1 ± .08 Pale yellow1011 1 ± .08 Yellow1012 1 ± .08 Brown1012 1 ± .08 Dark brown1013 1 ± .08 Black & brittle1011 2 ± .04 Transparent1011 2 ± .04 Translucent1012 2 ± .04 Pale yellow1012 2 ± .04 Yellow1013 2 ± .04 Light brown

0 Transparent1011 1 ± .08 Pale yellow1012 1 ± .08 Yellow1011 4 ± .02 Translucent1011 4 ± .02 Pale yellow1012 4 ± .02 Yellow1012 4 ± .02 Light brown1013 4 ± .02 Brown

200 300 400 500 600 700 8000

1

2

3

4

5

cb

a

Ab

sorp

tio

n

Wave length (nm)

Fig. 1(c). UV–Vis spectra of CR-39 irradiated with 120 MeV copper ionsat: (a) virgin, (b) fluence = 5 · 1011 and (c) 5 · 1012 ions cm�2.

4

5

ion

352 L. Singh et al. / Nucl. Instr. and Meth. in Phys. Res. B 255 (2007) 350–356

3. Results and discussion

3.1. Optical analysis

The irradiation of the polymers with 64Cu9+ (120 MeV)ions leads to pronounced coloration effects, starting fromlight brown to almost black at the highest fluence of1 · 1013 ions cm�2. On the other hand, the colorationeffect of 12C5+ (70 MeV) is not that much pronounced.Figs. 1(a),1(b),1(c) and 1(d) presents the spectra of virginand irradiated polymer samples at varying ion fluences. Ashift in the absorption from the ultraviolet to the visibleregion is observed on irradiation. One of the possibilitiesbehind this behaviour is interpreted as the formation ofextended system of conjugated bonds i.e. the formationof carbon clusters [14]. In the studied range of wavelengththe absorption bands are associated p � p* electronic tran-sitions [23]. This type of transitions occurs in the moleculesi.e. in the compounds containing double or triple bondsand also in aromatics. The excitation of p electron requires

200 300 400 500 600 700 8000

1

2

3

4

5

f

e

dc

b

a

Ab

sorp

tio

n

Wave length (nm)

Fig. 1(a). UV–Vis spectra of PS irradiated with 120 MeV copper ions at:(a) virgin, (b) fluence = 1 · 1011, (c) 5 · 1011, (d) 1 · 1012, (e) 5 · 1012 and(f) 1 · 1013 ions cm�2.

200 300 400 500 600 700 8000

1

2

3

4

5

a

f

e

dc

b

Ab

sorp

tio

n

Wave length (nm)

Fig. 1(b). UV–Vis spectra of PS irradiated with 70 MeV carbon ions at:(a) virgin, (b) fluence = 1 · 1011, (c) 5 · 1011, (d) 1 · 1012, (e) 5 · 1012 and(f) 1 · 1013.

200 300 400 500 600 700 8000

1

2

3 fedcba

Ab

sorp

t

Wave length (nm)

Fig. 1(d). UV–Vis spectra of CR-39 irradiated with 70 MeV carbon ionsat: (a) virgin, (b) fluence = 1 · 1011, (c) 5 · 1011, (d) 1 · 1012, (e) 5 · 1012

and (f) 1 · 1013 ions cm�2.

smaller energy and hence, transition of this type occurs atlonger wavelength.

In the high absorption region (where absorption is asso-ciated with interband transitions), the absorption coeffi-cient a(m) was given in the quadratic form by Tauc et al.[24] and discussed in the more general form by Davis andMott [25], who use the equation in the form

aðmÞ ¼ Baðhm� EgÞn

hm; ð1Þ

where Ba is a constant, Eg is the optical band gap, a(m) isthe absorption coefficient at a frequency of m and n is an in-dex which can assume values of 0.5 for allowed direct tran-sitions, 1.5 for direct forbidden transitions, 2 for allowedindirect transitions and 3 for forbidden indirect transitions.We are using n = 2, which is valid for the materials havinga(m) greater then 104 cm�1 [18]. The value of Eg in any or-ganic material is obtained by plotting (ahm)1/2 versus hm andextrapolating the linear region of the plot (ahm)1/2 to zero isused to define the so-called optical band gap in polymers.

The values of Eg for virgin as well as irradiated samplesare reported in Tables 2a and 2b. It is found that there is adecreasing trend of energy gap with the increase in ionfluence. For 64Cu9+ the optical energy gap decreases by

Fig. 2. The variation of number of rings (M) in the cluster with ion fluencein PS.

Table 2aThe variation of band gap energy and Urbach’s energy with irradiation in case of polystyrene, along with the number of rings (M) in the biggest clusterembedded

Fluence of Cu9+

(ion cm�2)Band gap energy(eV)

Urbach’s energy(eV)

M Fluence of C5+



M

0 4.36 0.06 �1 0 4.36 0.06 �11 · 1011 4.30 0.66 �1 1 · 1011 4.32 0.08 �15 · 1011 2.54 0.44 �3 5 · 1011 4.31 0.30 �11 · 1012 2.44 0.34 �3 1 · 1012 4.27 0.35 �15 · 1012 2.19 0.31 �4 5 · 1012 3.78 0.41 �11 · 1013 1.73 0.34 �6 1 · 1013 2.50 0.40 �3

Table 2bThe variation of band gap energy and Urbach’s energy with irradiation in case of CR-39, along with the number of carbon atoms (N) per conjugatedlength

Fluence of Cu9+



N Fluence of C5+



N

0 5.50 0.77 �3 0 5.50 0.77 �31 · 1011 – – – 1 · 1011 4.94 0.75 �45 · 1011 3.08 0.49 �6 5 · 1011 4.88 0.47 �41 · 1012 – – – 1 · 1012 3.36 0.45 �55 · 1012 2.75 0.41 �7 5 · 1012 3.17 0.43 �61 · 1013 – – – 1 · 1013 2.94 0.38 �6


almost 61% in polystyrene at the highest fluence of1 · 1013 ions cm�2. Similarly, a decrease in band gap of43% in polystyrene and 47% in CR-39 are observed, on12C5+ ion irradiation. The behavior of the optical gap ofion-irradiated samples can be explained following themodel of Robertson and O 0Reilly [17].

The number of carbon hexagon rings in the cluster (M)is then found from the relation [17]

Eg � 2jbj=ffiffiffiffiffi

Mp

; ð2Þ

where jbj is a bond integral that represents the interactionenergy of two p atomic orbitals. A theoretical value for jbjproposed by Robertson and O 0Reilly is 2.9 eV, whichaccording to Compagnini et al. [26] is an overestimationof the true value. So, the best-fit value of jbj given by themis 2.2 eV. In the present study on the aromatic polymer, Eq.(2) has been used to calculate the values of M and itsbehaviour with ion fluence is summarized in Table 2a.Fig. 2 reveals that the number of rings (M) increases withthe increase of ion fluence in irradiated PS. It is found thatthe variation of M is more prominent (1–6) in case of64Cu9+ (120 MeV) ion irradiated PS as compared to (1–3)for 12C5+ (70 MeV) ion irradiated target.

On the other hand for a linear structure, the number ofcarbon atoms per conjugation length N is given by [17]

N ¼ 2fp=Eg; ð3Þ

where 2f gives the band structure energy of the pair ofadjacent p sites. The value of f is taken to be �2.9 eV asit is associated with p! p* optical transitions in –C=C–structure. As the shift of the absorption edge can be attrib-uted to an increase of the conjugation length without for-

mation of new linear conjugated structure, Eq. (3) isapplied in the present study. The values of N are given inTable 2b. In 64Cu9+ (120 MeV) ion irradiated CR-39, thenumber of carbon atoms is increased from 3 to 7 withthe increase of fluence from 0 to 5 · 1012 ions cm�2. How-ever, N is raised up to 6 when the same polymer film is irra-diated with 12C5+ (70 MeV) ion under similar conditions.

The irregularities in the band gap level of the polymers isdefined in terms of Urbach’s energy and is calculated fromthe inverse of the slope of the linear part of the curve lnaversus hm (eV) where a being the absorption coefficient[27]. The calculated values are reported in the Tables 2aand 2b along with the band gap energy of the polymers.

4000 3600 3200 2800 2400 20000

120

3451

2190

3296

Tra

nsm

itta

nce

Wave number (cm-1)

Pristine 1 x 1012 ions cm-2

1 x 1013 ions cm-2

2000 1800 1600 1400 1200 1000 800 6000

120 Pristine 1 x 1012 ions cm-2

1 x 1013 ions cm-2

Tra

nsm

itta

nce

Wave number (cm-1)

a

b

Fig. 3. FTIR spectra of PS irradiated with 120 MeV copper ions. (a)ranges from 2000 to 4000 cm�1 and (b) ranges from 500 to 2000 cm�1. Theblack line corresponds to pristine sample, while red and green line arerepresenting the irradiated PS with fluence. (For interpretation of thereferences in colour in this figure legend, the reader is referred to the webversion of this article.)


The energy has reduced to 0.38 eV from its initial value of0.77 eV in case of CR-39 on irradiation with carbon ions.Similar kind of trend is obtained with heavy ions. Thissharp decrease indicates the regularization of the bandgap energy level. The results also reveal that in pristinePS, the Urbach’s energy has been found to be very smalli.e. 0.06 eV which may be due to semi-crystalline natureof polystyrene and on irradiation with carbon ions, theincrease of irregularities in band gap has been observed.This is attributed to the reason that carbon irradiationhas enhanced the diffusion rate of p-electrons to the forbid-den level of the PS. On irradiation with copper ions,Urbach’s energy first increases up to 0.66 eV at the fluenceof 1 · 1011 ions cm�2 and then decreases with the furtherincrease of fluence.

3.2. Chemical analysis

The nature of chemical bonds of polymers can be stud-ied through the characterization of the vibrational modesdetermined by infrared spectroscopy. A complex moleculehas a large number of vibration modes, which involve thewhole molecule. To a good approximation, however, someof these molecular vibrations are associated with the vibra-tions of individual bonds of functional groups while othermust be considered as vibrations of the whole molecule.The soggy vibrations of these molecules give rise to theabsorption bands at low energies falling in the region ofinfrared radiations [27]. In general the first evidence ofthe modification of an irradiated polymer is the change inintensity of the infrared bands. In case of polystyrene, theirradiation of 62Cu9+ (120 MeV) has done noticeable mod-ifications in FTIR spectra with the increase of fluence(Fig. 3). In addition to overall increase in intensity, somenew bonds also appear in the spectra. Even though the irra-diation was carried out in vacuum, the oxidized com-pounds are supposed to be created when the sampleswere put in contact with the air after irradiation. The oxy-gen from air reacts with the radiation-induced radicals togive C=O, –OH and C–O bands [28].

The new band at 3458 cm�1 represents the formation ofalcohol or phenol group (i.e. –OH group). This appearanceof medium broad band in the region 3550–3200 cm�1 is dueto intermolecular hydrogen bonding (i.e. O–H stretchingvibrations), which increases with the increase of fluence,at the expense of free hydroxyl band. This is also corrobo-rated with increase of C–O stretching vibrations in the1260–1000 cm�1 region of the spectrum which is also sug-gesting the existence of alcohols/phenols.

As reported in papers [29,30], the band at 3296 cm�1 wasconsidered because of R–C � C–H. A similar medium bandobserved in the present case at 3296 cm�1 could be assignedto C–H stretching vibration of alkyne group. A very weakband attributing the stretching of C � C bond of alkynes isalso observed in the spectra at 2260–2100 cm�1. The widthof this band indicates that both monosubstituted alkyneR–C � CH and disubstituted alkynes R–C � C–R0 are

formed. The formation of alkyne group is the result of break-ing of phenyl ring. However, this formation of alkyne groupsin chain (Eq. (4)) or even the production of small fragments,such as phenylacetylene (Eq. (5)), trapped in the film cannotbe excluded [30].

ð4Þ

ð5Þ

The C=C stretching mode of unconjugated alkenes usu-ally shows moderate to weak band at 1667–1640 cm�1.Increase in intensity of band has been observed in thisregion, that may be due to the increase in number of mon-osubstituted, disubstituted, trans-, or -cis-alkenes, orvinyldene alkenes, which usually absorb in this region.

It is well known that PS give very high radiation resis-tance to low (dE/dX)e and for observing changes in theintensity of the infrared bands high doses (in the order oftens of MGy) are required [30]. Consequently, 70 MeV car-bon is not able to produce much change in the FTIR spec-tra of polystyrene up to the fluence of 1 · 1013 ions cm�2

except the formation of new band at 3440 cm�1 (Fig. 4),

3500 3000 2500 2000 1500

2350

Tra

nsm

itta

nce

Wave number (cm-1)

3079

2756

2217

1581

Fig. 5. FTIR spectra of pristine CR-39 (full line curve) and of sampleirradiated with copper ions of 120 MeV with fluence of 5 · 1012 (dottedline curve) ions cm�2.

Fig. 4. FTIR spectra of pristine polystyrene (full line curve) and of sampleirradiated with carbon ions of 70 MeV with fluence of 1 · 1013 (dotted linecurve) ions cm�2. Fig. 6. FTIR spectra of pristine CR-39 (full line curve) and of samples

irradiated 70 MeV carbon ion with fluence of 1 · 1012 (dotted line curve)and 1 · 1013 (dashed line curve) ions cm�2.


which is representing the initiation of formation of –OHgroup.

Fig. 5 represents the FTIR spectra of pristine and cop-per irradiated (fluence = 5 · 1012 ions cm�2) CR-39. Inthe pristine sample there is a band at 1581 cm�1 which isassigned to –C=C– vibrations. The band at 2217 cm�1 isbecause of CO vibration, which is due to the decomposi-tion of carbonate group into CO2 and CO on irradiation.The production of CO2 is also confirmed from the increasein the intensity of band at 2350 cm�1. The shoulder at2756 cm�1 is representing the symmetric stretching of satu-rated C–H bands. The stretching vibration of unsaturated=C–H is represented by the band on the shorter wave-length side i.e. 3079 cm�1.

Fig. 6 represents the FTIR spectra of 12C5+ (70 MeV,4 pnA) ion irradiated CR–39. The peaks at 2339 cm�1

and 654 cm�1 are suggesting the formation of CO2 duringirradiation, which increases with the increment in exposuretime. The formation of CO2in CR-39 by c-ray irradiationhas also been reported [31]. The increase in intensity ofabsorption band at 3400–3600 cm�1 region is observed,which may be due to the scission taking place at –CH2–O–CH2 leading to the formation of hydroxyl group. Thisband is becoming broader with the increase of ion fluence.

A noticeable change in 600–800 cm�1 regions owing to eth-ylenic twisting (–CH=CH2) has also been observed.

We have observed less modifications with Cu than Cions because the range of Cu ion of 120 MeV in CR-39 is24.93 lm [22] and the thickness of sample is 250 lm there-fore most of the sample remains unmodified. However, Cion of 70 MeV has entered deep inside the sample(�134.33 lm), therefore inducing more modifications ascompared to Cu ion.

4. Conclusion

UV–Vis and FTIR studies show that there are signifi-cant modifications in the optical and chemical propertiesof both CR-39 and PS polymers on irradiation with64Cu9+ (120 MeV) and 12C5+ (70 MeV) ions. The opticalband gap decreases and this decrease is more pronouncedwith Cu-ion beam, owing to high electronic energy loss.The high value of energy deposition leads to heavy reorien-tation of polymeric material, which further results in theformation of carbon clusters. It has become clear fromthe calculations that cluster formation is more effectivewith Cu-ions in both of the polymers used.

In PS, the formation of hydroxyl (–OH), alkene (C=C)and alkyne (C � C) groups has been noticed with copperion bombardment, whereas little change in absorptionpeaks is observed on carbon ion irradiation. In case ofCR-39, it has been concluded that there are changes inthe intensity of the bands on irradiation without appear-ance of any new band.

Acknowledgements

The authors wish to thank to Mr. Sandeep Chopra, Mr.Fouran Singh and to the other staff members of InterUniversity Accelerator Centre, New Delhi for their helpduring irradiation and also for providing UV–Vis andFTIR facilities. Thanks are also due to Dr. MPS Isher,Deptt. of Pharmaceutical Sciences and Dr. S.S. Chimny,


Department of Chemistry, G.N.D.U. Amritsar for discus-sion on the FTIR spectra.

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Opto-chemical response of CR-39 and polystyrene to swift heavy ion irradiation