Surface modi®cation of polyethylene terephthalate implanted byargon ions 1

Youmei Sun a,*, Changlin Li a, Zhu Zhiyong a, Weiming Liu b, Shengrong Yang b

a Institute of Modern Physics, Chinese Academy of Sciences, P.O. Box 31, Lanzhou 730000, Chinab Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China

Abstract

Modi®cation of polyethylene terephthalate (PET) was carried out by 120 keV Ar�, 240 keV Ar2� and 360 keV Ar3�

ions implantation. From measurement of UV absorption, optical band gaps under various implantation conditions

were deduced with Tauc's plot. A dramatic decrease about 8 orders of magnitude of the sheet resistivity (qs) was ob-

tained and explained by the progressive narrowing of the band gap. Friction and wear behaviors of argon ion implanted

PET against steel ball were investigated. The change of surface atom and molecular radical of irradiated PET was char-

acterized by X-ray photoelectron spectroscopy. All results suggest that irradiation damage attributes to energy trans-

ferred by electronic excitation or ionization. The conducting property induced by implantation is attributed to the

formation of conduction states in the pristine band gap. Ó 1998 Elsevier Science B.V.

PACS: 79.20.RF; 61.41.te

Keywords: Surface modi®cation; Argon ion implantation; Polyethylene terephthalate

1. Introduction

Ion implantation is useful for many insulatingpolymers to enhance their electrical conductivityand modify their optical and mechanical property.Two major types of chemical modi®cation havebeen suggested in implanted polymers. The ®rstis crosslinking in which additional chemical bonds

are formed between neighboring polymer chains.The second is chain scission in which bonds alongthe polymer are destroyed. Calcagno [1] classi®edthe e�ects of ion irradiation on polymer into threedomains according to the energy density De � USe

(eV/cm3), given by the product of the ion ¯uence U(ions/cm2) and the electronic energy loss Se (eV/cm). Many spectroscopic techniques have beenemployed to characterize the modi®cation inducedby ion bombardment on polymeric ®lm. Howeverthe chemical nature of the treated-polymer makesit di�cult to be uniquely assessed. In particular,whether there is a formation or not of a graph-ite-like phase is a controversial point.

Nuclear Instruments and Methods in Physics Research B 135 (1998) 517±522

* Corresponding author. Fax: +86 931 888 1100; e-mail:

hiam@ns.lzb.ac.cn.1 Work is supported by foundation of Chinese Academy of

Sciences.

0168-583X/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved.

PII S 0 1 6 8 - 5 8 3 X ( 9 7 ) 0 0 6 3 1 - 9

2. Results and discussion

Polyethylene terephthalate (PET, (C10H8O4)n)is a semicrystalline polymer which has the biggeststretch strength of all thermoplasts. The samplesused in our test are sheets with a thickness of15 lm and a density of 1.397 g/cm3 made inShanghai, China. They were implanted with 120keV Ar�, 240 keV Ar2� and 360 keV Ar3� usingbeam current density of the order of 0.1 lA/cm2

to the ¯uences of 1 ´ 1013, 1 ´ 1014, 1 ´ 1015,3 ´ 1015, 5 ´ 1015 and 1 ´ 1016 ions/cm2, respec-tively, at a 200 KV ion implanter of Lanzhou In-stitute of Modern Physics. In order to directlymonitor the ¯uence from the stainless steel sup-port of sample and pledge the beam uniformity,the ion beam implanted to the 4 ´ 4 cm2 samplewas uniformly scanned over an area of 5 ´ 5cm2. During implantation, the vacuum of the tar-get chamber was about 133 lPa. After implanta-tion, the initially achromatic PET gradually showa dark graphite-like luster with increasing ¯uenceand charge state.

2.1. Ion beam induced conductivity

The sheet resistivities qs (X/h) of all PET sam-ples were measured with three annular electrodessystem by Model ZC36 1017 X Hyper-resistanceapparatus. From the resistance reading numberRs of the instrument, the outer diameter of mea-suring electrode D and inner diameter of the highvoltage electrode d, the sheet resistivity qs (X/h)was deduced by qs � 2pRs= ln�D=d�. The measur-ing system was shielded in a metal box to avoiddisruption of environment. The basic systematicerror of the sheet resistivity is between 10% and20% which comes from the Rs. The intrinsic sheetresistivity of PET is about 1.7 ´ 1016(X/h). Fig. 1shows the dose dependence of sheet resistivity qs

(X/h) for PET implanted with di�erent charge ar-gon ions, which demonstrates that ion implanta-tion causes an increase of approximately 8 ordersof magnitude and there is a relationship of qs

(Ar�) > qs (Ar2�) > qs (Ar3�). The surface resisti-vity decreases with ¯uence increasing and saturatesunder high ¯uence implantation.

2.2. Relation between optical and electrical proper-ties

UV±Visible absorption spectra of implantedPET were measured in the wavelength domain190±700 nm by double light beams referred tothe pristine PET ®lm. Davenas [2] demonstratedthat the modi®cation of the optical properties ofimplantation polymer could be interpreted withthe model developed for hydrogenated carbon®lms and their optical gap could be deduced fromTauc's plot [3]. The Tauc's relation illustrates avariation of the absorption edge of UV±Visible ab-sorption spectra as a function of the photon ener-gy in compliance with �aE�1=2 / �E ÿ Eg�; fromwhich the optical gap Eg may be easily deduced.As the absorption coe�cient a is in direct propor-tion to the optical density OD, the Tauc's relationcould be described as �OD=k�1=2 / �E ÿ Eg�. Fromthe absorption edge of the measuring spectra ofPET implanted with 240 keV Ar2� at di�erent ¯ue-nces, the Eg could be acquired for various ¯uencesby extrapolation of the linear part (Fig. 2). Themeasuring error of the OD is less than 1% andthe ®tting error of the linear part is less than the3%, so the error in Eg is about 4%. There are sim-ilar rules of ¯uence dependence for 120 keV Ar�

and 360 keV Ar3� implantation with 240 keVAr2�. The optical gap of the pristine PET is 2.67eV, but 0.78, 0.51 and 0.38 eV for Ar�, Ar2� andAr3� implantation at 1 ´ 1016 ions/cm2, whichmeans that there is a narrower gap for higher

Fig. 1. Sheet resistivity as a function of ¯uences at di�erent

charge state argon ions.

518 Y. Sun et al. / Nucl. Instr. and Meth. in Phys. Res. B 135 (1998) 517±522

charge ion implantation. In order to show the rela-tion between optical and electrical properties, wecompared optical gap Eg and sheet resistivity qs

for 240 keV Ar2� implantation in Fig. 3, whichdemonstrated that the position of the absorptionedge could provide a criterion of conductivityand the improvement of the conductivity couldbe explained by the progressive narrowing of theband gap.

2.3. Friction and wear measurement

Friction and wear tests were carried out on PET®lms using Kyowa Model DF.PM one-way recip-rocating friction tester with a 3.0 mm diameterGcr15 bearing steel (SAE 52100 steel) ball and100 g load at an average sliding velocity 90 mm/

min for a total of 100 cycles. The friction coe�-cients for di�erent Ar2� ¯uences (1±7 in Fig. 4)and 5 ´ 1015 Ar�/cm2 (8 in Fig. 4) and 5 ´ 1015

Ar3�/cm2 (9 in Fig. 4) were measured with a sys-tematic uncertainty between 5% and 10%. Fig. 4depicts that the friction coe�cient of the implantedPET has increased and this value is larger at lowerimplantation ¯uences. This is di�erent from im-planted Kapton [4,5] but is in agreement with im-planted PET and polycarbonate (PC) [6,7]. Itseems that friction and wear behaviors are not sen-sitive to ion charge at the same ¯uence 5 ´ 1015

ions/cm2 (6, 8 and 9 in Fig. 4). For the sliding sys-tem of polymer against steel, polymer is usuallytransferred to the surface of the steel ball duringfriction processes. The wear is ultimately a mani-festation of adhesion bond breakage, it is believedthat strong covalently bonded crosslinking is re-sponsible for the improvement of wear resistancein implanted polymers. The crosslinking inducedby implantation could increase the cohesivestrength of the polymer. As a result, the frictionforce increases and results in the friction coe�cientincrease to overcome the adhesive between mole-cules of the polymer. Friction and wear behaviorsare very complex phenomena which depend onload, speed and surface asperities etc. Althoughthe modi®ed polymers have shown a remarkablechange of wear resistance, the mechanism of the

Fig. 3. The relation between the absorption edge and conduc-

tivity for Ar2� implantation.

Fig. 4. Friction coe�cient of PET against steel under 100 g load

after 100 cycles for di�erent implantation conditions. (1) Pris-

tine, (2) 1 ´ 1013Ar2�/cm2, (3) 1 ´ 1014Ar2�/cm2, (4)

1 ´ 1015Ar2�/cm2, (5) 3 ´ 1015Ar2�/cm2, (6) 5 ´ 1015Ar2�/cm2,

(7) 1 ´ 1016Ar2�/cm2, (8) 5 ´ 1015Ar�/cm2, (9) 5 ´ 1015Ar3�/

cm2.

Fig. 2. Optical absorption spectra and Tauc plot method of

PET ®lms implanted by 240 keV Ar2� with di�erent ¯uences.

Y. Sun et al. / Nucl. Instr. and Meth. in Phys. Res. B 135 (1998) 517±522 519

improvement of the friction and wear is still notclear and needs further study.

2.4. Chemical structure analysis

The XPS measurements were carried out on aVG ESCALAB 220i-XL spectrometer ®tted witha monochromatized AlKa source (1486.6 eV).The X-ray incident angle was 45° and electrontake-o� angle was zero degree. A ¯ood gun wasused to cancel the charging-up e�ect during mea-surement. The individual peaks of the ESCA ofC1s and O1s were ®tted with multiple Gaussianfunction with background subtraction (Figs. 5and 6) corresponding to the implantation with var-ious charge ions at the same ¯uences 5 ´ 1015 ions/cm2. The data from the pristine sample could be ®t-ted with four distinct peaks for C1s (288.4, 286.4,284.3 and 282.3 eV) and three peaks for O1s

(532.9, 531.4 and 529.4 eV) (cf. Table 1). Table 1suggests that the C±H, C±O, and C@O bonds scis-sion induces the emission of oxygen and hydrogengaseous molecules; oxygen and hydrogen concen-tration decrease result in the carbon enrichment af-ter implantation. The bond C@O is destroyed moresensitively than the CH2±O, and higher charge ion

is much more destructive than lower one, becauseof its greater Coulomb interaction. The crosslink-ing leads to the enhancement reticulation of ben-zene ring.

3. Conclusion

From the measurement above, we ®nd thatthere are more improvement e�ects for highercharge argon ion except for the wear property,which can be explained by high energy densityDe. At low ¯uence (<1 ´ 1014 ions/cm2,De � 1023 eV/cm3) implantation, the little changeof the optical gap (Fig. 2) suggests that new bondsor chain scissions formation occurs. With the in-creasing of ¯uences (�1015 ions/cm2, De � 1024

eV/cm3), the continuous rearrangement induces acomplete change of the polymer molecular struc-ture; which causes Eg to change rapidly. At thehigher ¯uences (>5 ´ 1015 ions/cm2, De � 1025

eV/cm3), the complete overlap between the iontracks produces a stable new material which losesmemory of its original structure and stoichiomet-ry, Eg saturate. The dependence of all these pro-cesses on energy density is not the same for

Fig. 5. The C1s ESCA spectra and ®tting spectra corresponding to di�erent charge ion implantation at 5 ´ 1015 ions/cm2 ¯uence.

520 Y. Sun et al. / Nucl. Instr. and Meth. in Phys. Res. B 135 (1998) 517±522

di�erent ions, which demonstrates that energydensity is not the only relevant factor in the poly-mer modi®cations. During the irradiation, theemission of various gaseous molecular speciesleads to rearrangement, forming clusters of threeor more atoms; the crosslinking enhances reticula-tion of aromatic ring.

The increase of optical absorption and the ap-pearance of an electric conductivity are the mainconsequences of the damage accumulation in theimplantation layer. The disorder leads to the for-mation of bands of localized states in the gap,which accounts for the modi®cation of electronicconductivity. The decreased resistivities are also

Table 1

Surface atomic and bond concentration percentage for di�erent charge argon ions

Atom Fitted peak

(eV)

Probable bond states Pristine Ar� Ar2� Ar3�

Bond a (%) R b (%) Bond (%) R (%) Bond (%) R (%) Bond (%) R (%)

C1S 288.4 Carbonyl C@O 13.2 55 6.2 83 5.6 82 7.2 74

286.4 C±O 11.8 8.5 8.2 10.7

284.3 Benzene ring 58.6 61.3 86.2 81.2

282.3 Polluted 16.3 23.9 0 0

O1S 532.9 C±O 23.5 45 13.7 17 16.2 18 17.4 26

531.4 Carbonyl C@O 61.7 63.4 83.8 82.6

529.4 Polluted 14.8 22.8 0 0

a It expresses ®tting area percentage of the bond state, its relative error is about 10%.b Where R is the relative normal area percentage of C1S and O1S, its relative error is about 20%.

Fig. 6. The O1s ESCA spectra and ®tting spectra at the same condition with Fig. 5.

Y. Sun et al. / Nucl. Instr. and Meth. in Phys. Res. B 135 (1998) 517±522 521

related to the increase of carbon and decrease ofoxygen in the surface of PET ®lm.

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