Interaction of chlorine radicals with polyethylene and hydrocarbon thin films under vacuum conditions––a comparison with atomic oxygen reactivity

Surface Science 543 (2003) 75–86

www.elsevier.com/locate/susc

Interaction of chlorine radicals with polyethyleneand hydrocarbon thin films under vacuum

conditions––a comparison with atomic oxygen reactivity

Jessica Torres, C.C. Perry, A.J. Wagner, D. Howard Fairbrother *

Department of Chemistry, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA

Received 22 January 2003; accepted for publication 24 July 2003

Abstract

The surface reactions of atomic chlorine and oxygen with hydrocarbon-based polymers and organic thin films under

vacuum conditions have been investigated with in situ X-ray photoelectron spectroscopy (XPS). The interaction of

chlorine radicals (Cl(2P)) with polyethylene (PE) under vacuum conditions produces a partially chlorinated layer

containing both CCl and CCl2 groups whose concentration was maximized at the surface. Compared to higher-pressure

photochlorination experiments where the flux of chlorine atoms is higher, the maximum extent of PE chlorination as

measured by the C:Cl XPS ratio and the evolution of the C(1s) region was reduced in the present study while the surface

selectivity of the reaction was enhanced. This influence of chlorine atom flux on the extent of chlorination and surface

selectivity can be rationalized by a simple stochastic model of the PE chlorination process that incorporates steric effects

associated with the production of mono and dichlorinated carbon atoms as well as cross-linking reactions between

carbon-containing radicals. During the reaction of PE with atomic oxygen (O(3P)), a concentration gradient of oxygen-

containing carbon functionality is also observed in the near surface region. Experiments carried out on hydrocarbon

thin films based on self-assembled monolayers (SAMs) reveal that chlorination proceeds without erosion. In contrast,

the incorporation of new carbon containing-oxygen functionalities during reactions of hydrocarbon films with atomic

oxygen occurs in competition with carbon erosion.

� 2003 Elsevier B.V. All rights reserved.

Keywords: Surface chemical reaction; Chlorine; Self-assembly; Oxygen

1. Introduction

The interaction of atomic species (radicals) witha variety of different surfaces is important in a

number of technologically significant processes.

For example, plasmas are one of the most popular

* Corresponding author. Tel.: +1-410-516-4328; fax: +1-410-

516-8420.

E-mail address: [email protected] (D.H. Fairbrother).

0039-6028/$ - see front matter � 2003 Elsevier B.V. All rights reserv

doi:10.1016/S0039-6028(03)00992-0

methods of modifying a polymer�s chemical and

physical surface characteristics, such as wettability,

adhesion, biocompatibility [1] and gas permeability[2], without changing bulk properties. Within the

plasma, the concentration of reactive neutrals and

radicals are typically estimated to be 3–4 orders of

magnitude greater than their ion counterparts [3].

Chlorine plasmas for example, are used to modify a

polymer�s gas permeability [4,5] while in the mi-

croelectronics industry chlorine-based plasmas are

ed.

mail to: [email protected]

76 J. Torres et al. / Surface Science 543 (2003) 75–86

used to etch and clean semiconductors and metals

such as Si [6], GaAs, AlGaAs [7,8], Pt [9], and Al

[10]. The interaction between gas-phase radical

species and solid substrates during chemical vapor

deposition is also a central process in the growth of

a variety of semiconductor materials [11–14].Studies on the reactions of radical species with

surfaces have, however, been limited by the need to

generate clean controllable sources of neutral rad-

icals while maintaining the capability to monitor

changes in the physical and chemical composition

of the interface.

The interaction of atomic chlorine with hydro-

carbon surfaces has been studied by X-rayphotoelectron spectroscopy (XPS) during the

photochemical gas phase chlorination of polyeth-

ylene (PE) (–CH2–CH2–)n using Cl2 at pressures

between 10 and 500 Torr [15–18]. For example,

Elman et al. [17] showed that during chlorination

of PE under atmospheric pressures, CCl and CCl2species were produced with monochlorinated spe-

cies dominating, ultimately producing a CC/CH2:CCl:CCl2 ratio of 1:1:0.5. The uptake of

chlorine was found to slow drastically at longer

exposures, although angle resolved XPS measure-

ments indicated that chlorination was uniform

throughout the sampling depth (10–70 �AA). In a

later study, using a combination of XPS, attenu-

ated total reflectance (ATR) IR and gravimetric

measurements, McCarthy and co-workers [15]showed that the depth as well as the extent of

chlorination could be controlled by adjusting the

Cl2 pressure, photointensity and exposure. Em-

ploying a combination of ATR and UV–VIS

measurements, Vernekar and co-workers [18]

showed that gas-phase photochlorination of PE

was accompanied by simultaneous dehydrochlori-

nation. In related studies, Chidsey and co-workers[19] examined the surface functionalization of

methyl-terminated alkyl self-assembled monolay-

ers (SAM) on silicon during gas-phase photo-

chlorination (PCl2 � 0:2 Torr). Results from this

study indicated that chlorination of the monolayer

under these conditions was limited. More than

50% of the original carbon atoms remained unat-

tached to chlorine with a steric preference forchlorination at the methyl ends of the alkyl chains,

evidenced by ATR measurements.

In this study, we present results on the exposure

of atomic chlorine (generated from the thermal

dissociation of molecular chlorine) to hydrocar-

bon surfaces under vacuum conditions. A deter-

mination of the processes that accompany the

interaction of chlorine radicals with hydrocarbonsurfaces under these low-pressure conditions was

motivated by the need to better understand the

reactions of chlorine-based plasmas with poly-

meric substrates and also to explore the possibility

that this approach could be employed as a route

for selective surface functionalization.

Under the low-pressure conditions that char-

acterize our investigation, the exposure of PE tochlorine atoms produces a depth dependent con-

centration gradient of CCl and CCl2 species in the

near surface region. Compared to previous higher-

pressure photochlorination experiments on PE

[15,17], the maximum chlorine content obtained

was significantly reduced while the surface selec-

tivity was enhanced. A simple stochastic kinetic

model, which considered steric effects on thechlorination as well as cross-linking reactions be-

tween carbon containing radicals, was employed

to model the chlorination of PE. To develop a

more comprehensive understanding of radical/

polymer surface reaction dynamics, the reactivity

of chlorine atoms with hydrocarbon substrates has

also been compared to those associated with

atomic oxygen. During their interaction with PE,both atomic chlorine and atomic oxygen produced

a depth dependent concentration of new species in

the near surface region. Separate experiments

carried out on SAMs, used as models for poly-

meric interfaces, reveal that while chlorination

proceeds without significant substrate erosion, the

grafting of new oxygen-containing functionality

during reactions with atomic oxygen is accompa-nied by carbon loss from the overlayer.

2. Experimental

In situ XPS measurements were carried out in

the same ultra-high vacuum (UHV) chamber

(Pbase � 5� 10�9 Torr) as the chlorine radicalsource, as described previously [20]. XPS mea-

surements were performed with a Physical Elec-

J. Torres et al. / Surface Science 543 (2003) 75–86 77

tronics 04-500 dual anode source, using Mg Ka

(1253.6 eV) irradiation. All XP spectra were col-

lected at 15 kV and 300 W with a take off angle of

45� from the sample normal unless otherwise no-

ted. Samples were mounted in a carousel stage

with xyz translational and rotational capabilities.Binding energy scales were referenced to the CC/

CH2 peak in the C(1s) region at 284.6 eV [21]. The

C(1s) and Cl(2s) peaks were best fit using mixed

Gaussian–Lorentzian curves in conjunction with

Shirley background subtraction. The reliability of

the fitting protocol employed in Fig. 1 was verified

by comparing the Cl/C ratio obtained from the fits

in the C(1s) region shown in Fig. 1 against the Cl/C ratio calculated from the integrated C(1s) and

Cl(2s) XPS areas. 1 Results from this analysis re-

vealed a good qualitative agreement between

the Cl/C ratios obtained using these two methods.

The validity of the angle resolved XPS measure-

ments reported in the present study were veri-

fied by measurements on the polymer nylon-6

(NH(CH2)5(C@O))n, which revealed an angle in-dependent N(1s)/O(1s) XPS ratio and C(1s) spec-

tral envelope. ATR measurements were made with

a Spectra Tech Thunderdomee accessory using a

Mattson Infinity Series FTIR spectrometer. Sto-

chastic kinetic simulations were performed using a

software package [22] that utilizes the algorithm

developed by Gillespie [23,24].

2.1. Polyethylene sample preparation

A PE bar (McMaster Carr, high-density tech-

nical grade) was cut with a razor and washed

in 2,2,4-trimethylpentane to remove adventitious

carbon and oxygen impurities. Typical XPS sur-

veys showed <5% oxygen impurities in the native

PE samples.

2.2. Radical source

Atomic chlorine and oxygen were generated

using a Thermal Gas Cracker TC-50 (Oxford

1 This comparison is facilitated by the proximity of the C(1s)

and Cl(2s) binding energies (284.6 vs. 271 eV) that ensures the

relative surface sensitivity of these peak areas can be directly

compared.

Applied Research). This gas cracker works by

dissociating molecular gases such as chlorine or

oxygen into a stream of atomic, low-energy reac-

tive species to produce a mixture of Cl(2P) and Cl2,

or O(3P) and O2, hereafter referred to as atomic

Cl or atomic O respectively. A description of theradical source can be found in a previous publi-

cation [20]. The gas cracker was operated at 30 W

with a chlorine pressure of �5 · 10�8 Torr, and at

�7 · 10�7 Torr during oxygen exposure. Samples

were placed in line of sight with the effluent from

the gas cracker (target-to-sample distance �5 cm).

2.3. Preparation of self-assembled monolayers

Gold substrates were initially cleaned by 2 keV

Arþ ion sputtering and then dipped into a 5 mM

solution of hexadecane thiol (CH3(CH2)15SH) in

hexanes for 10–12 h to prepare an alkanethiolate

SAM (C16-SAM). Samples were then washed in

ethanol, water and hexanes and placed in the XPS

analysis chamber on a UHV carousel stage via afast entry load lock system. X-ray irradiation is

known to initiate electron-stimulated C–H, C–C

and S–Au bond breaking within alkanethiolate

SAMs [25–27] compromising the structural integ-

rity of the SAM and modifying the elemental

composition [25]. To minimize the effects of X-ray

irradiation in the present investigation, XPS

analysis was carried out on SAMs only after ex-posure to atomic chlorine and oxygen. Further-

more, individual SAMs were used for each atomic

chlorine/oxygen experiment and then discarded;

thus, the results shown in this investigation cor-

respond to single exposures of atomic chlorine/

oxygen to numerous C16-SAMs.

2.4. Atomic force microscopy (AFM) measure-

ments

Ex situ AFM experiments were carried out using

a Burleigh Metris 2000 NC instrument operating

in contact mode as described previously [28]. Sili-

con probes with a resonant frequency of 11–16

kHz (1:1 conical tip; 100 �AA) and a nominal stiff-

ness of 0.1–0.34 Nm�1 were used. A 6.62 · 5.83lm2 scan area was typically imaged. Consecu-

tive measurements on the same regions produced

C(1s)

Binding Energy (eV)282285288291

0 min

2 min

60 min

240 min

10 min

Cl(2s)


Cl/Cl2

ExposureTime

x 0.7

CC/CH2CHClCCl2 C(1s)

Satellites

C-Cl

Fig. 1. Variation in the C(1s) and Cl(2s) XPS regions as a function of chlorine atom exposure to polyethylene (PE) measured at a take

off angle of 45�. The C(1s) region was fit using a combination of CC/CH2 (284.6 eV), CHCl (286.1 eV) and CCl2 (287.7eV) species. The

Cl(2s) region was fitted with one peak associated with CClx species (x ¼ 1, 2) centered at �271 eV. In both the C(1s) and Cl(2s) regions,

raw data is shown as filled circles with fits as dashed lines, and backgrounds and envelopes as solid lines. Consecutive XP spectra on PE

samples exposed to the same chlorine atom exposure exhibited invariant C(1s) and Cl(2s) spectral envelopes, indicating that over the

timescale of XPS acquisition, X-ray exposure was not responsible for any measurable dechlorination.


identical AFM images, indicating the lack of tip-

induced damage during data acquisition. AFM

images of native PE shows that the surface is

composed of globular features (typical size

0.30 ± 0.06 lm) consistent with previous AFM

studies on native PE [29]. Following exposure tothe chlorine radical source the globular fea-

tures that characterized the native PE substrate

became less distinct and the chlorinated PE sur-

face developed a more fibrous appearance. Chlo-

rination of PE, however, did not proceed with any

significant change to the rms roughness of the

surface.

3. Results

The evolution of the C(1s) and Cl(2s) XPS re-

gions of PE as a function of exposure to the atomic

Cl source are shown in Fig. 1. Initially, PE exhibits

a single peak in the C(1s) region at 284.6 eV [17]corresponding to CC/CH2 species, accompanied

by the associated Mg Ka2 and Mg Ka3 satellite

peaks at 276.2 and 274.4 eV (shown in the Cl(2s)

region). Exposure of PE to atomic Cl resulted in a

broadening of the C(1s) spectral envelope towards

higher binding energies and the appearance of a

C–Cl peak in the Cl(2s) region at �271 eV asso-


ciated with CClx (x ¼ 1, 2) species [17]. Fig. 1

shows that the evolution of the C(1s) region during

exposure to atomic chlorine could be well fitted to

a combination of CC/CH2 (284.6 eV), CHCl (286.1

eV) and CCl2 (287.4 eV) species [17,19]. It should

also be noted that in separate experiments whenPE was exposed to Cl2 alone at similar pressures to

those used in Fig. 1, no changes in the C(1s) or

Cl(2s) XPS peaks were observed over a period of

several hours.

Cl/C

Rat

io

0.0

0.2

0.4

0.6

0.8

Cl/Cl2 Exposu0 50 100

Cl/C

Rat

io

0.0

0.2

0.4

0.6

0.8

1.0

0

20

40

60

80

100

Per

cen

tC

ompo

siti

on

Fig. 2. (a) Variation in the distribution of carbon species (CC/CH2 (�(take off angle of 45�) as a function of exposure to the atomic chlor

function of exposure to the atomic chlorine source measured as a fun

the Cl(2s) and C(1s) XPS areas. Data shown as black circles (�) wer

black triangles (.) at 10� take off angles of detection with respect

stochastic model. The solid and dashed lines correspond to the low-p

black circles (�) correspond to the Cl:C ratio calculated from the area

Details can be found in Section 4.

Fig. 2(a) shows the variation in the distribution

of CC/CH2, CCl and CCl2 species derived from

analysis of the C(1s) line shape (measured at a take

off angle of 45�) as a function of exposure to the

atomic chlorine source. Fig. 2(a) shows that ex-

posure of PE to the chlorine radical source is ac-companied by a decrease in the concentration of

CC/CH2 species over �70 min and the appear-

ance of mono and dichlorinated carbon species.

The rate of chlorine uptake into PE, however,

re Time ( min)150 200 250

(a)

(b)

(c)

), CCl (.) and CCl2 (�)) determined from the C(1s) XP spectra

ine source; (b) variation in the Cl/C ratio of a PE sample as a

ction of XPS take off angle. The Cl/C ratio was calculated from

e acquired at 80� (grazing angles), white circles (�) at 45� andto the sample normal and (c) Cl:C ratios calculated from the

ressure and high-pressure regimes, respectively. Data shown in

of the fitted peaks in the C(1s) XPS region presented in Fig. 2(a).


decreases rapidly during exposure to the chlorine

atom source. Indeed, analysis of Fig. 1 indicates

that for exposure times greater than 70 min, the

composition of the film as measured by XPS re-

mained constant with 62.5% CC/CH2, 29.4%

CHCl and 8.1% CCl2 species. Fig. 2(b) shows theuptake of chlorine into the PE surface as a func-

tion of atomic Cl exposure measured by the vari-

ation in the Cl(2s)/C(1s) XPS ratios, recorded at

three different take off angles (80�, 45� and 10�)measured with respect to the sample normal. A

comparison of the angle resolved XPS data shows

that although the Cl(2s)/C(1s) ratio exhibits a

similar dependence on atomic Cl exposure for thedifferent take off angles, the Cl(2s)/C(1s) ratio was

always largest for data acquired at an 80� take off

angle and smallest for data acquired at 10�. Fig.2(c) shows a comparison of the calculated Cl(2s)/

C(1s) ratios in the case of low (solid line) and high

(dotted line) chlorine atom fluxes based on the

stochastic model developed in Section 4. Experi-

mental data on the variation in the Cl(2s)/C(1s)

10º

45º

80º


0.28

0.38

0.46

Cl/C

CC/CH2CHCl

CCl2 Take OAngle

(a)

Fig. 3. Angle dependent measurements of the PE C(1s) XPS region

chlorine at 5 · 10�8 Torr and (b) atomic oxygen at 7 · 10�7 Torr. The

backgrounds and envelopes are shown as solid lines.

ratio as a function of exposure time is also shown

(solid circles).

Fig. 3 shows the C(1s) XPS region of PE sur-

faces following a saturation exposure to (a) atomic

Cl and (b) atomic O, recorded as a function of take

off angle. Fig. 3(a) was obtained after exposing PEto atomic Cl for 60 min at a pressure of 5 · 10�8

Torr, and was fitted using the same parameters

and peaks observed in Fig. 1. Fig. 3(b) was ob-

tained after exposing PE to atomic oxygen for 450

min at 7 · 10�7 Torr, and was fitted to include CC/

CH2 (284.6 eV), C–O (286.1 eV), C@O (287.5 eV)

and O–C@O (288.9 eV) species [20]. Fig. 3 shows

that the C(1s) region is sensitive to the take offangle after exposure to either atomic Cl or atomic

O with the concentration of Cl or O-containing

species greatest at a detection angle of 80�. Indeed,for PE exposed to atomic chlorine the concentra-

tion of CHCl species is approximately equal to the

CC/CH2 species at a detection angle of 80� (Fig.

3(a)). The idea that the concentration of oxygen

and chlorine containing species increases as a


0.39

0.49

0.59

O/C

CC/CH2O-C=OC=O

C-Off(b)

for separate samples after saturation exposures to: (a) atomic

raw XPS data is shown as filled circles with fits as dashed lines;


function of increasing XPS take off angle is also

supported by the measured variation in the Cl(2s)/

C(1s) and O(1s)/C(1s) XPS ratios, shown in Fig.

3(a) and (b) respectively.

Fig. 4 shows the effects of atomic Cl (Fig. 4(a))

and atomic O (Fig. 4(b)) exposure on the chemicalcomposition of hexadecanethiolate SAMs (C16-

SAM) adsorbed on gold. Exposures to atomic Cl

were performed at a pressure of 2 · 10�7 Torr,

while exposures to atomic O were performed at a

pressure of 7 · 10�7 Torr. Fig. 4(a) illustrates that

during the interaction of atomic Cl with C16-

SAMs, the C/Au ratio remains relatively constant

(left-hand axis), while the Cl(2p) peak area (right-hand axis) increases rapidly before reaching a

constant level. In contrast, Fig. 4(b) illustrates that

exposure of C16-SAM to atomic O results in a

steady decrease in the C/Au XPS ratio, while the

(a)

Cl/Cl2 Exposure Time (min)200180160140120100806040200

C/A

u ra

tio

CC

lx -Cl(2p) X

PS A

rea

(b)

O/O2 Exposure Time (min)0 100 200 300 400 500 600

C/A

u ra

tio

C-O

O(1s) X

PS A

rea

Fig. 4. Variation in the XPS area of the: (a) CClx (x ¼ 1 or 2)

(�) peak area in the Cl(2p) region and the C(1s)/Au(4f) ratio

(�) of an alkanethiolate C16-SAM during exposure to atomic

chlorine (Cl(2P)) and (b) C–O (�) peak in the O(1s) region and

the C(1s)/Au(4f) ratio (�) of an alkanethiolate C16-SAM during

exposure to atomic oxygen(O(3P)).

C–O O(1s) XPS peak area initially increases before

decreasing upon prolonged exposures.

4. Discussion

The present investigation details the surface

reactions of atomic chlorine with PE under vac-

uum conditions. In addition, the use of an alkan-

ethiolate SAM enables us to compare and contrast

the reactivity of atomic chlorine and atomic oxy-

gen with hydrocarbon-based substrates.

Figs. 1 and 2 illustrate that the reaction of

chlorine radicals with PE (CH2–CH2)n under vac-uum conditions produces CCl and CCl2 species.

The lack of reactivity of PE towards Cl2 gas alone

under these low-pressure conditions on the time-

scale of the present experiment (several hours)

demonstrates that chlorine radicals are required to

initiate the reaction. In PE, reaction is initiated

solely by Cl-mediated hydrogen abstraction from

C–H bonds, generating volatile HCl species [2,30];

–CH2–þ Cl� ! –C�

H–þHCl " ð1Þ

Reactions involving the alkyl radical are then re-

sponsible for generating CCl groups, thus

–C�

H–þ Cl�=Cl2 ! –CHCl–þ Cl� ð2Þ

Subsequent reactions involving C–Cl containing

species lead to the formation of CCl2 species

–CHCl–þ Cl� ! –C�

Cl–þHCl " ð3Þ

–C�

Cl–þ Cl�=Cl2 ! –CCl2–þ Cl� ð4ÞDespite the sequential nature of chlorine addi-

tion, Figs. 1 and 2 reveal that even after a satu-

ration exposure of atomic chlorine, most of the

carbon atoms within the XPS analysis region re-

main free of C–Cl linkages. This lack of com-plete chlorination has been observed in previous

photochlorination studies of PE and has been as-

cribed to steric effects due to the presence of bulky

CCl and CCl2 groups that reduce the efficiency of

subsequent chlorination steps [17]. Thus, a semi-

quantitative non-linear least squares fit to the data

shown in Fig. 2 using the approach outlined by

Elman et al. [17], yields a pseudo first-order rate


constant for hydrogen abstraction from methylene

groups (reaction (1)) of 1.17 · 10�3 s�1. In contrast,

the pseudo first-order rate constant for the loss of

monochlorinated species (–CHCl–) as a result of

hydrogen abstraction (reaction (3)) has an upper

limit of 6 · 10�6 s�1.The most significant difference between the ex-

periments reported in the present study and

photochlorination experiments carried out at at-

mospheric pressures is the lower incident chlorine

atom flux, illustrated by the reduced rate of CH2

group loss during chlorination. Thus, under our

experimental conditions, the pseudo first-order

rate constant for the loss of methylene groups(1.17 · 10�3 s�1) is an order of magnitude less than

the reported value by Elman et al. (1.5 · 10�2 s�1)

[17]. In these higher-pressure chlorination studies

the maximum chlorine uptake was determined by

XPS to be CC/CH2:CCl:CCl2�1:1:0.5 [15] and the

C(1s) XPS profile was independent of the take off

angle. In contrast, Figs. 1–3(a) reveal that the

maximum chlorine uptake is reduced under ourlower-pressure conditions, along with an increased

surface selectivity, evidenced by the sensitivity of

the C(1s) spectral profiles to the XPS take off an-

gle. The surface selectivity of the chlorination

process under our experimental conditions was

further evidenced by the fact that ATR measure-

ments failed to detect any IR intensity between 600

and 800 cm�1 associated with C–Cl stretchingmodes.

The greater surface selectivity and lower maxi-

mum chlorine uptake observed for PE chlorination

in the present investigation is in fact in agreement

with the previously observed effect of chlorine

pressure (50 vs. 760 Torr) on the photochlorina-

tion of PE [15]. Thus, the maximum chlorine up-

take determined by XPS (sensitive to the chlorinecontent on the nm length scale) was found to be

similar at both 50 and 760 Torr of Cl2, although

the maximum extent of chlorination measured by

ATR (sensitive to chlorination on the lm length

scale) and from gravimetric measurements was

significantly enhanced at 760 Torr [15]. Despite the

different means used to generate atomic chlorine in

the present study (thermal rather than photo-chemical), it therefore appears that the incident

chlorine radical flux is a significant factor in de-

termining both the maximum chlorine content and

surface selectivity of the reaction.

If steric effects alone were responsible for the

observed absence of complete chlorination in the

near surface region, the extent of chlorination

should be independent of the incident chlorineradical flux. Consequently, the influence of radical

flux on the chlorination process suggests that other

factors are operative. One possible explanation

that accounts for the influence of radical flux on

the chlorination process is the role of cross-linking

reactions (e.g. C�

Hþ C�

H ! CH–CH), since they

serve to limit the concentration of otherwise

available alkyl radicals for subsequent chlorina-tion. Evidence for the role of cross-linking reac-

tions in radical interactions with hydrocarbon

films can be found in a study by Kluth et al. [31] on

the reactions of H atoms with alkanethiolate

SAMs, where the observed decrease in contact

angle with increasing hydrogen atom exposure was

interpreted as indirect evidence for the presence of

alkyl radical cross-linking reactions. To test theidea that cross-linking reactions can play a role in

the chlorination process we developed a simple

stochastic model of PE chlorination assuming a

constant radical flux and a chlorination process

based on reactions (1)–(4). Steric effects were

simulated by assuming that hydrogen abstraction

from CHCl and chlorine addition to C�

Cl only

occurs when these species are adjacent to –CH2–groups. In addition, the production of CCl2 species

is assumed to sterically hinder reactions of adja-

cent CH2 groups as suggested previously [17].

Three types of cross-linking reactions were in-

cluded, specifically:

C .

H+ C

H

. C

H

C

Hð5Þ

C .

H

C

Cl

.+ C

H

C

Clð6Þ

C .

Cl

C

Cl

.+ C

Cl

C

Clð7Þ

Table 1

Rate constants used in the kinetic stochastic simulation

Reaction Rate constant

(1) 6.3 · 10�4 s�1

(2) 8.3 · 10�3 s�1

(3) 8.3 · 10�9 s�1

(4) 5 · 10�3 s�1

(5) 4.2 · 10�6 cm2 mol�1 s�1

(6) 4.2 · 10�6 cm2 mol�1 s�1

(7) 4.2 · 10�6 cm2 mol�1 s�1


The rate constants employed in this stochastic

model are shown in Table 1. The agreement be-

tween the Cl(2s)/C(1s) ratios measured experi-

mentally at a take off angle of 45� and those

calculated from this model were used as a guide tothe overall accuracy of the model. In addition, the

rate of hydrogen abstraction from methylene

groups (reaction (1)) was constrained to lie within a

factor of two of the value measured experimentally

from the loss of CC/CH2 groups during chlorina-

tion (Fig. 2(a)) [15]. Furthermore, chlorine addi-

tion reactions (steps (2) and (4)) were assumed to

have rate constants at least one order of magnitudehigher than the H abstraction reactions (steps (1)

and (3)). Reactions (1)–(4) that involve chlorine

atoms were treated as pseudo-first-order processes

due to the constant chlorine radical flux during

experiments, while reactions (5)–(7) were treated as

bimolecular second order processes. A comparison

of the Cl(2s)/C(1s) ratios measured experimentally

and those derived from the kinetic model outlinedin steps (1)–(7) is shown in Fig. 2(c).

Fig. 2(c) illustrates the qualitative agreement

between the Cl(2s)/C(1s) ratios measured experi-

mentally and those determined from the kinetic

model as a function of chlorine atom exposure

as well as the maximum extent of chlorination.

In addition, the limiting distribution of unreacted

carbon atoms (CH2/CC) as well as the mono(–CHCl–) and dichlorinated (–CCl2–) species can

be reproduced from this simulation. Thus, employ-

ing the rate constants listed in Table 1, the limiting

composition of the chlorinated overlayer was cal-

culated to be 65.2% CH2, 24.4% CHCl and 10.4%

CCl2, which compares favorably to the 62.5% CC/

CH2, 29.4% CHCl and 8.1% CCl2 obtained ex-

perimentally.

To simulate the effect of increasing chlorine

radical flux, the rate constants for those reactions

involving chlorine atoms were increased by a fac-

tor of 10 (reactions (1)–(4)). This value was cho-

sen to reflect the relative increase in the rate of

methylene group loss measured in higher-pressurephotochlorination experiments [15,17]. Under these

conditions of higher (fixed) incident chlorine rad-

ical flux, a more rapid rate and higher saturation

level of chlorination was observed (Fig. 2(c)),

consistent with previous photochlorination exper-

iments on PE [17]. In addition, the limiting com-

position of the chlorinated overlayer calculated

from our stochastic model changed to 46.6% CC/CH2, 37.7% CHCl and 15.7% CCl2 reflecting an

increase in the extent of chlorination as cross-

linking processes become less significant. This

change in the distribution of CC/CH2, CHCl and

CCl2 species is qualitatively similar to the limiting

values of 40% CC/CH2, 40% CHCl and 20% CCl2obtained by Elman et al. [17] in their higher-pres-

sure study. It should be noted that the chemicalcomposition of the film for both low- and high-

chlorine atom fluxes are derived from XPS mea-

surements of the C(1s) region on high-density PE

recorded at comparable take off angles.

The dependence of the total Cl uptake on the

incident chlorine radical flux results from the fact

that the –C�

H– group never reaches a steady-state

concentration. Under these conditions, the productpartitioning between –C

�

H– group chlorination

(reaction (2)) and bimolecular cross-linking pro-

cesses (reactions (5)–(7)) is sensitive to the incident

chlorine radical flux, effectively limiting the maxi-

mum chlorine content at lower incident fluxes. This

fact is illustrated in Fig. 2(c), where the Cl/C ratio

calculated from the model is shown in the low-

pressure limit (solid line) and the high-pressure limit(dashed line). Despite the inherent simplicity of our

model, the results clearly point to the role that

cross-linking reactions can play in limiting the ex-

tent of chlorination at lower incident radical fluxes.

During chlorination the effective flux of chlo-

rine atoms will in fact decrease as a function of

increasing distance below the PE surface. Thus,

the constant flux of chlorine radicals assumed inthe previous section should be regarded as an av-

erage flux within the PE substrate. As a result of


this depth dependence on the chlorine flux, the

extent of chlorination is also expected to decrease

below the surface as the significance of cross-

linking reactions increases, producing a concen-

tration gradient of CClx (x ¼ 1, 2) species. For the

low chlorine fluxes that characterize the presentstudy, this concentration gradient of CClx (x ¼ 1,

2) species is sufficiently localized in the near sur-

face region that it can be detected by angle re-

solved XPS measurements (Figs. 2 and 3). In

contrast, the C(1s) XPS profile was found to be

independent of the take off angle for higher-pres-

sure (flux) photochlorination experiments on PE.

The greater surface selectively of PE chlorinationunder our conditions of lower-chlorine radical flux

is also consistent with the lack of observable ATR

intensity associated with C–Cl incorporation

into the film, in contrast to ATR results obtained

under higher-pressure photochlorination condi-

tions [15]. It should be noted, however, that the

lower incident kinetic energy anticipated for chlo-

rine radicals produced from thermal rather thanphotochemical dissociation of Cl2 may also con-

tribute to the enhanced surface selectivity and

lower maximum chlorine uptake observed in the

present investigation.

In general, results from the present investiga-

tion suggest that free radical reactions with poly-

mers under vacuum conditions, where the flux of

reactive species is low, may offer a general route tomodify polymer surfaces with an extremely high

degree of surface selectivity. In polymer surface

modification processes, the ability to control sur-

face selectivity is important in the development

of optimal treatment strategies for a number of

properties, including gas permeability. Similarly,

the reaction of chlorine atoms with hydrocarbon

films under these low-pressure conditions may of-fer a means to selectively chlorinate the very top-

most layers of ordered molecular assemblies such

as SAMs as a convenient way to produce func-

tionalized monolayers.

4.1. A comparison of chlorine and oxygen radical

interactions with hydrocarbon thin films

Fig. 3 shows that the interaction of atomic

chlorine and atomic oxygen with PE substrates

both lead to a concentration gradient of chlorine

and oxygen containing species within the near

surface region. Despite these similarities, experi-

ments carried out on SAMs indicate that distinctly

different reaction pathways characterize the inter-

action of atomic chlorine and atomic oxygen withhydrocarbon surfaces.

In this investigation, SAMs are employed as

models for polymeric interfaces [20], where we

have exploited the fact that changes in the film�sthickness can be conveniently followed by moni-

toring the C(1s)/Au(4f) ratio, an advantage not

possible in experiments on bulk polymeric sub-

strates. Furthermore, the packing density of al-kanethiolate SAMs is comparable to PE [32]. In

the present study, Fig. 4(a) indicates that the re-

action with Cl radicals proceeds in the absence

of C–C bond cleavage. Chlorine radical-induced

carbon-carbon bond cleavage would produce a

decrease in the C(1s)/Au(4f) ratio due to the pro-

duction of volatile carbon-containing fragments

that were no longer tethered to the underlyingsubstrate [26]. This behavior is inconsistent with

the essentially constant C(1s)/Au(4f) ratio ob-

served during the interaction of atomic chlorine

with the SAM (Fig. 4(a)). The absence of –CCl3group production in this and other related studies

[15,17] is also consistent with the idea that chlorine

radicals can activate C–H bond cleavage via hy-

drogen atom abstraction but are unable to cleaveC–C bonds. The inability of atomic chlorine to

induce efficient C–C bond cleavage is also similar

to the results obtained on the interaction of fluo-

rine atoms with alkanethiolate SAMs [33] and in-

dicates that during chlorine and fluorine plasma

treatments, the removal of hydrocarbon contami-

nation is a result of ion bombardment.

In contrast to the reaction of atomic Cl withhydrocarbon films, Fig. 4(b) shows that the C(1s)/

Au(4f) ratio decreases monotonically during the

interaction of C16-SAMs with atomic oxygen, in-

dicating the presence of substrate erosion. This is

also reflected by the variation in the C–O(1s) XPS

area during exposure to atomic oxygen. Initially

the C–O(1s) signal increases as oxygen-containing

functional groups are formed at the hydrocarbonsurface, while upon prolonged exposure to atomic

oxygen the C–O(1s) signal decreases as volatile


carbon containing oxygen species (e.g. CO2) de-

sorb from the film [20]. The ability of atomic ox-

ygen to etch the hydrocarbon surface is related to

its ability to form linkages such as R–O–(C@O)–R

that can then react further via decarboxylation to

produce volatile carbon containing species, re-sulting in the formation of a steady-state etch

front. In contrast, the reactions of atomic chlorine

with hydrocarbon films are limited to those illus-

trated by reactions (1)–(4). Thus, the thin layer of

carbon-containing oxygen functionality that de-

velops during the interaction of atomic oxygen

with hydrocarbon films is at least in part a con-

sequence of the continuous etching of the film. Incontrast, the surface selectivity during chlorination

is postulated to derive from a combination of

steric and kinetic factors associated with the

chlorination process.

Based upon the reactivity of atomic chlorine

and oxygen with the prototypical C16-SAMs, a

model of radical interaction with PE can be de-

veloped; this is shown schematically in Fig. 5. Inthe case of atomic chlorine reactions with PE, a

(a)

(b)

Fig. 5. Schematic depiction of PE surface reactions during interaction

The evolution of the PE surface is depicted following specified radical

been depicted in an ordered array for ease of visualization.

partially chlorinated overlayer is produced con-

taining both CCl and CCl2 groups in the absence

of substrate erosion, with the extent of chlorina-

tion greatest at the vacuum/film interface (Fig.

5(a)). The reactions of atomic oxygen (O3P) with

hydrocarbon films in contrast, results in oxygenincorporation into the surface, followed by sub-

strate erosion with the ejection of volatile carbon

containing oxygen species (e.g. CO2) (Fig. 5(b)).

Thus, while the uptake of chlorine atoms from the

gas phase only occurs up until a point when the Cl/

C ratio reaches its limiting value, oxygen atoms

react continuously with the hydrocarbon film as

erosion proceeds.

5. Conclusions

Exposure of PE to atomic chlorine (Cl(2P))

under vacuum conditions leads to the production

of CCl and CCl2 groups. Compared to higher-

pressure studies on PE photochlorination, themaximum extent of chlorination is reduced while

with: (a) atomic chlorine (Cl(2P)) and (b) atomic oxygen (O(3P)).

exposures. It should be noted that the hydrocarbon chains have


the surface selectivity is enhanced. These results

have been rationalized on the basis of a simple

stochastic model of the PE chlorination process

that incorporates steric effects associated with the

production of mono and dichlorinated carbon at-

oms as well as cross-linking reactions betweencarbon-containing radicals. The surface selectivity

of the chlorination process is manifested by angle

resolved XPS measurements that illustrate the in-

homogeneous distribution of CCl and CCl2 groups

within the near surface region. A similar concen-

tration gradient is observed during the reaction of

atomic oxygen (O(3P)) with PE. Differences in the

reactivity of atomic chlorine and oxygen with hy-drocarbon films are manifested, however, in ex-

periments carried out on SAMs, used as models

for polymeric interfaces. Results from these studies

reveal that chlorination proceeds without sub-

strate erosion while the incorporation of new oxy-

gen functionality during reactions with atomic

oxygen competes with carbon loss from the film.

Acknowledgements

Support for this research was provided by a

National Science Foundation CAREER award (#

9985372) and a grant from the Petroleum Research

Fund (PRF # 35281–G5, G6) administered

through the American Chemical Society.

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Interaction of chlorine radicals with polyethylene and hydrocarbon thin films under vacuum conditions––a comparison with atomic oxygen reactivity