a simple method to fabricate a conductive polymer micropattern on an organic polymer substrate

A Simple Method to Fabricate a Conductive Polymer

Micropattern on an Organic Polymer Substrate

Peng Yang, Jingyi Xie, Wantai Yang*

Department of Polymer Science, Beijing University of Chemical Technology, Beijing 100029, P. R. China;Key Laboratory of Science and Technology of Controllable Chemical Reactions, Ministry of Education, Beijing 100029,P. R. ChinaFax: þ86 010 64416338; E-mail: [email protected]

Received: November 5, 2005; Accepted: December 21, 2005; DOI: 10.1002/marc.200500759

Keywords: conducting polymers; hydrophilicity; hydrophobicity; micropatterns; surfaces

Introduction

In past years, conjugated organic polymers such as poly-

aniline (PANI) and polypyrrole have been considered as

potential alternatives to metals and semiconductors. The

patterning of conductive polymers is often needed in the

design of circuits, elements, or display panels, and has been

achieved by using a variety of techniques, such as the dip

coating of a conductive PANI gel,[1] microcontact printing

through self-assembled monolayers (SAM),[2–6] electro-

polymerization,[5,6] enzyme-catalyzed polymerization,[7]

printing,[8–10] lithography,[11–14] microfluidic molding

(MMIC),[14–16] controlled electrophoretic patterning,[17]

and so on. Most of this research has concentrated on the

patterning of inorganic or metal surfaces, because of the

good surface chemical reactivity and conductivity. How-

ever, at present, the development of an approach to cons-

truct such a pattern on an organic polymer substrate is more

attractive because of the demand of flexible, disposable, or

portable devices. The bottlenecks for it are the chemical

inertness and insulating property of most polymer surfaces,

which render most of the current methods unusable. Garnier

et al. fabricated an all-plastics field-effect transistor by

directly printing a conductive graphite-based polymer ink

on a poly(ethylene terephthalate) (PET) surface.[9] Alter-

natively, Hohnholz and MacDiarmid found that the

selective deposition of PANI could be achieved on a com-

mon ink region, which was directly printed on a PET

substrate using an office printer.[10] Furthermore, White-

sides, MacDiarmid and co-workers found that on a

Summary: Under UV irradiation plus a photomask, a hy-drophilic/hydrophobic hybrid polymer surface is created bysandwiching an ammonium persulfate solution between twopolymer films. It is demonstrated that an effective conductivePANI micropattern can be fabricated on such a wettabilitypatterned surface. For PET, a stable negative micropatterncould be formed directly by the selective deposition of PANIonto the hydrophobic region. Alternatively, for PP or PI,direct deposition of PANI is non-selective, however, thePANI layer remains preferentially on the hydrophilic regionby peeling off the PANI layer on the hydrophobic region toform a positive micropattern.

Schematic illustration of the procedure used for the formationof a conducting polymer (PANI) micropattern on a wet-tability patterned polymer substrate.

Macromol. Rapid Commun. 2006, 27, 418–423 � 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

418 DOI: 10.1002/marc.200500759 Communication

hydrophilic glass surface, hydrophobic PANI could be

selectively deposited on a patterned hydrophobic SAM

region, as a result of the ‘‘like dissolves like’’ principle.[4]

However, although SAMs could also be extended onto PET

through an aminolysis reaction,[18] the same strategy is

difficult to extend to other polymer substrates without ester

functionalities.

Here, an attempt is made to find a simple and general

method to create such a wettability pattern on various

polymer surfaces, by using confined photocatalytic oxida-

tion (CPO), a method developed in our lab.[19] During this

reaction, sulfate anion groups (SO4�) are implanted into

various polymer substrates including polyolefins, poly-

esters, Nylon, and rubber. High hydrophilicity can be

obtained under UV irradiation in a few seconds. Because of

the hydrophobicity (>1008) of most of the polymer surfaces

used in present work, a simple photomask is used to pattern

the irradiated region, that is, the irradiated part becomes

hydrophilic and the unmodified original surface (unirra-

diated part) remains hydrophobic. Such a surface can be

used to fabricate an effective PANI micropattern on three

polymer substrates including biaxial oriented PET, poly-

(propylene) (BOPP), and polyimide (PI).

Experimental Part

The Procedures of CPO

Briefly (see details elsewhere[19]), a thin layer of an ammoniumpersulfate (APS) aqueous solution (30 wt.-%) was sandwichedbetween two polymer films (BOPP as top film), and certain UVirradiation (high pressure mercury lamp: 1 000 W, UV inten-sity: 8 000 mm � cm�2, irradiation time: 120 s) was conducted.For a wettability patterned surface, a photomask (shown inFigure2a, below)wasalsoused. The resultingwettability patternwas viewed optically by the condensation of saturated watervapor on the substrate below the dew point. The sample wasfurther used for the following PANI deposition without delay.

Polymerization and Deposition of PANI

The procedures followed strictly referred to the reports ofWhitesides, MacDiarmid and co-workers.[4] In brief, thesample was floated on an aqueous solution containing aniline(4 mL) and 1 M HCl (200 mL), with the modified surfacedownwards. Another solution containing APS (2.3 g) and 1 M

HCl (100 mL) was added to initiate polymerization. The filmswere taken out after different time and placed in an aqueoussolution of aniline (8 mL) in 1 M HCl (200 mL) for 30 min toform the emeraldine (EM) salt of PANI. The films were thenimmersed in 1 M HCl for 1 min and dried under nitrogen. TheEM salt was converted into the EM base by soaking the as-deposited film in double distilled water for 10 h.

The Formation of Conductive Polymer Patterns

The conductive polymer pattern was formed directly on thePET surface after taking it out from the solution and washing it

(strategy I). To form the pattern on the deposited BOPP or PIfilms, a piece of 3M Scotch1 adhesive tape, Scotch1 CrystalClear Tape (Cinta Cristal, CC1920-Bx), was placed on thedeposited film, pressed gently to achieve a homogeneous con-tact between the tape and the film, and then peeled off quickly(strategy II).

Characterization

Contact angles were measured by placing a 0.5 mL drop ofdistilled water on a film from OCA20 (Dataphysics Co.,Germany). The UV/vis absorption spectrum was recordedusing a GBC Cintra 20 spectrophotometer (Australia). Micro-scopic observation was carried out on a Nikon TE2000-ssystem (Tokyo, Japan). Scanning electron microscopy (SEM)was performed with a S250HK3 (Cambridge, UK) instrument.The conductivity of the PANI layer deposited on the PI surfacewas determined by the two-probe method under ambientconditions.

Results and Discussion

The synthetic strategies used are depicted in Figure 1. First,

an APS aqueous solution is sandwiched between two

polymer films (Figure 1a), and a photomask (Figure 2a) is

placed on the top film to control the reaction region. After

the reaction, the irradiated region changes to be highly

hydrophilic through the introduction of sulfate anion groups

(Figure 1b). The formed wettability pattern can be viewed

optically (Figure 2b), where a water strip is formed on the

hydrophilic region. Based on such a wettability patterned

surface, two approaches are found to be suitable to fabricate

the conductive polymer patterns onto organic polymer sub-

strates. For PET, a stable negative pattern can be formed

directly through the selective deposition of PANI onto the

hydrophobic regions (static water contact angle, �758)interspaced by the high hydrophilic regions (�158, irradi-

ated part) (strategy I, Figure 1c).[4] On the other hand, for

BOPP or PI, the first-step deposition of PANI does not have

obvious selectivity between the medium hydrophilic

(�508) and hydrophobic (�1008) regions (Figure 1d).

Alternatively, a positive PANI pattern can be obtained by

peeling off the adhesive tape on the deposited film to

remove the PANI layer on the hydrophobic region, while the

PANI layer on the hydrophilic region remains (strategy II,

Figure 1e).

The deposition difference of PANI between the hydro-

philic and hydrophobic surfaces is evaluated by monitoring

the UV/vis spectra evolution of PANI deposition (Figure 3).

PANI, in its EM salt form, is deposited on the treated

(hydrophilic) or original (hydrophobic) surfaces, which is

reflected by the two characteristic peaks near 400 and

800 nm, respectively (Figure 3a, solid curve).[20–23] The

EM salt could be converted into the EM base, which is

characterized by a shift of the peaks to near 350 and 650 nm

(Figure 3a, dot curve).[20–23] The intensity change of the

A Simple Method to Fabricate a Conductive Polymer Micropattern on an Organic Polymer Substrate 419

Macromol. Rapid Commun. 2006, 27, 418–423 www.mrc-journal.de � 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

characteristic peaks is proportional to the thickness of

the PANI layer,[24] which can be used to investigate the

deposition kinetics. For PET (Figure 3b), before 15 min, the

peak intensity at 400 nm (A400) increases with the depo-

sition time, whether on the treated hydrophilic (�158) or

original hydrophobic (�758) surfaces. The original hydro-

phobic surface has a higher affinity to PANI than the

hydrophilic surface, and shows a faster deposition speed.

For example, at 5 min, the intensity on the hydrophobic

surface is greater than that on the hydrophilic surface by a

factor of 3. The selectivity decreases as the polymerization

proceeds, and disappears after 15 min. From then on, the

depositions on the two kinds of surfaces are equivalent and

stable without obvious intensity change. This trend is

similar to that obtained on the hydrophobic/hydrophilic

hybrid SAM-modified surface,[4,5] and can be described as

follows: initially, the hydrophobic polymerization product,

PANI, is deposited preferentially on the unirradiated

regions and finally, further polymerization results in the

deposition on both unirradiated (hydrophobic) and irradi-

ated (hydrophilic) regions of the surface.[5] In contrast, for

BOPP (Figure 3c), the initial selectivity is not obvious:

before 15 min, the intensity increases on both the original

hydrophobic (�1008) and treated hydrophilic (�508) surfa-

ces with a similar speed, and after 15 min, the value is stable

without obvious change. The final intensity is nearly the

same on both the hydrophobic and hydrophilic surfaces.

This kinetics reflect that, from the beginning, PANI simul-

taneously deposits on both the hydrophobic and hydrophilic

UV irradiation

hydrophobic region

Polymer

PolymerAPS solution(a)

(b)

sulfate anion group (hydrophilic region)

strategy I strategy II

selective

deposition

nonselective

depostion

adhesive tape peeling

(c) (d)

(e)positive pattern

negative pattern

conducting polymer

Figure 1. Schematic illustration of the procedure used for theformation of a conducting polymer (PANI) micropattern on awettability patterned polymer substrate. a) A thin layer ofpersulfate ammonium (APS) aqueous solution is sandwichedbetween two polymer films, and selected UV irradiation isconducted by using a photomask. b) CPO takes place in theirradiated region, and the resulting wettability patterned surface isutilized to fabricate the PANI micropattern through two strategies.c). Strategy I: direct selective deposition of PANI on the patternedPET surface, where PANI is selectively adsorbed onto thehydrophobic region (negative pattern). d,e). Strategy II: PANI isdeposited onto the patterned BOPP or PI surface without obviousselectivity (d), and the PANI micropattern could be fabricated bypeeling off the adhesive tape, where PANI on the hydrophobicregion is removed, while PANI on hydrophilic region remains(positive pattern).

Figure 2. The images of a PANI pattern fabricated on PET andBOPP substrates by two strategies. a) The photomask used, wherewhite strips (I) show channels (areas to be UV-irradiated). b) Thecondensation pattern of water vapor on the wettability patternedsurface. c) The PANI pattern on a PET substrate through strategy I(direct deposition for 5 min). d) After the direct deposition for60 min, PANI is non-selectively deposited onto a BOPP substratewith no pattern formation. e) The PANI pattern on the BOPPsubstrate is obtained by peeling off the adhesive on the depositedfilm d (strategy II).

420 P. Yang, J. Xie, W. Yang


regions and this process lasts to the end of the deposition. It

seems that a similar deposition trend to BOPP should exist

on PI, because of both surfaces possessing nearly the same

wettability. However, it is found that the deposition on PI is

slightly different from that on BOPP, and shows a weak

selectivity between the original hydrophobic and treated

hydrophilic surfaces. For example, at 5 min, the peak

intensity change near 800 nm (A800) for the hydrophilic

surface is greater than that on the hydrophobic surface by a

factor of 1.8 (Figure 2d). After 15 min, the deposition is

stable and the final deposition amount on the hydrophilic

surface is slightly higher than that on the hydrophobic

surface.

Based on the above results, strategy I and II are devised in

order to fabricate an effective PANI pattern on the organic

substrates. For PET, the deposition selectivity between the

original and treated surfaces is utilized to obtain the PANI

pattern directly. The photomask shown in Figure 2a is used

to form a treated/original hybrid surface, and the deposition

time is selected as 5 min. Under these conditions, a PANI

strip micropattern can be obtained repetitiously with an

average width of 81.9 mm (Figure 2c), which exceeds the

designed feature (70 mm) of the shielding region (II in

Figure 2a) in the photomask by 17%. The thickness of the

PANI strip is about 50 nm. A longer deposition time can not

be used to obtain a high quality PANI pattern with increased

thickness, because of a quick and large decrease of the

deposition selectivity after 5 min (Figure 3b). In contrast,

for BOPP, the PANI pattern could not be formed directly on

a treated/original hybrid surface because of a lack of depo-

sition selectivity. For instance, the as-deposited film at

60 min showed a homogeneous deposition without the

formation of a pattern (Figure 2d). Alternatively, using

strategy II, the positive pattern can be obtained through the

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0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

(a)

Abs

Wavelength (nm)

EM salt EM base

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0.2

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on hydrophobic surface on hydrophilic surface

A40

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Deposition time (min)

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0.6 on hydrophobic surface on hydrophilic surface

A80

0

Deposition time (min)(d)

Figure 3. UV/vis spectroscopic change of PANI deposited on the polymer substrate. a) UV/vis spectra change from EM salt (solid curve)to EM base form (dot curve). b) The peak intensity change at 400 nm with the proceeding of polymerization on the PET substrate. c) Thepeak intensity change at 400 nm with the proceeding of polymerization on the BOPP substrate. d) The peak intensity change at 800 nm withthe proceeding of polymerization on the PI substrate.



adhesive peeling on the as-deposited film (Figure 2e). This

adhesive peeling method is fit for the as-deposited film

obtained at any time, and the stable deposition amount

(about 75 nm in thickness) can be achieved after 15 min.

The resulting PANI strip is 52.6mm wide, which approaches

the designed feature (50 mm) of the channels (I in Figure 2a)

in the photomask. For PI, although a very faint PANI pattern

can be formed if the deposition time is less than 15 min, the

clear PANI pattern is only formed on the PI surface after

the similar adhesive peeling on BOPP (Figure 4a),a and the

resulting PANI strip is 51.3 mm wide, which approaches the

design feature (50 mm). In addition to the positive patterns

on BOPP (Figure 4c) and PI, the complementary negative

patterns can be formed on the adhesive tape (Figure 4b,d).

The deposition difference of PANI on the three substrates

should be attributed to the wettability effect on these sur-

faces. The higher hydrophilicity (�158) made the irradiated

PET surface resistant to the PANI deposition more

effectively than BOPP or PI with a lower hydrophilicity

(�508), accordingly, the deposition only has an effective

selectivity on PET during the initial stage of the deposition.

In fact, when a hydrophilic SAM or a hydroxy-terminated

glass surface is used to effectively shield the PANI depo-

sition, the corresponding static water contact angle (CA) is

just in the range of �3–108, which approaches our value

(�158). It is well documented that the adhesion strength of

PANI is mainly determined by the surface free energy. A

high-energy, hydrophilic surface promotes the adhesion,

while a low-energy, hydrophobic surface is passive to the

adhesion.[4,5] A quantitative equation is introduced to

describe it, on the basis of the Young-Dupre equation:

DGa ¼ �glð1 þ cos yÞ ð1Þ

where DGa is the adhesion free energy, y is the liquid

contact angle on the surface, and gl is the surface tension of

the liquid.[25] From this equation, the adhesion strength on

the surfaces that have a high CA should be lower than that

on the surface that have a low CA. Accordingly, it is

possible that PANI on the hydrophobic region is removed

during peeling off the adhesive if the force is higher than the

adhesion free energy, while PANI on the hydrophilic region

remains. In this system, the CA on the unirradiated BOPP or

PI surface just approaches the value obtained on a CH3-

terminated SAM surface (�1050–1108), where the depo-

sition of PANI is easy to remove by peeling off the adhesive,

as demonstrated by others[4,5] and by the above. Wrighton

et al. report that a PANI layer on bare gold is also easy to

peel off by the adhesive.[5] The CA on such a gold surface isFigure 4. Optical photos of positive patterns on the substrate(a,c) and negative patterns on the adhesive tape (b, d). a) Thepositive pattern on a PI substrate after peeling. b) Thecorresponding negative pattern on the adhesive tape thataccompanied the formation of (a). c) The positive pattern on theBOPP substrate after peeling. d) The corresponding negativepattern on the adhesive tape that accompanied the formation of (c).The deposition time is 90 min for the formation of the PANI patternon PI and BOPP substrates.

a In Figure 4a, it appears as though some pollution hascontaminated the PI surface. However, the same pollution isfound to exist on the original surface, even after extensivecleaning. Accordingly, it is concluded that the contaminant inFigure 4a is not from the fabrication process of the PANIpattern, but mainly from the original surface.

422 P. Yang, J. Xie, W. Yang


just �508, which is lower than that on an unirradiated PET

surface (�758), which seems to indicate that the PANI on

the unirradiated PET surface can also be peeled off. How-

ever, the experimental results prove that the deposition on

PET could not be removed by the adhesive tape. This may

be attributable to a better compatibility of PANI with PET

than gold, for example, possible hydrogen-bonding inter-

actions between PET and PANI chains might enhance the

adhesion strength.

Although Whitesides et al[4] report that there is a great

conductivity difference of the deposited PANI between the

hydrophobic and hydrophilic regions, here it is found that

on the PI substrate, such a difference does not exist, and a

similar conductivity of about 10 S � cm�1 (the correspond-

ing thickness of the PANI layer is about 75 nm) can be

obtained on two types of surfaces.

In conclusion, a simple and low cost method to fabricate

PANI micropatterns on three typical polymer substrates,

including PET, BOPP, and PI, has been developed. Because

this approach is just based on a wettability patterned poly-

mer surface, which is conveniently fabricated by a simple

and general CPO reaction, it is believable that the fabri-

cation of a PANI pattern could be extended onto various

organic polymer substrates, which is very meaningful to the

large-scale fabrication of flexible, disposable, or portable

all-plastics electrical, optical devices.

Acknowledgements: The authors acknowledge funding of theMajor Project (50433040) from National Natural ScienceFoundation of China (NSFC) and the Major Project(XK100100433) for Polymer Chemistry and Physics SubjectConstruction from Beijing Municipal Education Commission(BMEC).

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a simple method to fabricate a conductive polymer micropattern on an organic polymer substrate

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