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A smart surface prepared using the switchable superhydrophobicity of neatelectrospun intrinsically electroactive polyimide fiber mats

Chang-Jian Weng,a Yu-Sian Jhuo,a Chi-Hao Chang,a Chun-Fang Feng,a Chi-Wei Peng,a Chung-Feng Dai,b

Jui-Ming Yeh*a and Yen Weic

Received 13th June 2011, Accepted 5th August 2011

DOI: 10.1039/c1sm06097j

An electroactive polyimide fiber (EPF) mat based on conjugated segments of electroactive amino-

capped aniline trimer (ACAT) as a diamine and 4,40-(4,40-sopropylidenediphenoxy)-bis(phthalicanhydride) (BSAA) as a dianhydride was successfully prepared by electro-spin technology with

electrochemical activity and dopable properties, which were similar to polyaniline. The degree of

electrochemical activity and dopable properties can be tuned by varying the content of ACAT existing

in the as-prepared electro-spun EPF mats. After doping with perfluorooctanesulfonic acid (PFOS), the

water contact angle of EPF surface is increased from hydrophobicity at 133� to superhydrophobicity at

155�. It is interesting that the EPF mat undergoes a switchable process from superhydrophobicity to

superhydrophilicity via doping with PFOS and de-doping with ammonium gas.

1. Introduction

Smart surfaces with reversibly switchable wettability, especially

switching between superhydrophobicity and super-

hydrophilicity, are of great importance due to their numerous

industrial applications.1–3 It is well know that surface free energy

and geometrical structure play key roles in the wettability of solid

substrates. The significant effect of the micro- (or nano-) surface

roughness of inorganic oxides or organic compounds is high-

lighted when fabricating superhydrophobic and super-

hydrophilic reversible switched surfaces.

A variety of stimulus tactics have therefore been developed for

controlling the wettability by way of light irradiation,4,5 thermal

treatment,6,7 treatment with acid solution,8–10 and other

approaches.11–14 With the development of smart materials, a dual

controllable wettability triggered by both temperature/pH, and

temperature/light irradiation has been demonstrated.15,16 Atten-

tion has also been paid to the wettability of conducting polymers,

which is a crucial parameter for controllable separation and

application of conducting polymers as sensors.17–21 For instance,

Berggren et al. 18 reported a wettability switch based on

a combination of a solid polymer electrolyte and a conducting

polymer, such as PANI, or poly(3-hexylthiophene), demon-

strating electrochemical control of wettability. Yan et al.

demonstrated19 that a polypyrrole film could be switched from

superhydrophobicity to superhydrophilicity by changing

aDepartment of Chemistry and Center for Nanotechnology, Chung-YuanChristian University, Chung Li, 32023, Taiwan, R. O. C. E-mail:Juiming@cycu.edu.tw; Fax: +886-3-265-3399; Tel: +886-3-2653340bTaiwan Textile Research Institute, Tucheng, 23674, Taiwan, R. O. C.cThe Tsinghua Center for Frontier Polymer Research Department ofChemistry, Tsinghua University, Beijing, 100084, China

This journal is ª The Royal Society of Chemistry 2011

electrical potential. Jiang et al. reported22,23 that PANI fibers

could be tuned from superhydrophobicity to superhydrophilicity

with PFOS and PFSEA as a dopant, respectively.

However, to the best of our knowledge, the wettability tran-

sition between superhydrophobicity and superhydrophilicity for

a non-conductive polymer, such as polyimide, has remained

a challenging task. Recently, research activities in terms of oli-

goanilines with well-defined structures for modelling the elec-

tronic, magnetic and optical properties of polyaniline have

emerged.24 Moreover, oligoanilines-derived electroactive poly-

mers have also attracted considerable research interests.25–28

Herein, we envisioned that electroactive polyimide might

reveal reversibly switchable wettability because it exhibits

dopable properties similar to polyaniline. Electrochemical

activity and dopable properties of as-prepared electroactive

polyimide fiber mats can be tuned by varying the content of

amino-capped aniline trimer (ACAT) existed in the polyimide

main chain. Therefore, in this work, we present the first report of

the reversibly switchable superhydrophobicity of neat electro-

spun intrinsically electroactive polyimide fiber (EPF) mats. The

wettability of electroactive polyimide fiber mats could be tuned

by varying the content of the ACAT segment existing in the

polymer main chain. The reversible wettability of EPF mat from

superhydrophobicity to superhydrophilicity could be reversibly

switched by doping with perfluorooctanesulfonic acid (PFOS)

and dedoping with ammonia gas.

2. Experimental

Chemicals and instrumentations

Aniline (Sigma-Aldrich) was distilled prior to use. 2,2-Bis[4-

(4-aminophenoxy)phenyl]hexafluoropropane (BDAF); Fluka,

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Buchs, Switzerland), 4,40-diaminodiphenylamine sulfate (Sigma-

Aldrich), N,N-dimethylacetamide (DMAc; Riedel-deHa€en),

tetrahydrofuran (THF; Riedel-deHa€en), 4,40-(4,40-iso-propylidenediphenoxy)-bis(phthalic anhydride)(BSAA, Sigma-

Aldrich, 97%), sodium chloride (NaCl, Sigma-Aldrich), per-

fluorooctanesulfonic acid (PFOS; Fluka, 40%), ammonium

hydroxide (NH4OH; Riedel-deHa€en, 35%) and ammonium per-

sulfate (APS, Sigma-Aldrich, 97%) were used as received without

further purification. All the reagents were reagent grade unless

otherwise stated.

Mass spectra were run on a Bruker Daltonics IT mass spec-

trometer model Esquire 2000 (Leipzig, German) with an Agilent

ESI source (model G1607-6001). 1H NMR spectra were run on

a Bruker 300 spectrometer, referenced to internal standard of

tetramethylsilane (TMS), DMSO was used as the solvent.

Attenuated total reflectance FTIR spectra was obtained at

a resolution of 4.0 cm�1 with a FT/IR spectrometer (FT/IR-4100)

at room temperature ranged from 4000 cm�1 to 650 cm�1. The

surface morphology of materials was characterized by scanning

electron microscopy (SEM), (Hitachi S-2300). Contact angles of

samples were measured using a First Ten Angstroms FTA 125.

Synthesis of N,N0-bis(40-aminophenyl)-1,4-quinonenediimine

(ACAT)

4,40-Diaminodiphenylamine sulfate (23.65 g, 0.796 mol) and

aniline (7.40 g, 0.796 mol) were dissolved in aqueous HCl solu-

tion (1.0 M, 800 mL) containing 75 g of NaCl. A solution of

ammonium persulfate (18.00 g, 0.789 mol) in aqueous HCl

(1.0 M, 200 mL) was added via a dropping funnel into the above

solution at an operational temperature of �5 �C with a dropping

rate of approximately 60 drops min�1. The reaction mixture was

stirred for additional 1 h at �5 �C. The resulting precipitate was

collected by filtration and washed with aqueous HCl solution

(1.0 M, 400 mL) precooled to 0 �C. The solid product was

washed with 10%NH4OH solution (100 mL) and a large amount

of distilled water and then dried in a vacuum oven at 50 �Covernight. The ACAT was obtained as a bulk solid. 11.8 g

(51.5%). 1H NMR (300 MHz, DMSO-d6): d 6.95 (s, 4H), 6.80–

6.60 (dd, 8H), 5.42 (s, 4H). MS (ESI) calcd. for C18H16N4 [M]:

288.14. Found 289.1 [M + H]+.

Scheme 1 A schematic representation of the synthesis of electroactive

polyamic acid and electroactive polyimides.

Synthesis of electroactive polyamic acid (EPAA), electroactive

copolyamic acid (ECPAA) and non-electroactive polyamic acid

(NEPAA)

Electrospun process was performed by using electroactive pol-

yamic acid (EPAA), electroactive copolyamic acid (ECPAA) and

non-electroactive polyamic acid (NEPAA) solution (10 wt%) as

an electrospun solution. Three polyamic acid were prepared

using 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane

(BDAF), N,N0-bis(40-aminophenyl)-l,4-quinonenediimine

(ACAT), and 4,40-(4,40-isopropylidenediphenoxy) bis(phthalic

anhydride) (BSAA) in different feeding ratios. The typical

procedure to prepare EPAA and NEPAA was using BSAA

(0.26 g, 0.5 mmole) and ACAT (0.14 g, 0.5 mmole) or BDAF

(0.26 g, 0.5 mmole) in 4 g of THF/DMAc (1 : 1) at room

temperature. The electroactive copolyamic acid (ECPAA) solu-

tion was synthesized by a well-developed procedure using BSAA

10314 | Soft Matter, 2011, 7, 10313–10318

(0.26 g, 0.5mmole), ACAT (0.072 g, 0.25 mmole) and BDAF

(0.13 g, 0.25 mmole) in 4 g THF/DMAc (1 : 1) at room

temperature. The as-prepared mixture was then stirred for an

additional 2 h at room temperature, as shown in Scheme 1.

Preparation of electroactive PAA fiber membranes

The electrospun solution was synthesized upward and its corre-

sponding molecular weight: NEPAA (Mw¼ 29 814; PDI¼ 1.95),

ECPAA (Mw ¼ 66 501; PDI ¼ 3.275) and EPAA (Mw ¼ 92 097;

PDI ¼ 2.138) were measured by GPC (0.1 wt% in NMP

solution).

Fig.1 showed the general electrospin process, which consisted

of a high-voltage power supply and a collector plate. The

spraying solution was put into a syringe with a 13 mm inner

diameter and a distance of 14 cm from the tip of syringe (anode)

to the collector (cathode). The high-voltage DC electric field

strength controlled by a high-voltage DC power supply was fixed

at 20 kV. The typical ejection rate of solution from the syringe is

0.02 mL min�1 and the electrospinning was conducted in air.

Removal of solvent and imidization of EPAA, ECPAA and

NEPAA fiber mats were performed concurrently by heating

them under air flow at 250 �C for 2 h. After imidization, the EPF,

ECPF and NEPF can be obtained.

Wettability switching experiments

As-prepared EPF was immersed into a solution of PFOS (2 wt%

in water) for about 2 h and then followed by washing with water

and air-drying at room temperature. The superhydrophobic of

EPF mat obtained. Ammonia was added into a closed vessel

using syringe, and then ammonia vapor was filled into the

This journal is ª The Royal Society of Chemistry 2011

Fig. 1 A schematic of the electrospin setup with modified auxiliary

electrode and field-controllable target electrode.

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chamber. After the superhydrophobic EPF mat was exposed to

dry ammonia vapor in chamber for �1 min, the super-

hydrophobic of EPF was obtained.

Fig. 2 FTIR spectra of (a) NEPF, (b) ECPF, (c) EPF.

Fig. 3 SEM of (a) NEPAA, (b) ECPAA, (c

This journal is ª The Royal Society of Chemistry 2011

3. Results and discussion

For the studies on FTIR spectroscopy, the EPF mat prepared

from thermal imidization of ACAT and BSAA exhibited an

obvious carbonyl asymmetric stretching of nC]O located at 1850

and 1778 cm�1 and symmetric stretching of nC]O located at

1720 cm�1.

The characteristic peak near 738 cm�1 due to the imide ring

deformation of the EPF mat was also found in the IR spectra.

Moreover, the FTIR spectroscopy of the ECPF mat revealed

a decrease in the intensity ratio of nC]O (1850 cm�1)/nC]O

(1778 cm�1), indicating a decrease in the feed composition ratio

of ACAT/BDAF existing in the as-prepared ECPF mat and this

ratio permits us to estimate the extent of oxidation of the poly-

mer. However, the NEPF mats lack the characteristic asym-

metric stretching of nC]O (1850 cm�1) of ACAT, as shown in

Fig. 2.

As important aspects, effects of the electroactive polyamic acid

fibers and electroactive polyimide fibers with different ACAT

content on morphologies have been studied, and SEM is used to

characterize their morphologies. SEM micrographs of all speci-

mens as showed in Fig. 3. Average diameters of electroactive

polyamic acid fibers which with different ACAT contents ranged

from about 420 to 560 nm. However, the fibers didn’t shrank

obviously after thermal condensation process and them still

uniform and continuous after imidization.

According to the proposed mechanism for the electrochemical

redox reaction of aniline oligomers,29 the electrochemical

behavior of all obtained specimens was examined in a strongly

acidic electrolyte composed of 1.0 M H2SO4 aqueous solution.

All samples were formed by electrospinning on the surface of a Pt

working electrode and followed by thermal conversion into

polyimide through imidization. Fig. 4 shows the cyclic voltam-

metry of EPF, ECPF and NEPF obtained using a saturated

calomel electrode (SCE) as the reference electrode with a scan

rate of 50 mV s�1. Under these conditions, cyclic voltammetry of

EPF showed a pair of reversible redox peaks, which were similar

to many longer oligomers that undergo a two-electron-transfer

process.30 For the studies of EPF, a single oxidation peak was

found to occur at a lower potential (582 mV) as compared to that

of polyaniline (800 mV). We considered that the oxidation peak

) EPAA, (d) NEPF, (e) ECPF, (f) EPF.

Soft Matter, 2011, 7, 10313–10318 | 10315

Fig. 4 Cyclic voltammetry of the polyimide, shown as (a) NEPF, (b)

ECPF and (c) EPF above, was measured in aqueous H2SO4 (1.0 M) with

a scan rate of 50 mV s�1. The proposed oxidation mechanism is shown

below.

Fig. 5 UV-vis spectra of EPF: (a) NH3 dedoped, (b) PFOS doped.

Fig. 6 SEM images of (a) ECPF, (b) EPF; SEM fluorine mapping

images of (c) ECPF, (d) EPF.

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located at �582 mV was assigned to the transition from the

reduced state to the oxidized state of EPF. It should be noted that

the EPF having higher content of ACAT exhibited a larger redox

current at 0.4 mA cm�2 (i.e., higher electroactivity) compared to

that of ECPF.

However, NEPF showed null zero redox current. This implied

that the incorporation of ACAT into polyimide may introduce

electroactivity into as-prepared EPF. Moreover, incorporation

of higher ACAT content may yield polyimide with higher elec-

tro-activity.

The EPF mat that has alternating conjugated segments

(ACAT) and non-conjugated segments (BSAA) in the main

chain still exhibits reversible dopable response properties. In this

study, perfluorooctanesulfonic acid (PFOS) was used as a dopant

to dope an EPF mat and examine its result in the UV-visible

spectrum. For an un-doped EPFmat, the quinoid (Q) absorption

peak located at ca. 509 nm and the benzenoid (B) peak located at

ca. 312 nm were assigned as the two characteristic peaks in the

emeraldine base (EB) form of ACAT, which were associated with

the transition ofpb–pq from a benzene unit to a quinone unit and

from a p–p* transition in a benzene unit, respectively.31 After

EPF was doped by perfluorooctanesulfonic acid (PFOS), the new

bands were observed at around 415 and 750–800 nm. The peak

appearing at 415 nm was probably due to the formation of

polarons (radical cations) caused by electron transition from

quinoid to benzoid units. The peak found at 800 nm with a long

tail was assigned to the polaron transition, which typically

characterizes protonation,22,23 and is identical to that of the

emeraldine salt (ES) form of the EPF mat. (Fig. 5)

In order to determine how the morphologies and elemental

distribution of the different ACAT contents in the polymer chain

can tune the contact angle (CA) by using PFOS as dopant, SEM

10316 | Soft Matter, 2011, 7, 10313–10318

imaging and SEM fluorine mapping imaging were performed.

SEM images of ECPI and EPI, which were doped with PFOS, are

illustrated in Fig. 6(a) and Fig. 6(b), respectively. EDX mapping

of an image in a SEM image of EPF and ECPF, which were

doped with a PFOS support, can provide information on the

distribution of fluorine from the PFOS dopant, as shown in

Fig. 6(c) and Fig. 6(d). It should be noted that more dense

fluorine distributions are observed for higher ACAT content

samples. Thus, these morphological studies demonstrated that

the EPF reveals a higher dopable degree than the EPCF because

of the more conjugated segment (ACAT) in the polyimide main

chain.

Since PFOS has dual doping and superhydrophobic functions,

reversible wettability of the surface of EPF may be realized

through a doping process. This prediction has been demon-

strated by measurements of the static CA of doped EPF and

ECPF. Fig. 7 gives the relationship between the content of

ACAT and CAs for all PFOS-doped specimens. It should be

noted that the CA of an electroactive polyimide fiber mat could

be tuned by varying the ACAT content while it was doped with

PFOS. Because the NEPF was without ACAT in the polymer

main chain, the CA of NEPF was only 106� and it cannot be

doped by PFOS. However, in ECPF and EPF, it is clear that the

CA increases from 117� and 131� to 133� and 155� with the

This journal is ª The Royal Society of Chemistry 2011

Fig. 7 The water contact angle of NEPF, ECPF and EPF.

Fig. 9 Reversible superhydrophobic–superhydrophilic conversion of

the EPF through doping with PFOS and de-doping by ammonia gas.

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increase in contents of ACAT, respectively. This implied that the

wettability and dopable degree of the electroactive polymer fiber

can be controlled by the ACAT content in the polymer main

chain.

The EPF mat presents superhydrophobicity with a static CA

as high as 155�. Obviously, superhydrophobicity is resulted from

the synergistic effects of the pore structure of the EPF mat and

a low surface energy of the PFOS dopant.

To give a surface revealing superhydrophobic properties, the

EPFmat was required to be doped with PFOS. As is well known,

the difference between advancing (qA) and receding contact

angles (qR) of a surface is used to understand contact angle

hysteresis.32 Fig. 8 shows the advancing and receding contact

angle for NEPI, ECPF and EPF mats doped with PFOS. The

ECPF doped with PFOS had advancing (qA) and receding (qR)

contact angles of 142� and 128�, respectively.After increasing the ACAT content in polyimide main chain

(EPF), both advancing and receding contacts angles increased up

to 158� and 150�. It is noteworthy that contact angle hysteresis

was reduced from 14� to 8�, respectively, which can be attributed

to the increase of doping degree due to the electroactive poly-

imide fiber mats doped with PFOS.

After the EPF mat was exposed to dry ammonia gas for 1 min,

interestingly, the water droplet on the surface of the mat spread

out, which results in a CA of about 0�. After the EPF mat was

immersed into a solution of PFOS (2 wt%) for about 2 h and then

Fig. 8 Advancing and receding water contact angles of the NEPF,

ECPF and EPF fiber mats.

This journal is ª The Royal Society of Chemistry 2011

dried in air, the EPF mat still gained superhydrophobic prop-

erties. These observations indicate that the doping/dedoping

process results in the reversible wettability changing from

superhydrophobicity to superhydrophilicity. The reversibility of

the surface wettability for the EPF mat can be repeated several

times without any change in wetting property as shown in Fig. 9.

The reversibility of the wettability observed for the EPF mat

can be explained as follows: The PFOS, composed of a hydro-

philic SO3H polar head and a hydrophobic perfluorinated

carbon tail, is not only reserved as a dopant, but also brings

about hydrophobic/hydrophilic function. However, the PFOS

molecules are then no longer locked to the ACAT backbone in

the polyimide main chain and it can move more freely.22,23

When EPF was doped with PFOS, the hydrophobic per-

fluorinated carbon tail was exposed at the outermost surface, the

superhydrophobic surface was obtained, as shown in Fig. 10(a).

After dedoping EPF with ammonia gas, the hydrophilic SO3�

groups can rotate and are exposed at the outmost surface, thus

resulting in conversion of the superhydrophobicity to super-

hydrophilicity. (Fig. 10(b))

Fig. 10 A proposed mechanism for the reversible wettability switching

by EPF (a) PFOS-doped and (b) NH3 de-doped.

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Conclusions

In summary, we have demonstrated an electrospin technique for

synthesizing an EPF mat, which has not only reversible response

properties but also has reversible wettablity that can be switched

from superhydrophobicity to superhydrophilicity by doping with

PFOS and dedoping with ammonia gas. Interestingly, the degree

of electrochemical activity and dopable properties can be

reversibly tuned by varying the content of ACAT existing in the

as-prepared electrospun polyimide fiber mats, based on SEM

fluorine element mapping results. Using NEPF, the CA of NEPF

was only 106� and it cannot be doped by PFOS. However, using

ECPF and EPF, the CA can be increased from 117� to and 155�

with the increase in the varying contents of ACAT. The revers-

ible wettability changing from superhydrophobicity to super-

hydrophilicity can be repeated several times by doping with

PFOS and de-doping with ammonia gas.

Acknowledgements

The financial support of this research by the Ministry of

Education, Taiwan, R. O. C., NSC 98-2113-M-033-001-MY3

and department of chemistry at CYCU., CYCU-98-CR-CH are

gratefully acknowledged.

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