SuperHydrophobic and SuperHydrophilic Polycarbonate by Tailoring Chemistry and Nano-texture with...

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SuperHydrophobic and SuperHydrophilicPolycarbonate by Tailoring Chemistry andNano-texture with Plasma Processing

Fabio Palumbo,* Rosa Di Mundo, Davide Cappelluti, Riccardo d’Agostino

Versatility of plasma processing is demonstrated for the preparation of nanotextured poly-carbonate surfaces with superior wettability properties. Tens of nanometers wide pillarstructures are produced on the polymer by means of oxygen plasma etching, inducing onthe surface a pronounced hydrophilic behavior. Aswell known from literature, however, treatedsurfaces undergo a fast hydrophobic recovery,but a post-deposition process can ensure the for-mation of transparent and stable superhy-drophylic or superhydrophobic surfaces.

Introduction

The importance of the role played by morphology of

materials in controlling the surface properties is well

known and has been worth of a huge amount of papers in

the last 10 years. Materials engineers try to copy from the

biological world the ability in developing macroscopic

smart properties, such as self-cleaning,[1–6] super-adhe-

sion,[6] anti-reflection,[7–10] friction reduction in fluids.[6] by

the optimization of surface micro-, nano-texturing. This

approach is generally called ‘‘biomimetics,’’ and it can find

application both in high added value technologies, such as

those for the development of MEMS devices, drug delivery,

advanced optoelectronics, and in some more traditional

and low-cost products: e.g., windows (buildings and

transportation), technical clothes, glasses.[11,12]

F. Palumbo, R. d’AgostinoIstituto di Metodologie Inorganiche e dei Plasmi(IMIP)-CNR, 70126Bari, ItalyE-mail: [email protected]. Di Mundo, D. Cappelluti, R. d’AgostinoDipartimento di Chimica, Universita di Bari, 70126 Bari, ItalyR. d’AgostinoPlasma Solution srl, via Orabona 4, 70126 Bari, Italy

Plasma Process. Polym. 2011, 8, 118–126

� 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonline

The most investigated properties in this scenario are

those related to the control of wettability: super-hydro-

phobicity/hydrophilicity and switching from one character

to the other.[13] On the other hand any texturing process,

even if intended for the optimization of other kind of

properties, e.g., moth-eye antireflection, inevitably leads to

a deep modification of the wettability behavior. As a

consequence a good comprehension of the wettability of

rough surfaces is important along with the elaboration of

facile nanotexturing methods which can easily add value to

material surfaces.

The effect of nanotexturing for hydrophobic surfaces,

giving rise to the so called ‘‘lotus effect,’’ is well understood,

even if in recent years a fervent discussion is animating the

community on the goodness of Wenzel and Cassie–Baxter

models for the interpretation of the results.[14–20] Indepen-

dently from the validity of the two models, it is generally

accepted that:

(i) s

librar

mooth conventional surfaces cannot present static

water contact angle (WCA), ustat, higher than 1208, e.g.,

Teflon,

(ii) a
n ultrahydrophobic/water repellent surface should
have a ustat higher than 1508, with small difference

y.com DOI: 10.1002/ppap.201000098

Plasm

� 20

SuperHydrophobic / SuperHydrophylic Polycarbonate by Tailoring Chemistry and Nanotexture

between advancing and receding WCAs, respectively,

uadv and urec,

(iii) s
uch regime (often called ‘‘air pocket,’’ or ‘‘fakir,’’ or
‘‘non-wet’’ state) is obtained only on textured surfaces

and it is characterized by the suspension of the drop

onto the surface protrusions.

When the liquid is water, in ‘‘air pocket’’ state, the drop

touch the solid only for a fraction namelyfs, while the rest is

in contact with air, as described by the Cassie–Baxter

equation

a

11

cosuCB ¼ fs cosuflat þ fs�1 (1)

where uCB and uflat are the WCA for the rough and the flat

surface, respectively. Only in this regime a water repellent

self cleaning behavior is attained. In fact, since the shear

work of adhesion, Ws, of a liquid drop on a surface is

proportional to the difference of receding and advancing

angle as follows (g lv is the liquid surface tension)

Ws ¼ g lv cosurec�cosuadvð Þ (2)

the drop will freely move on the surface, only if the WCA

hysteresis is low. Thus, the water drop rolling on the surface

will drag the dirt with itself, cleaning the surface. It should

be stressed that on such superhydrophobic surfaces also

the tensile work,Wt, necessary to remove the drop from the

surface is low since proportional to cosurec according to the

following expression[16]

Wt ¼ g lv ðcosurec þ 1Þ (3)

It is useful to mention that textured superhydrophobic

surfaces can present additional properties such as oleo-

phobicity[21] and anti-icing.[22–24] The latter is particularly

appealing for the power stations and airplanes in cold

regions, to reduce the adhesion of freezing rain.

The effect of texturing on the opposite side of wettability,

superhydrophilicity, is less investigated even if it can be

relevant for some important applications

such as diagnostics kit, sensors, cells

manipulation, antifogging.[13,25–30]

Both models, that of Wenzel and of

Cassie–Baxter, predict a decrease in WCA

when passing from flat to textured

surfaces. However, a critical value of

the equilibrium contact angle (on a flat

surface), uc, exists defined as[31–34]

cosuc ¼1�fs

r�fs

(3)

Figure 1. Scheme of the reactor for plasma texturing.
(r> 1, roughness of the surface) below
which an impregnated surface is formed

Process. Polym. 2011, 8, 118–126

WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

and water is sucked between protrusions. Under this

condition the drop of water stands on a composite material

consisting in solid and liquid, a Cassie–Baxter hydrophilic

regime. It is clear from Equation (3) that tuning the

geometric parameters of the surface morphology (rough-

ness and area fraction of protrusions) allows to control the

hydrophilic behavior of the material.

In this scenario, it is relevant to develop easy processes for

nanotexturing of polymers appealing for industrial appli-

cations. Soft nanolithography, a technology considered in

this field, generally passes through multistep, often time

consuming and not simply scalable, e.g., in large scale, low

added value products. Plasma processes, instead, such as

dry etching, allows for easy and versatile texturing, even of

large area polymeric surfaces. Various papers can be found

in the recent scientific literature with regard to plasma

etching/roughening of polyethylene,[35] polymethyl-

methacrylate,[8–10,25,26] cellulose,[27] polystyrene,[1,36–39]

zeonex,[8,10] silicone,[2,5] plasma deposited fluoropoly-

mer,[40] and SU-8 resist.[41]

In this paper, we focus our attention on the potential of

plasma nanotexturing in controlling extreme wettability of

polycarbonate (PC), which is a polymer particularly

appealing for several applications from automobile/air-

plane glasses to medical devices, lenses, and optics. Few

papers concern texturing of PC,[28,42] and mostly do not

explore capabilities of plasma processing. It will be shown

that plasma processes are versatile in preparing stable

nano-textured polymeric surfaces with super-hydropho-

bic/hydrophilic behavior, switching from fluorocarbon (or

silicone) to silicon oxide-like based chemistry.

Experimental Part

Preparation of Nanotextured Surfaces

Polycarbonate (PC) substrates (Makrolon1 1 mm thick), purchased

from GoodFellow, were cut in 1�1 cm2 slices, sonicated in

isopropylic alcohol, and dried before processing.

www.plasma-polymers.org 119

120

F. Palumbo, R. Di Mundo, D. Cappelluti, R. d’Agostino

Nanotextured PC was prepared in the plasma reactor sketched in

Figure 1. Briefly, it consists in a home-made capacitive coupling (CC)

apparatus with two parallel stainless steel electrodes with 2 cm

gap, the upper one ground and the lower one connected to a

radiofrequency (RF, 13.56 MHz, Caesar Dressler) power supply via a

matching network. Both electrodes had 140 cm2 of surface area and

were included in a glass vacuum chamber evacuated with a

rotative pump (base pressure 0.13 Pa). The gas flow rate was

controlled by means of electronic gas flow meters, and vacuum

measured with a baratron gauge (MKS Instruments).

The nanotexturing process was carried out feeding the reactor

with 10 sccm of O2 at a working pressure of 13.3 Pa. Experiments

were carried out keeping the substrate on the bottom electrode,

igniting the discharge at RF power in the range 50–200 W, at

variable time 1–20 min.

Deposition of Hydro-phylic,-phobic Coatings

Once etched with the O2 plasma, the samples were coated with

different films according to the experimental conditions reported

in Table 1. For the fluorocarbon coating, CFx, the recipe optimized in

reference[38,43] on nanotextured polystyrene, based on plasma fed

with perfluoro cyclobutane (c-C4F8), was used. In the case of

organosilicon films two kinds of coatings have been optimized

leading, respectively, to an hydrophobic chemistry, SiOChy-phobic,

and hydrophilic one, SiOxhy-philic. For the former a protocol was

developed consisting in direct plasma polymerization from a feed

containing hexamethyldisiloxane (HMDSO) as monomer. On the

other hand for the deposition of SiOxhy-philic oxygen and Ar were

added to the monomer, in order to consume the organic radicals.

Even if it could be possible to coat the nanotextured samples in

the same reactor used for the plasma etching, it was preferred to

study the plasma etching and deposition processes in distinct

reactors. In particular for the CFx deposition a reactive ion

etching configured reactor was considered, described in detail in

reference[36] Briefly it consists in a cylindrical stainless steel

chamber bearing a top shower headed ground electrode and a

bottom RF one, on which the samples were placed. In the case of the

silicon containing coatings the process was carried out in an RF

powered CC plasma reactor better illustrated in reference,[44]

where the sample holder is the ground bottom electrode.

Samples Characterization

Surface morphology of uncoated and coated samples was

investigated by means of scanning electron microscopy (SEM,

Table 1. Experimental condition of coating deposition.

Coating Gas

feed

Flow

rate

sccm

Power

W

Pressure

Pa

CFx c-C4F8 10 100 10.6

SiOChy-phobic HMDSO 3 100 13.3

SiOxhy-philic HMDSO/

O2/Ar

1.5/100/

50

200 13.3


� 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Zeiss EVO 40VP): specimens were gold-metalized by sputter coating

(BioRad Polaron) and observed at 20 kV. An image processing

software (ImageJ, http://rsbweb.nih.gov/) was utilized to evaluate

from the acquired top-view images the total top area fraction and

density of the structures, by thresholding every image in the gray-

scale and making it binary. The same software has been utilized to

calculate the average height and width of the structures.

X-ray photoelectron spectroscopy (XPS) analyses of textured PC

were carried out by means of Thermo Electron Corporation Theta

Probe Spectrometer with a monochromatic Al Ka X-ray source

(1 486.6 eV) at a spot size of 400mm and a take-off angle of 378.Analysiswere carried outwithin2 h from the processing, transferring

the samples from the plasma chamber to the XPS one by means of

commercial plastic boxes, hence exposing the surface at atmosphere.

Survey (0–1 200 eV) and high resolution spectra (C1s, O1s, F1s) were

acquired at a pass energy of 200 and 150 eV, respectively. Sample

charging was corrected with respect to the position of the C�C(H)

component in the C1s signal. Best fitting of the C1s XPS signal was

executed by means of the instrument software (Avantage), in six

components corresponding to: C�C(H) (C1, 285�0.1 eV),C�O (C2,

286.5�0.3 eV), C¼O (C3, 288.0� 0.3 eV), O�C¼O (C4, 289.5� 0.3 eV),

O�C(O)�O (C5, 290.5�0.3 eV) and shake up (C6, 291.5�0.3 eV). A

fourier transform infrared (FTIR) absorption spectroscopy analysis of

coatings deposited on polished crystalline silicon was carried out by

means of a Bruker Equinox 55 interferometer.

Water contact angles (WCAs) have been measured by means of a

Rame-Hart 100 goniometer, with the sessile drop method in

dynamic mode. Advancing and receding angles were measured by

placing a droplet of 1ml on the surface, then increasing the volume

to 4ml, finally decreasing it. Advancing angles (ua) are the

maximum angles observed during the droplet growth. Receding

contact angles (ur) are the ones just before the contact surface

reduction. Each WCA value has been averaged from measurements

of four drops with an estimated maximum error of 28.

Results and Discussion

O2 Plasma Nanotexturing of Polycarbonate

Following the work carried out on polystyrene,[1,36–39] and

analogously to what has been reported by other research

groups on polymethylmethacrylate,[8–10,25,26] plasma pro-

cessing of PC with an etching gas feed, like oxygen, can

effectively un-homogeneously affect the polymer surface

generating nanostructures. This is depicted in Figure 2

reporting the SEM top and tilted views images of PC treated

at 100 W for different process duration (1–20 min). For

comparison also the picture of the pristine substrate is

shown, where a substantial flat surface can be observed.

The first formation of nanostructures has been evidenced

for an etching duration of 3 min: dots start to uniformly

populate the PC surface. When the process proceeds, taller

nanostructures are formed reaching heights of almost 1mm

for a 20 min treatment. In particular a forest of nanopillars

can be observed at 15 and 20 min of treatment. Comparing

the top view with the tilted one, it can be stressed that the

pillars though narrow (width of about 70 nm), agglomerate

DOI: 10.1002/ppap.201000098

Figure 2. Scanning electron microscopy (SEM) pictures (top andtilted views) of untreated and plasma textured PC at differentprocessing time, at 100W RF power.


forming bigger clusters of pillars (even 300 nm wide). In

Table 2 the height (H) and width (D) of the pillars are

reported, along with the density (d), and area fraction of the

structures, from ImageJ elaboration of SEM images

analysis. It can be observed that, apparently, the density

of the structures drastically decreases when the treatment

time is increased, but the percentage of covered surface is

not changing significantly, in the range 5–20 min. In fact

the software analysis does not distinguish the single units

in the clusters of pillars for samples treated for time longer

than 5 min and as a consequence it indicates a decrease of

the structure density. Nevertheless, we can conclude that

for etching time longer than 5 min, the percentage of area

covered by the structures do not change significantly, but

the surface consists of clusters separated by isolated pillars.

This result seems to indicate that during the process part

of the grown pillars are consumed while others acts as

agglomeration center for the growth of taller structures.

In Figure 3 SEM pictures of PC treated for 5 min at different

RF power are reported, while the quantitative information

are reported in Table 1. Comparing with the surface obtained

at 100 W (Figure 2), it can be noted that lowering the power

leads to a reduction of the extent of the etching process and in

turn of the height of nanostructures (not quantifiable from

the SEM pictures), as it can be expected.



On the other hand when the RF power is increased, a

certain improvement in the etching process in terms of

nanostructures height is obtained at 150 W, but at 200 W

structures are slightly lower and their spacing increases.

This is also testified by the decrease of the percentage of area

fraction of cluster detected by image J analysis.

Ion bombardment surely will play a role in the texturing,

and raising the RF power typically led to an increase of the

bias voltage on the substrates. In the 50–200 W range the

voltages passed from �40 to �105 V, which were slightly

lower with respect to the bias values externally applied

in the system described in reference[25] for polymethyl-

methacrylate plasma texturing.

The SEM investigation has demonstrated that O2 plasma

is effective in nanotexturing PC and an RF power of 100 W

is sufficient for producing hundreds of nanometers tall

structures in one step.

For a characterization of the chemical modification of the

surface, XPS analysis has been carried out. In Figure 4 the

best-fitted C1s signal of pristine PC as well as of plasma

treated ones at 100 W for different time are reported; the

quantitative atomic concentrations can be found therein. It

can be observed that, as a consequence of the reaction with

the plasma, oxygen is incorporated in the surface of the

polymer (13% on the pristine polymer to about 32% for a

20 min treatment) and new chemical groups are formed,

carbonyl and carboxylic carbon. It is interesting to report

here that on treated surfaces contaminations of chromium

and of iron have been found, in a total amount of 3.2� 1.0%.

The contamination most likely is due to unavoidable

sputtering of the metal (stainless steel) electrode during

the plasma treatment. This finding leads to the following

comments:

(i) s
ince the XPS signals of metals denote the formation of
oxides, the total oxygen concentration determined by

means of XPS is not all directly related to the

modification of the polymeric chains.

(ii) t
he metal contamination can have a role in the etching
process.

Concerning the first comment, for a better evaluation of

the chemical modification of the polymer chains, it is

necessary to look at the trend of the C1s fitting components.

First of all, the total percent of oxidized carbon, COx groups,

increases from 9% of whole carbon in native PC to 49% in the

5 min treated one, then it slightly decreases for longer

treatment time. Secondly, within the extensive oxidation

of the 5 min treated polymer surface, beside the introduc-

tion of new groups like C¼O and C(O)�O, C�O groups are

incremented. For longer treatment time the contribution of

C�O decreases markedly, while the components relative to

carbon with higher degree of oxidation show only slight

reduction. On the other hand the relative contribution of


Table 2. Morphological features determined from SEM pictures processed with Image J software.

RF power Treatment time H D d Area fraction

W min nm nm mm�2 %

100 3 // 23� 6 1 100� 200 44

5 86� 8 49� 8 270� 54 24

15 600� 80 73� 10 60� 12 23

20 980� 70 61� 8 39� 8 27

50 // 18� 6 860� 150 17

150 5 110� 30 58� 6 100� 20 18

200 105� 20 66� 11 30� 6 10

Figure 3. Scanning electron microscopy (SEM) pictures (top andtilted views) of untreated PC plasma textured for 5min atdifferent processing RF power.

Figure 4. XPS C1s signal of plasma textured PC treated at differentprocessing time at 100W RF power. Atomic % for carbon andoxygen is reported therein: residuary % is due to metal atoms (Feand Cr).

122


hydrocarbon carbon (C�C, C�H) increases. This evidence

likely indicates that, for long processing time (higher than

5 min), during the plasma etching oxygen converts carbon

with low oxidation degree (C�O), in groups with an higher

oxidation state (C¼O, C(O)�O and O�C(O)�O). Since the

latters are continuously formed during the process, but they

can also react giving volatile species (such as carbon mon-

oxide and dioxide, or formic aldehyde), their content on the

surface likely remains constant, as highlighted by the C1s

best fitting. The described removal of the oxidized moieties

will leave un-oxidized carbon (hydrocarbon, C�C/C�H) on

the surface, testified by the relative increase of the corre-

sponding component in Figure 4.

As for the metal content, it should be stressed that, even

though plasma nanotexturing has been often mentioned in

recent literature for different polymers, few suggestions

have been given to explain the origin of such process.

Considering that also other authors[25] pointed out to the



presence of metals on plasma nanotextured polymers, and

in our previous work similar contamination was found in

textured polystyrene,[10] it is possible to argue that this

contamination can play a role in the etching process. In fact,

metal sputtered away from the electrodes, and eventually

from the chamber walls (the latter contribution excluded

for the reactor used in this work, being of glass), can be

DOI: 10.1002/ppap.201000098


randomly deposited onto the polymeric surface during the

plasma process. Since the deposited metal cannot be

ablated by the oxygen plasma, it will act as a physical

nanomask for the polymer etching, leading to the texturing

of the samples. A preliminary verification of this hypothesis

has been carried out by fully covering the RF electrode with

a quartz disk: in this case neither texture nor metals (even

silicon) were observed. Aware that this experiment is not

exhaustive, since quartz can alter the plasma character-

istics, the plasma processing has been tested in a broader

range of experimental parameter doubling time and/or

power, but it was not possible to reproduce the surface

texturing.

Since the application of this plasma texturing is the

control of the wettability of PC, the evaluation of the WCA of

treated PC has been carried out. The testing was focused

onto surfaces produced at different treatment time and the

results are illustrated in Table 3, reporting advancing and

receding WCA of as treated surfaces and of pristine PC

substrates.

When processed with an oxygen plasma the PC surface

becomes very hydrophilic, with values of about 118 and 38,for the advancing and receding contact angle, respectively,

whatever the treatment time considered is. It can also be

observed that, while the difference between uadv and urec for

the untreated PC is quite large, these values are very close

for the treated surface as it could be expected for surfaces

with a substantial hydrophilic character: the water drop

advancing on a nonwet zone does not probe large energy

differences while leaving the wet regions, as it can happen

in the case of hydrophobic surfaces. Raising power to 200 W

does not give further improvement in polymer wettability.

The superhydrophilic character can be attributed to the

hydrophilic groups grafted on the polymer surface

(included the low amount of metal oxide clusters), but

surely texturing gives an important contribution.

Table 3. Water contact angle (WCA) values for untreated and O2plasma treated PC.

RF

Power

Treatment

time

As treated After 21 d

W min uadv (-) urec (-) uadv (-) urec (-)

100 5 9 3 63 13

10 11 3 71 16

15 10 3 71 12

20 12 3 78 19

150 5 11 4 66 15

200 5 13 4 64 16

Untreated 93 75 // //



Though PC surface is extremely hydrophilic as soon as

treated, it undergoes typical hydrophobic recovery: in fact

21 d after the processing, advancing WCA raised to about

608, reaching 788 for the sample treated for 20 min. This

confirms that this kind of process is unsuitable for

producing stable hydrophilic polymeric surface. Also urec

denotes aging of the surface however in 21 d do not pass the

value of 208. This can be explained considering that the

receding angle is more sensitive to the high energy grafted

functionalities, and, as a matter of fact, remaining lower, it

keeps the memory of the modification, suggesting that high

energy groups remain within the reorganized surface.

The observation that aging seems to be more evident in

the samples with the most extensive texturing deserves

some further comments. In fact, comparing such results to

give a rationale on the aging of O2 plasma treated samples is

complex, since the process is altering not only the surface

chemistry, but also the morphology (increasing time leads

to taller structures) and, as thoroughly explained in this

paper, the real WCA value deeply depends on texturing.

Coming back to hydrophobic recovery, Tsougeni et al.[25]

indicated that (even if expensive in terms of time) oxygen

plasma etching lasting 60 min on PMMA, can delay the

hydrophobic recovery, and the polymer remains stably

hydrophilic for 20 d, but becoming inevitably hydrophobic

in the following period of observation. It is possible that in

that case the long treatment time could modify a thicker

region of the surface, inhibiting the immediate aging of the

material.

Coating of Nanotextured Polycarbonate

Once textured the PC substrates have been coated by film,

20 nm thick, with different chemical composition in order

to control the wettability behavior and to avoid aging

phenomena. The comparison has been carried out for the

polymer textured at 100 W with 5 and 20 min O2 etching.

In order to get water repellency both a fluorocarbon and a

silicone-like chemistry have been utilized for the deposition

and tested, while for the superhydrophilic behavior a silica-

like coating has been chosen. The chemical composition of

the selected plasma coatings can be observed in Figure 5

where the infrared absorption spectra are reported. The

fluorocarbon coating (CFx) spectrum (Figure 5a) shows the

typical absorption band at 1 250 cm�1 of CFx stretching

modes and the amorphous band at 750 cm�1. In the case of

the silicon containing coatings (Figure 5b and c) the huge

difference in chemistry when O2 is present is evident:

oxygen addition scavenges hydrocarbon groups leading to

an inorganic coating (SiOxhy-philic).[45] In the case of the

SiOxhy-philic coatings the Si�OH absorption band at

930 cm�1 is present supporting the hydrophilic character

of the coating. On the other hand the FTIR spectrum of the

SiOChy-phobic film presents the typical features of silicone-


Figure 5. FTIR spectra of (a) CFx, (b) SiOChy-phobic, and(c) SiOxhy-philic coatings.

124


like coatings: Si(CH3)x group absorption bands (1 260,

860 cm�1) and CHx stretching.

The hydrophobic behavior of the CFx and SiOChy-phobic

coatings can be appreciated onto flat polished silicone

substrates as reported in Table 4. It can also be observed that

the flat coatings obtained from the organosilicon/O2

discharge lead to a hydrophilic surface.

As expected the deposition onto nanotextured PC largely

modifies the wetting behavior. Care was taken to verify by

SEM observation that coatings did not alter morphological

profile of the textured substrate. Considering the hydro-

phobic surfaces, it can be observed that even the 5 min

etched PC exhibits an increase of both advancing and

receding WCA, with a reduction of the hysteresis. The

20 min etched PC with SiOChy-phobic coating shows the

highest water repellent behavior, in fact both uadv and urec

have the highest values. This can be better emphasized

considering that the shear adhesion work is proportional to

the difference of the cosine of advancing and receding

contact angles (Equation 2), DcosQ, and the corresponding

values for the surfaces described in this paper are reported

Table 4. Water contact angle (WCA) and hysteresis of plasma coate

Coating Flat

Qadv Qrec DcosQ Qadv

CFx 118 90 0.470 162

SiOChy-phobic 118 101 0.280 162

SiOxhy-philic 36 26 0.090 9



in Table 4. It can be observed that such difference (and in

turn the adhesion) is by far the lowest for the hydrophobic

silicone-like chemistry used. It should also be stressed that

the corresponding tensile work of adhesion (Equation 3),

which is proportional to cosurec is the lowest. However, this

result is in contrast with what found on the 5 min textured

surface indicating a more hydrophobic value for the CFx-

based chemistry. Even if the surfaces were observed with

SEM to check for morphological differences after coating, it

is likely that, since the features obtained for the 5 min

etched PC are quite small, more accurate morphological

analysis could evidence differences for the two different

hydrophobic coatings.

A further comment should be done for SiOChy-phobic

deposited on the 5 min textured surface: even if the

difference between uadv and urec is close to the value

obtained on the flat surface, the shear work of adhesion is

more than halved and the corresponding tensile one

should be much lower (more negative value of cosurec).

This consideration can be useful for understanding that

the use of the mere difference between uadv and urec to

describe the wettability dynamics of hydrophobic surfaces

could be inappropriate.

Corresponding WCAs for coating deposited onto 20 min

etched PC are displayed in Figure 6, where pictures taken

from the WCA goniometer are reported. The slight

difference in urec between the fluorocarbon and the

silicon-based coating can be appreciated. Finally, it should

be commented that even after 5 months the wettability

properties of such surfaces do not change.

Turning attention to the hydrophilic textured surfaces,

SiOxhy-philic, two features are important: a drastic reduction

of WCA results comparable, or even lower, with the values

obtained for the etched PC, and the almost negligible WCA

hysteresis. In particular for the 20 min etched surface uadv

and urec as low as 48 and 28 are obtained, respectively; the

corresponding image of the drop for WCA measurement is

shown in Figure 6. Furthermore, as expected in super-

hydrophilic Cassie–Baxter regime, the impregnation phe-

nomenon was observed. Some comments could also be

done in this case concerning the work of adhesion. As

reported in Table 4 the shear contribution is very low since

proportional to DcosQ, and in fact easy sliding of the flat

d and textured PC.

5min texturing 20min texturing

Qrec DcosQ Qadv Qrec DcosQ

154 0.052 169 160 0.042

148 0.103 173 171 0.005

6 0.007 4 2 0.002

DOI: 10.1002/ppap.201000098

Figure 6. Pictures taken from WCA measurement on coated PCtextured for 20min in oxygen plasma.


drop of water could be observed when slightly tilting the

substrate holder. On the other hand the force necessary to

pull out the drop (tensile work of adhesion) is much higher

than in the hydrophobic case since cosurec is close to 1. This

data fulfill the description of McCarthy et al.[18,19] who

refers to this behavior as ‘‘tensile hydrophilic’’ for surfaces

having WCA lower than 908 and low hysteresis, where

water drops can easily move on the surface, but cannot fall

down when the substrate is held upside down. To conclude

the description of such superhydrophilic surface stability in

air has been evaluated measuring WCA after the deposition

of the SiOxhy-philic for the 5 min textured surface on a period

of 5 months: the advancing contact angle do not passed 108,indicating no aging (in terms of wettability).

Notwithstanding the encouraging results, it should be

considered that, mainly in the superhydrophilic case, the

character could be not reversible: it can be supposed

that upon water contact the pillars could change the

morphology, and even aggregation promoted, as shown by

Journet et al. in reference[46] or Bico et al. in reference[47] In

this case wettability could change on samples dried after

wetting. However to this aspect of the topic a following

study will be dedicated to understand the phenomena but

also for searching methods to get reusable surfaces.

Conclusion

This work highlights the suitability and effectiveness of

plasma processing in preparing both superhydrophobic

and superhydrophylic surfaces from PC. It is demonstrated



that a certain control of polymer nanostructure size can be

accomplished by tuning the experimental plasma para-

meters during oxygen etching. It is also shown how

coupling PECVD with previous plasma etching can support

matching the wettability criteria with surface chemistry

needs switching from silicone- to silica-like and to

fluorocarbon composition.

It should also be underlined that this work give a relevant

experimental contribution to the investigation of super-

hydrophilic nanotextured polymeric surfaces, a field yet not

fully exploited with respect to the superhydrophobic one.

Acknowledgements: Carmine Urso is kindly acknowledged forthe experimental assistance.

Received: July 21, 2010; Revised: November 6, 2010; Accepted:November 9, 2010; DOI: 10.1002/ppap.201000098

Keywords: nanostructures; plasma etching; polycarbonates;superhydrophilic; superhydrophobic; surfaces

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DOI: 10.1002/ppap.201000098

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