fabrication of polyethylene superhydrophobic surfaces by stretching-controlled micromolding

Full Paper

Fabrication of Polyethylene SuperhydrophobicSurfaces by Stretching-ControlledMicromolding

Jie Feng,* Mingda Huang, Xin Qian

This paper describes a novel process that is stretching-controlled thermal micromolding, tofabricate bionic superhydrophobicpolyethylenefilms. Low-densitypolyethylenewas thermallypressed inavacuumovenontoPDMSstamps replicated fromlotus leaves.Afterbeingcooledandpeeled off from the stamps, the polyethylene films withsuperhydrophobic surfacewere created, exhibiting awatercontact angle of 154.1� 3.58 and a rolling angle of�78. SEMimaging showed that the superhydrophobic surface hadmicro-papillas much higher than those on the lotus leaf,demonstrating the papillas had been stretched longer fromthe holes on the stamp during the separating process. Thisstudy shows that micromolding is a promising techniquefor largescaleproductionofsuperhydrophobicfilms,even ifthe holes on the mold are not deep enough.

Introduction

Superhydrophobic surfaces have attracted great attention

in recent years for their wide applications in water

repellency, anti-fog, anti-oxidation and self-cleaning.[1–3]

Many natural plant leaves have superhydrophobic proper-

ties due to their hierarchical structures. Inspired by this

principle, attempts to produce artificial superhydrophobic

films have been intensively exploited in recent years.[4–14]

These attempts generally combine appropriate surface

roughness with low-surface-energy materials but mainly

focus on creating micro-/nanostructures on hydrophobic

substrates.

So far, the reported methods to prepare superhydro-

phobic surfaces include phase separation,[4] electrochemi-

cal deposition,[5] chemical vapor deposition,[6] fluorinated

J. Feng, M. Huang, X. QianDepartment of Materials Science & Engineering, Zhejiang Uni-versity of Technology, Hangzhou, 310014, P. R. ChinaE-mail: [email protected]

Macromol. Mater. Eng. 2009, 294, 295–300

� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

materials roughening,[7] template-based extrusion,[8]

solvent-induced crystallization,[9] Teflon film extending,[10]

polymer electrostatic spinning,[11] microsphere/inorganic

materials arraying,[12] etching,[13] lithography,[14] etc.

Some of these methods drove the fundamental research

and some of them hold great promise for the development

of industrial products. However, most of these promising

methods could not be combined with present commercial

techniques for producing plastic films such as the

manufacturing of polyolefin film by the cast method.

During the past decade, soft lithography as a new

microfabrication technique has been well developed to

prepare various patterned or roughened surfaces by using

elastomeric stamps.[15] By using the stamps replicated

from lotus leaf and an epoxy-based azo polymer solution

as ‘‘ink’’, Wang’s group[16] fabricated mimicked super-

hydrophobic surfaces through pressing the featured faces

of the stamps against ‘‘inked’’ substrates and drying under

a proper condition after peeling off the stamps. Similar

bionic superhydrophobic surfaces have also been created

by Hang Ji et al.,[17] where they first replicated a negative

DOI: 10.1002/mame.200800331 295

J. Feng, M. Huang, X. Qian

Figure 1. Scheme of stretching-controlled micromolding processfor fabricating polyethylene superhydrophobic surfaces.

296

poly(dimethylsiloxane) (PDMS) template from fresh lotus

leaf and then performed a second replication still using

PDMS on this negative template by adding an antistick

monolayer in advance. In a more recent report,[18] highly

hydrophobic surfaces have been prepared directly by

replicating plant leaf surfaces with a nickel mold through

UV-nanoimprinting lithography.

Polyolefin is widely used in applications where hydro-

phobicity and clean surfaces are required. Its films are

generally produced by the melt ‘‘cast method’’. If we can

make the mold used in cast method similar to the negative

template replicated from lotus leaf, the polyolefin films

with hierarchical structure and superhydrophobicity

would be obtained in large scale. Based on this idea,

thermal-press micromolding was studied as a model of the

modified cast method. The resulted superhydrophobic

films micromolded in a vacuum oven were found

possessing surface micro-papillas much higher than those

on the films micromolded in a general oven due to

stretching caused by interaction between the polyolefin

and the PDMS stamp. This finding implies that the cast

method may be modified to produce superhydrophobic

polymer films, even when the holes on the mold are not

deep enough.

Experimental Part

The PDMS stamp was fabricated using the method described by

Hang Ji.[17] Briefly, the mixture of elastomer base and curing agent

(Sylgard 184, Dow Corning) with a proper ratio of 10:1 was cast

onto a fresh lotus leaf collected from Hangzhou West Lake. After

solidification at room temperature for 24 h and being peeled off

from the lotus leaf, the PDMS stamp with a complementary

topographic surface structure of the lotus leaf was created. The

thickness of the stamp can be controlled by the thickness of the

annular mold fixed on the lotus leaf in advance. A PDMS stamp

with length, width and thickness of 10 cm �10 cm �0.2 cm was

used in the following micromolding procedure.

Low-density polyethylene (LDPE, N220, SINOPEC Shanghai

Petrochemical Co. Ltd.) was selected to be hydrophobilized due

to its wide application, high commercial value and low melting

temperature (Tm� 140 8C). In the thermal micromolding process,

the LDPE particles were first accumulated on the featured surface

of the PDMS stamp that was placed on a piece of glass slide, and

then another piece of glass slide was put on the LDPE and two

pinchcocks were used to apply a certain pressure (�500 g � cm�2)

between the LDPE and the PDMS stamp. Then the whole construct

was incubated in a vacuum oven (�0.1 MPa) for 20 min at

temperatures up to 220 8C. After cooling to room temperature, the

PDMS stamp was peeled off and LDPE films with topographic

surfaces were obtained. As a control, a similar process was

performed (�500 g � cm�2, 220 8C, 20 min), but using a general

oven, for checking the difference of the resulted films (Figure 1).

Surface morphologies of the PDMS stamps and the micro-

molded LDPE films were imaged by a field emission scanning



electron microscopy (FE-SEM, Hitachi S-4700) with the accelerat-

ing voltage of 15 kV. All the samples prepared for the SEM study

were sputtered with thin layers of platinum about 10 nm in

thickness. The water contact angles (CAs) were measured with a

sessile drop method using a DSA10-MK2 contact angle measuring

system from Kruss. The droplet volume was 4 mL. For those

droplets that could not be adsorbed from the needle tip onto the

surface, due to too hydrophobic a tweezer with acuate tip was

used to transmit them with a certain force. The rolling angle was

defined as the surface inclining angle at which the droplets began

to roll down and measured by inclining the superhydrophobic

surfaces gradually from 08 to higher angles till the droplets rolled.

All measurements were carried out under ambient conditions and

the final data were averaged by six measurements. Dynamic

contact angle was also given by following the CAs changes within

10 min to further confirm the superhydrophobicity.

Results and Discussion

Although some researchers have already confirmed the

feasibility of replicating PDMS negative stamps from fresh

lotus leaves, here we still present the SEM images of the

stamps that we made. Completely reverse with the

papillas on the lotus leaf, randomly but proportionally

spaced microholes were observed on the PDMS stamp

(Figure 2). Higher-magnification images (see inset) show

that the average diameter of the cone-shaped holes near

the top of the surface is �8 mm and the depth is �8 mm.

This corresponds with the well known sizes of the papillas

on the lotus leaf,[2] demonstrating that the hierarchical

structure on the lotus leaves had been accurately

transferred onto the negative stamps. However, how to

further transfer the PDMS structure onto polymer films as

accurately as possible is still a challenge.

Wang et al.,[16] Hang et al.[17] and Losic et al.[19] have

successively created bionic superhydrophobic surfaces by

casting and curing liquid pre-polymers on PDMS stamps

DOI: 10.1002/mame.200800331

Fabrication of Polyethylene Superhydrophobic Surfaces by . . .

Figure 3. SEM images of LDPE films micromolded in vacuum oven(a), general oven (b) or by first melting LDPE in vacuum oven andthen pressing in general oven (c).

Figure 2. SEM images of the PDMS stamp replicated from a lotusleaf. The inset is a highmagnification of a single hole viewed fromsection.

replicated from the fresh lotus leaves. However, realizing

superhydrophobicity directly by thermal-pressing poly-

mer onto microstructured mold is still a challenge,[20]

while this may be the most prospective method for

producing superhydrophobic films industrially. The factors

influencing the success of micromolding are probably the

distortion caused by the high viscosity of the melted

polymer and the air trapping within the stamp holes. To

eliminate the effect of these factors, vacuum was used here

to remove the trapped air and fill the melted polymer into

the stamp holes as deeply as possible.

Figure 3 shows the topographical structures of the

resulted LDPE films. It can be seen that two different kinds

of papillas were formed when using different ovens for

micromolding. The papillas on LDPE films micromolded in

the vacuum oven were much higher and slimmer (height

30–40 mm, diameter 4–6 mm) than those on the lotus leaf.

However, the papillas on films micromolded in the general

oven seemed to be as low as those on the lotus leaves

(height �8 mm, diameter 6–8 mm). The higher papillas

should be formed by sufficient filling of the melted

polymer into the holes of the stamp and subsequent

stretching from the holes during separating operation. The

interaction caused by firm and all-sided contact at

polyethylene-PDMS holes interface should be responsible

for such stretching. The lower papillas should be formed

with little stretching caused by weak interaction. Peeling

off operation also demonstrated the above discussed

analysis. It is found that it is easy to peel off the stamp

from the films micromolded in the general oven while this

is difficult for the films molded in the vacuum oven.

To further demonstrate above inference, a similar

process was performed later. Briefly, the LDPE was first

melted on the featured surface of the PDMS stamp in a

vacuum oven at the temperatures up to 220 8C. Then

the vacuum oven was opened and a certain pressure

(�500 g � cm�2, holding for 10 min) was applied between

the melted polyethylene and the PDMS stamp via two



pieces of glass slides and two pinchcocks. After cooling to

room temperature, the stamp was peeled off and the LDPE

film with topographic surface structure was obtained. The

SEM image (Figure 3c) shows that two kinds of papillas

were formed simultaneously on the same film: higher ones

in most of the areas (middle) and lower ones in the little

edge area. The higher papillas should be formed by

vacuum assistance while the lower papillas on the edge

should be formed without vacuum application since it was

after the vacuum oven was opened that the pressing

operation was performed. They corresponded to the melted

LDPE extruded by pressure from the stamp middle to outside.

Surface hydrophobicity, especially superhydrophobi-

city, is a very straightforward property of most solid

www.mme-journal.de 297


Figure 5. The rolling angle (the inset image) and the dynamiccontact angle of LDPE films micromolded in vacuum oven.

298

surfaces. Stream from a water bottle or even a water pipe

can be used to qualitatively estimate surface super-

hydrophobicity. However, a more quantitative under-

standing of the superhydrophobic surface should be made

by contact angle measurements. Figure 4 is the CCD

camera images of 4 mL water droplets on LDPE films

micromolded in a vacuum and a general oven. The

corresponding CAs were 154.1� 3.58 and 137.0� 2.78,respectively. In addition, water drops were found rapidly

skipped over a slightly inclined (�78) LDPE film micro-

molded in vacuum oven but the same phenomenon has

not been found on the LDPE film micromolded in the

general oven demonstrating that the former surfaces were

superhydrophobic (Figure 5, see inset). Dynamic contact

angle study further confirmed that this superhydropho-

bicity was stable (Figure 5). Table 1 gives the CAs of LDPE

films micromolded at different temperature (180 8C, 200 8Cand 220 8C) but all in general oven. It can be seen that these

CAs ascended as the temperature increased but all were

lower than 1508, demonstrating viscosity and filling extent

do have important influence on the surface microstructure

and hydrophobicity.

The LDPE films micromolded either in vacuum oven or in

general oven both showed higher surface hydrophobicity

than the smooth film (CA 998). This can be explained by

Figure 4. The CCD camera images of 4 mL water droplets on LDPEfilms micromolded in vacuum (a) or general (b) oven.



Wenzel’s theory and Cassie’s composite contact theory.[21]

According to these two classical theories, the apparent CAs

(ur) can be given as the functions of the geometric

parameters of the microstructures on the surface:

Tabtur

Tem

-C

180

200

220

cos uwr ¼ 1 þ 4H=a

ð1þb=aÞ2

� �cos ue (1)

cos ucr ¼ 1þcos ueð1þb=aÞ2 � 1 (2)

where a, b and H can be seen as the papilla’s width, space

and height, respectively. ue is the intrinsic CA of smooth

LDPE surface (998).According to the Equation (1), if b/a on micromolded

LDPE films with higher (H/a¼ 8) and lower papillas (H/

a¼ 1.5) were estimated as 4 and 2 respectively, their

corresponding theoretical Wenzel’s CAs were calculated as

1118 and 1028. While according to the Equation (2), their

apparent Cassie’s CAs could be calculated as 1658 and 1558,respectively. The experimental results were 154.1� 3.58and 137.0� 2.78 correspondingly, both much closer to

Cassie’s theoretical predictions than to Wenzel’s predic-

tions, demonstrating the water droplets on both two kinds

of films should have trapped much air (Figure 6). The

higher CA value on the LDPE films micromolded in the

le 1. The CAs of LDPE filmsmicromolded at different tempera-e, however prepared all in the general oven.

perature Contact angle

-

126.4� 3.1

134.4� 3.4

137.0� 2.7

DOI: 10.1002/mame.200800331

Fabrication of Polyethylene Superhydrophobic Surfaces by . . .

Figure 6. Scheme of water droplets on LDPE films with high (a) orlow (b) papillas.

vacuum oven should attribute to the higher ratio of gas-

liquid contact area to solid-liquid area (Figure 6a). Similar

explanation have also been reported by Feng et al.,[22]

Patankar et al.,[23] and Hess et al.[24].

So far, lots of attempts to mimick lotus leaf have

achieved great success, however, a simple and economical

procedure still remains to be found. Compared with other

soft lithography techniques such as cast molding or

imprinting liquid curable pre-polymers, thermal replica

molding plastic polymers may be the most prospective

technique for producing superhydrophobic surface in a

large scale because it is closest to the present commercial

technique for manufacturing polymer films, e.g., the cast

method. However, researches found it difficult to get

superhydrophobic surfaces by simply hot-pressing melted

polymers onto negatively microfabricated molds.[20,25]

This may be caused by the incorrect replication due to

the high melting viscosity: nano structures could not be

replicated easily.

A notable feature of our work is that it implied us

Cassie’s papilla can be produced by stretching-controlled

micromolding process, even the microholes on the stamp

or on the mold are not deep enough compared with the

resulted papillas. This is a meaningful finding because

fabricating deep holes on industrial mold is not an easy

task,[26,27] generally they are produced by stainless

materials. If we could obtain randomly but statistically

proportional spaced microholes on stainless rolling molds,

LDPE or other polymer superhydrophobic films would be

produced by stretch-molding in large scale, just like

producing common plastic films by using the traditional

cast method. Research is currently being carried out with

the aim of finding a feasible process by which such a

stainless rolling mold could be produced.



Conclusion

A new approach, the stretching-controlled thermal micro-

molding, has been developed to fabricate polyethylene

superhydrophobic surfaces. It mainly applies the interac-

tion between the mold and the LDPE to remarkably stretch

the papilla and thus endow the LDPE films superhydro-

phobicity. Although this approach is exemplified by using

a PDMS stamp and vacuum assistant filling, it can be very

possibly expanded into systems using stainless rolling

mold and normal pressure based on the same ‘‘interaction-

controlled stretching’’ principle. This study would be

helpful to develop an industrial technique for mass-

producing superhydrophobic polymeric films.

Acknowledgements: The financial support from the NationalNatural Science Foundation of China (Grant No. 50703037),Zhejiang Natural Science Foundation (Y407256) and ZhejiangQianjiang Project (2007R10005) is gratefully acknowledged.

Received: November 14, 2008; Revised: February 3, 2009;Accepted: February 3, 2009; DOI: 10.1002/mame.200800331

Keywords: LDPE; micromolding; stretching; superhydrophobic

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DOI: 10.1002/mame.200800331

fabrication of polyethylene superhydrophobic surfaces by stretching-controlled micromolding

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