Full Paper
Formation of Ordered Two-Dimensional
Polymer Latticeworks With Polygonal Meshes
by Self-Organized Anisotropic Mass Transfera
Wei Li, Yaru Nie, Junhu Zhang, Difu Zhu, Xiao Li, Haizhu Sun, Kui Yu,
Bai Yang*
This manuscript addresses the formation of self-assembled two-dimensional (2D) polymer
latticeworks with multiple polygonal meshes packed in various ordered arrays. Firstly, ordered
arrays of water droplets were formed in the hydrophilic regions of patterned self-assembled
monolayers (SAMs) consisting of isolated hydrophilic circles surrounded by a continuous
hydrophobic region. After dip-coating this water-patterned surface into a polymer solution in
chloroform, dewetting of the polymer solution led to the formation of a crater-like porous
polymer film. Next, the resulting polymer film with round pores arranged in a 2D ordered
array was subjected to a thermal treatment carried out at a temperature higher than the
glass-transition temperature (Tg) of the polymer. The thermal annealing process resulted in a
morphological transformation from circular pores into polygonal meshes packed in either a
similar or different ordered array. This morphological transition is self-organized, involving
mass transfer, an anisotropic process, and is controlled by the minimization of the Gibbs free
energy. An empirical equation was established to guide the experiments. Thus, the patterned
features of the polymer meshes
can be designed via the ordered
arrays of the hydrophilic circles
of the SAMs as well as by experimental parameters such as the
concentration of the polymer
solution. The formation of the
polymer latticework with polygonal meshes reveals that selforganized mass transfer can be
applied in micropatterning by
elaborate experimental design.
W. Li, Y. Nie, J. Zhang, D. Zhu, X. Li, H. Sun, B. Yang
Key Lab for Supramolecular Structure and Materials, College of
Chemistry, Jilin University, Changchun 130012, P. R. China
Fax: þ86 0431 8519 3423; E-mail: [email protected]
K. Yu
Steacie Institute for Molecular Sciences, National Research
Council, 100 Sussex Drive, Ottawa K1A 0R6, Canada
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a
: Supporting information for this article is available at the bottom
of the article’s abstract page, which can be accessed from the
journal’s homepage at http://www.mcp-journal.de, or from the
author.
DOI: 10.1002/macp.200700469
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W. Li et al.
Introduction
In modern science and technology, patterning technologies have been extensively applied in various fields[1] such
as the fabrication of optical devices,[2] biochips,[3] electronic devices,[4] sensors,[5,6] actuators,[7] and microelectronic
circuits.[8] Traditional photolithographic technology as
well as several advanced lithographic technologies[9,10]
have been well developed over the past few decades. These
techniques are very successful in producing structures and
patterns for diverse applications. However, they suffer
from high cost and a lack of flexibility for application to the
broader range of materials of growing modern technological interests. The need to overcome these problems has
led to the emergence of many alternative techniques, such
as soft lithography,[11] self-assembly,[12] and embossing.[13] These new techniques have attracted significant
attention because they offer the possibility of patterning
various materials over large areas at lower cost and higher
throughput.
In recent years, there has been an increasing interest in
polymeric materials with ordered microstructures, due to
their potential applications in many areas.[2–7] Polymeric
materials usually exhibit unique properties, such as a glass
transition and polymer compatibility, as compared to
other materials. Therefore, in addition to conventional
lithography, patterning techniques employing polymers
have been developed, in addition to increasing our
knowledge on controlling the surface morphology of
various polymeric assemblies. More recently, some
researchers have begun to focus on the surface instabilities
of viscous polymer thin films. Such instabilities often lead
to disordered structures and can be controlled by elaborate
experimental design. As an example, Lee et al. demonstrated that the instability of anisotropic buckling in a thin
polymer film could be employed to pattern polymeric
materials.[14,15] The utilization of the phase-separation
behavior of both polymer mixtures and block copolymers
has also been reported, where the instabilities of phase
separation are usually controlled by an underlying prepatterned variation of surface energies[16] or strong
electrical fields.[17] Schäffer showed that sub-micrometer
polymer patterns can be fabricated by controlling both the
laterally-modulated destabilizing force and the destabilization of a thin polymer film that is exposed to an external
electric field or a high-temperature gradient.[18,19] Moreover, to induce the surface pattern of polymeric films,
recent studies have indicated that the utilization of
instabilities in dewetting processes may be a promising
strategy.[20–23]
Herein, we report our study on the self-organized
formation of two-dimensional (2D) polymer latticeworks
with multiple polygonal meshes arranged in ordered
arrays. The process is straightforward and consists of the
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dewetting of a polymer solution leading to a ‘‘crater-like’’
porous polymeric film with a subsequent thermal treatment that results in a morphological transition, and thus
the formation of an ordered polymer latticework with
polygonal meshes. The dewetting process of the polymer
solution takes place on a water-patterned substrate which
is generated via the condensation of water droplets on
underlying patterned self-assembled monolayers (SAMs)
consisting of hydrophilic domains distributed in a hydrophobic matrix.[20,23] The morphological transition was an
anisotropic process with mass redistribution in directions
determined by the features of the porous polymeric film,
which replicated the original chemical pattern on SAMs.
This mass-transfer process was a self-organized process
controlled by the minimization of the Gibbs free energy.
Therefore, various polygonal features of the polymeric
meshes could be fabricated simply by adjusting the
original SAMs on gold substrates.
Results and Discussion
Formation of Ordered Polymer Latticeworks with
Regular Polygonal Meshes on Patterned SAMs
Figure 1 shows a schematic of the overall strategy for the
fabrication of ordered polymer latticeworks with polygonal meshes on gold substrates. The fabrication of these
Figure 1. Schematic outline of the procedure used to fabricate
ordered polymer latticeworks with regular polygonal meshes
supported on gold substrates.
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Formation of Ordered Two-Dimensional Polymer Latticeworks . . .
polymer latticeworks involves two main stages: the
formation of a polymer film with circular pores arranged
in an ordered array and the subsequent self-organization
of the polymer film into a polymer latticework with
polygonal meshes packed in a similar or different array.
The first stage is achieved by dewetting a polymer solution
coated on water-patterned SAMs, the second stage takes
place during a thermal annealing process carried out above
the glass-transition temperature (Tg) of the polymer.
Previous studies have demonstrated that with techniques, such as conventional lithography,[24] microcontact
printing (mCP) and vapor deposition,[25] various solid
surfaces can be patterned at micrometer-sized areas with
different wettabilities. When a liquid film is deposited on
these surfaces, the liquid will selectively dewet specific
areas and wet the remaining areas, transferring the
patterns on solid surfaces into liquid microstructures. In
our experimental approach, patterned SAMs with domains
of different wettabilities on flat gold substrates were
fabricated through mCP,[26] with water condensation taking
place on the hydrophilic areas. As regards the patterned
SAMs shown in Figure 1, the isolated and circular domains
are COOH-terminated hydrophilic SAM regions, surrounded by a continuous matrix of CH3-terminated
SAM. Exposing the patterned SAMs to a water-saturated N2
flow formed water droplets on the hydrophilic regions in
ordered arrays, replicating the original SAM pattern due to
the selective wetting of water on the two different surface
domains. This principle has been successfully applied to
generate liquid-patterned condensation figures[25,26] and
2D inverse patterns can be further fabricated by dewetting
these condensation figures from immiscible solutions of
materials.[20] As shown in Figure 1, the water-patterned
SAMs on gold substrates were dipped into a polystyrene
(PS) solution in chloroform and immediately withdrawn.
After the complete evaporation of chloroform and water, a
PS film with circular pores arranged in an ordered array
remained. Our previous results have revealed that the
polymer concentration in the chloroform solution plays an
important role in this dewetting process.[23] Ordered arrays
of polymer rings were formed instead of the porous
polymer films when the solution concentration was low
enough due to the occurrence of a second dewetting
process during solvent evaporation. For the PS solution
used in the present study, with a molecular weight of
280 000 (Mw ), the critical concentration was 2 mg mL1.[23]
Accordingly, the concentrations of the PS solutions used
here were always higher than 3 mg mL1 to attain polymer films with ordered pores.
Figure 2A shows a field-emission scanning electron
microscopy (FESEM) image of a porous PS film created from
an 8 mg mL1 PS solution in chloroform. The circular areas
with higher transmission in the central part as compared
to the periphery are holes that replicate the condensation
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Figure 2. (A) FESEM image of the porous polymer film; the inset
gives a sketch of the underlying chemical pattern; (B) FESEM
image of the polymer latticework with regular square meshes
after thermal annealing; the inset shows a magnified image. The
sample was fabricated by dewetting from an 8 mg mL1 PS
solution.
figure after water evaporation. The ordered array of water
droplets replicates the hydrophilic areas of the patterned
SAMs; thus, the diameter of the holes in the PS film can
be readily controlled using different mCP patterns. The
corresponding chemical pattern is presented in the inset of
Figure 2: the diameter of the hydrophilic circles is 10 mm
and the nearest center-to-center distance is 14 mm.
Figure 2B shows an image of the polymer latticework
morphology that resulted by annealing the porous
polymer film shown in Figure 2A thermally for two hours.
During the process of thermal annealing to fabricate
polymeric microstructures, the polymer film is softened at
a temperature much higher than the Tg of the polymer to
attain fluidity.[27] In the present study, thermal treatment
was carried out at 150 8C, whereas the Tg of the PS used
is 100 8C. Figure 2 clearly displays the morphological
transition from polymer film with circular pores arranged
in a square array to a polymer latticework consisting
of square meshes. Figure 2B is indicative of the level
of perfection, ordering, scale, and areas that could be
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Figure 3. Multiform polymer latticeworks with regular polygonal meshes fabricated by this method (A) hexagons meshes; (B) triangles
meshes; (C) rhombuses meshes; (D) trapezoids and pentagons meshes. The insets demonstrate sketches of the chemical pattern
predesigned for these structures. All of the samples in this Figure were fabricated from an 11 mg mL1 PS solution.
achieved routinely using our process. An FESEM image with a
higher magnification is shown in the inset of Figure 2B. The
width of the square meshes is approximately 1.6 mm,
which is much less than 4 mm, the closest distance
between two adjacent hydrophilic circles on the original
patterned SAMs.
In order to understand the dynamic development of the
morphological transition from circular pores to square
meshes during the thermal treatments, we recorded this
process in situ with an optical microscope equipped with a
CCD camera (the video file is available as Supporting
Information). The optical microscopy demonstrates that
the circular pores of the polymeric film are dilated during
the annealing process. Whereas the holes are dilated in all
directions, the dilation is anisotropic in nature, with four
directions demonstrating a relatively faster rate of
dilation. The four directions of faster dilation are indicated
by the black square and black arrows in the inset of Figure
2A. These directions correspond to the diagonals of the
black square that is formed from four symmetry centers of
the square interstices among the four adjacent circular
holes. Consequently, the circular holes with a closest
distance of 4 mm are transformed to square meshes with a
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much smaller frame width as a result of the dilation
process.
The in situ monitoring of the annealing process strongly
suggests that the morphological transition stems from an
anisotropic dilation of the circular pores. Additionally, the
dilation is influenced by the arranged features of the
circular pores, which comes from the hydrophilic circular
regions of the printed SAMs. Therefore, the final morphological features of the final polymer latticework can readily
be tuned with suitable mCP patterns. The insets in
Figure 3(A–D) demonstrate four examples of patterns
and the corresponding polymer latticework morphologies
obtained are shown in Figure 3(A–D), respectively. The
hydrophilic circles are 10 mm in diameter, with a shortest
center-to-center distance of 14 mm, however they are
designed with different ordered arrays. Multiple polygonal
meshes such as hexagons, triangles, rhombuses, trapezoids
and pentagons were successfully fabricated by generating
water condensation figures on these patterns and applying
an 11 mg mL1 PS solution. Black arrows and black
polygons are shown in the insets to indicate the directions
of faster dilation. For the mCP patterns shown in Figure 3A
and Figure 3B, the circular holes have six and three
DOI: 10.1002/macp.200700469
Formation of Ordered Two-Dimensional Polymer Latticeworks . . .
directions separately of faster dilation during the dilation
process. This ultimately results in hexagonal and triangular meshes. There are four directions of faster dilation for
the circular holes shown in Figure 3C, with two of those
directions being faster than the other two (denoted by the
long black arrows in the inset of Figure 3C). As these two
directions offered more space for dilation, the resulting
pattern that formed was a rhomboidal mesh. For the
Figure 4. (A) AFM image of porous polymer film; the diameter of the original underlying hydrophilic circles is 10 mm; (B) AFM image of the
morphology after thermal annealing corresponding to the sample in (A); (C) AFM image of porous polymer film, the diameter of the original
underlying hydrophilic circles is 8 mm; (D) AFM image of the morphology after thermal annealing corresponding to the sample in (C). The
period of the chemical pattern for the samples in this Figure is 14 mm. All of the samples were fabricated from a 13 mg mL1 PS solution.
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porous polymer film formed from the pattern shown in
the inset of Figure 3D, there are two types of dilation: one
labeled with dark solid arrows, the other indicated by dark
dashed arrows. The former has one direction with the
fastest rate of dilation and four directions with the second
fastest rate of dilation, while the latter has two directions
with the fastest rate of dilation. Accordingly, the resulting
polymer latticework has an ordered array of pentagonal
and trapezoidal meshes together.
As the FESEM images in Figure 3 indicate, the formation
of these polymer meshes with polygonal features packed
in different ordered arrays demonstrates that the ordered
arrays of the circular pores of the polymer films affect their
self-organization with anisotropic mass transfer during
thermal treatments. Thus, the geometry of the ordered
polymer meshes can be controlled through the proper
design of the chemical patterns on the original SAMs.
Mechanism of the Formation of Polymer Latticeworks
with Polygonal Meshes
Figure 4A and Figure 4B show atomic force microscopy
(AFM) height-mode images of two samples produced by
dewetting from a 13 mg mL1 PS solution, with the latter
sample produced by thermal annealing for 2 h after
dewetting to generate the polymer latticework structure.
The pattern features of the underlying SAMs are the same
as in Figure 2. Figure 4A clearly shows that the circular
pores are crater-like with high rims and low interstices
between adjacent pores. Moreover, height analysis reveals
the thickness between neighboring pores along the shorter
line in Figure 4A is greater than that along the other line. It
is obvious that polymer films become softened and mobile
during the thermal-annealing process. Thermodynamically, the stability of a fluid film is assumed to be governed
by the change in Gibbs free energy[28] and a thermodynamic system will spontaneously tend to go to the
lowest Gibbs-free-energy state. Accordingly, during the
thermal treatment, the crater-like porous PS film that is in
a relatively-higher Gibbs-free-energy state has a microstructure that is difficult to preserve and surface tension
will result in shrinkage of the surface area as a result of the
minimization of surface free energy. Previous work in
the literature have reported that polymer films can be
dewetted from the substrate by an annealing-initiated
process.[15] Here, polymer transferred from the higher rims
to lower film regions together with volume shrinkage and
dewetting process afterwards to reduce the Gibbs free
energy of the system, to initiate the subsequent dilation of
the circular holes when the polymer film was annealed
above its Tg.
Additionally, the morphological features of the porous
polymer film give an anisotropic character to the dilation
process. As observed in Figure 4A, the average film
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thickness is different in different areas of the film. The
film becomes thinner when it is further away from
the circular rims. Specifically, the symmetry centers of the
square interstices among the four adjacent circular holes
have the smallest film thickness for the mCP pattern used
in Figure 4A. Obviously, the height intercept between the
rims and the polymeric valley of this crater-like morphology favors mass transfer in the dewetting process during
the annealing. Polymer will shrink and dewet from the
substrate faster in the directions pointing to thinner areas
to decrease the Gibbs free energy more efficiently.
Therefore, the dilation rate in the direction of the green
line that points to the symmetry centers of the square
interstices with a larger height intercept is faster than the
direction of the red line which points to the interstices just
between two neighboring holes. This self-organizing effect
directly drew the circular hole to the regular square
meshes (see the white arrows in Figure 4A) and other
geometries shown in Figure 3. Figure 4B demonstrates that
the height intercept disappeared and the height of the
polymer latticework structure was uniform after the
thermal annealing. This indicates that the driving effect
for the formation of polygonal meshes by the height
intercept of the crater-like porous polymer film disappeared after 2 h of thermal annealing. It is worth pointing
out that the polymer latticework structure is still not in the
lowest Gibbs free energy state; namely, it is only an
intermediate state. The annealing time should be chosen
carefully as less thermal annealing time was needed when
a polymer with lower Tg was used or if the thermal
annealing was performed at a higher temperature. Excess
thermal treatment would finally lead to disordered
structures. The FESEM image of a sample that was
annealed at 150 8C for 24 h is available as Supporting
Information.
In our experiments, another important prerequisite for
the formation of polymer latticework structures is that
the diameter of the circular hole must be larger than
2/3 the pattern period, the shortest center-to-center
distance between two adjacent holes, to guarantee that
there are enough film height intercepts in different
directions. If the diameter of the holes was less than this
value, the intercept was almost the same in all directions
and the circular holes exhibited an isotropic dilation
process in this case. Figure 4C and Figure 4D show AFM
height-mode images of two samples, both produced from
the same experimental conditions as the samples in
Figure 4A and Figure 4B. The diameter of the hydrophilic
circles used here was 8 mm, while the period remained
14 mm. From the height analysis of Figure 4C, we observe
that the intercepts are uniform in all directions. Thus,
the holes had an isotropic dilation process to form the
morphology in Figure 4D such that only a porous polymer
film with larger holes was fabricated after thermal
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annealing instead of polygonal polymer meshes. In this
situation, because the height intercept has disappeared, a
latticework structure with regular polygonal meshes could
not be formed, even after further thermal treatment. PS is
used as an example here, but our method could also be
extended to other polymeric materials. The only requirement seems to be that these materials have fixed Tg and
can be dissolved in a volatile organic solvent that is
immiscible with water.
Factors Influencing the Final Polymer Morphology
For many applications that utilize polymer-patterning
techniques, it is necessary for the morphological polymer
features to have the possibility of being easily adjusted. In
our method, the height and the width of polymer frames in
the fabricated latticework structure with polygonal
meshes were strongly influenced by the solution concentration and the pattern features of the original SAMs.
Figure 5 shows AFM height-mode images of four samples
produced from different experimental conditions.
The conditions for the samples shown in Figure 5A and
Figure 5B were the same, except for the solution
concentration, which was 6 mg mL1 and 15 mg mL1
respectively. The underlying scale of the surface chemical
pattern that consisted of hydrophilic circles was 10 mm
in diameter, separated by 4 mm. These two images indicate
that the dimensions of the polymer latticework frame
tended to increase both in width and height with
increasing concentration of the polymer solution concentration. Figure 5C has the same solution concentration, and
the same mCP pattern features as Figure 5A, however the
arrays of the hydrophilic circles were designed to form a
hexagonal latticework structure. Both the height and the
width of the polymer latticework frame decreased in this
Figure 5. (A) AFM image of a latticework structure fabricated by this method. The concentration of PS solution was 6 mg mL1, the
underlying chemical pattern was designed for square meshes. Hydrophilic circles were 10 mm in diameter and separated by 4 mm; (B) AFM
image of a latticework structure. The experimental conditions are same as for (A) except that the solution concentration was 15 mg mL1;
(C) AFM image of a latticework structure. The experimental conditions are same as for (A) except that the underlying chemical pattern was
designed for hexagon meshes; (D) AFM image of a latticework structure. The experimental conditions are same as for (A) except that the
hydrophilic circles were 12 mm in diameter and separated by 5 mm.
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W. Li et al.
Figure 6. (A) Sectional sketches of the polymer latticework structure and the dip-coating film of polymer solution on the water-patterned
substrate; (B) Curve of wh and C for different underlying chemical patterns.
case. In Figure 5D, the diameter of the underlying
hydrophilic circles was 12 mm and the period changed to
17 mm, with other conditions remaining the same as for
the sample shown in Figure 5A. The dimensions of the
polymer latticework frame were increased remarkably
as compared to Figure 5A. These phenomena can be
explained by a derived empirical equation that can also
be used to guide other experiments in our process. As
indicated in Figure 6A, Stage I is a cross-sectional sketch of
dip-coating a PS solution film on the water-patterned
substrate and stage II represents a polymer latticework
structure with polygonal meshes after thermal annealing.
Because of the law of conservation of matter, the polymer
mass in stage I should be equal to that in stage II. After
mathematical derivation, the empirical equation is in the
form:
wh ¼ 0:55
CPðH 0:67qrÞ
rpolymer
(1)
Where w is the width of the latticework frame, h is the
height of the latticework frame, C is the concentration of
the polymer solution, P is the period of both the underlying
chemical pattern and the final polygonal latticework
structure, H is the coating solution thickness, rpolymer is the
polymer density, q is the ratio of the total hydrophilic area
and the total substrate area, and r is the radius of the
hydrophilic circle. The details of the derivation are given in
the Supporting Information. Equation 1 clearly elucidates
the association between the wh of the final latticework
frame and the experimental conditions such as C, P, q and r.
The variable q here was varied for different modes of
design arrangement of the hydrophilic circles on the
chemical patterns, such as those shown in Figure 2 and
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Figure 3. It represents the influence of the underlying
chemical pattern and can readily be calculated for a fixed
pattern. H remains unknown in this empirical equation.
Usually, the thickness of a dip-coating film on a flat,
homogeneous substrate can be calculated by the Landau-Levich equation[29,30] when the capillary numbers mU/
s 1, which gives:
H ¼ 0:944
2
ðmUÞ =3
1
1
s =6 ðrGÞ =2
(2)
In Equation (2), m is the solution viscosity, U is the
withdrawal speed, s is the surface tension at the solution/
air interface, r is the fluid density and G is the gravitational
constant. The coating thickness is mainly defined by the
withdrawal speed, the solid content and the viscosity of
the liquid.[31] In the current work, all of the polymer
solutions were dilute polymer solutions (3 mg mL1 to 25
mg mL1 in chloroform solution with a solid content of
0.2-1%). m, r and s vary little in this concentration range
when the molecular weight of the polymer is only 280 000.
Moreover, the height of the water droplets strongly
influences H in our experiments and it was directly
dependent on the value, r. Based on the above analysis, H
will change little for a fixed chemical pattern and a
constant withdrawal speed; wh was in direct proportion to
C according to Equation (1) in the concentration range we
used here. The experimental results agreed well with this
conclusion. Figure 6B shows the relationship of wh and C
for different chemical pattern dimensions and arrangement modes. For a fixed pattern, we can find two points on
the straight line and forecast the concentration that is
needed for a given value of wh.
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Formation of Ordered Two-Dimensional Polymer Latticeworks . . .
Figure 7. (A) Photograph of PS film with perforated latticework structure floating on water surface; (B) Optical image of PS film with
latticework structure transferred onto Si substrate; (C) FESEM image of a PS latticework structure with square meshes on curved plane; (D)
Optical image of PDMS stamp fabricated by replica molding from a PS latticework structure on a Si substrate.
The physical meaning of wh is the cross-sectional area of
the frames of the as-prepared polygonal latticework
structure; thus, we can control the features of the
cross-sectional area of the latticework frames by changing
C, P, r and q according to the empirical equation, Equation
(1). This is especially useful for materials whose functionality is strongly influenced by their cross-sectional
area, such as conductive polymers. Experimental results
also indicated that w and h have the same variability, that
is to say, they increase and decrease together with changes
in C, P, r and q.
Applications of the Polymer Latticework Structures
with Polygonal Meshes
The polymer latticework structures with polygonal
meshes were formed during thermal treatment; thus,
there is no chemical bonding between the polymer film
and the underlying substrate. Because gold surfaces were
used as the substrates, first to generate the chemical
pattern of the patterned SAMs, and subsequently to
prepare the polymer latticework film, the film will lift off
of the substrate by etching the underlying Au layer. Since
most polymers are acid-resistant, we chose dilute a.u. regia
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to etch the gold layer.[32] Other etching solutions can also
be used according to previous work.[33,34] The porous
polymer films always float on the top of the water surface,
making it easy to separate them from the underlying
substrate. Figure 7A is a photograph of the PS film with
a square latticework structure floating on the water
surface after etching. Color scattering that results from the
microstructure of the film can be seen in this photo. The
area of the obtained polymer film with ordered latticework
structures could be up to several square centimeters
through careful control of the experimental process. Here,
the polygonal meshes are perforated, completely-open
microstructures. Generally, when using photolithography
and etching, the fabrication of completely-open ‘‘through’’
structures requires multiple lithographic steps, thus
increasing fabrication costs.[35,36] Our method offers a
facile way to fabricate similar structures. After peeling-off
the films from Au substrates, we can retrieve the films on
other substrates; even curved faces could be applied.
Figure 7B demonstrates an optical image of the polymer
latticework film transferred onto Si substrates. The film
was transferred into double layers by repeating this
operation for the purpose of proving that the structure
presented here was really transferred to the Si substrate. In
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W. Li et al.
Figure 7C, the polymer latticework structure was transferred to a curved face. During the transfer procedure from
the water surface, water always filled the interface
between the polymer film and the substrates just after
the film was retrieved, whereas the surface tension of the
water ensured that the soft polymer film kept its
morphology and did not fold. Figure 7B and Figure 7C
show that the polygonal latticework structure could be
maintained well after the complete evaporation of the
water. For the reason that it is difficult for traditional
photolithographic strategies to fabricate microstructures
on curved surfaces, our method also provides a pathway to
curved surface micropatterning. Figure 7D shows an
optical image of a polydimethylsiloxane (PDMS) stamp
that was fabricated by replica molding from a PS latticework structure with square meshes. These ordered
polymer latticework structures are also expected to find
applications in device manufacturing, biological applications or as etching/plating protection films in relief
morphology fabrication.
cover slides with surfaces that had been primed with a thin layer
of chromium. Patterned SAMs on a gold surface were prepared by
microcontact printing (mCP) following the printing method
for alkanethiols on gold as described by Kumar et al.[37]
16-Mercaptohexadecanoic acid (Aldrich) was used as the ink to
generate the dispersed COOH-terminated hydrophilic SAM.
Hexadecanethiol (Aldrich) was used to form a continuous
hydrophobic CH3-terminated SAM. The patterned SAMs were
carefully washed with chloroform; then, N2 flow saturated with
water vapor was streamed across the surface of the patterned
SAMs through a glass nozzle immediately after the chloroform
evaporated. The inspection of the water-condensation process was
carried out in situ using optical microscopy. Ordered water-droplet
arrays were formed on the substrate after this procedure. This
water-droplet-covered gold substrate was dipped into a PS
(Aldrich) solution in chloroform and withdrawn immediately at
a speed of 1–3 cm s1. The solution concentration was ranged
from 3 to 25 mg mL1. After chloroform and water completely
evaporated at room temperature, an ordered porous PS film
remained. This porous film was thermally annealed at 150 8C,
leading to the polymer latticework structures with polygonal
meshes. The annealing time was typically 2 h and then the sample
was cooled to below its Tg to freeze the structure.
Conclusion
In summary, we have demonstrated a simple method to
fabricate polymer latticework structures with multiple
polygonal meshes. Experimental results show that the
formation of the final polygonal meshes should result from
a Gibbs-free-energy-controlled anisotropic mass-transfer
process. The different mass-transfer rates in different
directions were induced by crater-like polymer morphologies. This method could be extended to other polymers,
and different shapes of the polymer meshes were acquired
by simply adjusting the dimension features of the
chemical pattern on the underlying Au substrates. We
also derived an empirical equation to partly guide the final
polymer latticework morphology. Moreover, the polymer
structures we fabricated could be peeled off and transferred to other substrates for further applications. These
polymer latticework morphologies with polygonal meshes
may be promising for ‘‘through’’ microstructure fabrication of polymers, both on even and curved surfaces, or
other device-oriented applications. Our strategy utilizing a
self-organized mass-transfer process opens a new alternative for polymer micropatterning techniques.
Experimental Part
Preparation of the Polymer Latticework with Regular
Polygonal Meshes
The overall procedure for fabricating polymer latticeworks with
polygonal meshes on patterned SAMs is depicted in Figure 1. Thin
films of gold were prepared using thermal evaporation onto glass
256
Macromol. Chem. Phys. 2008, 209, 247–257
ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Etching of the Underlying Au Substrate and
Polymer-Film Transfer
Etching experiments were carried out by immersing the polymerpatterned Au substrates into a.u. regia (3 HCl þ 1 HNO3) for 4 h to
etch away the underlying Au film.[32] In order to make the etching
process react less violently, the a.u. regia solution was diluted by
de-ionized water to 1:10 (v:v) prior to etching. The polymer film
with latticework structures was detached from the substrate and
floated on a water surface after this etching process and it could be
retrieved and transferred onto other substrates. Before transferring, the substrates needed to be cleaned with appropriate
solvents.
Characterization
Scanning electron microscopy was done on a JEOL JSM 6700F
field-emission scanning electron microscope (FESEM), and samples
were sputtered with a layer of gold (10 nm thick) prior to
imaging. Atomic force microscopy (AFM) observations of the
sample surfaces were carried out with a commercial instrument
(Digital Instrument, Nanoscope IIIa, Multimode), and all of the
images were obtained in contact mode under ambient conditions
at room temperature. Triangular Si3N4 cantilevers with pyramidal
tips purchased from Nanosensor were used to image the polymer
films. The optical microscopy experiments were carried out on an
Olympus BX51 microscope in reflection mode. The pictures were
captured with an MVC1000 USB2.0 megapixel camera and the
videos were recorded with a Panasonic color charge-coupled
device (CCD) and digitized with a frame grabber.
Acknowledgements: This work has been supported by the
National Natural Science Foundation of China (No. 20534040),
DOI: 10.1002/macp.200700469
Formation of Ordered Two-Dimensional Polymer Latticeworks . . .
the National Basic Research Program of China (2007CB936402), the
program for Changjiang Scholars and Innovative Research Team in
University (No. IRT0422) and the program of Introducing Talents of
Discipline to Universities (B06009).
Received: August 15, 2007; Revised: November 6, 2007; Accepted:
November 8, 2007; DOI: 10.1002/macp.200700469
Keywords: annealing; dewetting; micropatterning; monolayers;
self-assembly
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