Communication
Development of Nanodomain and Fractal
Morphologies in Solvent Annealed Block
Copolymer Thin Films
Juan Peng, Yanchun Han,* Wolfgang Knoll, Dong Ha Kim*
We have systematically studied the thin film morphologies of asymmetric polystyreneblock-poly(ethylene oxide) (PS-b-PEO) diblock copolymer subjected to solvent vapors of varying
selectivity for the constituent blocks. Upon a short treatment in neutral or PS-selective vapor,
the film exhibited a highly ordered
array of hexagonally packed, cylindrical microdomains. In the case of
PEO selective vapor annealing, such
ordered cylindrical microdomains
were not obtained. Instead, fractal
patterns on the microscale were
observed and their growth processes
investigated. Furthermore, hierarchical structures could be obtained if
the fractal pattern was exposed to
neutral or PS selective vapor.
Introduction
Block copolymers have attracted significant attention as
nanostructured materials, since they self-assemble into
J. Peng, W. Knoll, D. H. Kim
Max Planck Institute for Polymer Research, Ackermannweg 10,
55128 Mainz, Germany
Y. Han
State Key Laboratory of Polymer Physics and Chemistry,
Changchun Institute of Applied Chemistry, Graduate School of
the Chinese Academy of Sciences, Chinese Academy of Sciences,
Changchun 130022, P. R. China
Fax: þ86-431-5262126; E-mail: [email protected]
D. H. Kim
Division of Nano Sciences and Department of Chemistry, Ewha
Womans University, 111 Daehyun-Dong, Seodaemun-Gu,
Seoul 120750, Korea
Fax: þ82-2-3277-3419; E-mail: [email protected]
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well defined nanoscale morphologies, with their shape,
size and spacing being determined by the relative volume
fraction of the constituent blocks and their respective
molecular masses.[1–3] This versatility makes block copolymers ideal candidates for use as functional nanomaterials in recent nanoscience and nanotechnology.[4–6] Interfacial energies[7] and commensurability[8] determine the
behavior of the block copolymer thin films. By applying
external fields, such as electric fields,[9] controlled interfacial interactions,[10] shear,[11] chemically patterned
substrates,[12] or crystallization,[13,14] the perpendicular
orientation of microdomains can be readily achieved in
thin block copolymer films.
Increasingly, solvent induced ordering has been used to
produce ordered nanodomain morphologies with controlled orientation.[15–24] The solvent vapor adjusts the
interfacial energy of constituent blocks at the air/film
interface, which allows the control of orientation normal
DOI: 10.1002/marc.200700206
Development of Nanodomain and Fractal Morphologies . . .
to the film surface. For a given copolymer system, a
particular solvent may be classified as neutral or selective,
according to whether it is (i) a good solvent for both blocks,
or (ii) a good solvent for one but a poor or non-solvent for
the other.[25,26] There is growing interests in the effect of
solvent selectivity on the resulting block copolymer
morphologies.[18] In general, a neutral solvent distributes
itself nearly equally between microdomains and can
screen the unfavorable contact of blocks. By manipulating
the solvent selectivity, the degree of microphase separation can be shifted. Previous work on the effects of solvent
selectivity has focused on dilute solutions where the
selectivity drives micellization.[27] In contrast, less attention has been devoted to solvent selectivity on solvent
annealed block copolymer thin films.
In the present paper, we adjust the interfacial energy at
air/film surface by solvent vapor and explore the effect of
solvent selectivity on solvent annealed PS-b-PEO thin
films. In comparision to the classical polystyrene-blockpoly(methyl methacrylate) (PS-b-PMMA) system, the longrange order in PS-b-PEO may arise from the stronger
nonfavorable interactions between PS and PEO. Three
solvents were chosen, from neutral to different block
selective. We first compare the nanostructures in thin films
exposed to solvent vapors of different selectivity, and the
effect of mixed solvent vapors. Then, fractal patterns on
the microscale are presented after a PEO selective solvent
vapor treatment and morphological evolution is discussed.
Finally, we report on hierarchical structures exhibiting
fractal patterns and cylindrical nanodomains simultaneously. Since PEO is water soluble, the oriented arrays of
nanocylinders have the potential application in water
permeable membranes with nanoscopic channels.
Experimental Part
An asymmetric diblock copolymer of PS and PEO, denoted
PS-b-PEO, with a molecular weight of 25 400 g mol1, a polydispersity of 1.05 and a PEO volume fraction of 0.25 was purchased
from Polymer Source, Inc. and used as received. Solutions of the
copolymer (1 wt.-%) in benzene were spin coated onto silicon
substrates. The film thickness was 40 nm determined by a
surface profiler (Tencor P-10, KLA, Tencor). The spin-coated films
were exposed to three different solvent vapors (benzene, toluene,
water) in closed vessels at room temperature (22 8C). In addition,
some samples were also exposed to mixed benzene/water or
toluene/water vapors with an initial volume of 50/50 for comparison. After a certain exposure time, the sample was removed to
an ambient atmosphere.
Atomic force microscopy (AFM) images were obtained in both
height and phase contrast modes using a Digital Instruments
Dimension 3100 scanning force microscope in the tapping mode.
Etched silicon tips on a cantilever (Nanoprobe) with spring constants ranging between 33.2 N m1 and 65.7 N m1 (as specified
by the manufacturer) were used. Optical images were obtained
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using an optical microscope (Axioskop, Zeiss, Germany) in
reflection mode.
Results and Discussion
This section is divided into two parts: first, we describe the
different nanostructures that are formed in PS-b-PEO thin
films after annealing in solvents with varying selectivity.
Second, we focus on the fractal growth process after PEO
selective solvent annealing and a possible mechanism for
the fractal growth is proposed. Finally, a hierarchical
structure is also demonstrated by exposing the fractal
pattern to neutral or toluene vapor.
Nanostructures Formed in PS-b-PEO Films by Solvent
Vapor with Varying Selectivity
For a given system, a solvent that is good for one block can
be classified as neutral or selective, according to whether it
is good or a non-solvent for the other block. The relative
affinity of solvents for each block is governed by the
polymer-solvent interaction parameter, xP-S (P denotes
polymer and S denotes solvent).[28] For non-polar systems:
xPS ¼ VS ðdS dP Þ2 =RT þ 0:34
(1)
where Vs is the molar volume of the solvent, R is the gas
constant, T is the temperature and dS and dP are the
solubility parameters of the solvent and polymer, respectively. For polar systems:
xPS ¼ VS ½ðddS ddP Þ2 þ ðdpS dpP Þ2 =RT
(2)
where dd is the dispersion solubility parameter and dp is the
polar solubility parameter. In the present work, the
solubility parameters of PS and PEO are dpS ¼ 18.6 (J cm3)1/2 and dPEO ¼ 20.5 (J cm3)1/2, respectively.[28] The
calculated xP-S values for different polymer-solvent pairs at
room temperature (22 8C) are listed in Table 1. Using the
Flory-Huggins criterion for complete polymer-solvent
miscibility, i.e., xP-S < 0.5, benzene is a good solvent for
both PS and PEO blocks. Toluene is a selective solvent for
Table 1. Polymer-solvent interaction parameters (xP-S) calculated
from different polymer-solvent pairs.
a)
Benzene
Toluene
Water
PS
0.34
0.35
4.40a)
PEO
0.43
0.54
1.26
Obtained from Polymer Handbook (T ¼ 435 K).[28]
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J. Peng, Y. Han, W. Knoll, D. H. Kim
PS. Though xPEO-water > 0.5, water
is a good solvent for PEO at room
temperature.[28]
Neutral Benzene Vapor
Annealing
The as-cast PS-b-PEO thin films
show a disordered wormlike
pattern [Figure 1(a)] due to fast
solvent evaporation, giving the
chains insufficient time to rearrange to attain an equilibrium
morphology. Upon exposure to
benzene vapor for 10 min, the
film showed a highly ordered
array of cylindrical domains
[Figure 1(b)]. A fast fourier transform (FFT) of the AFM image
shown in the inset of Figure 1(b)
indicates hexagonally packed
cylinders with few defects.
According to the volume ratio
of 25% of the PEO segment, it is
concluded that the darker areas
represent the PEO nanodomains.
As reported by Lin and coworkers,[29] the cylindrical PEO
domains span the entire film
thickness and the orientation
normal to the film surface can
be achived in films with thickness many times the period of
the bulk copolymer.a The average
diameter of 20.1 nm and a lattice
spacing of 27.1 nm were derived
from this AFM image. During
a
Figure 1. AFM phase images of PS-b-PEO thin films prepared under different conditions: (a) as-cast
film; (b) annealed in neutral benzene vapor for 10 min; (c) annealed in PS selective toluene vapor
for 5 min; (d) annealed in PEO selective water vapor for 167 h; (e) annealed in mixed toluene/
water vapor for 5 h. Fast Fourier Transform (FFT) pattern in the inset of Figure 1(b) indicates a
perfect hexagonal arrangement of nanocylinders. The phase scale is shown in the inset. Image
sizes are on a 1 mm 1 mm scale.
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By varying the solution concentration and the spinning speed, films
with four different thicknesses (16,
68, 135, and 900 nm) were also
achieved. The former three films
showed similar morphologies after
subsequent solvent annealing compared to 40 nm-thick films, while the
latter showed cylindrical domains of
PEO were oriented parallel to the
surface of the film. It indicates nanoscopic cylindrical PEO domains can
be produced normal to the PS-b-PEO
film with thickness several times the
period of the copolymer. However,
when the film was very thick, the
external field (solvent vapor) is not
strong enough to orient cylindrical
domains normal to the film surface.
DOI: 10.1002/marc.200700206
Development of Nanodomain and Fractal Morphologies . . .
solvent annealing, the high mobility of both the PS and the
PEO, along with the strong repulsion between the two
blocks leads to a high degree of long range lateral order.
The cylindrical PEO domains appear dark in the AFM phase
images, indicating that PEO is softer than PS matrix.
Consequently, PEO has not crystallized upon solvent
annealing.
PS Selective Toluene Vapor Annealing
water soluble, some amount of water is contained in the
PEO domains during the annealing process, leading to an
increase in the diameter of the nanoscopic cylindrical
domains and the repeat period of the lattice to 31.4 and
41.2 nm, respectively. In addition, the phase contrast seen
in the image (508) is much higher than those in Figure 1(b)
and 1(c), which is due to the softer PEO domains swollen by
water. With the mixed benzene/water vapor treatment,
Next, we present nanostructures of the
PS-b-PEO films annealed by toluene
vapor. Cylindrical nanodomains with
a hexagonal packed array were also
observed, similar to the nanostructure
induced by benzene vapor [Figure 1(c)].
Since toluene is a good solvent for PS
but a poor solvent for PEO, this indicates
that the movement of the major PS
block has a critical effect on the
formation of cylindrical domains with
long range order.
PEO Selective Water Vapor Annealing
The above results indicate that benzene or toluene vapor confers enough
chain mobility to the PS block in the
cylindrical morphology formation.
Therefore, it is not surprising to see
that upon PEO selective water vapor
annealing, regardless of the length
of solvent vapor treatment, the thin
film nanostructure remains almost
unchanged compared to the as-cast
film [Figure 1(d)]. It was interesting to
find that, during water vapor annealing, some polymer droplets were
formed which acted as nuclei for the
following development of the complex
fractal pattern. The growth process
and formation mechanism will be
discussed later.
Mixed Benzene/Water or Toluene/
Water Vapor Annealing
The nanostructures in thin PS-b-PEO
films induced by mixed benzene/
water or toluene/water vapor were
also investigated. Ordered arrays of
hexagonally packed cylinders are
observed by exposure of the films to
mixed toluene/water vapors similar to
those formed by pure benzene or
toluene vapor annealing, as shown
in Figure 1(e). Since the PEO block is
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Figure 2. Optical micrographs in reflection mode for PS-b-PEO thin films after (a) water
vapor annealing for 24 h, removal of water vapor, and storage in air for (b) 48 and (c) 960 h,
respectively. (a’), (b’) and (c’) are AFM height images of PS-b-PEO thin films after (a’) water
vapor annealing for 117 h and removal of water vapor, and storage in air for (b’) 24 h and (c’)
84 h, respectively.
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J. Peng, Y. Han, W. Knoll, D. H. Kim
we could also get results similar to Figure 1(e) (images not
shown). Compared to pure benzene or toluene vapor
annealing, the stability of PS-b-PEO films during mixed
vapor annealing is much higher. For example, if the
PS-b-PEO film was annealed in benzene vapor for 5 h,
the film dewetted, some large droplets formed, and
cylindrical domains were destroyed. In contrast, the
PS-b- PEO film was still stable and cylindrical domains
remained unchanged by benzene/water mixed vapor
annealing.
Nanostructure Formation Mechanisms
We further investigated the differences in surface morphologies observed after benzene, toluene or water vapor
annealing. This difference can be understood by considering the polymer-solvent interaction parameter (xP-S)
values for different polymer-solvent pairs. If asymmetric
PS-b-PEO is cast on a Si substrate with a SiOx surface layer,
due to the preferential interaction of the minor PEO block
with the substrate and the lower surface energy of the
major PS block (g PS ¼ 33 mN m1 < g PEO ¼ 43 mN m1),
most PS segregates to the air interface to form a PS-rich
layer. The wormlike structure indicates uncompleted
microphase separation between PS and PEO. If exposed
to solvent vapor, the film is covered with solvent molecules. The interfacial energy at vapor/film is different from
that in air or in vacuum and it can be adjusted by varying
the solvent vapor used for annealing. During benzene or
toluene vapor annealing, the solvent molecules induce
sufficient chain mobility for the PS chain and the strong
non-favorable interactions between PS and PEO blocks lead
to rapid further microphase separation. The ordering is
initiated both at the surface and the bottom of the film and
propagates laterally throughout the entire film.
If treated in PEO selective water vapor, two factors
prevent the formation of ordered cylindrical nanostructures.
Firstly, if the PEO blocks are swollen by water, there would
be no direct contact between most of the underlying PEO
blocks and the water vapor due to the presence of a PS-rich
layer near the air/film interface. Thus, the solvent must
diffuse through the upper PS-rich layer in order to reach the
underlying PEO blocks, resulting in a delay in the system’s
response. Secondly, during water vapor annealing, the major
PS blocks do not have enough mobility to reconstruct
themselves and the mobility of the minor PEO blocks is not
enough to induce the rearrangement of the entire film.
between 7.0 and 9.0 mm [Figure 2(a)]. The droplets remain
unchanged during further annealing in water vapor for a
long time. However, it is interesting to find that the
droplets develop into fractal patterns after storing the film
in air for different lengths of time [Figure 2(b) and 2(c)].
The typical size of the fully grown fractal patterns exceeds
300 mm.
AFM measurements were also carried out in order to
follow the evolution of the fractal structure and to obtain
more accurate information on the profile of the film.
Figure 2(a’) shows a nearly round droplet with a uniform
front on the film surface, with a diameter of 7.1 mm and a
height of 468 nm. As time lapsed, the round droplet
became anisotropic [Figure 2(b’)]. Some small drops or
fingers were generated near the contact line, giving rise
eventually to a fractal pattern [Figure 2(c’)]. By AFM
Fractal Pattern Growth after PEO Selective Water
Vapor Annealing
Fractal Pattern Growth Process
As mentioned above, some polymer droplets are formed on
the film surface during water vapor annealing with sizes
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Figure 3. (a) Analysis of the fractal dimension of a fully developed
fractal pattern (inset) using the box count method. The slope
yields the fractal dimension df. (b) Time evolution of the fractal
dimension of fractal patterns in PS-b-PEO films after water vapor
annealing and storage in air.
DOI: 10.1002/marc.200700206
Development of Nanodomain and Fractal Morphologies . . .
sectional profile analysis, the height of
the fractal pattern was found to be
about 9 nm.
Fractals are generally observed in
far from equilibrium growth phenomena. In our systems, the fractals
are quite similar in shape to nonequilibrium patterns observed in other
systems, such as dendritic crystallization, electrode position, viscous fingering and dielectric breakdown.[30] As
a result, we performed the fractal
dimension analysis for an isolated
fully developed fractal pattern using
the box count method,[30] with the
results shown in Figure 3(a). The
fractal dimension df can be obtained
from the scaling relationship of
NðlÞ ldf , where N(l) is the number
of boxes of size l that contain at least
Figure 4. A typical hierarchical pattern in a PS-b-PEO thin film after annealing the fractal
part of the fractal pattern region. From
pattern using toluene vapor for 10 min.
the slope in Figure 3(a), a fractal
dimension of df ¼ 1.66 was obtained.
This result is in exact agreement with
lower than room temperature, the PEO block is also mobile
the theoretical fractal dimension result of Muthukuin the air. The water contained in the film gradually
mar:[31]
evaporates and causes an anisotropic stress throughout
2
the film, thereby destabilizing the initially uniform front of
df ¼ ðd þ 1Þ=ðd þ 1Þ
(3)
the droplets and inducing heterogeneity of the polymer
mobility. The evolution of the droplet front becomes
where d ¼ 2 is the space dimension.
anisotropic and spreading fronts develop faster than the
Image analysis was then performed in order to obtain
adjacent regions, forming fractal patterns. While in the
quantitative information on the growth process of fractal
water vapor environment, the water contained in the film
patterns. Figure 3(b) shows the time evolution of the
does not evaporate. Therefore, droplets remain unchanged
fractal dimensions of the fractal patterns after water vapor
and no fractal patterns form.
annealing and being stored in air. It can be seen that
the fractal dimension gradually increases from 0.85 at
Hierarchical Micro/Nanostructure
short storage time to 1.67 at long storage time. The
increase of the fractal dimension is an indication of
Hierarchical structures are important forms among varigaining fractality.
ous unconventional morphologies. These structures are
Fractal Pattern Growth Mechanism
Based on our observations, we present a tentative
mechanism to interpret the formation of fractal patterns.
The water contained in the film is believed to play a critical
role in the fractal pattern formation. Microscopically
heterogeneous stress is believed to trigger the formation
of fractal patterns and the heterogeneity comes from the
water contained in the film. Since PEO is water soluble, it is
swollen in the presence of water vapor with a high
mobility after some time and PS is mobile to some degree.
PS and PEO aggregate with each other to form droplets like
a melt which contain much water inside. If the film is
stored in the ambient atmosphere, since the Tg of PEO is
Macromol. Rapid Commun. 2007, 28, 1422–1428
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made up of building units at different length scales. From
the above results it is concluded that upon neutral benzene
or PS selective toluene vapor annealing, the film exhibits a
highly ordered array of hexagonally packed, cylindrical
microdomains, while after PEO selective water vapor
annealing, no such cylindrical microdomains are formed.
Instead, fractal patterns on the micron scale are observed.
Several questions naturally arise:
(i) What would happen if one put the film with a fractal
pattern into benzene or toluene vapor?
(ii) Can the fractal pattern on the microscale and
cylindrical domains on the nanoscale be combined
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J. Peng, Y. Han, W. Knoll, D. H. Kim
to form a hierarchical pattern at multiple length
scales?
To answer these questions, the PS-b-PEO films with
fractal patterns were exposed to benzene or toluene vapor
and examined by both optical microscopy and AFM. A
typical hierarchical pattern is shown in Figure 4. Hexagonally packed cylindrical microdomains appear both
inside and outside of the fractal pattern. It should be noted
that the fractal pattern was not destroyed by the short
benzene or toluene vapor treatment employed in this
experiment. Such a complex evolution of micro/nanostructured morphology is of fundamental scientific interest,
and may offer useful templates for unconventional
functional nanostructures.
Conclusion
Thin film morphologies of asymmetric PS-b-PEO diblock
copolymer were investigated after annealing by solvent
vapors with neutral and different block selectivity. After
neutral benzene and PS selective toluene vapor annealing,
highly ordered arrays of hexagonally packed cylindrical
PEO domains were produced in a PS matrix. Ordered
cylindrical nanostructures could not be obtained by PEO
selective water vapor annealing, indicating that the
movement of the PS block has a major effect on the
formation of ordered arrays of cylindrical nanodomains.
Mixed benzene/water or toluene/water vapor annealing
proved to be a strategy for manipulating the cylindrical
domain size and the repeat period of the lattice.
It was interesting to observe the development of a
fractal pattern after PEO selective water vapor annealing.
The polymer droplets formed during water vapor act as
nuclei and the anisotropy of the polymer mobility induced
by the water contained in the film plays a critical role in
fractal pattern formation. Further, by exposing the film
with a fractal pattern to benzene or toluene vapor
annealing, hierarchical patterns with well defined cylindrical microdomains could be produced both within and
outside the fractal pattern. Our results open up new
possibilities for the construction of complex controllable
structures by a simple ‘‘bottom up’’ approach.
Acknowledgements: J. Peng is grateful to the Science and
Technology Cooperative Program between the Chinese Academy
of Sciences and the Max Planck Gesellschaft and Alexander von
Humboldt Foundation. This work was supported by the Korea
Research Foundation and grant funded by the Korean Government
(MOEHRD, Basic Research Promotion Fund) (KRF-2006-003D00138). The authors are indebted to Dr. Jun Fu and King Hang
Aaron Lau for helpful discussion.
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Received: March 18, 2007; Revised: April 27, 2007; Accepted: April
30, 2007; DOI: 10.1002/marc.200700206
Keywords: atomic force microscopy; fractal patterns; hierarchical nanostructures; PS-b-PEO block copolymers; solvent annealing
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DOI: 10.1002/marc.200700206