Fabrication of thin film surface templates from two immiscible polymers by phase separation and phototethering

Article abstract of DOI:10.1246/cl.2010.254

Thin layer surface templates were fabricated by spin coating two immiscible polymers onto a photoreactive substrate, followed by photoirradiation and removal of the unreacted polymer with solvent. Phase-separation occurs during the spin coating process and the separated structure was tethered onto the substrate surface by photoirradiation, to yield a thin surface pattern on the substrate. The surface patterns thus obtained were robust, applicable to large areas, and could be tuned at will by designing component polymers.

Full text of DOI:10.1246/cl.2010.254

doi:10.1246/cl.2010.254
254
Published on the web February 16, 2010
Fabrication of Thin Film Surface Templates from Two Immiscible Polymers
by Phase Separation and Phototethering
Koichi Kawamura,*1 Kazuhiro Yokoi,1 and Mitsuhiro Fujita2
1Synthetic Organic Chemistry Laboratories, FUJIFILM Corporation,
577 Ushijima, Kaisei, Ashigarakami-gun, Kanagawa 258-8577
2Aalysis Technology Center, FUJIFILM Corporation, 210 Nakananuma, Minamiashigara 250-0193
(Received December 11, 2009; CL-091103; E-mail: kouichi_kawamura@fujifilm.co.jp)
∗
∗
Thin layer surface templates were fabricated by spin coating
∗
∗
30
O
70
50
O
50
two immiscible polymers onto a photoreactive substrate,
followed by photoirradiation and removal of the unreacted
polymer with solvent. Phase-separation occurs during the spin
coating process and the separated structure was tethered onto the
substrate surface by photoirradiation, to yield a thin surface
pattern on the substrate. The surface patterns thus obtained were
robust, applicable to large areas, and could be tuned at will by
designing component polymers.
O
O
O
O
O
O
O
O
O
n-Bu
C6F13
Si
Si
O
O
n
n=11
P2
P1
CCl3
N
N
Cl3Si
O
N
CCl3
A1
Surface patterning for the fabrication of structures at
micrometer and/or nanometer scale is receiving great attention,
particularly for its increasing use in surface-directed phase
separation. In this technique, the domains of a phase-separating
mixture of polymers in a thin film can be guided into arbitrary
structures onto a surface by designing a prepatterned variation of
surface energies. This allows these surface patterns to be readily
transferred to a two-component polymer film.1,2 This approach
might provide a simple means for fabricating polymer-based
microelectronic circuits and is expected to play an important role
in future technological applications.1,3-8 For instance, Friend
et al. have reported a control of the phase separation of conjugate
polymer blends by introducing a chemical pattern onto the
substrate for surface energy modification, which has led
successfully to improved LED performance.9,10
Various methods can be used to produce periodic patterns
of chemically heterogeneous substrates. The most versatile are
those of microcontact printing,1,2,4,5,9 or a phase-separated
Langmuir-Blodgett (LB) film.11-14 These procedures are capable
of covering a metal substrate with patterns of thin (ca. 2 nm)
self-assembled monolayers (SAMs). However, in order to realize
their potential, it is necessary to have the ability to fabricate the
surface pattern over a larger area, more simply and with polarity
and morphological control.
Scheme 1. Schematic representation of surface pattering of phase-
separated polymers.
common solvent (1 wt % in 2-butanone), a drop of the solution
was placed onto the substrate, and the film was prepared by spin-
coating (1500 rpm, 20 s) to form a layer about 30 nm thick. As
most chemically different polymers are immiscible due to the
their much reduced entropy of mixing compared to their low
molecular weight analogs, demixing takes place during the spin-
coating procedure.4,17-19 By then photoirradiating the film for
10 s through a glass filter (transparent over 320 nm, 32 mW at
365 nm), a radical was generated from the photoinitiator on the
substrate and this reacted with an unsaturated group of the
polymer next to the substrate, causing phototethering. After
removal of unreacted polymers with a solvent, an ultrathin
polymer film (ca. 4 nm) was fixed onto the silicon surface. If the
polymers next to the substrate have a phase-separated structure,
that structure will be fixed onto the surface of the substrate after
removing unreacted polymers.
Experimental results are as follows. As P1 and P2 are
strongly incompatible, coexisting phases of the almost pure
polymer components were formed after spin-coating. (Figure 1a,
as a reference). After photoirradiation and removing unreacted
polymers (Figure 1b), the AFM image revealed almost the same
morphology, but a flatter, planar structure was observed, as
evident from surface average roughness (Ra). The ellipsometric
thickness of the spin cast film (Figure 1a) was 26 nm, whereas
that of the irradiated film (Figure 1b) was much thinner,
estimated about 4 nm by ellipsometry (DHA-XA/S4, Mizojiri
Opt Co.). These results indicate that the radicals photogenerated
on the surface reacted with the unsaturated pendant group of the
polymers next to the substrate, resulting in the formation of a
thin layer between the substrate and the polymers. Because of
the phase-separated structures in the polymer layer, a thin layer
structure next to the substrate was transferred and fixed on the
surface.
In this letter, we report a new method for preparation of
surface templates that utilizes phase separation of two immis-
cible polymers and phototethering onto a photoreactive substrate
surface.
Scheme 1 shows the structures of the compounds used in
this study and the immobilization process used to prepare a
phase-separated polymer thin film onto a photoreactive silicon
wafer. As shown in Scheme 1, two copolyacrylates, P1 (Mw =
34700, Mw/Mn = 1.7) and P2 (Mw = 27300, Mw/Mn = 1.4),
each having a polymerizable unsaturated pendant group, were
prepared.15 A silicon substrate was treated to immobilize a
photoinitiator onto the substrate by dipping a silicon wafer in
a 0.1 wt % toluene solution of A1 for 3 h.16 The experimental
procedure was simple: the two polymers were dissolved in a
Chem. Lett. 2010, 39, 254-256
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

255
7
6
5
4
3
2
1
0
(a)
(b)
P2
P1
0
10
20
30
Figure 1. AFM images of phase-separated films. (a) As spin-cast
(reference). Thickness: 26 nm, Ra: 4.9 nm. (b) Surface pattern obtained
after photoirradiation (>320 nm) of spin-cast film and removal of
unreacted upper layer by washing in 2-butane. Thickness: 5 nm, Ra: 1.2
nm. Scanned area 5 ¯m © 5 ¯m. AFM: E-seep/NanoNavi, SII-NT Co.
exposure time / s
Figure 3. Ellipsometric thickness of individual polymers (P1 or P2)
after irradiation (>320 nm) and washing with a solvent.
(a)
(b)
(c)
Scheme 2. Fabrication of opposite surface patterns (b), (c), from a
phase-separated polymers film. (a) as spin-coated, (b), (c) after photo-
tethering. Surface pattern depends on which polymer has reactive
pendant group.
Figure 2. AFM images (5 ¯m © 5 ¯m) of surface pattern at various
weight ratio in P1:P2 after spin-coating, photoirradiation and solvent
washing: (a) 2:1, (b) 1:1, and (c) 1:2. The area ratios of the darker and
the brighter portions are 7:3, 5:5, and 3:7, and ellipsometric thickness is
3.9 « 0.3, 4.3 « 0.1, and 4.8 « 0.1 nm, respectively.
(A)
(B)
PS
P2
+
∗
∗
∗
∗
11
89
50
50
O
Figure 2 shows the AFM topographic images (5 ¯m © 5 ¯m)
of thin films obtained from the mixed polymers with different
weight fractions. The brighter portions were about 2 nm higher
than the darker portions. The mixing ratios of P1:P2 in
Figures 2a, 2b, and 2c were 2:1, 1:1, and 1:2, respectively. By
measuring the area of darker and brighter portions by a threshold
method using the grain analysis function generated by computer
software (SPIWin), the area ratios were found to be 7:3, 5:5, and
3:7, respectively, which were in good agreement with the weight
fraction of the component polymers. This result indicates that
brighter regions correspond to P2 and darker portions corre-
spond to P1.
+
O
O
O
O
O
n-C4H9
O
C6F13
O
O
HN
P3
P4
O
Figure 4. Structures of polymers for surface patterning.
to the polymer surface pattern. The remarkably stable surface
and strong anchoring of these polymer patterns to the substrate
are a manifestation of the covalent bonding between the
polymers and the substrate.
The layer thickness of the individual polymers was
confirmed (Figure 3). Either P1 or P2 formed a uniform thin
layer and the thickness was slightly higher for P2 than for P1, in
accord with the results obtained in Figure 2.
Our methodology can be extended to control the morphol-
ogy and polarity of surface patterns. As shown in Scheme 2,
a phase-separated film consisting of two immiscible polymers
(Scheme 2a) can form a surface pattern (b) or reversed pattern
(c) after photoirradiation and washing with a solvent, depending
on which polymer has a reactive pendant group.
Our idea was realized by designing new polymers. As
shown in Figure 4, a combination of polystyrene (PS, Aldrich,
Mw = 194000, Mw/Mn = 1.3) and a fluoro-polymer having an
unsaturated reactive pendant group P2 (weight ratio 1:2, case A)
and a combination of polystyrene having the unsaturated
reactive group P3 (Mw = 143000, Mw/Mn = 2.2) and fluoro-
polymer P4 (Mw = 13600, Mw/Mn = 1.9, weight ratio 1:2, case
B) were employed. In both cases, two polymers are dissolved in
a common solvent (2-butanone, 1 wt %) and spin-coated onto a
photoinitiator-tethered silicon wafer.
The mechanism of polymer dewetting and three-dimen-
sional distribution of phase-separated structure by spin-coating
was extensively studied by Steiner et al.18 They observed
different structures on an air surface (e.g., islands and plateaus
consisting of two polymers) and next to the substrate (e.g., a
homogeneous thin layer consisting of either polymer), due to the
difference of the surface energy of two polymers. Taking this
into consideration, it is interesting to note here that a domain
structure that was proportional to the feed ratio of the two
component polymers was observed at the near surface (ca. 4 nm)
of the substrate in our experiments.
In order to confirm the robustness of the thin films,
ultrasonic washing for extended period of time (ca. 10 min) in
2-butanone was carried out repeatedly, with no apparent damage
The AFM image of spin-casted film of PS/P2 combinations
(1:2 weight ratio, Figure 5a) showed a column-like structure
Chem. Lett. 2010, 39, 254-256
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

256
2
3
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(a)
(b)
(c)
4
5
6
7
8
9
Figure 5. AFM images (20 ¯m © 20 ¯m). (a) PS + P2 (1:2): as spin-
cast; reference. (b) PS + P2 (1:2): spin-casting, photoirradiation
(>320 nm, 30 s) and washing by 2-butanone. (c) P3 + P4 (1:2): same
procedure as in (b). Vertical scale is 73, 10, and 5 nm respectively.
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2530.
15 Synthesis of P2: A mixture of 2-(2-bromoisobutylyoxy)ethyl meth-
acrylate (27.9 g, 0.1 mol), 2-(perfluorohexyl)ethyl methacrylate
(43.2 g, 0.1 mol, M-1620, Daikin Co.) were dissolved in 166 g of 2-
butanone and heated to 75 °C in a nitrogen atmosphere. Subsequently,
0.345 g of dimethyl 2,2¤-azobis(2-methylpropionate) (V601, Wako
Co.) was slowly added, and the reaction mixture was further reacted at
75 °C. After 12 h, the reaction mixture was cooled to room temper-
ature and a solution of 50.2 g of 1,8-diazabicyclo[5.4.0]undecene-7
(DBU) and 0.23 g of 4-methoxyphenol in 2-butanone (100 g) was
added to the reaction mixture and this was left to stir for 12 h at room
temperature. After neutralizing with trifluoroacetic acid, the reaction
mixture was poured into water and left for 30 min with vigorous
agitation. After drying under vacuum, a sticky white solid was
obtained. 1H NMR (CDCl3, ¤): 0.8-1.2 (m, 4H), 1.94 (m, 6H), 2.45 (t,
2H), 4.40-4.45 (t, 6H), 5.60 (s, 1H), 6.12 (s, 1H). Polymer P1 was
synthesized in a similar manner using poly(dimethylsiloxypropyl
methacrylate) (FM0711, Tisso Co., Mw = 1000) as the monomer.
with PS existing in the brighter, peripheral regions, as suggested
by the increased peripheral region with an increase of PS content
(PS/P2 = 1:1). The peripheral region was higher than the
circular region at about 30 nm. Upon photoirradiation of this
film and removing the unreacted polymers by ultrasonication in
2-butanone, a reversed column-like structure appeared, with
relatively flat shape (Figure 5b). The height of the circular
domains was 3 nm higher than the surrounding regions. Because
only P2 had a reactive pendant group, it is reasonable to assume
that the circular domain consisting of the fluoro-polymer P2 was
tethered onto the surface of the substrate by chemical bonds and
the excess P2 and the unreacted PS were removed by the solvent
treatment.
A reversed pattern would be expected by changing the
unsaturated pendant group from the fluoro-polymer to PS (case
B, in Figure 4). As shown in Figure 5c, after photoirradiation
and dissolution of a spin-cast film (P3 + P4, 1:2 weight ratio),
a thin peripheral structure (ca. 1 nm height) of PS, which is
reversed pattern of Figure 5b, was revealed on the substrate
surface. These results indicate that the morphology of a surface
pattern could be tuned simply by designing the reactivity of two
polymers.
In conclusion, we demonstrated a novel method of
fabricating patterned-polymer thin films using separation of
two immiscible polymers followed by photografting of the
polymers on the solid surface. Our methodology has the
following advantages: (1) The surface patterns are robust, due
to the covalent bonding between the polymer and the substrate.
(2) The method can be applied to a large area with a short
processing time, due to the rapid spin-coating process. (3) The
size and shape of the domains can be tuned by designing
component polymers and the mixing ratio. This technique will
be important in the fabrication of patterned materials at the
micrometer or nanometer scale for application to electronic
devices, sensors, and for protein adsorption. Further study to
extend our approach to a large number of binary polymer pairs
on a variety of different substrates is in progress.
M
w = 34700. P3 was obtained by polymerization of styrene and 2-
hydroxyethyl methacrylate by V601, followed by condensation with
2-isocyanatoethyl methacrylate using bismuth octyric acid (U600
Nitto Kasei Co.) as a catalyst. P4 was synthesized by polymerization
from 2-(perfluorohexyl)ethyl methacrylate and n-butyl methacrylate
using V601.
16 Synthesis of A1: A mixture of 2.42 g of 4¤-(10-undecenyl)-4-{[2,6-
bis(trichloromethyl)-1.3.5-tiazine-4-yl]}biphenyl, 0.9g of the trichloro-
silane and 0.06 g of Speier’s catalyst (10 wt % of H2PtCl6/6H2O in
isopropanol) in dry THF was stirred overnight at room temperature.
When the reaction was complete according to 1H NMR, the solvent
and the excess of trichlorosilane were removed under reduced
pressure, leaving a slightly yellow solid, which was very sensitive to
1
moisture and stored under Ar atmosphere. H NMR (CDCl3, ¤): 1.2-
1.8 (m, 12H), 1.85 (qq, 2H), 2.0 (dt, 2H), 4.03 (t, 2H), 7.02 (d, 2H),
7.64 (d, 2H), 7.80 (d, 2H), 8.75 (d, 2H), HRMS (Bruker-Daltonics)
calcd for C28H31Cl9N3OSi [M + H]+, m/z 767.943, found. 767.943.
17 S. Y. Heriot, R. A. L. Jones, Nat. Mater. 2005, 4, 782.
18 S. Walheim, M. Böltau, J. Mlynek, G. Krausch, U. Steiner, Macro-
molecules 1997, 30, 4995.
We acknowledge Hidehiro Suzuki at WDB Co., Ltd and
Ayako Nakamura at Analysis Technology Center, FUJIFILM for
technical assistance during this study.
References and Notes
1
M. Böltau, S. Walheim, J. Mlynek, G. Krausch, U. Steiner, Nature
1998, 391, 877.
19 S. Walheim, E. Schäffer, J. Mlynek, U. Steiner, Science 1999, 283,
520.
Chem. Lett. 2010, 39, 254-256
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/