Defect-free nanoporous thin films from ABC triblock copolymers

Article abstract of DOI:10.1021/ja0608141

The self-assembly of triblock copolymers of poly(ethylene oxide-b-methyl methacrylate-b-styrene) (PEO-b-PMMA-b-PS), where PS is the major component and PMMA and PEO are minor components, provides a robust route to highly ordered, nanoporous arrays with cylindrical pores of 10-15 nm that show promise in block copolymer lithography. These ABC triblock copolymers were synthesized by controlled living radical polymerization, and after solvent annealing, thin films showing defect-free cylindrical microdomains were obtained. The key to the successful generation of highly regular, porous thin films is the use of PMMA as a photodegradable mid-block which leads to nanoporous structures with an unprecedented degree of lateral order. The power of using a triblock copolymer when compared to a traditional diblock copolymer is evidenced by the ability to exploit and combine the advantages of two separate diblock copolymer systems, the high degree of lateral ordering inherent in PS-b-PEO diblocks plus the facile degradability of PS-b-PMMA diblock copolymer systems, while negating the corresponding disadvantages, poor degradability in PS-b-PEO systems and no long-range order for PS-b-PMMA diblocks.

Full text of DOI:10.1021/ja0608141

Published on Web 05/19/2006
Defect-Free Nanoporous Thin Films from ABC Triblock
Copolymers
Joona Bang,†,‡ Seung Hyun Kim, Eric Drockenmuller, Matthew J. Misner,
§
|
§
,
§
,‡
Thomas P. Russell,* and Craig J. Hawker*
Department of Chemical and Biological Engineering, Korea UniVersity, Seoul 136-713, Republic
of Korea, Materials Research Laboratory and Department of Chemistry and Biochemistry,
UniVersity of California, Santa Barbara, California 93106, Department of Polymer Science and
Engineering, UniVersity of Massachusetts, Amherst, Massachusetts 01003, and Laboratoire des
Mat e´ riaux Polym e` res et Biomat e´ riaux - UMR CNRS 5627, UniVersit e´ Claude Bernard Lyon, 15,
Bd Latarjet, F- 69 622 VILLEURBANNE, France
Received February 2, 2006; E-mail: russell@mail.pse.umass.edu; hawker@mrl.ucsb.edu
Abstract: The self-assembly of triblock copolymers of poly(ethylene oxide-b-methyl methacrylate-b-styrene)
(PEO-b-PMMA-b-PS), where PS is the major component and PMMA and PEO are minor components,
provides a robust route to highly ordered, nanoporous arrays with cylindrical pores of 10-15 nm that show
promise in block copolymer lithography. These ABC triblock copolymers were synthesized by controlled
living radical polymerization, and after solvent annealing, thin films showing defect-free cylindrical
microdomains were obtained. The key to the successful generation of highly regular, porous thin films is
the use of PMMA as a photodegradable mid-block which leads to nanoporous structures with an
unprecedented degree of lateral order. The power of using a triblock copolymer when compared to a
traditional diblock copolymer is evidenced by the ability to exploit and combine the advantages of two
separate diblock copolymer systems, the high degree of lateral ordering inherent in PS-b-PEO diblocks
plus the facile degradability of PS-b-PMMA diblock copolymer systems, while negating the corresponding
disadvantages, poor degradability in PS-b-PEO systems and no long-range order for PS-b-PMMA diblocks.
Introduction
size and, to a lesser extent, the two-dimensional arrangement
of nanoscale features. By simply changing the molecular weight
The self-assembly of block copolymers has attracted consid-
erable attention as a simple, versatile route to nanostructured
and/or the relative ratio of constituent blocks, the morphology
and size scales of the features can be controlled. In terms of
thin films, the nanoporous materials derived from block
copolymers have also been actively studied as nano-templates,
membranes, separation media, high surface area support for
1
materials and has led to the development of block copolymer
lithography as a viable technique for the fabrication of micro-
electronic devices.
block copolymer lithography allows exquisite control over the
2-15
As a “bottom up” molecular system,
1
,8,12,14,16-28
catalysis, and sensors.
†
Korea University.
University of California.
University of Massachusetts.
Universit e´ Claude Bernard Lyon.
Several routes to generate nanoporous films via block
copolymer self-assembly have been reported since Nakahama
and co-workers first demonstrated the formation of nanoporous
polymer films from a siloxane-functionalized poly(styrene-b-
‡
§
|
(
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7622
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J. AM. CHEM. SOC. 2006, 128, 7622-7629
10.1021/ja0608141 CCC: $33.50 © 2006 American Chemical Society

Thin Films from ABC Triblock Copolymers
A R T I C L E S
the minor component selectively after self-assembly and, as a
result, generate nanoscopic pores in a matrix of the majority
component. A wide variety of diblock copolymers have been
used, in some cases incorporating functionality to cross-link the
majority component or homopolymers to modify the morphol-
ogy. In contrast, only a small number of ABC-triblock copoly-
ordering. The use of triblock copolymers provides a simple route
to the generation of well-ordered nanoscopic templates that can
be potentially used for applications requiring the precise control
of location and orientation of nanoscopic domains, such as high-
density, addressable storage media or for registration with an
underlying pattern.
27,28
mers
have been studied, which is surprising given their richer
Results and Discussion
phase morphologies. One of the major shortcomings with all
of these systems is the presence of defects or grain boundaries
in the nanoporous array, which limits the range of applications
for these nanoscale templates to those that do not require
registration or alignment.
The synthesis of a library of ABC triblock copolymers, poly-
(
ethylene oxide-b-methyl methacrylate-b-styrene) (PEO-b-
PMMA-b-PS), with various PMMA block lengths necessitated
the use of a living free radical polymerization process. Due to
its synthetic versatility and ability to polymerize methyl
methacrylate (MMA) under controlled conditions, reversible
addition fragmentation chain transfer (RAFT) polymerization32
was the living free radical polymerization procedure of choice.
The sequence of polymerization was therefore ethylene oxide,
followed by MMA and styrene with the volume fraction of PEO
and PMMA blocks being controlled to allow the formation of
cylindrical PEO/PMMA microdomains.
To overcome the drawback of lateral order and defects, we
have recently demonstrated that the self-assembly of polystyrene-
b-poly(ethylene oxide) (PS-b-PEO) diblock copolymers leads
to a highly ordered arrays of cylindrical microdomains of PEO
in a PS matrix. By the use of solvent evaporation and annealing,
defect-free arrays can be achieved over large lateral areas (ca.
3
0,31
1
(
0 µm × 10 µm).
In comparison to the classical poly-
styrene-b-methyl methacrylate) (PS-b-PMMA) diblock copoly-
In designing the RAFT macroinitiator based on PEO, one of
the major hurdles is the limited range of RAFT agents that act
as efficient initiators for the controlled polymerization of methyl
methacrylate. The tedious and low-yielding synthesis of tertiary,
dithioester RAFT agents derived from AIBN derivatives was
circumvented by the recent development by a variety of groups33
of stabilized secondary, dithioester RAFT agents. These dithioester
derivatives are based on the realization that a tertiary radical is
not required for efficient initiation of MMA if two radical
stabilizing groups are present. As a result, synthetic accessibility
of these MMA-capable RAFT agents is greatly enhanced and a
variety of efficient functionalization strategies are now available.
To facilitate the synthesis of RAFT macroinitiators, well-defined
monomethoxy-terminated PEO derivatives were used as starting
materials, and the desired PEO-RAFT macroinitiator was
prepared by end-group modification via a two-step procedure
involving the initial esterification of monomethoxy PEO with
R-bromophenylacetic acid to give the phenyl acetate derivative,
mer system, the long-range order in PS-b-PEO arises from the
stronger nonfavorable interactions between PS and PEO,
combined with the directionality of solvent evaporation. Al-
though a high degree of long-range lateral order of PEO
cylindrical microdomains is obtained with few defects, it is
extremely difficult to selectively remove the PEO block by
simple etching processes. Hillmyer and co-workers recently
reported the successful use of iodic acid to achieve this.
However, the degradation requires immersion in a 57 wt %
aqueous HI solution at 60 °C for 5 days and was only applicable
to bulk samples and not thin films. This is in direct contrast to
the classical PS-b-PMMA system discussed above where
irradiation with UV light for less than 10 min leads to
degradation of the PMMA and generation of porosity.
In this manuscript, a strategy that combines the facile
degradation of PS-b-PMMA with the long-range lateral order
of PS-b-PEO-based systems to give defect-free, porous thin films
suitable for block copolymer lithography is presented. By
employing a triblock copolymer system, it is envisaged that the
advantages of different diblock copolymer systems can be
combined while at the same time negating the corresponding
disadvantages such as poor degradability in PS-b-PEO diblocks
and a lack of long-range order for PS-b-PMMA systems. To
achieve these desired characteristics, the required triblock
copolymers have a general PS-b-PMMA-b-PEO structure in
which the central PMMA block imparts degradability to the
system while the terminal PEO block permits long-range
2
8
1
. Coupling of 1 with the anion derived from the reaction of
phenylmagnesium bromide with carbon disulfide gives the
dithioester, 2. The RAFT agent at the chain end of 2 is stabilized
by both the phenyl ring as well as the ester group and is capable
of controlling the polymerization of MMA (Scheme 1). The
incorporation and quantification of the RAFT group at the
termini of the PEO chains was confirmed by a variety of
techniques. Size-exclusion chromatography in combination with
a photodiode array detector confirmed the presence of the
dithioester group through the characteristic UV absorbance of
the dithioester group at ca. 320 nm. Quantification of chain-
(
(
(
(
22) Mao, H.; Arrechea, P. L.; Bailey, T. S.; Johnson, B. J. S.; Hillmyer, M. A.
Faraday Discuss. 2005, 128, 149-162.
1
end functionalization could be accomplished by H NMR and
23) Li, M.; Douki, K.; Goto, K.; Li, X.; Coenjarts, C.; Smilgies, D. M.; Ober,
C. K. Chem. Mater. 2004, 16, 3800-3808.
integration of the unique signals for the CH-S group at 5.73
ppm and the aromatic protons ortho to the CdS group at 8.02
ppm followed by comparison with the resonances for the PEO
chain (Figure 1a).
Reaction of 2 with MMA and AIBN at 70 °C gave the diblock
copolymer, 3, which was then used to initiate the polymerization
24) Liang, C.; Hong, K.; Guiochon, G. A.; Mays, J. W.; Dai, S. Angew. Chem.,
Int. Ed. 2004, 43, 5785-5789.
25) Pai, R. A.; Humayun, R.; Schulberg, M. T.; Sengupta, A.; Sun, J.-N.;
Watkins, J. J. Science 2004, 303, 507-511.
(
(
26) Xu, Y.; Gu, W.; Gin, D. L. J. Am. Chem. Soc. 2004, 126, 1616-1617.
27) Sugihara, S.; Kanaoka, S.; Aoshima, S., J. Polym. Sci., Part A: Polym.
Chem. 2004, 42, 2601-2611.
(
28) Rzayev, J.; Hillmyer, M. A. Macromolecules 2005, 38, 3-5; Rzayev, J.;
Hillmyer, M. A. J. Am. Chem. Soc. 2005, 127, 13373-13379; Mao, H.;
Hillmyer, M. A. Soft Matter 2006, 2, 57.
(32) Monteiro, M. J. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 5643-
5651; Perrier, S.; Takolpuckdee, P. J. Polym. Sci., Part A: Polym. Chem.
2005, 43, 5347-5393.
(33) Li, C.; Benicewicz, B. C. J. Polym. Sci., Part A: Polym. Chem. 2005, 43,
1535-1543; Takolpuckdee, P.; Mars, C. A.; Perrier, S.; Archibald, S. J.
Macromolecules 2005, 38, 1057-1060.
(
(
29) Lee, J. S.; Hirao, A.; Nakahama, S. Macromolecules 1988, 21, 274-276.
30) Kim, S. H.; Misner, M. J.; Xu, T.; Kimura, M.; Russell, T. P. AdV. Mater.
2
004, 16, 226-231.
(
31) Kim, S. H.; Misner, M. J.; Russell, T. P. AdV. Mater. 2004, 16, 2119-
2
123.
J. AM. CHEM. SOC.
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VOL. 128, NO. 23, 2006 7623

A R T I C L E S
Bang et al.
Scheme 1. Synthesis of PEO-b-PMMA-b-PS Triblock Copolymers, 4, by RAFT Polymerization
of styrene to give the desired triblock copolymer, 4. The fidelity
of the growth process could be confirmed by SEC, which shows
the controlled increase in molecular weight on chain extension
from the PEO-RAFT macroinitiator, 2, to the PEO-PMMA
diblock, 3, and finally to the PEO-b-PMMA-b-PS triblock
copolymers, 4 (Figure 2). The relative amount of each block
and their associated molecular weights could be determined by
no detectable amounts of homopolymer or diblock copolymer
contaminants.
Initial morphological studies of the ABC triblock copolymers
involved SAXS experiments on bulk samples, and representative
examples are shown in Figure 3. The three PEO-b-PMMA-b-
PS triblock copolymers, having volume fraction of PS between
7
4 and 84% (4b, 4e, and 4h), exhibited a cylindrical micro-
1
H NMR based on the known molecular weight of 5000 amu
for the starting PEO block (n ) 112) (Figure 1b).
domain morphology as evidenced by higher order reflections
at q values of 1:x3:x4:x7:x9 relative to the first-order
reflection. The other triblocks, such as 4a which had the lowest
weight fraction of PS, exhibited only a first-order peak. The
absence of higher-order reflections is presumably due to the
lack of the long-range order in these bulk samples.
The fidelity of the RAFT polymerization process allowed full
characterization of all the triblock copolymers synthesized, and
the stability of the intermediate macroinitiator and subsequent
diblock copolymers permitted the molecular weight of the
PMMA and PS blocks to be accurately controlled. As a result,
a library of triblock copolymers were prepared in which the
overall bulk morphologies were maintained as a bulk polystyrene
domain with cylindrical microdomains of PMMA/PEO (Table
To investigate the morphology of the corresponding thin
films, scanning force microscopy, SFM, was used, and Figure
4
shows the SFM phase images of a selection of three triblock
copolymers on silicon substrates, 4a, 4b, and 4e, shortly after
spin-coating. The resulting structures are poorly defined, and it
is difficult to identify any morphology or orientation of the
microdomains. However, if the films are annealed in the
presence of benzene vapor and a controlled humidity atmosphere
(70-90% relative humidity), well-defined structures are ob-
served, where arrays of nanoscopic, cylindrical microdomains
of the minor components are seen at the surface of films for all
three triblocks (Figure 5a-c). As these systems are intended
as templates for block copolymer lithography, it was important
to examine the nanostructure present at the substrate surface as
1).
Interestingly, the richer phase behavior of ABC triblock
copolymers permitted a range of nanostructures to be studied
within this overall cylindrical domain space. This allows the
effect of morphology and phase separation of the PMMA and
PEO blocks on the thin-film behavior and degradability to be
examined by varying the molecular weight of the middle PMMA
block from 1500 to 16 000 amu. The overall molecular weights
of 4 were therefore 20 000-121 000 g/mol, and in all cases,
narrow polydispersity indexes (PDI < 1.1) were obtained with
7624 J. AM. CHEM. SOC.
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VOL. 128, NO. 23, 2006

Thin Films from ABC Triblock Copolymers
A R T I C L E S
Figure 1. 1H NMR spectrum of (a) PEO-RAFT macroinitiator, 2, and (b) PS-b-PMMA-b-PEO triblock copolymer, 4. (Inset to a) Magnification of the end
group of PEO-RAFT macroinitiator.
Figure 2. SEC traces for (a) PEO-RAFT, 2 (Mn ) 5400 amu; PDI ) 1.04); (b) PS-b-PEO diblock, 3 (Mn ) 6900 amu; PDI ) 1.06); and (c) PS-b-
PMMA-b-PEO triblock copolymer, 4a (Mn ) 20 800 amu; PDI ) 1.05).
Table 1. Sample Characteristics of PS-b-PMMA-b-PEO Triblock
Copolymer
side by SFM. Significantly, the same structure was observed at
this interface as compared to the surface, which indicates that
the cylindrical domains span the entire thickness of films and
are oriented normal to the film surface. Of perhaps greater
importance is the observation that these triblock copolymers
films develop a defect-free, hexagonally packed cylindrical array
as was observed for the PS-b-PEO diblock copolymers.34 It is
noteworthy that the incorporation of the middle block has little
effect on the ordering behavior of PS and PEO, which suggests
that a wide range of structural and functional units may be
incorporated as a middle block without disrupting long-range
ordering, further enhancing the potential applications of these
systems.
M
PS
M
PMMA
M
PEO
D
c-c
(nm)a
sample
(kDa)
(kDa)
(kDa)
f
PS
f
PMMA
f
PEO
M
w
/M
n
4
4
4
4
4
4
4
4
a
b
c
d
e
f
13.5
19.4
22.6
35.5
32.4
45.0
50.6
100.0
1.5
2.6
3.0
5.0
6.0
6.5
11.7
16.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
0.69 0.07 0.24
0.74 0.08 0.18
0.76 0.09 0.16
0.80 0.10 0.10
0.77 0.13 0.11
0.81 0.10 0.09
0.77 0.16 0.07
0.84 0.12 0.04
1.03
1.03
1.05
1.06
1.05
1.05
1.03
1.07
22.9
27.6
28.1
33.3
36.4
37.3
48.5
58.7
g
h
a
Measured by SAXS (Dc-c ) 4π/ 3q*).
x
well, because it may be significantly different from the surface
structure at the air interface.
The wafer-polymer interface was therefore examined by first
removing the substrate and floating of the polymeric film using
HF followed by flipping of the film and examining the “bottom”
(34) As with the case of PEO-b-PS diblock copolymers, it should be also noted
that a certain level of the relative humidity also plays an important role on
the long-ranged ordering. For example, the solvent annealing below the
relative humidity of 70% does not induce the long-ranged ordering. In this
case, all cylinders were oriented horizontally to the substrate (data not
shown). An optimal relative humidity on the long-ranged ordering is
between 70 and 90% for these PEO-b-PMMA-b-PS triblock copolymers.
J. AM. CHEM. SOC.
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VOL. 128, NO. 23, 2006 7625

A R T I C L E S
Bang et al.
removed at all (Figure 6a,b). The same results were observed
for different film thickness and for even longer UV exposure
times. When the length of the PMMA block increases (4d),
essentially all of the microdomains were removed (Figure 6c),
and for 4e and 4g, which have PMMA blocks of 6000 and
1
1 700 amu respectively, it was found that a nanoporous
structure was successfully produced after UV irradiation.
Significantly, this irradiation and subsequent washing did not
induce any change in the film structure, and highly oriented,
defect-free, nanoporous films were prepared (Figure 6c,d). It is
also evident from all of the TEM images that the nanopores
span the entire film. However, it is apparent from these samples
that there is an optimal molecular weight for the triblock
copolymer systems. For the nanoporous template obtained from
4
h, which has the longest PMMA block and the largest overall
molecular weight, cylinder domains were still oriented perpen-
dicularly to the substrate; however, the long-ranged ordering
was not attained, which may be due to a reduced mobility for
these higher molecular weights (Figure 6f).
Figure 3. SAXS traces illustrating the cylindrical morphologies for PEO-
b-PMMA-b-PS triblock copolymers 4a, 4b, 4e, and 4h. The arrows indicate
From these results, it is apparent that PS-b-PMMA-b-PEO
triblock copolymers can be used to generate highly ordered
nanoporous templates with both a minimum and maximum
length of the PMMA block for defect-free porous structures after
UV irradiation. To understand the origin of this lower critical
molecular weight (<5.0 kDa) in greater detail, thin-film
morphologies of a range of samples before UV exposure were
examined. As a representative example, Figure 7a,b shows the
TEM images of 4b (PMMA ) 2.6 kDa and PEO ) 5.0 kDa)
and 4e (PMMA ) 6.0 kDa and PEO ) 5.0 kDa) prior to UV
irradiation, respectively. It is immediately apparent that the two
triblock copolymers exhibit totally different phase behavior with
the lower molecular weight derivative, 4b, showing a single
diffuse cylindrical phase, whereas the higher molecular weight
triblock, 4e, reveals a structured core-shell morphology (see
schematic representations). Examination of the DSC traces for
these triblock copolymers provides further evidence to support
the morphology. As can be seen in Figure 8, for a constant
molecular weight of PEO (5.0 kDa), the crystallinity of the PEO
block decreases significantly as the molecular weight of the
PMMA block increases from 1.5 to 2.6 kDa and is totally absent
at 6.0 kDa. The miscibility of PMMA and PEO for the lower
molecular weight blocks found in 4a allows the formation of a
single PMMA-PEO cylindrical domain, and the additional
chain length of PMMA confers enough chain mobility for the
PEO chain to crystallize. As the molecular weight of the PMMA
block increases, the miscibility decreases, which results in a
phase separated core-shell structure for which the confinement
the possible reflections at q*/q spacings of 1:
drical morphology.
x
3:
x
4:
x
7: 9 for cylin-
x
The perfection of order is dramatically shown in the
triangulation map in Figure 5d-f, where every domain center
is connected by a line to the adjacent domain centers. Each
“
perfect” domain with six nearest neighbors is colored blue,
and defects with five or seven nearest neighbors are colored
red and yellow, respectively. As seen in Figure 5d, every domain
has six nearest neighbors, and this high degree of lateral order
was found to extend over large areas for all triblock copolymers.
The domain spacing calculated from the SFM images exactly
matches that measured by SAXS (see Table 1). However, there
is not enough phase contrast to identify all three phases in the
triblock copolymer film by SFM, and it is impossible to state
whether the cylindrical microdomains are composed of a single
mixed PEO/PMMA phase or distinct PEO and PMMA phases.
The selective removal of the minor components, cross-linking
of the major component, and generation of a nanoporous
structure in PS-b-PMMA-b-PEO triblock copolymers films were
achieved by deep UV irradiation followed by acetic acid rinsing.
This treatment is well-known to degrade and remove PMMA
but does not induce any cleavage or degradation in PS-b-PEO
diblock copolymer films. Figure 6 shows TEM images of six
triblock copolymer films after UV irradiation and acetic acid
rinsing.
For 4a and 4b, contrary to expectations, the minor compo-
nents were only partially removed and in some areas not
Figure 4. SFM phase images for PEO-b-PMMA-b-PS triblock copolymer thin films of (a) 4a, (b) 4b, and (c) 4e after spin-coating.
626 J. AM. CHEM. SOC. VOL. 128, NO. 23, 2006
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9

Thin Films from ABC Triblock Copolymers
A R T I C L E S
Figure 5. SFM phase images for PEO-b-PMMA-b-PS triblock copolymer thin films of (a) 4a, (b) 4b, and (c) 4e after annealing for 12 h in a benzene vapor
with controlled humidity. (d), (e), and (f) correspond to triangulation images of SFM images shown in (a), (b), and (c), respectively.
Figure 6. TEM images (top view) for PEO-b-PMMA-b-PS triblock copolymer thin films of (a) 4a, (b) 4b, (c) 4d, (d) 4e, (e) 4f, and (f) 4h after solvent
annealing, followed by UV irradiation. The bright circles correspond to nanopores where the cylindrical microdomains are removed after UV irradiation.
of the PEO to the central domain does not allow the PEO chains
to crystallize. This transition from phase-mixed to phase-
separated microdomains also provides a rationale for the changes
observed for photodegradation of the triblock copolymers.
Previous work has shown that the photochemical degradation
of PMMA is severely retarded in the presence of solvents such
as tetrahydrofuran, which contain an ether linkage, as well as
in polymer blends with poly(vinyl acetate).35 It is therefore
proposed that a similar effect is operating in these triblock
copolymer systems where the photodegradation of PMMA is
retarded at lower molecular weights of PMMA due to the
formation of a miscible blend with PEO and only when complete
phase separation occurs does the central PMMA block undergo
efficient photodegradation.
These results further highlight the importance of structural
control in polymer materials where small changes in the chain
length of the PMMA block leads to dramatically different
(35) Kaminska, A.; Swiatek, M. J. Therm. Anal. 1996, 46, 13830-1390.
J. AM. CHEM. SOC.
9
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A R T I C L E S
Bang et al.
Figure 7. TEM images (top view) for PEO-b-PMMA-b-PS triblock copolymer thin films of (a) 4b and (b) 4e after solvent annealing. The schematics below
illustrate the possible structures of the cylindrical microdomains.
UV irradiation to degrade the PMMA block and generate an
almost perfect hexagonal array of 10-15 nm diameter cylindri-
cal holes. Significantly, this behavior was found to depend
critically on the molecular weight and phase behavior of the
central PMMA block, and only when the PMMA block formed
a distinct microdomain did facile degradation occur. When the
PMMA is miscible with PEO, phase separation occurs to give
well-ordered arrays of PMMA/PEO cylinders in a PS matrix;
however, photochemical degradation of the PMMA chains is
retarded, and no template is formed. The ability to tune the
complex phase morphologies of triblock copolymer systems by
synthetically varying the block lengths and chemical nature of
each block is a powerful tool for the fabrication of functional
nanostructures. By combining the desired features of multiple
block copolymers into a single system, greater variability and
nanostructural control is obtained when compared to traditional
diblock copolymer systems.
Figure 8. DSC traces for PEO-b-PMMA-b-PS triblock copolymer thin
films of (a) 4a, (b) 4b, and (c) 4e.
Experimental Section
Materials were characterized by 1H nuclear magnetic resonance
(NMR), using a Bruker 200 MHz spectrometer with the residual solvent
signal as an internal reference. Gel permeation chromatography was
performed in THF on a Waters chromatograph equipped with four 5-µm
Waters columns (300 × 7.7 mm) connected in series with increasing
pore size (100, 1000, 10 000, and 1 000 000 Å). Waters 410 differential
refractometer index (DRI) and 996 photodiode array detectors were
employed. The molecular weights of the polymers were calculated
relative to linear PMMA and PS standards. The modulated differential
scanning calorimetry (DSC) measurements were performed with a TA
performance in block copolymer lithography. Only when the
PMMA block forms a separate phase in a cylindrical core-
shell morphology can UV irradiation effectively degrade the
PMMA block leading to generation of the block copolymer
template.
Conclusion
Exploiting the facile degradability of PS-b-PMMA block
copolymers, combined with the long-range ordering of PS-b-
PEO systems, has allowed the development of a versatile
methodology for the fabrication of highly ordered arrays of
nanopores via the self-assembly of triblock copolymers. The
synthesis of PS-b-PMMA-b-PEO triblock copolymers by RAFT
polymerization allows the middle PMMA block to function as
a sacrificial block while the terminal PEO block directs phase
separation. This allows templates for block copolymer lithog-
raphy to be prepared by initial solvent annealing followed by
-
1
Instruments, DSC 2920, and with a ramp rate of 4° min . Glass-
transition temperatures (T ) were taken as the midpoint of the inflection
g
tangent, upon the third heating scan. Infrared spectra (IR) were obtained
on Perkin-Elmer Spectrum BX FT-IR system using either diffuse
reflectance sampling accessories or drop deposition onto NaCl plates.
4
-(Dimethylamino)pyridinium-4-toluenesulfonate (DPTS) was pre-
pared through the reaction of 4-(dimethylamino)pyridine (DMAP) and
anhydrous p-toluenesulfonic acid (PTSA) in benzene according to the
7628 J. AM. CHEM. SOC.
9
VOL. 128, NO. 23, 2006

Thin Films from ABC Triblock Copolymers
A R T I C L E S
literature.36 PEO was dried by dissolving in toluene, followed by
azeotropic distillation at 60 °C; styrene was stored over CaH and then
2
vacuum distilled, and 2,2′-azobisisobutyronitrile (AIBN) was recrystal-
lized from methanol. All other reagents were purchased from com-
mercial sources and used as received unless otherwise noted.
R-Bromo-phenylacetate Terminated Poly(ethylene oxide), 1.
g, 29 mmol), was dissolved in benzene (1 mL), and the reaction mixture
was degassed by three freeze-pump-thaw cycles. The glass ampule
was sealed under vacuum and then heated at 115 °C for 20 h
(conversion of styrene was ca. 70%). The reaction mixture was
dissolved in dichloromethane (5 mL) and precipitated into hexane (250
mL). To remove any unfunctionalized PEO homopolymers, the
precipitate was further dissolved in dichloromethane (5 mL) and
reprecipitated in methanol (250 mL) to yield purified the triblock
Monomethoxy poly(ethylene oxide) (M
g, 2.00 mmol) and DMAP (49 mg, 0.4 mmol) were dissolved in toluene
20 mL) and dried by azeotropic distillation. To this solution was added
n
) 5 000, PDI ) 1.05) (10.0
1
(
copolymer, 4, as a pink solid. Yield: 65%; H NMR (200 MHz, CDCl )
3
dichloromethane (30 mL), R-bromophenylacetic acid (1.29 g, 6.00
mmol), DPTS (295.0 mg, 1 mmol), and dicyclohexylcarbodiimide
δ: 0.74-2.30 (m, CH , CH, CH ), 3.45-3.64 (s, OCH ), 3.64-3.78
2
3
3
(s, OCH ), and 6.28-7.34 (m, ArH); M ) 45 500; PDI ) 1.06.
2
n
(DCC) (2.07 g, 10 mmol). The reaction mixture was then stirred at
Preparation of the Thin Films. General Procedures. The PEO-
b-PMMA-b-PS triblock copolymers were spin coated from benzene
solutions onto silicon substrates and then annealed in a benzene
atmosphere, where the relative humidity was controlled between 70
and 90%. The film thickness was controlled by adjusting the solution
room temperature for 12 h, filtered, and added dropwise to hexane (500
mL). The crude product was filtered, and the desired functionalized
PEO derivative, 1, was further purified by precipitation into hexane.
1
Yield: 67%. M
.24-4.05 (s, OCH
R-(Dithiobenzoate)-phenylacetate Terminated PEO, RAFT-PEO
n
: 5 400. PDI: 1.04. H NMR (200 MHz, CDCl
3
) δ:
3
2
), 5.40 (s, CH), and 7.32-7.61 (m, ArH).
30
concentration and the spinning speed. To cleave the PMMA block,
the copolymer films were irradiated by deep UV light with a wavelength
Macroinitiator, 2. Carbon disulfide (165 mg, 0.13 mL, 2.2 mmol) was
added dropwise to a solution of phenylmagnesium bromide (0.73 mL
of a 3.0 M solution in diethyl ether, 2.2 mmol) in dry tetrahydrofuran
-2
of 254 nm at a dose of 25 J cm (XX-15S; UVP Inc.) under vacuum
for 10 min, rinsed in acetic acid, and then rinsed in water.
Characterization. To determine the bulk morphologies for triblock
copolymers, small-angle X-ray scattering (SAXS) was performed using
a rotating anode with CuΚ X-rays (λ ) 1.54 Å). A sample-to-detector
R
distance of 1.7 m was used to access the necessary q range. The two-
dimensional (2-D) SAXS images were azimuthally averaged to produce
one-dimensional profiles of intensity, I, vs wavevector, q, using the
(30 mL) at 50 °C. The reaction mixture was stirred for 30 min under
nitrogen, resulting in a dark-red solution. To this solution was added
the functionalized PEO, 1 (5.5 g, 1.1 mmol) dissolved in dry
tetrahydrofuran (10 mL), and the reaction mixture was heated at reflux
for 24 h. The solution was then filtered and precipitated into hexane
(
500 mL) to yield the PEO-RAFT macroinitiator, 2, as a pink solid.
2
-D data reduction program FIT2D.
1
Yield: 84%. M
3
8
n
: 5,400. PDI: 1.04. H NMR (200 MHz, CDCl
.25-4.06 (s, OCH ), 5.74 (s, CH), 7.31-7.60 (m, ArH), and 7.95 -
.05 (d, ArCSS).
PEO-b-PMMA-RAFT Diblock Copolymer, 3. A mixture of the
3
) δ:
Scanning force microscopy (SFM) images were obtained in both
2
the height and phase-contrast mode using a Digital Instruments
Dimension 3000 scanning force microscope in the tapping mode. Films
for transmission electron microscopy (TEM) were prepared on silicon
substrates having a thick layer of silicon oxide. These films were floated
onto the surface of a 5 wt % HF solution, transferred to a water bath,
and then picked up on a Cu grid. A JEOL 100CX electron microscope
operating at 100 kV was used to examine the morphology.
PEO-RAFT macroinitiator, 2 (1.0 g, 0.2 mmol), MMA (2.0 g, 20
mmol), and AIBN (5 mg, 0.03 mmol) was dissolved in benzene (1
mL) and degassed by performing the three freeze-pump-thaw cycles.
The glass ampule was sealed under vacuum and then heated to 70 °C
for 5 h (conversion of MMA was ca. 50%). After this time, the viscous
reaction mixture was dissolved in dichloromethane (5 mL) and
precipitated into hexane (400 mL) to give the diblock copolymer, 3,
Acknowledgment. This work is based upon work supported
by the National Science Foundation under the MRSEC program
(UCSB MRL, DMR-0520415 and UMass MRSEC, DMR-
0213695), the Army Research Office through its MURI
program, and the Department of Energy Office of Basic Energy
Science.
1
as a pink solid. Yield: 43%. H NMR (200 MHz, CDCl
.08 (m, CH , CH, CH ), 3.45-3.60 (s, OCH ), and 3.64-3.80 (s,
OCH ); M ) 10 200; PDI ) 1.05.
3
) δ: 0.65-
2
2
3
3
2
n
PEO-b-PMMA-b-PS Triblock Copolymers, 4. The RAFT-
terminated diblock copolymer, 3 (0.5 g, 0.05 mmol) and styrene (3.0
(36) Moore, J. S.; Stupp, S. I. Macromolecules 1990, 23, 65-70.
JA0608141
J. AM. CHEM. SOC.
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VOL. 128, NO. 23, 2006 7629