Anisotropic self-assembly of gold nanoparticle grafted with polyisoprene and polystyrene having symmetric polymer composition

Article abstract of DOI:10.1021/ja402412q

Methodology to self-assemble metal nanoparticles into three-dimensional mesoscale patterns is a fundamental technique to construct functional materials. Here, we demonstrate that hybridizing an immiscible polymer pair with a metal nanoparticle allows the hybrid to self-assemble in the film, resulting in spontaneous alignment of the nanoparticles at the phase-separated interface formed by the constituent polymers. Organic-inorganic hybrids composed of polyisoprene, polystyrene, and gold nanoparticle were prepared by multistep "grafting-to" method coupled with alkyne-azide click reaction. The polymer composition can be controlled by the feed ratio of gold nanoparticle to azide ligands. The gold nanoparticle hybrid with symmetric polymer composition forms an "alternating lamellar" structure of polyisoprene and polystyrene, where the gold nanoparticles were forced into the phase-separated interfaces.

Full text of DOI:10.1021/ja402412q

Communication
pubs.acs.org/JACS
Anisotropic Self-Assembly of Gold Nanoparticle Grafted with
Polyisoprene and Polystyrene Having Symmetric Polymer
Composition
Tatsuhiro Nakano, Daisuke Kawaguchi,§ and Yushu Matsushita*
Department of Applied Chemistry, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
S
* Supporting Information
as templates for assembly of nanoparticles without possessing
their own ability to self-assemble.
ABSTRACT: Methodology to self-assemble metal nano-
particles into three-dimensional mesoscale patterns is a
fundamental technique to construct functional materials.
Here, we demonstrate that hybridizing an immiscible
polymer pair with a metal nanoparticle allows the hybrid to
self-assemble in the film, resulting in spontaneous
alignment of the nanoparticles at the phase-separated
interface formed by the constituent polymers. Organic−
inorganic hybrids composed of polyisoprene, polystyrene,
and gold nanoparticle were prepared by multistep
“grafting-to” method coupled with alkyne−azide click
reaction. The polymer composition can be controlled by
the feed ratio of gold nanoparticle to azide ligands. The
gold nanoparticle hybrid with symmetric polymer
composition forms an “alternating lamellar” structure of
polyisoprene and polystyrene, where the gold nano-
particles were forced into the phase-separated interfaces.
To the contrary, if nanoparticles are able to hybridize with
immiscible polymers with more than two components, the
nanoparticle−polymer hybrids can be expected to attain the
ability to self-assemble into ordered structures with a
nanometer scale and their nanoparticles are forced into the
phase-separated interfaces without templates. Such hybrids
must be similar to multicomponent star polymers which form
microphase-separated structures and their junction points align
in the interfaces.23
Here we demonstrate how to hybridize AuNPs with two
immiscible polymers composed of polyisoprene (I) and
polystyrene (S) controlling the polymer composition via
multistep “grafting-to” methods using amine-thiol ligand
exchange and alkyne−azide click reaction. The hybrid
comprised of AuNP, I and S (AuNP-IS) with symmetric
polymer composition forms a lamellar structure of I and S and
the AuNPs are located at the I/S phase-separated interface.
Thiol-terminated polyisoprene (I-SH) and ethynyl-termi-
nated polystyrene (S-yne) with Mw of 55 kDa and 60 kDa,
respectively, were prepared by living anionic polymerizations
and a subsequent chain end modification. 1-Azidoundecan-11-
thiol (Az-SH) was prepared following the previous reports24,25
and used as a ligand having a clickable site. AuNP capped with
dodecylamine ligands were prepared by single-phase ap-
t is of importance to control the spatial distribution of
Inanoparticles to construct functional materials such as all-
optical storage, photovoltaic and plasmonic devices.1 Many
efforts have been devoted to control the spatial distribution of
nanoparticles at the nanometer scale using self-assembly of
layer-by-layer of polyelectrolytes,2−4 synthetic polymers5−7 and
DNA,8,9 etc. Among these methods, block copolymers have
most often been utilized for the fabrication because they form
mesoscale three-dimensional ordered structures in bulk so-
called microphase-separated structures. The basic manner of
using the microphase-separated structures is either to
incorporate nanoparticles into microdomains10−14 or to make
them segregated in the phase-separated interfaces.11,13−16 For
the former case, two methods have been employed: one is
blending nanoparticles with block copolymers11−13,17 and
another is in situ synthesis of nanoparticles in micro-
domains.18,19 For the latter case, the selective spatial
distribution of nanoparticles near the polymer/polymer inter-
face has been achieved by utilizing interfacial segregation of
nanoparticles in microphase-separated structures, which is
controlled by the combination of loss entropy and thermody-
namic interactions between nanoparticle surface and constitu-
ent polymers.11,13−16 Recently, the segregation of gold
nanoparticles (AuNPs) was accomplished in the middle of
microdomains using supramolecular block-graft copoly-
mers.20−22 However, in all cases, block copolymers were used
proach,26 whose average diameter is 2.8
0.3 nm evaluated
by TEM observation (Figure S1, Supporting Information). The
hybridization of AuNP with I and S was conducted as follows.
First, the ligand-exchange reaction from dodecylamine to Az-
SH was conducted varying the Az-SH/AuNP feed ratio.
Successively, I-SH was grafted onto the AuNP surface covered
with the mixed ligands of dodecyamine and Az-SH. In this case,
dodecylamine is selectively substituted for I-SH due to the
difference in the bond strengths of amine-gold and thiol-gold.
S-yne was grafted onto the hybrids composed of AuNP and I-
SH via alkyne−azide click reaction between Az-SH and S-yne.
The crude hybrid products of AuNP-IS were purified by
fractional precipitation and preparative gel permeation
chromatography (GPC) to obtain AuNP-IS hybrids with no
residual homopolymers. The preparation and characterization
of AuNP-IS is described in detail in the Supporting
Information. The AuNP-IS hybrids were described with φs
Received: March 7, 2013
Published: April 24, 2013
© 2013 American Chemical Society
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dx.doi.org/10.1021/ja402412q | J. Am. Chem. Soc. 2013, 135, 6798−6801

Journal of the American Chemical Society
Communication
value, where φs is the average volume fraction of the S
1
component determined by H NMR and mass densities of I
and S.
Figure 1 shows the relationship between Az-SH/AuNP feed
ratio and polymer composition in AuNP-IS hybrids. The φs
○
Figure 1. Volume fraction of S (φs, ) and total grafting density of I
●
and S chains on AuNP (σ, ) as a function of the feed ratio of Az-SH/
AuNP. The average diameter of AuNP is 2.8 0.3 nm. The Mws of I
and S are 55k Da and 60k Da, respectively.
Figure 2. TEM micrographs of solvent-annealed AuNP-IS hybrid film.
(a) AuNP-IS-0.48 and b; its magnified one. Since the samples were
stained with OsO4, I and S phases appear dark and bright, respectively.
AuNPs appear black dots in b due to the highest electron density
among the components. (c and d) reconstructed TEMT images,
where the white dots correspond to AuNPs. The observing directions
in c and d are defined as edge-view and through-view, respectively. d is
obtained by rotating c by 90° about the vertical axis and by extracting
the information about the AuNPs distributed in the same plane.
value linearly increases with increasing the Az-SH/AuNP feed
ratio whereas the overall grafting density(σ) is constant at 1.4
0.1 chains nm−2, that is, independent of the feed ratio. This
means that the amount of the grafted I-SH chains can be
determined by the amount of the grafted Az-SH, and that the
amount of the grafted S chains is also limited by the excluded
volume effect of I chains preexisted on the AuNP surface.
Picking up the AuNP-IS hybrid with φs = 0.48 (AuNP-IS-0.48)
as an example, the numbers of the grafted S and I chains are
estimated to be 15 each since the surface area of AuNP is
calculated to be 24.6 nm2 based on the average AuNP diameter.
Hence, AuNP-IS-0.48 can be regarded as “the two-component
30-arm star copolymers with AuNP as a junction point”.
The structure of AuNP-IS-0.48 was examined using trans-
mission electron microscopy (TEM) (Figure 2). Figure 2a
shows an alternating lamellar structure consisting of I and S
phases and having a domain spacing (D) of ∼55 nm. Figure 2b
represents the enlarged TEM image of AuNP-IS-0.48, where
AuNPs are observed as black dots at the interfaces between I
and S phases observed as gray and white, respectively. To
confirm the spatial dispersion of AuNPs in the film evidently,
we observed a nonstained AuNP-IS-0.48 sample by trans-
mission electron tomography (TEMT). A snapshot of the
reconstructed TEMT image displays that AuNPs are aligned in
the longitudinal direction (Figure 2c), where this observing
direction is defined as edge-view. This is consistent with Figure
2a, which shows that AuNPs are forced into the I/S interfaces.
On the other hand, a snapshot of the reconstructed TEMT
image observed from through-view represents the two-dimen-
sional dispersion of AuNPs with the average nearest neighbor
distance of ∼15 nm (Figure 2d). Hence, these results clearly
demonstrate that the AuNPs are anisotropically distributed at
the phase-separated interface in alternating lamellar structures
formed by the constituent grafted polymers of I and S onto
AuNP.
direction, the integer order peaks were observed (Figure 3b).
Note that the intensities of even number peaks are relatively
higher than those of odd number ones. This is in stark contrast
to the SAXS pattern from lamellar structures of symmetric
diblock copolymers showing that the relative intensities of odd
number peaks are much higher than those of even number
peaks. This result clearly indicates that the regular array of
AuNPs in mesoscale leads to the characteristic SAXS profile.
To quantify the structure of AuNP-IS-0.48, the SAXS
intensity profile for the transverse direction was analyzed
using the paracrystal method.27−29 The microphase-separated
structure of AuNP-IS-0.48 observed from the edge-view
direction can be regarded as ABCB four-layer type lamellar
structure of an ABC triblock copolymer based on TEM image
shown in Figure 2. Assuming that the number of lamellae in a
grain is large enough, the SAXS intensity (I(q)) of the lamellar
structure can be approximated by
I(q) ∝ P(q)Z(q)q−2
(1)
where q is scattering vector defined by q = (4π/λ)sin θ, λ and
2θ being the wavelength of the X-ray and the scattering angle,
respectively. P(q) and Z(q) are the form factor and the lattice
factor, respectively. The form factor of the one-dimensional
particle of ABCB type lamella (P(q)Lam_ABCB) is given by29
P(q)LamABCB
2
The structure of AuNP-IS-0.48 in large area was also
examined by small-angle X-ray scattering (SAXS). The
corresponding 2D SAXS pattern of AuNP-IS-0.48 from edge-
view geometry depicts a strong anisotropy shown in Figure 3a.
Taking the sector average (azimuthal angle μ = 0 5° and 180
5°) for the AuNP-IS-0.48 with respect to the transverse
sin(qx/2)
(qx/2)
σ2q2
2
sin(qy/2)
(qy/2)
σ2q2
2
⎧
⎨
⎩
⎫
⎬
⎭
⎛
⎝
⎞
⎟
⎠
⎛
⎝
⎞
⎟
⎠
⎜
⎜
=
k1
exp −
− k2
exp −
∬
2
⎧
⎪
2 ⎫
⎪
⎧
⎫
−[x − (2d2 + d3)]
−(y − d3)
2σy2
⎨
⎩
⎬
exp
⎨
⎪
⎩
⎬
⎪
⎭
× exp
dydx
2σx2
⎭
(2)
6799
dx.doi.org/10.1021/ja402412q | J. Am. Chem. Soc. 2013, 135, 6798−6801

Journal of the American Chemical Society
Communication
2
⎡
⎤
3
⎢
⎥
⎦
P(q)AuNP = k3
[sin(qR) − qRcos(qR)]
q3R3
⎣
(6)
where R is the radius of the AuNP and k3 is the electron density
difference between AuNP and I or S phases. In this case, since
the electron density of AuNP is much higher than those of I
and S phases, the electron density difference between I and S
phases is assumed to be negligibly small. On the whole, the
total SAXS intensity (I(q)) is given by,
I(q) = A × (P(q)LamABCB × Z(q)Lam + P(q)AuNP
where A is a scale factor.
)
(7)
The solid line in Figure 3b is calculated from eqs 1−7 and is
in good agreement with the experimental values. Hence, it is
reasonable to consider that one-dimensional electron density
profile shown in Figure 3c reflects the actual concentration
profile. The thickness of AuNP phase evaluated by this method
is 2.3 nm, which corresponds to the diameter of AuNP. This
fact strongly indicates that AuNP aligns at S/I interfaces in a
broad range. Furthermore, the thicknesses of the I and S phases
are 23.6 and 24.8 nm, respectively, resulting in D of 53.0 nm.
̅
Hence, SAXS data of AuNP-IS-0.48 are in good agreement with
TEM observation.
In contrast to the SAXS pattern from edge-view, the 2D
SAXS pattern from through-view shows an isotropic pattern
(Figure 3d). Figure 3e shows the circular-averaged SAXS
intensities obtained from Figure 3d. A broad peak was observed
at around q ≈ 0.5 nm−1, and the associated correlation length
calculated from d = 2π/q is 13 nm. This is again in good
agreement with the interparticle distance of AuNPs evaluated
from the TEM image shown in Figure 2d. Besides, the
longitudinal sector-averaged 1D SAXS profile taken from
Figure 3a is similar to the circular-averaged SAXS profile
from through-view. Hence, these results strongly support the
self-assembled structure of AuNP-IS-0.48 schematically shown
in Figure 3f.
In conclusion, we have demonstrated a concept for preparing
metal nanoparticle−polymer hybrids composed of AuNP,
polyisoprene and polystyrene by multistep “grafting-to”
methods and alkyne−azide click reaction. Their polymer
composition can be controlled by varying the Az-SH/AuNP
feed ratio, where the φs value linearly increases with increasing
the feed ratio. The AuNP-IS hybrid with symmetric polymer
composition forms an “alternating lamellar” structure of I and S
phases, where the AuNPs are selectively located at the I/S
interfaces because AuNP acts as a pseudojunction point of a
multiarm star copolymer. These results constitute an important
step toward making functional materials such as plasmonic
devices by self-assembly.
Figure 3. SAXS patterns of AuNP-IS hybrid with φs = 0.48. (a) 2D
SAXS pattern from edge-view showing an anisotropic pattern. (b)
Sector-averaged intensities with the sector angle of 5° with respect to
the transverse direction. In b, the open circles are experimental values
and the solid line is calculated one from eqs 1∼7 based on the model
1D electron density profile shown in c. (d) 2D SAXS pattern from
through-view shows an isotropic pattern. (e) Comparison of the
circular-averaged intensities from d with the sector-averaged ones from
a with the sector angle of 5° with respect to the longitudinal
direction. (f) Self-assembled structure of AuNP-IS-0.48.
where d1, d2, and d3 are the thicknesses of I phase, S phase,
AuNP phase, and interface, respectively. k1 denotes the electron
density difference between AuNP and I phase while k2 is that
between S and I phases (cf. Figure 3c). σ is related to the
interfacial thickness t by the relation, t = (2π)1/2σ. The
integration interval is 0−2d3 for y and 0−2(2d2 + d3) for x. The
lattice factor for one-dimensional lamellar structure (Z(q)Lam
)
with domain spacing (D = d + 2d + d ) is given by27−29
̅
1
2
3
1 − |F(q)|2
ASSOCIATED CONTENT
* Supporting Information
Detailed synthetic procedures, experimental techniques. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Z(q)Lam
=
■
1 − 2|F(q)|cos(qD) + |F(q)|2
̅
(3)
S
2
2 2
⎛
⎜
⎝
⎞
⎟
⎠
−g D q
̅
|F(q)| = exp
2
(4)
(5)
g ≡ σD/D
AUTHOR INFORMATION
Corresponding Author
yushu@apchem.nagoya-u.ac.jp
■
where g is Hosemann’s g parameter defined by eq 5. σD is the
standard deviation of D assuming a Gaussian distribution of D.
In addition, the scattering from each AuNP also contributes to
the total SAXS intensity. The form factor for the AuNP-
(P(q)AuNP) is given by
Present Address
§Education Center for Global Leaders in Molecular Systems for
Devices, Kyushu University, Japan
6800
dx.doi.org/10.1021/ja402412q | J. Am. Chem. Soc. 2013, 135, 6798−6801

Journal of the American Chemical Society
Communication
(26) Jana, N. R.; Peng, X. G. J. Am. Chem. Soc. 2003, 125, 14280.
(27) Sakurai, S.; Okamoto, S.; Kawamura, T.; Hashimoto, T. J. Appl.
Crystallogr. 1991, 24, 679.
Notes
The authors declare no competing financial interest.
(28) Shibayama, M.; Hashimoto, T. Macromolecules 1986, 19, 740.
(29) Tanaka, Y.; Hasegawa, H.; Hashimoto, T.; Ribbe, A.; Sugiyama,
K.; Hirao, A.; Nakahama, S. Polym. J. 1999, 31, 989.
ACKNOWLEDGMENTS
■
This research was in part supported by the Grant-in-Aids for
young scientist (A) (No.22685013) and scientific research (A)
(No.22245038) from the Ministry of Education, Culture,
Sports, Science and Technology (MEXT), Japan. Small-angle
X-ray scattering measurements were carried out at Photon
Factory (PF), Institute of Materials Structure Science, High
Energy Accelerator Research Organization, Japan (Program
No. 2012G628 and 2010G642). We thank Kunihito Koumoto,
Chunlei Wan and Keiko Ohta in Department of Applied
Chemistry, Nagoya University for helping TGA measurements.
We are also grateful to Shigeo Arai and Kohei Matsuoka for the
reconstruction of 3D-TEM tomography. D.K. thanks Truong
Cao Tue for the valuable discussion.
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