Oriented nanoporous lamellar organosilicates templated from topologically unsymmetrical dendritic-linear block copolymers

Article abstract of DOI:10.1002/anie.200501577

Dendrimers between the sheets: Environmentally responsive dendritic-linear block copolymers, based on poly(ethylene oxide) and a dendron derived from 2,2′-bis(hydroxymethyl)propionic acid (see structure), were used to organize organosilicate vitrificates into nanostructured lamellar morphologies. Upon thermolysis of the template, a perforated porous lamellar structure (4 nm) between organosilicate sheets (6-9 nm) was obtained. (Chemical Equation Presented).

Full text of DOI:10.1002/anie.200501577

Communications
Template Synthesis
DOI: 10.1002/anie.200501577
Oriented Nanoporous Lamellar Organosilicates
Templated from Topologically Unsymmetrical
Dendritic-Linear Block Copolymers**
Teddie Magbitang, Victor Y. Lee, Jennifer N. Cha,
Hsiao-Lin Wang, W. Richard Chung, Robert D. Miller,
Geraud Dubois, Willi Volksen, Ho-Cheol Kim,* and
James L. Hedrick*
Microstructured porous materials have potential application
in a variety of areas, including separation techniques,
catalysis, sensing, coatings, microelectronics, and electroop-
[
*] T. Magbitang, V. Y. Lee, Dr. J. N. Cha, Dr. R. D. Miller, Dr. G. Dubois,
Dr. W. Volksen, Dr. H.-C. Kim, Dr. J. L. Hedrick
IBM Almaden ResearchCenter
650 Harry Road, San Jose, CA 95120 (USA)
Fax: (+1)408-927-3310
E-mail: hedrick@almaden.ibm.com
H.-L. Wang, Prof. W. R. Chung
Department of Chemical and Materials Engineering
San Jose State University
San Jose, CA (USA)
[**] The authors acknowledge support from the NSF Center for Polymer
Interfaces and Macromolecular Assemblies (CPIMA: NSF-DMR-
0213618) and the National Institute of Standards and Technology,
US Department of Commerce, for the use of the neutron research
facilities. This investigation also utilized facilities supported in part
by the National Science Foundation under Agreement No. DMR-
9
986442. We thank Robert M. Briber, Hongxia Feng, and Zhaoliang
Lin for their assistance with the small-angle neutron scattering
SANS) experiments. Portions of this research were carried out at
(
the Stanford Synchrotron Radiation Laboratory, a national user
facility operated by Stanford University on behalf of the US
Department of Energy, Office of Basic Energy Sciences. The SSRL
Structural Molecular Biology Program is supported by the Depart-
ment of Energy, Office of Biological and Environmental Research,
and by the National Institutes of Health, National Center for
Research Resources, Biomedical Technology Program. We thank
Mike F. Toney, Hiro Tsuruta, and Igor Smolsky for their assistance
witht he small-angle X-ray scattering (SAXS) experiments and
Philip M. Rice, Leslie E. Thompson, and Eugene A. Delenia for the
transmission electron microscopy (TEM) studies.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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[
1]
tics. Silica-based mesoporous materials, prepared by the
assembly of periodic inorganic and surfactant-based struc-
tures, have received considerable attention because of the
ture-directing agents for organosilicates to generate a unique,
oriented perforated porous lamellar morphology.
PEO hydroxy-functionalized oligomers (M = 4800 and
w
[
2]
À1
diverse collection of possible pore sizes and shapes. More
recently, this concept has been extended to include lyotropic
liquid crystalline assemblies, ionic liquids, cyclodextrin-
based materials, triphenylene as an electron donor (charge-
transfer agents), phthalocyanine-based amphiphiles, and
9800 gmol ; polydispersity indices (PDI) = 1.03 and 1.05,
respectively) were chosen as the linear component of the
copolymer, as PEO has been shown to be miscible with the
[
3]
[4]
[
5]
methyl silsequioxane (MSSQ) prepolymer over wide compo-
[
6]
[7]
[8c,g]
sitional and molecular-weight ranges.
Dendrons derived
[
8]
block copolymers
as structure-directing agents. Block
from bis(MPA) were chosen as they decomposed cleanly and
[
16]
copolymers have the advantage of fine-tuning the polymer/
solvent/silica phase behavior through variation of the mono-
mer type, volume composition, or macromolecular architec-
quantitatively upon thermolysis; furthermore, they can be
prepared by both divergent and convergent methods and
provide exquisite NMR spectroscopic handles to follow the
[
9]
[17]
ture.
various chemical transformations. Frechet and co-workers
We have recently described the generation of nanoporous
organosilicates by using block copolymers that exhibit uni-
molecular self-organization within a vitrifying organosilicate
matrix. This process requires an environmentally responsive,
star-shaped macromolecule that organizes the organosilicate
vitrificate into nanostructures through a matrix-mediated
collapse of the interior core of the core–corona polymeric
structure. The insoluble core is rendered compatible in the
thermosetting matrixand dispersed through the outer corona
showed that hydroxy-terminated PEO is an excellent scaffold
for the divergent growth of dendrons using acetal-protected
anhydride derivatives of bis(MPA) and also importantly
demonstrated that the PEO facilitates product purification
[
18]
through precipitation. This divergent-growth approach was
employed using acetonide-protected bis(MPA) anhydride,
prepared from the self-condensation of the acetonide-pro-
tected bis(MPA) using N,N’-dicyclohexylcarbodiimide
(DCC), in the presence of dimethylaminopyridine (DMAP;
0.25 mol% with respect to the anhydride) in a solvent mixture
of CH Cl /pyridine (90:10). An approximate 2–4-fold excess
[
8c,g]
designed to be miscible with the matrix.
The unimolecular
nature of the amphiphile eliminates the complexdynamic
assembly that characterizes most amphiphiles, and the
curvature constraints of this architecture generate non-
interconnected spherical pores up to high volume fractions
2
2
of anhydride was used to insure quantitative conversion of the
terminal hydroxy groups, and the reaction was performed at
room temperature (approximately 15–18 h) under nitrogen
(Scheme 1). Upon completion, the excess anhydride was
treated with methanol to facilitate purification as the by-
products generated are soluble in diethyl ether and the PEO
copolymers do not allow polymer precipitation. Deprotection
of the acetonide groups was accomplished by using Dowex
50W-X2 acidic ion-exchange resin in methanol at 548C for
5 h. This general procedure was repeated for each generation
and for both poly(ethylene) glycol (PEG) molecular weights
surveyed, thus ultimately producing up to sixgenerations of
bis(MPA) dendron “block length” in the copolymers. These
(
approximately 50%) upon porogen burn out. Herein, the
synthesis of amphiphiles that are between conventional
surfactants and block copolymers, with the shape of the
former and the size of the latter, is described with the
objective of templating a wide variety of porous morphologies
through an amphiphile, organosilicate self-assembly process.
[
10]
Amphiphilic dendrons as described by Percec et al. are an
example of a system that self-assembles into a variety of
nanostructures. Alternatively, dendritic linear hybrid AB
amphiphilic block copolymers are also likely candidates, as
[
11]
13
Frechet, Meijer, Stupp, Hawker, and others have shown
that such architectures respond to changes in their environ-
ment through changes in the type and shape of micelles
formed. In addition, these copolymers assemble into well-
defined Langmuir–Blodgett monolayers that are capable of
the formation of nanostructured materials with unusual
transformations were followed using C NMR spectroscopy,
as the quaternary carbon atom of bis(MPA) is sensitive to the
substitution of the neighboring hydroxy groups, and the
spectra associated with dendritic, linear, and terminal sub-
stitution have been identified by model-compound studies.
The functionalization and deprotection steps are quantitative
irrespective of the generation (Figure 1). The quaternary
carbon atom of the acetonide-protected first generation
occurs at d ꢀ 42 ppm, which on deprotection shifts to
d ꢀ 50.5 ppm. Subsequent functionalization of the first gen-
eration produces the acetonide-protected second generation
(d ꢀ 42 ppm) and the inner first generation (d ꢀ 46 ppm);
incomplete transformations are readily detected. The surface
hydroxy groups of the dendrons for the fourth to sixth
generations were subsequently functionalized by esterifica-
tion with either protiated or deuterated heptanoic acid in
CH Cl using DCC/pyridinium 4-toluenesulfonate (DPTS)/
[
12]
properties and morphologies.
Wiesner and co-workers
used such amphiphilic dendrimers as structure-directing
agents for aluminosilicate-based materials and generated
hybrids with a wide variety of nanostructured morpholo-
[
13]
gies. As the shape of low-molecular-weight surfactants has
a pronounced effect on the type of aggregate structure formed
in silica-surfactant self-assembly, we felt that a dendritic linear
amphiphilic copolymer provides an excellent platform from
which to tailor the size and shape of the copolymer. Herein,
the responsiveness to change in the environment of the
copolymer can be tuned by the type, surface functionality, and
dendrimer generation together with the structure and molec-
2
2
[
19,20] 13
DMAP.
C NMR spectroscopy of the terminal quater-
[
14]
ular weight of the linear component. Herein, we describe
the use of amphiphilic dendritic linear copolymers, based on
poly(ethylene oxide) (PEO) and a dendron derived from 2,2’-
nary carbon atoms of bis(MPA) clearly showed quantitative
conversion into the esterification product (Figure 1). Func-
tionalization of the polymer chain with acetonide–bis(MPA)
(d ꢀ 42 ppm), followed by its deprotection (d ꢀ 50.5 ppm), is
[
15]
bis(hydroxymethyl)propionic acid (bis(MPA)),
as struc-
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Communications
Scheme 1.
clearly seen for each generation (up to generation 5). The
The amphiphilic polymers were dissolved in a solution
containing the MSSQ prepolymer in 1-methoxy-2-propanol,
and the resulting solution was spun on a silicon wafer to
produce thin films that were cured to 4308C to effect the first
network formation of the MSSQ and decomposition of the
sacrificial copolymer template. Representative tan d curves
obtained from DMA for the 1a/MSSQ hybrid (40 wt.%
copolymer loading) after curing to 808C (minimal advance-
ment in MSSQ molecular weight) and 1508C (significant
advancement in molecular weight of MSSQ, that is, network
formation) are presented in Figure 3 (also see the Supporting
Information) together with the copolymer for comparison.
Irrespective of the cure temperature, a two-phase structure is
fifth generation is then deprotected and functionalized with
heptanoic acid (d ꢀ 46 ppm). The general characteristics of
the fourth–sixth generation copolymers prepared from each
of the PEG oligomers, including the molecular weight and
weight fraction of the dendron and the molecular weight and
polydispersity of the entire copolymer, are given in Table 1.
The size-exclusion chromatograms of the fourth–sixth gen-
eration copolymers 1a–c (see Figure 1 and the Supporting
Information) and (2a–c) clearly demonstrate monomodal,
narrowly dispersed products (Figure 2). Thin films of these
block copolymers show microphase-separated morphologies,
as evidenced by the presence of two T values in the dynamic
g
mechanical analysis (DMA) spectra for the third–sixth
generation copolymers (Figure 3). Wiesner and co-workers
studied the morphologies of similar macromolecular archi-
clearly evident in the hybrid. The retention of the T value at
g
À258C associated with the bis(MPA) dendron phase and the
disappearance of the PEO transition (approximately À558C)
supports the hypothesis that a two-phase structure is present
[
21]
tectures in detail.
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1
3
Figure 1. C NMR spectra of copolymers 1a–c.
Figure 2. Size-exclusive chromatograms of copolymers 2a–c.
Table 1: Characteristics of dendritic-linear amphiphilic copolymers.
[
a]
[b]
Sample PEO
Dendron
Dendron Mn
Mn
PDI
À1
À1
segment generation fraction
[gmol
]
[gmol ]
À1
Mn
gmol
(M [gmol ])
n
À1
[
]
1
1
1
2
2
2
a
b
c
a
b
c
4800
4800
4800
9800
9800
9800
4
5
6
4
5
6
(4030) 45
9000
13000
16800
14000
18000
21500
9700
11300
14000
16500
18200
20000
1.08
1.03
1.04
1.05
1.05
1.04
(8200) 63
(12000) 71
(4030) 29
(8200) 45
(12000) 55
1
[
a] Detected by H NMR spectroscopy. [b] Detected by gel-permeation
chromatography.
even prior to cure. Although it is difficult to assign with
Figure 3. DMA of copolymer 1a (c), 1a/MSSQ mixture cured to
08C (d) and 2508C (a).
certainty, we believe it is likely that the T value at 508C is the
g
8
mixed T values of the PEG and MSSQ phase as they are
g
miscible. Small-angle neutron scattering (SANS) measure-
ments of 1a (in which the chain ends are selectively decorated
with deuterated heptanoic acid) confirm the two-phase
structure of the hybrid containing 40 wt.% copolymer (see
the Supporting Information). This structure, generated upon
spin coating, is thought to occur through a self-assembly
process facilitated by the selective solubilization of the PEO
by MSSQ and subsequent collapse of the dendron (namely,
the structure-directing agent). Further heating of hybrid
samples to 4308C removes the block-copolymer template to
create a porous thin film. For example, a porous film derived
from a hybrid containing 40 wt.% 1a has a refractive indexof
surveyed, all refractive indices and dielectric constants were
comparable for similar loading levels.
Cross-sectional transmission electron micrography
(TEM) images of MSSQ mixtures containing 20, 40, and
60 wt.% of 1b are shown in Figure 4. The sample of 20 wt.%
shows a porous structure (pore sizes: 4–5 nm) with a shape
that resides at the interface between the spherical and
cylindrical morphologies. Conversely, mixtures that contain
the higher volume fractions of copolymer manifest a nano-
porous lamellar structure that is oriented parallel to the
surface, and this morphology is maintained over a wide
compositional range (approximately 35–70%). Small-angle
X-ray scattering (SAXS) measurements on the porous films
show an increasing scattering intensity with porogen volume
1
.24 and a dielectric constant of 2.00, in comparison to the
dense MSSQ matrixwith values of 1.37 and 2.85, respectively.
Irrespective of the PEG block length or dendron generation
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Figure 5. SAXS profiles of porous organosilicates: a) SAXS profiles as
a function of weight fraction of G4-PEO5K; b) SAXS profiles of porous
films generated with40 wt.% of copolymer. T he samples were
prepared by thermal heating to 4508C after spin-casting.
condition (40 wt.% loading of 1b), thus indicating a layered
structure with porosity oriented parallel to the substrate
(Figure 6). The first-order Bragg peak corresponds to a period
of approximately 107 Š (2p/q). The higher-order peaks
correspond to periods of approximately 56 and 38 Š,
respectively.
Figure 4. TEM images of copolymer 1b withmixtures of MSSQ
containing a) 20, b) 40, and c) 60 wt.% copolymer.
fraction of 1a (Figure 5). At a loading of 40 wt.% and above,
a strong interpore correlation is observed with peaks that
correspond to approximately 129 Š (2p/q), and no significant
change was observed by increasing the copolymer loading to
Figure 6. XR profile of porous organosilicate thin films generated with
40 wt.% 1b. The samples were prepared by thermal heating to 4508C
after spin-casting.
6
0 wt.%. Figure 5b shows the effect of PEO molecular weight
and dendron generation on the pore structure. An increase in
interpore distance from 129 to 137 Š was observed for a
loading of 40 wt.% with increasing PEO molecular weight
from 5000 to 10000. The SAXS profiles for 2a and 2b show
the interpore distance increases from 137 to 197 Š as the
dendron generation is varied from the fourth to the fifth.
Although a porous morphology was observed for all calcined
samples, the well-defined lamellar structure oriented parallel
to the surface was observed only for the copolymer that
contained the fifth generation dendron, irrespective of the
PEO block length. The lamellar morphology oriented parallel
to the surface was further confirmed by X-ray reflectivity
In summary, environmentally responsive, topologically
unsymmetrical dendritic linear diblock copolymers with a
compatiblilizing linear component were used to organize a
MSSQ vitrificate into nanostructures through a matrix-
mediated collapse of the interior core. DMA and SANS
measurements strongly suggest that the dendron is phase
separated in the as-cast film, and serves as the macro-
molecular template for a porous lamellar morphology (XR
and SAXS) at copolymer compositions above 35% volume
fraction. The bulky nature of the dendron may also affect
their ability to pack efficiently into the traditional spherical
micelles that can form from diblock copolymers in selective
solvents. The formation of elongated micellar structures, or
(
XR) by the strong scattering that satisfies the Bragg
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[
11b,12c]
9
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2
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Published online: October 25, 2005
Keywords: dendrimers · hybrids · lamellar morphology ·
.
organosilicates · template synthesis
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Supporting Information.
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