Functional nanocomposites prepared by self-assembly and polymerization of diacetylene surfactants and silicic acid

Article abstract of DOI:10.1021/ja027332j

Conjugated polymer/silica nanocomposites with hexagonal, cubic, or lamellar mesoscopic order were synthesized by self-assembly using polymerizable amphiphilic diacetylene molecules as both structure-directing agents and monomers. The self-assembly procedu

Full text of DOI:10.1021/ja027332j

Published on Web 01/08/2003
Functional Nanocomposites Prepared by Self-Assembly and
Polymerization of Diacetylene Surfactants and Silicic Acid
†
†,‡,|
Mengcheng Lu, Jinman Huang, Raid Haddad,†
†,¶
†,#
Yi Yang, Yunfeng Lu,
†
†
†
§
George Xomeritakis, Nanguo Liu, Anthony P. Malanoski, Dietmar Sturmayr,
Hongyou Fan, Darryl Y. Sasaki, Roger A. Assink, John A. Shelnutt,
Frank van Swol,†,‡ Gabriel P. Lopez, Alan R. Burns, and C. Jeffrey Brinker*
‡
‡
‡
‡
†
‡
,†,‡
Contribution from the Center for Micro-Engineered Materials and Department of Chemical and
Nuclear Engineering, The UniVersity of New Mexico, Albuquerque, New Mexico 87131,
Sandia National Laboratories, Albuquerque, New Mexico 87185, and Institute of
Inorganic Chemistry, Technical UniVersity of Vienna, A 1060 Vienna, Austria
Received June 17, 2002 ; E-mail: cjbrink@sandia.gov
Abstract: Conjugated polymer/silica nanocomposites with hexagonal, cubic, or lamellar mesoscopic order
were synthesized by self-assembly using polymerizable amphiphilic diacetylene molecules as both structure-
directing agents and monomers. The self-assembly procedure is rapid and incorporates the organic
monomers uniformly within a highly ordered, inorganic environment. By tailoring the size of the oligo-
(ethylene glycol) headgroup of the diacetylene-containing surfactant, we varied the resulting self-assembled
mesophases of the composite material. The nanostructured inorganic host altered the diacetylene
polymerization behavior, and the resulting nanocomposites show unique thermo-, mechano-, and solvato-
chromic properties. Polymerization of the incorporated surfactants resulted in polydiacetylene (PDA)/silica
nanocomposites that were optically transparent and mechanically robust. Molecular modeling and quantum
calculations and 13C spin-lattice relaxation times (T
1
) of the PDA/silica nanocomposites indicated that the
surfactant monomers can be uniformly organized into precise spatial arrangements prior to polymerization.
Nanoindentation and gas transport experiments showed that these nanocomposite films have increased
hardness and reduced permeability as compared to pure PDA. Our work demonstrates polymerizable
surfactant/silica self-assembly to be an efficient, general approach to the formation of nanostructured
conjugated polymers. The nanostructured inorganic framework serves to protect, stabilize, and orient the
polymer, mediate its performance, and provide sufficient mechanical and chemical stability to enable
integration of conjugated polymers into devices and microsystems.
Introduction
exhibit highly developed but single functions. Here we report
an efficient, evaporation-induced self-assembly procedure to
Natural materials such as bone and shell teach us that
combining hard and soft materials in periodic architectures over
multiple length scales can result in composite materials with
enhanced mechanical properties such as toughness, strength, and
hardness. From a materials science viewpoint, such synergistic
natural composites have served as a holy grail of design and
construction. However, from an engineering viewpoint, emulat-
ing natural synthetic pathways is generally either impossible
or impractical, while conventional composite processing affords
insufficient structural control on the meso- and microscales.
Also, because of the inherent specificity of the evolutionary
selection process, natural materials, like biopolymers, typically
prepare mechanically robust, multifunctional polymer/silica
nanocomposites with hexagonal, cubic, or lamellar mesoscopic
order, using polymerizable surfactant molecules as both structure-
directing agents and monomers. Through incorporation of
diacetylenic units in the hydrophobic surfactant tails, in situ
polymerization enabled the formation of conjugated polymer
(
polydiacetylene) nanocomposites that have no biological
counterparts.
Because of extended π-electron delocalization along their
backbones, conjugated organic polymers exhibit electronic and
optical properties of interest for applications ranging from light-
1
emitting diodes to biomolecular sensors. For example, in blue-
colored polydiacetylene (PDA), the optical absorption blue-shifts
dramatically when stress is applied to the backbone via the
pendant side chains, and this thermally, mechanically, or
chemically induced chromatic (blue f red) response has been
explored as a colorimetric transduction scheme in a variety of
†
The University of New Mexico.
Sandia National Laboratories.
Technical University of Vienna.
Present address: Chemical Engineering Department, Tulane University,
‡
§
|
New Orleans, LA 70118.
¶
Present address: Intel Corp. RA1-234, 5200 NE Elam Yong Parkway,
2
,3
Hillsboro, OR 97124.
chemically and physically based sensor designs. However,
#
Present address: Ultraphotonics, 48611 Warm Springs Blvd., Fremont,
CA 94539.
(1) Charych, D.; Nagy, J.; Spevak, W.; Bednarski, M. Science 1993, 261, 585.
10.1021/ja027332j CCC: $25.00 © 2003 American Chemical Society
J. AM. CHEM. SOC. 2003, 125, 1269-1277
9
1269

A R T I C L E S
Yang et al.
to improve further the optical and electronic performance of
conjugated polymers and enable their integration into devices,
it may be necessary to incorporate them in nanoengineered
architectures that could provide alignment, control charge and
energy transfer, mediate conformational changes, and impart
needed mechanical and chemical stability. With respect to PDA,
it is anticipated that confinement and ordering of the diacetylenic
monomers within nanopores could alter their polymerization
pathway vis- a` -vis bulk or LB-prepared PDA. The confinement
and proximity of the inorganic walls are also anticipated to
influence the nature of the chromatic response and its revers-
ibility.
More generally, it is well established that for conjugated
polymers dissolved in solution or prepared as films, bends or
twists of the conjugated polymer chain result in breaks in the π
conjugation, resulting in a wide range of conjugation lengths.
Because the excitation energy is a strong function of conjugation
length, the polymer displays an inhomogeneous distribution of
physical properties such as absorption energy. Also, because
defects require hopping across chains, the charge carriers
Figure 1. Molecular structures of DA surfactants (1 and 2) and polym-
erization of DA/silica nanocomposites. (a) Film cross section near the final
stage of drying shows an oriented hexagonal mesostructure and hypothetical
arrangement of DA surfactants adjacent to the cylindrically structured silicic
acid framework. (b) Hypothetical structure of polymerized PDA/silica
nanocomposite formed upon exposure to UV light and continued acid-
catalyzed siloxane condensation (adapted from ref 7: Nature 2001, 410,
913).
enhance the performance of optoelectronic devices and to
increase their chemical and mechanical stabilities.
In this paper, we discuss a hybrid organic/inorganic self-
assembly process in which polymerizable amphiphilic diacety-
lene molecules serve as both structure-directing agents and
monomers. Our self-assembly approach is rapid and prepositions
both organic and inorganic precursors into highly ordered three-
dimensional arrangements. In situ polymerization allows for the
uniform and controlled incorporation of organic polymers within
a highly ordered, inorganic environment. The resulting poly-
diacetylene/silica nanocomposites are optically transparent and
mechanically robust. NMR and theoretical calculations were
used to model the arrangement of the surfactant molecules
within the nanostructured silica host. As compared to the
corresponding ordered, pure polymer films prepared by Lang-
(
electrons or holes), while being able to move rapidly along a
chain, move slowly as a whole in amorphous polymer films.
Finally, when the carriers do recombine to form excitons, rapid
interchain energy transfer may cause the newly formed exciton
to migrate to a low energy defect or trap site, quenching the
physical process such as luminescence. It is therefore critical
to be able to orient individual polymer chains and, in the
meantime, be able to separate them beyond the effective radius
of the interchain dipolar interactions that are critical in interchain
energy transfer. To address these problems, Tolbert and co-
workers recently demonstrated control of energy transfer in a
poly[2-methoxy-5-(2′-ethyl-hexyloxy-)-1,4-phenylene vinylene]
(
posite, prepared by MEH-PPV infiltration of a preformed,
oriented hexagonal, silica mesophase, was heterogeneous,
exhibiting two distinct conjugated polymer environments, viz.
polymers inside and outside the hexagonally arranged pore
channels of the silica particles. In general, because polymer
infiltration into a preformed porous nanostructure depends on
its partitioning from solvent, we expect it to be difficult to
control polymer concentration, orientation, and uniformity in
the corresponding nanocomposite. Further, when the nanostruc-
ture pore size is less than the radius of gyration of the solvated
polymer, infiltration proceeds by a reptation mechanism in which
the polymer threads itself through the pore, requiring rather long
4
4
4
MEH-PPV)/silica nanocomposite. However, this nanocom-
5
muir-Blodgett deposition, nanostructuring alters the diacety-
lene polymerization behavior. The hybrid organic/inorganic
nanocomposite materials exhibit useful thermo-, solvato-, and
mechanochromic properties. They also have increased hardness
and reduced gas permeability as compared to the parent polymer,
which should enhance their mechanical and chemical stabilities
and aid in their integration into devices.
Experimental Section
See also Supporting Information for details.
The diacetylene (DA) surfactants (denoted 1 in Figure 1) were
prepared by coupling tri-, tetra-, penta-, and decaethylene glycols with
the acid chloride of 10,12-pentacosadiynoic acid (PCA) (denoted 2 in
Figure 1). Precursor solutions were synthesized from tetraethyl ortho-
silicate (TEOS, Si(OC H ) ), diacetylenic (DA-EO ) surfactants (1 with
2 5 4 n
n ) 3, 4, 5, 10, 2, or combinations thereof), and HCl catalyst prepared
4
processing times at elevated temperatures. In some cases, PDA
being a good example, conjugated polymers are completely
insoluble, making polymer infiltration through a solution phase
impossible.
in a tetrahydrofuran (THF)/water solvent. The final reactant mole ratios
A related issue relevant to conjugated polymer devices is their
environmental sensitivity, especially to oxygen. In that polymer/
inorganic composites are considered promising candidates for
barrier films for food packaging, pharmaceuticals, and corrosion
inhibition, we envision that nanostructuring with relatively
impermeable silicon dioxide layers or walls could reduce
permeability while maintaining optical transparency. Conjugated
polymer/inorganic nanocomposites therefore may serve both to
were 1 TEOS:31.4-84.3 THF:4.96 H O:0.013 HCl:0.06-1.41 DA
surfactant.
2
Our approach to conjugated polymer/silica nanocomposites employs
a series of oligoethylene glycol functionalized diacetylenic surfactants
both as amphiphiles to direct the self-assembly of thin film silica
6
mesophases and as monomeric precursors of the conjugated polymer,
PDA. Beginning with a homogeneous solution of silicic acid and
surfactant prepared in a THF/water solvent with initial surfactant
(
5) Sasaki, D. Y.; Carpick, R. W.; Burns, A. R. J. Colloid Interface Sci. 2000,
(
(
(
2) Cheng, Q.; Yamamoto, M.; Stevens, R. Langmuir 2000, 16, 5333.
229, 490.
3) Cheng, Q.; Stevens, R. Langmuir 1998, 14, 1974.
(6) Lu, Y. F.; Ganguli, R.; Drewien, C. A.; Anderson, M. T.; Brinker, C. J.;
Gong, W.; Guo, Y.; Soyez, H.; Dunn, B.; Huang, M. H.; Zink, J. I. Nature
1997, 389, 364.
4) Nguyen, T.-Q.; Wu, J.; Doan, V.; Schwartz, B. J.; Tolbert, S. H. Science
2
000, 288, 652.
1270 J. AM. CHEM. SOC.
9
VOL. 125, NO. 5, 2003

Functional Nanocomposites
concentration c (e.g., 0.034 M for DA-EO
A R T I C L E S
o
5
) much less than the critical
follows. First, a cluster algorithm was developed to identify all
of the surfactants that make up a particular micelle. Next, we
determined the principal axes of the inertia tensor of the cluster.
The axis of the cylindrical micelle is determined as the inertia
tensor’s eigenvector with the smallest eigenvalue. We then
sought to represent each H4T4 surfactant by a vector. Although,
in principle, one could use end-to-end vectors, we decided that
a superior method is to again use the inertia tensor approach,
this time applied to each individual surfactant. In this fashion,
we can represent each DA (i.e., H T ) molecule by a unique
vector that gives the molecule’s orientation and is centered on
the molecule’s center of mass. Finally, a measure of orientational
order of the H4T4 surfactants in the cylindrical micelle was
obtained by forming the inner product of the molecule vector
and the vector describing the direction of cylinder axis. We
collected histograms of this inner product distribution, which
is, apart from a constant, the cosine of the angle between the
molecular axis and the micellar axis.
surfactant micelle concentration cmc, we use evaporative dip-coating,
spin-coating, or casting procedures to prepare thin films on silicon 〈100〉
7
or fused silica substrates. During deposition, preferential evaporation
of THF concentrates the depositing film in water and nonvolatile silica
and surfactant species. The progressively increasing surfactant con-
centration drives self-assembly of diacetylene/silica micelles and their
further organization into ordered liquid crystalline mesophases. Shape
and concentration of the DA surfactants influence the mesophase
obtained (lamellar, hexagonal, or cubic). Ultraviolet (UV) light-initiated
polymerization of the DA units, accompanied by catalyst-promoted
siloxane condensation, topochemically convert the colorless mesophase
into the blue PDA/silica nanocomposite, preserving the highly ordered,
self-assembled architecture (Figure 1).
4
4
Films were prepared by casting, spin-coating at 2000 rpm, or dip-
coating at a rate of 7.6-40 cm/min in an ambient atmosphere (23-25
°C, and relative humidity between 5 and 40%). Polymerization of PDA
to the blue form was done by UV exposure at 254 nm for times ranging
from 30 s to 30 min. Subsequent transformation to the red form was
accomplished by heating at 100 °C for times ranging from 30 s to 2
min or by exposure to solvent. A solvatochromic transition of blue
PDA/silica nanocomposite films to the corresponding red films was
accomplished by immersion of the blue films in the series of solvents,
hexane, 2-propanol, acetone, ethanol, methanol, or dimethylformamide.
Immersion times were 3 s followed by drying in nitrogen at room
temperature.
Molecular Modeling and Quantum Calculations. The
coarse-grained lattice calculations provide valuable insight into
the local packing and orientation of a collection of spontaneously
self-assembled DA molecules. To assess the effects of curvature
on the individual PDA oligomers, we undertook detailed
molecular simulations with detailed force fields to provide
realistic backbone structures whose electronic structures could
then be obtained from semiempirical quantum mechanical
calculations. Present computational limitations do not allow for
a detailed treatment of an entire micelle of (partially) polym-
erized DAs. Instead, we study single PDA oligomers. The
molecular configurations of single PDA oligomers were gener-
ated using molecular mechanics (MM) and dynamics (MD)
performed with the PolyGraf software package (version 3.21)
Modeling Section
Lattice Monte Carlo Simulation. The detailed packing and
orientational information of DA monomer surfactants in cylin-
drical micelles was studied with a 3D three-component lattice
8
model of a water/ethanol/surfactant system. As shown previ-
9,10
ously by Larson, this type of modeling is sufficiently coarse-
grained to allow for the sampling of the spontaneous self-
assembly process and the resulting 3D structures, yet sufficiently
detailed to investigate the orientation of DA within the cylindri-
cal micelle. Our lattice model represents small molecules (i.e.,
ethanol and water) as single sites on a simple cubic lattice with
interaction potentials based on the single fluid phase diagram
2
for isolated DA molecules and with Cerius (version 4.0) for
periodic calculations of molecules confined to silica pores. The
particular force field used for the atomic interactions was
Dreiding II with the dielectric constant set to 2.64. Partial
charges in the PDA oligomer were assigned using the charge
equilibration method. A MD simulation in the NVT ensemble
was performed for 500 ps, during which the temperature (T)
was fixed at 300 K. Structures were written out periodically at
0.1 ps intervals.
11
and employs Lorentz-Berthelot mixing rules for the cross
interactions.11 The surfactant molecules are represented by a
polymer chain, a collection of beads with various patterns of
connectivity. Here we represent the DA molecule by a simple
linear chain consisting of four hydrophilic beads (“heads”, H)
and four hydrophobic beads (“tails”, T), or H4T4. NVT Monte
Semiempirical quantum calculations were performed on
selected saved structures. To reduce the computational effort,
we first “trimmed” the PDA structures. That is, the DA
backbone of the PDA oligomers was terminated at the first
methylene group of each side chain by substituting a hydrogen
atom in its place. On the basis of the trimmed structures obtained
from the MD simulation, we calculated UV-visible spectra of
the PDA oligomers using the transition wavelengths and
3
Carlo (MC) simulations on a lattice of size 32 with periodic
boundary conditions were started from a random configuration.
7
After an initial equilibration of 10 attempted MC moves at high
7
temperature, the temperature was dropped in five stages of 10 -
8
1
0 attempted MC moves, and subsequent self-assembly was
followed until equilibrium was reached after approximately
8
3
× 10 MC moves. The initial composition was located in an
area of the three-component phase diagram of H4T4 that our
previous simulations had identified as the hexagonal phase. The
description of the orientation of H4T4 surfactants in cylindrical
micelles requires an objective analysis and was determined as
oscillator strengths obtained from semiempirical ZINDO/S
12,13
calculations
performed using the HyperChem 5.02 software
package, which provides the orbitals, energies, and transition
energies and dipoles. ZINDO/S is known to give accurate
spectroscopic predictions for these types of molecules when a
sufficient number of HOMOs and LUMOs are included in the
configuration interaction part of the calculation, as is the case
here.
(
7) Lu, Y. F.; Yang, Y.; Sellinger, A.; Lu, M. C.; Huang, J. M.; Fan, H. Y.;
Haddad, R.; Lopez, G.; Burns, A. R.; Sasaki, D. Y.; Shelnutt, J. A.; Brinker,
C. J. Nature 2001, 410, 913.
(8) Rankin, S. E.; Malanoski, A. P.; van Swol, F. Mater. Res. Soc. Proc. 2001,
6
36, D1.2.1.
(
9) Larson, R. G. J. Chem. Phys. 1988, 89, 1642.
(
10) Larson, R. G. J. Phys. II France 1996, 6, 1441.
11) Hansen, J.-P.; McDonald, I. R. Theory of Simple Liquids, 2nd ed.; Academic
Press: London, England, 1986.
(
(12) Ridley, J. E.; Zerner, M. C. Theor. Chim. Acta 1973, 32, 111.
(13) Bacon, A.; Zerner, M. C. Theor. Chim. Acta 1979, 53, 21.
J. AM. CHEM. SOC.
9
VOL. 125, NO. 5, 2003 1271

A R T I C L E S
Yang et al.
Figure 2. DA surfactants serve both as amphiphiles to direct the self-
assembly of the silicic acid mesostructure and as monomeric precursors of
the conjugated polymer, PDA. Increasing the number, n, of EO subunits
comprising the hydrophilic surfactant headgroup resulted in the formation
of higher-curvature mesophases: lamellar (n ) 3) f hexagonal (n ) 5) f
cubic (n ) 10) due to a decreasing value of the surfactant packing parameter
14,15
g.
However, larger headgroups (n ) 10) also served as spacers,
preventing polymerization of the pure DA-EO10 surfactant. Addition of
surfactants with smaller headgroups (e.g., 1 with n ) 3 or 5, or 2) was
necessary to form PDA in the cubic system.
Figure 4. Representative TEM images of nanocomposite thin films
(
prepared as in Figure 3b and c) and particles (formed by a related aerosol-
17
assisted EISA approach ). (a) [110]-Oriented hexagonally ordered nano-
composite film (inset shows [001]-orientation) prepared using 2.23% DA-
EO5. (b, c) Two orientations ([100] and [111]) of a cubic nanocomposite
film prepared using 2.23% DA-EO10. (d) PDA/silica nanocomposite particles
prepared using an aerosol-assisted evaporation-induced self-assembly
technique.
surfactant) shows a lamellar mesostructure with a (001) dif-
fraction peak at 47 Å. The presence of higher order diffraction
peaks, particularly at (003), indicates the formation of highly
ordered alternating silica/PDA layers. On the other hand, the
suppression of the even order peaks, such as (002) and (004),
16
indicates comparable thicknesses of the silica and polymerized
DA surfactant sublayers. When 2.23% of DA-EO5 was used to
prepare the film, a hexagonal mesophase with the cylinder axes
oriented parallel to the substrate surface resulted (Figure 3b),
with strong (100) and (200) diffraction peaks at 50 and 25 Å,
respectively. When a film was prepared using 2.23% DA-EO10
surfactant, a face centered cubic mesophase was detected (Figure
3c) with unit cell parameter a ) 108.2 Å.
Figure 3. X-ray diffraction (XRD) patterns of nanocomposite thin films.
(a) Film prepared using 1.86% of DA-EO3 (weight % DA-EO3 relative to
the solution weight without surfactant) shows a lamellar mesostructure with
unit cell dimension of 47 Å. (b) Film prepared with DA-EO5 (2.23%) shows
a hexagonal mesophase with strong (100) and (200) diffraction peaks at
In a hexagonally ordered nanocomposite film prepared using
5
0 and 25 Å, respectively. (c) Film prepared using 2.23% DA-EO10
2
.23% DA-EO5 surfactant, there can be seen a striped pattern
surfactant exhibits a face centered cubic mesophase with unit cell parameter
a ) 108.2 Å.
(Figure 4a) that is consistent with a [110]-oriented hexagonal
mesophase with repeat distance around 45 Å and regions of
the corresponding hexagonally patterned mesostructure char-
acteristic of the [001]-orientation (inset). The smaller spacing
measured by TEM versus XRD (45 Å vs 50 Å, compare Figures
Results and Discussion
Mesostructures. The choice of surfactant greatly influences
the resultant mesostructure (Figure 2). This is evident from the
XRD patterns and TEM micrographs shown in Figures 3 and 4
for nanocomposites prepared from diacetylenic surfactants with
tri- (n ) 3), penta- (n ) 5), and decaethylene (n ) 10) glycol
headgroups. Increasing values of “n” increase the surfactant
headgroup area ao. This in turn reduces the value of the
4
a and 3b) may be due to the over-focus condition used to
achieve high contrast in TEM, high vacuum, and/or electron-
beam-induced shrinkage (normal to the substrate), which can
occur in general for uncalcined, mesostructured materials. For
the cubic nanocomposite film prepared using 2.23% DA-EO10
surfactant, TEM imaging (Figure 4b) showed a highly ordered
surfactant packing parameter, g ) V/aol, where V is the surfactant
[100] orientation with unit cell parameter of 108 Å. The TEM
volume, and l is the tail length,1
4,15
favoring the formation of
image of the corresponding [111] orientation with repeat
distance of 62 Å is shown in Figure 4c. Introduction of DA-
EO3, DA-EO5, or PCA as cosurfactants with smaller headgroups
progressively higher curvature mesophases: lamellar (n ) 3)
f hexagonal (n ) 5) f cubic (n ) 10) (illustrated in Figure
2
). In Figure 3a, a film prepared using 1.86% of DA-EO3
(30-50 wt % relative to EO-DA10) was necessary to polymerize
(weight % DA-EO3 relative to the solution weight without
PDA in the blue or red form within the cubic mesophase. The
XRD patterns and TEM images of the cubic mesostructures were
(
14) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press:
San Diego, CA, 1992.
(15) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc. 1976, 2,
(16) Als-Nielsen, J.; McMorrow, D. Elements of Modern X-ray Physics; John
1
525.
Wiley & Sons, Ltd.: Chichester, England, 2000.
1272 J. AM. CHEM. SOC.
9
VOL. 125, NO. 5, 2003

Functional Nanocomposites
A R T I C L E S
1
3
Table 1.
C NMR Spin-Lattice Relaxation Time (T1) of the
DA-EO5 Monomer and (DA-EO5)/Silica Nanocomposites
T
1
,
1
T ,
carbon
CtC-
CtC-
chemical shift
DA-EO
5
nanocomposites
-
-
77.1 ppm
66.2 ppm
70.6 ppm
29.4 ppm
400 ms
551 ms
162 ms
193 ms
950 ms
897 ms
307 ms
305 ms
(
CH2-CH2-O)-
CH2
quite comparable with and without the addition of cosurfactants
DA-EO3 or DA-EO5 or PCA). Figure 4d shows PDA/silica
(
nanocomposite particles prepared using an aerosol-assisted
evaporation-induced self-assembly technique.17 The particles
exhibit an ordered multilamellar exterior and a disordered
wormlike mesostructured interior, consistent with a radially
directed self-assembly process.17
Figure 5. Patterned polymerization induced by UV irradiation and
thermochromic and solvatochromic transition of hexagonal PDA/silica
nanocomposite films. (a) Schematic representation of the UV patterning
procedure. (b) Optical image of patterned blue/colorless film formed by
UV irradiation for 2 min. (c) Patterned red/colorless film formed by heating
the film in (b) to 100 °C for 1 min. (d) Patterned red/blue film formed by
UV exposure of film in (c) for 2 min. (e) UV-visible spectra of the blue
PDA/silica hexagonally ordered nanocomposite, the corresponding red film
formed by heating at 100 °C, and the fluorescence emission spectrum of
the red film (excited at 500 nm). Film formed from DA-EO5. (f) Differential
absorbance at 645 nm resulting from the solvatochromic transition of blue
PDA/silica nanocomposite films to the corresponding red films. Without
consideration of the nonpolar solvent, hexane, the regression equation is
abs645 nm ) 0.003 (dielectric constant)-0.005, correlation coefficient ) 0.992
(adapted from ref 7: Nature 2001, 410, 913).
From the highly ordered nanocomposite mesostructures
observed by TEM, we infer that the surfactant monomers/
structure-directing agents are uniformly organized into precise
spatial arrangements prior to polymerization. The 1 C spin-
lattice relaxation time (T1) is a measure of the carbon chain
mobility. Table 1 shows a comparison of the T1’s of different
carbons species before and after being assembled into the
mesopores. These carbon species exhibit well-defined reso-
nances and provide a representative sample of the various
portions of the molecule. If the monomer and silica were a
simple mechanical mixture with macroscopic regions of homo-
geneity, then the local mobility would be unaffected, and the
T1 would be unchanged. However, we observed significant
increases of the T1’s of the monomer when it was incorporated
into the silica host. The T1’s of the selected carbons increased
by approximately a factor of 2, indicating that the mobility of
the monomer molecule has been significantly restricted in the
silica nanocomposite in agreement with the gas permeability
results (see below). This restriction is presumably related to the
molecular ordering that has taken place in the vicinity of the
rigid silica network.
3
thermochromic blue f red transition (Figure 5c), the unpo-
lymerized regions again remaining clear. A second UV exposure
without the mask polymerized the clear regions, resulting in
the red and blue pattern (Figure 5d). Figure 5e shows the
corresponding absorption spectra of the blue and red PDA/silica
films along with the photoluminescence spectrum of the red
film. The blue form of PDA is not fluorescent, and this
difference is of obvious importance for thermal, mechanical,
and chemical sensors.
Whereas lamellar mesophases (prepared using DA-EO3)
exhibit polymerization and thermochromic behavior qualitatively
similar to that of hexagonal mesostructures, cubic mesostructures
Such spatial arrangements (also discussed later in the Mo-
lecular Modeling section) establish the proximity (topochem-
istry) of the reactive diacetylenic moieties and thus influence
strongly the PDA polymerization process. This is best illustrated
by comparing the polymerization behaviors, as evidenced by
the blue (or red) color, of nanocomposite films prepared with
different mesostructures (i.e., the hexagonal, cubic, and lamellar
mesostructures shown in Figures 3 and 4) and contrasting these
behaviors with those of planar self-assembled monolayer and
trilayer films formed by Langmuir-Blodgett deposition of the
neat DA surfactants.
Spatial and Mesostructural Control of Polymerization
Behavior. Figure 5 shows spatial patterning of both PDA
polymerization and the thermochromic blue f red transition.
A nanocomposite film with a hexagonal mesostructure was
formed by spin-coating a sol prepared with 2.23% of DA-EO5
onto a glass substrate. A 2 min UV exposure through a mask
(prepared using DA-EO10) and Langmuir monolayers and
trilayers (prepared using neat 1 with n ) 3, 5, or 10) remain
colorless upon UV exposure and during heating (blue and red
forms of other monolayer and trilayers of polymerizable
diacetylene surfactants (e.g., PCA and PCAEA) are readily
5
distinguishable at the air-water interface ). The different
behaviors of lamellar and Langmuir films emphasize the
importance of the nanostructured inorganic host on PDA
polymerization. In both systems, the diacetylenic surfactants are
organized into highly oriented planar configurations with the
EO headgroups disposed toward the hydrophilic interface, either
water (Langmuir films) or polysilicic acid (nanocomposites).
Despite these similar organizations, we postulate that Langmuir
films do not polymerize because the reactive DA moieties are
spaced too far apart as indicated by the molecular areas
measured at 30 mN/m in a Langmuir trough: DA-EO1 ) 24
(Figure 5a) resulted in the patterned polymerization of PDA as
evident from the blue regions, the corresponding clear unpo-
lymerized regions having been masked (Figure 5b). A subse-
quent heat treatment to 100 °C for 1 min resulted in a
2
2
2
Å , DA-EO3 ) 38 Å , DA-EO5 ) 46 Å . Closer spacing of the
EO headgroups (and correspondingly the diacetylenic moieties)
within the self-assembled nanocomposites (lamellar, hexagonal,
or cubic) may be anticipated from the requirement for charge
(
17) Lu, Y. F.; Fan, H. Y.; Stump, A.; Ward, T. L.; Rieker, T.; Brinker, C. J.
+
-
δ+
Nature 1999, 398, 223.
density matching at the R-EOn-y[EO‚H3O ]y‚‚yX ‚‚wI in-
J. AM. CHEM. SOC.
9
VOL. 125, NO. 5, 2003 1273

A R T I C L E S
Yang et al.
tion.22 This points to a significant difference in the organization
and polymerization of the DA surfactants located at the curved
silicic acid interface as compared to that on the flat water
surface.
The importance of the proximity of the DA moieties on
polymerization is further illustrated by our inability to polymer-
ize DA within the cubic mesophase prepared with n ) 10 (see
Figure 3c and Figure 4b,c). The large EO10 surfactant head-
groups are necessary to direct the formation of the cubic
mesophase, but they serve as spacers, preventing polymerization.
Introduction of 1, with n ) 3 or 5, or 2 as cosurfactants with
smaller headgroups (30-50 wt % relative to EO-DA10) allows
us to form cubic mesophases and to polymerize the mixed
surfactant assemblies into the blue and red forms of PDA. It is
2
3
expected that, due to phase separation, this is not possible in
Langmuir monolayers or LB films.
Figure 6. Stress development in a hexagonally ordered PDA/silica
nanocomposite film upon UV irradiation and exposure to solvent (film
formed from DA-EO4 and stress measured using a cantilever beam
The polymerization of the surfactant is highly dependent upon
the topological alignment of diacetylenic units within the
2
4,25
19,20
supramolecular assembly.
Additionally, for colorimetric
technique
). The film was cast on the beam at time ) 0. Region I: Silica
condensation generated a tensile stress in the film. Region II: Compressive
stress developed upon UV-promoted PDA polymerization (film turned blue).
Region III: UV irradiation was turned off. Region IV: The film was
exposed to THF vapor, resulting in the development of a compressive
solvation stress and inducing the solvatochromic transition to the red
fluorescent form. Region V: The film was reexposed to air, reducing the
solvation stress. The net residual stress approaches a value close to that
observed prior to THF exposure.
materials to form (especially blue colored materials), the degree
of polymerization and conjugation length of the ene-yne
26
backbone must be considerable. This indicates that the micellar
structures that template the silica sol-gel material must contain
highly oriented, densely packed surfactants. This packing will
yield facile topochemical polymerization of the diacetylene units
to give colored polydiacetylene. Figure 1 illustrates a postulated
lengthwise polymerization of surfactants within a cylindrical
rodlike micelle to yield the blue form of PDA. Circumferential
polymerization, although favored on the basis of the proximity
of reactive DA moieties, would impose a high curvature on the
PDA backbone due to the small pore size of the mesostructured
host (2.3-3.1 nm diameter, see Figure 7).
-
terface (where R ) DA, y e n, X ) Cl , Iδ+ ) the partial
charge of the silica framework, and w is the “concentration” of
the framework needed for the charge balance) that reduces the
optimal EO headgroup area ao.18
The closer spacing of the EO headgroups is reflected in the
magnitude of biaxial stress resulting from PDA polymerization
measured using a cantilever beam technique.19-21 Figure 6
Physical Properties of PDA/Silica Nanocomposites. Modu-
lus (6.6 ( 2.00 GPa) and hardness (0.47 ( 0.2 GPa) values
measured for the PDA/silica nanocomposites by nanoindentation
are comparable to those of calcined mesoporous silica films
that have been successfully integrated into microelectronic
shows the stress development in a hexagonally ordered PDA/
silica nanocomposite film (formed from DA-EO4) upon UV
irradiation and exposure to solvent. In the first stage (Region I
in Figure 6), condensation of the silica framework of the
nanocomposite generated a tensile stress in the film. We then
observed development of compressive stress upon UV-promoted
polymerization (film turns blue, Region II in Figure 6) and
during the solvatochromic (Region IV in Figure 6) or thermo-
chromic blue f red transformation, indicating a net expansion
of the PDA relative to the silica host during polymerization and
the chromatic transition. Qualitatively similar stress behavior
was found for hexagonally ordered nanocomposites formed from
DA-EO5/silica and cubic films formed from mixtures of DA-
EO3 and DA-EO10 as well as for other solvents that induce the
solvatochromic transition.
2
7
devices as low dielectric constant films (Y.L. and C.J.B.,
unpublished). SAW-based nitrogen sorption experiments28 show
Type II sorption isotherms (Figure 7a-d), indicating that the
as-deposited film and polymerized PDA/silica nanocomposite
have no internal porosity accessible to N2 at -196 °C,29
indicating that the PDA completely fills the pore channels.
The corresponding permeability data for hexagonally ordered
PDA/silica nanocomposites (deposited on a commercial tubular
3
0
ceramic membrane support ) have been measured, and the
results are shown in Table 2 along with values for a poly-
(dodecyl methacrylate) (PDM)/silica nanocomposite film3 and
1,32
The corresponding neat DA surfactants prepared as Langmuir
monolayers do not polymerize and develop no stress upon UV
irradiation. Polymerizable DA Langmuir monolayers and mul-
tilayers (prepared using different surfactants with smaller
headgroups) contract upon polymerization, and the magnitude
of this contraction is used to assess the extent of polymeriza-
(
22) Day, D.; Ringsdorf, H. Polym. Sci.: Polym. Lett. Ed. 1978, 205.
23) Gaines, G. L. Insoluble Monolayers at Liquid-Gas Interfaces; John Wiley
& Sons: New York, 1966; pp 281-300.
(
(24) Menzel, H.; Mowery, M. D.; Cai, M.; Evans, C. E. J. Phys. Chem. B 1998,
102, 9550.
(25) Collins, M. J. Polym. Sci., Part B: Polym. Phys. 1988, 26, 367.
(
26) Kuriyama, K.; Kikuchi, H.; Kajiyama, T. Langmuir 1988, 4, 6468.
27) Lu, Y. F.; Fan, H. Y.; Doke, N.; Loy, D. A.; Assink, R. A.; LaVan, D. A.;
Brinker, C. J. J. Am. Chem. Soc. 2000, 122, 5258.
(
(
18) Monnier, A.; Schuth, F.; Huo, Q.; Kumar, D.; Margolese, D.; Maxwell, R.
(28) Frye, G. C.; Ricco, A. J.; Martin, S. J.; Brinker, C. J. MRS Symp. Proc.
1988, 121, 349-354.
(29) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area, and Porosity;
Academic Press Inc.: New York, 1982.
(30) Tsai, C. Y.; Tam, S. Y.; Lu, Y. F.; Brinker, C. J. J. Membr. Sci. 2000,
169, 255-268.
(31) Sellinger, A.; Weiss, P. M.; Nguyen, A.; Lu, Y. F.; Assink, R. A.; Gong,
W.; Brinker, C. J. Nature 1998, 394, 256.
S.; Stucky, G. D.; Krishnamurty, M.; Petroff, P.; Firouzi, A.; M., J. Science
1
993, 261, 1299.
(
(
(
19) Samuel, J.; Brinker, C. J.; Frink, L. J. D.; van Swol, F. Langmuir 1998,
1
0, 2602.
20) Lu, M. C. Drying and Calcination of Sol Gel Coatings; The University of
New Mexico: Albuquerque, NM, 2001.
21) Stoney, G. G. Proc. R. Soc. London 1909, A82, 172.
1274 J. AM. CHEM. SOC.
9
VOL. 125, NO. 5, 2003

Functional Nanocomposites
A R T I C L E S
Figure 7. N2 sorption isotherms of hexagonally ordered nanocomposite
films acquired using a surface acoustic wave (SAW)-based technique.28
Traces a and b: Type II adsorption-desorption isotherms of as-deposited
film. Traces c and d: Type II adsorption-desorption isotherms of polym-
erized PDA/silica nanocomposite film. Traces e and f: Type IV adsorption-
desorption isotherms of the mesoporous silica film after PDA removal by
calcination at 450 °C for 3 h in air. Inset shows a pore size distribution
based on adsorption data (trace e) using the BJH model. Traces g and h:
Type IV adsorption-desorption isotherms of the mesoporous silica film
after PDA removal at room temperature using an O2 plasma treatment (Fan,
H., unpublished). Films were prepared using 2.23 wt % DA-EO5 surfactant.
The lower amount of adsorption for isotherms e and f relative to g and h
reflects loss of porosity upon calcinations and differences in original film
thickness.
Figure 8. SA in a three-component mixture of water/ethanol and a
surfactant that mimics diacetylene. (a) A space-filled rendition of the
surfactant tails (yellow) and a stick representation of the headgroups (blue).
(b) A head-on view, vector representation of the surfactants (blue) and the
cylinder axes (black). (c) As (b) but now a side view, indicating preferential
ordering. (d) Probability distribution of surfactant orientations. We plot the
cosine of the angle between the surfactant axis and the cylinder axis. Note
that we observe a peak near the origin and around 50°.
2
9
films on SAW devices and used an oxygen plasma treatment
to remove the PDA at room temperature, thereby minimizing
film shrinkage. The N2 sorption isotherm of the resulting
mesoporous film is shown in Figure 7g,h. It is of Type IV and
Table 2. Permeabilities of PDA/Silica, PDM/Silica
Nanocomposites, and Polymers
2
9
exhibits a minor amount of hysteresis. BJH analysis of the
adsorption isotherm indicates a pore size of 3.1 nm. The
hysteresis and smaller pore size measured using the desorption
isotherm may be indicative of incomplete removal of the PDA,
creating pore restrictions. PDA removal by calcination at 450
permeability [barrer]
10
2
[
1 B ) 10- cc cm/cm s cmHg]
material
PDA/silica
temp
He
CO
2
N
2
CH
4
25 °C
00 °C
60 °C
25°C
35 °C
35 °C
0.10
0.11
0.15
0.08
54.5
7.4
0.038
0.091
0.173
0.12
265
2.96
0.022
0.029
0.048
0.05
18.4
0.24
0.030
0.039
0.074
0.067
55.9
0.53
°C for 3 h in air resulted in a Type IV isotherm typical of highly
1
1
ordered, hexagonal mesoporous silicas (Figure 7e,f). The pore
diameter, 2.25 nm, calculated from the adsorption isotherm is
smaller than that from TEM and XRD due to shrinkage during
calcination. The inset figure shows a quite narrow pore size
distribution calculated from the adsorption data (Figure 7e).
Molecular Modeling. The final structures obtained from the
lattice MC simulation are depicted in Figure 8a, which shows
hexagonally stacked cylinders. To better visualize the cylinders,
we use a space-filled mode for the drawing of the tail group
PDM/silica32
poly(decyl-acrylate)
poly(octadecyl-acrylate)
3
3
3
3
two pure polyacrylate films33 for comparison. It can be seen
that the intrinsic permeability of nanostructured polymer/silica
coatings is significantly lower than the values reported in the
literature for pure polymers,33 despite the very small thickness
of both the PDA/silica film (56 nm) and the PDM/silica film
(yellow), use a wireframe mode for the headgroup (blue), and
(150 nm). The preliminary results here indeed suggest that such
do not show the small molecules (water and ethanol). The
headgroup faces the aqueous solution. The very tip of the
headgroup is intimately intertwined with the aqueous solution.
This figure clearly illustrates that, although we can easily
identify the cylindrical micelles, it is impossible to assess the
orientation of individual DAs from this type of representation.
In Figure 8b and c, we show the results of the orientational
order analysis. The black arrows indicate the direction and
location of the cylinder axes, while blue unit vectors represent
the orientation of some of the individual DA molecules. Each
unit vector is located at the center of mass of a DA surfactant.
For clarity, we do not show all of the unit vectors, but instead
we have chosen those vectors that have a tilt angle close to 50°
polymer/silica coatings have great potential for barrier properties
and can be considered an alternative to presently practiced
polymer-based barriers and flake films.34 The low values of both
adsorption and permeability indicate that the composite archi-
tecture may impart some degree of oxidation resistance (im-
portant for extension to other conjugated polymer nanocom-
posites).
To estimate the size of the pore channels in which PDA is
confined, we prepared hexagonal PDA/silica nanocomposite
(
32) Brinker, C. J.; Bond, E. M.; Chen, H.; Yang, Y.; Xomeritakis, G.; Fan, H.
Y.; Yu, K.; Pardey, R. Corrosion 2002, submitted.
(33) Mogri, Z.; Paul, D. R. J. Membr. Sci. 2000, 175, 253.
34) Yang, C.; Nuxoll, E. E.; Cussler, E. L. AIChE J. 2001, 47, 295.
(
J. AM. CHEM. SOC.
9
VOL. 125, NO. 5, 2003 1275

A R T I C L E S
see below). The orientational vectors indicate that DA surfac-
Yang et al.
(
tants are tilted with respect to the cylinder axis. This is very
obvious when a cylinder is viewed from the side, as in Figure
8c. A view straight down the cylinder axis (Figure 8b) reveals
that the projections of the orientational vectors point radially
toward the center. This implies that the orientational vectors
tend to have no tangential component. To quantify the tilt, we
calculated the inner product of the orientational vector with the
direction of the cylinder axis, which gives the cosine of the tilt
angle. A histogram of an ensemble average over all of the DAs
is shown in Figure 8d. The probability distribution shows two
peaks. One peak, near cos(θ) ) 0, with the smaller integral
represents a small fraction of radial alignment. The major peak,
which covers the region of approximately 0.5 < cos(θ) < 0.8
Figure 9. (a) Model of a 16-unit cyclic oligomer of PDA (16-mer) inside
a silica pore. (b) PDA 16-mer after molecular dynamics and partial energy
minimization. (c) 16-mer trimmed for quantum mechanical simulation with
ZINDO/S. (d) and (e) The simulated molecular orbitals for the 16-mer and
(i.e., 37° < θ < 60°), represents the majority of DAs and im-
plies a predominant tilt angle of roughly 50°. Note, of course,
that this tilt angle can be accomplished in two ways as is obvious
from Figure 7c (unit vectors tilting to the left or right). Typically,
the radially aligned DAs are found where regions of different
tilt meet.
14-mer, respectively, and (f) the associated electronic spectra (compare with
Figure 5e).
the minimum number of DA units in a ring of blue form PDA,
isolated PDA rings (Figure 9b) with 14 and 16 units (i.e., 14-
mer and 16-mer) were constructed in an ordered conformation
and energy-minimized using MM. Typically, 10 000 iterations
were performed, which brings the average force down well
below the typical minimization criterion of 0.1 kcal/mol. The
diameters across the rings were measured: 20 Å for the 14-
mer and 23 Å for the 16-mer. Electronic properties of the
energy-minimized and terminated structures (Figure 9c) were
obtained by applying quantum mechanical methods (Figure
9d,e). We found that an energy-minimized cyclic PDA 16-mer
can exhibit absorption behavior consistent with that of the blue
form of PDA. On the other hand, rings of 14 units only give
rise to the red form because of the significant disruption of the
conjugation (Figure 9e).
Although DA surfactants organized into ellipsoidal liposomes
(
40 nm × 15 nm) are known to polymerize in the blue and red
35
forms, we questioned whether much more highly curved
circumferential arrangements of DA surfactants such as those
found from the lattice simulations (within spherical or cylindrical
micelles) could polymerize with the proper ene-yne conjugation
to produce colored materials. To test this, we performed a
detailed simulation of cylindrically arranged DA monomers
(
initially energy-minimized in isolation) confined to a silica pore.
2
The pore was constructed (using Cerius ) by creating a
cylindrical pore (30 Å diameter) in the center of a crystalline
SiO2 cylinder (63 Å long, 30 Å inner diameter, 9 Å wall
thickness, cut out from a SiO2 crystal structure cube of side 63
Å). Dangling Si and O atoms of the inner surface were
terminated with appropriate hydrogen and hydroxyl groups.
Inspired by the MC results, we investigated the packing of DA
monomers within the 30 Å pores. Fifty-four DA monomers were
arranged into three concentric cones of 18 monomers each with
headgroups facing, but not interpenetrating, the pore wall and
the tails sloping toward the pore axis. Periodic boundary
Using the results for the isolated ring with 16 units, we placed
a single 16-unit ring inside the SiO pore described above (see
2
Figure 9a) and allowed it to undergo potential energy minimiza-
tion (MM) for 5000 iterations. The resulting conformation was
then analyzed with quantum mechanical methods. The simulated
spectra were similar to the results for the isolated 16-unit ring,
36
conditions and the Minimum Image Convention were em-
ployed in all three orthogonal directions of the cell during a
indicating that the presence of the SiO pore walls does not
2
significantly alter the ring conformation.
5
00 ps MD simulation during which the SiO2 framework
The above calculations demonstrate that a ring PDA oligomer
of sufficient size can give rise to both the blue and the red form.
This in no way establishes that PDA confined to nanoporous
positions were held fixed. The MD calculations indicated that
cyclic arrangements of up to 19 DA monomers around the
periphery of the 30 Å diameter pore are possible.
SiO exclusively or indeed ever adopts this structure. Most
2
In forming rings of polymer in pores, the ring diameter is
constrained by that of the pore. As we have seen from the lattice
MC simulation, the pore diameter is determined by the self-
assembly of the DAs and depends on their size as well as their
packing, as expressed for instance by the tilt angle. Experimental
information about the pore diameter can be obtained from both
N2 sorption experiments and TEM studies. The PDA ring
diameter will affect the “bending” between units and hence the
electron conjugation and the resulting color. It is therefore of
interest to determine the smallest possible ring that could still
exhibit sufficient conjugation to achieve colored states. To find
likely, other nonlinear conformations, including elliptical rings
37
or a helical structure, will also allow for sufficient conjugation
to exhibit both the blue and the red forms. Such nonlinear
conformations are presumably rare for Langmuir monolayers
and LB films, but the modeling clearly favors such structures
over linear polymerization down the pore axis.
Chromatic Transition Characteristics. The blue PDA/silica
nanocomposites exhibit solvatochromic, thermochromic, and
mechanochromic properties. When contacted with the series of
polar solvents, 2-propanol, acetone, ethanol, methanol, dimeth-
ylformamide, the films transform to the red, fluorescent form,
and the differential absorbance at 645 nm (absorbanceblue -
absorbancered) scales linearly with the dielectric constant of the
(35) Spevak, W.; Nagy, J. O.; Charych, D. H.; Schaefer, M. E.; Gilbert, J. H.;
Bednarski, M. D. J. Am. Chem. Soc. 1993, 115, 1146.
(
36) Allen, M. P.; Tildesley, D. J. Computer Simulation of Liquids; Oxford
University Press: Oxford, England, 1989.
(37) Frankel, D. A.; O’Brien, D. F. J. Am. Chem. Soc. 1991, 113, 7436.
1276 J. AM. CHEM. SOC.
9
VOL. 125, NO. 5, 2003

Functional Nanocomposites
A R T I C L E S
self-assembly schemes developed recently7,40,41 represents a
general, efficient route to the formation of robust multifunctional
nanocomposites. Using this approach, we fabricated PDA/silica
nanocomposites, which exhibit unique thermo-, mechano-, and
solvatochromic properties. The choice of surfactant greatly
influences the resultant mesostructure: lamellar (n ) 3) f
hexagonal (n ) 5) f cubic (n ) 10). The resulting nanostruc-
tured inorganic host also altered the diacetylene polymerization
behavior: the use of large EO10 surfactant headgroups is
necessary to direct the formation of the cubic mesophase, but
they also serve as spacers, preventing polymerization. NMR,
molecular modeling, and quantum calculation results indicated
that DA surfactant monomers can be uniformly organized into
precise spatial arrangements prior to polymerization, and the
DA surfactants are tilted with respect to the cylinder axes in
the cylindrical micelles. The calculations also demonstrated that
a ring of PDA oligomer of sufficient size can give rise to both
the blue and the red forms. The relations between solvatochro-
mic properties and the dielectric constants of different solvents,
the different fluorescent properties between the blue and red
form PDA/silica nanocomposites, and the good mechanical
properties of the nanocompite films make the PDA/silica
nanocomposite films good potential candidates in sensing. The
hybrid organic/inorganic nanocomposite films also have very
low permeabilities as compared to that of polymer coatings and
could be useful for integration of environmentally sensitive
conjugated polymers and for optically transparent, self-reporting
barrier materials, in general.
Figure 10. Rapidly reversible thermochromic behavior of PDA/silica
nanocomposite film prepared using 2.23% DA-EO5 on a filter paper. (a)
Blue film; (b) purple film; (c) red film; (d) purple film; (e) blue film. The
film was heated using a heat gun at t ) 0, and the heat source was withdrawn
at t ) 11 s.
solvent: ∆abs645 nm ) 0.003 (dielectric constant)-0.005, cor-
relation coefficient ) 0.992 (see Figure 5f), suggesting applica-
tions in sensing. We postulate that polar solvents diffuse within
the polar, hydrophilic EOn pendant side chains. Accompanying
solvation stresses are transferred to the PDA backbone (evidence
for solvation stresses are presented in Figure 6), reducing its
conjugation length, therefore inducing the blue to red transfor-
38
mation as understood by recent molecular mechanics simula-
3
9
tions. The solvatochromic behavior shows very slow revers-
ibility. Heating to temperatures in excess of 47°C can also cause
the blue-to-red transformation. This thermochromic behavior
is rapidly reversible (several seconds) and is recorded via a CCD
camera using an Adobe Primere 4.0 software (see Supporting
Information: a movie shows the whole thermochromic transi-
tion.) Figure 10 shows snapshots extracted from this movie that
represent the different stages (colors) of a rapid reversible
thermochromic transition. This may arise from hydrogen-
bonding interactions38 of the pendant side chains with the silanol
moieties of the surrounding inorganic mesophase, supplying a
restoring force and enabling recovery of the original side chain
orientation. Mechanical abrasion of the blue nanocomposite film
causes local transformation to the red fluorescent form. Thus,
we envision tribochromic barrier coatings that could sense
excessive mechanical damage.
Acknowledgment. We thank Dr. Dhaval Doshi and Dr.
Evelyn Bond for technical discussions and Chris Hartshorn and
Dr. Tom Buchheit for the nanoindentation results. This work
was supported by the U.S. Department of Energy Basic Energy
Sciences Program, the DOE Nanoscience Engineering and
Technology Program DE-FG03-02ER15368, the Air Force
Office of Scientific Research F49620-01-1-0168 and MURI
F49602-01-1-0352, the National Aeronautics and Space Ad-
ministration NAG5-8821, the Sandia National Laboratories
Laboratory-Directed Research and Development Program, the
UNM/NSF Center for Micro-engineered Materials. and the
Defense Advanced Research Projects Agency Bio-Weapons
Defense Program. TEM investigations were performed in the
Department of Earth and Planetary Sciences at the University
of New Mexico. Sandia is a multiprogram laboratory operated
by Sandia Corp., a Lockheed-Martin Co., for the U.S. DOE
under Contract DE-AC04-94AL85000.
Conclusion
The use of polymerizable surfactants as both structure-
directing agents and monomers in the various evaporation-driven
Supporting Information Available: Experimental section
(
PDF) and a movie showing the rapidly reversible thermochro-
(
38) Patel, G. N.; Chance, R. R.; Witt, J. D. J. Chem. Phys. 1979, 70, 4387.
39) Burns, A. R.; Carpick, R. W.; Sasaki, D. Y.; Shelnutt, J. A.; Haddad, R.
Tribiology Lett. 2001, 10, 89.
mic behavior of a PDA/silica nanocomposites film deposited
on a filter paper (QT). This material is available free of charge
via the Internet at http://pubs.acs.org.
(
(
40) Fan, H. Y.; Lu, Y. F.; Stump, A.; Reed, S. T.; Baer, T.; Schunk, R.; Perez-
Luna, V.; Lopez, G. P.; Brinker, C. J. Nature 2000, 405, 56.
41) Brinker, C. J.; Lu, Y. F.; Sellinger, A.; Fan, H. Y. AdV. Mater. 1999, 11,
(
5
79.
JA027332J
J. AM. CHEM. SOC.
9
VOL. 125, NO. 5, 2003 1277