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DAVIS
RESEARCH GROUP Our group's
research concerns the self-assembly of polymers at interfaces and
in solution and the fabrication of nanostructured, functional polymer
films. Self-assembled structures are interesting in that, with the
proper chemical design, small molecules and macromolecules will spontaneously
form nanostructures that have great scientific and technological interest.
A common theme in this work is the physical and colloid chemistry
of polymer solutions and suspensions and adsorption and self-assembly
at interfaces. Experimental techniques used by our group include dynamic
and static light scattering, ellipsometry, electrophoresis, atomic
force microscopy, electron microscopy, X‑ray photoelectron spectroscopy,
and rheological measurements of complex fluids. In addition, there
are close ties with several interdisciplinary research groups at Virginia
Tech including the Center for Self-Assembled Nanostructures and Devices
(CSAND) and the Macromolecules and Interfaces Institute (MII). Current
projects are described below. |
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AREAS
OF RESEARCH Water-Soluble Synthetic and Bioderived Block Copolymers -
Solution Properties and Self-Assembly at Interfaces - joint with Prof. K.E. Van
Cott (Chemical Engineering, University of Nebraska-Lincoln), Prof.
W.A. Ducker (Chemistry), and Prof. J.S. Riffle (Chemistry). The control of chemistry at interfaces is critical
for many technologies including those used to make biomaterials, microelectronics,
adhesives, ceramics, and advanced, functional coatings. In many cases,
these materials require the production and processing of particles
with sizes ranging from 5 nm to 10 microns. A common problem for these
applications lies in controlling the chemistry at a surface - often
a fluid-solid interface. When the interface is on the surface of a
particle, the selective control of surface chemistry can directly
affect particle aggregation that is particularly important in suspensions
with high particle concentrations. Particle aggregation leads to higher
viscosities, the onset of a yield stress, and, in many cases, sedimentation
of the particles. Regardless of the particular details of the application,
controlling particle aggregation in suspensions is critical for proper
processability. |
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Block copolymers can
adsorb at solid-liquid interfaces to form self-assembled brush layers. The
resulting surface forces generated by the adsorbed polymer layers can have a
profound effect on adhesion processes, on particle dispersion, and on
suspension rheology. A block copolymer stabilizer has an anchor block that
adsorbs strongly onto a surface while the tail block remains solvated. Mutual
repulsion between tethered tail segments leads to tail extension, thus
generating a well-ordered brush layer with a thickness defining the range of
surface forces. Surfaces coated with properly designed block copolymers
experience strong repulsive forces due to these highly extended tails, thus
offering the most efficient approach to particle stabilization which can
reduce a suspension’s viscosity by orders of magnitude, thus making it
possible to process the suspension. We are investigating two classes of novel
block copolymers. |
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The first class of block
copolymers consists of synthetic polyamino acids - proteins. We have
recently shown that these have the potential for forming brush layers
on metal oxide surfaces in contact with water.[1] Based on these findings,
the novel fusion protein, shown below, was designed with a thioredoxin
head group coupled to a linear proline linker chain that is terminated
with a short glutamic acid sequence. We have recently demonstrated
that this fusion protein performs as it was designed to do - it adsorbs
at a positively charged surface from an aqueous solution and forms
a self-assembled layer that generates a repulsive steric force.[2]
The ability to control the chemistry of biosynthetically produced
block copolymers of amino acids at the repeat unit level offers exciting
possibilities for developing new families of polymers that can modify
the surface properties of selected substrates for a variety of applications. |
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The second class of block
copolymers, synthesized by Professor Riffle at Virginia Tech, consists
of blocks of poly(ethylene oxide) (PEO) and ion-containing blocks.
For example, we are studying triblocks consisting of PEO endblocks
and hydrophobic center blocks that contain carboxylic acid groups.
We have found that these water-soluble triblock copolymers undergo
a reversible micelle transition at low pH and/or low temperature due
to a subtle interplay between hydrogen bonding, electrostatic, and
hydrophobic interactions. In addition, these copolymers preferentially
adsorb onto positively charged surfaces such as Fe3O4
at pH ~7 and form brushlike layers that impart steric stabilization.[3]
We seek to understand the relations between polymer structure, solution
properties, and interfacial adsorption leading to brush formation
and particle stabilization. |
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References: [1] "Adsorption of Novel Block Copolymers for
Steric Stabilization and Flocculation of Colloidal Particles in Aqueous
Environments", J. Krsmanovic, Ph.D. thesis, Virginia Tech, 2003. [2] "Unnatural Proteins for the Control of
Surface Forces", A. Tulpar, D. B. Henderson, M. Mao, B. Caba,
R. M. Davis, K. E. Van Cott, and W. A. Ducker, Langmuir, accepted. [3] "Adsorption of brush-forming triblock copolymers
on metal oxides surfaces", B. L. Caba, J.L. Krsmanovic, A.Y.
Carmichael, J.S. Riffle, and R.M. Davis, in preparation |
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Self-Assembly of Polymeric Films for Nonlinear Optical
Applications - joint with Prof. K.E.
Van Cott (Chemical Engineering, University of Nebraska-Lincoln), Prof. J.R.
Heflin (Physics), and Prof. H.W. Gibson (Chemistry). Next-generation optical communications systems and
computers require the conversion of electrical signals to optical
signals at high modulation frequencies and at low cost. The electrical-to-optical
conversion step can be accomplished with materials that exhibit the
nonlinear optic (NLO) effect. This refers to the change of a material's
optical properties (e.g., refractive index, absorbance) in response
to an applied electric field or incident light. An electro-optic (EO)
modulator, shown below, is typically made in the form of a thin film
optical waveguide, typically 1 micron or more in thickness, through
which light is passed while a modulated, external electric field is
imposed on the film. If the applied voltage is modulated, then the
transmitted optical signal will have that same modulation. In essence,
an electrical signal can be converted into an optical signal. We are
working on a novel form of an EO modulator made of self-assembled
polymeric films that contain highly oriented, polarizable chromophores.
Compared to existing technologies
based on inorganic materials, the proposed organic devices would result
in faster and less expensive fiber optic communications components.
Current devices operate at a signal modulation rate of 40 GHz while
the devices from the proposed materials will operate at rates >
150 GHz. |
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Electro-optic
Modulator Based on a Mach-Zehnder Interferometer |
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We have recently
developed a new self-assembly technique with the potential to manufacture
nonlinear optical devices rapidly and economically. [1,2,3] These films are
created through alternate deposition of polyanions and polycations onto a
substrate. This process creates multiple bilayers of material, which, through
many dipping steps, creates a film thick enough for an optical waveguide. In
order to achieve the second order NLO properties desired from these films,
NLO-active chromophores are incorporated into the films using novel
self-assembly techniques that involve electrostatic and covalent
interactions. For modulator devices, the chromophore must be highly
polarizable and oriented in the film. Using readily available chromophores
developed originally for dyeing textiles, we have developed films with
outstanding temporal and thermal stability and which have an electro-optic
coefficient (a key measure of EO response) that is 50% of the
state-of-the-art lithium niobate material. Our objectives are to develop
better chromophores and to understand the role of polymer and chromophore
structure necessary to obtain the necessary orientation and to find the
deposition conditions needed to fabricate films for modulators that are
superior to those made from inorganic crystals. |
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References: [1] "Layer-by-Layer Deposition and Ordering
of Low Molecular Weight Dye Molecules for Second Order Nonlinear Optics",
M.Guzy, P. Neyman, C. Brands, J.R. Heflin, H.W. Gibson, R.M. Davis,
K. Van Cott, Angewandte Chemie, Int. Ed., 41: 3236-3238, (2002). [2] "Characterization of the Purity and Stability
of Commercially Available Dichlorotriazine Chromophores Used in Nonlinear
Optical Materials", K.E. Van Cott, T. Amos, H.W. Gibson, R.M.
Davis, J.R. Heflin, Dyes and Pigments, 58, 145-155, (2003). [3] "Self-Assembly of Organic Films: Covalent/Ionic
Self-Assembly for Nonlinear Optical Materials", M. Guzy, P.J.
Neyman, C. Brands, J.R. Heflin, H. Gibson, R.M. Davis, K.E. Van Cott,
in The Dekker Encyclopedia of Nanoscience and Nanotechnology, Marcel
Dekker, (2004). |
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Polymer-Fullerene Photovoltaic Cells - joint with Prof. J.R.
Heflin (Physics) As part of the worldwide effort towards renewable
energy sources, organic photovoltaic materials are intensely studied because
of the potential for lightweight, flexible, inexpensive, efficient solar
cells. A major advance in polymeric photovoltaic devices was achieved with
the observation of photogenerated charge separation in
poly(para-phenylenevinylene) (PPV)-C60 composites. Upon
photoexcitation, rapid electron transfer occurs from the polymer to the high
electron affinity C60. However, photoexcited electron-hole pairs
at distances larger than ~10 nm from the fullerene acceptor recombine before
charge separation occurs, yielding photoluminescence. Thus, in an organic
solar cell consisting of a layer of conducting polymer followed by a layer of
C60, only the material within 10 nm of the interface results in
efficient optical to electrical energy conversion. Nanoscale compositional
control of the electron donor and acceptor species is important to optimizing
the performance of polymeric photovoltaics. We have recently demonstrated a
new approach for developing efficient organic photovoltaic devices by using
thermal processing of thin films of organic, electrically conducting polymers
to form interdiffused donor-acceptor layers.[1,2,3,4] Starting from a
standard bilayer system of spin-cast poly(methoxy-phenylenevinylene),
MEH-PPV, followed by an evaporated layer of C60, the film was then
heated above the MEH-PPV glass transition temperature (230oC) to
enhance the diffusion of the fullerene into the polymer, resulting in a
concentration gradient structure. Because
the fullerene acceptor is distributed throughout the film, exciton
recombination is dramatically reduced, resulting in a decrease in the
photoluminescence by more than 50X and an increase in the photocurrent by a
factor of 10 throughout much of the visible spectrum. These results
demonstrate that thermally-controlled interdiffusion is an extremely
promising route for nanoscale control of the composition of organic
photovoltaic films and for the manufacture of efficient organic solar cell
devices. |
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Our objectives are to explore new electron
donor/acceptor chemistries and to optimize the processing parameters to
further increase the photovoltaic response. Recent morphological studies
using TEM and Auger spectroscopy demonstrate that there is substantial room
for improvement through use of better miscible donor-acceptor combinations.
We seek to further improve photovoltaic response for these films by making
films with new component materials, by further studying thermal processing
parameters, and by varying layer thicknesses. |
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References: [1] "Creation of a gradient polymer-fullerene
interface in photovoltaic devices by thermally-controlled interdiffusion",
M. Drees, K. Premaratne, W. Graupner, J.R. Heflin, R.M. Davis, D.
Marciu, M. Miller, App. Phys. Lett., 81(24), 4607-4609, (2002). [2] "Thickness Dependence,
In Situ Measurements, and Morphology of Thermally-Controlled Interdiffusion
in Polymer- C60 Photovoltaic Devices" M. Drees, R.M. Davis, J.R.
Heflin, Physical Review B, 69, 165320 (pp1-6), (2004). [3] "Improved Morphology
of Polymer-Fullerene Photovoltaic Devices with Thermally-Induced Concentration
Gradients", M. Drees, R.M. Davis, J.R. Heflin, Journal of Applied
Physics, accepted. |
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CURRENT MEMBERS OF THE RESEARCH GROUP Beth L.
Caba
(Ph.D. student, MACRO program; interfacial and solution properties of
hydrophilic block copolymers) Akhilesh
Garg (Ph.D. student, Chemical Engineering; self-assembly of polymeric
nonlinear optical films) Qian Zhang (Ph.D. student,
Chemistry, co-advised with Prof. J.S. Riffle; solution and interfacial
properties of block copolymers for stabilization of magnetic oxide particles) |
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