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STIMULI-RESPONSIVE POLYMERS
The development of stimuli-responsive
polymers is another broad area that is attracting our
interest. One fundamental scientific aspect that
stimulates our group in this context is the exploitation
of non-covalent supramolecular interactions in polymer
matrices. The approach is the basis for the development
and application of functional polymer blends.
Rather than designing and synthesizing new, complex
functional macromolecules “from scratch”, minor
fractions of "functional additives" are blended with
"inert matrix polymers" in order to create (often after
rather specific processing protocols) new and novel
materials with unique or unusual property matrices.
Several different stimuli-responsive materials platforms
are currently under investigation and development in our
laboratory:
Polymer
Chameleons: Smart Polymers with Self-Assessing
Capabilities |
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A current core project of our
laboratory is focused on the development of
“self-assessing polymers”. We are interested in
materials with integrated sensors, which
visually indicate stimuli such as mechanical
deformation.
The approach pioneered by our
group is based on the incorporation of small
amounts of specially tailored fluorescent dyes
into conventional polymers (e.g. PE, PP, PET)
and relies on the formation of nano-scale
aggregates of these sensor molecules in the
polymer matrix. Mechanical deformation
transforms the nano-phase-separated systems into
molecular mixtures. Because our oligo(p-phenylene
vinylene) sensor molecules are designed to form
excimers when aggregated, this transition is
concomitant with a pronounced shift of the
material’s emission colour. We are developing a
predictive understanding for the various factors
that govern the supramolecular architectures of
these polymeric guest-host systems; fully
understanding aspects that govern the formation
of the nano-scale dye aggregates in the host
polymer and their dispersion upon deformation,
for example, is one major objective of our
research. The sensing scheme, on the other hand,
is useful for a plethora of technological
applications
that range from
tamper-evident packaging materials to
smart fishing lines.
We expanded the technology
platform to the detection of stimuli other than
deformation. For example an inverted mechanism,
i.e. the phase separation of initially
molecularly mixed blends of excimer-forming dyes
and glassy or semicrysalline host polymers, with
a glass transition in a temperature regime of
interest, represents another versatile sensing
mechanism, which is useful for the fabrication
of threshold temperature sensors
or time-temperature indicators. In
this case, the sensing scheme relies on
kinetically trapping thermodynamically unstable
molecular mixtures of the sensor dyes in the
glassy amorphous phase of the polymer.
Subjecting these materials to temperatures above
Tg leads to permanent and pronounced
changes of their PL emission spectra, as a
result of phase separation and excimer
formation.
We have recently demonstrated
the adaptation of the concept to
chromophore systems which change their
absorption properties and lead to a colour
change that can be readily detected by the
unassisted eye (i.e., without the need of
triggering fluorescence.
If you are interested in
applying this technology, please
contact us.
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Schematic representation of
excimer (de)formation, the general sensing
approach employed in our polymer chameleons.
Laboratory samples of a
fishing line with built-in fluorescent
deformation sensor (left) and time-temperature
integrator (right).
Pictures of time-temperature
indicators / threshold temperature sensors based
on polymers with excimer-forming fluorescent
dyes.
Pictures of a polymer
time-temperature indicator comprising a
chromogenic dye.
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Selected Recent Publications:
Crenshaw, B.; Burnworth, M.; Khariwala, D.; Hiltner, P.A.;
Mather, P.T.; Simha, R.; Weder, C.;
Deformation-Induced
Color Changes in Mechanochromic Polyethylene Blends;
Macromolecules 2007, 40, 2400-2408
Kunzelman, J.; Crenshaw, B.; Kinami,
M.; Weder, C.;
Self-Assembly and Dispersion of
Chromogenic Molecules: A versatile and General Approach for
Self-Assessing Polymers; Macromol. Rapid Commun.
2006, 27, 1981-1987.
Cover Picture.
Kinami, M.;
Crenshaw, B.; Weder, C.; Polyesters with Built-In
Deformation and Threshold Temperature Sensors; Chem.
Mater.
2006, 18, 946-955.
Multi-Issue Cover Picture
Crenshaw, B.; Weder, C.;
Deformation-Induced Color Changes in Melt-Processed Photoluminescent Polymer Blends; Chem. Mater. 2003,
15, 4717-4724.
Löwe,
C.; Weder, C.; Photoluminescent Polymer Blends and Uses
Therefore; US 7,223,988 (2007). (to Case
Western Reserve University).
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Functional Molecules in Nanostructured
Hosts
Another interdisciplinary
research program in the area of
stimuli-responsive materials is
based on the general idea to incorporate
stimuli-responsive molecules into nano-scale
templates which feature periodic structures.
This approach affords hybrid materials in
which the functionality of small organic
molecules is married with the geometric effects
of the template, resulting in new materials
which display properties that are absent in the
respective constituents. In collaboration with
Dr. Martin
Steinhart (Max Planck Institute of Microstructure
Physics, Halle, Germany) and Professor J.H.
Wendorff (U. Marburg) we studied the formation
of nanowires and nanotubes by the wetting of
porous alumina or silicon substrates with
discotic liquid crystals. We demonstrated that
aligned liquid crystalline nanowires within
ordered porous alumina templates show a
pronounced texture on a macroscopic scale. The
texture is the result of a complex interplay of
the pore geometry, interfacial phenomena, and
the thermal history.
These studies form the basis of a recently
awarded
NSF Materials World Network grant, which
promotes cooperative activities in materials
research between US investigators and their
counterparts abroad. Together with Prof.
Dr. Kenneth Singer
(Physics Department, Case),
Dr. Martin
Steinhart
and
Prof. Ralf Wehrspohn (University of
Paderborn) we began to design, prepare and study
Photonic Crystals (PCs) with a Dynamic
or Nonlinear Band Gap. These structured
materials form the physical basis for ultra-fast
optical switches, which are of considerable
technological interest and could enable a
plethora of applications that range from
high-density optical data processing to adaptive
optics to high-throughput communications
systems.
Another example for the
exploitation of the optical
bandgap displayed by nano-structured
architectures are co-extruded multilayer polymer
films, which we study in collaboration with Profs.
Anne Hiltner and
Eric Baer (Macromolecular Science and
Engineering, CASE). Their unique enabling
processing technology
makes it possible to fabricate large-area,
robust, multilayered polymer films with periodic
refractive index modulation through the film
thickness. Introduction of functional molecules
into these films leads to planar optical
elements with new and novel optical properties.
Employing photo-reactive additives, our initial
collaborative work in this arena focused on
the exploration of photo-patternable
light-reflecting multilayer polymer films, which
irreversibly change their optical
characteristics upon exposure to suitable,
high-intensity irradiation. With the notion that
interfaces play a key role in many optical and
electronic effects, we dramatically broadened
the scope of this activity, which has evolved
into one of three research platforms (Optical
and Electronic Materials and Devices) of the
recently awarded
NSF Science and Technology Center for Layered
Polymeric Systems (CLiPS).
Selected Recent
Publications:
Tangirala, R.; Baer, E.; Hiltner, A.; Weder, C.; Design
and Applications of Photopatternable Nanomaterials; Adv.
Funct. Mater. 2004, 14, 595-604.
Steinhart, M.; Zimmermann, S.; Schaper, A.K.;
Ogawa, T.; Tsuji, M.; Gösele, U.; Weder, C.; Wendorff, J.H.;
Morphology of Polymer/Liquid Crystal Nanocomposite Tubes;
Adv. Funct. Mater. 2005, 15, 1656-1664.
Steinhart, M.; Zimmermann, S.; Göring, P.; Schaper,
A.K.; Gösele, U.; Weder, C.; Wendorff, J.H.; Liquid
Crystalline Nanowires in Porous Alumina: Geometric
Confinement versus Influence of Pore Walls; Nano Lett.
2005, 5, 429-434.
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Ion-Conducting
Nanocomposites
Nother
thrust of our
activities in the arena of stimuli-responsive
materials is the field of ion-conducting polymers.
We have embarked on the
investigation of a hitherto unknown family of
novel sulfonated poly(p-arylene alkylene)
and are investigating the suitability of these
materials for use in polymer-based proton
exchange membranes (PEMs), which are one of the
key performance limiting technologies in fuel
cells.
We have successfully
developed strategies for the synthesis of these
materials and prepared a variety of copolymers
with different compositions.
Preliminary experiments
indicate that PEMs based on these new polymers
display interesting ion transport
characteristics. |
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Proton conductivity (20°C) of sulfonated
poly(arylene alkylene)s (see inset) as a
finction of composition and relative humidity. |
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Another recently initiated research
program sets out to explore and exploit ion-conducting solid polymer
electrolytes that combine high ionic conductivity with
good mechanical properties, we prepared and investigated nanocomposites of LiClO4-doped ethylene oxide-epichlorohydrin
(EO-EPI) copolymers and nano-scale cellulose whiskers
derived from tunicates. We have shown that homogeneous
nanocomposite films based on EO-EPI copolymers, LiClO4
and tunicate whiskers can be produced by
solution-casting THF/water mixtures comprising these
components and subsequent compression-moulding. The
Young's moduli of the nanocomposites thus produced are
increased by a factor of up to >50, when compared to the
copolymers, while the electrical conductivities
experience only comparably small reductions upon
introduction of the whiskers. The nanocomposite with the
best combination of conductivity (1.6•10-4 S/cm at room
temperature and a relative humidity of 70 %) and Young's
modulus (7 MPa) was obtained with a copolymer having an
EO-EPI ratio of 84:16, a whisker content of 10 % w/w and
a LiClO4 concentration of 5.8 % w/w.
Selected Recent
Publications:
Schroers, M.; Kokil, A.; Weder, C.;
Solid Polymer Electrolytes based on Nanocomposites of
Ethylene Oxide-Epichlorohydrin Copolymers and Cellulose
Whiskers; J. Appl. Polym. Sci. 2004, 93, 2883-2888.
Schroers, M.; Kokil, A.; Weder, C.;
Polymer Electrolytes with Improved Mechanical
Properties; Polym. Prepr. (Am. Chem. Soc., Div. Polym.
Chem.) 2004, 45(1), 54. |
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