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Overview
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The properties of materials
depend upon their structure and, to a considerable extent, upon the dynamics
of the atoms and molecules that comprise them. For example, a rubbery polymer
at room temperature will become rigid at sufficiently low temperature; the
slowing of molecular motions in the polymer is responsible for this change.
Quantitatively, over a 20 K temperature range, the force required to deform
the material will increase by a factor of 104, yet the static
structure changes only slightly and no thermodynamic phase transition
occurs. As another example, in a microphase separated block copolymer
system, the efficiency of small molecule transport in the two phases can
differ by a factor of 1010 as a result of the different dynamics
in those phases. Such differences can be exploited in synthetic membranes.
Our research attempts to develop a molecular-level understanding of
dynamics in polymeric materials and low molecular weight glass formers. We
try to understand why particular dynamics are observed. What is it about
the structure of the material and the potentials which govern the
interaction of the atoms which makes dynamics fast or slow in a given
system? As devices move closer to the nanometer length scale, the knowledge
obtained from our molecular level experiments will become more
essential to the correct functioning of these devices.
Our research is funded primarily by the National Science
Foundation and we gratefully acknowledge this support.
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Polymer
dynamics in multicomponent systems. |
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The long stringy nature of polymer molecules gives them properties that
cannot be found in any other material. We work to tie together the
viewpoints of polymer physicists and polymer chemists. Physicists
(stereotypically) are looking for universal properties, i.e., in what ways
do polymer molecules all act the same? Chemists tend to emphasize the
peculiarities of polymer behavior and ascribe them to particular details of
the molecular structure. When possible, we try to integrate these
perspectives, attempting to understand from the molecular structure exactly
how the “universal” properties will be exhibited.
Almost all applications of polymers utilize
copolymers, polymer blends, or
composites, as opposed to homopolymers. Miscible polymer blends are a good
starting point for understanding the properties of these composite systems;
they are also technologically important and have flow properties which
cannot currently be predicted.
In most polymer melts
and blends,
conformational(segmental) dynamics are the fundamental motions which drive longer
length scale rearrangements. These dynamics, which occur on the length
scale of a few Angstroms, ultimately control the viscosity, elasticity, and
glass transition of the system. In polymer blends, the segmental dynamics of
each component are modified from the values which they have as pure
components and in general are not equal to each other. Composition
fluctuations thus lead to spatial variations in the mobility of the blend.
Two major questions regarding polymer blends are:
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1) How can the segmental dynamics of each component in the blend be
predicted from the properties of the pure homopolymers?
2) Given knowledge of the segmental dynamics of each component, how can the
long time transport and flow properties of the blend be predicted?
NMR measurements have a special role to play in the study of polymer
blends, since isotopic labelling allows each component to be selectively
interrogated. However, the successful application of NMR relaxation time
measurements to polymer blends requires the ability to turn relaxation
parameters into a quantitative description of the distribution of segmental
relaxation times. |

Simulation of Polyisoprene / Polystyrene Blends
by Roland
Faller (UC-Davis) |
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Using multi-field measurements, we have
recently shown that this is possible in both single component systems and in
miscible blends and block copolymers. An essential feature of our approach
is the comparison with large scale molecular dynamics computer simulations.
These simulations provide a check on the interpretation of our measurements,
while the measurements also check the simulations.
Recent
powerpoint presentation on polymer mixture dynamics

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Supercooled
liquids/diffusion in thin films.
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If crystallization is avoided
upon cooling, a liquid will become more viscous and gradually transform into
an amorphous solid which we call a glass. Part of the interest in this
process is that a new type of matter results by a purely kinetic process,
i.e., the glass transition does not result from changes in structure.
Glasses of many types (polymeric, inorganic, saccharide...) play important
roles in technologies. |
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Spatially heterogeneous dynamics
Simulation by Glotzer group (U. of Michigan)
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Much of the phenomenology of supercooled liquids and glasses transcends a
particular class of materials. We study molecules such as o-terphenyl and
glycerol; these are arguably the simplest systems which supercool and form
glasses. These are ideal systems for uncovering the essential features of
glass formation and insights from the study of these materials can be
transferred to technologically relevant systems.As an example, our
studies on low molecular weight glass formers have suggested the explanation
for the anomalously fast diffusion of solvents, antioxidants, and
plasticizers through polymers, as well as some puzzling rheological
properties.
Other areas where an understanding of mobility of supercooled
liquids is essential include |
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pharmaceuticals (will an amorphous drug crystallize during
storage?), enzyme and tissue preservation (can water be removed from a saccharide solution to yield a glassy matrix while preserving biological
structures?), and food products (can crystallization of LifeSavers be
suppressed?). |
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In the last few years, we have shown that, as the glass transition is
approached, dynamics become increasingly spatially heterogeneous, i.e., the
dynamics in one region of the sample may be orders of magnitude faster than
the dynamics a few nanometers way. One important consequence of this
heterogeneity is that translational diffusion coefficients can be 1000 times
larger than expected. Our current work involves vapor deposition of
thin films of deuterated and hydrogenous glass formers onto a cold substrate,
followed by subsequent interdiffusion. Using neutron reflectivity (NR)
and thermally programmed desorption (TPD), these experiments
have extended the range of measured diffusion coefficients |
Self-diffusion of tris-naphthyl
benzene
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by 6 orders of magnitude.
Neutron reflectivity measurements provide a direct test for the presence of
nanometer-size dynamic heterogeneities.
Powerpoint
presentation tutorial on dynamics in supercooled liquids and polymers
Powerpoint presentation on supercooled liquids (ACS meeting, Sept
2003)
Recent
powerpoint presentation on "Unanswered questions about supercooled liquids
and glasses"

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Creation
of Exceptionally Stable Glasses |
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We have prepared what are very likely
the most stable glasses ever made in a laboratory. In an afternoon, we can
make glasses that would require at least 4000 years to prepare using any
published methodology. Our glasses have useful material properties. For
example, because they are more dense and energetically more stable than
ordinary glasses, they resist crystallization and water uptake. These stable
glasses may have immediate technological relevance, e.g., to stabilize
amorphous pharmaceuticals. More fundamentally, as compared to ordinary
glasses, our glasses are much deeper in the energy landscape that controls
the thermodynamics and kinetics of an amorphous system. There has been
intense recent activity aimed at understanding fundamental issues of glass
formation, such as the Kauzmann entropy paradox, in terms of the energy landscape.
Our stable
glasses are ideal for exploring these fundamental issues.
We have prepared stables glasses of two organic molecules by
vapor deposition onto substrates held somewhat below the glass transition
temperature. In this temperature range, the top few nanometers of a glass
can be substantially more mobile than the interior of the glass. If
deposition is sufficiently slow, molecules sticking to the surface have the
opportunity to sample many configurations before they get buried and
immobilized. This configuration sampling allows the top layer to attain or
nearly attain equilibrium at the substrate temperature. Subsequent layers
are similarly equilibrated. In this way, the usual kinetic constraints to
the production of stable glasses can be avoided. Given the deep connections
between glasses and other energy
landscape problems (e.g., protein folding), we
hope that this work have a multidisciplinary impact. This is a collaborative
effort with Robert McMahon and Lian Yu (UW-Madison) and Sushil Satija (NIST
Center for Neutron Research). |
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PowerPoint presentation on "Creation of Exceptionally Hiking down the
energy landscape:
How vapor deposition and surface mobility produce exceptionally stable
glasses"
PowerPoint presentation on
"Exceptionally stable organic glasses with low enthalpy and
high kinetic stability prepared by vapor deposition" (APS meeting,
March 2008)
PowerPoint
presentation on "Comparison of surface mobility on polymeric and low
molecular weight glass-formers" (APS meeting, March 2008)

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Crystal
growth from glasses and supercooled liquids |
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Amorphous materials are
often useful as precursors to crystalline materials. For example, if we can
learn to control crystallization in amorphous aluminum alloys, we should be
able to produce a material stronger (and significantly lighter!) than steel.
In other cases, spontaneous crystallization destroys the useful properties
of amorphous materials. The pharmaceutical industry has developed many drugs
that cannot be marketed because the crystalline form of these drugs is not
sufficiently soluble. It has been shown that drugs delivered as glasses can
be more than 20 times as active as the crystalline form, because of enhanced
solubility. However, this is only possible if the glassy drug is stable in
the package for two years without crystallizing. |
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Currently no one can
accurately predict crystallization under the conditions relevant to these
applications – the current theory cannot predict correctly either nucleation
rates or crystal growth rates. For example, the accompanying figure shows
that the kinetic part of crystal growth rates is not inversely proportional
to the viscosity, as predicted by current theory. There are good reasons to
believe that these failures occur because of the heterogeneous dynamics
described above. The current theory treats the disordered state as
dynamically uniform.
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With Professor Lian Yu and his group (in the School of Pharmacy), we are
studying crystal growth in thin films of organic glasses. We have found
evidence for fast crystal growth at the surface. |
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Another interesting feature of this surface
crystal growth in the way in which the crystals grow more than 100 nm out of
the surface. This striking behavior seems to require either that the crystal
is pushed out of the glass or that molecules climb up the sides of the
crystal to attach to the top.
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PowerPoint presentation on Influence of heterogeneous dynamics on
crystallization
PowerPoint
presentation on Fundamental Studies of Crystallization of Organic Materials
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Dynamics
in actively-deformed polymer glasses |
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Consider polycarbonate (the polymer used in safety
glasses) as a representative polymer glass. When polycarbonate is stretched
slightly (less than 1%) at room temperature, it responds like a very stiff,
ideal spring. When the force is released, the material returns to its
original state. At larger deformation, polycarbonate “yields” and can be
pulled without further increase of the applied
force. If the force is released
in this regime, the glass does not return to its original length but has
acquired a “permanent set”. If polycarbonate is deformed beyond 130% of its
original length, it typically fractures and breaks. This sequence of events
is the response of a “tough” material and it is completely different than
the brittle response of a crystalline solid. Qualitatively, the macroscopic
behavior of a tough material is not so different than a metal spring of
everyday experience. After a small deformation, the spring can snap back to
its original length. Larger deformations result in a bent spring that
remains extended even without any applied force.
We wish to understand the microscopic mechanism that allows polymer glasses
to “flow” (deform) under conditions where mobility is otherwise absent. We
have built an apparatus to measure the deformation-induced mobility of
polymer glasses and nanocomposites. It has been previously established that
the reorientation of dilute dye molecules (on the time scale of thousands of
seconds) can monitor the segmental dynamics of a polymer melt. Here we
utilize this method to measure mobility during tensile deformation of a
free-standing poly(methyl methacrylate) glass. We have observed increases in
mobility during deformation over a range of temperatures (from Tg-10 K to Tg
-30 K), with larger changes at lower temperatures.
At Tg – 18 K, with a
strain rate of 10-5/s, segmental mobility increases slowly at first and then
dramatically, so that the increase in mobility during deformation reaches a
factor of about 200. After removing the stress, we observe that the enhanced
mobility disappears slowly. These measurements are being compared to
continuum and mesoscopic models of polymer glass dynamics and rheology. This
is a collaborative effort with Juan de Pablo (UW-Madison), James Caruthers
(Purdue), and Ken Schweizer (Illinois).

PowerPoint presentation on
"Dynamics in Actively
Deformed Polymer Glasses"
PowerPoint presentation on
"Dye Reorientation as a Probe of Stress induced Mobility in PMMA Glasses"
(APS
meeting, March 2008)

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