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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 by the National Science Foundation
and the Department of Energy. 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.
PowerPoint presentation on "Segmental and terminal dynamics in miscible
polymer blends containing polystyrene or polyisoprene"
PowerPoint presentation on "Two DSC Glass Transitions in Miscible Blends
of Polyisoprene/Poly(4-tert-butyl styrenen)"

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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), secondary ion mass spectrometry (SIMS),
and thermally programmed desorption (TPD), these experiments have
extended the range of |
Self-diffusion of tris-naphthyl
benzene
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measured diffusion coefficients by 6 orders of magnitude for four different
organic glass formers.
PowerPoint
presentation tutorial on dynamics in supercooled liquids and polymers
PowerPoint presentation on
"Translational diffusion (and crystal growth rates) in supercooled liquids
and glasses: Influence of spatially heterogeneous dynamics"
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 methodology other than the one that we have developed. Our glasses have useful material properties.
For
example, because they are more dense and lower in enthalpy than ordinary
glasses, they resist crystallization and water uptake. These stable
glasses may have immediate technological relevance, e.g., to optimize charge
mobility in organic electronics. 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 "Hiking
down the energy landscape: Mobile surfaces allow the preparation of
stable glasses via physical vapor deposition"
Poster of
"AC Nanocalorimetry of Indomethacin and tris-Napthylbenzene"
PowerPoint
presentation on "Comparison of surface mobility on polymeric and low
molecular weight glass-formers" (APS meeting, March 2008)
PowerPoint presentation on "Neutron
and x-ray scattering measurements on extraordinarily stable molecular
glasses"
Poster of "Mobile
surfaces allow the preparation of extraordinary organic glasses by vapor
deposition"

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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.
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
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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.
With Professor Yu, we are also studying a remarkable
aspect of crystal growth in glasses. The accompanying figure shows crystal
growth rates in ortho-terphenyl. The behavior above 250 K is well
understood, at least at a qualitative level. As the temperature approaches
the melting point, the crystal growth rate goes to zero because of
thermodynamics. As the temperature approaches Tg from above, the crystal
growth rate gets small because molecular motion is needed to form crystals
and the rate of motion is slowing. Thus the sharp increase in the crystal
growth rate below 250 K is quite unexpected. This fast growth process
persists well into the glass state below Tg. How is it that molecules that
cannot move (because they are in a glass) manage to reorganize into a
crystal?! |
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PowerPoint presentation
on "Fast crystal growth from single component organic glasses"
Poster on "Fast crystal growth in
organic glasses"
PowerPoint presentation on Influence of heterogeneous dynamics on
crystallization
PowerPoint
presentation on Fundamental Studies of Crystallization of Organic Materials
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In many situations dynamics near a glassy polymer surface are important.
For example, polymer structures with dimensions less than 50 nm are
projected to be routinely prepared by lithography within 6 years.
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Collapse behavior of resist structure |
These small structures are weaker than would be
expected based on the properties of larger polymer samples; this weakness
will interfere with many possible applications. Our current hypothesis is
that this weakness results from dynamics near free surfaces being orders of
magnitude faster than those away from the interface. We use confocal
microscopy to measure the dynamics of dye molecules near the surface of
small polymer structures. The
rotational and translational motion of the dyes can be quantitatively
related to the polymer dynamics near the surface. |
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PowerPoint
presentation on "Direct measurement of molecular motion in freestanding
polystyrene thin films (and other polymers too)" ACS 2011

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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. 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
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during deformation
over a range of temperatures (from Tg-5 K to Tg -20
K), with larger changes at lower temperatures. Increasesin mobility of
up to a factor of 2000 have been observed. The data are consistent with the
idea that deformation induces mobility that allows flow to occur, as shown
in the accompanying figure.
These measurements are being compared to continuum and mesoscopic models of
polymer glass dynamics and rheology.
We are working
to understand changes in mobility during more complex deformations. A
particular important goal is to understand how mobility changes the position
of the polymer system on the energy landscape.
This is a collaborative effort with Juan de Pablo (UW-Madison), James
Caruthers (Purdue), and Ken Schweizer (Illinois). |
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PowerPoint presentation on
"How Stress Unjams a Polymer Glass:
Direct Measurement of Molecular Mobility During Active Deformation"
PowerPoint presentation on
"Direct Measurement of Molecular Mobility in Actively Deformed Polymer
Glasses"
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