A Cold Neutron Chopper Spectrometer
for
the
Spallation
Neutron Source
1 Introduction. 2
2 Scientific Case. 2
2.1 Quantum Liquids. 3
2.2 Rotational Tunneling Spectroscopy. 4
2.3 Magnetism.. 6
2.4 Sepctroscopy: Adsorbates at
Interfaces and Nanoporous Materials. 7
2.5 Dynamics of Water 8
2.5.1 Confined and Interfacial Water 8
2.5.2 Environmental Chemistry: Water and Reactions
at the Water-Solid Interface. 9
2.5.3 Solvent Structure and Dynamics in
Restricted Environments. 10
2.6 Complex Fluids. 11
2.6.1 Lubrication, Selective Adsorption
and Diffusion at Interfaces. 11
2.6.2 Polymer Dynamics. 13
2.6.3 Diffusion in Bilogical Gels. 14
3 Referencers. 15
We propose the formation of an Instrument Development Team
to pursue the construction and operation of a high resolution multi-chopper
spectrometer. This instrument would have
variable resolution and would simultaneously cover a broad range of both energy
and momentum transfer. The resolution of
the instrument, and the energy-momentum range covered, allow it to address a
broad variety of scientific problems in areas as diverse as biomaterials,
polymers, cements, magnetic system, geological systems, and quantum liquids.
Furthermore, it will have 2 orders of magnitude more flux on sample than
existing disc chopper spectrometers allowing it to address problems not
possible at present.
The proposed instrument with energy resolution that is
variable between 10 and 100 meV, over an incident energy (Ei) range of 220 meV,
is a high priority of the Instrument Oversight Committee[i]
(IOC) and the user community[ii] of the Spallation
Neutron Source (SNS) since it address a broad range of science. In addition, the spectrometer provides energy
resolution and momentum transfer (Q) ranges complimentary to the Backscattering
and Fermi chopper instruments planned for the
SNS.
The design of the proposed instrument is similar to IN5 at ILL
and DCS at NIST. However, the instrument
we propose will have much greater intensity, greater angular coverage at low
angles above and below the scattering plane, and will utilize position
sensitive detectors throughout. Thus, we
expect that this instrument will support a broad range of science ranging from
the motion of water in cements to single crystal studies of magnetic systems.
In this proposal we first describe some of the scientific
areas on which the proposed instrument will have a major impact. We then describe the proposed instrument and
our plan of action for constructing this instrument. We then describe the conceptual design of the
instrument and its performance characteristics. Finally, we describe the
structure of the IDT, the estimated cost of the instrument and the expected
funding for construction of the instrument.
An important aspect of this proposal is that we are planning
a two phase construction project. The
first phase would see a limited capability instrument at IPNS in three years. The second phase would see the instrument
fully operational at SNS on day one. We
believe that this two phase approach, while increasing costs slightly, offer
many overwhelming benefits. Among these
are an extended commissioning period, an invaluable opportunity for the SNS to
debug and evaluate systems such as detector electronics, data acquisition
systems, and chopper controls, a chance to develop data analysis and
visualization software, the opportunity to commission sample environment
equipment, the ability to train students and develop a user community, and, if
all goes well, the opportunity to carry out some science. We believe that the advantages of this plan
to the SNS, the IDT, and the neutron scattering community far outweigh the
additional costs it imposes.
The proposed instrument will addresses a broad range of
scientific topics ranging from quantum liquids to magnetism to the dynamics of
water in a wide range of environments from biological systems to cements. These topics
are at the forefront in many different areas such as condensed matter physics,
materials science, chemistry, biology and environmental science. The proposed spectrometer, with its high
intensity and adjustable resolution, will provide a powerful and flexibile tool for addressing these problems. In the remainder of this section we describe
just a few of the possible areas where this instrument will have a major
impact.
The atomic-scale dynamical responses of liquid and solid 3He
and 4He are of special interest because they are strongly
interacting quantum systems. Both liquid 3He and 4He
exhibit superfluid properties at low temperatures and
serve as model systems for studies of strongly interacting fluids1-3, critical phenomena4,5, superconductivity6 and even - in the case of He
- for exploring big bang dynamics relating to the birth of the universe. The
spectrometer proposed here would be without equal as a device for measuring the
dynamical structure factor, S(Q, w), of quantum liquids in the wave vector
range 0.1 <Q<3.0 -1 where collective excitations, such as phonons, rotons
and zero-sound excitations, are observed in quantum liquids.
Figure 2‑1 S(f,E), with background subtracted, for two scattering
angles, 75.32o and 80.28o, located near the (bulk) roton momentum (1.92 -1). Shaded areas indicate additional scattering
intensity lower than the bulk roton. Dashed lines denote fits to each individual
scattering component (long dash: 3-D roton, short
dash: multiphonon, bold dash: 2-D roton)
and the solid line is the sum of these components. Inset: the dispersion of the 3-D and 2-D roton (shaded region) extracted from the scattering data.
Quantum liquids will undoubtedly continue to be of
widespread interest for many years to come. Currently, for instance, there is
much effort devoted to exploring the effects of disorder on the superfluid phase transition7-9. Disorder can have a profound
impact on phase changes, even to the extent of altering the universality class
of the transition. Spatial disorder can be introduced by confining liquid
helium in porous media, such as aerogels and xerogels. A multi-chopper spectrometer is ideal for
observing the small changes in the microscopic excitations that appear to be
associated with the modified transport properties of such systems. For example,
recent studies of these systems have reported the appearance of a new
excitation the two-dimensional roton. This
excitation has a lower energy than the roton
associated with the bulk liquid. The observation of this new excitation allows
the modified properties of confined helium, such as its heat capacity and superfluid fraction, to be understood in terms of the
microscopic excitations of the system.
A different kind of disorder can be introduced into superfluid 4He by the addition of 3He.
This statistical type of disorder is very different from spatial disorder
because 3He atoms are fermions and therefore are not participants in
the Bose-Einstein condensation, which is responsible
for the superfluid properties of 4He.
There is very little understanding of the microscopic effects of spatial and
statistical disorder at the present time. High quality measurements are
expected to be essential as further guides to the theory. Also multiphonon effects are important in distinguishing between
the various theories.
Add discussion of measurements at negative pressures (ref
Maris)
Hindered rotation and quantum rotational tunneling measured
via inelastic neutron scattering can provide extremely detailed information on
the shape and magnitude of intermolecular potentials10. Methods such as electron energy loss
spectroscopy, helium atom scattering, and optical techniques can provide
information on the higher energy molecular motions but do not provide the same
sensitivity to the details of intermolecular potentials as high resolution
neutron spectroscopy. Therefore neutron
scattering is an ideal measurement technique for developing a fundamental
understanding of intermolecular forces.
One of the recent developments in rotational tunneling
spectroscopy (RTS) is its application to the study of adsorbed molecules11-14. These porous materials are playing an
increasingly important role in the exploration of fundamental scientific issues
related to diverse technological applications including adhesion, lubrication, tribology and engineering of materials. The surface interactions and finite size
effects both play a key role in the qualitative modification of the properties
of materials contained within the porous host.
The rotational dynamics of the adsorbed species can be affected directly
by the interactions with the surface so the resulting neutron spectra reflect
the changes in the intermolecular potential.
Methane adsorbed on various surfaces and into porous hosts (Figure 2‑2) is an excellent example in which a detailed
understanding of changes to the intermolecular orientational
potential due to the presence of a surface has been obtained12,13. RTS has also been used to illustrate that the
effects of disorder on the tunnel spectrum in a simple molecular solid confined
to a porous host14 are similar to those observed
in a glassy polymer system15 where the structural disorder
yields a distribution of local orientational
potentials. These applications of RTS
are useful in obtaining a fundamental understanding of the role of nanoscale disorder in the quantum dynamics and static
structure of the adsorbed molecular system.
Figure 2‑2 INS spectrum of methane in a controlled pore glass
taken with the high resolution chopper spectrometer MIBEMOL12. The excellent energy resolution was necessary
to resolve the peaks at 77 meV and 145 meV.
Figure 2‑3 The top panel shows a
contour plot of the inelastic scattering from H2 in Vycor
at various pore fillings at 6 K. The
bottom panle is a cut through the contour plot at
f=0.65 (indicated by the line).
Another example, which exploits the broad energy range
available with a multi-chopper spectrometer, is hydrogen adsorbed on surfaces. Free H2 is well described as a
quantum rigid rotor with discrete energy levels labeled by the rotational
quantum number, J, and energies given by EJ = BJ (J + 1) where B is
the rotational constant which is equal to 7.35 meV
for H2. These energy levels
are relatively insensitive to interactions that bind the molecule to a
surface. However, they are very
sensitive to orientaional interactions which cause a
shift in the rotational energies from their free molecule value. Figure
2‑3 shows the scattering from H2 adsorbed on
porous glass as a function of hydrogen filling.
The evolution of the system as the filling is increased clearly
illustrates the crossover from surface dominated behavior to bulk like behavior.
Studying the rotational dynamics of adsorbed species
requires both high intensity neutron beams because of the relatively dilute
samples, and good energy resolution (dE<100
meV) in order
to resolve the low-energy tunneling modes.
The multi-chopper spectrometer at the Spallation
Neutron Source will be capable of providing the high intensity and excellent
energy resolution necessary to study these interesting systems.
Despite many years of study, the fields of superconductivity
and magnetism continue to provide new surprises as exemplified, for example, by
the recent discoveries of high temperature superconductivity, giant magnetoresistance and colossal magnetoresistance.
Apart from their basic scientific interest, magnetic materials in which
electron correlations produce macroscopic effects are central to new
developments in technologically important areas such as power transmission and
information storage.
A high-resolution inelastic scattering spectrometer provides
a powerful tool for the study of magnetic systems. In such forefront areas such as the study of
quantum magnetism an instrument capable of measuring with energy resolutions of
tens of meV
and momentum transfers of 0.1 to several -1 is ideally suited to
the study of low energy magnetic excitations, and subtle changes induced by
small shifts of temperature or field.
Many of the most interesting problems now under investigation, including
quantum criticality, spin gaps, and non-Fermi-liquid
materials, demand an instrument with these capabilities.
In addition to the one-magnon mode
shown here, there is a much weaker two-magnon mode
near energy transfers of 1 meV. This has been observed17 using a cold triple axis
spectrometer. Our proposed instrument
would have the resolution and intensity to characterize both excitations
simultaneously. The quasi two-dimensional
antiferromagnet K2V3O8
has a small energy gap of order 100 meV, with dispersion ranging up to a few meV. The dispersion measured at IRIS18 is shown in Figure 2‑5. Neutron
diffraction19 has revealed that a small
magnetic field closes the gap and leads to an exotic spin-reorientation
transition. The proposed instrument at
SNS would allow for simultaneous measurements of the gap and full dispersion as
a function of applied field and temperature, offering the possibility of a much
better understanding of quantum critical phenomena associated with the closing
of spin gaps by applied magnetic fields.
As a final example of a physical system where the new
instrument would have an impact, we consider the non-Fermi-Liquid
material UCu5-xPdx, for which measure-ments
on different machines20 had to be combined to obtain
evidence for unconventional scaling of the magnetic fluctuations. Better resolution combined with a large
bandwidth would enable similar studies to be carried out on a single machine.
Accounting for the additional intensity available at SNS, one may also consider
studies with smaller samples. All of
these factors point to some
remarkable
opportunities for forefront research on magnetic materials using the proposed
instrument.
Figure 2‑4 One-magnon mode in the alternating chain material Cu(NO3)2D2O measured using IRIS.
Figure 2‑5 Dispersion
of magnetic excitations in the quasi-two dimensional antiferromagnet
K2V3O8 as seen with the IRIS spectrometer.
2.4 Sepctroscopy: Adsorbates at
Interfaces and Nanoporous Materials
The chemical changes that occur at interfaces during
heterogeneous catalysis using zeolites (for example) represents
a challenge to experimental and theoretical techniques. The shape selectivity
of these materials derives from the shape and size of the voids and pores in
these inorganic crystals. The interaction of adsorbates
with the active sites and the diffusion of reactants and products in the
internal voids enhance the production of specific products. These are commonly
used to provide basic feed stocks for the chemical industry. Neutron scattering
is well suited as a probe of these properties. Vibrational
spectroscopy studies of adsorbates located at
preferred sites in the catalyst allow for the in-situ
characterization of reaction intermediates. The weak interaction of the neutron
with most isotopes allows for bulk measurements, focusing on the
intra-crystalline species.
The vibrational spectroscopy of adsorbates is mainly concerned with the lower frequency intramolecular and external modes of the adsorbate. When a molecule attaches to a characteristic
site in the adsorbate, the strength of that
interaction can be probed using vibrational spectroscopy. It is also in principle possible
to probe the dynamics of adsorbents such as zeolites, where the frequency and
amplitude of the framework motion is related to inter-void diffusion. The
straightforward amplitude weighting of the neutron intensities is of
considerable benefit in such studies. At higher frequencies, the vibrational spectroscopy of adsorbates
that form intermediates (e.g. proton transfer from H-ZSM-5 to bases) can
provide considerable insight into the nature of the chemical forces which shape
the catalytic activity of such materials.
For the case of diffusion, neutron scattering can provide
dynamical information on the characteristic length scales of zeolite structures, and the time-scale for site-site movement.
A characterization of such microscopic details regarding the adsorbate-zeolite interaction provides a quantitative basis
for the understanding of the performance of these catalysts, in addition to the
important question of the verification of computational modeling approaches.
Although some characterization of adsorbates
is currently possible, the experiments are typically limited to adsorbates containing hydrogen. Even in such favorable
cases (hydrogen incoherent neutron scattering cross-section is 16x greater than most other atoms) it is often not possible
to obtain sufficient signal from such dilute systems. It is often also the case
that the resolution of the instrument limits the interpretation of the data. A
significant increase in intensity is required, some of which could be
sacrificed in those cases where resolution is of importance. For the dynamics
of non-hydrogenous adsorbents and adsorbates, extra
intensity is a fundamental requirement for further development in this field.
The planned multi-chopper instrument is well suited to
studies of adsorbates in nanoporous
compounds as the length scales probed by such an instrument spans the range of
characteristic lengths in, for instance, zeolites. The time scale of the
diffusive motion in nanopores is typically suited to
the good energy resolution envisaged for CNCS, and the energy transfer range
will be very useful to explore unusual non-Lorentzian
broadening effects. The low frequency vibrational
motion characteristic of the interaction of the adsorbate
with the nanopores typically occurs in the range 5-20
meV,
and this energy range will be accessible on CNCS. Furthermore, the higher
momentum transfers up to about 4 -1, and the better Q resolution obtainable using the proposed spectrometer will
make coherent quasiclastic scattering studies a
realistic option. This will also be aided by the expected improvements in
signal-to-noise.
Questions regarding the nature of diffusive motions of water
in confined geometry and on surfaces of macromolecules have not been answered
definitively in spite of the fact that it has been studied by every generation
of neutron physicists,. The reason is that these pioneers had access to only
quasi-elastic spectrometers with resolutions of about 100 meV.
They were forced to analyze their data with a model consisting of an elastic
peak plus quasielastic contributions (all Lorentzians). It was not until Bellissent
and Chen carried out studies with a resolution of 10 mev (IN5 at ILL)
that we discovered that the quasi-elastic peak was not Lorentzian21. In fact, at lower
temperatures, it was very well approximated by the Fourier transform of a
stretched exponential relaxation function. Furthermore, there was no need of
postulating the existence of an elastic peak, because the Fourier transform of
the stretched exponential has a cusp at the elastic position, and it has a
broad wing which can also be fitted with Lorentzians21,22. This is significant because
the mode coupling theory shows that the lineshape is
not necessarily Lorentzian. Spectrometers capable of studying the line
shape are essential if the motion of water is to be understood. For this to be possible, the resolution has
to be below 15 eV. One cannot forget that water
molecules also undergo simultaneous rotational motion but those contributions
are much broader. That means an ideal QENS instrument should have a variable
energy resolution ranging from say 5 to 30 meV and the energy range covered (the dynamic range) should
be as broad as possible because one is dealing with a broad shoulder.
The wave vector transfer range of 0.1-4.0 -1 is well suited to the study of
diffusive motions of
water. For a non-Lorentzian line shape the
signal-to-noise ratio is very important as Fourier transform of a stretched
exponential has a long tail which is difficult to resolve from a flat
background. This will allow a broad
range of to problems to be addressed.
These include:
- Coherent
dynamics of heavy water in porous glass where a semi-localized, mixed, longitudinal-transverse
branch of short-wave length sound
dispersion relation may exist. Experimentally, the existence of a branch
of propagating high frequency sound has been observed23. Also, supercooled D2O in Vycor has exhibited
clear evidence of an alpha relaxation as predicted by MD 24,25.
- Short-time
confined diffusional motions of lipid molecules
as well as the hydration
water on lipid bilayers without
and with peptide channels imbedded in them (experiments carried out using
back scattering and Fermi chopper
spectrometers at NIST recently).
- Single-particle, relaxational
dynamics of hydration water in Portland cement as functions of
temperature, additives, and aging
of the cement paste (experiments
carried out using MIBEMOL multi-chopper spectrometer at Saclay, France
and a Disk-chopper Spectrometer at NIST recently).
Environmental chemistry has emerged recently as an important
discipline within chemistry and, in its broadest definition, includes the study
of chemical processes in atmospheric, aquatic, marine, soil, geological,
biological and related engineered systems. A common component in all these
systems is water, where it has been estimated that there is 3.2x1020
g of water in the biosphere. Furthermore, water is a major component of living
organisms, e.g., comprising upwards of 70% by weight of the human body. As a
result, most environmental chemical and biochemical reactions proceed in
aqueous systems.
Because of its ubiquity in the environment, its importance
to biological functioning, and its host of unique properties, water is the most
extensively studied of all compounds.
Much of what is known about the structural and dynamical
properties of water has been determined using neutron scattering techniques. In
fact, the first diffraction study of liquids was conducted in 1946 using water.
Since that time, the radial distribution function for liquid water, as
determined using neutron scattering, has been a standard against which all
molecular models of liquid water have been compared. The ability or inability
of a given potential function to reproduce the experimentally-derived radial
distribution function is a convenient and powerful test of that model
potentials ability to reproduce the density and structural features of liquid
water. A second test of a molecular dynamics-type computational model is its
ability to properly reproduce the self-diffusion coefficient of water; here
again, neutron scattering studies have played a significant role, where quasielastic neutron scattering studies have quantified the
temperature and pressure dependence of the translational
and rotational mobility of water. Similar studies have been conducted using the
solid form(s) of water, and related neutron scattering studies have quantified
ion solvation and the structure and properties of waler at surfaces. Of particular importance to
environmental chemistry are reactions at surfaces. It has been estimated that
there exists ~1018
m2 of inorganic, organic and biological surfaces in soils and
waters. Soils can have specific surface areas of <1 to >100 m2/g, and the role of surfaces in atmospheric
reactions is well-known. A consequence of the abundance of both surfaces and
water in the environment is that many important chemical processes in the
environment occur at the solid-water interface. Examples of such processes
include the dissolution and precipitation of minerals and the sorption and
release of contaminants. Some argue that life itself began at the mineral-water
interface.
Such reactions are ultimately important to a wide range of
environmental quality and related health issues, and are important to waste
containment, water treatment, contaminant transport, and a host of other
natural and engineered processes.
Application of neutron scattering techniques will provide
new insights into the environmental chemistry of water and reactions at the
water-solid and other interfaces. Quasielastic
neutron scattering has been used to quantify the translational
and rotational mobility of water in layer silicate clay minerals, zeolites and
other solid phases. Nevertheless, this approach has not been used extensively
by environmental chemists, and there is considerable scope for further work,
particularly in the area of organic contaminants. Finally, as described above,
the capability for vibrational spectroscopy of
adsorbed species should offer needed details about the state of adsorbed
molecules, particularly for systems not amenable to traditional photon-based
techniques.
CNCS is well suited for such studies because of its momentum
transfer range, which probes on length scales of a fraction of an angstrom, to
length scales which are characteristic of the cross-over from three to
two-dimensional behavior in layered compounds such as clays. Furthermore, it is
likely that the use of coherent quasielastic
scattering will provide further insight into the transport properties of adsorbates, and the excellant
Q-resolution will be an important asset.
The nature of solvent structure and mobility can change
drastically when the solvent is in a restricted environment. The aim of my research
is learn how solvent structure and dynamics vary for molecules at surfaces,
especially molecules at the liquid-liquid interface of reverse micelles. There
are several different goals for our studies. We aim to learn how the specific
interactions that molecules experience at interfaces influence the solvent
structure and dynamics. We can then use this information to predict the role of
restriction in chemical reactivity in these environments. We have designed
further experiments to create and investigate novel reverse micelles that solubilize nonaqueous polar
solvents and water/polar solvent mixtures in a nonpolar
continuous phase. Such studies, that vary each component of the reverse micellar solution, will reveal the role of polar and nonpolar solvent and the role of the surfactant in reverse
micelle formation and create new restricted media for chemical reactions.
Our initial experiments have focused on water in Aerosol-OT
reverse micelles in isooctane. The water/Aerosol-OT/isooctane water-in-oil reverse
micellar solutions have been exceptionally well
characterized by various methods. Our studies have shown that the water in
smaller sized micelles is largely bound to the Aerosol-OT polar head group
while in larger micefles a water pool approaching
bulk characteristics is observed; the dynamics of the water molecules vary with
micellar size. The efficacy of quasielastic neutron scattering (QNS) for determining water
motion in. Aerosol 01 (AOl)
reverse micelles has recently been demonstrated. Our experiments at Colorado
State University
probe the dynamics of polar solvation indirectly via
a probe molecule located inside the reverse micelles. To learn more about these
systems, we have initiated a collaboration to probe the translational
and rotational motion of water in various AOl
reverse micelles.
Systems that we plan to explore include a range of polar
solvents, polar solvent/water mixtures, surfactants and nonpolar
solvents. The first investigations will focus on Aerosol 01 microemulsions
sequestering nonaqueous polar solvents. However, many
other systems are planned. Polar solvent properties such as dielectric
constant, surface tension and hydrogen bonding will be varied to discover their
role in reverse micelle formation. Potential polar solvents include methanol, acetonitrile, nitromethane, dimethylsulfoxide, formamide, N-methylformamide, N-methylacetamide,
dimethylformamide, propylene carbonate, ethylene
glycol, propylene glycol, and glycerol. Surfactants will be varied to explore
the role of the surfactant head group interactions. In particular, anionic,
cationic, nonionic and zwitterionic surfactants will
be used to form microemulsions. Furthermore, the
influence of nonpolar shape will be explored. Some nonpolar solvents to be studies include straight chain alkanes such as decane, branched alkanes such as isooctane, cyclic alkanes
such as cyclohexane, and haloalkanes
such as carbon tetrachloride.
QNS measurements on these systems can reveal basic
information about the relative contribution of rotational and translational motion to ultrafast
polar solvation dynamics, an issue of intense
discussion among researchers studying solvation
dynamics. In solvation dynamics experiments, the
experimental results cannot distinguish contributions from solvent rotations
and translations. In contrast, the QNS technique provides information about
both solvent rotational and translational motions.
Therefore, it may be possible to use the QNS results to determine the relative
contribution of rotation and translation to solvation
dynamics. Solvation dynamics measurements show that
water is essentially immobilized in the smallest AOl reverse micelles but that for slightly larger
reverse micelles, an ultrafast, bulk-like component
appears, increasing with micellar size. QNS measurements
will reveal whether both rotational and translational
motion increase commensurately as the micelles grow.
The proposed research has the potential for tremendous
impact on nanomaterial synthesis. New restricted
environments will be discovered in which nonaqueous
chemistry can be scaled down to nanoscopic
proportions. This proposed research will not only push forward the limits of nanoscale chemistry, it will also provide in depth
understanding of the forces driving reverse micellization.
By solubilizing liquids in addition to water,
specific molecular interactions can be correlated with micelle formation.
Selective adsorption of a single chemical species onto a
solid surface from a multicomponent fluid mixture is
a phenomenon of wide interest in materials science and has far-reaching
technological applications. An example of particular interest occurs in soft
materials and involves the adsorption of long flexible-chain molecules. Such
polymers at solid interfaces are often used as coatings, lubricants, and
adhesives. A microscopic understanding of their selective adsorption could
potentially lead to significant advances in these technologies.
As discussed in a number of reviews considerable progress has been made in understanding the structure and
thermodynamics of polymer mixtures near interfaces. This work has lead to very
interesting studies of polymer kinetics near interfaces at the macroscopic
level. However, understanding of the microscopic dynamics of these systems is
only at a very early stage of development. Some of the fundamental questions to
be addressed are: how do long, flexible molecules anchor, diffuse, and desorb from a surface. At what rate do these motions occur
and on what do these rates depend? In particular, how does the presence of
solvent molecules in the interfacial region affect these motions?
The dynamical complexity of selective adsorption derives
from the size and flexibility of the polymer molecules which introduces several
time and length scales into the problem as well as the number of competing
interactions involving the polymer, solvent, and interface. In the last five
years, probably the most significant advances in studying polymer dynamics near
interfaces have been made with computer simulations. The simulations that have
treated the dynamics of the selective adsorption process in the most detail
have necessarily used solutions of relatively short molecular chains with the
molecules of choice being alkanes having the
structure [CnH2~+2j. At present, intermediate-sized alkanes (20 < n <40) are at the limit of available computer power for
simulations of fluid structure and dynamics near interfaces.
One of the most detailed studies of the dynamics of
selective adsorption at the atomic level was performed by Xia
and Landman. In their investigation of the
segregation of a binary alkane mixture on a gold
substrate, they found evidence of a dynamical adsorption mechanism involving a
sequential, segmental, reptation-like penetration and
adsorption process of the long-alkane chain. Such a
mechanism had been proposed and discussed on the basis of macroscopic
experimental studies of the adsorption-desorption
kinetics of longer polymer chains. Thus, there is reason to believe that a
quantitative understanding of the microscopic structure and dynamics of these
model systems will aid in interpreting selective adsorption of more complex
polymers.
Despite the effort which has been invested in simulations of
the dynamics of flexible chains near interfaces, there have been strikingly few
experimental studies with which to compare them. We describe here a
comprehensive research program which responds to these recent developments in
selective adsorption and builds on our own experimental investigations of the
structure and dynamics of alkane films. It is
comprised of quasielastic neutron scattering (QNS)
experiments and computer simulations to probe, at the atomic level, the
dynamics of binary alkane solutions near a solid
interface. QNS is well matched to measure the diffusion rates which are
calculated for the adsorbed intermediate-sized alkanes.
In addition, structural studies will include new experiments utilizing th~ techniques of synchrotron
x-ray scattering and ellipsometry as well as an
extension of our neutron diffraction experiments. Knowledge of the equilibrium
structure of these alkane solutions provides a
critical benchmark for validating the computer simulations and is a
prerequisite for studies of molecular diffusive motion in these mixtures by quasielastic neutron scattering.
An extremely important stimulus for the computer simulations
of flexible chain molecules near interfaces has been experiments utilizing the
surface force apparatus (SFA) which have measured the solvation
forces between parallel surfaces enclosing a variety of complex fluids. These
studies have demonstrated the remarkable effects of confinement on the
structure and dynamics of fluids near a solid interface. However, the SFA
experiments can only probe atomic length scales in a direction normal to the
solid interface and are sensitive to the dynamic response of a confined liquid
to shear on macroscopic time scales. Our neutron and x-ray diffraction
experiments will probe molecular ordering in adsorbed films in directions both
parallel and perpendicular to a solid interface, while our quasielastic
neutron scattering experiments will probe molecular diffusive motion in these
films.
Although our principal focus in this project is the study of
selective adsorption of intermediate-sized linear alkanes
as prototypes for a variety of longer chain molecules (polymers), these
molecules are themselves of technological interest as lubricants. In fact, many
commercial lubricants are mixtures of linear and branched alkanes
so that it is of considerable interest to investigate selective adsorption in
these systems. Thus far, studies have centered on the degree of layering that
occurs in alkane fluids near surfaces, the dependence
of this layering behavior on molecular architecture, and the implications of
confinement induced ordering on the lubricating properties of films. Several
very recent computer simulations of intermediate-sized alkanes
have compared the structure and dynamics of linear and branched alkanes near surfaces as well as in the bulk melts.
However, there is only a small database of diffusion constants from NMR
experiments on bulk samples for testing these simulations. The QNS experiments
on intermediate-sized atkanes (20 < n < 40) which we are proposing will measure
the diffusion rates of pure phases of linear and branched alkanes
adsorbed on graphite as functions of temperature and film thickness for
comparison with these simulations. Furthermore, our ellipsometry
and synchrotron x-ray specular reflectivity
experiments will allow us to compare the measured 4ensity profiles of linear
and branched alkane films adsorbed on a
single-crystal metal surface to those calculated in simulations. These
experiments will be followed by QNS experiments to investigate the dynamics of
linear/branched alkane mixtures.
A signature of the long-chain nature of polymers is that
dynamic processes in polymer melts and solutions occur not only on the picosecond (ps) time scale
typically seen in low molecular weight organic liquids (ascribable in polymers
to solvent dynamics and repeat-unit scale dynamics), but also on much longer
time scales (>nanoseconds) resulting from coordinated niotions
of the chains. The manner in which local motions in polymers, such as
conformational librations and rotations, are related to the coordinated, long
time/large length scale motions is an important fundamental question that
remains unresolved. The strong coupling of spatial and time
scales manifest in polymer dynamics makes neutron scattering techiqu~es the preferred methods in. exploring the
mechanisms of polymer dynamics. The unique ability of neutrons to
variably probe distances on the order of interatomic
spacing has led to a greatly increased understanding of local dynamics in
polymers. Mechanistic understanding of these motions has been greatly
facilitated by quantitatively accurate molecular dynamics simulations of
polymer systems identical to those investigated experimentally. For the longer
time/large length scale processes, neutrons have also played an important role,
again greatly facilitated by quantitative comparison with simulation.
One approach is to focus on elucidating the mechanistic
connection between local and chain dynamics in polymer melts, glasses and
solutions through combined experimental and simulation studies. The systems of
interest include glass forming polymers which exhibits relaxational
bifurcation into a high-frequency and a low-frequency process as the
temperature is reduced toward the glass transition4 the structure
and dynamics of aqueous solutions of hydrophilic amphiphilic
polymers, and the structure and dynamics of polymer electrolytes. In the latter
materials, strong intermolecular interactions strongly influence the
conformations and dynamics of the polymer chains on time and length scales
accessible to neutron scattering. Molecular dynamics simulations can provide
detailed predictions of the motion of the atoms on the length and time scales
explored on CNCS.
Fundamental studies of local and chain dynamics in polymers
would be greatly facilitated by access an instrument such as CNCS, for the
following types of measurements:
- Incoherent
scattering of the 10 ps to 1 ns time scale to
study conformational and hydration/solvent dynamics.
- Coherent
QENS capabilities on the 10 ps to 1 ns time
scale in order to study complexation dynamics in
polymer electrolytes.
- Low-frequency
inelastic scattering to study unusual vibrational
features such as the Boson peak (typically below 10 meV)
The proposed momentum transfer range and energy resolution
of the CNCS spectrometer is well suited these types of studies, down to
approximately 100 ns. It will not be able to probe times down to 1 ns, but that is typically the domain of high-resolution
backscattering and neutron spin echo instruments at reactor sources. The energy
transfer range of up to 20 meV neutron energy loss,
in conjunction with a good energy resolution will make CNCS an excellent choice
for studies of the problems such as the relationship between the onset of
diffusive motion and vibrational modes such as the
Boson peak.
Hyaluronan (hA)
is a naturally occurring polysaccharide and is an important molecule in
biological systems. HA is a major component of the connective-tissue
polysaccharides (or glycosaminogLycans), and in
interstitial matrices. It is abundant in a number of biological fluids such as
vitreous humour, synovial
fluids, and umbilical cord fluids. The interstitial matrix contains
approximately three-quarters of the total extracellular
fluid, and the regulation of the water content of the matrix may play an
important role in determining blood volume, especially during emergencies such
as hemorrhage or dehydration. HA gels exhibit high water retaining and inbibition capacity and HA of high molecular weight (MW>
106 Da) forms a gel even at rather low
concentrations (~1% HA by weight). The specific roles of HA gels include the
influence of the flow of water (hydraulic conductivity) and the inbibition of water in the tissue through relaxation of its
concentration gradient (mutual diffusion).
One area of great interest is the physical properties of the
gels and solutions formed by HA and HA/protein (such as collagen). In
particular, we have concentrated on the mobility and transport of water in
these systems. Studies of water mobility in HA gels, the role of HA in the
resistance to water flow, and the interaction of water with the polysaccharide
molecules are required to understand the physico-chemical
basis of the physiological function of HA. In addition there is evidence in the
literature that supports a hypothesis that the release of fluid sequestered in
the HA-based interstitial matrix into the blood space is dependent on depolymerization of HA by the action of the enzyme hyaluronidase (HAdase).
Quasielastic neutron scattering
(QNS) has been used to probe the dynamics of water molecules in HA gels and
gels treated with HAdase. Water that is tightly bound
to the biopolymer appears immobile on the time scale of our experiments. A
second, majority population of water is also present which exhibits diffusion
rates that are reduced relative to water in its bulk liquid state. Time
constants for mobile water in the gels range from 0.5 to 1 psec for rotational motion, and 1
to 5 psec for the time between translational jumps. The mean jump distance typically
varies from between 2 and 3 .
A CNCS represents an ideal instrument to probe the translational dynamics, as it probes the time range from 1
to 30 psec and length scales ranging from 2 to 30 .
3
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The
potential impact of the instrument we propose on studies of magnetic systems is
hinted at by results obtained to date using the IRIS spectrometer at the