-MRS-
Chairs
Bill Carty
Dept of Ceramic Engr
Alfred Univ
Alfred, NY 14802
607-871-2451
Seth Fraden
Physics Dept
Brandeis Univ
MS 057
Waltham, MA 02254
781-736-2888
Terry Garino
Materials Processing Dept
Sandia National Labs
MS 1411
Albuquerque, NM 87185-1411
505-845-8762
Robert Prud'homme
Dept of Chem Engr
Princeton Univ
A301 Equad
Princeton, NJ 08544
609-258-4577
* Invited paper
SESSION HH1: GELS
Chair: Robert K. Prud'homme
Monday Morning, April 5, 1999
Salon 13 (M)
8:30 AM *HH1.1
RHEOLOGY OF PHYSICAL GELATION. H. Henning Winter , Dept.
Chemical Engineering and Dept. Polymer Science and Engr., Univ. of Massachusetts,
Amherst, MA.
Polymers at their transition from liquid to solid (critical
gels) exhibit a distinct relaxation pattern which allows detection of the
gel point and direct observation of gelation dynamics. The slow molecular
dynamics of critical gels is governed by a power law distribution of relaxation
modes. The time dependent relaxation modulus, G(t)=St-n, decays with a
relaxation exponent n and front factor S (stiffness of critical gel). This
can be measured, for instance, by mechanical spectroscopy at low frequencies
which shows distinct power laws of dynamic moduli, G' G wn, and a frequency-independent
loss tangent, G/G'. Not only is rheology a sensitive measure of molecular
mobility at a wide range of length scales. Rheology has also become more
quantitative due to the recent discovery of the rheological laws which
govern the gel point. Universality of the transition behavior has been
found for chemical and physical gelation (Winter HH, Mours M, Advances
in Polymer Science 134:165-234, 1997). The finite life time of physical
junctions imposes a problem if the ratio of life time and experimental
time (gel number) is within a decade below or above order unity. Two physical
gels will be discussed: crystallizing popyolefins and micro-phase separating
block copolymers.
9:00 AM HH1.2
COLLOIDAL GELS OF TETHERED SILICA PARTICLES IN POLAR
MEDIA: DIRECT CORRELATION BETWEEN DYNAMIC RHEOLOGY AND INTERACTION PARAMETERS.
Srinivasa R. Raghavan, Saad A. Khan , North Carolina State University,
Department of Chemical Engineering, Raleigh, NC; Jun Hou, Gregory L. Baker,
Michigan State University, Department of Chemistry, East Lansing, MI.
Colloidal interactions between particles dispersed in
a liquid can be suitably tailored by modifying the surface chemistry of
the particles. In the case of fumed silica particles, the surface can be
systematically altered from hydrophilic to hydrophobic by replacing a portion
of the original silanol (Si-OH) groups by non-polar alkyl chains. In this
study, we probe the effect of surface modification of fumed silica on their
rheology and microstructure in polar media. Variables of interest include
the length of tethered alkyl chain and the extent of surface coverage.
For the continuous phase, we examine a range of polyether liquids comprising
different architectures and molecular weights. We find that when the alkyl
chains are C8 or longer, and are attached at saturation levels,
a dense non-polar surface layer is formed on each silica unit. Such particles
experience strong interactions in polar media, leading to the formation
of a volume-filling gel network. We show that these interactions arise
as a result of a negative free energy of mixing between the tethered chains,
owing to the mismatch in chemical nature between chains and solvent. In
this flocculation process van der Waals interactions between the particles
play a negligible role. We also find that the greater the mismatch between
particle surface and liquid, the greater the ``stickiness'' of the surface
chains and correspondingly, the higher the elastic modulus (G
)
of the fumed silica network. This leads to a unique correlation between
G and a term comprising the
parameter for the chain-solvent pair. An approximate but useful form of
this correlation can be written as G
(s - m)4
where the latter expression characterizes the mismatch in solubility parameters
between the surface chains (s)
and the liquid medium (m).
9:15 AM HH1.3
MORPHOLOGICAL AND PROPERTY CHARACTERISTICS OF BLOCK COPOLYMER
GELS AND MESOGELS. Jonathan Laurer, Nicole Jackson, Megan King, Saad Khan,
Richard Spontak , NC State Univ, Depts of Materials Science & Engineering
and Chemical Engineering, Raleigh, NC; Scott White, Becton Dickinson Technologies,
Polymer Science & Technology Dept, Research Triangle Park, NC; Steven
Smith, Procter & Gamble Company, Corporate Research Div, Cincinnati,
OH.
Triblock copolymers in the presence of a midblock-selective
solvent are capable of forming thermo-reversible (physical) gels if the
endblocks are sufficiently rigid to serve as physical crosslink sites.
Such networked materials are of fundamental importance in the study of
macromolecular self-assembly, as well as commercial importance in the production
of shock-absorbing media and sealants. In this work, the morphological
features and mechanical properties of several copolymer-solvent systems
in which gelation occurs due to glassy styrenic endblocks are described.
The copolymer midblocks investigated here include isoprene, ethylene-co-propylene
and ethylene-co-butylene. Material and process variables to be discussed
include midblock-solvent compatibility, temperature stability, shear orientation,
blend composition and endblock reinforcement (with polyphenylene oxide).
Images of the microstructural elements (typically micelles) in these blends
have been obtained by energy-filtered and cryofracture-replication transmission
electron microscopy, and accompanying mechanical properties have been measured
by dynamic rheology. In addition to copolymer-solvent blends allowed to
reach near-equilibrium, the preparation and characteristics of solvent-rich
nonequilibrium mesogels, i.e., anisotropic physical gels composed of highly
swollen glassy bilayers, will be presented.
9:30 AM HH1.4
GEL-LIKE MODES IN POLY(ETHYLENE OXIDE) MELTS. R. Walkenhorst,
J.C. Selser , G. Piet, University of Nevada, Las Vegas, Physics Department,
Las Vegas, NV.
Two slow, long-ranged relaxations in melts of poly(ethylene
oxide) were studied using the noninvasive dynamic light scattering technique
of photon correlation spectroscopy. Measurements were made for two PEO
samples, one with a molecular weight distinctly above the entanglement
value and a second sample witha molecular weight distinctly below the entanglement
value. Based on several lines of evidence, it is concluded that the faster
of these relaxations, which can be identified with the cluster or Fischer
mode observed earlier in other homopolymer melts, results from the gel-like
behavior of a physical network formed by the polymer in the melt while
larger scale rearrangements in the polymer network structure are responsible
for the slower relaxation.
9:45 AM HH1.5
HIGHLY SWOLLEN HYDROGELS: RHEOLOGY AND PHASE BEHAVIOR.
H. Jiang, Anteon Inc., Dayton, OH; W. Su, TMCI, Dayton, OH; P.T. Mather
, T.J. Bunning, AFRL/MLBP and MLPJ, Materials and Manufacturing Directorate,
WPAFB, OH.
We have investigated the phase transition of water within
polyacrylate/chitosan blend hydrogels, with focus on the effect of crosslinking
extent, by means of oscillatory shear rheology. Interest in these materials
stems from our recent report of excellent laser damage resistance where
the measured laser damage threshold (LDT) is better than BK7 glass and
quartz, and 20 to 35 times higher than commercial PMMA. A strong correlation
between the hydrogel viscoelastic properties and water phase behavior is
found, with the presence of the hydrophilic moieties on the macromolecular
chains and the restriction of the polymer network influencing this correlation.
Trends among the measured shear storage modulus (Gí), shear loss modulus
(Gî), and shear loss tangent (tan d) are shown to be related to the mobility
of water within the gels. While an increase in the extent of crosslinking
is found to enhance the gel storage modulus at room temperature, the storage
modulus below the freezing point of water is found to decrease. Our results
are discussed with respect to the influence of polymer microstructure on
water mobility, confinement, and crystallization.
10:30 AM HH1.6
INTERACTION POTENTIAL BETWEEN POLYAMPHOLYTE STABILIZED
COLLOIDS. Christos N. Likos, Hartmut Loewen , Institut fuer Theoretische
Physik II, Heinrich-Heine-Universitaet Duesseldorf, Universitaetsstrasse,
Duesseldorf, GERMANY; Abe Vaynberg, Norman J. Wagner, Center for Molecular
and Engineering Thermodynamics, Dept. of Chemical Engineering, University
of Delaware, Newark, DE.
To understand stabilizing effects of gelatin we explore
how the adsorbed gelatin affects the interparticle effective pair potential.
Experimental measurements of the structure factor of colloidal dispersions
(60nm and 100nm diameter) stabilized with gelatin are analyzed to obtain
effective interparticle potentials. We use liquid integral equation theories
to fit the experimental structure factors to a simple model proposed by
Kamiyama and Israelachvili [Macromolecules 25: 5081 (1995)]. To this end,
we start with SANS profiles obtained with solutions of bare, charged latex
particles at different concentrations and fit them using a DLVO potential.
Having determined the various parameters characterizing the bare latex
particles, we proceed with the gelatin-coated solutions, where an additional
``compressional'' energy arising from the brushes is considered. In this
manner, the stabilizing potential of an adsorbed polyampholyte is quantified
These results are used to understand measurments of the stability of gelatin
stabilized dispersions under flow and their rheology.
10:45 AM HH1.7
THE STRUCTURE OF ADSORBED GELATIN ON FLAT SURFACES AND
COLLOIDS. K. Abraham Vaynberg , Norman J. Wagner, Heiko Ahrens, Dirk Eck,
Christiane A. Helm, Christos N. Likos, Hartmut Lowen.
Polyampholytes, such as gelatin, are well known stabilizing
colloids used extensively in the chemical and food industries. In this
work we explore gelatin interfacial layer properties by a number of methods
in order to facilitate the understanding of gelatin stabilization. Measurements
of the interfacial gelatin layer were done at the air-water and air-arachidic
acid interface. With X-ray reflectivity, the electron density profiles
of the adsorbed layers were measured. For the air/water interface, the
electron density profile consisted of two parts, a 1.5 nm thick concentrated
gelatin layer, and then a long trail (7 - 9nm). Only a diffuse tail was
found in the case of adsorption to the air-arachidic acid interface. These
layer structures were compared to previous results of the gelatin structure
on polymer colloids with various surface properties by small angle neutron
scattering (SANS) and dynamic light scattering (DLS) (Vaynberg et al. JCIS,
205: 131-140 (1998)). SANS measurements of the structure factor of concentrated
colloidal dispersions with adsorbed gelatin layers provide a further method
to characterize the adsorbed layer. Monte Carlo modeling of the structure
factors yields interparticle forces that could be understood in terms of
the adsorbed layer structure. These results help explain experimental measurements
of the strong stabilizing properties of gelatin in shear flows and at rest.
11:00 AM HH1.8
MICROSCOPIC AND BULK PROPERTIES OF CYANOGEL SYSTEMS:
A NOVEL AQUEOUS SOL-GEL SYSTEM BASED ON THE CHEMISTRY OF TRANSITION METAL
CYANOMETALATE COMPLEXES. Andrew Bocarsly , Ileana Flintoff, Jochim Gross,
George Scherer, Stefanie Sharp, Jennifer Wilson, Princeton University,
Dept. of Chemistry, Princeton, NJ.
Recently, we reported that a new broad class of hydrogels
can be synthesized based on the reaction of tetrachlorometalates with a
wide variety of transition metal cyanometalate complexes in an aqueous
environment. Such gels are composed of a coordination polymer having a
star morphology. We term these materials `cyanogels', based on the use
of a bridging cyanide ligand as a structural motif within the gel. Cyanogel
formation follows a classic sol-gel synthesis pathway with gel products
typically containing 95% water.
Due to its unusual chemical stability, the gel synthesized from tetrachloropalladate
and hexacyanocobaltate has been the focus of our characterization efforts.
Palladium cobalt cyanogels can be dehydrated to form xerogels, or solvent
exchanged using supercirtical carbon dioxide to form aerogels. These materials
are microporous offering interesting gas adsorption properties. The bulk
and microscopic properties of the solvent free gel materials have been
examined using a variety of spectroscopic techniques in conjunction with
electron microscopy and mechanial perturbation experiments. Based on these
experiments the gels appear collodial in nature with two different length
scales for porosity; one at the molecular limit the other a nanometer scale.
The bulk properties of the palladium.cobalt cyanogel have been examined
using beam bending experiments. In addition to permiability information,
these experiments have produced the mechanical moduli for this material.
Available results allow a comparision of this material to the better studies
silica gel system.
11:15 AM HH1.9
LAMELLAR HYDROGELS FROM CATANIONIC SURFACTANT MIXTURES
AND SIMPLE AMPHIPHILIC POLYMERS. Bret A. Coldren , Hee-Tae Jung, Joseph
A. Zasadzinski, Materials Research Laboratory and Department of Chemical
Engineering, University of California, Santa Barbara, CA; Hidetaka Iwai,
Kao Corporation, JAPAN.
Highly swollen bilayers of dimyristoylphosphatidylcholine,
pentanol and polyethylene glycol polymer-lipid (PEG-lipid) form gel-like
phases with up to 90% water. Freeze-fracture electron microscopy shows
that these gels are induced by the proliferation of interconnected, high
curvature liquid crystalline defects which are stabilized by the steric
requirements of the wedge-shaped PEG-lipid. This novel behavior can be
generalized to create similar hydrogels using dilute lamellar phases from
cationic and anionic surfactant mixtures. These catanionic mixtures form
lamellar phases of up to 80% water, in addition to spontaneous vesicle
phases at lower surfactant concentrations. Electron microscopy shows addition
of PEG-lipid to these dilute catanionic lamellar phases induces similar
interconnected defects, resulting in gelation. This finding supports theoretical
predictions and experimental observations that catanionic bilayers have
a characteristically low bending modulus. Additionally, we find that it
is possible to replace the PEG-lipid molecules with simple block copolymers
and nonionic surfactants of comparable molecular weight. The lower costs
and enhanced chemical stability associated with catanionic surfactants
and block copolymers makes these novel gel materials potentially useful
as thickening agents, personal care products, detergents, etc., where viscosity
control at high water fractions is desirable.
11:30 AM HH1.10
LYOTROPIC LIQUID CRYSTALLINE POLYMER GELS-A NEW THERMOTROPIC
MATERIAL. Arno Seeboth , Hans-Rainer Holzbauer, Jörg Kriwanek, Th.
Fischer, WITEGA, Dept. Adv. Mater., Berlin, GERMANY.
In recent years thermotropic materials have met with growing
interest. Such materials (for example polymer blends or gels can be used
as thermotropic materials for intelligent shade-giver (light valve) or
Large Area Displays. The aim of this work was to examine the possibility
of preparing gels with new optical properties. Of particular interest were
materials where the optical transparency of the system varies with temperature
without the gelation behaviour being perturbed. The optical behaviour of
lyotropic liquid crystalline polymer gel networks have been described by
us. These gels change their optical properties extremely when temperature
increases or decreases by few degrees. A phase transition between the lyotropic
liquid crystalline and isoptropic phase in the system is the cause for
the optical effect. The contrast ratio of the aqueous gel (1 wt
polyethylene glycol and 12 wt
polyvinyl alcohol) in the temperature range between 27C
and 33C is nearly 85: 1. This
is a similar contrast ratio to that found for a phase transition from a
thermotropic nematic phase to an isotropic phase or in electro-optically
addressed LCD's. It is higher than that of photo-addressed LCD's. However,
for all known thermotropic materials only one mode transition (clear-cloudy,
or cloudy-clear) takes place with increasing or decreasing temperature.
Therefore additional studies were done to examine the possibility of preparing
gel networks that exhibit multiple phase transitions in connection with
changing the transparency of the system depending on temperature variation.
Furthermore, preliminary investigations show, that the gels also suitable
as hybrid light- and heat valves.
11:45 AM HH1.11
REVERSIBLE COLOR CHANGES IN AQUEOUS POLYMER GEL NETWORKS.
Jörg Kriwanek , Arno Seeboth, WITEGA, Department Advance Materials
Ltd, Berlin, GERMANY.
The aim of the work is to examine the possibility of preparing
gels with markedly reversible color changes with temperature (thermochromism)
in the range of 5 to 80.
Such gels are interesting for the production of new color filters, as temperature
sensors, and especially for uses in intelligent windows. We have found,
that some pyridinium N-phenolate betaines (so called Reichardt dyes) exhibit
thermochromic effects in aqueous polyvinylalcohol (PVA)- borax gel networks:
For instance the Reichardt dye ET(30), by addition of
N-tetradecylbetaine as surfactant solubilized in a PVA-borax gel, change
its color gradually from near colorless at 10
to deep violett at 80 at pH =
8.5. Additionally to the changes in the absorption intensities the absorption
maxima are shifted bathochrom from 537 nm (10)
to 555 nm (80). The color changes
in dependence with temperature can be explained primarily by reversible
protonation of the phenolate structure and forming of the corresponding
phenole structure of the dye molecules, since the phenole structure is
preferred in the microenviroment of the gel network at lower tempertures.
By heating the gel network is gradually destroyed and the phenolate structures
is forming which exhibit in the UV/Vis-absorption spectra the CT-transition
from the phenolate to the pyridinium part of the molecule.
SESSION HH2: GELS AND SPHERICAL COLLOIDS
Chair: Bill Carty
Monday Afternoon, April 5, 1999
Salon 13 (M)
1:30 PM HH2.1
GELS FORMED FROM ASSOCIATING POLYMERS IN SOLUTION. Subramanian
Kesevan, Robert K. Prud'homme , Princeton University, Princeton, NJ.
Guar and hydroxypropyl guar (HPG) polymer solutions can
be crosslinked with borate ions to form gels. The gels are not true gels,
but association fluids with tunable relaxation times. Chemical equilibria
involving boric acid, borate ions, and borate ions associated with cis-diol
sites on the polysaccharide chains determine the number of crosslinks and,
thus, the rheology of the borate gels. Measurements were performed over
the temperature range from 15 C to 65 C, and the pH range from 6.35 to
9.5. The storage and loss moduli obey time-temperature superposition so
that master curves can be constructed over 6 decades in frequency. The
shift factors along the time axis follow a single exponential Arrhenius
form.. The activation energies for guar and HPG are identical, which is
consistent with the cis-diols on each polymer being the active site for
crosslinking. Also, the data at different pH values can be superimposed
by time-pH shifting to produce master curves. The reduced moduli data follow
a single time constant Maxwell model. The results are interpretted using
the model of Leibler, et al. for the rheology of associating polymer systems.
1:45 PM HH2.2
SIMULATION OF HARD PARTICLES IN A PHASE SEPARATING BINARY
FLUID. Valerly V. Ginzburg, Feng Qiu, Marco Paniconi, David Jasnow and
Anna C. Balazs , University of Pittsburgh, Depts of Physics and Chemical
Engineering, Pittsburgh, PA.
We simulate the motion of spherical particles in a mixture
of two immiscible fluids. As the particles are diffusing through the medium,
the fluids are undergoing phase separation. In addition, one of the fluids
wets the surface of the mesoscopic particles. Thus, the system exhibits
a competition between spinodal decomposition and wetting. By combining
a cell dynamical systems model with the Langevin dynamics for particles,
we show that the addition of hard particles significantly changes both
the speed and the morphology of the phase separation. The domain growth
rate shows a nontrivial dependence on the particle density, in qualitative
agreement with earlier experimental studies. We also consider the effect
of shear on the morphology and rate of domain growth in this system. We
find that applying an externally imposed shear provides a distinct means
of tailoring the characteristics of the mixture.
2:00 PM HH2.3
DYNAMIC LIGHT SCATTERING OF COMPLEX FLUIDS. Thomas A.P.
Seery , Amit Sehgal, R.A. Weiss and Kaushik Chakrabarty University of Connecticut,
Polymer Program, Storrs, CT.
Recent efforts to apply dynamic light scattering to the
study of complex systems has led to the development of novel experimental
approaches as well as advances in data analysis. Studies of associating
systems that exhibit multiple relaxations have been examined using a combination
of dynamic light scattering and total intensity measurements to place contributions
of each mode of scattering on an absolute intensity scale. Polyelectrolytes
in high dielectric constant solvents and ionomers in non-polar solvents
are two systems where recent advances have been made using this approach.
In addition, studies of conducting polymers as well as heme containing
proteins in solution have led to efforts to address the inherent problems
in scattering from light absorbing solutions. Absorption of the incident
light produces a cascade of related physical phenomena that greatly complicate
the acquisition and interpretation of data. The primary effect is local
heating of the solution that generates a temperature gradient. The temperature
gradient in turn creates a gradient in refractive index, density, and viscosity
that are accompanied by thermal lensing and convective flow. The observation
of these effects as a function of input power allows for interpretation
of data. The unique oscillations seen in the correlation functions may
also provide a diagnostic of the presence or absence of bimodal size distributions.
2:15 PM HH2.4
STRUCTURAL STUDIES OF COLLOIDAL GOLD-SILICA AEROGELS.
R.M. Stroud , M.L. Anderson, C.A. Morris, C.I. Merzbacher and D.R. Rolison,
Naval Research Laboratory, Washington, DC.
Colloidal metal aerogels are composite materials that
combine the porosity of the >85% free-space aerogel structure with the
optical and chemical properties of metal nanoparticles. This combination
of properties is desirable for sensing, catalytic and electrocatalytic
applications. The properties of a specific colloidal metal aerogel depend
on the domain size and pore structure of the aerogel matrix, the size and
composition of the metal nanoparticles, and also the matrix environment
surrounding the metal. We have prepared a series of base-catalyzed silica
aerogels with gold colloids ranging in size from 5 nm to 100 nm. The aerogels
have been characterized using scanning and transmission electron microscopy
in order to determine the aerogel domain size, the metal colloid size,
and the spatial distribution of the colloids. Distinct morphologies, in
which the colloid size is smaller, equal to, or larger than the silica
domain size have been identified. The accessibility of the gold surface
to species introduced from solution, which is a critical feature for many
applications, will be discussed in terms of the morphology of the metal
colloid-aerogel matrix.
2:30 PM *HH2.5
VISCOELASTIC PROPERTIES OF PARTICLE GELS. Lennart Bergström
, Joseph A. Yanez and Eric Laarz, Institute for Surface Chemistry, Stockholm,
SWEDEN.
The effect of strength of attraction and volume fraction
on the mechanical properties of alumina particle networks were investigated.
Alumina particle gels were formed reversibly and in situ in the rheometer
by cooling alumina particle suspensions with adsorbed poly(12-hydroxy stearic
acid) suspended in a marginal solvent, hexanol. The collapse of the polymer
layer with decreasing solvency (temperature) induces flocculation when
the long-range van der Waals force overcomes the remaining steric repulsion.
The gelation temperature depends on volume fraction. At the gel temperature,
Tgel, the gel becomes predominantly elastic; at temperatures below Tgel,
the elasticity increases with decreasing temperature. We will discuss the
functional form of the volume fraction scaling of the elastic modulus,
measured over a wide range of volume fraction (0.2 < f< 0.425) and
temperature (10-14C), based on
current theories.
3:30 PM *HH2.6
HARD SPHERE SUSPENSION CRYSTALLIZATION KINETICS. Zhengdong
Cheng , Paul M. Chaikin, Princeton Univ, Dept of Physics, Princeton, NJ;
William B. Russel, Princeton Univ, Dept of Chemical Engineering, Princeton,
NJ.
Kinetics of crystallization of colloidal hard sphere suspension
are investigated using time resolved Bragg scattering. Three samples of
PMMA/PHSA, 600 nm in diameter, in decalin/tetralin with volume fractions ,
were studied in m g. The effect of gravity was clarified by comparison
with studies on ground with the same samples. The Bragg scattering from
the close packing planes and the diffusive scattering from the stacking
disorder were captured by a CCD camera. As FCC 200 peak shows up in all
three samples during the coarsening process, indicating FCC might be the
equilibrium structure of the hard sphere crystal. The crystallites initially
compressed by the surrounding liquid, expanded as the crystallization progressed,
as measured by position of the main peak. The integrated intensity of the
main peak measures the crystalline fraction of the sample, while the width
determines the crystallite size. The ratio of these two quantities indicates
the number of crystallites, which reflects the nucleation or coarsening
processes.
4:00 PM HH2.7
MONODISPERSE SILICA AND ZnS PARTICLES WITH CONTINUOUSLY
VARIABLE SHAPE MADE BY ION IRRADIATION OF MICRO-SPHERES. E. Snoeks ,a
A. van Blaaderen,a,b M.L. Brongersma,a
T. van Dillen,a C.M. van Kats,b K.
Velikov,b and A. Polmana; a
FOM-Institute for Atomic and Molecular Physics, Amsterdam, THE NETHERLANDS;
b Van 't Hoff Laboratorium, Debye Institute, Utrecht,
THE NETHERLANDS.
An ion-irradiation induced anisotropic plastic deformation
effect is employed to continuously and controlably change the shape spherically
shaped micro-particles. Oblate and prolate spheroids are obtained. Silica
and ZnS spheres were obtained with diameters of nm
and nm with a polydispersity
in size of , and placed on the
surface of a Si substrate. The spheres on the substrate were then irradiated
with Xe ions at an energy of MeV,
to fluences ranging from
to ions/cm2;
and analysed by scanning electron microscopy (SEM) under various tilt angles.
A spectacular change in shape is observed for both small and large particles
of ZnS and silica: oblate particles (smarty shape) are obtained with an
aspect ratio of upto 3.2 for the highest fluence. When viewed along the
ion beam direction, the particles are circular with a diameter of upto
1.6 times larger than the original, while in the plane perpendicular to
the ion beam they are elliptically shaped with the shortest cross section
0.5 times the original diameter. Prolate spheroids (cigar-shape) are made
by a second irradiation of oblate particles under a direction perpendicular
to the original ion beam. At MeV
all Xe ions pass through the spheres, each ion depositing about MeV
into a local cylindrically shaped spike in the micro-spheres. In the spike
region, the temperature rises by more than K
within picoseconds after ion impact. The hot region expands, and causes
a small anisotropic plastic deformation for each incoming ion. The macroscopically
oberved deformation is a cummulative result of over a million of ion impacts,
and therefore does not lead to additional polydispersity. This method to
produce non-spherical particles with continuously variable shape will open
paths to experimental studies on the properties of crystallization and
optical scattering from photonic crystals of non-spherical particles. Optical
scattering measurements are underway.
4:15 PM HH2.8
X-RAY PHOTON CORRELATION SPECTROSCOPY STUDY OF THE MICROSCOPIC
DYNAMICS OF POLYSTYRENE LATEX SUSPENDED IN GLYCEROL: DIFFUSIVE DYNAMICS
AND THE ROLE OF CONFINEMENT. D. Lumma * 1, L.B. Lurio*
1, A.R. Sandy* 1, P. Falus1, M.A. Borthwick1,
S.G.J. Mochrie1, M. Sutton2, J.F. Pelletier2,
A. Malik3, G.B. Stephenson3, L.J. Regan4-1Center
for Materials Science and Engineering, Massachusetts Institute of Technology,
Cambridge, MA; 2Physics Department, McGill University, Montréal,
CANADA; 3Materials Science Division, Argonne National Laboratory,
Argonne, IL; 4Department of Molecular Biophysics and Biochemistry,
Yale University, New Haven, CT. *These authors contributed equally
to this work.
The dynamics of polystyrene spheres []
suspended in glycerol have been studied, using the emerging technique of
X-ray photon correlation spectroscopy [XPCS]. Our experiments investigate
suspensions of various concentrations from the dilute limit to the onset
of crystallization , extend over one order of magnitude in length scales
[],
and span four orders of magnitude in the time domain [].
In dilute suspensions, the dynamic behavior is well-described by simple
Brownian motion. By contrast, in highly-concentrated suspensions, the inverse
diffusion coefficient, 1/D, displays a strong wavevector dependence
and mimics the static structure factor, S(Q). These results
demonstrate how suspended particles adopt different microscopic dynamics
in response to the confinement imposed by their neighbors.
Our time-correlation functions exhibit high signal-to-noise
levels, almost comparable to those of dynamic light scattering [DLS]. Yet
unlike DLS, XPCS is capable of characterizing opaque systems, and moreover
of supplying S(Q) over a wide range in reciprocal space.
This work marks a crucial step in the progression of XPCS towards becoming
a routine characterization technique for low-energy dynamics. As such,
it will have major impact on future investigations of the microscopic dynamics
of dense colloidal and micellar systems, block- and copolymer blends, and
entangled homopolymer melts.
4:30 PM HH2.9
DETERMINATION OF PARTICLE SIZE DISTRIBUTIONS IN CONCENTRATED
SLURRIES FROM ACOUSTIC AND OPTICAL ATTENUATION DATA. Christopher A. White,
Melanie L. Carasso , Sanjay S. Patel, Jorge L. Valdes, Bell Laboratories
Lucent Technologies, Murray Hill, NJ.
Concentrated suspensions containing particles of radius
10 nm to 10 microns are important for many industrial processes including
chemical mechanical polishing and sol-gel chemistry. Acoustic attenuation
and optical turbidity provide sensitive measures of the particle size distribution
in these suspensions at process concentrations. We have developed a robust,
stable, and fast inversion method for the determination of particle size
distributions from measured attenuation spectra. The inversion method can
be applied to both optical and acoustic attenuation measurements. Theoretical
predictions using experimental data demonstrate that this new algorithm
outperforms commercially available methods for the determination of particle
size distributions from attenuation data. The level of characterization
afforded by this new algorithm provides a unique opportunity for greater
understanding and control of manufacturing processes.
4:45 PM HH2.10
MECHANICAL PROPERTIES OF SILICA AEROGELS BY DEPTH-SENSING
MICRO-INDENTATION. Magda Moner-Girona, Anna Roig , Elies Molins, Carles
Miravitlles, Institut de Ciència de Materials (CSIC), Barcelona,
SPAIN; Elena Martínez, Joan Esteve, Univ of Barcelona, Dept of Applied
Physics and Electronics, Barcelona, SPAIN; Joan Llibre, Carburos Metálicos
SA, R&D, Barcelona, SPAIN.
Silica aerogels are highly porous materials with nanometer
scale pore diameters. Samples were synthesized by supercritical solvent
evacuation from a wet gel. Because very small loads are sufficient to crack
these fragile materials, standard Vickers and Knoop tests are not sensitive
enough to measure their mechanical properties. Therefore, we used a new
dynamical method called Depth-Sensing Micro-Indentation (DSMI). The DSMI
method allows one to simultaneously measure the applied load and the penetration
depth of a Berkovich diamond tip, hysteresis load displacement curves are
obtained. Very small indentation loads (1mN)
com be applied and the load and depth resolutions are better than 1N
and 0.1nm respectively. Young's modulus (E), hardness and the elastic recovery
parameter (ERP) were measured in samples that were synthesized by a variety
of the sol-gel processes. Results were analyzed as functions of density
(), morphology and pore size distribution.
A relation of the type, E
with 2.9 was found for
the acetone-synthesized series. As a function of aerogel density, two different
regimes of mechanical behavior are observed. The lowest density aerogels
are elastic but the denser aerogels are elasto-plastic materials.
SESSION HH3: MAGNETORHEOLOGICAL FLUIDS/
ELECTRORHEOLOGICAL FLUIDS
Chair: Terry J. Garino
Tuesday Morning, April 6, 1999
Salon 13 (M)
8:30 AM *HH3.1
MR FLUID SPONGES AND DEVICES. J. David Carlson , Lord
Corporation, Materials Division, Cary, NC.
Stable, high-strength, magnetorheological (MR) fluids
that enable the benefits of controllable fluid technology are now commercially
available. Rotary MR fluid brakes have been manufactured and sold in modest
numbers into the exercise equipment market since 1995 as easily controlled,
variable resistance elements in stair-climber and cycling machines. Early
in 1998 a real-time, commercial vibration control system based on linear
MR fluid shock absorbers became available for use in the seats of large
Class 8, i.e., eighteen wheeler, trucks. Recently, a new way of using MR
fluids in which the fluid is contained in an absorbent matrix has been
developed. Such MR fluid sponge devices enable low-cost, controllable fluid
applications by eliminating the need for most of the high-cost components
normally associated with a fluid filled device. MR fluid sponge devices
compliment existing ways of using MR fluids. They are particularly appropriate
for less demanding, low-force applications where a high degree of control
is desired. MR fluid sponge devices contain MR fluid that is constrained
by capillary action in an absorbent matrix such as a sponge, open-celled
foam, felt or fabric. The sponge serves to keep the MR fluid located in
the active region of the device where the magnetic field is applied. The
sponge allows a minimum volume of MR fluid to be operated in a direct shear
mode without seals, bearings or precision mechanical tolerances. They are
not susceptible to gravitational settling or sedimentation of the MR fluid
suspension. Examples of a variety of recently developed MR fluid sponge
devices will be described.
9:00 AM HH3.2
AGGREGATION KINETICS AND STABILITY OF STRUCTURES FORMED
BY MAGNETIC MICROSPHERE. Weijia Wen , F. Kun, K.F. Pal, D.W. Zheng and
K.N. Tu.
Rings, chains, and clusters formed by magnetization-controllable
microspheres suspended in the liquid at zero field are reported in this
paper. The magnetic microspheres were made by plating glass beads with
Ni film of a critical thickness. We found that the ring leading to magnetic
flux closure is the most stable configuration. At high concentrations all
individual rings, chains and clusters join together to form a net-like
structure. A computer simulation based on the dipole-dipole interaction,
without thermal noise has been carried out and the results are in good
agreement with the experimental observations. Based on an analytic approach
we give a simple explanation of the formation and stability of rings. In
addition, the chain-column transition under an external magnetic field
has also been investigated and it is found that the ground state of magnetorheological
fluids is BCT structure
9:15 AM HH3.3
STRUCTURES OF A MAGNETORHEOLOGICAL FLUID. Gerald Gulley
, Sandia National Labs, Albuquerque, NM; Rongjia Tao, Southern Illinois
University, Dept of Physics, Carbondale, IL.
Extensive computer simulations were carried out to find
the underlying structures of a magnetorheological fluid. The simulations
took into account dipolar forces, the viscous drag force, and Brownian
motion. Three structures were found: the bct lattice, the liquid state,
and a columnar structure that bears resemblance to the worm-like structure
often found in magnetorheological fluids.
9:30 AM HH3.4
EFFECTS OF STRUCTURES ON RHEOLOGY IN A MODEL MAGNETORHEOLOGICAL
FLUID. Mustapha Chaker , Neal Breslin and Jing Liu, Department of Physics
and Astronomy, California State University, Long Beach, CA.
An externally applied magnetic field can induce different
types of structures in a magnetorheological (MR) fluid ranging from separated
columns to a worm pattern of bent walls. The type of structure obtained
depends upon not only the ramping rate of the magnetic field, but also
on the particle volume fraction. In this work, we study the influence of
these structures on rheology using magnetic polystyrene colloids. We found
that these changes of structure in the fluid lead to variations in its
apparent viscosity and the yield stress. Using a controlled-strain-rate
rheometer, the apparent viscosity and yield stress as a function of the
magnetic field strength and the particle volume fraction were obtained.
As suspected, experimental results show that for a given shear rate, the
apparent viscosity increases with increasing magnetic field. However, after
a certain time, a restructuring occurs within the fluid, which in turn
causes the viscosity to drop abruptly before leveling off. This variation
profile of the viscosity as a function of time will be reported as our
on-going comprehensive studies of the effects of structures on rheology
in our model magnetorheological fluid.
9:45 AM HH3.5
ORIGIN OF CORROSION IN MAGNETORHEOLOGICAL FLUIDS USED
FOR OPTICAL FINISHING. Irina A. Kozhinova a, Steven R.
Arrasmitha, Leslie L. Gregga, Stephen
D. Jacobsa,b, Univ. of Rochester, Center for Optics
Manufacturinga and Laboratory for Laser Energeticsb,
Rochester, NY.
Finish polishing of optical glasses, crystals, and ceramics
is one of the most promising uses of magnetorheological fluids. Commercial,
computer numerically controlled (CNC) magnetorheological finishing (MRF)
machines have recently come on-line in U.S. optics companies to produce
precision flat, spherical and aspheric optical components. MRF eliminates
subsurface damage, smoothes rms microroughness to less than 10,
and corrects p-v surface figure errors to 0.03
in minutes. Fundamental to the success of this technology is a stable magnetorheological
(MR) fluid composed of 36 vol. carbonyl
iron (CI) and 6 vol. cerium oxide in
water. Invariance in the viscosity, and hence material removal efficiency,
of this high solids concentration fluid throughout a work day is critical
to stability and repeatability of the process. This is achieved with a
fluid containment system that provides constant agitation and circulation
to prevent sedimentation, constant temperature control with a heat exchanger,
and use of an automated dripper system to make up for water lost through
evaporation.Current MRF machine practice calls for periodic replacement
of the MR fluid because it develops an unpleasant odor. Analysis of the
used MR fluid shows evidence of corrosion. Here we present results from
a study of corrosion in aqueous based MR fluids. Details of a spectrophotometric
technique for quantifying iron ion corrosion product concentration in the
supernatant are given. We find that the corrosion resistance of the CI
powder itself is good. Experiments show that accelerated corrosion of the
CI in the MR fluid is caused by additives incorporated into the cerium
oxide abrasive.
10:30 AM HH3.6
DESIGN AND TESTING OF A NEW MAGNETORHEOMETER. Aric B.
Shorey a,b, Stephen D. Jacobsa,b,c,
William I. Kordonskid, Sergei R. Gorodkine,
Kevin M. Kwongb, Univ of Rochester Materials Science
Programa, Center for Optics Manufacturingb
and Laboratory for Laser Energeticsc, Rochester, NY;
QED Technologies, LLCd, Rochester, NY; Belarus Academy
of Sciences, A.V. Luikov, Heat and Mass Transfer Institute, Minsk, BELARUSe.
Magnetorheological Finishing (MRF) is a newly developed
and recently commercialized method for finishing optical components. The
MR fluid is a water based suspension consisting of magnetic carbonyl iron
(CI), nonmagnetic abrasives and various stabilizers. This fluid stiffens
under the action of a magnetic field to provide a compliant, abrasive lap
that is used in the computer-controlled MRF process. The increase in stiffness
can be attributed to magnetic particle alignment along field lines which
tends to create a chain structure. This means the bulk properties of the
fluid will be anisotropic, and so it is important that the material be
tested in the same orientation that it is to be used. There were several
issues that were addressed in the manufacture of this device. The design
of the pole pieces for the electromagnet proved difficult. The fluid must
be subjected to a field that is symmetric in one direction, provides a
gradient in a second direction while it is sheared in a third direction.
Commercial software was used to model the field and aid in the design of
the pole pieces. Slight variations in the pole design gave significant
variations in field distribution as well as measured fluid properties.
This will be demonstrated through experimental results. A second major
issue that was addressed is compliance with the no slip boundary condition
at the moving boundaries confining the fluid. The novel solution to this
problem will also be discussed.
10:45 AM HH3.7
DYNAMICS OF A CONCENTRATED FERROFLUID EMULSION STUDIED
BY DIFFUSING-WAVE SPECTROSCOPY. Serge Cutillas and Jing Liu, Department
of Physics and Astronomy, California State University, Long Beach, CA.
A ferrofluid emulsion is used as a model magnetorheological
fluid to study the effect of a dipole-dipole interaction on particle dynamics
in concentrated particle region. When a magnetic field is applied, an induced
dipolar moment appears on each droplet of ferrofluid dispersed in water.
Dipolar forces between particles lead to an important structure change
inside the emulsion. The emulsion goes from a simple colloidal system to
complex aggregates like chains, columns or walls oriented to the magnetic
field direction. In most current MR fluids applications, particle concentration
is normally high. Thus, Diffusing Wave Spectroscopy is used to probe particle
dynamics in these concentrated dipolar fluids. We show that the electric
field autocorrelation function from multiple scattering light is not a
simple decaying exponential but has a long tail following the decay at
short times. This behavior is interpreted by the binding nature of particle
interactions. Particles are trapped by the presence of the others all around
them. This situation is very similar to that of colloidal crystals. However,
essential differences exist since particle interactions are not controlled
by typical isotropic interaction resulted from ionic or particle concentration,
but rather by magnetic dipole-dipole energy that is anisotropic. From correlation
function, the mean square displacement of particles in aggregates will
be measured as a function of time. As the magnetic field strength and the
particle volume fraction increases, we expect that the mean square displacement
decreases reflecting the liquid to solid phase transition. Real time video
microscopy is also used to correlate the structure with the dynamic measurements.
Since a link is possible between dynamic properties measured optically
and rheological properties, we will also compare our results with mechanical
measurements of rheological properties in the same frequency range.
11:00 AM HH3.8
MAGNETORHEOLOGICAL MEDIUM BASED ON POLYMERIC COMPOSITIONS
WITH FERROUS OXIDES. T.G. Lazareva , Institute of General and Inorganic
Chemistry of the National Academy of Sciences of Belarus, Minsk, BELARUS.
The basic moments of synthesis of fillers of magnetorheological
media based on iron oxides are discussed. An influence of the synthesis
conditions, namely, a composition of the compounds, pH and a temperature
of the solution, the presence of surfactants, precipitators on the structure
and properties (magnetic, sorption properties) of the fillers are evaluated.
The sorption capacity of various fillers with respect to a series of such
water-soluble polymer dopants as cellulose esters, polyvinyl alcohol is
determined. The conditions of synthesis of carbon-mineral fillers and their
properties depending on the type of polymer dopants in synthesis, their
concentration and heat treatment temperature are considered. A comparative
study is made of magnetorheological properties of the compositions in oils
depending on the type, structure, and properties of the fillers, the presence
of surfactants, moisture content of the fillers. Recommendations are given
on the synthesis of optimum magnetorhelological fillers of the investigated
compounds. The main regularities of the formation of the compositions based
on magnetic fillers (Fe3O4, -Fe2O3)
and binders - water-soluble carboxymethyl cellulose, methyl cellulose,
polyvinyl alcohol phenol-formaldehyde, epoxyacrylic and pentaphthalic resins
have been considered using the rheological method. The flow curves have
been estimated depending on composition structure, type of binder and filler,
composition temperature, size of filler's particles, it's porosity, the
introduction of joining agents. The influence of the filler on the composition's
viscosity and type of solution's flow has been estimated. The concentrational
intervals of gelatinization depending on weight correlations of binder
and filler have been found out. The phenomenological Akay-Leslie reological
equation of state for anisotropic viscoelastic liquids (A-L model) was
chosen for describing the flow resistance of the compositions. The data
available as to the initial composition viscosity have been classified
based on types of coherence characterized by the indication of degree of
concentrational dependence of viscosity. The magnetorheological effect
of compositions depending on the type of composition, weight correlation
of the filler and binder, temperature and the type of coherence has been
estimated. The conclusion has been made as to the influence of type of
coherence (which depends on intermolecular interaction at the phases boundary
line: polymeric solution - fillers and on initial solution structure) on
magnetorheological effect.
11:15 AM HH3.9
SOUND PROPAGATION IN A MAGNETORHEOLOGICAL FLUID. Yuri
Nahmad, Camilo Arancibia, Carlos Ruiz , Dept of Applied Physics, CINVESTAV-IPN,
Merida, Yucatan, MEXICO.
The purpose of this report is to discuss experimental
evidence of two long wavelength longitudinal acoustic modes propagating
through a fluid made of hydrogen-reduced iron particles suspended in glycerine.
The first mode travels through glycerine channels within the suspension
whereas the second one propagates through the fibrillated structure formed
by the iron particles. This second mode is very strong in amplitude and
travels at very low speeds, depending on the applied magnetic field. The
behavior of the system resembles the propagation of sound through a fluid-saturated
porous material. However, some distinct physical effects are observed,
like the structural saturation and the rheological hysteresis of the velocity
of sound in the suspension. A time-of-flight acoustic technique, which
consists in producing and detecting a pulse of pressure within the suspension,
is used. The pulse is created by a plane piezo-electric transducer and
detected by a similar piezo crystal.
11:30 AM *HH3.10
ASPECTS OF NONLINEAR CONDUCTION IN ELECTRORHEOLOGICAL
SUSPENSIONS. P.J. Rankin, Y.M. Shkel, D.J. Klingenberg , Dept. of Chemical
Engineering, University of Wisconsin, Madison, WI; J.L. Shohet, Dept. of
Electrical and Computer Engineering, University of Wisconsin, Madison,
WI.
It is well known that the field-controlled stress transfer
properties of electrorheological suspensions arise from the electrostatic
interactions between the polarized particles. Idealized models of particle
polarization have successfully reproduced, at least qualitatively, a wide
variety of experimental observations. However, nonideal features of particle
polarization are apparent, especially at large electric field strengths
where applications are likely to operate. Nonlinear conduction within the
suspension is believed to dominate such nonidealities, giving rise to increased
current and reduced stress transfer. We will present results from various
experiments aimed at understanding the origin of nonlinear conduction in
electrorheological suspensions. Varying the particles' surface chemistry
can have a pronounced effect on the nonlinear character of the electrorheological
response, suggesting that charge transport at the particle surface is of
paramount importance. Transient electrical measurements reveal information
about the polarization kinetics, providing additional insight into the
physical chemistry of nonlinear conduction.
SESSION HH4: ELECTRORHEOLOGICAL FLUIDS/
SUSPENSIONS OF RODS
Chair: J. David Carlson
Tuesday Afternoon, April 6, 1999
Salon 13 (M)
1:30 PM *HH4.1
THE NONLINEAR RHEOLOGY OF ELECTRORHEOLOGICAL FLUIDS.
James E. Martin , Sandia National Laboratories, Albuquerque, NM.
When a colloidal suspension is exposed to a uniaxial electric
field the polarized particles chain along field lines causing a macroscopic
solidification of the fluid, the basis of the so-called electrorheological
(ER) effect. Likewise, in a rotating electric field particles form sheets
in the plane of the field, which we call the rotary ER effect. Both of
these fluids exhibit a nonlinear, shear thinning rheology, due to shear-induced
structural relaxations. Because the fluid stress can be controlled by the
applied field, a number of applications are possible, including electromechanical
actuators, clutches, and active vibration dampers. To design these devices,
and to develop effective control loop algorithms, it is necessary to understand
the strongly nonlinear rheology of these fluids. We have used time-resolved,
two-dimensional light scattering on a concentrated colloidal silica fluid
in steady and oscillatory shear to demonstrate that the fragmentation and
aggregation of chain-like particle microstructures is the cause of flow
nonlinearities. We show that the light scattering is an indirect measure
of the fluid stress. These observations form the basis of a kinetic chain
model we developed to describe the nonlinear dynamics of the microstructures
in ER fluids in nonstationary shear flows. Understanding the microstructural
dynamics then leads us to a theory of the macroscopic rheology of these
fluids in nonstationary, low Reynolds number flows. Finally, we have conducted
extensive large-scale (1000-10000 particles) simulations of these fluids
in steady and oscillatory shear, and will compare these results to theory
and experiment.
2:00 PM HH4.2
STRUCTURE EVOLUTION AND CORRESPONDING ELECTRORHEOLOGICAL
EFFECT. Weijia Wen , D.W. Zheng and K.N. Tu, Department of Materials Science
and Engineering University of California at Los Angeles, CA.
We present an investigation about chain/column evolution
and the corresponding electrorheological (ER) effect performed with glass/oil
ER fluid. Our results demonstrate that once the field applied to the ER
fluids surpasses a certain time period, the particles begin aggregating
to form chains. It is found that both the initial response time and the
chain's formation time depend on the conductivity of the microspheres.
The overall trend is that the higher the micro-sphere's conductivity, the
shorter the response time and the stronger the particles' interaction under
the same external electric field. Moreover, These chains then coarsen and
eventually form columns in the direction of the external field. We found
that different column structures can be obtained depending on how the electric
field is applied to the ER fluid. Only a loose column structure can be
achieved if a square pulse field is applied to the ER fluid, yet a compact
column is formed when the field strength is increased slowly. We have measured
the ER effect with a sensitive yield-stress testing device as the structure
varies. The results indicate that there exist three increasing tendencies
of interaction among particles corresponding to three processes of sequential
transition between states; they are (1) random spatial configuration to
chain, (2) chain to metastable column, and (3) metastable column to stable
column.
2:15 PM HH4.3
CONFOCAL MICROSCOPY STUDY OF ELECTRORHEOLOGICAL FLUIDS.
Ujitha Dassanayake, Seth Fraden , Complex Fluids Group, Brandeis University,
Waltham, MA; Alfons van Blaaderen, FOM Inst. for Atomic and Molecular Physics,
Amsterdam, NETHERLANDS.
Specially synthesized fluorescently labelled core-shell
silica spheres were used as a model electrorheological fluid to experimentally
explore structure formation and evolution under conditions of no shear.
Using Laser Scanning Confocal Microscopy we measured the location of each
colloid in three dimensions. We observed the equilibrium body-centered
tetragonal phase and several non-equilibrium structures such as sheet-like
labrynths or isolated chains of colloids. The formation of non-equilibrium
structures was studied as a function of the volume fraction, electric field
strength and starting configuration of the colloid. We compare our observations
to previous experiments, simulations and calculations.
2:30 PM HH4.4
A THERMODYNAMIC APPROACH TO FIELD-INDUCED STRESSES IN
ELECTRO- AND MAGNETOACTIVE COMPOSITES. Yuri M. Shkel and Daniel J. Klingenberg
, Dept. of Chemical Engineering, Univ. of Wisconsin, Madison, WI.
Field-induced stresses give rise to electro- and magnetoactive
behavior in a variety of soft materials, such as electro- and magnetostrictive
elastomers, and electro- and magnetorheological suspensions and gels. The
study of electro- and magnetoactivity of stiff materials has traditionally
focused on their apparent responses to applied fields. Such an approach
is not appropriated for soft materials, where the deformation arising from
field-induced forces is strongly affected by the conditions at the boundaries.
We employ an alternative approach where field-induced stresses are determined
directly. These stresses are related to the system free energy through
thermodynamic variables that describe how the permittivitty (permeability)
depends on material deformation. This reduces such problems as determining
field-induced deformations or rheological properties to determining well-defined
material parameters that are independent of boundary conditions. In this
presentation, we will describe this thermodynamic formalism and illustrate
examples where electroactive responses are described in terms of these
thermodynamic parameters. We will show how these parameters can be determined
via dielectric measurements, and present results for several different
systems. We will also describe how these parameters can be calculated from
microscopic model of the materials. Our results illustrate that electroactivity
is very sensitive to the material microstructure, particularly the degree
of anisotropy. For example, electrostrictive deformations of anisotropic
structures are predicted to be more than order of magnitude larger than
the response of an isotropic medium with the same chemical composition.
The approach is quite general, capable of describing a variety of systems,
such as polymer films, liquid crystals, as well as electro- and magnetorheological
composites.
2:45 PM HH4.5
THE MOTION EQUATION OF ELECTRORHEOLOGICAL FLUID. Albert
A. Mokeev , Vitebsk Univ, Vitebsk, Blearuss, Andrew A. Mokeev, Moscow Univ,
Moscow, RUSSIA.
The current of the electrorheological suspensions in an
electrical field is by sequence of breaks of a virtual lattice of a chains
of particles, connected among themselves and with electrodes - walls of
the channel - by the electrostatic interaction and them reconstructionÕs
for small intervals of time about relacsation time is lower on current
from others fragments of chains, that creates a pressure of resistance
to current, not dependent on current velocity, similarly to a pressure
of a dry friction.
Units of the particles which have been not included in
a virtual lattice, as a result of collisions between by itself create resistance
of a viscous friction, which pressure, proportional velocity of shift,
exponential depends on a difference of bond energy of units and kinetic
energy of their relative movement referred to their average energy, similar
thermal. The particles not included in a virtual lattice and in units,
electrically interact with collisions also will form a suspension component,
rendering resistance to current proportional to its velocity, and linearly
growing with growth of an intensity E of an electrical field. Total pressure
of resistance to current linearly growing with growth E, that as against
results of known theoretical works will be coordinated with the experienced
data, with small velocity of current grows almost linearly with grow of
a current velocity and after a failing site aspires again to whether -
linearly to grow appropriate much to smaller viscousity.
The equation of a movement of electrorheological suspension
with such rheological law has the kind similar to Nouet-Stoks equation,
in which the effective structural viscosity whether the sum of the exponential
function from a difference of value g1, proportional to the bond energy
of units both square of a gradient of velocity and their function, so with
small gradients - near to an axis of the channel, where the velocity is
constant in process of shift from an axis, it transforms in the Nouet-Stoks
equation with a large viscosity, and with large gradients of velocity -
near to walls channel again coincides with the Nouet-Stoks equation with
the small viscosity. The profile of velocity is a conjugate of the flat
and abrupt parabolas, in comaxis - cylindrical slot-hole channel, in cylindrical
coordinates this equation has a kind
P- the difference of pressure, Uo- electrical voltage
between walls of the channel, Eo- intensity of an electrical field with
which the saturation of polarization of particles initiates, o
- viscosity in absence of an electrical field, B(E) - linear component
of viscosity, g1(E) - gradient of speeds appropriate to bond energy of
unite, go- to average their energy.
3:30 PM *HH4.6
THE RANDOM DENSE ROD PACKING. Albert P. Philipse , Van
't Hoff Laboratory for Physical and Colloid Chemistry, Debye Institute,
Utrecht University, Utrecht, THE NETHTERLANDS.
The experimentally observed asymptotic scaling (L/D)
= constant, for the random packing volume fraction ()
of rods with high aspect ratio (L/D) is shown to be simply related to (the
distribution of) particle contacts in the packing. Consequences of this
geometrical analysis are discussed for anisotropic colloids and powders.
It is argued, for example, that many random rod packings are actually meta-stable
rod glasses with respect to an ordered (nematic) structure.
4:00 PM *HH4.7
MATERIALS FROM CHIRAL ROD-LIKE CELLULOSE SPECIES. Derek
G. Gray , Pulp and Paper Research Centre, McGill University, Montreal,
CANADA.
Cellulose is the most abundant material in the biosphere,
and cellulose fibres are constituents of wood, wood composites, paper and
textiles. Natural cellulose fibres often show evidence of chiral structure.
For example, the thickest layer in wood cell walls consists of cellulose
microfibrils twisted into a shallow right-handed helix. This chiral structure
results in a chiral response to changes in relative humidity. Quite different
chiral structures and properties are displayed when natural cellulose fibres
are broken down chemically to give rod-like colloidal particles. An aqueous
suspension of these cellulose crystallites forms a chiral nematic liquid
crystalline phase in dilute aqueous suspension. Removal of water gives
a solid chiral nematic film composed of pure cellulose with the optical
properties of cholesteric liquid crystals. Some other properties of cellulosic
materials that result from of the chiral rod-like nature of their constituents
will be discussed.
4:30 PM HH4.8
COLLOIDAL DYNAMICS IN A LIQUID CRYSTAL. Roan Lavery ,
Wilson Poon, Jason Crain, Univ of Edinburgh, Dept of Physics and Astronomy,
Edinburgh, UNITED KINGDOM.
We present the results of a study of the translational
diffusion of colloidal PMMA spheres in the isotropic phase of a liquid
crystal. A significant increase in the hydrodynamic radius is seen as the
nematic phase is approached. We attribute these observations to an increasingly
ordered layer of liquid crystal molecules forming at the surface of the
particles, an effect known as orientational pre-wetting. In addition, we
present evidence that a small coating of another solvent can be formed
around the particles during preparation and this coating will remain stable
while the particles are dispersed in solution. This extra coating has the
effect of altering the boundary conditions at the surface of the particles
from the stick boundary condition, implicit in the Stokes-Einstein expression,
to partial slip boundary conditions and consequently changing the dynamics
dramatically. This is seen in both mesogenic and non-mesogenic solvents.
For the case of a liquid crystal solvent the coating also affects the ability
of the liquid crystal molecules to order at the surface of the particles,
and therefore their ability to act as defect sites. This means that the
isotropic-nematic phase transition temperature will be relativley unaltered
by the presence of the particles. It is shown that larger coatings lead
to a smaller reduction in the phase transition temperature and a decrease
in orientational pre-wetting as the nematic phase is approached.
4:45 PM HH4.9
ENGINEERING ENTROPY: BUILDING ORDER WITH DISORDER. Zvonimir
Dogic, Marie Adams, Seth Fraden , Complex Fluids Group, Department of Physics,
Brandeis University, Waltham, MA.
We study mixtures of colloidal suspensions of rodlike
and spherical colloids under conditions approximating hard particle fluids,
meaning the colloids behave as impenetrable objects with no other interaction
potential. These simple, hard particle fluids exhibit unexpectedly rich
phase behavior ranging from liquid crystalline phases, to bulk separation
of the rods and spheres, and to microphase separation of the rods and spheres
in alternating layers. Through experiment, simulation, and theory we explore
how phase behavior is controlled solely by the shape and concentration
of the particles. Our studies suggest that the observed phase behavior
is entropically driven by a competition between the entropy of mixing and
packing of the particles. Because all molecules have a hard particle core,
the phenomena are likely to be quite general, applying also for example
to low-molecular mass liquid crystals, amphiphiles and block copolymers,
to bioseparation methods and DNA partitioning in prokaryotes, and to protein
crystallization, and the manufacture of composite materials.
SESSION HH5: PAPER, FIBER, POLYMERS
Chair: Seth Fraden
Wednesday Morning, April 7, 1999
Salon 13 (M)
8:30 AM *HH5.1
Abstract Withdrawn.
9:00 AM *HH5.2
THE ROLE OF FIBRE FLOCCULATION IN PAPER STRUCTURE. R.J.
Kerekes , Pulp and Paper Institute of Canada, Pulp and Paper Centre, University
of British Columbia, Vancouver, CANADA.
Pulp fibres flocculate into aggregates which impart a
characteristic non-uniformity to paper on a scale of several millimetres.
This non-uniformity, visible in transmitted light, diminishes the physical
and optical properties of paper. Consequently, minimization of fibre flocculation
has long been an objective of papermaking.
It was established many years ago that mechanical rather
than colloidal forces governed fibre flocculation in the shear flows used
in papermaking. However, the process by which individual flocs form and
the conditions required for their creation have only recently been investigated
in detail. This paper will review recent research on this topic at the
University of British Columbia.
The paper will focus on the formation of fibre flocs,
their propreties, and the role they play in the micro-uniformity of paper.
Specific attention will be paid to the effect of fibre length, coarseness
(weight per unit length), and refining on fibre flocculation in paper structure.
9:30 AM *HH5.3
TENSILE STRENGTH OF WET PAPER . Andre Schroder , Djamel
Bensarsa, Ludwik Leibler, CNRS/ELF Atochem, Levallois-Perret, FRANCE.
Fiber bonds play a major role in the in-plane tensile
strength of paper. It is well known that the Young's modulus (E) of a well-bonded
dry paper sheet is one third of the Young's modulus (Ef) of its individual
fibers. However, this relation is not valid for wet paper. We have measured
in-plane Young's (E) and shear (G') moduli of wet paper sheets. Various
wetting liquids have been studied. We have observed that the shear modulus
(G') varyied over two decades from 0.1 MPa to 10 Mpa. We have also measured
the Young's modulus (Ef) of wet individual fibers, which was found to be
much higher than (G'). We find that the Young's modulus (E) of these various
wet papers is proportional to their shear modulus (G'). Hence, we conclude
that the Young's modulus (E) of wet paper is governed by the shear modulus
of fibre-fibre crossings, rather than by the friction between fibers in
the bonding region, as has been hitherto assumed. We can interprete quantitatively
our results by considering a simple model for the fiber network. Moreover,
the present study enables us to rationalize the role of wet strength additives,
which in particular increase the shear modulus of fiber-fiber crossings.
10:30 AM *HH5.4
SIMULATIONS OF FLEXIBLE FIBER SUSPENSIONS. C.F. Schmid,
L. Switzer, D.J. Klingenberg , Dept. of Chemical Engineering, University
of Wisconsin, Madison, WI.
A simulation method is employed to investigate particle
aggregation phenomena in fiber suspensions undergoing low Reynolds number
flow. In contrast to aggregation in colloidal dispersions that arises from
van der Waals attractive forces between particles, aggregation in elongated
particle suspensions is attributed primarily to mechanical effects. Microscopic
fiber features such as aspect ratio, intrinsic shape, flexibility, and
surface roughness are thought to play a role in mechanical aggregation.
Experiments aimed at elucidating the relationships between microscopic
fiber features and macroscopic flocculation behavior are difficult as fibers
are small, opaque, and rapidly moving in most applications. Thus, simulations
that provide detailed information about the structure (fiber orientations,
positions) and mechanics (interparticle forces, velocities) in the suspension
can provide insight into the aggregation process. In this simulation technique,
a fiber is modeled as a chain of rigid rods connected by elastic hinges.
Fibers interact via excluded volume and frictional contact forces. The
effects of such fiber features as elastic fiber deformation, irregular
fiber equilibrium shape, and interparticle friction on the flocculation
process are addressed. Interesting similarities between the rheology of
aggregating fiber suspensions and micelle solutions that form shear-induced
structures will be discussed.
11:00 AM HH5.5
ATTRACTIVE INTERACTIONS BETWEEN FILAMENTOUS BIOPOLYMERS
OF LIKE CHARGES. Jay X. Tang , Paul A. Janmey, Brigham and Women's Hospital,
Harvard Medical School, Boston, MA; Alexander Lyubartsev, Lars Nordenskiold,
Stockholm University, Physical Chemistry, Stockholm, SWEDEN.
Interactions between charged macromolecules in solution
are essential for the material properties of soft condensed matters including
colloidal suspensions, polyelectrolytes, and other complex fluids. Our
recent studies focus on the polyelectrolyte nature of a class of filamentous
biopolymers, with the goal of best defining attractive interactions between
them. Our approach is to compare results from experimental studies that
define many features of counterion mediated lateral aggregation with Monte-Carlo
simulations considering realistic ion size and molecular structures. Filamentous
phages fd and M13, which are identical filaments with exception of different
surface charge, are selected as model biopolymers for a quantitative comparison
between simulation results and the experimental data. In such a context,
we will discuss a number of intriguing features including sharpness of
bundling transition, variation of the threshold condition, re-solubilazation,
limited bundle size, and ion specificity. The knowledge obtained using
model biopolymers is aimed at enhancing our overall understanding of both
biological and industrial materials, and to provide valuable guidance for
rational drug design, paper and food processing, etc.
11:15 AM HH5.6
AVOIDANCE MODEL FOR CHARGED SPHERES AND RODS BEYOND THE
DILUTE LIMIT. Judith Herzfeld , Eric M. Kramer, Jining Han, Brandeis University,
Dept of Chemistry, Waltham, MA.
The effects of soft repulsions are calculated using a
relatively simple and intuitive model that takes account of local order.
Comparison with exact results in the dilute limit indicates that the avoidance
model is remarkably accurate for charged spheres. For charged spherocylinders,
where further approximations are needed, the accuracy is less satisfactory,
but still useful. Two advantages of the avoidance model are the ease of
application to high concentrations and the concomitant estimation of local
order. The local order is shown to decrease with increasing concentration,
except for jumps at the long-range ordering transitions. The predicted
shift of the isotropic-nematic transition to lower concentrations mimics
the observed effect of electrostatic repulsions in suspensions of virus
particles. Positional ordering transitions are also predicted to be shifted
to lower concentrations, consistent with the observed behavior of charged
lyotropics. This work was supported by National Institutes of Health grant
HL36546 and National Research Service Award GM18932.
11:30 AM *HH5.7 SHAPE-DEPENDENT COLLOIDAL SEPARATIONS.
Thomas G. Mason , Exxon Research and Engineering Co., Corporate Research,
Annandale, NJ.
Dispersions consisting of both micron-sized disks and
spheres in a micellar solution exhibit novel aggregation phenomena arising
from entropic excluded volume interactions. By adjusting the micelle concentration,
these attractive interactions can be controlled so that they induce a preferential
aggregation of the disks into striking columnar structures, leaving the
spheres unaggregated. These columnar aggregates of disks are spatially
segregated from the spheres by creaming, providing a novel mechanism for
shape-dependent colloidal separations.
SESSION HH6: POSTER SESSION
Wednesday Evening, April 7, 1999
8:00 P.M.
Salon 7 (M)
HH6.1
KINETICS OF AGGREGATION IN ASSOCIATING SULFONATED POLYSTYRENE
IONOMER SOLUTIONS. Laura Harris , Robert A. Weiss and Thomas A.P. Seery,
University of Connecticut, Chemical Engineering Department, Chemistry Department
and Polymer Program, Storrs, CT.
There have been remarkably few studies of associating
ionomers given their industrial significance. Solvent studies are extremely
useful in developing an understanding of how isolated ionomer chains interact,
which may then help explain bulk or solid phase interactions. Previous
investigation of the aggregation behavior of solutions of the acid form
of sulfonated polystyrene in decalin have characterized the phase behavior
as a function of molecular weight and percent sulfonation. These studies
have shown the existence of large aggregates that pre-associate above the
macrophase separation temperature. Dynamic light scattering has shown a
temperature and concentration dependence on the nature of the aggregates
formed in solution. Parallel measurements on matched pairs of the ionomer
and the parent homopolymer provide the means to isolate the effect of ionic
moieties and compensate for variations in solvent quality or viscosity.
The system under consideration has a macrophase separation line near the
theta condition for the parent homopolymer so that chain associations are
being probe under conditions where the backbone sections between ionic
groups should be ideal gaussian subchains. Current efforts are focused
on the rate of aggregate formation. By conducting dynamic temperature jump
studies, we will gain an understanding of the kinetics of the microphase
aggregate formation which appears to provide the driving force for phase
separation.
HH6.2
ASSOCIATIVE POLYMERS BEARING n-ALKYL HYDROPHOBES: RHEOLOGICAL
EVIDENCE FOR MICROGEL-LIKE BEHAVIOR IN ALKALINE MEDIA. Srinivasa R. Raghavan,
Saad A. Khan , North Carolina State University, Dept of Chemical Engineering,
Raleigh, NC; Robert J. English, The University of Leeds, Dept of Color
Chemistry, Leeds, UNITED KINGDOM; Richard D. Jenkins, Union Carbide Asia
Pacific Inc, SINGAPORE.
Rheological techniques are used to probe the behavior
of hydrophobic alkali-swellable emulsion (HASE) polymers, bearing n-alkyl
hydrophobes, in aqueous alkaline media. The polymers possess a comb-like
architecture with a polyelectrolyte backbone (ethyl acrylate-co-methacrylic
acid) and hydrophobes (ca. 10 per polymer chain) tethered to the backbone
via polyether side chains. The size of the hydrophobes is varied from n-C8
to n-C20 in this study. We demonstrate that, at such a level
of hydrophobic modification, and at relatively high polymer concentrations,
the microstructure in these polymer systems is akin to that existing in
concentrated microgels. Thus, the original polymer latex particles swell
extensively in alkaline media and disintegrate to form a system of close-packed,
compressible (``soft'') aggregates. This is reflected in the rheological
response of the system, where we observe a ``pseudo-yield stress'' coupled
with considerable shear-thinning under steady shear, and a characteristic
power-law behavior of the dynamic elastic (G') and viscous (G'') moduli
with frequency ()under oscillatory shear (G', G'' 0.4)
persisting to very long time scales. Concentration-independent master curves
are obtained for the elastic modulus G', with the level of G' increasing
with hydrophobe size. The similarity in the dynamic response suggests that
there exists a qualitative equivalence in microstructure over the range
of systems, the only difference being the ``softness'' or compressibility
of the particles. Data from this study are also contrasted with those for
a similar HASE polymer bearing a smaller number of alkylaryl hydrophobes
[English et al. (1997)]. In the latter case, the rheology can be interpreted
in terms of hydrophobic associations and chain entanglements occurring
in solution. Thus, subtle variations in molecular architecture are shown
to cause significant differences in morphology and microstructure for these
polymer systems.
HH6.3
POLYURETHANE GELS IN THE CONTROLLED ADMINISTRATION OF
ANTICANCER DRUGS. Erkesh O. Batyrbekov , Rinat Iskakov, Bulat A. Zhubanov,
Institute for Chemical Sciences, Almaty, KAZAKSTAN.
Polymeric gels on the base of segmented polyurethanes
are widely used as biomedical materials because of their excellent physico-mechanical
properties. The most predominant factor is a phase separation being typical
for these polymers. One allows mainly to regulate the release profile of
incorporated drug. In the order to clarify the release of anticancer drugs,
such as cyclophosphamide and blemicine, from segmented polyurethanes the
supermolecular structure of microdomain polymer and the interaction between
the drug and polyurethane were preliminary studied. Supermolecular structure
of polyurethane gels was described with x-ray scattering ir-spectroscopy
and proton NMR study. It was established the phase separation in polyurethane
is intensified by means of increase of molecular weight of soft segments.
The drug-concentrated domains of hard segments are microheterogeneously
dispersed in the amorphous soft segments controlled a diffusion of drug
from gels. These results indicate that supermolecular structure design
of segmented polyurethane allows to control the drug release from the polymer
matrix.
System Administrator
3/8/1999