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Research Interests
Hydrodynamics of colloidal suspensions; viscosity and
thermodynamics of polymer solutions; chemical kinetics and
statistics of synthetic and biological macromolecules;
statistical thermodynamics and the thermal and pressure
properties of polymer melt, glass crystal, and
nanocomposites; phase equilibria in polymer mixtures; the
glassy state: steady state and relaxational properties;
positron annihilation spectroscopy.
Overview of Research
The general characteristic has been the search for
theoretical quantitative rationalization, prediction and its
test, and pertinent experimentation. The first endeavor was
the extension of Einstein's viscosity theory in two
directions. First to moderate concentrations by the
introduction of hydrodynamic interactions, second to
non-spherical particles. A further modification,
particularly suitable to highly concentrated suspensions
followed later. It should be noted that the concentration
problem has continued to receive attention in the literature
up to the present day. The second aspect has been successful
in analyses of rod-like suspensions and of protein
solutions. Continuing with viscosities of flexible polymers
in solution, extensive measurements in good and poor
solvents over a range of molar masses served to establish
corresponding states representations valid over wide
concentration ranges. This served to arrive at empirical
representations of the combined effects of hydrodynamic
interactions and conformational changes, and to relate
viscosity to solution thermodynamics. An early theory of
adsorption of flexible chain molecules on solid surfaces
represents another example of a combination of chain
statistics and thermodynamics. In the area of chemical
kinetics and statistics, degradation processes have been a
central subject. First there was a statistics of random
scission, followed by a most general kinetic formulation of
a step process, pertinent for hydrolytic reactions of
synthetic and biological macromolecules. Next came a
kinetics of depolymerization as a chain reaction. The
predictions of this theory have been extensively examined,
primarily in pyrolysis. The essential is the competition
between a depropagation or unzipping and an inter and
intramolecular chain transfer step. This accounts for the
wide spectrum in respect to overall rate, molar mass changes
and monomer yields, depending on polymer structure. Turning
to biological macromolecules, a series of studies deals with
sequence analysis and neighbor effects in polynucleotides
and proteins, template conditioned replication, and the
kinetics of cooperative transformations. The corresponding
equilibrium had been treated as a Ising problem. The kinetic
solutions now obtained lead in the limit to the classical
results, but are obtained by different methods. An extensive
range of experimental and theoretical research is concerned
with the physical properties of the bulk polymer, primarily,
melt and glass. This has involved thermal expansivities in a
study of subglass relaxations down to liquid He
temperatures. Moreover the pressure-volume-temperature
relations were accurately measured. Parallel theoretical
studies treat the low temperature glass and extensively the
polymer melt. The statistical thermodynamics of a
lattice-hole model showed considerable quantitative success
in the prediction of thermal and pressure properties, and
resulted in generalizations to mixtures including most
recently nanocomposites and related phase equilibria, and to
modifications appropriate for the glassy state. An
ingredient of the theory is the temperature and pressure
dependent hole fraction, a particular free volume quantity.
This has led to a theory of thermoelastic properties of the
glass. Moreover it has provided a link to other free volume
dependent properties, namely melt viscosity and viscoelastic
relaxation, and it has been the starting point for a
dynamics of volume relaxation. We note finally the relation
to free volume quantities, derived by means of positron
spectroscopy.
Current Activity
Nanocomposites
with L.A.Utracki, Natl.Res. Council,Canada.
The pressure-volume-temperature relations of a polyamid/exfoliated
clay combination with varying compositions was determined.
Excellent agreement with the predictions of our lattice-hole
theory is observed. The model assumes a gradient of
molecular mobility, starting with a layer of attached chains
and reaching asymptotically with increasing distance the
level of pure polymer. Thus the energetic particle-matrix
interactions depend on interparticle distances and hence on
composition. The free volume is reduced by about 15% in the
composite, indicating as a consequence significant changes
in mechanical and in transport properties.
Solubility parameter and
statistical thermodynamics
in collaboration with L. A. Utracki.
This quantity, a cohesive energy density (ced), has played
in the past a semiempirical role in gauging miscibility in
polymer solutions and blends. For a volatile liquid it is
obtainable from the heat of vaporization. For a polymer it
has been derived from a variety of measurements in a series
of solvents or swelling agents. The ced of the solvent
exhibiting an extremum in the particular property is then
adopted as the value for the polymer solute. Values reported
in the literature refer to ambient pressure and a standard
temperature of 250C, but with considerable variations
depending on method employed. These procedures are to be
questioned on two grounds. First, a temperature of 250C
implies different conditions for different systems, being
above or below Tg or Tm. Second, the environment and hence
the intermolecular interactions in the melt and solution
states differ. We compute the solubility parameter directly
by means of our lattice-hole theory. At 250C the values are
considerably higher than the tabulated values. Choosing
alternatively an "internal" characteristic temperature of
Tg+300 K, good correlations between computed and listed
values obtain. This implies that an elevated temperature is
required to generate corresponding levels of molecular
mobility and packing in solution and in bulk, or more simply
in free volume levels.
Positron and thermodynamic free
volume
with G. Consolati, Politecnico di Milano and B. Olson.
Positron free volumes are conventionally extracted from
measured lifetimes by assuming spherical cavities. In a
series of polystyrene fractions of varying molar mass
differences between these and free volumes derived from
thermal expansion data are observed. We show that these can
be reconciled by allowing for molar mass dependent
anisotropies (disks or prisms) of the cavities.
Gas solubility in polymers
with C. B. Park, University of Toronto.
Detailed experimental data of CD2 in polystyrene and
polycarbonate are analyzed in terms of our theory.
Recent Publications
"Statistical Thermodynamics Predictions of the Solubility
Parameter," with L. A.Utracki, Polymer International, 53,
279 (2004).
“The Two Critical Concentrations in Polymer Science,”
with R. Koningsveld, H. Berghmans, and W. H. Stockmayer, J.
Phys. Chem., to be published (2004).
“The Two Critical Concentrations in Polymer Science,” R.
Koningsveld, H. Berghmans, R. Simha, and W.H. Stockmayer,
Journal of Physical Chemistry B, 108(41),
16168-16173, October 14, 2004.
“Pressure-volume-temperature Dependence of Polypropylene/Organoclay
Nanocomposites,” L.A. Utracki and R. Simha,
Macromolecules, 37(26), 10123-10133, December 28,
2004.
Awards
Inaugural Raymond F. Boyer Distinguished Lecturer, Case
Western Reserve University, 2000
Polymer Physics Prize, Am. Phys. Soc
A. Cressy Morrison Prize, N.Y. Acad. Sci.
Bingham Medal, Society of Rheology
Distinguished Service Award, Washington Academy of Sciences
Meritorious Service Award, U.S. Dept. of Commerce
Superior Accomplishment Awards, Natl. Bureau of Standards
Certificate of Recognition, NASA.
Visiting Professorships: Dresden, Eindhoven, Freiburg,
Toulouse
Fellowships: Columbia University, Lalor Foundation, J.F.
Kennedy Memorial Foundation, British Science Research
Council
Fellow: AAAS; Am. Inst. Chem.; Am. Phys. Soc.; N.Y. Acad.
Sci. Washington Acad. Sci. Ed. Board, J. Polymer Sci. |
