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EPI Research Areas and Programs
The
breadth of the research in the Emulsion Polymers Institute is
reflected in a wide range of efforts in the preparation,
characterization, and application of polymer latexes. A major
commitment exists in a number of research areas including the
kinetics and mechanisms of a wide variety of polymerization
processes, the study of particle morphology and its development, the
preparation, stabilization, polymerization, and application of
miniemulsions, film formation and crosslinking, the role of
surfactants and stabilizers in emulsion polymerization, and the
preparation of large-particle-size monodisperse latexes. We have a
continuing commitment in other research areas including
characterization of particle size and size distributions through
chromatographic separation techniques, reactor modeling, control and
scale-up, surface and interface characterization,
flocculation/coagulation studies, impact modifiers, the microscopy
of polymer colloids, nanoparticles, hybrid composite particles,
among other topics. Specific projects can vary depending on the
combined interests of industry through our Liaison Program and our
faculty and research staff. Some research is carried out in
collaboration with other research organizations and faculty members
at Lehigh. Collaboration with industry ranges from membership in our
Liaison Program to sponsorship of graduate student fellowships to
contract research programs. The descriptions which follow are not
intended to be an exhaustive review of the Institute's research, but
rather to provide some insight into its many activities.
Kinetics and Mechanisms
of Emulsion Polymerization Processes
An understanding of
various polymerization processes are gained through studies
of the kinetics and mechanisms of the events which occur
during the given process. The complexity of heterogeneous
chemical reactions typified by emulsion polymerization makes
this understanding difficult, and after fifty years of study
since the first quantification by Smith & Ewart, it is still
incomplete. As part of most research projects in the EPI,
the kinetics and mechanisms involved in preparing polymer
colloids are investigated to varying degrees. Reactions
include not only variations on the emulsion polymerization
process such as seeded, semi-continuous, and continuous
processes but also miniemulsion, microemulsion, living free
radical, and dispersion polymerizations.
The role of initiators
(oil- and water-soluble), surfactants (ionic, nonionic,
polymeric, and reactive), and comonomers on the rate of
polymerization, the particle size and number, and the
particle morphology and surface characteristics are part of
many of our investigations. In addition, mathematical
modeling based on our current understanding is frequently
used to advance this understanding through comparison to
experimental data. Prediction continues to be the long-term
goal of these mechanistic studies.
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| Schematic representation of
conventional emulsion polymerization (from C.
Anderson) |
Polymerization
kinetics are traditionally measured by gravimetry, gas
chromatography, or dilatometry. Each of these techniques
measures quantities related to the conversion of monomer to
polymer. Gravimetry and gas chromatography are limited in
accuracy and the number of data points that can be obtained
in a particular reaction, while dilatometry requires a
specialty reactor which differs significantly in
configuration and operating characteristics from
conventional stirred reactors.
Reaction calorimetry
provides an alternative technique with the advantage of
continuous monitoring of the heat of reaction in a stirred
tank reactor. The reaction rate is directly obtained from
the heat data, and this is integrated to obtain
conversion-time information. We are using the Mettler RC1
reaction calorimeter to monitor emulsion polymerization and
copolymerization reactions. The level of detail in the
polymerization kinetics allows close examination of all
stages of these reactions. Our particular interests include
the nucleation stage, the relationship between the heat
evolved and the number of particles generated, the effect of
process variables such as agitation (high shear vs. high
mixing; especially important in reactor scale-up), and the
evaluation of the development of copolymer composition from
the heat of reaction data.
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| Use of reaction calorimetry to
study the mechanism of the emulsion polymerization
of styrene. Intervals I and III are the same as
described by the classical Smith-Ewart theory, while
Stage 2 was observed originally by Varela de la Rosa
in the EPI. In Stage 2, the number of particles (Np)
and heat of reaction (Qr, reflecting the
polymerization rate, Rp) increase at a
slower rate compared to Interval I. |
Latexes prepared using
multiple monomers can contain particles with uniform
copolymer composition or having morphologies ranging from
core-shell to "half-moon", "sandwich-like",
"raspberry-like", or even separate particles of each
polymer. These structures can have a profound influence on
the final properties of the material. Industrially,
composite latexes are extremely important, with applications
including exterior paints, binders and coatings, impact
modifiers, and pressure-sensitive adhesives. The development
and control of particle morphology has been, and continues
to be, an important area of investigation within the EPI.
The understanding derived from these studies will enable the
preparation of a wide range of composite latexes of
technological importance.
Recent work has
encompassed both theoretical and experimental approaches to
understand morphology development. Thermodynamic expressions
derived for the various morphologies point out the
importance of interfacial tensions between the phases as
determinants of particle morphology. These are dependent on
the polymer, surfactant, and incorporation of surface
charges resulting from the initiator species. The role of
grafting reactions and the incorporation of compatibilizing
agents into the latex particles, such as macromonomers, are
the subject of current investigations. In addition to
observing the morphology of individual latex particles using
techniques such as transmission electron microscopy in
conjunction with a given staining technique, NMR has
also been used extensively to investigate particle
morphology. 1H spin-diffusion NMR measurements
have been used to help determine the presence and extent of
polymer microdomains within a latex particle, especially
when a macromonomer has been used as a formulation component
in the polymerization. (H)T 1r
NMR
relaxation measurements have also been used to
determine the interphase thicknesses in multilayer composite
latex particles.
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Morphology of
composite latex particles and thermodynamic
prediction and control of particle morphology. |
Miniemulsions: Formation,
Stabilization, Polymerization, and Applications
Miniemulsions are
oil-in-water emulsions prepared using a mixed emulsifier
system consisting of an ionic surfactant and a cosurfactant,
which is either a fatty alcohol (such as cetyl alcohol) or a
long chain alkane (such as hexadecane). These emulsions are
characterized by exceptional stability with droplet sizes
ranging from 50 to 400 nm. Both "artificial" and "synthetic"
latexes are created through use of miniemulsion technology.
The former refers to latexes prepared by emulsifying polymer
solutions with subsequent removal of the solvent, allowing
the preparation of latexes of polymers which cannot be
prepared by emulsion polymerization (e.g., epoxies). The
latter refers to latexes prepared by emulsion
polymerization.
Miniemulsion research is
proceeding in a number of areas including the stabilizer
system, the fate of monomer droplets during polymerization,
copolymerization, mathematical modeling, and the mechanism
to explain the phenomenon of enhanced droplet nucleation
where the addition of a small amount of polymer to the
monomer droplets allows for a large increase in the number
of miniemulsion droplets which become polymer particles
during polymerization and the subsequent large increase on
the miniemulsion polymerization rate. The phase behavior and
properties of mixed surfactant systems have been studied by rheological measurements, optical and electron microscopy,
small angle neutron scattering, and differential scanning
calorimetry. The size and stability of monomer droplets is
being investigated prior to polymerization using capillary
hydrodynamic fractionation (CHDF), acoustic attenuation
spectroscopy (APS) and electron microscopy.
The evolution of the particle size distribution during the
course of miniemulsion polymerizations is being followed to
gain a more detailed knowledge of the polymerization process
with respect to the nucleation stage and the disappearance
of the droplets. One exciting area of research in the
EPI is the application of miniemulsion technology to living
free radical polymerization where molecular weight may be
controlled in the resulting latex particles. Studies
relating to the miniemulsion copolymerization behavior of
monomer pairs such as vinyl acetate/vinyl 2-ethylhexanoate
and styrene/n-butyl acrylate have been conducted.
In addition, miniemulsion technology is currently being
applied in application areas such as the encapsulation of
inorganic pigments (such as titanium dioxide) and in the
preparation of hybrid ("artificial") composite latexes.
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Schematic of
procedure to encapsulate titanium dioxide pigment by
miniemulsion polymerization. |
Film Formation and Crosslinking from Latex Systems
Commercial latexes are
often required to form continuous films in applications such
as coatings and adhesives. Typically, film formation occurs
by the evaporation of water and the coalescence of the
polymer particles. However, the latter can either occur by
the interdiffusion of polymer chains from neighboring
particles or by some kind of physical or chemical bonding at
the particle-particle interface (e.g., interfacial
crosslinking). Our research interests include studies of a
variety of types of latexes and their films in terms of the
location of reactive functional groups that are capable of
undergoing crosslinking reactions and the mechanical
strength development of the films.
Early work in the
Institute involved both theoretical and experimental studies
on the drying of latex films and the coalescence of the
polymer particles. We have recently been revisiting the
drying stage during the film formation process in latex
blend systems comprised of well-defined monodisperse "hard"
(polystyrene) and "soft" (n-butyl methacrylate-co-n-butyl
acrylate) latex particles, with and without carboxyl groups
present. The morphology of the dried film is studied
using electron microscopy and atomic force microscopy. We
have also investigated the influence of
crosslinking during latex film formation and have developed
a "macromonomer" crosslinker to be used as a model in
crosslinking studies to achieve a "looser" crosslinked
network structure to be compared with conventional
crosslinkers such as divinylbenzene which result in
"tighter" crosslinked networks. The influence of the degree
of crosslinking was evaluated in terms of the mechanical
properties of the resulting films and correlated with film
morphology. Recently a study was completed to investigate
the competition between interparticle crosslinking and
interdiffusion in developing strength in films prepared from
reactive latex blends. Several studies also examined the
role of pigment and latex binder interfaces during film
formation in pigmented latex systems.
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Mechanism of
latex film formation. |
Role of Surfactants
The single ingredient
in an emulsion polymerization which offers the widest
variation is the surfactant or the system of surfactants.
Control of the particle size and size distribution,
stability, latex surface tension, and rheological properties
are primarily controlled by the selection of the type(s) and
amounts of this material. The EPI has engaged in many
studies of the role of many different surfactants in emulsification
and emulsion polymerization as well as seeded, microemulsion,
miniemulsion, inverse emulsion, dispersion, and suspension
polymerizations. Recent work includes detailed studies on
the behavior of a variety of polymerizable surfactants in the emulsion polymerization of styrene or vinyl
acetate/butyl acrylate copolymers in terms of the
incorporation of the polymerizable surfactant into the
surface of the particles and its effect on the mechanism and
kinetics of the polymerization. Grafting of vinyl acetate
onto poly(vinyl alcohol) polymeric stabilizer is the subject
of a recent research program to better understand particle nucleation
in this system with important industrial relevance.
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AFM (atomic force micrographs) of polystyrene
latex particles prepared with various
polymerization surfactants. TREM LF-40 (Cognis)
is a reactive surfactant.
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Preparation of
Large-Particle-Size Monodisperse Latexes
Monodisperse latexes are widely used
and can be found in many scientific and commercial
applications. A variety of techniques are now applied to
prepare particles with submicron to submillimeter sizes
having narrow particle size distributions. These include
conventional and seeded emulsion polymerization, emulsifier
free polymerization, dispersion and suspension
polymerizations.
From 1978 to 1985, the Emulsion
Polymers Institute in conjunction with the National
Aeronautics and Space Administration (NASA) undertook the
preparation of large-particle-size monodisperse latexes
(2-100 micron) in the microgravity environment of the Space
Shuttle Orbiters Columbia and Challenger. The gravitational
effects of creaming and settling were thus alleviated
allowing the successful preparation of monodisperse
particles in the size range of 5 to 30 micron diameter by
the method of successive seeding. Two microgravity products,
10 micron and 30 m micron polystyrene latexes, were
subsequently certified as Standard Reference Materials by
the National Institute of Standards and Technology (NIST,
formerly the National Bureau of Standards). The 10 micron
particles represent the first commercial product made in
space.
The further development of
ground-based techniques have advanced our ability to prepare
large-size latex particles. Seeded polymerizations were
improved through recipe modifications (e.g., stabilizer
system) and the use of a novel rotating cylindrical reactor
designed to simulate microgravity conditions for sedimenting
species resulting in particles ranging in size up to 100
micron.
Alternately, dispersion polymerization
in organic media was applied to prepare monodisperse latexes
up to 10 micron diameter. This method is simple in practice
and in principle can be applied to a wide variety of
monomers. Polystyrene, poly(methyl methacrylate), poly(n-butyl
acrylate), and polybutadiene particles are prepared by this
method. The kinetics and mechanisms of these reactions, as
well as their application, are currently under study.
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Preparation of
large-particle-size monodisperse latexes: The first
commercial products prepared in space. |
Although not a
specific research project per se, the application of
microscopy serves as an essential component of many research
programs within the EPI. Traditionally, transmission
electron microscopy (TEM) has been used to obtain particle
size distributions of latexes through the somewhat tedious
process of measuring the diameters of hundreds or even
thousands of particle images on electron micrographs. Larger
size particles are imaged and measured using scanning
electron microscopy (SEM).
Developments, such as
preferential staining, microtoming, and cryomicroscopy,
however, have opened many more avenues for investigation.
Our understanding of the development of structure in
composite latex particles continues to advance through use
of these tools.
Microscopes and sample
preparation instruments and techniques continue to be
improved and invented. The recent explosion in the
development of scanning probe microscopes is one example
with Lehigh acquiring an atomic force/scanning tunneling
microscope (AFM/STM). These expanding capabilities are vital
to the continuing study of latexes in the Institute.
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Examples of
polymer and particle morphologies by microscopy. |
NSF-Funded Research
As part of our
outreach efforts, a presentation in Adobe PDF format
summarizing the research results that were obtained in the
course of our 3 year NSF Grant (# CTS-9980208, "Kinetics and
Mechanism of Stable Free Radical Miniemulsion Polymerization
(SFRMP) and Application to the Preparation of Nanostructured
Latex Particles", Provost and Professor Mohamed S. El-Aasser,
Principal Investigator), may be obtained by
clicking here.
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