UNHLATEXTM KMORPH

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UNHLATEXTM KMORPH

• The KMORPH program predicts the morphology that
  will be likely when the reaction kinetics proceed
  faster than the rate at which phase separation
  and rearrangement can occur, resulting in
  non-equilibrium morphologies.
• The prediction is based on calculating the
  distance that entering radicals can penetrate
  into seed particles during the second stage
  polymerization. The spatial distribution for
  radicals determines where phase separation of the
  second stage polymer will occur within the seed
  particle.
• This is a task that requires one to first model
  the reaction kinetics for the seeded
  polymerization and has caused KMORPH to develop
  into a fairly complete kinetic model.

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The user has the choice of a number of monomer
and polymer types to form a great variety of
commercially important seed and second stage
(co)polymers. As with EQMORPH, the relevant
physical parameters and rate coefficients for
these monomers and their polymers are contained
within a database built into the program. Users
also have the ability to simulate additional
monomers, polymers, initiators or surfactants not
included within the database.
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In a manner similar to EQMORPH, this screen
contains variables that can be controlled by the
person performing the polymerization. The
polymerization can be run either in a batch or
semicontinuous manner, with monomer (neat or
pre-emulsified) and initiator being fed over time
as desired. Reaction conditions, such as
temperature, seed latex characteristics, the
amounts of the various reactants, and the feed
rates are set in this window.
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Morphology predictions are made by determining
the location within the seed particles where the
second stage polymer is formed. In many cases,
this may be restricted to the outer regions of
the particles due to slow diffusion rates for the
second stage radicals after entering the
particles from the water phase. A picture of the
predicted morphology is constructed based on the
probability distribution for termination events
as a function of the radial distance within the
particle This distribution is calculated by
considering the random diffusion of polymer
radicals within the seed particles during
polymerization. An example is shown above for a
case where the second stage radicals penetrated
substantially into the seed particles, but full
penetration to the center of the particles not
possible.
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The numerous results of the kinetic simulation
can be displayed in graphical form. These include
monomer concentrations, radicals per particle,
molecular weights (of both growing and dead
polymer), copolymer compositions, propagation and
termination rate coefficients, monomer diffusion
coefficients, and many others. The above Figure
compares the predicted conversion rate versus
experiment data for the batch, seeded emulsion
polymerization of styrene at two different
temperatures. These simulation are performed
without the need for any adjustable parameters to
allow the program to be used in a truly
predictive manner.
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The predicted particle morphology simulates a
microtomed section of the latex particle. The
section thickness (percentage of the particle
diameter) can be varied in the program and its
effect on the resulting picture of the latex
particle is shown here. It is clear that the
thickness can affect how the particle appears in
the TEM image. This clearly illustrates the care
that must be taken when interpreting experimental
TEM images to make conclusions about particle
morphology.
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This example represents a semi-batch
polymerization of styrene in a Poly(methyl
acrylate-co-methyl methacrylate) seed latex at
70C. The predicted conversion rate corresponds
very closely to the experimental data. These
kinetic simulations determine the conditions
within the latex particle during the
polymerization that are required to make the
morphology prediction.
For a section thickness of approximately 90nm
The actual TEM image is also shown to the right,
along with the KMORPH prediction. The dark areas
are the polystyrene (stained with RuO4) domains
within the lighter seed copolymer. The KMORPH
simulation predicts the same type of complex,
non-equilibrium structure as observed in the TEM
image.
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Continuing Efforts for UNHLATEXTM KMORPH

• KMORPH is evolving through various stages of
  refinement. Continuing efforts will include a
  more detailed consideration of phase
  rearrangement and consolidation of domains within
  the particle during and after reaction. This
  will always include significant amounts of
  experimentation, both as a reality check for the
  morphology predictions, as well as to incorporate
  new science into the program.
• Refinements will include further consideration of
  acid comonomer systems, crosslinking, chain
  transfer etc.