The Polymer Research Group (PRG) at the University
of New Hampshire has established an advanced research effort
in emulsion polymers. Our present work on latex particle morphology
control suggests that major advances can be achieved in this area
in partnership with industry. Working together with a group of companies, the
PRG advances this science and creates new software tools to
be used by the industrial partners.
In our "integrated consortium",
technology transfer takes place in a continuous
manner. In addition to the traditional consortium objectives of
performing advanced research, and providing early access to research
results and graduate students, we provide interactive computer
software with periodic upgrades as the work progresses. We have
already developed several versions of the morphology software package in our
series and it is available to all consortium members.
The science upon which this software has been established is somewhat
complex, but with an interactive tool one can apply the science
in rather simple ways. We will train industry representatives
to use the software and expect them to apply it to their own research
and development situations.
The consortium members take an active role
in the effort by supporting us financially and by providing at
least one of their staff to advise us and follow our advances,
and to be a conduit for the transfer of technology. As we expect
the results of our work to be a significant advance in the prediction,
control, and analysis of latex particle morphology, the payback
for our industrial partners can be very significant. Industry
members which remain integrated in the consortium
have greatly enhanced knowledge in a difficult subject area,
and apply continuously improved software tools to the
design of new latex products and processes, to troubleshoot development
and manufacturing problems, and to provide for a more complete
understanding of the characteristics of existing products. This
results in the saving of time and money. Member firms also have access
to pre- and post doctoral scientists and engineers with a great depth
of knowledge in emulsion polymers and who are or will
be eventually seeking employment in industry.
Phase equilibrium predictions are
required at each stage of the polymerization during which the
copolymer composition must be continuously determined. Given the
broad range of copolymer systems of interest to the emulsion polymer
community, generalization of the results is of great importance.
Here is a good example of where the potential insight gained through
molecular modelling can be of great assistance.
Most often, and especially under industrially relevant conditions, thermodynamic equilibrium is not achieved. Diffusion controlled kinetically limited structures are most prevalent in industry and the field in general. The ability to understand the development of latex particle morphology under dynamic processing conditions is thus the ultimate goal of this program. Dynamic (i.e. non-equilibrium) considerations require the simultaneous application of polymerization reaction kinetics and phase separation kinetics in a dispersed phase environment marked by diffusive transport of both monomer and polymer.
The polymerization kinetics for composite latex particles forces one to consider a system composed of three phases, two polymer and one aqueous. Within this environment free radical entry and exit take place, as well as the initiation, propagation and termination reactions. In this modeling, we follow the "life" course of each growing co-polymer chain, from its inception in the continuous aqueous phase (for systems intiated by water soluble initiator, such as a persulfate), through its aqueous phase kinetics and growth to a "z-mer" oligomeric length sufficient for surface activity on the seed polymer/water interface, through its continued propagation to a length and hydrophobicity that it can penetrate into the seed polymer, through its continued growth within the seed polymer environment, until its eventual termination of growth. All the while, each growing chain is susceptible to the various forms of chain transfer, grafting, branching, crosslinking, and termination which can each in their own way have impact on the evolution of molecular architecture and affect the overall molecular weight distribution of the system. Moreover, for systems where the second stage polymer is of sufficiently different polarity than the seed polymer it is growing within, the tendency for phase separation and diffusion necessary for phase rearrangement also must be considered.
The resulting nanostructured latex particle morphology is derived from a complex interplay between the polymerization reaction kinetics, diffusion capabilities of the polymeric chains as a function of their chain length and architecture, and the competition between the thermodynamic driving force for phase separation and diffusion limitations imposed by the kinetic timeframe.
Nucleation and growth of phase separated domains
resulting from polymerization within latex particles needs to
be considered within an environment in which there is diffusive
transport driven by interfacial tension gradients. The occluded
domains will undergo Brownian motion, volumetric growth by polymerization
and Ostwald ripening, leading to collision and coalescence. These
processes need to be quantified by rate equations which can be
simultaneously considered with those of the reaction kinetics.
While there remains a great deal of work to be done in this area, the
perspectives provided by our present and continually improving
understanding of dynamic morphology development are embodied well in our current software offerings.
Since many particle morphologies are such that they
present a variety of apparent structures when viewed by transmission
electron microscopy (TEM), we have enhanced our software
to simulate the multiple projections that a structured particle
can make onto the TEM screen. This simulation can then be compared
with the experimental micrograph in order to improve the understanding
of the TEM analysis.
We are always interested to increase our circle of members. New members must make an initial commitment of three years, then can remain in the Consortium on a year to year basis. Details of the current and proposed research programs of the PRG can be obtained by contacting us directly. We encourage your questions and invite you to join with us in these research endeavors.
Download a brochure describing our Industrial Consortium (PDF)