UNH Latex Industrial Consortium: Preface

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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 UNHLATEX® software 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.

Executive Summary

The Polymer Research Group (PRG) at the University of New Hampshire is dedicated to the pursuit of basic research in areas of industrial importance. For over 20 years the PRG has been involved in research and technology development in emulsion polymers and other dispersed phase systems, with focus on latex particle morphology control and interfacial science. The present status of our research in latex morphology, grafting reactions, and interfacial phenomena has allowed us to launch a major research effort in the dynamic simulation of latex particle morphology development during reaction (kinetically controlled) and latex aging after reaction. Such a research program requires the integration of reaction kinetics, interfacial science, grafting reactions, and molecular modelling at polymeric interfaces. The output of the research effort is in the form of predictive models and experimental evidence, embodied within interactive computer software. The software is in a form suitable for industrial scientists and engineers to use in the design and interpretation of experiments, as an aid in the development of new processes to manufacture new and existing latex products, and in providing continuing education in a rather complex subject area for technically based employees.

Latex Particle Morphology Control -- Equilibrium Perspective

For a number of years the PRG has been very active in developing predictive models for latex morphology control. This work originally focused on the conditions at thermodynamic equilibrium, characteristic of processing conditions where the phase separation kinetics are faster than the speed of reaction within the latex particles. Successful models have been produced which describe the transition of the morphology as conversion increases and which respond to changes in concentration of single surfactants, changes in the types of co-polymers used as seeds and as second stage polymers, changes in the types of monomers added to the seed latex, changes in the degree of crosslinking of the seed polymer, and to a more limited extent, changes in the SO 4- end group concentrations at the particle surface.

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.

Dynamic Latex Particle Morphology Control -- Kinetically Limited Structures

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.

Computer Software Development

We continue to develop computer software for simulation of both the kinetic/diffusion controlled latex particle morphology and for the (less often achieved) equilibrium favored latex particle morphology for a particular recipe and process employed in the latex synthesis. These software packages, ongoing in development, are respectively entitled UNHLATEX®_KMORPH (kinetically controlled) and UNHLATEX®_EQMORPH (thermodynamically controlled), and are both used to model the morphology (development) of structured particles. The programs have reached a high level of sophistication allowing dynamic interaction with the user and the capability for one to extract the experimental conditions necessary to achieve a targeted morphology. Details on each of these simulation programs can be found in the Software Development sections of our Research pages (Tsavalas , Sundberg).

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.

Industrial Consortium

In order to carry out the research objectives cited above, we developed an industrial consortium in January of 1995, which is still strongly active today, to guide our work and to provide financial support.  Industrial members contribute $25,000/year (with a 3 year initial commitment) to staff the research programs with pre- and post doctoral scientists and engineers. The consortium members provide broad guidance to the research directions through discussion at annual board meetings. In addition to the benefits of receiving research results, consortium members receive UNHLATEX® software free of charge, frequent reports, interaction with the research students and staff, and preferred access and financial discounts for advanced workshops in emulsion polymers and interfacial science. Furthermore, to preserve your competitive advantage, UNHLATEX® software are not available to non-members of the consortium.

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)