Progress in Predicting Latex-Particle Morphology and Projections for

Chapter 3 Progress in Predicting Latex-Particle Morphology and Projections for the Future 1

Yvon G. Durant and Donald C. Sundberg

Downloaded by CORNELL UNIV on September 7, 2016 | http://pubs.acs.org Publication Date: April 1, 1997 | doi: 10.1021/bk-1997-0663.ch003

Polymer Research Group, Department of Chemical Engineering, University of New Hampshire, Durham, NH 03824

The ability to predict the structure of composite latex particles has improved markedly since the introduction of free energy analyses of the phase separated particles. Such analyses have highlighted the importance of the interfacial tensions at the polymer/water and polymer/polymer interfaces. The variation of these interfacial tensions with important parameters such as surfactant level, polymer type, monomer concentration, and initiator end groups can be utilized to predict equilibrium particle morphology in a straightforward manner. A l l possible structures can be displayed on a surface energy map and the morphology with the lowest free energy can be easily located. The present challenge is to be able to determine the simultaneous effect of all of the important variables on the various interfacial tensions. The future challenge is to move away from the thermodynamic equilibrium constraints of the free energy approach and to describe the dynamic changes of the internal particle phase structure resulting from polymerization reaction within the particle.

The importance of latex particle morphology to finished goods properties has been known for a long time. Two good examples of this are impact modified thermoplastics, such as A B S , and many types of water borne coatings. However it has only been in the past decade or so that there has been much progress reported in the predictive capability for latex morphology, while the need to learn what controls the morphology has been recognized for a long time. Experimental approaches to identify the important parameters controlling the morphology (1-10) have been helpful and have also resulted in the identification of particle structures such as coreshell, inverted core-shell, hemisphere, occluded, sandwich, raspberry, etc. Predictive 1

Corresponding author

© 1997 American Chemical Society

Glass; Technology for Waterborne Coatings ACS Symposium Series; American Chemical Society: Washington, DC, 1997.


3. DURANT & SUNDBERG

Predicting Latex-Particle Morphology

45

approaches (11-23) began to appear later and have principally been restricted to equilibrium conditions. Dynamic analyses are now becoming available (24, 25) but are quite limited at the present time, as expected for such a complex subject. In the present discussion we limit our descriptions to the conditions of thermodynamic equilibrium. Torza and Mason (26) can be credited with describing the first predictive analysis about 25 years ago. They utilized the spreading coefficients concept to write inequalities which rank the total surface energy of a limited number of particle structures. They then applied this analysis to two component oil droplets dispersed in water and were very successful in predicting the correct morphologies. Nearly 15 years later our group presented a more general free energy analysis (11-13) so as to be able to deal with a wider variety of particle structures. Since that time others (15, 16, 19-22) have made contributions along the same lines. In all of these approaches it readily becomes clear that the limiting factor is the ability to understand the influence of the many experimental variables upon the various interfaces within and at the external surface of the particle. The purpose of this paper is to outline some of the advances in these areas and to show their application to experimental latices. Free Energy Analysis of Particle Morphology When there are no elastic forces involved in the latex particle, unlike that existing in crosslinked polymer particles, the free energy analysis amounts to the consideration of the hypothetical change in the Gibbs free energy in transitioning from two bulk phase polymers to a two component particle dispersed in water. The water may or may not contain surfactant or other components such as salts. The above transition involves the formation of interfaces within the particle and at its boundary with the water phase, and the free energy change can simply be written in terms of the interfacial tensions and areas as A G = Σ Yi A,. A n efficient way to do such calculations is to consider the change from bulk phase polymers to composite particles of every conceivable shape and to compare the free energies of all of the different particle structures. The preferred equilibrium morphology will be that with the lowest free energy. The free energy change equation for each structure is different from another only due to the different geometries of the particles if the interfacial tensions are assumed to be independent of particle structure, as is the case for most situations. This was shown clearly by Sundberg et al (13) where the free energy equations were presented for a variety of different morphologies. Equation (1) is an example of this for the transition to a core-shell (CS) particle in which the seed latex particle remains as the core and the second stage polymer forms the shell.

AG

=γ

2

ρ ν ρ ζ

+γ £ΐ-φρ} * Ρ2/

- //η/*

(1)

where y , y , and γ /ρ are the interfacial tensions at the seed polymer/water, second stage polymer/water, and seed polymer/second stage polymer interfaces, respectively. In this equation φ is the volume fraction of the second stage polymer in the particle, and Aq is the surface area oif the initial seed polymer. Similar equations p1/w

P2/w

Ρ1

2



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TECHNOLOGY FOR WATERBORNE COATINGS

can be written for any other particle structure, although they may be quite complex due to the particular geometry of the structure. Three component particles can also be described by similar equations (23). A more elegant approach has been developed by Durant (27) in which all fully phase separated, two component particles were geometrically placed on a multi-axis plane. This "topological map" is arranged according to the angles associated with the phase boundary intersections in the particle and is pictured in Figure 1. At the far left of the plane we have a CS particle, while at the far right we have an inverted core-shell (ICS) particle. In between we have any variety of other fully phase separated particles. When the free energies associated with each of these particle structures (one can choose to do many hundreds via computer) are calculated, an energy surface can be constructed and when graphically presented, the differences in particle energies are readily apparent. A computer software, UNHLATEX™, has been developed in order to make the computations rapid and efficient. Figure 2 is a sample graphical output from the software for a particular set of interfacial tensions and at a stage ratio (second stage polymer to seed polymer) of 300%. The circle with a cross inside of it signifies the point at which the energy surface is at its minimum and indicates the preferred particle morphology. In this example it turns out to be an hemisphere particle.

Figure 1. Topological map. Dark phases represent the seed polymer and graphases the second stage polymer.



Figure 2. Free energy surface for a polystyrene seed, η-butyl methacrylate system, with a stage ratio of 300% and a concentration of SDS surfactant of 50% of the C M C .


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Progress in Predicting Latex-Particle Morphology and Projections for

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