Donald C. Sundberg

Latex Particle Morphology, Fundamental Aspects: A Review

The control of particle morphology is an essential part of producing high quality latex products for applications in coatings, adhesives, impact modifiers, and medical diagnostics, among others. A great variety of formulation and process variables are available to manipulate the particle structure and many different morphologies have been created. Techniques to characterize these morphologies are varied, but electron microscopy of both whole and sectioned particles is the most common one used. Atomic force microscopy is gaining in utility and often two or more characterization methods are simultaneously used to gain clarity of interpretation. A great deal of basic understanding of the factors controlling the morphology has been achieved by applying equilibrium thermodynamics to phase separated particles in aqueous media. Interfacial tensions at the polymer‐water interface and at the polymer–polymer interface, along with crosslinking density are found to be the dominant factors controlling the equilibrium morphology. In turn there is a significant number of formulation variables which determine the interfacial tensions and the crosslinking density. Successful models have been developed and applied to a number of different polymer systems. Much less progress has been made in understanding the development of non‐equilibrium morphologies where the possible number of particle structures is essentially infinite. Here, characterization techniques become somewhat less precise in identifying exact structure and further work is needed to advance this capability. In the dynamic reaction environment of the latex process the morphology develops within very viscous phases and is a result of competitive reaction and diffusion processes. Some progress has been made to quantitatively describe these phenomena, but more work is needed. This is even more evident when extension to carboxylic and hybrid (e.g. polyurethane/acrylic) latices is desired.

Microencapsulation of emulsified oil droplets by in-situ vinyl polymerization

This paper reports on the preparation and evaluation of microcapsules formed by the polymerization of methyl methacrylate in the presence of an oil/water emulsion. The oil phase was composed of an alkane (n-decane or hexadecane) and the oil/water emulsions were stabilized by a variety of emulsifiers. Both oil-soluble and water-soluble initiators were used and the monomer was introduced by either dissolving it in the oil or by feeding it through the water. The primary objective of the work was to study the effects of both formulation and process variables on the morphological characteristics of the polymer/oil composite particle. Our experimental findings indicate that it is not a trivial task to assure that the polymer is formed at the interface in such a way that it envelopes the oil droplet. It was found that the type of emulsifier used is crucial in determining the success of the encapsulation process.

Conversion dependent morphology predictions for composite emulsion polymers: 1. Synthetic latices

Abstract Phase structure develops within composite latex particles during the polymerization process and is potentially dependent upon both the latex recipe and the polymerization process characteristics. An equilibrium thermodynamic approach is presented to predict the particle morphology as a function of the extent of conversion of a seed latex polymerization reaction. The discussion highlights the role of the monomer as it influences the phase compositions and interfacial tensions throughout the polymerization. It is found that a number of different particle morphologies possess nearly the same total interfacial energy throughout a significant portion of the polymerization reaction and that it is quite likely that occluded structures will form in addition to the more fully phase-separated structures, such as core-shell and hemispheres. Detailed methods to predict the probabilities of forming a variety of different morphologies are presented.

Estimating diffusion coefficients for small molecules in polymers and polymer solutions

The diffusion coefficient for small molecules (solvent or monomer) through polymer solutions in the vicinity of the glass transition are known to change by as much as six orders of magnitude with only a small change in polymer concentration. Experimental measurements are difficult in this region and consequently there are data for only a limited number of systems. A rather simple method to estimate these diffusion coefficients for the rubbery, glass transition, and glassy regions as a function of polymer concentration and application temperature is presented. While the method is empirical in nature, it is based on carefully executed experimental studies, sound scaling laws, and agrees extremely well with free volume theories in the rubbery region. The method only requires a knowledge of the pure polymer glass transition temperature in order to estimate the diffusivity of molecules like styrenic and acrylic monomers (molecular weight of approximately 100 g/mol) at any polymer concentration and for temperatures above and below the polymer glass point.

Non-equilibrium particle morphology development in seeded emulsion polymerization. 1: penetration of monomer and radicals as a function of monomer feed rate during second stage polymerization

Abstract Starve feeding of monomers is often used in an attempt to control latex particle morphology, especially when non-equilibrium structures are desired. For the case of a polar seed polymer and a non-polar second stage polymer, we have analyzed the relative probabilities of reaction and diffusion of polymer radicals and monomers as they penetrate the seed particle. The resultant penetration ratios (for polymer radicals and monomers) and fractional penetration values (depth of penetration) correlate well with a number of different non-equilibrium morphologies obtained from a wide variety of experimental reaction conditions. We conclude that the lack of polymer radical penetration is responsible for non-equilibrium core-shell structures for the glassy PMMA seed/PS system, while the styrene monomer easily penetrates the entire particle, even at very slow monomer feed rates. When the polar, low T g PMA is substituted for the PMMA seed, the polymer radicals cannot be excluded from the particle center and an inverted core-shell equilibrium structure is obtained at all monomer feed rates.

Resonant soft x-ray scattering from structured polymer nanoparticles

The application of resonant soft x-ray scattering to chemically heterogeneous soft condensed matter materials is presented. Two structured styrene-acrylic polymer composite latex particles ∼230nm in diameter were utilized to delineate the potential utility of this technique. Angular scans at photon energies corresponding to strong scattering contrast between specific chemical moieties made it possible to infer the effective radii that correspond to the two polymer phases in the nanoparticles. The results show that resonant soft x-ray scattering should be a powerful complementary tool to neutron and hard x-ray scattering for the characterization of structured soft condensed matter nanomaterials.

Hydroplasticization of Polymers: Model Predictions and Application to Emulsion Polymers

The plasticization of a polymer by solvent has a dramatic impact on both its thermal and mechanical behavior. With increasing demand for zero volatile organic compound materials and coatings, water is often the sole solvent used both in the polymer synthesis and in formulation and application; latex colloids derived from emulsion polymerization are a good example. The impact of water on the glass transition temperature of a polymer thus becomes a critical physical property to predict. It has been shown here that in order to do so, one simply needs the dry state glass transition temperature (T(g)) of the (co)polymer, the T(g) of water, and the saturated weight fraction of water for the sample in question. Facile calculation of the later can be achieved using water sorption data and the group additivity method. With these readily available data, we show that a form of the Flory-Fox equation can be used to predict the hydroplasticized state of copolymers in exceptional agreement with direct experimental measurement. Furthermore, extending the prediction to include the impact of the degree of ionization for pH responsive components, only with extra knowledge of the pK(a), was also validated by experiment.

Conversion dependent morphology predictions for composite emulsion polymers: 2. Artificial latices

Abstract An equilibrium thermodynamic analysis of composite particle morphology development within artificial latices is presented. The analysis emphasizes the role of the solvent upon phase compositions and interfacial tensions, and predictions of the favoured morphology are made as a function of the extent of solvent removal. Experimental observations of the morphology of poly(methyl methacrylate)/polystyrene composite particles agree well with the predictions and demonstrate the significant role that the surfactant can have upon the preferred particle structure. Consideration is given to the choice of solvent used to produce the artificial latex and it is predicted that the preferred particle morphology is unlikely to be dependent upon the type of solvent used in the process.

Dynamic Modeling of Non‐equilibrium Latex Particle Morphology Development During Seeded Emulsion Polymerization

We have developed software to simulate the development of non‐equilibrium latex particle morphologies produced by seeded emulsion polymerization. The diffusion of second stage polymer radicals within seed particles controls the development of morphology in a large number of systems. Knowledge of the conditions present within the latex particles during the reaction is required in order to model this diffusion process, and this makes it necessary to first simulate the kinetics of the polymerization. The program considers both the water phase and particle phase reactions, and can simulate polymerizations carried out under either batch or semi‐batch conditions. The model predictions agree well with experimental results both in terms of the polymerization kinetics and the development of particle morphology.

Control of particle morphology and film structures of carboxylated poly (n butylacrylate)/poly (methyl methacrylate) composite latex particles

Particles with a soft (s) core of poly (n-butyl acrylate)/poly (methyl methacrylate) (PBA/PMMA) copolymer and a hard (h) shell of PMMA were synthesized via a two-stage polymerization process. Two synthesis parameters were investigated: (i) the phase ratio of the core and the shell; and (ii) the compatibility of the two phases. The s/h phase ratio was varied from 100:0 to 0:100. The compatibility between the two phases was changed by (i) using acrylic acid (AA); (ii) by using pure PBA as core material; and (iii) by cross-linking the shell. Particle morphology was characterized by atomic force microscopy (AFM) on freeze-dried and on tempered single particles. The degree of coverage was found to depend on the shell content and the phase compatibility. The results are in good agreement with findings from transmission electron microscopy and solid state NMR given in Acta Polymerica [50 (1999) 347]. The experimental results are compared to predictions from simulation work on the particle morphology based on thermodynamic and kinetic considerations. The second part of the paper focuses on the phase distribution and the film morphology of films formed by the structured particles. Phase distribution at the surfaces, degree of film formation and the phase distribution in the bulk are characterized by AFM, cross-correlated and compared to the findings regarding particle structure in the first part of the paper. This approach is to our knowledge unique regarding its completeness and new in its methodology. The microscopic results concerning the bulk, the single particle and the surface properties are correlated to macroscopic properties like the minimum film forming temperature, pendulum hardness and gloss.