Jeffrey M. Stubbs

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.

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.

Penetrant diffusion in poly(methyl methacrylate) near Tg: dependence on temperature and polymer weight fraction

Forced Rayleigh scattering was employed to measure diffusion coefficients for a ternary system comprising camphorquinone or diacetyl in poly(methyl methacrylate) with methyl isobutyrate as the diluent. Measurements were made at polymer weight fractions, w(p), over the range 0.78-0.90 and temperature 25-50 degrees C for camphorquinone (which spans the glass transition regime), and at w(p) = 0.9 and 0.95 and at room temperature for diacetyl. Results for camphorquinone show a strong temperature dependence, with diffusion coefficients changing as much as two orders of magnitude between 25 and 50 degrees C, and showing high activation energies for diffusion approximate to 125 kJ mol(-1). No sharp change in the activation energy for diffusion was observed as the glass transition is crossed. The pre-exponential factor for the diffusion coefficient decreased dramatically through the glass transition. The diffusion coefficients of small penetrants in glassy polymers appear strongly dependent on the molecular size

Fundamental studies on morphology control for latex systems with application to waterborne coatings: The effect of polymer radical mobility in latex particles during polymerization

This work reports on the development of composite latex particle morphology in seeded, semicontinuous emulsion polymerization. Previous work has explained particle morphology development in terms of the ability of second stage polymer radicals to diffuse into, or “penetrate,” the seed particles. This article investigates two separate effects that essentially alter the mobility of the second stage polymer radicals. The first is the initiator type, specifically nonionic versus ionic initiators, and the second is the use of chain transfer agent (CTA). It is shown that ionic initiators make it more likely to obtain core shell morphologies by decreasing the penetration of radicals, but only under a narrow set of conditions. Chain transfer agents, on the other hand, allow for more deviation from core shell morphologies by increasing the penetration of radicals, but significant changes were only observed at the highest concentration of CTA studied. Therefore, both factors are fairly subtle and effects are only observed under specific conditions.