Particle Swelling

Seymour and coworkers (27,28,29,30) actually used these composition gradients to prepare block copolymers by swelling particles containing occluded (i.e., living) macroradicals with a second monomer. Such block copolymers were prepared from occluded vinylacetate, methyl methacrylate, and acrylonitrile macroradicals, and the yield of block copolymers was studied as a function of the solubility and rate of diffusion of the swelling monomer in the particles. 

For the present purpose, discussions will he restricted to cases where the transport of Z( through the continuous phase is the rate-determining step. The rate of transport from the bulk of the continuous phase to the surface of the swelling particles (phase a) is then given by (Smoluchow i, 1918)... 

Values for the propagation rate constant can be determined from bulk or solution experiments. Values of k have been published for a wide variety of monomers as a function of temperature. With standard emulsion polymerization recipes the value of [M]p is determined from equilibrium swelling measurements if a free monomer phase is present and by a mass balance if all the monomer is in the polymer particles. One normally assumes that [M] is not dependent on particle size in latexes comprised of different-sized particles. This assumption will be questionable in some systems, especially those involving high-swelling particles. 

The simplest description of swelling is based on equilibrium thermodynamics (Equation 25.21), and considers as the driving force the affinity of the monomer to the polymer via the Flory-Huggins theory of polymer solutions and as counteracting contribution the increase in the interfacial free energy due to the growth of swelling particles. Equation 25.21 was derived by Morton-Kaizerman-Altier (MKA equation) over 50 years ago [22, 23] ... 

Y and Z-j. If the seed has been prepared with an ionic initiator, the latter case may even involve a surface layer of an ionic high polymer surfactant on the swelling particles. This may cause the transport of Z- across the interface to be the rate determining step, thus leading to a decrease in the rate of absorption. 

Small, hard particles and powders, with dimensions of the order of 1 /rm can of course be fixed in a flat layer, as described in Section 6.5.2, but they then may manifest a roughness which invariably results in measured contact angles that are too high (Chaudhury, 1984). Porous materials have the same drawback, as have pressed pellets of particles, with the added difliculty that the contact angle liquid tends to disappear into such porous bodies, by capillarity, before they can be properly measured. However, non-swelling particles, powders or porous solids can still be used in contact angle measurements, by wicking. 

Evidently the excess dissipation of energy is proportional to the square of the conduction current and inversely proportional to the conductivity X. For a suspension of spherical non-swelling particles with radius a, VoN SMOLifCHOWSKi gave the following equation. 

The explanation of low-pressure hysteresis proposed by Amell and McDermott some thirty years ago was formulated in terms of the swelling of the particles which accompanies adsorption. The swelling distorts the structure, for example by prising apart weak junctions between primary... 

Rigid particles other than unsolvated spheres. It is easy to conclude qualitatively that either solvation or ellipticity (or both) produces a friction factor which is larger than that obtained for a nonsolvated sphere of the same mass. This conclusion is illustrated in Fig. 9.10, which shows the swelling of a sphere due to solvation and also the spherical excluded volume that an ellipsoidal particle requires to rotate through all possible orientations. 

Since polymer swelling is poor and the aqueous solubiUty of acrylonitrile is relatively high, the tendency for radical capture is limited. Consequentiy, the rate of particle nucleation is high throughout the course of the polymerization, and particle growth occurs predominantiy by a process of agglomeration of primary particles. Unlike emulsion particles of a readily swollen polymer, such as polystyrene, the acrylonitrile aqueous dispersion polymer particles are massive agglomerates of primary particles which are approximately 100 nm in diameter. 

Both mechanical and chemical action promote ink detachment from cellulose fibers during pulping. Mechanical action includes interfiber abrasion and fiber flexing and bending. Chemical action includes fiber swelling and surfactant-promoted ink particle emulsification and solubilization. 

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