Monomer mutual solubility with polymer

Indeed, in the world of tomorrow we can expect new aspects of polymer solids to extend the conventional and successful structure ideas of this century. These, of course, were the recognition as molecular identities of the chains of repeating chemical monomers. The circumstances of those entities have resulted in interesting concepts of solubilities, viscosity, and other mechanics, and especially thermodynamic limitations m mutual solubility or comparability of polymer mixtures. But we have known for decades that even homogeneous regular chain polymers such as Carothers polyesters and polyamides formed solids with manifold imperfections and irregularities, such as order-disorder crystal configurations.(22,23)... 

A simple expression governs the solubility of a liquid solute in a solvent, provided the solvent is practically insoluble in the liquid solute and that, again, only dispersion forces are operative between them. The first condition yields for the activity of the solute in its practically neat liquid phase, as well as in the saturated solution in equilibrium with it, to a2 1 and In a2 0. This dispenses effectively with the first term on the right hand side of Eq. (2.10). For a given liquid solute, the solubility parameter of the solvent dictates the solubility and constitutes entirely the solvent effect on it. This fact has found much application in the determination of the solubilities of certain liquid polymers in various solvents, the mole fraction x2 and volume V2 then pertain to the monomer of the solute. If, however, the solvent is also soluble in the liquid solute, as is the case when a solvent is capable of swelling a polymer, then the mutual solubility is given by ... 

There are some requirements for monomers used for the emulsion polymerization. The primary requirement for monomers is that they must have a limited water solubility and be soluble in the formed polymer. However, the solubility in water should not be too high, otherwise this monomer would tend to polymerize in the water phase. In the mechanism for emulsion polymerization one of the driving forces is the absorption of monomer into the polymeric particles if the monomer and polymer are not mutually soluble then this process will not be efficient. Many different vinyl monomers are currently used in practical emulsion polymerization, including acrylates, methacrylates, St, AN (in copolymers), VAc, isoprene, and 1,3-butadiene. In addition, the monomers would not react with water, surfactants and other additives. Table 11.6 presents the propagation rate coefficients of various monomers examined in emulsion polymerization. 

The solubility of polymer melts in single component and in binary monomeric liquid mixtures is so widely known and frequently reviewed both with respect to practical applications, especially for polymer fractionation, and with respect to theory that little need be said here. The properties of monomer liquids and of polymer melt which determine their mutual miscibility have been assembled on Table 3 and are seen to be quite similar to those which determine the interaction between gases and polymer melts on Figure 4. 

The first term on the RHS of (III-8) represents mutual termination of radicals in the polymer particles (i.e. second order termination). The second term represents a first order termination of radicals in the polymer particles by monomer soluble impurities (MSI), which are present in the polymer particles due to their transfer in there with monomer during the monomer diffusion phase from monomer droplets. 

Hydrophobically associating polymers consist primarily of water-soluble monomer units with a small number of water-insoluble monomer units. Synthesis of high-molecular-weight random copolymers of acrylamide and alkylacrylamides required a novel aqueous surfactant micellar solution polymerization (2-4) because of the mutual immiscibility of the water-soluble and hydrophobic monomers. The use of surfactant micelles enabled solubilization of the hydrophobic monomer (alkylacrylamide [R]) into the aqueous phase containing the water-soluble monomer (acrylamide [AM]). The resulting RAM polymer after isolation provided homogeneous aqueous solutions. 

Harkins calculated from the solubility of styrene in water (0.00368 mol dm at 50 °C [50]) that there are 4 x 10 molecules dm . In a 3% solution of potassium dodecanoate there are about 1 x 10 micelles dm , but with 61 molecules per micelle with an unswollen radius of 2.1 nm the cross-sectional area of the monomer-swollen micelles exceeds that of the styrene molecules by a factor of at least 12. Hence the micelles are more likely to capture initiator radicals produced in the aqueous phase. Polymerization within the micelles must be much faster than in the water because the concentration of styrene will be much the same as in bulk (8.5 mol dm ). The molar mass of the polystyrene produced is much larger than the molar mass of all the styrene molecules solubilized in a micelle thus, the monomer must be able to diffuse through the aqueous phase from other micelles and monomer droplets to allow the polymer radical to continue to grow until it is finally terminated by the entry of another initiator radical from the aqueous phase. Under the standard conditions of the mutual recipe (Table 4.1) there is 180 g water to 100 g styrene taking the emulsion droplets to have a radius of 1 pm, the ratio of the total cross-sectional areas of droplets to micelles to monomer molecules is about 1 30 2.5. The ratio of total surface areas would be even more heavily biased in favour of micelles. Hence it is probable that many more radicals will be captured from the aqueous phase by the micelles than by the emulsion droplets or than react with the monomer molecules in aqueous solution. 

The relatively large monomer droplets (generally 2-5ym in diameter) have too small a surface area to capture radicals from the aqueous phase and therefore serve as reservoirs for the diffusion of monomer through the aqueous phase to the pol3onerizing oligomeric radicals, micelles, or polymer particles. Despite the unfavorable statistical probabilities, however, some monomer droplets capture radicals and polymerize to form microscopic or near-microscopic particles (14), and some of these particles which are entirely separate from the main particle size distribution are formed in most batch polymerizations. Polymerization in monomer droplets becomes much more significant when the size of the emulsion droplets is decreased. The use of ionic emulsifier-fatty alcohol mixtures (13) and, later, ionic emulsifier-alkane mixtures (15), allows the preparation of 0.1-0.2ym size styrene monomer droplets, which compete favorably with initiation in micelles and in the aqueous phase as the mechanism of particle nucleation. The mechanism of formation of these "mini-emulsions" has been attributed to the very low solubility of the fatty alcohols and alkanes in water (16) or to the formation of crystalline complexes between the ionic emulsifiers and fatty alcohols (17) the two mechanisms are not mutually exclusive. Thus this mechanism pertains only to special systems. 

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