Monomer solubility laws

The second important characteristic of the micellar solution that relates to solubilization is the micelle size. Poor aqueous soluble compounds are solubilized either within the hydrocarbon core of the micelle or, very commonly, within the head group layer at the surface of the micelle or in the palisade portion of the micelle. Predictions of the micelle size have relied on the use of empirical relationships employed within a thermodynamic model, for instance the law of mass action where micellization is in equilibrium with the associated and unassociated (monomer) surfactant molecules (Attwood and Florence, 1983). 

Small organic molecules such as monomers will obey the same laws when present in the very low concentrations which are involved in practice. It is interesting to compare solubilities, for example under one atmosphere of nitrc en, a typical rubber will dissolve about 60 ppm, most polymers dissolve similar amounts. This is actually within the range of residual monomer contents under consideration in most toxic monomer problems. 

It is assumed in the above and following discussions that the activities occurring in the law of mass action equation can be replaced by molar concentrations. The assumption is exact when a polymer melt is polymerized to a polymer soluble in the monomer without interaction between the components, that is, when the activities are equal to unity. Nonideal behavior is treated in Section 16.2.5. 

The rate law for DADMAC polymerization in an aqueous system, when persulfate is used, is not simple Rp = (S208 )° (DADMAC) . A combination of complicated initiation reactions and dimeric DADMAC interactions can account for the unusually high exponent of the DADMAC concentration (156). High monomer concentrations (>1.5 mol/L) used in commercial processes result in greater rates of polymerization and higher molecular weights. PDADMAC with low residual unreacted monomer can be manufactured in water using either persulfate addition or ammonium persulfate with sodium metabisulfite (157). Polymerization of DADMAC has also been studied in water-in-oil emulsion in a continuous stirred tank reactor (158). In that case, the oil-soluble initiator. 

For slurry reactors, the concentration of monomer in the solvent should be used for calculation of kp. This, however, has not been the usual practice in the literature. Usually polymerization rates are normalized with respect to the total pressure or the monomer partial pressure in the head-space of the slurry reactor. The total pressure can differ significantly from the monomer pressure if the solvent vapor pressure is appreciable at the operating temperature. Use of the monomer partial pressure is somewhat better since the concentration of monomer in the solvent is directly proportional to the monomer partial pressure in the pressure range where Henry s law is applicable. However, comparisons of activities in slurry reactors based on the partial pressure of the monomer above the solvent are only valid for a single solvent since the solubility of the monomer is a function of the solvent. 

Johnson, Christian, and Affsprung determined the solubility of water in liquids as a function of the total vapor pressure at 25°C in the low-water-concentration region. The liquids were nitrobenzene, dibutyl phthalate, tributyl phosphate, N,N-dimethylaniline, tributylamine, aniline, N,N-dimethylcy-clohexylamine, benzyl alcohol, cyclohexanone, and methyl phenyl ketone. These authors do not refer to the structure of water itself. They assumed that water molecules are present in solution as an equilibrium mixture of monomers and not more than two polymeric species each of which obeys Henry s law. ... 

Another method used to improve the fracture energy of BMI resins consists in mixing the thermosetting material with linear thermoplastic polymers. This can be illusirated by the behavior of mixtures containing Compimide 796 and TM 123 BMI resins with SABIC Ultem 1000 , poly(ether-imide) 26 (Fig. 9.13). The critical stress intensity factor ATic of the Unear polymer is six times higher than that of the BMI matrix and does follow the mixture law for all BMI/Ultem combinations. The linear polyimide can also be added as 20—40 p.m spherical particles to the BMI resin before it is polymerized. In another example, particles of a soluble precyclized polyimide (Ciba Geigy XU 218 ) are dispersed in BMI monomer, which is subsequently melted at 177 °C and cured at 232 °C. The two-phase material obtained by this means exhibits a glass transition temperature of 348 °C compared with only 260 °C for the BMI resin without polyimide additive. 

---