Polymer solubility criteria

Polymer-solvent interactions and solubility testing were summarised by Staal [26]. 

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More fundamental treatments of polymer solubility go back to the lattice theory developed independently and almost simultaneously by Flory (13) and Huggins (14) in 1942. By imagining the solvent molecules and polymer chain segments to be distributed on a lattice, they statistically evaluated the entropy of solution. The enthalpy of solution was characterized by %, the Flory-Huggins interaction parameter, which is related to solubility parameters by equation 5. For high molecular weight polymers in monomeric solvents, the Flory-Huggins solubility criterion is x A 0.5. 

The critical value of %i sufficient for solubility of polymer having large molecular mass is 0.5. Good solvents have a low %i value. %i is a popular practical solubility criterion and comprehensive compilations of these values have been published. ... 

Mutual miscibility between plasticizers and polymers is an important criterion from a practical point of view. If a polymer is soluble in a plasticizer at a high concentration of the polymer, the plasticizer is said to be a primary plasticizer. When the polymer solubility is limited in the technologically important high concentration range, the plasticizer is called a secondary plasticizer. In this case, two phases are present after the plasticization process one phase where the polymer is only slightly plasticized and one phase where it is completely... 

The first criterion was associated with improved secondary and tertiary petroleum recovery processes. This is the justification for the patent appHcations issued to the Dow (50) and Exxon (51) corporations. The additional costs of production and the increased adsorption of such modified water-soluble polymers are detrimental to the commercial appHcation of such polymers and even the academic studies in this area have decreased in recent years. 

Lohse et al. have summarized the results of recent work in this area [21]. The focus of the work is obtaining the interaction parameter x of the Hory-Huggins-Stavermann equation for the free energy of mixing per unit volume for a polymer blend. For two polymers to be miscible, the interaction parameter has to be very small, of the order of 0.01. The interaction density coefficient X = ( y/y)R7 , a more relevant term, is directly measured by SANS using random phase approximation study. It may be related to the square of the Hildebrand solubility parameter (d) difference which is an established criterion for polymer-polymer miscibility ... 

The analytical method developed in the BCR project (Franz and Rijk, 1997) to determine residual carbonyl chloride monomer in polymers was pre-validated by two laboratories and found appropriate for the quantitative determination of carbonyl chloride with a LOD = 0.3 mg/kg below and in the range of the restriction criterion of 1 mg/kg polymer, with observed repeatability values of r = 0.23 and 0.32 mg carbonyl chloride/kg polymer, respectively. The method is applicable to polycarbonate as well as to other polymers and copolymers where these are soluble in methylene chloride. 

It follows from Eq. (7.12) that only positive values of y are permitted, whereas it was mentioned above that the criterion for complete solvent-polymer miscibility is yn < 0.5. The conclusion is that the difference in solubility parameters of solvent and polymer must be small. If we assume that Vs 80 cm3/mol = 0.8 x 10-4 m3/mol, then at room temperature, the maximum value of l<5p-<5sl would be 4 (MJ/m3)1/2 = 2 (cal/cm3)1/2. This number, of course, depends strongly on the liquid molar volume. 

It was concluded that definition of asphaltenes based only on solubility is not a satisfactory criterion and that the behavior of asphaltenes in chromatographic separations is incompatible with such structures where the polymer units are interconnected predominantly by a-bonds. The asphaltenes are a complex state of aggregation best represented by the stacked cluster structure (micelle), which, however, cannot explain some of the GPC behavior of very dilute asphaltene solutions. 

The differences in the polymerization kinetics and colloidal behavior of polymerization systems based on monomers of different polarity may be illustrated (Bakaeva et al., 1966 Yeliseyeva and Bakaeva, 1968) by the polymerization of the model monomers, methyl acrylate and butyl methacrylate, at various concentrations of sodium alkylsulfonate (C,5H3 S03Na). The fact that the solubility of the monomers in water differs by two orders of magnitude (5.2 and 0,08/ , respectively) was used as a criterion of polarity. An additional advantage to comparing these two monomers is that their polymers have rather close glass transition temperatures which is important for coalescence of particles at later stages of polymerization. 

At pressures above these limits, the solubility of CO2 in the polymer phase remains relatively constant as seen on the right hand branches of Figures 4 and 5. This condition affects the distribution of CO2 and polymer between the two phases. When the composition of the polymer phase is almost constant, a preferential partitioning of CO2 into the SCF phase drives a certain amount of polymer from the polymer phase into the SCF phase based on the criterion of phase equilibria. This effect together with the solvent density increase due to pressure cause the enhancement of the solubility of polymer in the SCF phase as observed on the left hand branches of Figures 4 and 5. 

There are some who question the usefulness of the Flory-Huggins solubility parameter for problems related to the solubilization of polymers, although it is agreed that it is useful for study of the thermodynamics of dilute solutions. Barton (1975) has referred to literature that cites its shortcomings as a practical criterion of solubility. Some of these are ... 

The simplest criterion for a theoretical calculation of the miscibility of the hydrophobic portions of small molecules with the hydrophobic microphase of a membrane polymer appears to be a partial solubility parameter that is, calculation of the solubility parameter of only the hydrophobic portions of the molecules. This can be done using a group contribution approach in which each group (for example, a CHg-group) in the molecule contributes a certain attractive force and a certain volume to the molecule. In this way, a solubility parameter may be estimated for either a portion of a molecule or for a whole molecule. 

Insufficient chemical resistance of a blend at times leads to its rejection for use in an aggressive chemical environment, although it possesses an excellent combination of mechanical properties. Thus chemical and solvent effects on polymer blends are important factors that frequently determine blends applicability. Attention has been given to chemical resistance of blends starting from the fundamental concept of the solubility parameters. Apart from the chemical and environmental restrictions, thermal resistance of a polymer blend is often a major criterion for its applicability. Thus, the thermal conductivity, heat capacity and heat deflection temperature of polymeric materials are discussed in separate sections. 

To test these predictions, we employed a technique that is based on yet another prediction of the theory already mentioned above, namely, that water existing in the state of polarized multilayers has reduced solubility for large molecules and hydrated ions including Na salts, sugars, and free amino acids. This criterion of testing has the following merits experimental simplicity (equilibrium dialysis) and above all, its unambiguity. Thus, if water in a certain model solutions has a p-value for a Na salt (p-value the apparent equilibrium distribution coefficient of a probe substance like Na between the water under study and the reference normal dilute solution outside the dialysis bag) equal to 0.5, one can then state unequivocally that at least 50% of the water has been affected by the proteins or polymer present. 

The polymer superstructure influences its solubility. Askadskii and Matveev proposed a new criterion of solubility for linear polymers based on interaction of forces of a surface tension on wetting. The solubility parameter of polymer should be lower or equal to the work of rupture by solvent of a bond relative to a volume unit of the bond element. The condition of solubility can be expressed as follows ... 

The value of equilibrium swelling can be a practical criterion of solubility. Good solubility of linear polymers is expected if the value of equilibrium swelling is of the order of 300-400%. The high resistance of polymers to solvents indicates that the equilibrium swelling does not exceed several percent. 

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