Solvents for Common Polymers

Aqueous media, such as emulsion, suspension, and dispersion polymerization, are by far the most widely used in the acryUc fiber industry. Water acts as a convenient heat-transfer and cooling medium and the polymer is easily recovered by filtration or centrifugation. Fiber producers that use aqueous solutions of thiocyanate or zinc chloride as the solvent for the polymer have an additional benefit. In such cases the reaction medium can be converted directiy to dope to save the costs of polymer recovery. Aqueous emulsions are less common. This type of process is used primarily for modacryUc compositions, such as Dynel. Even in such processes the emulsifier is used at very low levels, giving a polymerization medium with characteristics of both a suspension and a tme emulsion. 

The single most important consideration in nonaqueous GPC is sample solubility. Although adsorption is not an infrequent problem, finding a solvent for a polymer is usually the hard step in analysis. The most common solvents for nonaqueous GPC are toluene, tetrahydrofuran, chloroform, and DMF. A number of potentially useful solvents are toxic, corrosive, or expensive,... 

Table 3.50 Possible solvent/nonsolvent combinations for common polymers...

Styrene-divinylbenzene gels are usually used for the separation of synthetic molecules dissolved in an organic solvent (Fig. 7.2). Tetrahydrofuran, a good solvent for most polymers, is frequently used as the mobile phase. However, chloroform, toluene or hot trichlorobenzene are also used to dissolve polymers that are not soluble in common solvents. 

Precipitation is a common technique used to isolate and purify polymers. For synthetic polymers, this often involves dissolving the polymer in an organic liquid that is a good solvent for the polymer and then adding this solution slowly to a large excess of a poor solvent (or nonsolvent), a liquid in which the polymer is insoluble. Introduction to the poor solvent causes the polymer chains to collapse, aggregate, and come out of solution. 

Experimental methods for solubility parameters of polymers commonly involve observing the swelling of the polymer as solvent is added. After performing this experiment with a number of solvents with different solubility parameters, the solvent which leads to the greatest degree of swelling for the polymer is the best solvent for that polymer. Since a / value of zero in Equation (2D-9) indicates the degree of solubility of a polymer in a solvent,... 

Microspheres can be made by dispersing or dissolving the drug in the polymer solution, and adding a second liquid that is not a non-solvent for the polymer but provides a limited solubility for the polymer solvent. There are two common methods of producing microparticles the solvent deposition method and the solvent evaporation method. 

The preceding discussion implies that compatible polymers should often induce heteroflocculation when particles homosterically stabilized by them are mixed. This should be especially evident with polymer pairs that form coprecipitates or coacervates, and in solvents that are not very good solvents for either polymer. Such behaviour contrasts markedly with the mixing of particles homosterically stabilized in the same dispersion medium by incompatible polymers, for which stability is predicted to occur. Selective flocculation seems feasible irrespective of the sign of Xz3, but will undoubtedly be more common in systems for which xzz is positive. 

One should keep in mind that mole fraction-based activity coefficients /a become very small values for common polymer solutions and reach a value of zero forMs ->oo, which means a limited applicability at least to oligomer solutions. Therefore, the common literature provides only mass fraction-based activity coefficients for (high-molecular) polymer -i- (low-molecular) solvent pairs. The molar mass Mb of the polymeric liqitid is an average value (M ) according to the usual molar-mass distribution of polymers. 

Interaction parameter second virial coefficient and exponent a in the viscosity law characterize thermodynamic quality of the solvent for given polymer. The most common in use is the exponent a, as it is needed to express the hydrodynamic volume of macromolecules and also because its values are known... 

Another specific phenomenon in polymer solutions is the effect of cosolvency when a mixture of two nonsolvents forms a solvent or even a thermodynamically very good solvent for polymer under study. Much less common is the occurence of co-nonsolvency, when a mixture of two solvents for given polymer constitutes its precipitant. Both cosolvency and co-nonsolvency result from solvent - solvent interactions and from the entropy changes of polymer coils induced with their interaction with solvent molecules. It is advisable to consider these unexpected phenomena when working with rrtixed mobile phases. 

The limitations in postsynthesis processability are due to the chain stiffness and interchain interactions that render these materials insoluble in common solvents. For example, polymers can become crosslinked, highly branched, or electrostatically crosslinked due to polaron/bipol-aron charge interactions. The chemical or ionic crosslinking renders the polymer intractable. However, several approaches have been adopted to facilitate solution processability including ... 

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