Nonsolvents

Both thermodynamic and kinetic aspects of mixed systems (e.g., the precipitation step in wet spinning) involve the properties of the other components (solvent and nonsolvent in wet spinning) as well as the polymer. 

Based on these ideas, the intrinsic viscosity (in 0 concentration units) has been evaluated for ellipsoids of revolution. Figure 9.3 shows [77] versus a/b for oblate and prolate ellipsoids according to the Simha theory. Note that the intrinsic viscosity of serum albumin from Example 9.1-3.7(1.34) = 4.96 in volume fraction units-is also consistent with, say, a nonsolvated oblate ellipsoid of axial ratio about 5. 

The first term reflects the fact that, in practice, volume fraction is not the concentration unit ordinarily used. Even for nonsolvated spheres, some factors will modify the Einstein 2.5 term merely as a result of reconciling practical concentration units with

[Pg.597]

The effect of ellipticity also increases [77] above the 2.5 value obtained for spheres. Analytical functions as well as graphical representations like Fig. 9.3 are available to describe this effect in terms of the axial ratios of the particles. In principle, therefore, a/b values for nonsolvated, rigid particles can be estimated from experimental [77] values. 

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 f is a measurable quantity for, say, a protein, and since the latter can be considered to fail into category (3) in general, the friction factor provides some information regarding the eilipticity and/or solvation of the molecule. In the following discussion we attach the subscript 0 to both the friction factor and the associated radius of a nonsolvated spherical particle and use f and R without subscripts to signify these quantities in the general case. Because of Stokes law, we write... 

The dependence of f/fo on solvation and eilipticity has been worked out in detail. Since the situation parallels the way [77] exceeds its value for nonsolvated spheres, we shall not elaborate on the details, but merely summarize the conclusions ... 

If the hemoglobin molecule were a nonsolvated sphere, we could write for its volume... 

Frictional coefficients of f, solvated protein, and fo, nonsolvated sphere. 

DMAC and nonpolar solvents form synergistic mixtures which dissolve high molecular weight vinyl chloride homopolymers. For example, a mixture of DMAC with an equal volume of carbon disulfide [75-15-0] a nonsolvent, dissolves 14 wt % of Geon 101 vinyl chloride homopolymer at room temperature, whereas the solubUity of Geon 101 ia DMAC alone is about 5 wt % (15). 

Phase Separation. Microporous polymer systems consisting of essentially spherical, intercoimected voids, with a narrow range of pore and ceU-size distribution have been produced from a variety of thermoplastic resins by the phase-separation technique (127). If a polyolefin or polystyrene is insoluble in a solvent at low temperature but soluble at high temperatures, the solvent can be used to prepare a microporous polymer. When the solutions, containing 10—70% polymer, are cooled to ambient temperatures, the polymer separates as a second phase. The remaining nonsolvent can then be extracted from the solid material with common organic solvents. These microporous polymers may be useful in microfiltrations or as controlled-release carriers for a variety of chemicals. 

When pure needle-like crystals of -aminobenzoyl chloride are polymerized in a high temperature, nonsolvent process, or alow temperature, slurry process, polymer is obtained which maintains the needle-like appearance of monomer. PBA of inherent viscosity, 4.1 dL/g, has been obtained in a hexane slurry with pyridine as the acid acceptor. Therefore PBA of fiber-forming molecular weight can be prepared in the soHd state. 

Fibers spun by this method may be isotropic or asymmetric, with dense or porous walls, depending on the dope composition. An isotropic porous membrane results from spinning solutions at the point of incipient gelation. The dope mixture comprises a polymer, a solvent, and a nonsolvent, which are spun into an evaporative column. Because of the rapid evaporation of the solvent component, the spinning dope solidifies almost immediately upon emergence from the spinneret in contact with the gas phase. The amount of time between the solution s exit from the spinneret and its entrance into the coagulation bath has been found to be a critical variable. Asymmetric fibers result from an inherently more compatible solvent/nonsolvent composition, ie, a composition containing lower nonsolvent concentrations. The nature of the exterior skin (dense or porous) of the fiber is also controlled by the dope composition. 

Other than fuel, the largest volume appHcation for hexane is in extraction of oil from seeds, eg, soybeans, cottonseed, safflower seed, peanuts, rapeseed, etc. Hexane has been found ideal for these appHcations because of its high solvency for oil, low boiling point, and low cost. Its narrow boiling range minimises losses, and its low benzene content minimises toxicity. These same properties also make hexane a desirable solvent and reaction medium in the manufacture of polyolefins, synthetic mbbers, and some pharmaceuticals. The solvent serves as catalyst carrier and, in some systems, assists in molecular weight regulation by precipitation of the polymer as it reaches a certain molecular size. However, most solution polymerization processes are fairly old it is likely that those processes will be replaced by more efficient nonsolvent processes in time. 

HoUow-fiber fabrication methods can be divided into two classes (61). The most common is solution spinning, in which a 20—30% polymer solution is extmded and precipitated into a bath of a nonsolvent, generally water. Solution spinning allows fibers with the asymmetric Loeb-Soufirajan stmcture to be made. An alternative technique is melt spinning, in which a hot polymer melt is extmded from an appropriate die and is then cooled and sohdified in air or a quench tank. Melt-spun fibers are usually relatively dense and have lower fluxes than solution-spun fibers, but because the fiber can be stretched after it leaves the die, very fine fibers can be made. Melt spinning can also be used with polymers such as poly(trimethylpentene), which are not soluble in convenient solvents and are difficult to form by wet spinning. 

Methacrylic acid polymer is iasoluble ia the monomer, which may result ia the plugging of transfer lines and vent systems. Polymers of the lower alkyl esters are often soluble ia the parent monomer and may be detected by an iacrease ia solution viscosity. Alternatively, dilution with a nonsolvent for the polymer such as methanol results ia the formation of haze and can be used as a diagnostic tool for determining presence of polymer. 

Liquid poHshes and waxes containing 10 wt % or more petroleum distillates must be contained in childproof packaging (61). General experience indicates that natural waxes and polyethylene waxes are nontoxic (62). Although nonsolvent floor poHshes are relatively nontoxic, concern for floor waxes continues to be sHp-resistance (63,64). 

Poly(ethylene oxide) associates in solution with certain electrolytes (48—52). For example, high molecular weight species of poly(ethylene oxide) readily dissolve in methanol that contains 0.5 wt % KI, although the resin does not remain in methanol solution at room temperature. This salting-in effect has been attributed to ion binding, which prevents coagulation in the nonsolvent. Complexes with electrolytes, in particular lithium salts, have received widespread attention on account of the potential for using these materials in a polymeric battery. The performance of soHd electrolytes based on poly(ethylene oxide) in terms of ion transport and conductivity has been discussed (53—58). The use of complexes of poly(ethylene oxide) in analytical chemistry has also been reviewed (59). 

Polymer Suspensions. Poly(ethylene oxide) resins ate commercially available as fine granular soHds. However, the polymer can be dispersed in a nonsolvent to provide better metering into various systems. Production processes involve the use of high shear mixers to disperse the soHds in a nonsolvent vehicle (72—74). 

In terms of general solvency, solvents may be described as active solvents, latent solvents, or diluents. This differentiation is particularly popular in coatings applications, but the designations are useful for almost any solvent appHcation. Active solvents are strong solvents for the particular solute in the apphcation, and are most commonly ketones or esters. Latent solvents function as active solvents in the presence of a strong active solvent. Alcohols exhibit this effect in nitrocellulose and acryUc resin solutions. Diluents, most often hydrocarbons, are nonsolvents for the solute in the apphcation. 

Details for the nonsolvent batch oleum sulfonation process for the production of BAB sulfonic acid have been described, including an exceUent critique of processing variables (257). Relatively low reaction temperatures (ca 25—30°C) are necessary in order to obtain acceptable colored sulfonate, which requires refrigerated cooling (Table 9, example D). 

Typically, a polymer is first dissolved in a mixture of miscible solvents and nonsolvents. This mixture (lacquer solution) is frequently a better polymer solvent than any of the components (3,4). The lacquer solution is dearated and spread as a thin film on a suitable support. The surface of the film is then placed in contact with a nonsolvent diluent (precipitant) miscible with the solvent. This precipitates or gels the surface almost instantaneously. 

Poly(vinyhdene chloride) also dissolves readily in certain solvent mixtures (82). One component must be a sulfoxide or A/,Al-diaIk5lamide. Effective cosolvents are less polar and have cycHc stmctures. They include aUphatic and aromatic hydrocarbons, ethers, sulfides, and ketones. Acidic or hydrogen-bonding solvents have an opposite effect, rendering the polar aprotic component less effective. Both hydrocarbons and strong hydrogen-bonding solvents are nonsolvents for PVDC. 

---