Nonsolvent dilution

Figure 9.23 Batch crystallization by nonsolvent dilution (drown-out crystallization).

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. 

The critical points occur at low polymer concentrations—the lower the higher the molecular weight. As the molecular weight goes to infinity (see dotted curve in Fig. 123,a), the critical point moves into the nonsolvent-solvent axis i.e., the critical concentration of polymer is zero in the limit of infinite molecular weight. Near the critical point when X is large but finite, both phases are dilute in polymer. 

Mergence of the binodial with the nonsolvent-solvent axis shows that the polymer concentration in the more dilute phase becomes vanishingly small when the proportion of nonsolvent exceeds appreciably that at the critical point. These features clearly parallel those observed in two-component systems, with the nonsolvent-solvent ratio assuming the role of temperature in the latter. It may be shown that they are not critically dependent on the particular values assigned to the... 

They showed further that the limiting slope (RTA2) of the plot of the osmotic pressure-concentration ratio tz/c against the polymer concentration in a binary solvent mixture should be proportional to the value of the quantity on the left side of Eq. (17),f with V2 representing the volume fraction of solvent in the nonsolvent-solvent mixture which is in osmotic equilibrium with the solution. The composition of the liquid medium outside the polymer molecules in a dilute solution must likewise be given by V2. The composition of the solvent mixture within the domains of the polymer molecules may differ slightly from that outside owing to selective absorption of solvent in preference to the nonsolvent. This internal composition is not directly of concern here. If the solution is made sufficiently dilute, the external nonsolvent-solvent composition v2 = l—Vi) will be practically equal to the over-all solvent composition for the solution as a whole. Hence... 

Polymerization in dilute solution also allows a very straightforward isolation and purification of the microgels. After polymerization, the resulting microgels can be conveniently precipitated from the reaction solution by using suitable nonsolvents for the microgel molecules. The resulting powders can be filtered off, dried, and redispersed in suitable solvents when needed. 

The reaction is exothermic and proceeds rapidly at room temperature. The polymerization is generally performed by passing oxygen or air through a stirred solution of the catalyst and monomer in an appropriate solvent. When the desired molecular weight is attained, the polymer is isolated by dilution of the reaction mixture with a nonsolvent for the polymer. The precipitated polymer is then removed by filtration, washed thoroughly and dried. The polymer is soluble in most aromatic hydrocarbons and chlorinated hydrocarbons and insoluble in alcohols, ketones and aliphatic hydrocarbons. 

The solution was diluted with cyclohexane, a solvent for polystyrene and a nonsolvent for polymethylmethacrylate, and the product thereafter precipitated by addition to methanol THF/cyclohexane/methanol = 1 2 10 (by volume). The product was finally dried to constant weight. 

When the diluent is a nonsolvent, there is no comparable swollen state and the final structure is one in which large entangled nuclei are connected by a relatively small number of coiled and crumpled internuclear chains and the polymer and diluent phases are segregated. As with the solvent-modified materials, collapse of the system of interconnected nuclei occurs as the diluent is removed, but the large size of the nuclei will lead to the appearance of macroporosity at considerably lower DVB contents and dilutions. In fact, the macroporosity appears at lower DVB contents... 

Q < 6 (the lower the infinite dilution activity coefficient of the solvent, the greater the solvency of a chemical). Values of the infinite dilution activity coefficient above 10 indicate nonsolvency. In the intermediate region, it is difficult to conclude if the specific chemical is a solvent or a nonsolvent. 

If a polystyrene latex that is stabilized solely by an electrostatic mechanism is coagulated by the addition of electrolyte, that coagulation is usually irreversible to subsequent dilution. In contrast, sterically stabilized dispersions can usually be flocculated by the addition to the dispersion medium of a nonsolvent for the stabilizing moieties mere dilution of the concentration of the nonsolvent to a suitably low value is often sufficient to induce the particles to redisperse spontaneously. 

The process uses very dilute solutions, normally in the concentration range of 2 between 10" and 10. In what is called the cloud-point titration method, dilute polymer solutions are titrated with nonsolvent at constant temperature to the first cloud point. The volume fraction 3 of nonsolvent to give the first cloud point is plotted against the logarithm of the volume fraction 2 of the polymer at the cloud point. The extrapolated straight line obtained for a hoinologous series at a point on the 2 = 1 axis was found experimentally and theoretically to be equivalent to (3)0 (Figure 6-19). (3)e corresponds to the solvent-precipitant theta mixture for the polymer at this temperature. 

The polymers formed by this process can be recovered from the latexes by precipitation in a large excess of nonsolvent and drying under vacuum. They are further characterized in dilute solutions by means of techniques such as light scattering, gas permeation chromatography (GPC), or viscometry. 

The reaction may be carried out in the presence of pyridine that acts as a catalyst and as an HCl scavenger. Often, a chlorinated solvent is used as a diluent for the pyridine. Phosgene is bubbled through a solution of the diphenol at 25-35 The pyridine hydrochloride precipitates out and, after washing the pyridine solution with dilute HCl and water, the polymer is precipitated with a nonsolvent. 

Is there a difference in results for the van der Waals and mixing rules The answer is yes, but there is no preponderance of accuracy in one form or another. The van der Waals mixing rules, with two binary parameters, is quite satisfactory for nonassociating, nonsolvating systems. But no mixing rule is adequate for components with hydrogen bonds in dilute solution. The accuracy of the descriptions of excess volume was uneven, with no apparent pattern of success or failure. 

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