Solubility of Polymers in Solvents

In many applications it is of prime importance that plasticizer and polymer be completely miscible at the molecular level. Thermodynamic criteria for the solubility of additives in polymers are the same as those given for simple liquid mixtures [2-6]. When two liquid substances form a stable solution, classical thermodynamics shows that the free enthalpy of mixing, AG , is negative [7]. Furthermore, one can write 

In the case of an ideal solution, = 0, and since A5m is always positive, the value of AG is negative. When the solution does not behave ideally, a simple relation for Mlm has been proposed the van Laar expression for any two-component system based on the cell model for a liquid mixture [8,9], 

The quantity ASm in Eq. (1.6) is now defined as the configurational entropy change due to mixing. So when Aca 0, temperature becomes a very important factor for the complete miscibility of two liquids. 

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CH Ochi, K., Saito, T., and Kojima, K., Determination of solubilities of polymers in solvents by a laser scattering technique. Fluid Phase Equil, 158-160, 847, 1999. 

The Hildebrand solubility parameters 6 can be predicted on the basis of the solubility of polymers in solvents with known Hildebrand solubility parameters. 

Hydrogen bonding between these polar species also plays a part in distorting the relationship between the solubility parameter and the actual solubility of polymer in solvent. The presence of crystallinity in the polymer also affects this relationship, leading to anomalous results. Many film forming polymers have solubility parameters in the range 8.5-10.0. Solvents in this range are preferred. 

Increasing the fluorine content of a polymer molecule increases its chemi-eal, solvent and flame resistance and photo stability imporves its electrical properties such as lower dielectric constant, lower coefficient of friction, raises melting point, increases its thermal stability and weakens its mechanical properties. Solubility of polymers in solvents usually decreases by increasing its fluorine content. 

Diphenyl compound (i.e. linkage bet >een rings) Melting range of polymer (°C) Solubility of polymer in ordinary solvents... 

Suitable gases in the form of supercritical fluids represent clean solvents/ carriers, which neither leave residues nor impose an environmental load. A number of factors determine the solubility of polymers in supercritical carbon dioxides (SCCO2) and these are given in Table 3.1. 

There already exists a substantial literature devoted to the estimation of various material properties with the help of additive structual increments (Reid et. al, 1987, Van Krevelen, 1990). The regular solution theory in combination with additive structural increments has a wide application for estimating the relative solubilities of organic substances in polymers and the solubility of polymers in various solvents (Barton, 1983) and will be described later in this chapter. When estimating partition coefficient values, one is quickly confronted with this method s application limits, particularly with polar and non-polar structures, for example the partitioning of substances between polyolefins and alcohol (Baner and Piringer, 1991). 

Evidently, the most important application of the solubility parameters to be discussed in this chapter is the prediction of the solubility of polymers in various solvents. A first requirement of mutual solubility is that the solubility parameter of the polymer <5P and that of the solvent <5S do not differ too much. This requirement, however, is not sufficient. There are combinations of polymer and solvent for which solvent for which rip <5S, but yet do... 

Also for the solubility (degree of swelling) of polymers in solvents it holds in general that this is only appreciable if the specific cohesion of polymer and solvent do not differ too much13. Thus rubber (VU/V 7.9) is soluble in most solvents... 

Wolf, B. A. Blaum, G., "Measured and Calculated Solubility of Polymers in Mixed Solvents Monotony and Cosolvency," J. Polym. Sci., 13, 1115 (1975). 

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. 

When this theory was used to predict the solubility of polymers in a variety of solvents, it was only partially successful. It was apparent that other intermolecular forces were at work which could not be calculated by this simple procedure. Hydrogen bonding, probably the strongest type of intermolecular force in a nonelectrolyte, was the clue for making solubility parameter theory work. 

Hildebrands solubility parameter (3) is the most useful index of the solubility of resins in solvents and plasticizers. With volatile materials it is derived from the change of vapor pressure with changes in temperature and can be measured very accurately. The parameter of the polymer is the same as that of its monomer. For many hydrocarbon resins, monomers may enter into the polymer, and the complete composition of the resin is not known. In such cases the solubility parameter is estimated from the range of the parameters of the solvents for the polymer. 

There are several different methods to separate PNIPAM-supported catalysts from the reaction mixtures. Both liquid-solid separations and liquid-liquid separations can be used. The most frequently used liquid-solid separation method takes advantage of the varying solubility of polymers in different solvents. For example, PNIP AM can be precipitated from THF into hexanes. PNI-PAM copolymers also exhibit lower critical solution temperature (LCST) behavior. Specifically, PNIPAM and its copolymers can be prepared such that these polymers are soluble in water at low temperature but precipitate when heated up. This property may be used as either a purification method or a separation tech-nique.[l 1] A thermomorphic system is a liquid-liquid biphasic system developed in our group. It uses various solvent mixtures with temperature-dependent miscibility to effect separation of catalysts from substrates and products, as shown in Figure 2. 

Numerous sources with compiled solubility parameters are available for commercial solvents [1, 2] and polymers [2, 73] Although their use affords qualitative results, they are commonly used in industry to predict the miscibility of polymers in solvents [39, 77]. For solvents ranked according to their solubility parameter, those in close proximity may show a comparable solubility behavior, whereas those that are far apart may show substantial differences. 

The compact core of a micelle is characterized by a uniform polymer density, fiXBs), chemical potential per monomer unit, plb Xbs), and excess free energy per unit area of the core-water interface, kBTy xBs)- Here, Xbs T) is the Hory-Huggins parameter of monomer (B)-solvent (5) interaction, and Xbs T) > Xbs 0) = 1/2 under poor solvent conditions for the monomer units of block B. Although the solubility of polymers in organic solvents usually decreases with a decrease in temperature, dxiT)fdT < 0, the situation is more complex in aqueous solutions. In particular, it appears that the solubility of thermosensitive block B in water typically decreases with an increase in temperature [11], and hence dXBsiJ)/dT>0. In this case, the collapse of blocks B and the aggregation of the block copolymers into micelles occur at r > LCST, where LCST is the lower critical solution temperature. 

According to Gee, a dependence of an equilibrium swelling of polymers in solvents on their solubility parameters is expressed by a curve with a maximum where flie abseissa is equal to the solubility parameter of the polymer. For exact evaluation of a swelling degree is represented by an equation resembling the Gaussian function ... 

The goal of this review was to present aspects of the preferential adsorption phenomena of solvents and polymers with a focus on their thermodynamic aspects. The idea behind this was to attract the attention of polymeric physico-chemists to this area, which is sufficiently related to a lot of different effects. In fact, preferential adsorption occupies a special place in the solubility of polymers in mixed solvents. 

The most interesting supercritical solvent is CO2. However, in most cases the solubility of polymers in supercritical CO2 is very hmited, and great efforts have therefore been made to discover experimentally under which conditions the solubihty of polymers could be increased. 

Xiong, Y. (1993) Modeling solubility of polymers in pure and binary solvents at near- and supercritical conditions, M.Sc. Thesis, University of Maine, Orono, Maine, USA (Thesis Advisor E. Kiran). 

It is important to note here that for the non-network polymers to have quite significant intrinsic microporosily (IM), limited rotational freedom is an essential requirement/ It is also interesting that one can manipulate the solubility of PIMs in solvents by varying the ratio of the aromatic ort/zo-dihalide and catechol units in the network/ ... 

In order to try and develop more rapid methods for elucidating polymer solubility in any solvent an understanding of the characteristics of the solvent in question must be fully understood. A brief discussion concerning the solubility of polymers in CO2 will be presented followed by a novel high-throughput solubility screening method. 

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