Polymers, dissolution Thermodynamics

The solubility of hydrocarbons in rubbery polymers can be described in more detail by several theories of solutions using various criteria of thermodynamic affinity [7,25-28], of which the Flory-Huggins theory is the most popular one. It takes into account the volume content of the penetrant dissolved in the polymer and the change in the length of the polymer s thermodynamic segment as a result of dissolution [7]. However, it should be pointed out that to describe dissolution, a rehned dual-mode sorption model can be used, e.g., the model by Pace and Datyner [7,29,30]. 

In addition to this, blends of olefin polymers are thermodynamically incompatible systems. The incompatibility and the tendency of PO blends to create separate phases persist in their melts. The presence of two phases in a reacting system can affect variations in the favorable concentrations of low molecular weight reagents within the polymers owing to the specific absorption or their preferential dissolution in one of the polymers. Thus, in a grafting reaction of polar monomers within a heterophase system, the difference in solubilities of low molecular weight reactants... 

The dissolution of a polymer in a penetrant involves two transport processes, namely penetration of the solvent into the polymer, followed by disentanglement of the macromolecular chains. When an uncrosslinked, amorphous, glassy polymer is in contact with a thermodynamically compatible liquid (solvent), the latter diffuses into the polymer. A gel-like layer is formed adjacent to the solvent-polymer interface due to plasticization of the polymer by the solvent. After an induction time, the polymer is dissolved. A schematic diagram of solvent diffusion and polymer dissolution is shown in Fig. 1. However, there also exist cases where a polymer cracks when placed in a solvent. 

In addition to the above, polymer dissolution rate data have been used to determine glass transition temperature and other thermodynamic parameters associated with polymorphic changes [21]. Dissolution has also found a variety of uses in the pharmaceutical sciences. In the development of microcapsules for sustained release dosage forms [22], the mechanism of drug transport is governed by the dissolution of the polymer. Cooney [23] studied the dissolution of pharmaceutical tablets in the design of sustained release forms. Ozturk et al. [24, 25] showed that the dissolution of the polyacid, which is used in enteric-coated tablets, was the controlling step in the release kinetics mechanism. 

Solubility of polymers, that is, ability to dissolve spontaneously in low-molecular-weight solvent and to form thermodynamically stable molecular solutions, is determined by a delicate interplay between the gain in conformational entropy, translational and orientational entropy of solvating molecules of low-molecular-weight solvent, and enthalpy change upon polymer dissolution. Therefore, solubility of polymer in a particular solvent strongly depends on temperature. Usually, polymers with nonpolar monomer groups are more soluble in nonpolar solvents, whereas polymers whose... 

Finally, we note that a dynamic property such as the dissolution rate of a polymer into a solvent depends on more than equilibrium thermodynamic properties. In the case of polymer dissolution, mass transfer considerations such as polymer diffusion in the solvent are important. For example, when water, a nonsolvent, is added to 2-butanone, poly(methyl methacrylate) dissolves more rapidly despite the fact that the solvent mixture is thermodynamically poorer than 2-butanone by itself [29],... 

The general principle of solubility is that like dissolves like. Hence polar polymers dissolve most readily in polar solvents, aromatic polymers in aromatic solvents, and so on. This is reflected in the thermodynamics of dissolution. 

When using this approach to polymer solubility, we need to remember that the basis is thermodynamics. In other words, this approach gives information about the energetics of solubility, but does not give any insight in the kinetics of the process. In order to promote rapid dissolution, it may be more helpful to employ a solvent that is less good thermodynamically, but that consists of small, compact molecules that readily diffuse into the polymer and hence dissolve the polymer more quickly. 

Although many thermodynamic theories for the description of polymer solutions are known, there is still no full understanding of these systems and quite often, one needs application of empirical rules and conclusions by analogy. As a rough guide, some solvents and non-solvents are indicated in Table 2.6 (Sect. 2.2.5) for various polymers. However, not all combinations of solvent and nonsolvent lead to efficient purification of a polymer via dissolution and reprecipitation, and trial experiments are required therefore. 

The Boltzmann law computes to a configurational AS governed by Eq. (3.22). A configurational AS represents dissolution of a perfectly ordered, pure solid polymer in pure solvent (Allcock and Lampe, 1981). van Oss (1991) cautions against designating physical processes as AH- or AS-driven unless careful microcalorimetric measurements have been made, because many thermodynamic suppositions (imputed to modeling or intuition) have not been substantiated by experimentation. Although descriptive analyses of... 

This article reviews the phase behavior of polymer blends with special emphasis on blends of random copolymers. Thermodynamic issues are considered and then experimental results on miscibility and phase separation are summarized. Section 3 deals with characteristic features of both the liquid-liquid phase separation process and the reverse phenomenon of phase dissolution in blends. This also involves morphology control by definite phase decomposition. In Sect. 4 attention will be focused on flow-induced phase changes in polymer blends. Experimental results and theoretical approaches are outlined. 

Phase dissolution in polymer blends. The reverse process of phase separation is phase dissolution. Without loss of general validity, one may assume again that blends display LCST behavior. The primary objective is to study the kinetics of isothermal phase dissolution of phase-separated structures after a rapid temperature-jump from the two-phase region into the one-phase region below the lower critical solution temperature. Hence, phase-separated structures are dissolved by a continuous descent of the thermodynamic driving force responsible for the phase separation. The theory of phase separation may also be used to discuss the dynamics of phase dissolution. However, unlike the case of phase separation, the linearized theory now describes the late stage of phase dissolution where concentration gradients are sufficiently small. In the context of the Cahn theory, it follows for the decay rate R(q) of Eq. (29) [74]... 

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