Polymers in organic solvents and supercritical fluids

SANS Studies of Polymers in Organic Solvents and Supercritical Fluids in the Poor, Theta, and Good Solvent Domains... 

As noted previously, enzymes and cofactors do not normally dissolve in nonaqueous solvents. This can be altered by coating or covalently modifying the surfaces of these species with materials (often polymers) that are miscible with nonaqueous solvents. For example, lipid-coated enzymes can be dissolved in organic solvents and supercritical fluids (Figure 3.5a) [7]. Fluorinated coenzyme can be dissolved in fluorous solvents and supercritical CO (Figure 3.5b) [8]. 

These observations delineate an intrinsic analogy among the temperature behaviors of polymers in organic solvents, supercritical fluids, and polymer blends in which similar crossover phenomena are observed (see Section 7.6.2.1). Using the scaling variable r = (T — 7c)/(0 — T ), which accounts for the distances of the temperature both from T and from T, and normalizing the correlation... 

The supercritical fluid mefhod is a relafively new method, which can minimize the use of organic solvents and harsh manufacturing conditions taking advantage of two distinctive properties of supercritical fluids (i.e., high compressibility and liquid-like density). This method can be broadly divided into two parts rapid expansion of supercritical solutions (RESS), which utilizes the supercritical fluid (e.g., carbon dioxide) as a solvent for the polymer, " and supercritical antisolvent crystallization (SAS), using the fluid as an antisolvent that causes polymer precipitation. Recent reviews of the supercritical technology for particle production are available in the literature. ... 

In conventional polymerization processes in organic solvents, it is possible to follow the reaction rate by correlating the decrease in pressure of a supply of gaseous monomer to the conversion. Heller describes a method in which the decrease in pressure is correlated to the reaction rate with a virial equation of state [28]. A similar method can be used for reactions in supercritical media, which are often subject to a pressure change upon reaction. In this study, a model was developed to determine the reaction rate indirectly based on the measured pressure during polymerization and based on a description of the phase behavior of the polymer and supercritical fluid phase [9, 29]. 

The choice of solvents for enzymatic reaction has been widened from organic solvents to various types of solvents such as supercritical fluids, ionic liquids, etc. The enzymatic reaction in organic solvent has been reported already in 1970s, the first biocatalysis in ionic liquids [4] was in 2000, and the first biocatalysis in supercritical fluids [5] was in 1985. Currently four kinds of liquid or fluid solvents, aqueous, organic solvents, ionic liquids, and supercritical fluids, are available for biocatalysis as shown in Figure 3.2. Moreover, biphasic or triphasic solvent systems consisting of two or more kinds of the solvents are also often employed for biocatalysis. Solid phase of immobilized enzymes and/or hydrophobic polymer to adsorb substrate and product may also exist. The performance of a biocatalyst depends significantly on the solvent system. The best medium should provide optimum reaction rates and simplify work-up procedure to make the process both economical and environment friendly. 

A crystalline or semicrystalline state in polymers can be induced by thermal changes from a melt or from a glass, by strain, by organic vapors, or by Hquid solvents (40). Polymer crystallization can also be induced by compressed (or supercritical) gases, such as CO2 (41). The plasticization of a polymer by CO2 can increase the polymer segmental motions so that crystallization is kinetically possible. Because the amount of gas (or fluid) sorbed into the polymer is a dkect function of the pressure, the rate and extent of crystallization may be controUed by controlling the supercritical fluid pressure. As a result of this abiHty to induce crystallization, a history effect may be introduced into polymers. This can be an important consideration for polymer processing and gas permeation membranes. 

Gas AntisolventRecrystallizations. A limitation to the RESS process can be the low solubihty in the supercritical fluid. This is especially evident in polymer—supercritical fluid systems. In a novel process, sometimes termed gas antisolvent (GAS), a compressed fluid such as CO2 can be rapidly added to a solution of a crystalline soHd dissolved in an organic solvent (114). Carbon dioxide and most organic solvents exhibit full miscibility, whereas in this case the soHd solutes had limited solubihty in CO2. Thus, CO2 acts as an antisolvent to precipitate soHd crystals. Using C02 s adjustable solvent strength, the particle size and size distribution of final crystals may be finely controlled. Examples of GAS studies include the formation of monodisperse particles (<1 fiva) of a difficult-to-comminute explosive (114) recrystallization of -carotene and acetaminophen (86) salt nucleation and growth in supercritical water (115) and a study of the molecular thermodynamics of the GAS crystallization process (21). 

Applications The majority of SFE applications involves the extraction of dry solid matrices. Supercritical fluid extraction has demonstrated great utility for the extraction of organic analytes from a wide variety of solid matrices. The combination of fast extractions and easy solvent evaporation has resulted in numerous applications for SFE. Important areas of analytical SFE are environmental analysis (41 %), food analysis (38 %) and polymer characterisation (11%) [292], Determination of additives in polymers is considered attractive by SFE because (i) the SCF can more quickly permeate throughout the polymer matrix compared to conventional solvents, resulting in a rapid extraction (ii) the polymer matrix is (generally) not soluble in SCFs, so that polymer dissolution and subsequent precipitation are not necessary and (iii) organic solvents are not required, or are used only in very small quantities, reducing preparation time and disposal costs [359]. 

For the analysis of organic additives in polymeric materials, in most cases, prior extraction will be necessary. Depending on the nature of the additive, many different approaches are employed. These include soxhlet extraction with organic solvent or aqueous media, total sample dissolution followed by selective precipitation of the polymer leaving the additive in solution, assisted extraction using pressurised systems, ultrasonic agitation and the use of supercritical fluids. In trace analysis, solid phase extraction (SPME) from solution or solvent partition may be required to increase the analyte concentration. 

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