Swelling with supercritical solvents

As the pressure of the gas is increased, the solubility of the supercritical gas in the solid polymer increases resulting in swelling, a phenomena that could be advantageous in certain applications while its deleterious impact should be minimized, if not totally eliminated, in other instances. 

The sorption of supercritical solvent and flic resulting swelling could be very high, for example around 30% and 20% respectively for carbon dioxide in polymethylmefliacrylate (PMMA). In such instances, the experimental information could be summarized using polymer equation of states such as Sanchez-Lacombe where a single mixture fitting parameter is used.  

Swelling can be advantageous in that it enables pemieation and diffusion of the supercritical fluid into the polymer network. Fragrances, dyes, or medicinal substances loaded in the supercritical fluid can readily impregnate into the polymer and load the polymer with the aforementioned additives. Upon release of the pressure, only the supercritical fluid (i.e., carbon dioxide) flashes off. This avenue has led to a plethora of controlled (timed) release products. 

Naturally, swelling may be undesirable in many instances as well. For example, swelling of organic polymer based membranes decreased selectivity. Other possible deleterious effects could include malfunctions due to solubilization and swelling of sealants such as gaskets or o-rings. 

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There have been many useful attempts made to classify separation technologies. Supercritical fluids are applicable with both intra-phase and inter-phase separations. Due to the ease and flexibility in which a new phase can be formed for regeneration of the solvent, inter-phase is the more common. Furthermore, material solubility and swelling problems, particularly with organic-component based membranes, limit inter-phase separations. This is due to the enhanced solubility of these components in supercritical solvents. 

Supercritical carbon dioxide is generally a poor solvent for polymers (91). However, it does have the capacity to swell many polymers (92,93), and this can be of considerable advantage in blend formation. Watkins and McCarthy (32) found that the solubility of carbon dioxide in PCTFE reaches a maximum at a temperature of 313 K and a pressure of 10.4 MPa. This maximum represents a mass gain of 4.5% carbon dioxide in the host substrate PCTFE. Wissinger and Paulaitis (94) found that PMMA swells by about 20 vol % after reaching equilibrium in carbon dioxide at a temperature of 305.7 K. However, it is important to note that PMMA undergoes foaming with supercritical carbon dioxide treatment, as observed by Shieh et al. (95,96). 

A radical polymerisation can be carried out with a range of polymerisation techniques. Those with only a single phase present in the system are bulk and solution polymerisations, involving the monomer, a solvent if present and the initiator. By definition, the formed polymer in a bulk or solution polymerisation remains soluble (either in the monomer or the solvent). A precipitation polymerisation is one in which the system starts as a bulk or solution polymerisation, but the polymer precipitates from the continuous phase to form polymer particles which are not swollen with monomer. A precipitation polymerisation when the polymer particles swell with monomer is called dispersion polymerisation apart from polymerisation in the continuous phase, the polymer particles have an additional locus of polymerisation, and the particles in these systems are colloidally stabilised. Precipitation polymerisation is often performed in an aqueous medium (e.g. acrylonitrile polymerisation in water). Dispersion polymerisation is usually performed in organic solvents that are poor solvents for the formed polymer (supercritical or liquid carbon dioxide may also be used as a continuous medium for dispersion polymerisation). 

As an alternative to distillation, extraetion with a eo-solvent that is poorly mis-eible with the ionie liquid has often been used. There are many solvents that can be used to extract product from the ionic liquid phase, whether from a monophase reaction or from a partially miscible system. Typical solvents are alkanes and ethers (15). Supercritical CO2 (SCCO2) was recently shown to be a potential alternative solvent for extraction of organics from ionic liquids (22). CO2 has a remarkably high solubility in ionic liquids. The SCCO2 dissolves quite well in ionic liquids to facilitate extraction, but there is no appreciable ionic liquid solubilization in the CO2 phase in the supercritical state. As a result, pure products can be recovered. For example, about 0.5 mol fraction of CO2 was dissolved at 40°C and 50 bar pressure in [BMIMJPFe, but the total volume was only swelled by 10%. Therefore, supercritical CO2 may be applied to extract a wide variety of solutes from ionic liquids, without product contamination by the ionic liquid (29). 

In order to overcome the main limitations of the impregnation processes, connected to the limited solubility of the compounds in the supercritical fluids, Perman [68] proposed an alternative method. A supercritical impregnation process was coupled with a liquid solvent (preferentially water) to enhance the drug solubilization. The system composed of a liquid drug solution and the polymeric support was pressurized with the supercritical fluid. Consequently, the swelled polymer allows rapid diffusional transport of the solute into the polymeric substrate. In different examples, bovine serum albumin microspheres were impregnated with insulin, trypsin and gentamicin (see Table 9.9-5). 

The advantage of conducting the precipitation polymerization in supercritical fluids is the ease with which the unreacted monomer can be recovered from the reaction medium and the ease of recovering the produced polymer from the solvent. Free-radical polymerization in SCF hydrocarbon solvents makes use of the relationship between solvent power and SCF density to alter the threshold of precipitation of the polymer chains and also to minimize the swelling of the precipitate. This process produces polymers with controlled molecular weight with a narrow molecular weight distribution. 

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