Polymers in Supercritical Fluids

Hyperbranched polymers are more simple to produce on a large scale than dendrimers. Generally, a one-pot synthesis is used, yielding fewer regular structures and very broad molecular weight distributions (45). 

The average degree of branching, DB, of AB2 hyperbranched polymers is given by 

The maximum possible number of growth directions in AB2 systems is given by 

The DB values range from zero for the Unear polymer to unity for the perfect dendrimer structure. 

Classically pure substances are solid, hquid, or gaseous. A supercritical fluid is a pure substance compressed and heated above its critical point see Rgure 14.13 (46). The major characteristics of a supercritical fluid are hquidlike density, gaslike diffusivity and viscosity, and zero surface tension. 

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In the RESS method, the solute of interest is solubilized in a supercritical fluid, which is then rapidly expanded through a nozzle. As the fluid expands, it loses its solvent capabilities and the solute precipitates out. While this technique has the advantage of not using any organic solvent, it is restricted by the generally poor solubility of most polymers in supercritical fluids. Indeed, polymers generally have to be below 10,000 MW in order to be eligible for this method of particle production [126]. 

Several authors [3-9] studied the solubility of polymers in supercritical fluids due to research on fractionation of polymers. For solubility of SCF in polymers only limited number of experimental data are available till now [e.g. 4,5,10-12], Few data (for PEG S with molar mass up to 1000 g/mol) are available on the vapour-liquid phase equilibrium PEG -CO2 [13]. No data can be found on phase equilibrium solid-liquid for the binary PEG S -CO2. Experimental equipment and procedure for determination of phase equilibrium (vapour -liquid and solid -liquid) in the binary system PEG s -C02 are presented in [14]. It was found that the solubility of C02 in PEG is practically independent from the molecular mass of PEG and is influenced only by pressure and temperature of the system. 

In the literature, very little data on the solubility of polymers In supercritical fluids have been reported. The data are limited because experiments require precise detections of composition and molecular weight distribution. The study of the liquid solutes Is even more complicated than solid solutes. For solid solutes, the solid phase can be assumed to remain pure and only the supercritical fluid phase is then sampled. In case of liquid solutes, the supercritical fluid Is appreciably soluble in the solute and therefore both phases must be sampled. 

The phase behavior of polymers in supercritical fluid (SCF) solvents has been reviewed by Kirby and McHugh [8]. The effect of supercritical carbon dioxide on polymer-solvent mixtures is addressed elsewhere [5]. 

KIR Kirby, C.F. and McHugh, M.A., Phase behavior of polymers in supercritical fluid solvents, Chem. Rev., 99, 565, 1999. 

Since then. Dr. Woldfarth s main researeh has been related to polymer systems. Currently, his research topics are molecular thermodynamics, continuous thermodynamics, phase equilibria in polymer mixtures and solutions, polymers in supercritical fluids, PVT behavior and equations of state, and sorption properties of polymers, about which he has published approximately 100 original papers. He has written the following books Vapor-Liquid Equilibria of Binary Polymer Solutions, CRC Handbook of Thermodynamic Data of Copolymer Solutions, CRC Handbook of Thermodynamic Data of Aqueous Polymer Solutions, CRC Handbook of Thermodynamic Data of Polymer Solutions at Elevated Pressures, CRC Handbook of Enthalpy Data of Polymer-Solvent Systems, and CRC Handbook of Liquid-Liquid Equilibrium Data of Polymer Solutions. 

A. D. Shine, Polymers in supercritical fluids. Chapter 18 in Physical Properties of Polymers Handbook, J. E. Mark, Ed., American Institute of Physics, New York... 

H. Pohler and E. Kiran, Miscibility and phase separation of polymers in supercritical fluids Polymer-polymer-solvent and polymer-polymer-solvent ternary mixtures, presented at the AIChE Annual Meeting, Chicago, Illinois, November 10-15, 1996. 

SANS study of polymers in supercritical fluid and liquid solvents... 

Adidharma and Radosz provides an engineering form for such a copolymer SAFT approach. SAFT has successfully applied to correlate thermodynamic properties and phase behavior of pure liquid polymers and polymer solutions, including gas solubility and supercritical solutions by Radosz and coworkers Sadowski et al. applied SAFT to calculate solvent activities of polycarbonate solutions in various solvents and found that it may be necessary to refit the pure-component characteristic data of the polymer to some VLE-data of one binary polymer solution to calculate correct solvent activities, because otherwise demixing was calculated. GroB and Sadowski developed a Perturbed-Chain SAFT equation of state to improve for the chain behavior within the reference term to get better calculation results for the PVT - and VLE-behavior of polymer systems. McHugh and coworkers applied SAFT extensively to calculate the phase behavior of polymers in supercritical fluids, a comprehensive summary is given in the review by Kirby and McHugh. They also state that characteristic SAFT parameters for polymers from PVT-data lead to... 

Another important requirement for the development of new polymer processes based on SCCO2 is knowledge about the phase behavior of the mixture involved, which enables the process variables to be tuned properly to achieve maximum process efficiency. Determining parameters in the phase behavior of a system are the solvent quality, the molecular weight, chain branching, and chemical architecture of the polymer, as well as the effect of endgroups and the addition of a cosolvent or an antisolvent An overview of the available literature on the phase behavior of polymers in supercritical fluids has been published by Kirby and McHugh [50]. 

Modification of polymers in supercritical fluids is an attractive alternative for more conventional approaches such as solution modification and melt modification. Neither conventional technique is economically or ecologically attractive because of the hazardous waste in the form of organic solvents, together with left-over monomer(s) and initiator(s). Furthermore, much energy is required to remove the solvents at the end of the solution process, or undesired side reactions may occur at elevated temperatures necessary for melt modification. Modification of polymers in supercritical fluids is therefore an attractive alternative, since very mild reaction conditions can be applied, and side reactions are expected to be limited, if not avoided. After modification is completed, the solvent can be easily released by reducing the pressure, and non-reacted chemicals can be supercritically extracted with pure supercritical fluid. 

A number of equation of state theories have been used to model phase behavior of polymers in supercritical fluids. For example the lattice-fluid theory of Sanchez and Lacombe[4U 42] includes holes on the lattice in order to model compressibility. The lattice-fluid theory has been applied to model phase behavior of both homopolymers and copolymers in supercritical fluids[32, 38, 43, 44]. The statistical associating fluid theory (SAFT)[43,45-48] and corresponding state models[49] have also been employed to model compressible polymer-solvent mixtures. Figure 1 gives the pressure-concentration phase diagram for poly(dimethyI siloxane) in CO2 modeled with the lattice-fluid equation of state[50]. 

Polymer conformation is also a strong function of solvent quality. The conformation of polymers in supercritical CO2 has been studied with both X-ray[58] and small-angle neutron scattering[59]. The important connection between polymer conformation and phase behavior has been studied recently in Monte Carlo (MC) simulations of polymers in supercritical fluids[60-62]. A unique feature of these simulations is that the solvent is included explicitly to capture the effect of solvent compressibility on the phase behavior. 

M. A. Jacobs, Measurement and Modeling of Thermodynamic Properties for Processing of Polymers in Supercritical Fluids, PhD thesis, Technische Universiteit Findhoven, Findhoven, 2004. 

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