Critical phase behavior basics

Beyond the basics, there are a number of efforts underway to advance the role of critical phase behavior in chemical processing. A few of these are already making an impact, but most remain as concepts under development. We briefly summarize key elements of these and provide links that should facilitate a broader understanding on subjects of interest. This list does not aspire to be comprehensive. Rather, it reflects the author s biased impressions of subjects that might resonate with developers of chemical processes. It is hoped that this brief survey will improve with feedback from readers and the passage of time. 

Although investigated in lesser detail earlier, the science and technology of polymer blends had its emergence in the 1970 s. Many of the basic principles existed prior to that time e.g., Flory-Huggins thermodynamic principles as well as contributions by Guggenheim and Prigogine). Commercial blends existed for decades before, however the concept of miscibility, phase behavior, and the basic nature of polymer blends was not well understood or appreciated. An initial review of polymer blends [Bohn, 1968] listed only 12 miscible polymer pairs, some of which were minor variations in copolymer structure. The review also noted that UCST (upper critical solution... 

The first application we will discuss in this section is the influence of the pure component properties (basically the volumetric properties) on the phase behavior in a mixture of two homopolymers. A mixture for which all necessary information is available(7,10) is the mixture of Polystyrene(PS) with the Poly(vinyl methyl ether)(PVME). Scaling constants for PS are obtained from Reference 7 while scaling constants for PVME and binary parameters are obtained from Reference 10. In Figure 1 is shown the influence of the thermal expansion coefficient a of PS on the spinodal curve of the mixture of PS and PVME considered both monodisperse with molecular weight 51000. As shown in the Figure a small increase in the thermal expansion coefficient of PS of the order of 1 % enhances the compatibility of PS with PVME by raising the spinodal curve and the critical temperature by about 30 degrees. 

In the case of more water-soluble monomers and (amphiphilic) macromonomers, the Smith-Ewart [16] expression does not satisfactorily describe the particle nucleation. The HUFT [9,10] theory, however, satisfactorily describes the polymerization behavior or the particle nucleation of such unsaturated hydrophilic and amphiphilic monomers. The HUFT approach implies that primary particles are formed in the aqueous phase by precipitation of oligomer radicals above a critical chain length. The basic principals of the HUFT theory is that formation of primary particles will take place up to a point where the rate of formation of radicals in the aqueous phase is equal to the rate of disappearance of radicals by capture of radicals by particles already formed. Stabilization of primary particles in emulsifier-free emulsion polymerization may be achieved if the monomer (or macromonomer) contains surface active groups. Besides, the charged radical fragments of initiator increases the colloidal stability of the polymer particles. 

The first chapter by Levelt Sengers and the second chapter by Schneider and his coworkers introduce the basic concepts on supercritical fluids, fluid mixtures, and provide an overview of applications. Thermodynamics and phase equilibria in binary and ternary mixtures are treated in Chapter 3 by Gauter and Peters. Chapter 4 by Anisimov and Sengers describes the recent developments on crossover phenomena that attempt to bridge the gap between the behavior of fluids asymptotically close to the critical point with behavior away from criticality. 

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