Phase separation in polymers

Figure A2.5.27. The effective coexistence curve exponent P jj = d In v/d In i for a simple mixture N= 1) as a fimction of the temperature parameter i = t / (1 - t) calculated from crossover theory and compared with the corresponding curve from mean-field theory (i.e. from figure A2.5.15). Reproduced from [30], Povodyrev A A, Anisimov M A and Sengers J V 1999 Crossover Flory model for phase separation in polymer solutions Physica A 264 358, figure 3, by pennission of Elsevier Science.

Povodyrev A A, Anisimov M A and Sengers J V 1999 Crossover Fiery model for phase separation in polymer solutions Physica A 264 345-69... 

Figure 8.3b shows that phase separation in polymer mixtures results in two solution phases which are both dilute with respect to solute. Even the relatively more concentrated phase is only 10-20% by volume in polymer, while the more dilute phase is nearly pure solvent. The important thing to remember from both the theoretical and experimental curves of Fig. 8.3 is that both of the phases which separate contain some polymer. If it is the polymer-rich or precipitated phase that is subjected to further work-up, the method is called fractional precipitation. If the polymer-poor phase is the focus of attention, the method... 

An example of this case is a vinyl (A2 ) - divinyl (A4) polymerization. The assumption of an ideal polymerization means that we consider equal initial reactivities, absence of substitution effects, no intramolecular cycles in finite species, and no phase separation in polymer- and monomer-rich phases. These restrictions are so strong that it is almost impossible to give an actual example of a system exhibiting an ideal behavior. An A2 + A4 copolymerization with a very low concentration of A4 may exhibit a behavior that is close to the ideal one. But, in any case, the example developed in this section will show some of the characteristic features of network formation by a chainwise polymerization. 

Case Study 1 Flow-induced Phase Separation in Polymer Solutions... 

C. Rangel-Nafaile, A. Metzner, K. Wissbrun, Analysis of stress-induced phase separations in polymer solutions, Macromolecules, 17,1187 (1984). 

In this section we would like to deal with the kinetics of the liquid-liquid phase separation in polymer mixtures and the reverse phenomenon, the isothermal phase dissolution. Let us consider a blend which exhibits LCST behavior and which is initially in the one-phase region. If the temperature is raised setting the initially homogeneous system into the two-phase region then concentration fluctuations become unstable and phase separation starts. The driving force for this process is provided by the gradient of the chemical potential. The kinetics of phase dissolution, on the other hand, can be studied when phase-separated structures are transferred into the one-phase region below the LCST. 

In Ref. [120] the first time has been reported on flow-induced phase separation in polymer blends. When PS/PVME blends were exposed to shear or extensional flow at lower temperatures, 20 to 30 K below the equilibrium coexistence temperature, phase separation was observed in both flow regimes. As the authors suggest, the stress, rather than the deformation rate, appears to be the most important parameter in flow-induced phase separation. 

Our group was the first to report imaging with a diamond ATR accessory that provided a field of view of ca. 1 mm2 and the spatial resolution of ca. 15 pm without the use of an infrared microscope [18], The demonstration of the applicability of a diamond ATR accessory for FTIR imaging opened up a range of new opportunities in polymer research, from compaction of tablets [21-23] to studying phase separation in polymer blends subjected to supercritical fluids [24], This imaging approach was successfully utilised for the study of dissolution of tablets in aqueous solutions [25], We have also demonstrated macro... 

The microstructure of the multiphase media is often the product of phase transitions, e.g. (i) capillary condensation in the porous media, (ii) phase separation in polymer/polymer and polymer/solvent systems, (iii) nucleation and growth of bubbles in the porous media, (iv) solidification of the melt with a temporal three-phase microstructure (solid, melt, gas), and (v) dissolution, crystallization or precipitation. The subject of our interest is not only the topology of the resulting microstructured media, but also the dynamics of its evolution involving the formation and/or growth of new phases. 

---