Phase Separation of Polymer Solutions

By far the majority of polymeric membranes, including UF membranes and porous supports for RO, NF or PV composite membranes, are produced via phase separation. The TIPS process is typically used to prepare membranes with a macroporous barrier, that is, for MF, or as support for liquid membranes and as gas-liquid contactors. In technical manufacturing, the NIPS process is most frequently applied, and membranes with anisotropic cross-section are obtained. Often, 

Solvent/nonsolvent system. The solvent must be miscible with the nonsolvent (here an aqueous system). An aprotic polar solvent like N-methyl pyrrolidone (NMP), dimethyl formamide (DMF), dimethyl acetamide (DMAc) or dimethylsulfoxide (DMSO) is preferable for rapid precipitation (instantaneous demixing) upon immersion in the nonsolvent water. As a consequence, a high porosity anisotropic 

Exposure time of proto-membrane before precipitation. The effect of exposure to atmosphere before immersion is dependent on the solvent property (e.g., volatility, water absorption) and atmosphere property (e.g., temperature, humidity). This step (i.e., combination of EIPS or VIPS with NIPS cf. above) has significant effects on the characteristics of the skin layer and the degree of anisotropy of the resulting membrane [14]. 

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For preparative purposes batch fractionation is often employed. Although fractional crystallization may be included in a list of batch fractionation methods, we shall consider only those methods based on the phase separation of polymer solutions fractional precipitation and coacervate extraction. The general principles for these methods were presented in the last section. In this section we shall develop these ideas more fully with the objective of obtaining a more narrow distribution of molecular weights from a polydisperse system. Note that the final product of fractionation still contains a distribution of chain lengths however, the ratio M /M is smaller than for the unfractionated sample. 

The phase transitions, such as a phase separation of polymer solutions, a sol-gel transition, or a volume phase transition of gels, are always accompanied by conformation changes of polymers. Therefore, when the phase transitions are induced isothermally by external stimulation, the transitions cause efficient conformation changes. This contribution describes how such efficient stimuli-responsive polymer systems can be constructed. 

Emmerik, P. T. van Smolders, C. A., "Phase Separation of Polymer Solutions. The Calculation of the Cloudpoint curve with a Concentration and Temperature-Dependent Free Energy Correction Parameter," Eur. Polym. J., 9, 157 (1973). 

The formation of lyophilic colloidal systems may also take place via phase separation of polymer solutions. A typical phenomenon that occurs upon phase separation is the formation of the so-called coacervates, which are characteristic nuclei containing higher concentration of polymer, as compared to that in the medium surrounding them. It is speculated that coacervation was a second stage (after the formation of adsorption layers) in the ordered structuring of organic matter on Earth. 

There are several ways to prepare porous polymer scaffolds by controlled phase separation of polymer solutions in a polymer-rich phase and a polymer-poor phase. 

The Flory-Huggins lattice consideration of the polyelectrolyte solutions presented above incorrectly describes dilute polyelertrolyte solutions. In the Flory-Huggins approach, the monomers are uniformly distributed over the whole volrrme of the system, leading to underestimation of the effect of the short-range monomer-monomer interactions and of the intra-chain electrostatic interactions. A similar problem appears in the Flory-Huggins theory of phase separation of polymer solutions (see for discussion References 32 and 33). This leads to the incorrect expression for the low polymer density branch of the phase diagram. 

As an alternative approach, supercritical CO2 has recently been used as a nonsolvent to induce phase separation of polymer solution for the purpose of obtaining dry membranes [33-35]. Supercritical CO2 is nontoxic, nonflammable and of low cost. Supercritical CO2 can accelerate the drying process of membrane surfaces without collapsing the pore structures due to the absence of liquid-vapor interfaces. Thus, no additional post-treatment is needed for obtaining the dry membranes. This method can also be considered as environmentally friendly because the solvent that initially exists in the polymer solution can be easily recovered in the supercritical CO2 phase after the exchange of solvent and CO2. Although membranes prepared via this method showed porous and macrovoid-free structure, the obtained membrane pore sizes are relatively large, between 1 and 10 pm. 

The use of supercritical CO (SC CO ) as a nonsolvent to induce phase separation of polymer solution for the purpose of dry membranes has the following advantages in comparison with the traditional wet phase inversion method ... 

Now that you have the chemical potential, you can follow the same procedures you use for the lattice model of simple solutions to predict the colligative properties and phase separations of polymer solutions. 

An et al. 1997, Pressure dependence of the miscibility of poly(vinyl methyl ether) and pwlystyrene Theoretical representation, / Ghent. Phys. Vol.107, No.7, PP. 2597-2602. An Wolf, 1998, Combined effects of pressure and shear on the phase separation of polymer solutions, Macromolecuks, Vol.31, No. 14, PP. 4621-4625. 

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.

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

Rangel-Nafaile, C Metzner, A. B. Wissbrun, K. F., "Analysis of Stress-Induced Phase Separations in Polymer Solutions," Macromolecules, 17, 1187 (1984). 

First, we consider a mono-dispersed polystyrene of molecular weight M containing pendant azobenzene groups. When these groups are in the trans form, the polymer solution phase separates at t, which corresponds to Tg of Fig. 29. The isomerization of the chromophores from the trans to the cis form causes the phase separation temperature to rise to t which corresponds to Tg of Fig. This means that phase separation of the solution is induced between t, and t by ultraviolet irradiation, which causes the trans-cis isomerization. 

Further evidence to support the above hypothesis on the role of structure in phase separation of aqueous solutions is provided by the effect of additional alcohol. The amount of alcohol was increased from 3.0 to 5.0 gm/dl, the surfactant concentration kept constant, and the salinity varied. The addition of alcohol extended the range of salinity where the aqueous solutions are isotropic to 0.8 gm/dl NaCl. According to the above hypothesis, no phase separation should take place on addition of polymer to the isotropic solutions existing up to 0.8 gm/dl NaCl. Indeed, no phase separation was observed when as much as 1500 ppm Xanthan was added at such compositions. Thus, the addition of alcohol increases the critical electrolyte concentration for phase separation, an effect seen also by others (9). 

The phase diagram of polymer solutions is shown in Fig. 5.1, assuming the usual case of5>0in Eq. (4.31) (with x — BjT i decreasing function of temperature . In the poor solvent half of the diagram (at temperatures below 6) the binodal separates the two-phase region from the two singlephase regions. 

In this paper the discussion was concentrated mainly on the phase separation in polymer solutions due to the change of the composition of the mixture. The basic relations are also valid for phase separations induced by temperature changes, that is thermal gelation and can be applied to glass and metal alloys as well as to polymers. 

Tlhis study was made to reconcile the behavior of low molecular weight hydrocarbon resins and the behavior of their plasticizers with the solubility parameter and with the Flory-Huggins treatment of phase separation from polymer solutions. These resins are widely used industrially for coatings, floorings, adhesives, rubber compounds, and many other applications. Since they are usually hard and brittle, they are used with rubber, drying oils, plastics, or with plasticizers. 

Dodecyldimethyl-ammoniumethyl-methacrylate bromide Micellar Phase separation of polymer from the micellar solution >95% conversion [57]... 

In principle, this ratio can also affect the IP concentration profile in a quahta-tive sense. When considering a homogeneous epoxy-amine mixture just brought into contact with the surface of an adherend or a fiUer particle, preferential adsorption of the amine molecules will result in local variations of the amine/epoxy concentration ratio r (to be defined below). Assuming a comparably fast diffusion of the amine molecules, their quick enrichment at the interface will result in a near-interface zone of increased r values and an adjacent zone of amine depletion, i.e., with reduced r values. Similar concentration variations are dealt with in the field of surface-driven phase separation in polymer solutions and mixtures [8]. While the wetting layer is in local equihbrium with the depletion layer, the diffusion from the bulk down the concentration gradient into the latter feeds the growth of the former. 

The mechanism of stabilization of polymeric oil-in-oil emulsions consists of the following 1) Incompatibility of polymers causes the phenomenon of phase separation of polymers in solution, 2) this causes the force which drives the graft copolymer into the interface of polymeric oil-in-oil emulsions, and 3) this causes the formation of coalescence barriers. 

The phase separation of the solution is driven by the entropy of forming a liquid-crystalline phase at a high polymer concentration. However, Maier and Saupe [30] concluded that the formation of a nematic phase arises from orientation-dependent interaction of induced dipoles. From these considerations, it appears that both entropy and enthalpy contribute to the stability of mesophases, although in a different ratio for large and small molecules. Asymmetric at-... 

A number of experimental methods have been developed to determine polymer compatibility e.g., observation of phase separation in polymer solutions, mixed polymer solutions viscosity, and light... 

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