Polymer-rich phase

Phase Inversion (Solution Precipitation). Phase inversion, also known as solution precipitation or polymer precipitation, is the most important asymmetric membrane preparation method. In this process, a clear polymer solution is precipitated into two phases a soHd polymer-rich phase that forms the matrix of the membrane, and a Hquid polymer-poor phase that forms the membrane pores. If precipitation is rapid, the pore-forming Hquid droplets tend to be small and the membranes formed are markedly asymmetric. If precipitation is slow, the pore-forming Hquid droplets tend to agglomerate while the casting solution is stiU fluid, so that the final pores are relatively large and the membrane stmcture is more symmetrical. Polymer precipitation from a solution can be achieved in several ways, such as cooling, solvent evaporation, precipitation by immersion in water, or imbibition of... 

Macroscopically, the solvent and precipitant are no longer discontinuous at the polymer surface, but diffuse through it. The polymer film is a continuum with a surface rich in precipitant and poor in solvent. Microscopically, as the precipitant concentration increases, the polymer solution separates into two interspersed Hquid phases one rich in polymer and the other poor. The polymer concentration must be high enough to allow a continuous polymer-rich phase but not so high as to preclude a continuous polymer-poor phase. 

The thud step gives a polymer-rich phase forming the membrane, and a polymer-depleted phase forming the pores. The ultimate membrane structure results as a combination of phase separation and mass transfer, variation of the production conditions giving membranes with different separation characteristics. Most MF membranes have a systematic pore structure, and they can have porosity as high as 80%.11,12Figure 16.6 shows an atomic force microscope... 

The characteristic features of phase equilibria in polymer-solvent mixtures will be examined in the present section, the discussion being confined to systems having both phases completely liquid. Equilibria involving a polymer-rich phase in which the polymer is semicrystalline will be the subject of the following section. 

Tg (-22 °C) of a homogeneous 70/30 PNIPAM-water mixture. Observation of samples by scanning electron microscopy and optical microscopy revealed that the morphology of the polymer-rich phase is preserved only if the polymer solutions are brought to zone C. Polymer solutions heated to zone B undergo demixing upon quench-cooling [160]. Aqueous solutions of PVCL, PNIPMAM, and PNIPMA exhibit similar behaviour [157,158,369,370]. 

Partial vitrification may affect kinetic processes during the coil-globule transition. Thus, at very high dilution, macroscopic phase separation well above the LCST might be stopped by partial vitrification of the polymer-rich phase. At this point we can only speculate whether vitrification interferes with the coil-globule transition or not. This problem is open for discussion and needs experimental confirmation. 

Let us now consider a system composed of a polymer and a solvent. For compositions in between the inflection points, solvent molecules will diffuse into the solvent-rich phase, and the polymer molecules diffuse in the polymer-rich phase. Thus diffusion occurs against a concentration gradient. Therefore, this type of phase separation is known as up-hill diffusion. The up-hill diffusion leads to a spontaneous decomposition and it is therefore also named spinodal decomposition. The formation of two phases via spinodal decomposition occurs immediately upon reaching the spinodal decomposition region and does not require any activation energy. 

Graphically, the conditions for thermodynamic equilibrium are equal to two points which have a common tangent. These points give the composition of a polymer-rich phase (I) and a solvent-rich phase (II) that can coexist in thermodynamic equilibrium. The summation of such points is also called the coexistence curve or binodal line. 

The phase behavior of the polymers is also dependent on the type and concentration of salt present. Many times a sufficiently high concentration of salt in a single polymer solution can induce phase separation to form one salt-rich and one polymer-rich phase. Sodium and potassium phosphate are commonly used salts. 

It is interesting to frame these very tentative considerations in terms of rod diffusion, since this is the process by which the polymer-rich phase must be formed. However, care must be taken to isolate the effects of mutual diffusion of the collection of rods as a (phenomenological) response to a concentration (chemical potential) gradient and simple self diffusion of a single rod, which is the case treated by Doi and Edwards.(24)... 

Process in which a precipitant is added incrementally to a highly dilute polymer solution and the intensity of light scattered by, or the turbidity due to, the finely dispersed particles of the polymer-rich phase is measured as a function of the amount of precipitant added. 

Process in which a polymeric material, consisting of macromolecules differing in some characteristic affecting their solubility, is separated from a polymer-rich phase into fractions by successively increasing the solution power of the solvent, resulting in the repeated formation of a two phase system in which the more soluble components concentrate in the polymer-poor phase. 

In essence, the model divides a reactive polymer solution into a dispersed polymer-rich phase (phase 1), within which the concentration of functional groups is defined by the polymer morphology and structure, and a solvent-rich phase which contains no functional groups (phase 0). The individual polymer molecules are modeled as spheres of polymer-rich phase stuck at points of an imaginary lattice in solution. If the polymer concentration is sufficiently high, another phase enters the calculations which consists of overlapping polymer-rich spheres (phase 2). 

Within either of the polymer-rich phases, borate esters and diesters of the functional groups are assumed to form with the same association constants as observed for the independent functional groups in aqueous solution. The resulting equations simply describe the borate ester association constants as well as mass balances on boron and polymer-bound functional groups. Wise and Weber used the model to estimate association constants for the borate esters formed with the diols in PVA and to predict the gelation of PVA-borate solutions. As we have independently measured the association constants for the borate esters formed in this work, we have used the model to estimate the radius of gyration of the GP3 dendrimer and to predict both the boron speciation in borate/GP3 solutions and the efficacy of PAUF using these functional dendrimers. 

The only revision of the model which has been incorporated here is the formal description of the functional group concentration in the polymer rich phases. In our work with the dendrimer, nominally containing 128 terminal functional groups, we calculated the total ligand concentration within the polymer phase 1 to be... 

The thin lines, the so-called co-existence curves, give the composition of the ethylene-rich phase (Fig. 5.1-2, left) and that of the polymer-rich phase (Fig. 5.1-2, right) for different total concentrations of polymer. With decreasing pressure the concentration of polymer in the ethylene-rich phase decreases, and increases in the polymer-rich phase. 

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