Phase separation of a polymer solution

Phase separation is another method that can be employed for the preparation of nanoscale sttucmres and is frequently used to prepare three-dimensional tissue-engineering scaffolds. Phase separation of a polymer solution can produce a polymer-rich domain and a solvent-rich domain and this morphology can be fixed by quenching under low temperamre conditions. 

Liquid—Liquid Phase Separation, in contrast to solid-liquid phase separation, lowering temperature can induce liquid-liquid phase separation of a polymer solution with an upper critical solution temperature and when the crystallization temperature of the solvent is sufficiently lower than the phase separation temperature. In an equilibrium phase diagram of a polymer solution, the spin-odal curve divides the liquid-liquid phase separation region into two regions a thermodynamically metastable region (between the binodal and spinodal) and a thermodynamically unstable region (enclosed by the spinodal) (Fig. 11). Above the... 

Phase inversion involves the phase separation (i.e., demixing) of a homogeneous polymer solution followed by the fixation of structure through the solidification of the polymer matrix. Two main approaches that induce the phase separation of a polymer solution are as follows ... 

Separation of a polymer solution into two phases may also be brought... 

Figure 9 Qualitative phase diagram of a polymer solution showing phase separation both on heating (at the lower critical solution temperature) and on cooling (at the upper critical solution temperature). (From Ref. 31.)...

A new technique in which SC CO2 is used to induce the phase separation of the polymer solution has been recently proposed by different authors. Kho et al. [67] used compressed CO2 for the formation of Nylon 6 membranes. The process was... 

For many years polymeric membranes have been utilized widely for material separation without detailed characterization of the pore size and the pore size distribution. Most of the commercially available membranes are prepared by either a dry or a wet phase-inversion process. These membranes are formed by the phase separation of multicomponent polymer-solvent systems, the underlying principle being phase separation of the polymer solution. 

Next we come to phase transitions. Chapter 14 mentions the various phase transitions that may occur, such as crystallization, gas bubble formation, or separation of a polymer solution in two layers it then treats the nucleation phenomena that often initiate phase transitions. Chapter 15 discusses crystallization, a complicated phase transition of great importance in foods. It includes sections on crystallization of water, sugars, and triacylglycerols. Chapter 16 introduces glass transitions and the various changes that can occur upon freezing of aqueous systems. 

Phenomenological Description of Phase Separation. Phase separation due to thermal gelation, evaporation of solvent and addition of nonsolvent can be illustrated with the aid of the phase diagram of a polymer solution. 

Fig. 4 Schematic phase diagrams of a polymer solution showing LL phase separation with UCST behavior. Curve s is the spinodal, curve b is the binodal, and curve g is the glass transition temperature as a function of polymer concentration. BP indicates the Berghmans point, (a) LL phase separation is the only thermodynamic transformation of the system [17,25, 36]. (b) Curve c shows the crystallization temperature of a polymer fully miscible in a solvent as a function of concentration in the solution [17, 25], The LL phase coexistence curve (combined with vitrification) is a (classical) metastable process that lies beneath the crystallization curve c. In route 1, a polymer solution is supercooled at ALj, and the only active process is polymer crystallization. In route 2, the initially homogeneous solution is supercooled to a larger undercooling than namely AL2. Crystallization may compete either with LL phase separation when reaching point C, or LL phase separation coupled with vitrification when reaching point D. At C, crystallization may take place in the polymer-rich phase. At D, both LL phase separation and crystallization may become arrested by vitrification...

Another widely used approach in this area is a sol-gel process. In order to create surface roughness after deposition of thin films, a secondary component is included in the sol-gel deposition process which can be removed later by dissolution in hot water or sublimation. The removal of the secondary components gives porous structures. Subsequent lluorinated silane coating can render these sol-gel processed films superhydrophobic [81-83]. Microporous structures can be created through phase separation of organic polymer solutions and then used as a template for sol-gel processing of porous silica substrates. Ruorosilane treatment of these substrates produces superhydrophobic surfaces [84]. 

Composites of chitosan and P-TCP with improved compressive modulus and strength have been prepared by a soUd-Uquid phase separation of the polymer solution and evaporation of the solvent [8]. The composites exhibited bioactivity when immersed in SBF. Variation of polymer/filler ratio and development of different macroporous structures resulted in products with potential applications in tissue engineering. 

Coacervation n. The separation of a polymer solution into two or more liquid phases, one of which is a polymer-rich liquid. The term was introduced to distinguish this phenomenon from the precipitation of a polymer solute in solid form. The process is used in microencapsulation by emulsifying or dispersing the material to be encapsulated with a solution of the polymer. By changing the temperature or concentration of the mixture, or by adding another polymer or solvent, a phase separation may be induced and the polymeric portion forms a thin coating on the external surfaces of the particles. After further treatment to solidify the polymeric wall, the capsules can be isolated in powder form by filtration. It is an intermediate stage between sol and gel formation. 

Fig. 2. (a) Schematic phase diagram of a polymer solution, at constant pressure, using temperature T and volume fraction 0 of the polymer as variables. Phase separation into a dilute or semidilute solution (with volume fraction 0e ex) a more concentrated solution (with volume fraction 0 x)occurs for states (T, 0) that fall in the two-phase region underneath the coexistence curve (binodal). The binodal ends in the critical point 0c(AO, TdN), where N is the degree of polymerization of the polymer. For iV - oo the critical point merges with the theta point of the dilute solution, ie, T = 0,0c(iV oo) = 0. For further explanations cf text,... 

If the solvent is sufficiently poor, the polymer will phase separate into a dense polymer liquid and a dilute polymer solution in the bulk. The bulk phase diagram of a polymer solution is also depicted in Figure 1. The macroscopic phase coexistence of the two polymer phases, which are distinguished by their densities 2ind p oL/ terminates at a critical point with critical... 

The above results show two compositions on a binodial line, which are connected by a tie line. In other words, these compositions are in equilibrium with each other when phase separation of the polymer solution takes place. 

There is a lot of literature so far on the study of hydro-phobic interaction of ethyl(hydroxyethyl)cellulose (EHEC) with various ionic surfactants in water. The techniques used in these studies are of electromotive force of N-tetradecylpyridinium bromide [1], of thermal gelation with sodium dodecylsulfate (SDS) [2], of Fourier transform NMR in the presence of dodecyltrimethylammonium bromide [3], of phase separation diagram with various kinds of anionic and cationic surfactants [4], and so on. These studies have focused on the complex formation between EHEC and these surfactants and on the concomitant phase separation of the polymer solution. 

Figure 9.8 (a) Phase diagram of a polymer solution showing the phase separation... 

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