Demixing doping

It should be noted that the demixing of doped SnO during high-temperature annealing, which is accompanied by an increase in the concentration of the second phase, is a common phenomenon for metal oxides (Nowotny 1988). Usually it is stated that this effect takes place at 800-900 °C. However, on the basis of results of the annealing influence on the lattice parameters of doped SnO... 

Macrovoids formation can be avoided by delayed demixing, and increasing polymer and/or nonsolvent concentration in the polymeric dope. Other studies suggested to introduce solvent into the coagulation bath [21-23], to increase solvent evaporation time [24], to work with a S-NS pair with low miscibility [25] or to use organic additives such as polyvinylpyrrolidone (PVP) [26,27]. 

In the fabrication of microporous hollow-fiber membrane, the development of membrane pores can be correlated to thermodynamic and kinetic effects of the polymer dope. When additives were introduced into the polymer dope, two significant effects transpired. First, instantaneous demixing would occur, which was due to the thermodynamic enhancement of phase separation by reducing the miscibility of the solution dope with the nonsolvent. Second, the increased viscosity of the solution would cause kinetic hindrance against the phase separation process, which resulted in a delay of solution demixing [45]. 

In a thermodynamic context, instantaneous demixing is likely to take place when the initial composition of the homogeneous polymer dope is relatively close to its metastable or unstable state. This is because a little amount of nonsolvent is sufficient to induce phase separation. In addition to thermodynamics, the kinetics of membrane formation, which often refers to the rate of solvent-nonsolvent exchange during the phase inversion, is observed to govern the final membrane structure in many cases [14,15]. If the solvent-nonsolvent exchange rate is slow, delayed danix-ing will occur even when a homogeneous system is thermodynamically close to its metastable or unstable state and vice versa. 

Water is a strong nonsolvent to many polymer solutions, but it is possible to prepare a homogeneous polymer dope containing a small amount of water. Presence of water brings the dope closer to its metastable or unstable state, and thus facilitates instantaneous demixing during membrane formation. However, when the amount of water... 

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