Fabrication processes of membranes by phase separation

The majority of porous polymer membranes on the market are manufactured using a phase separation process. Phase separation (also known as phase inversion or demixing) results from a change in the thermodynamic state of an initially homogeneous polymer solution. The thermodynamic change of state can be induced by various methods:

• temperature variation ;

• differential evaporation of the solvent in a polymer/solvent/non-solvent ternary solution ;

• non-solvent intrusion into a polymer/solvent binary solution.

Phase inversion induces the creation of two phases: a polymer-poor phase and a polymer-rich phase, which grow by nucleation-growth or spinodal decomposition mechanisms to form the membrane architecture. After liquid-liquid demixing, the polymer-rich phase solidifies to form the membrane matrix. The lean phase is removed by successive washings, leaving the membrane pores.

The properties and performance of a membrane are closely linked to its morphology. The main objective of membrane manufacturers is therefore to control the texturing processes to give them ultra-, micro- or nanofiltration properties, while ensuring good mechanical, thermal and chemical resistance properties, or even to develop membranes with a surface layer offering properties suitable for reverse osmosis, gas separation or pervaporation . In this context, mastering the processes at the origin of phase separation, and in particular transfer phenomena, is crucial. This requires a detailed understanding of transport mechanisms in a polymer matrix. At this stage, a modeling approach can be useful, firstly to formalize the various coupled phenomena involved (thermodynamics, transfer, transport), and secondly to predict composition paths as a function of the chosen operating conditions. In addition, knowledge of the process parameters that will significantly influence membrane structuring during industrial production completes this cognitive approach.

This folder presents :

• the principles inherent in each phase separation process (thermodynamic aspects, implementation) ;

• industrial applications ;

• modeling approach.