Membrane Casting by Phase Inversion: Solution Preparation and Fabrication Guide

Membrane Casting by Phase Inversion: Solution Preparation and Fabrication Guide

Phase inversion is the most widely used technique for fabricating polymeric membranes, from microfiltration to ultrafiltration and the support layers of RO/NF thin film composite membranes. Understanding the casting process, solution preparation, and the factors that control membrane structure is essential for membrane researchers developing new membrane materials.

What Is Phase Inversion?

Phase inversion (also called phase separation) transforms a homogeneous polymer solution into a solid membrane with a controlled pore structure. The process involves inducing thermodynamic instability in the polymer solution, causing it to separate into a polymer-rich phase (which forms the membrane matrix) and a polymer-lean phase (which forms the pores).

Types of Phase Inversion

Non-Solvent Induced Phase Separation (NIPS)

The most common method. A cast polymer film is immersed in a non-solvent (typically water) bath. The exchange of solvent out of the film and non-solvent into the film causes rapid phase separation.

• Produces asymmetric membranes with a dense skin layer and porous sublayer

• Skin layer pore size controlled by polymer concentration and casting conditions

• Sublayer structure: finger-like macrovoids or sponge-like structure depending on formulation

• Standard method for PES, PSf, PVDF, CA, PAN, and PEI membranes

Thermally Induced Phase Separation (TIPS)

• Polymer dissolved in a high-boiling-point solvent at elevated temperature

• Cooling causes phase separation as solubility decreases

• Produces symmetric, sponge-like pore structures

• Used for PE, PP, and PVDF hollow fiber and flat sheet membranes

Vapor Induced Phase Separation (VIPS)

• Cast film exposed to humid air before immersion in non-solvent bath

• Water vapor absorption initiates slow phase separation from the top surface

• Produces membranes with larger surface pores and more open structure

• Often combined with NIPS (VIPS + NIPS) for membrane structure control

Solution Preparation (NIPS)

• Polymer selection: PES (typical 15-20 wt%), PVDF (12-18 wt%), PSf (15-22 wt%) are most common

• Solvent: NMP, DMF, DMAc, or DMSO. NMP is the most common for PES and PVDF

• Additives: PVP or PEG (2-5 wt%) as pore former and hydrophilizer; LiCl (1-3 wt%) to increase solution viscosity and suppress macrovoid formation

• Dissolution: Mix at 50-70°C for 12-24 hours with mechanical stirring until completely homogeneous

• Degassing: Remove air bubbles by standing or vacuum degassing for 4-12 hours before casting

Casting Procedure

• Substrate: Glass plate or non-woven polyester fabric (for supported membranes)

• Casting knife: Doctor blade with adjustable gap (50-300 μm typical)

• Casting speed: Manual or motorized; consistent speed ensures uniform thickness

• Exposure time: 0-60 seconds in air before immersion (controls skin layer density)

• Coagulation bath: DI water at controlled temperature (20-60°C)

• Post-treatment: Soak in fresh DI water for 24 hours to remove residual solvent; store in water or dry

Tech Inc. manufactures membrane casting equipment including automated casting machines, doctor blades, coagulation baths, and flat sheet test cells for evaluating the performance of lab-cast membranes. Our casting systems ensure reproducible membrane fabrication with precise control of casting thickness and speed.

Frequently Asked Questions

How do I control membrane pore size in NIPS?

Pore size is primarily controlled by polymer concentration (higher concentration = smaller pores), solvent/non-solvent system, additive type and concentration, coagulation bath composition and temperature, and air exposure time before immersion.

Why do my membranes have macrovoids?

Macrovoids (finger-like voids) form due to rapid demixing when there is a large thermodynamic driving force for phase separation. To suppress macrovoids: increase polymer concentration, add viscosity enhancers (LiCl), use a weaker non-solvent in the bath, increase bath temperature, or increase air exposure time before immersion.