Facilitated Structure Formation in Isoporous Block Copolymer Membranes upon Controlled Evaporation by Gas Flow

Abstract

The conventional fabrication of isoporous membranes via the evaporation-induced self-assembly of block copolymers in combination with non-solvent induced phase separation (SNIPS) is achieved under certain environmental conditions. In this study, we report a modification in the conventional fabrication process of (isoporous) flat sheet membranes in which the self-assembly of block copolymers is achieved by providing controlled evaporation conditions using gas flow and the process is introduced as gSNIPS. This fabrication approach can not only trigger and control the microphase separation but also provides isoporous structure formation in a much broader range of solution concentrations and casting parameters, as compared to fabrication under ambient, uncontrolled conditions. We systematically investigated the structure formation of the fabrication of integral asymmetric isoporous membranes by gSNIPS. A quantitative correlation between the evaporation conditions (causing solvent evaporation and temperature drop) and the self-assembly of block copolymers beginning from the top layer up to a certain depth, orientation of pores in the top layer and the substructure morphology has been discussed empirically.

Nonequilibrium Processes in Polymer Membrane Formation: Theory and Experiment

“…In most studies, the composition of the atmosphere (air) is not documented, although humidity can have a strong effect during the evaporation step. 311,343 As the evaporation of solvent is an endothermic process, the temperature may decrease significantly in thin films with an insufficient heat flow and heat capacity from the substrate. Therefore, temperature is changing across the film thickness (along the z-direction) and both, thermodynamics and chain relaxation time, are affected.…”

“…Although there are many experimental investigations on the preparation and characterization of polymer membranes with kinetically trapped structures, also from an experimental point of view there remain a number of open questions. In most studies, the composition of the atmosphere (air) is not documented, although humidity can have a strong effect during the evaporation step. , As the evaporation of solvent is an endothermic process, the temperature may decrease significantly in thin films with an insufficient heat flow and heat capacity from the substrate. Therefore, temperature is changing across the film thickness (along the z -direction) and both, thermodynamics and chain relaxation time, are affected.…”

“…This corresponds to the time region where a significant reduction of THF density is observed on the logarithmic time scale. Using a controlled flow of nitrogen above cast solutions of PS- block -P4VP, it was shown that the range of polymer density and the range of evaporation time can be significantly enlarged to lower polymer densities and shorter evaporation times compared to normal conditions, without affecting the pore size and order of the resulting membranes …”

How does Micro‐ and Macro‐Phase Separation of Block Copolymers Affect the Formation of Integral Asymmetric Isoporous Membranes? A Review on Effective Factors

“…Therefore, solvent evaporation can be facilitated using a forced convection by inducing air/gas flow, for example by casting the BCP solution in the fume hood. [54,154,155] This method can be also applied for a solvent-mixture containing THF, leading to a much more rapid structure formation, for which the isoporous structure remains stabilized for a longer time while keeping the pore size almost constant. [155] In general, solvent evaporation, prior to plunging the solutioncast film into non-solvent bath, induces micro-phase separation and influences the self-assembled BCP nanostructures morphology in addition to their orientation to the film surface.…”

Selective Swelling and Functionalization of Integral Asymmetric Isoporous Block Copolymer Membranes

“…In the last decade, the fabrication of SNIPS membranes both in flat sheet [ 43–47 ] and hollow fiber [ 25,48–54 ] geometry have been extensively explored. Empirical optimization of solvent mixture composition, polymer concentration, evaporation time, concentration of additives (if required), etc., are inevitable tasks of SNIPS membrane casting.…”