Self-assembly of block copolymers during hollow fiber spinning: anin situsmall-angle X-ray scattering study

Abstract

We investigated the self-assembly of block copolymers during hollow fiber membrane (HFM) fabrication by conducting in situ small angle X-ray scattering (SAXS) and ex situ scanning electron microscopy (SEM) studies. SAXS enables us to follow the structural rearrangements after extrusion at different distances from the spinning nozzle. The kinetics of the spinning process is examined as a function of the composition of block copolymer solutions and the spinning parameters. We studied the influence of the extrusion rate on the block copolymer microdomains and their self-assembly in weakly segregated and ordered solutions. The addition of magnesium acetate (MgAc2) leads to ordering of micelles in the block copolymer solution already at lower polymer concentrations and shows an increased number of micelles with larger domain spacing as compared to the pristine solution. The SAXS data show the effect of shear within the spinneret on the self-assembly of block copolymers and the kinetics of phase separation after extrusion.It is observed that the ordering of micelles in solutions is decreased as indicated by the loss of crystallinity while high extrusion rates orient the structures perpendicular to the fiber direction. The structural features obtained from in situ SAXS experiments are correlated to the structure in the block copolymer solutions in absence of shear and the morphologies in flat sheet and HF membranes obtained by ex situ SEM. This allows a systematic and comparative study of the effects varying the microdomain ordering within different block copolymer solutions and the formed membrane structures. KEYWORDS:block copolymer, self-assembly, in situ SAXS, isoporous membrane, isoporous hollow fiber membrane Highlight of work:Study of the self-assembly of block copolymers in ordered and weakly segregated solutions after extrusion during fabrication of isoporous hollow fiber membranes. Graphical Abstract/TOC

Kinetic state diagrams for a highly asymmetric block copolymer assembled in solution

“…Consequently, it is common to study the self-assembly mechanisms using in situ or ex situ analytical approaches that can resolve morphology evolution during self-assembly. [30][31][32][33] While in situ and time resolved ex situ studies provide great insight into the mechanism of self-assembly, they are typically only applied to a small number of assembly pathways, as their application is time-consuming and can require high-end instrumentation. Here we investigate the assembly of a kinetically controlled block copolymer using a kinetic state diagram approach.…”

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

“…Interestingly, as shown in Figure 7, for 18 wt% we observed the influence of gas flow not only as inhomogeneous structures on the top structure but also in the substructure near the top surface. The hexagonal packing of P4VP domains in the direction of flow can be observed in Figure 7a''. A similar coalescence of spherical micelles into cylinders and their hexagonal packing in the direction of flow was also observed due to shear during isoporous hollow fiber membrane fabrication [39]. This limits the gSNIPS method for solutions having lower polymer concentration.…”

“…The increase in rate and time of evaporation moves the segregation of microdomains in a certain depth of the substructure. The cylindrical progression of pores in a micellar solution having a spherical initial state is the result of coalescence of spheres layer by layer during solvent evaporation [39]. This phenomenon can also be facilitated by increasing the segregation strength, e.g., by selective swelling of pore-forming block by varying solvent(s) [27], by introducing additives to the pore-forming block [29,30,39] or by applying an external field [37].…”

“…Experimentally, many approaches like blending or additives in the casting solution [29][30][31][32][33][34][35][36], solution compositions [27], an electric field on as-cast polymer film [37], shear during fabrication [38,39], temperature [40], evaporation rate [41] and humidity [42] have been reported to influence the long-range order and the orientation of self-assembled nanostructures. Solvent evaporation has proven to be a remarkably effective tool for directing the self-assembled nanostructures in the block copolymer thin films.…”

New Insights into Water Treatment Materials with Chemically Sensitive Soft and Tender X-rays

“…Previous work has employed hard X-ray small angle scattering to track the in situ morphology evolution of solution-cast block copolymer membranes [41][42][43][44] (Figure 5a) and other functional polymers [45]. Non-solvent induced phase separation in combination with block copolymer self-assembly produces integral asymmetric membranes (IAMs) that have a selective, nanoporous thin layer atop a more porous, less ordered sponge-like layer [46].…”