Polymers are materials made of long, repeating chains of molecules. Credit to their nature, polymers can form porous architectures that possess attractive properties; such as, rich pore geometries, tunable pore sizes, and facile functionalization. As such, they have aroused tremendous attention over past decades. In particular, porous polymers offer various kinds of ordered structures and customizable properties. Recent research has revealed that block copolymers (BCPs) are important precursors to porous materials with well-defined, sub-100 nm structures. Among the existing pore-making strategies, the newly emerged selective swelling-induced pore formation is distinguished because it provides extremely convenient and nondestructive access to reversible nanopores with a variety of pore geometries. By this approach, uniform pores with ordered morphologies can be created throughout the entire volume of the BCPs, regardless of whether they are in the form of nanoscopic fibers, thin films or bulk materials including thick films and monoliths. In general, nanoporous materials derived from block copolymers with rich morphological diversity and designable functionality, have attracted extensive interest in membrane separation, drug delivery, and catalysis. Unfortunately, it is still challenging to realize the cavitation of dense BCPs through a fast and cost-effective strategy.
Overall, from a large-scale manufacturing stand point, it is desirable that a fast and efficient strategy to enable the selective swelling of BCPs be developed, so as to make possible the continuous production of nano-porous BCPs. In this view, the research group of Yong Wang at Nanjing Tech University in China developed a fast, convenient, and cost-effective strategy, namely, microwave-boosted selective swelling (MBSS), for the preparation of BCP-based nanoporous materials. Their work is currently published in the research journal, Macromolecules.
Specifically, the researchers proposed a microwave-boosted selective swelling that could introduce prominent porosities into BCPs within 30 s. To realize this, various empirical tests were implemented followed by characterization of the resultant samples. To be precise, SEM was used to observe surface and cross-sectional morphologies. Also, water permeance and rejection tests were conducted on thus-produced membranes.
The authors reported that when applying the microwave heating on swelling solvents, the dipole rotation of polar molecules lead to a drastic collision that gave a quick temperature increase. Besides, the furious rotation was seen to enable a fast diffusion of swelling solvents into BCPs, and also accelerated the selective swelling of BCPs, achieving a pronounced volume expansion to give nano-porosities in seconds.
In summary, the study developed a MBSS to rapidly introduce nanopores into amphiphilic BCPs within 30 s. The microwave heating on swelling agents was reported to facilitate the interaction between polymer chains and solvent molecules, leading to an extremely fast volume expansion of the dispersion phase. This strategy is greatly universal, capable of cavitating not only polystyrene-based BCPs but also highly rigid polysulfone-based BCPs. Very importantly, the microwave-based approach allows extremely facile preparation of BCP-based nanoporous materials, which is essential for their mass production and real-world applications.
Xiansong Shi, Xingyuan Wang, Yanjie Wang, and Yong Wang. Producing Nanoporosities in Block Copolymers within 30 s by Microwave-Boosted Selective Swelling. Macromolecules 2020, volume 53, page 3619−3626.