Conspectus
Porous organic polymers (POPs),
essentially including polymers
with intrinsic microporosity (PIMs), conjugated microporous polymers
(CMPs), covalent organic frameworks (COFs), hyper-cross-linked polymers
(HCPs) and so on, have recently attracted broad interest in many application
areas because of their structural diversity and functional tunability.
However, except for linear PIMs that can dissolve in organic solvents
for solution processing into membranes, most POPs are highly cross-linked
(hereafter termed CPOPs) and are synthesized as insoluble and unprocessable
powders, which prevent CPOPs in many applications. Developing methodologies
for solution processing CPOPs to high-quality membranes, monoliths,
and (aero)gels has been a major challenge in this field because of
the following issues. First, the inherently cross-linked structures
and the strong framework–framework interactions in CPOPs give
rise to very weak solvation of the frameworks, leading to easy aggregation
and precipitation in solutions. Next, to date, several methods for
preparing CPOP membranes have been proposed, but their conditions
vary with different systems, and there lacks a general strategy for
membrane formation of most CPOPs (or at least CPOPs of the same category).
Additionally, CPOP-based monoliths and (aero)gels are rarely reported,
and it has been considered difficult to control the hierarchical porosity
to form the monoliths and (aero)gels during the CPOP syntheses. Last,
the effects of the forms of membranes/(aero)gels on the transport
(electron, ion, and mass) properties have not been intensively investigated
for the lack of suitable systems. Therefore, since it was first announced
accompanied by the birth of CPOPs, research studies regarding solution-processed
CPOPs have been underexplored for a long time without significant
advances being achieved.
To break the unprocessable shackles
of CPOPs, our group started
to make contributions to this field in 2018. We developed two general
strategies, namely, “charge-induced dispersion (CID)”
and “thermal hyper-cross-linking (THC)” strategies,
to produce high-quality CPOP membranes and (aero)gels, respectively.
For the CID strategy, we found that the introduction of plenty of
charges to the frameworks of CPOPs substantially enhanced their interactions
with polar solvents, rendering the transparent, stable, and solution-like
CPOP sols which could be further processed into membranes. For the
THC strategy, we intensively investigated the gelation mechanism and
found that this system was synthetically controllable to produce CPOP
(aero)gels and could serve as a platform for hybridization with many
porous materials to achieve a molecular-level entanglement. Moreover,
we successfully demonstrated that the transport properties in the
CPOP membranes and gels were largely promoted by 1–2 orders
of magnitude compared to their powder forms, thereby expanding the
use of CPOP membranes and gels in the fields of electronic conduction,
proton conduction, iodine adsorption, and molecul...
