Porous polyimides have been considered
to be a promising material
class for gas capture and sequestration, leading to the synthesis
of a substantial number of individual networks with noteworthy sorption
properties. In spite of these efforts, the vision of a chemical control
of adsorption and desorption of small molecules, in particular, for
the competing uptake of technical relevant gas mixtures, is still
hardly investigated. Here, we present a systematic study of five new
polyimide networks based on a set of linkers with chemical functionalities
covering the full range from hydrophobic to hydrophilic interactions.
The corresponding microporous organic polyimides (MOPI-I to -V) were
synthesized successfully based on a condensation reaction between
amino and anhydride linker molecules in m-cresol
at high temperatures, resulting in cross-linking degrees beyond 95%
in all cases. Argon and carbon dioxide isotherms reveal surface areas
up to 940 m2/g with ultramicroporosity, about 50% microporosity
and high thermal stabilities under air with decomposition temperatures
up to 480 °C. Sorption screening for variable temperatures revealed
remarkable uptakes for carbon dioxide up to 3.8 mmol/g and water vapor
up to 19.5 mmol/g combined with a smooth gate opening around 0.25 p/p
0 for MOPI-IV. In contrast,
for MOPI-V the water vapor uptake decreases down to 7 mmol/g. Interestingly,
the trend of the selectivities calculated by IAST and Henry does not
correlate with the uptake behavior. For instance, MOPI-I and MOPI-III
exhibit with 78 and 13 the highest CO2 over N2 and CH4 Henry selectivities, although their CO2 uptake is around 3.0 mmol/g. In total, we attribute the sorption
properties for this class of materials mainly to the void size and
shape within the ultramicroporous region. The chemical environment
of the surfaces seems to have little influence on the uptake and a
stronger effect on the separation behavior.
Covalent organic frameworks (COFs) show advantageous characteristics, such as an ordered pore structure and a large surface area for gas storage and separation, energy storage, catalysis, and molecular separation. However, COFs usually exist as difficult-to-process powders, and preparing continuous, robust, flexible, foldable, and rollable COF membranes is still a challenge. Herein, such COF membranes with fiber morphology for the first time prepared via a newly introduced template-assisted framework process are reported. This method uses electrospun porous polymer membranes as a sacrificial large dimension template for making self-standing COF membranes. The porous COF fiber membranes, besides having high crystallinity, also show a large surface area (1153 m 2 g −1 ), good mechanical stability, excellent thermal stability, and flexibility. This study opens up the possibility of preparation of large dimension COF membranes and their derivatives in a simple way and hence shows promise in technical applications in separation, catalysis, and energy in the future.
GIWAXS measurements confirm end-on alignment for TPD-copolymers obtained from Stille polycondensation using fluorinated and non-fluorinated comonomers.
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