Microporous polyimides with high surface area and CO2 selectivity fabricated from cross-linkable linear polyimides.

Linear polyimides of intrinsic microporosity have been intensively investigated for gas separation due to their microporous structure and high surface area. The microporous structure in the linear polyimides of intrinsic microporosity comes from their contorted structure. Therefore, most linear polyimides without contorted structure do not have micropores. In this work, the microporous polyimides are constructed through the condensation of a cross-linkable dianhydride monomer with two novel nitrogen-rich diamine monomers and post crosslinking reaction. The linear polyimide precursors without contorted structure have the same main-chain structure. The introduction of crosslinked structure endow the crosslinked polyimides (PI-CLs) with microporous structure. The microporous structure in PI-CLs can be tuned by changing the substituents of the linear polyimide precursors. The PI-CLs have competitive CO2 uptake capacity (7.3-9.4 wt%) at 273 K and 1 bar. Particularly, the crosslinked polyimide containing trifluoromethyl groups (CF3-PI-CL) shows high CO2/N2 and CO2/CH4 selectivity (72 and 22) at 273 K, which are among the best results for reported porous materials. This work reveals that the introduction of crosslinked structure and changing substituents is an efficient method for constructing microporous polyimides with abundant micropores and excellent CO2 selective adsorption capacity. This method also has great potential for fabricating high-performance microporous polymers based on other linear polymers without rigid contorted structure.

Effect of External Electric Field on the Ordered Structure of Molecular Chains and Hole Mobility in Regioregular Poly(3-hexylthiophene) with Different Molecular Weights.

This research investigated the effect of a high-voltage external electric field on the ordered structure of molecular chains and hole mobility in regioregular poly(3-hexylthiophene) (P3HT) with different molecular weights through X-ray diffraction, atomic force microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, micro-Raman spectroscopy, UV-vis spectroscopy, photoluminescence spectroscopy, and organic field-effect transistors. The optimal magnitude of the external electric field was 5000 V/cm. With the optimized electric field applied to a series of P3HT films, the carrier mobility of all P3HT films increased, and the increase rate changed from 105% to 56%, closely depending on the increase in molecular weight from 33 kg/mol to 100 kg/mol. The results indicated that the increase in carrier mobility was attributed to the P3HT conformation order, which was controlled by the external electric field. Molecular weight was a critical factor in determining the P3HT conformation response to the external electric field. The external electric field orientated lower-molecular-weight (33 kg/mol) P3HT into ordered structures more obviously than higher-molecular-weight (100 kg/mol) P3HT. This research contributes to the understanding of the effect of an external electric field on the ordered structure of the chains and carrier mobility in P3HT with different molecular weights. This research also reveals the regularity and mechanism of the formation of ordered structures and essentially enhances the carrier mobility of P3HT films with different molecular weights, to fabricate photovoltaic devices with high efficiency, based on polymer physics.

Dynamic Evolution from Chain Disorder to Order of PTB7 Condensed State Structures under External Fields.

In this research, the effect of external fields (solvent, temperature, solution concentration, and external force) on dynamic evolution from chain disorder to order of poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2- b:4,5- b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4- b]thiophenediyl]] (PTB7) condensed state structures was explored by UV-vis absorption spectra, atomic force microscope, and transmission electron microscopy (TEM). It was found that PTB7 main chains presented amorphous conformations induced by the poor solvent 1,2-dichloroethane. However, the local ordered aggregation appeared in amorphous conformations when the solubility of the poor solvent was again lowered by reducing temperature. It is worth noting that the size of ordered aggregation was further increased with the decrease of solution concentration or increase of external force. It was found that there were two main PTB7 absorption peaks in the UV-vis absorption spectra; we denoted A0 -0 for the intensity of the lower energy absorption peak and A0-1 for the intensity of the higher energy absorption peak. The ratio R = A0-0/ A0-1 was used to characterize the dynamic evolution from disorder to order of the PTB7 condensed state structures in absorption spectra. It increased from 0.94 for PTB7 amorphous state to 1.25 for PTB7 large-size ordered aggregation. The dynamic evolution from chain disorder to order could also be distinctly observed by TEM. It was inferred that PTB7 condensed state structures (amorphous state, local ordered aggregation, and large-scale ordered aggregation) might exist simultaneously because of the complexity of copolymer conformations. This research is meaningful to establish physical basis for the molecule design and the synthesis of materials to enhance photoelectronic device efficiency based on condensed matter physics of conjugated polymer.