Zhihong Chang

Crosslinked polybenzimidazole via a Diels–Alder reaction for proton conducting membranes

The crosslinking of polybenzimidazole (PBI) is a potential strategy to improve the mechanical properties and dimensional stability of acid-doped membranes, as well as to retain additives in the membranes. An effective method to prepare crosslinked PBI with a well-defined structure via a Diels–Alder reaction between vinylbenzyl functionalized PBI (PBI–VB) and α,α′-difurfuryloxy-p-xylene (DFX) is proposed. The chemical structure of PBI–VB is confirmed by FTIR and 1H NMR. The model reaction of styrene and DFX is employed to clarify the crosslinking reaction of PBI and DFX. During the crosslinking process, three kinds of chemical reaction may happen. The first is a Diels–Alder reaction of DFX with the vinyl groups of PBI–VB. The second is the self-polymerization of vinyl groups. The third is the grafting of difuran groups via a Diels–Alder reaction. The first two reactions contribute the most to the crosslinking of the PBI membrane. With the addition of DFX, there is competition between these two kinds of crosslinking reactions. When the feed ratio of DFX is below 20%, the tensile strength of the crosslinked membranes increases with increasing content of DFX. The crosslinking of the membrane is mainly a results of Diels–Alder reactions. When the feed ratio of DFX exceeds 20%, the tensile strength decreases slightly. Besides the crosslinking via Diels–Alder reactions, the crosslinking of the membrane is also contributed by the self-polymerization of vinyl groups and the grafting of difuran groups. The crosslinked PBI membrane exhibits improved mechanical strength, higher physical and chemical stability, as well as higher phosphoric acid (PA) retention ability. After doping with PA, the crosslinked membrane exhibits good proton conductivity over a temperature range of 60 to 180 °C.

Highly efficient separation, enrichment, and recovery of peptides by silica-supported polyethylenimine.

Highly efficient and charge-selective adsorption and desorption of peptides at trace level by a solid-phase adsorbent is described. The adsorbent of SiO2@PEI is synthesized by covalent immobilization of branched polyethylenimines (PEI) exclusively on the outer surface of the porous silica particles (∼300 μm). For aqueous peptides (Mw = 600-3000 Da), SiO2@PEI can capture the negatively charged ones and leave the positively charged ones intact, and by adjusting pH of the system peptides with different isoelectric points (pIs) can be well separated. Targeted peptide at low abundance (at least as low as 0.1 mol % with respect to the highest one) can be well separated. The association constants of K > 10(12) M(-1) at pH > pI and K < 10(4) M(-1) at pH < pI are found; that is, selectivity > 10(8) is generally available. Thus, a peptide even at sub-femtomolar level can be extracted and eluted for analysis, and efficient recovery (79-92%) of the peptides is found. The extraction is mainly promoted by multisite electrostatic interaction, and the hydrophilic and cationic properties of PEI at low pH play a unique role in desorption efficiency and selectivity. The unbiased nature of this method renders the adsorbent applicable to the efficient separation of a broad spectrum of peptides, including those with similar pIs.