Structure and properties relationships for aromatic polyimides and their derived carbon membranes: experimental and simulation approaches.

The main objective of this study is to investigate the factors of the chemical structure and physical properties of rigid polyimides in determining the performance of derived carbon membranes through both the experimental and simulation methods. Four polyimides made of different dianhydrides were pyrolyzed at 550 and 800 degrees C under vacuum conditions. The resultant carbon membranes exhibit excellent gas separation performances beyond the traditional upper limit line for polymer membranes. The thermal stability and the fractional free volume (FFV) of polyimides were examined by a thermogravimetric analyzer and a density meter. The chain properties of polyimide, such as flatness, chain linearity, and mobility, were simulated using the Cerius(2) software. All above characterizations of polyimides were related to the microstructure and gas transport properties of the resultant carbon membranes. It was observed that the high FFV values and low thermal stability of polyimide produce carbon membranes with bigger pore and higher gas permeability at low pyrolysis temperatures. Therefore, polyimides with big thermally labile side groups should be preferred to prepare carbon membranes at low pyrolysis temperatures for high permeability applications. On the other side, since the flatness and in-plane orientation of precursors may lead carbon membranes to ordered structure, thus obtaining high gas selectivity, linear polyimides with more coplanar aromatic rings should be first choice to prepare carbon membranes at high pyrolysis temperatures for high selectivity applications. The location of the indan group also affects chain flatness and in-plane orientation. As a result, carbon membranes derived from the BTDA-DAI precursor have superior separation performance to those derived from Matrimid.

The facile synthesis of an aldehyde-containing graft copolymer membrane for covalent protein capture with retention of protein functionality.

The immobilization of biomolecules onto an insoluble carrier surface has always been a subject of great interest to enhance their resistance to pH and temperature, which aids in an increased activity lifespan as well as easy reuse of the said biomolecules. However, traditional methods are only able to provide single-layer biomolecular binding and require multiple chemical reactions to prepare the final substrate before the immobilization can be carried out properly. Here we report a facile one-step chemical synthesis of a new aldehyde-bearing graft copolymer via atom transfer radical polymerization (ATRP) for covalent protein capture in a multilayered approach to covalently capture bovine serum albumin (BSA) onto a polymeric membrane. The resultant protein-bound membrane illustrated the retention of BSA's stereoselective discrimination ability by binding to an excess of 2 mol of tryptophan/mol of BSA and demonstrated an enantioresolution of a 0.184 mM racemic tryptophan mixture with a time-averaged-separation factor of 2.9.

Acetylation of beta-cyclodextrin surface-functionalized cellulose dialysis membranes with enhanced chiral separation.

The enhanced enantiomeric separation of racemic phenylalanine solution has been demonstrated by the membrane-based chiral resolution method using an acetylated beta-cyclodextrin-immobilized cellulose dialysis membrane. Beta-cyclodextrin (CD) was first immobilized onto the surface of commercial cellulose dialysis membranes, followed by the acetylation reaction through the treatment of the membranes with acetic anhydride to form the chiral selective acetylated beta-cyclodextrin-immobilized cellulose dialysis membrane. The acetylated CD-immobilized membrane exhibits enantioselectivity in the range of 1.26-1.33 depending on the acetylation time. The improvement in enantioselectivity after acetylation was mainly attributed to the better discrimination ability of acetylated CD and the decrease in membrane pore size. Molecular modeling simulations indicate that the acetylation of hydroxyl groups would result in a CD conformation with torus distortions and would create higher steric hindrance for penetrants. As a result, compared to the original CD, the acetylated CD may have less effective binding but better discrimination of enantiomers. The energy drop is only 3 kcal/mol between different enantiomers before and after the binding of phenylalanine with an unmodified CD. The energy drop increases to 10 kcal/mol if acetylated CD is employed as the chiral selector, showing stronger characteristics for chiral selection.

Surface characterization, modification chemistry, and separation performance of polyimide and polyamidoamine dendrimer composite films.

6FDA-polyimide films modified by polyamidoamine (PAMAM) dendrimers with generations of 0, 1, and 2 are reported in this article. The actual molecular conformation and bulk size of these three generation dendrimers immobilized on polyimide surface were characterized by atomic force microscopy. After comparing with the results of dynamic simulation, we believe that the disk-shape cluster structure of dendrimers has been developed on the polymer surfaces. The amidation and cross-linking reaction between dendrimers and polyimide were examined and quantified by X-ray photoelectron spectroscopy, attenuated total reflection Fourier transform infrared spectroscopy, and gel content measurements. Modification time and the generations of PAMAM dendrimer have been verified as two important factors in determining the properties of modified polyimide films. These modified polyimide films exhibit excellent gas separation performance. The ideal selectivity of He/N(2) increases tremendously to about 200% as compared to that of the original polyimide film. Particularly, the separation performance of CO(2)/CH(4) gas pair can be improved beyond the upper bond limit possibly due to the strong interactions of dendrimer molecules with CO(2), which was verified by sorption tests.