ABSTRACT
Polyamic acid (PAA) is the precursor of polyimide (PI), and its solution's properties have a direct influence on the final performances of PI resins, films, or fibers. The viscosity loss of a PAA solution over time is notorious. A stability evaluation and revelation of the degradation mechanism of PAA in a solution based on variations of molecular parameters other than viscosity with storage time is necessary. In this study, a PAA solution was prepared through the polycondensation of 4,4'-(hexafluoroisopropene) diphthalic anhydride (6FDA) and 4,4'-diamino-2,2'-dimethylbiphenyl (DMB) in DMAc. The stability of a PAA solution stored at different temperatures (-18, -12, 4, and 25 °C) and different concentrations (12 wt% and 0.15 wt%) was systematically investigated by measuring the molecular parameters, including Mw, Mn, Mw/Mn, Rg, and [η], using gel permeation chromatography coupled with multiple detectors (GPC-RI-MALLS-VIS) in a mobile phase 0.02 M LiBr/0.20 M HAc/DMF. The stability of PAA in a concentrated solution decreased, as shown by the reduction ratio of Mw from 0%, 7.2%, and 34.7% to 83.8% and that of Mn from 0%, 4.7%, and 30.0% to 82.4% with an increase of temperature from -18, -12, and 4 to 25 °C, respectively, after storage for 139 days. The hydrolysis of PAA in a concentrated solution was accelerated at high temperatures. Notably, at 25 °C, the diluted solution was much less stable than the concentrated one and exhibited an almost linear degradation rate within 10 h. The Mw and Mn decreased rapidly by 52.8% and 48.7%, respectively, within 10 h. Such faster degradation was caused by a greater water ratio and less entanglement of chains in the diluted solution. The degradation of (6FDA-DMB) PAA in this study did not follow the chain length equilibration mechanism reported in literature, given that both Mw and Mn declined simultaneously during storage.
ABSTRACT
Metal-organic frameworks (MOFs) are advanced platforms for enzyme immobilization. Enzymes can be entrapped via either diffusion (into pre-formed MOFs) or co-crystallization. Enzyme co-crystallization with specific metals/ligands in the aqueous phase, also known as biomineralization, minimizes the enzyme loss compared to organic phase co-crystallization, removes the size limitation on enzymes and substrates, and can potentially broaden the application of enzyme@MOF composites. However, not all enzymes are stable/functional in the presence of excess metal ions and/or ligands currently available for co-crystallization. Furthermore, most current biomineralization-based MOFs have limited (acid) pH stability, making it necessary to explore other metal-ligand combinations that can also immobilize enzymes. Here, we report our discovery on the combination of five metal ions and two ligands that can form biocomposites with two model enzymes differing in size and hydrophobicity in the aqueous phase under ambient conditions. Surprisingly, most of the formed composites are single- or multiphase crystals, even though the reaction phase is aqueous, with the rest as amorphous powders. All 20 enzyme@MOF composites showed good to excellent reusability and were stable under weakly acidic pH values. The stability under weakly basic conditions depended upon the selection of enzyme and metal-ligand combinations, yet for both enzymes, 3-4 MOFs offered decent stability under basic conditions. This work initiates the expansion of the current "library" of metal-ligand selection for encapsulating/biomineralizing large enzymes/enzyme clusters, leading to customized encapsulation of enzymes according to enzyme stability, functionality, and optimal pH.