A comparative experimental study of the hygroscopic and mechanical behaviors of electrospun nanofiber membranes and solution-cast films of polybenzimidazole

This article reports a comparative experimental study of the hygroscopic and mechanical behaviors of electrospun polybenzimidazole (PBI) nanofiber membranes and solution-cast PBI films. As-electrospun nonwoven PBI nanofiber mats (with the nanofiber diameter of ~250 nm) were heat-pressed under controlled temperature, pressure and duration for the study; lab-made solution-cast PBI films and commercially available PBI films (the PBI Performance Product Inc., Charlotte, NC) were used as the control samples. Thermogravimetric and microtensile tests were utilized to characterize the hygroscopic (moisture absorption) and mechanical properties of the PBI nanofiber membranes at varying heat-pressing conditions, which were further compared to those of solution-cast PBI films. Experimental results indicated that the PBI nanofiber membranes carried slightly higher thermal stability and less hygroscopic properties than those of solution-cast PBI films. In addition, heat-pressing conditions significantly influenced the mechanical properties of the resulting PBI nanofiber membranes. The stiffness and tensile strength increase with increasing either the heat-pressing pressure or duration, and relevant mechanisms were explored. The present study provides a rational understanding of the hygroscopic and mechanical behaviors of electrospun PBI nanofiber membranes and solution-cast PBI films that are beneficial to their reliable cutting-edge applications in high-temperature filtration, polymer electrolyte membranes (PEMs), etc.     

FRCaMP,a Red Fluorescent Genetically Encoded Calcium Indicator Based on Calmodulin from Schizosaccharomyces Pombe Fungus

Red fluorescent genetically encoded calcium indicators (GECIs) have expanded the available pallet of colors used for the visualization of neuronal calcium activity in vivo. However, their calcium-binding domain is restricted by calmodulin from metazoans. In this study, we developed red GECI, called FRCaMP, using calmodulin (CaM) from Schizosaccharomyces pombe fungus as a calcium binding domain. Compared to the R-GECO1 indicator in vitro, the purified protein FRCaMP had similar spectral characteristics, brightness, and pH stability but a 1.3-fold lower ΔF/F calcium response and 2.6-fold tighter calcium affinity with K_d of 441 nM and 2.4–6.6-fold lower photostability. In the cytosol of cultured HeLa cells, FRCaMP visualized calcium transients with a ΔF/F dynamic range of 5.6, which was similar to that of R-GECO1. FRCaMP robustly visualized the spontaneous activity of neuronal cultures and had a similar ΔF/F dynamic range of 1.7 but 2.1-fold faster decay kinetics vs. NCaMP7. On electrically stimulated cultured neurons, FRCaMP demonstrated 1.8-fold faster decay kinetics and 1.7-fold lower ΔF/F values per one action potential of 0.23 compared to the NCaMP7 indicator. The fungus-originating CaM of the FRCaMP indicator version with a deleted M13-like peptide did not interact with the cytosolic environment of the HeLa cells in contrast to the metazoa-originating CaM of the similarly truncated version of the GCaMP6s indicator with a deleted M13-like peptide. Finally, we generated a split version of the FRCaMP indicator, which allowed the simultaneous detection of calcium transients and the heterodimerization of bJun/bFos interacting proteins in the nuclei of HeLa cells with a ΔF/F dynamic range of 9.4 and a contrast of 2.3–3.5, respectively.     

Kinetics of structuring in the sol-gel systems based on tetraethoxysilane with organic additives: II. Gels

This paper reports on the results of investigations into the rheological properties of gels that are prepared by aging sols based on water-alcohol solutions of tetraethoxysilane (TEOS) modified by a number of metal salts and organic polyhydroxyl compounds, such as glycerol, poly(ethylene glycol) (PEG-300), and hyperbranched aliphatic complex polyol polyethers (HBP-49, HBP-64). It is demonstrated that the strength of the gels formed can be controlled using these additives.     

Microstructure changes of polyurethane by inclusion of chemically modified carbon nanotubes at low filler contents

The surface of multi-walled carbon nanotubes (MWCNTs) was modified to introduce acidic groups in either covalent or van der Waals interaction bonding environments to establish cross-linking sites with a host polymer. Nanocomposites based on a polyurethane matrix (PU) containing chemically functionalised multi-walled carbon nanotubes (MWCNTs) have been shown to alter its mechanical performance depending on the nature of the surface functional groups on MWCNTs, which correlates to the type of bonding interaction of the surface group and also the dispersibility of MWCNTs and their influence on the domain structure of polyurethane. The stress at break for nanocomposites containing 0.25 wt% of acid-oxidised MWCNTs (MWCNT-ox), bearing covalently attached carboxylic, lactone and phenolic groups, was twice that of the native PU and Young’s Modulus for the nanocomposites increased by four times. Whereas when hemin, which contains carboxylic functionality, was immobilised to the surface of pure MWCNTs, the improvement in Young’s Modulus was only around twice that of pure PU. Differences in the disaggregation of MWCNTs into PU were observed between the samples as well as variation of the native domain structure of PU. The results also infer that the purification of MWCNTs from acid-oxidative lattice fragments (fulvic acids) is vital prior to conducting surface chemistry and polymerisation in order to ensure maximum mechanical performance enhancement in their reinforcement of the host polymer.