Within a polymer film, free-volume elements such as pores and channels typically have a wide range of sizes and topologies. This broad range of free-volume element sizes compromises a polymer's ability to perform molecular separations. We demonstrated free-volume structures in dense vitreous polymers that enable outstanding molecular and ionic transport and separation performance that surpasses the limits of conventional polymers. The unusual microstructure in these materials can be systematically tailored by thermally driven segment rearrangement. Free-volume topologies can be tailored by controlling the degree of rearrangement, flexibility of the original chain, and judicious inclusion of small templating molecules. This rational tailoring of free-volume element architecture provides a route for preparing high-performance polymers for molecular-scale separations.
Systemic translocation of RNA exerts non-cell-autonomous control over plant development and defense. Long-distance delivery of mRNA has been proven, but transport of small interfering RNA and microRNA remains to be demonstrated. Analyses performed on phloem sap collected from a range of plants identified populations of small RNA species. The dynamic nature of this population was reflected in its response to growth conditions and viral infection. The authenticity of these phloem small RNA molecules was confirmed by bioinformatic analysis; potential targets for a set of phloem small RNA species were identified. Heterografting studies, using spontaneously silencing coat protein (CP) plant lines, also established that transgene-derived siRNA move in the long-distance phloem and initiate CP gene silencing in the scion. Biochemical analysis of pumpkin (Cucurbita maxima) phloem sap led to the characterization of C. maxima Phloem SMALL RNA BINDING PROTEIN1 (CmPSRP1), a unique component of the protein machinery probably involved in small RNA trafficking. Equivalently sized small RNA binding proteins were detected in phloem sap from cucumber (Cucumis sativus) and lupin (Lupinus albus). PSRP1 binds selectively to 25-nucleotide single-stranded RNA species. Microinjection studies provided direct evidence that PSRP1 could mediate the cell-to-cell trafficking of 25-nucleotide single-stranded, but not doublestranded, RNA molecules. The potential role played by PSRP1 in long-distance transmission of silencing signals is discussed with respect to the pathways and mechanisms used by plants to exert systemic control over developmental and physiological processes.
A fundamental understanding of polymer microstructure is important in order to design novel polymer electrolyte membranes (PEMs) with excellent electrochemical performance and stabilities. Hydrocarbon-based polymers have distinct microstructure according to their chemical structure. The ionic clusters and/or channels play a critical role in PEMs, affecting ion conductivity and water transport, especially at medium temperature and low relative humidity (RH). In addition, physical properties such as water uptake and dimensional swelling behavior depend strongly on polymer morphology. Over the past few decades, much research has focused on the synthetic development and microstructural characterization of hydrocarbon-based PEM materials. Furthermore, blends, composites, pressing, shear field, electrical field, surface modification, and cross-linking have also been shown to be effective approaches to obtain/maintain well-defined PEM microstructure. This review summarizes recent work on developments in advanced PEMs with various chemical structures and architecture and the resulting polymer microstructures and morphologies that arise for potential application in fuel cell, lithium ion battery, redox flow battery, actuators, and electrodialysis.
Abstract-Sphingosine 1-phosphate (S1P), an abundant lipid mediator in plasma, regulates vascular and immune cells by activating S1P receptors. In this report, we investigated the mechanisms by which high plasma S1P levels are maintained in mice. We found that plasma S1P turns over rapidly with a half-life of Ϸ15 minutes, suggesting the existence of a high-capacity biosynthetic source(s). Transplantation of bone marrow from wild-type to Sphk1 Ϫ/Ϫ Sphk2 ϩ/Ϫ mice restored plasma S1P levels, suggesting that hematopoietic cells are capable of secreting S1P into plasma. However, plasma S1P levels were not appreciably altered in mice that were thrombocytopenic, anemic, or leukopenic. Surprisingly, reconstitution of Sphk1 Ϫ/Ϫ Sphk2 ϩ/Ϫ bone marrow cells into wild-type hosts failed to reduce plasma S1P, suggesting the existence of an additional, nonhematopoietic source for plasma S1P. Adenoviral expression of Sphk1 in the liver of Sphk1 Ϫ/Ϫ mice restored plasma S1P levels. In vitro, vascular endothelial cells, but not hepatocytes, secreted S1P in a constitutive manner. Interestingly, laminar shear stress downregulated the expression of S1P lyase (Sgpl) and S1P phosphatase-1 (Sgpp1) while concomitantly stimulating S1P release from endothelial cells in vitro. Modulation of expression of endothelial S1P lyase with small interfering RNA and adenoviral expression altered S1P secretion, suggesting an important role played by this enzyme. These data suggest that the vascular endothelium, in addition to the hematopoietic system, is a major contributor of plasma S1P. Key Words: sphingosine 1-phosphate (S1P) Ⅲ sphingosine kinase (Sphk) Ⅲ S1P lyase (Sgpl) Ⅲ plasma S1P gradient Ⅲ Shear stress T he bioactive lipid sphingosine 1-phosphate (S1P) is a potent regulator of numerous biological responses, the most well characterized being cardiovascular and immune effects. S1P binds to and activates a widely expressed family of G protein-coupled receptors, termed S1PRs. Intracellular signaling of these receptors are thought to mediate most of the effects of S1P.  S1P is abundant (0.1 to 1.2 mol/L) in plasma, where it is mainly bound to albumin and high-density lipoprotein (HDL). 5,6 Thus, S1P receptors on blood-borne cells are likely to be constitutively activated. In contrast, S1P levels in tissues are considerably lower (0.5 to 75 pmol/mg wet weight), although tissues with high blood content, such as spleen, are exceptions. This concentration difference of S1P between plasma and tissues has been termed the vascular S1P gradient, which was shown to be functionally important in lymphocyte egress from the lymphoid tissues and the thymus. 7,8 The regulation of S1P production and release is not well understood. Secretion of S1P is observed in a variety of cells including platelets, 9 -11 erythrocytes, 9,12,13 mononuclear cells, neutrophils, 9 mast cells, 14,15 and endothelial cells. 16 The concentration of S1P in the cell is determined by the activity of biosynthetic enzymes (sphingosine kinase [Sphk]-1 and -2) and the degradative ...
Conversion of hydroxyl-containing polyimides into polybenzoxazole can be achieved by thermal rearrangement of the aromatic polymer chain with decarboxylation at elevated temperature. Synthetic methods to prepare polyimide precursors are important for the resulting thermally rearranged (TR) polymer membranes. Here, we report on the effect of several imidization methods on the properties of TR polymer membranes. Thermal and chemical imidizations are the most common routes to prepare polyimides, and solution thermal imidization using an azeotrope is also widely used, especially to obtain soluble polyimide-containing functional groups. We report here on the syntheses of ortho-functional polyimides from 4,4′-hexafluoroisopropylidene diphthalic anhydrides and 2,2′-bis(3-amino-4-hydroxyphenyl)hexafluoropropane by three different imidization methods. Acetate-containing polyimides by chemical imidization and further silylation treatment as well as hydroxyl-containing polyimides by thermal and azeotropic imidization are characterized using thermogravimetric analysis, density, positron annihilation lifetime spectroscopy, and gas permeation property measurements. Comparison between the precursor polyimides and the resulting thermally rearranged polybenzoxazole (TR-PBO) membranes exhibited significant increase in fractional free volumes and cavity sizes followed by enhanced gas permeation properties.
A facile reactive seeding (RS) method was developed for the preparation of continuous MOF membranes on alumina porous supports, in which the porous support acted as the inorganic source reacting with the organic precursor to grow a seeding layer.
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