Cations play a complex structural role in oxide glasses, as they occur in different kinds of environments, which allow them to exert a contrasted influence on physical and chemical properties of these glasses. The combination of structural information given by a wide range of spectroscopic methods and by radiation scattering, combined with numerical modelling, has given insight on the structural organisation around these cations. Among these characteristic properties are unusually low-coordination numbers, such as 5-fold coordination, and the presence of extended ordered domains, in which cation polyhedra are edge- or corner-sharing. This review presents evidence for a structural control of several physical and chemical properties in oxide multicomponent glasses. The use of zinc as a stabilising glass component arises from its network-forming position, which implies the presence of low-charge cations in its surrounding and as a consequence decreases the concentration of modifier components. The compositional dependence of glass coloration by transition elements has been investigated thoroughly through the example of nickel in silicate and borate glasses. The wide range of coloration observed may be explained by the existence of three kinds of environments, with nickel occurring in 4-, 5-, and 6- coordination. The relationships of these sites with the medium-range organisation of the glasses have been understood by a combined use of EXAFS spectroscopy and neutron scattering with isotopic substitution. The two other examples that are presented to illustrate structure–property relationships concern the physical solubility of gazes in glasses and the alteration processes of glasses used as analogues of nuclear waste matrices. In this last example, the use of structural probes as zirconium illustrates the influence of the alteration solution on the process of glass corrosion and further development of a gel at the glass–solution interface. A comparison with the evolution of the surrounding of iron shows that the two major processes, hydrolysis/condensation and dissolution/precipitation, depend on the element considered. 相似文献
Poly(2‐hydroxyethyl methacrylate)‐block‐poly(N‐isopropylacrylamide) (PHEMA‐b‐PNIPAM) was prepared by controlled surface‐initiated ATRP from silicon substrates, and the resulting block copolymers were successfully converted into the corresponding PSEMA‐b‐PNIPAM by esterification of the hydroxy groups on the PHEMA block using excess of succinic anhydride. The PSEMA‐b‐PNIPAM block copolymer brushes respond to both temperature and pH stimuli. The double‐responsive behavior of the block copolymer brushes in solution was investigated by height imaging and force–distance measurements of AFM. The results clearly show the responsive behavior of the smart block copolymer brushes.
We report for the first time the combination of ATRP and ring‐opening episulfide polymerization as a means to synthesize polysulfide‐based low‐dispersity amphiphilic block copolymers. The most significant finding is the possibility to perform ATRP under mild conditions using poly(propylene sulfide) macroinitiators, apparently without any significant copper sequestration by the polysulfides. Using glycerol monomethacrylate (GMMA) as a hydrophilic monomer, the polymers self‐assembled in colloidal structures with a morphology depending on the PS/GMMA ratio, but also probably on GMMA degree of polymerization. We here also present a new AFM‐based method to calculate the average number of amphiphilic macromolecules per micelle. 相似文献
Fulvic acid amide (FAA) was synthesized with fulvic acid (FA) and urea. The structure of FAA was characterized by X‐ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. Poly(lactic acid)/fulvic acid amide (PLA/FAA) composites were prepared by melt blending and compression molding. The nucleation effect of FAA on PLA was investigated by differential scanning calorimetry and polarized optical microscopy. Structure‐property relationship of PLA/FAA composites showed that FAA accelerated crystallization rate of PLA and improved toughness of PLA. Rotational rheological behavior of PLA/FAA composites showed that FAA increased the storage modulus of PLA. Capillary rheological analysis showed that the apparent viscosities of PLA composites were highly increased after the introduction of the FAA nucleating agent. Moreover, thermogravimetric analysis demonstrated that thermal degradability of PLA/FAA composites has been increased significantly compared with the neat PLA. 相似文献
Crystal growth of the metal–organic framework MOF‐5 was studied by atomic force microscopy (AFM) for the first time. Growth under low supersaturation conditions was found to occur by a two‐dimensional or spiral crystal growth mechanism. Observation of developing nuclei during the former reveals growth occurs through a process of nucleation and spreading of metastable and stable sub‐layers revealing that MOFs may be considered as dense phase structures in terms of crystal growth, even though they contain sub‐layers consisting of ordered framework and disordered non‐framework components. These results also support the notion this may be a general mechanism of surface crystal growth at low supersaturation applicable to crystalline nanoporous materials. The crystal growth mechanism at the atomistic level was also seen to vary as a function of the growth solution Zn/H2bdc ratio producing square terraces with steps parallel to the <100> direction or rhombus‐shaped terraces with steps parallel to the <110> direction when the Zn/H2bdc ratio was >1 or about 1, respectively. The change in relative growth rates can be explained in terms of changes in the solution species concentrations and their influence on growth at different terrace growth sites. These results were successfully applied to the growth of as‐synthesized cube‐shaped crystals to increase expression of the {111} faces and to grow octahedral crystals of suitable quality to image using AFM. This modulator‐free route to control the crystal morphology of MOF‐5 crystals should be applicable to a wide variety of MOFs to achieve the desired morphological control for performance enhancement in applications. 相似文献
Identifying the form and role of the chemical species that traverse the stages of crystallization is critical to understanding the formation process of coordination polymers. Herein, we report the combined use of in situ atomic force microscopy and mass spectrometry to identify preformed, complex, cadmium 2‐ethylimidazole containing solution species in the growth solution of the cadmium 2‐ethylimidazolate metal–organic framework CdIF‐4, and show that they are critical in the surface nucleation for the crystal growth of this material. Surface nucleation appears to be instigated by these [Cdx(CH3CO2)y(C5H7N2/C5H8N2)z]‐containing solution species and not by sole addition of the ligand molecules. The CH3CO2? or Cd(CH3CO2)2 groups of the former are substituted subsequently as the framework growth proceeds. Our greater understanding of such solution species and their role in crystallization will guide future syntheses of designed functional coordination polymers. 相似文献