Lamellar structures thermal stability

Recently, Carreon and Guliants reported novel hexagonal, cubic and lamellar VPO phases, which displayed improved thermal stability, desirable chemistries (i.e. the P/V ratios and vanadium oxidation states), and pore structures for the partial oxidation of n-butane [143-145]. These novel VPO phases displayed the selectivities to maleic anhydride up to 40 mol. % at 673K at 10 % n-butane conversion [146]. A conventional organic VPO catalyst containing well-crystallized vanadyl(IV) pyrophosphate, the proposed active and selective phase for n-butane oxidation to maleic anhydride, displayed the selectivities to maleic anhydride 50 mol. % under the same reaction conditions. The low yields observed for mesoporous VPO catalysts confirmed the critical role of the vanadyl pyrophosphate phase (VO)2P207 in catalyzing the oxidation of -butane to maleic anhydride. Therefore, the amorphous nature of the mesoporous VPO... 

The X-ray crystal structure, electrical conductivity, and thermal stability of the lamellar compound Cgv.gCrOgClg have been determined. The compound, which is of the third stage (with a c-axis dimension of 14.87 A) was found to be unusually stable, and its conductivity as well as composition shows but slight change after complete evacuation of the reacting CrOgClg gas. 

Although the above-mentioned materials may not have a lamellar structure, they do show mesoporosity (see Figure 11.3). The calcined (600°C) ZSU-L material has a BET surface area of 276m g and exhibits a type-I isotherm with hysteresis loop of type H4, associated with narrow sHt-like pores. The average pore size estimated by the BJH method was 3.6nm. Little change was observed in the BET surface area and pore size of the ZSU-L powders calcined at 800 °C, which indicates that the materials have high thermal stability. 

In this example, the thermal stability decreased with increasing dendritic generation, for both the dendrons and the symmetric dendrimers. TEM and XRD studies indicated a lamellar structure. Interestingly, some of these asymmetric phase-separated dendrons also possess amphiphilic properties and... 

The lamellar structures exhibit poor thermal stability. Upon removal of the template by thermal treatment, the structure collapses resulting in a dense phase with little structural order or porosity. The lamellar phase can be stabiHzed by subsequent treatment using an alkoxide [36]. Removal of the template after this stabihzation treatment resulted in a structure having an X-ray diffraction pattern of only hOO peaks consistent with the lamellar configuration. The X-ray diffraction pattern is illustrated in Fig. 4. The transmission electron micrograph also shown in Fig. 4 is of uniform layers having an interlayer separation ( 40 A) consistent with the X-ray diffraction data. 

The layered silicate can improve thermal stability of collagen and depending on the collagen morphology (gel or colloidal solutions) and on the method used for silicate dispersion, a variety of lamellar structures and morphologies - intercalated or exfoliated- can be obtained (Vuluga et al., 2007). 

Both ternary nanocomposites (CG/PB/MA-MMA and CG/PB/MA-VA) present a partially exfoliated lamellar structure (Fig. 9). In the nanocomposites with hydrophobic comonomer (MMA) the degree of interaction is stronger at basic pH, thus we obtain partially exfoliated nanostructured biomaterials. When we use hydrophilic comonomer (VA), the degree of interaction is stronger at acidic pH. For biocompatibility tests, we select the nanocomposite with hydrophobic comonomer. The strong degree of interaction for the hydrophobic comonomer reflects itself in an improved thermal stability (Table 4). 

Contrary to what many people think, PLSNs are not a recent discovery. One of the earliest systematic studies of the interaction between a clay mineral and a macromolecule dates backs to 1949, when Bower described the absorption of DNA by montmorillonite. Even in the absence of X-ray diffraction (XRD) evidence, this finding implied insertion of the macromolecule in the lamellar structure of the silicate. In the case of synthetic polymers, Uskov found in 1960 that the softening point of polymethylmethacrylate derived by polymerisation of methylmethacrylate was raised by montmorillonite modified with octadecyl-ammonium, while in the following year Blumstein obtained a polymer inserted in the structure of a montmorillonite by polymerising a previously inserted vinyl monomer. In 1965 Blumstein first reported the improved thermal stability of a PMMA/clay nanocomposite. He showed that PMMA inserted between the lamellae of montmorillonite clay resisted thermal degradation under conditions that would otherwise completely degrade pure PMMA." ... 

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