Loss of crystallinity

The best combination of properties of polyethylene-based ionomers, such as stiffness, strength, transparency, and toughness, are realized at partial degrees of conversion of about 40-50% [13]. The initial increase in properties is a result of the presence of ionic interactions, which strengthen and stiffen the polymer. There is, however, some loss of crystallinity as a result of the presence of the ionic groups. When the loss of crystallin-... 

Melting. Loss of crystallinity of a solid reactant through melting, or eutectic formation, or dissolution in a product often results in an enhanced rate of decomposition as a consequence of the relaxation of the bonding forces responsible for lattice stabilization. The appearance of a... 

In order to get the pore system of zeolites available for adsorption and catalysis the template molecules have to be removed. This is generally done by calcination in air at temperatures up to 500 °C. A careful study (ref. 12) of the calcination of as-synthesized TPA-containing MFI-type single crystals by infrared spectroscopy and visible light microscopy showed that quat decomposition sets in around 350 °C. Sometimes special techniques are required, e.g. heating in an ammonia atmosphere (ref. 13) in the case of B-MFI (boron instead of aluminum present) to prevent loss of crystallinity of the zeolite during template quat removal. 

When applying zeolites with relatively low Si/Al ratio such as Y the hydrogen bromide formed in the reaction is a problem because of its strong adsorption accompanied by gradual loss of crystallinity of the zeolite. This problem has been addressed in several ways. 

Aluminum-deficient Y zeolites prepared by partial removal of aluminum with a chelating agent (e.g. EDTA) also show improved thermal and hydrothermal stability compared to the parent zeolite. The optimum stability was found in the range of 25 to 50 percent of framework A1 extraction (8). However, the maximum degree of dealumination is also affected by the SiO /Al O ratio in the parent zeolite a higher ratio appears to allow more advanced dealumination without loss of crystallinity (8,25,45). Above 50 or 60 percent dealumination, significant loss of crystallinity was observed. Calcination of the aluminum-deficient zeolite resulted in a material with a smaller unit cell size and lower ion-exchange capacity compared to the parent zeolite. 

Loss of crystalline structure as a function of temperature can also be measured ex situ by performing the heat treatment and then determining the change in zeolite content by X-ray diffraction. Such an analysis is shown in Figure 4.43 for... 

The FT-IR spectra of the resulting Gd(III)-exchanged NaY zeolites did not show any loss of crystallinity or dealumination during the gadolinium exchange. A detailed XRD study also confirmed the retained crystallinity nevertheless, there are significant changes in the relative intensities of certain refiections. 

Uses/Sources. Wood contains 50-70% cellulose cotton and other textile fibers of plant origin contain 65-95% rayon is prepared by dissolving natural cellulose and then precipitating it from solution, with some loss of crystallinity. Cellulose is made into cellophane film and is used to form fibers, resins, coatings and gums. 

Constant use of this liquid nitrogen delivery system requires control of the environment within the X-ray enclosure to eliminate condensation of water on the phase separator located above the crystal sample. If this water were to reach the sample, the crystal would experience a temporary increase in temperature that can result in loss of crystallinity. Dehumidification of the hutch eliminates this potential problem. Removal of ambient humidity has the added benefit of reducing the formation of ice on all components that use liquid nitrogen, particularly the dewars used to store crystals before and after X-ray analysis. 

Cooke, D., Gidley, M. J. (1992). Loss of crystalline and molecular order during starch gelatinisation origin of the enthalpic transition. Carbohydrate Research, 227, 103-112. 

The crystalline structure of modified zeolites determine a number of properties which are specific and favorable for catalytic reactions. The complete or partial loss of crystalline structure during catalytic reactions or regeneration is in most cases accompanied by decreased catalytic activity. Thermal stability or structural stability characteristics are therefore suitable for evaluating such catalysts or supported catalysts. 

Figure 2 illustrates the X-ray powder diffraction patterns for the as -synthesized and the calcined heterostructures. Both diffraction patterns show a 001 reflection indicating the presence of a layered structure with a basal spacing 36.8 A respectively 32.9 A. However, a decrease of 4 A in the gallery height, along with a small loss of crystallinity, can be observed upon calcination. 

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