Amorphous irreversible structural change

Investigations performed by Minchev et al (215) indicated that the framework of crystalline silicoaluminophosphates can be damaged upon the rehydration of the template-free material. In the case of rehydrated template-free H-SAPO-5 and H-SAPO-34, for example, a strong loss of the crystallinity occurs in the presence of water. However, the crystallinity can be completely restored after an additional dehydration at 823 K. Hydration of H-SAPO-37 at room temperature causes irreversible structural changes and leads to a material that is totally amorphous to X-ray diffraction (216). At temperatures of more than 345 K, template-free H-SAPO-37 exhibits a high stability toward hydration (216). 

In a series of publications by Ishii et al. [54-57], effects of the electric field on structural changes in the amorphous regions, accompanied by an additional relaxation process, were discussed. These effects are reflected in the angular dependence of the second moment at different temperatures. The separation of any orientational effects due to poling from stretching effects were made by the preparation of different sample types. The complications for such a separation arose from the facts that (i) mechanically induced effects on chain orientations are much larger than that of the (electric) dipole reorientation and (ii) after poling only a small irreversible electric polarisation remains. 

Thermoplastic polymers can be heated and cooled reversibly with no change to their chemical structure. Thermosets are processed or cured by a chemical reaction which is irreversible they can be softened by heating but do not return to their uncured state. The polymer type will dictate whether the compound is completely amorphous or partly crystalline at the operating temperature, and its intrinsic resistance to chemicals, mechanical stress and electrical stress. Degradation of the basic polymer, and, in particular, rupture of the main polymer chain or backbone, is the principal cause of reduction of tensile strength. 

Some polymorphic modifications can be converted from one to another by a change in temperature. Phase transitions can be also induced by an external stress field. Phase transitions under tensile stress can be observed in natural rubber when it orients and crystallizes under tension and reverts to its original amorphous state by relaxation (Mandelkem, 1964). Stress-induced transitions are also observed in some crystalline polymers, e.g. PBT (Jakeways etal., 1975 Yokouchi etal., 1976) and its block copolymers with polyftetramethylene oxide) (PTMO) (Tashiro et al, 1986), PEO (Takahashi et al., 1973 Tashiro Tadokoro, 1978), polyoxacyclobutane (Takahashi et al., 1980), PA6 (Miyasaka Ishikawa, 1968), PVF2 (Lando et al, 1966 Hasegawa et al, 1972), polypivalolactone (Prud homme Marchessault, 1974), keratin (Astbury Woods, 1933 Hearle et al, 1971), and others. These stress-induced phase transitions are either reversible, i.e. the crystal structure reverts to the original structure on relaxation, or irreversible, i.e. the newly formed structure does not revert after relaxation. Examples of the former include PBT, PEO and keratin. 

Polymerization reactions are in most cases conducted under conditions remote from polymer-monomer equilibrium, i.e., these reactions are, to a great extent, irreversible. A polymer can have different molecular and supramolecular structures for example it can be iso- or syndiotactic, amorphous, or crystalline. Differences in the chemical potential of polymers with different structures are small in comparison with the changes observed in the chemical potential at conversion of a monomer into a polymer. This means that the possibility for a polymer of a certain structure to be formed will be determined by kinetic causes the nature of the catalyst, solvent, etc. According to the scheme in Fig. 5, the polymer with structure 2 will be mainly produced. 

The uniqueness and versatility of carbonaceous porous materials is demonstrated by Mukai et al. (2004) in their attempt to reduce the phenomenon of irreversibility of the LIB. As indicated above, irreversibility is associated with the formation of solid electrolyte films on surfaces of carbons by an irreversible reaction of lithium ions with the electrolytes. For the isotropic porous carbons (not amorphous carbons as quoted by Mukai et al., 2004), the electrolyte film is formed preferentially in the entrances to the porosity (mainly microporosity). Should it be possible to prevent this deposition, then the irreversible component of battery performance could be reduced. It is established that increasing the heat treatment of carbons (normally beyond about 800 °C) decreases the pore dimensions, but at the same time there is reduction in volume of porosity which is available for lithium entry. Quite separately, Suzuki et al. (2003) report on the impossibility of bringing about a meaningful reduction in the irreversible component, maintaining the reversible component, by changing the porosity of the material. That is, an improvement automatically creates a deterioration. The use of an approach of carbon vapor deposition (as for pyrolytic carbons) has been tried whereby carbon is deposited in the entrances to the microporosity. There is no overall change to carbon structure. This method was successful but applications on an industrial scale are expensive. 

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