Temperature-induced transformations

The concept of a single structural transition in amorphous material, i.e., an amorphous-amorphous transition, was coined in 1985 by Mishima et al. [25] on the example of water. Today, in many respects the nature of pressure- and/or temperature-induced transformations in glasses and amorphous solids remains unclear. In most cases, the pressure treatment of glasses and amorphous solids results in residual densification. In the process, the densified glasses and... 

By comparison, other methods for the preparation of alloys are less used in the laboratory, although in special cases the optimum methods may involve reduction (chemical or electrolytic) of metallic compounds. In addition, some intermetallic compounds are best obtained as residues remaining after the corresponding basic alloys are dissolved. However, a knowledge of the temperature-induced transformations of the phase diagram is of the greatest importance in all cases thus, the literature references below must be consulted, if at all possible. 

An example of a similar type of cooperative transformation is shown in Fig. 3.17 for different molecular weight nucleic acids (obtained by shear degradation) from T2 bacteriophage.(62) This temperature induced transformation is quite clear in the figure. At low temperature all the appropriate bases are bonded, one to another at high temperature, in the disordered state, the bases are no longer bonded. The... 

Another property pecuHar to SMAs is the abiUty under certain conditions to exhibit superelastic behavior, also given the name linear superelasticity. This is distinguished from the pseudoelastic behavior, SIM. Many of the martensitic alloys, when deformed well beyond the point where the initial single coalesced martensite has formed, exhibit a stress-induced martensite-to-martensite transformation. In this mode of deformation, strain recovery occurs through the release of stress, not by a temperature-induced phase change, and recoverable strains in excess of 15% have been observed. This behavior has been exploited for medical devices. 

For example, the difference between the monoclinic CaC2 phases is the presence of only one type of acetylide ion in phase II and two distinct C2 species in phase III, as concluded from their crystal structures and from C-NMR studies (Fig. 8.3) [8]. The transformation between phases II and III is induced by heating phase II above 150°C until the metastable phase. III, is formed. Phase III remains stable even when being cooled down to room temperature. However, when the metastable phase III is ground in a mortar at room temperature, it transforms back into phase II. 

In situ XRD stndies of thermally and chemically induced transformations were monitored with a totally automated diffractometer-micrometer system (9). Samples were thermally ramped in 5°C increments from 30 to 160°C, maintained at 160°C for 0.5 hr, and then cooled in 5°C increments to 30°C under 500 torr hydrogen. 20 scans were obtained after each temperatnre increment. The individnal scans conld be stacked to show the temperature evolution of the stracture. 

It was found that in spite of the large excess of modifying amine (N-isopropyl-, -diethyl, -dipropyl, -diisopropyl, -n-hexyl, -cyclohexyl, -n-octyl), the extent of substitution did not exceed 5-10 molar %. For the case of the N-isopropyl derivative, i.e. [poly(AAm-co-NiPAAm)], the authors connected such results with the temperature-induced conformational transformation of partially hydrophobized copolymer acquiring the contracted conformation, "... which made it difficult for N-isopropylamine to react further with the amide groups [22], Unfortunately, no data on the solution behaviour of these interesting copolymers have been reported to date, although there is a high probability that they would demonstrate certain properties of the protein-like macromolecules. At least, in favour of similar supposition is supported by the results of our studies [23] of somewhat different PAAm partially hydrophobized derivative, whose preparation method is depicted in Scheme 3. 

Appelo CAJ, VanDerWeiden MJJ, Toumassat C, Charlet L (2002) Surface complexation of ferrous iron and carbonate on ferrihydrite and the mobilization of arsenic. Environ Sci Techno 36 3096-3103 Ardizzone S, Formaro L (1983) Temperature induced phase transformation of metastable Fe(OH), in the presence of ferrous ions. Mat Chem Phys 8 125-133... 

The transformation of famotidine from form B to form A was found to be controlled by pressure, temperature, and the interaction between those factors [252]. A subsequent study focused on identifying appropriate milling conditions to avoid conversion [253]. A study of milled Ti and C powders to form TiC found that Raman spectroscopy provided insights not available from XRD studies alone [254]. The successful complexation, or process-induced transformation, of nimodipine (NMD) and hydroxylpropyl-jS-cyclodextrin... 

Ardizzone, S. Eormaro, L. (1983) Temperature induced phase transformation of mestable Ee(OH)3 in the presence of ferrous ions. Mat. 

Eiser et al. (2009) have recently reported on the pH-induced transformation of a hard-boiled egg from a white, brittle particulate gel to a transparent, elastic polymer gel (see Figure 6.20). Eggs were incubated in their hard protective shells for up to 26 days in a strong alkaline solution (0.9 M NaOH + 0.5 M NaCl, pH 12) at room temperature. These harsh experimental conditions are apparently rather similar to those used in a traditional Chinese method developed over two thousand years ago as a way of preserving eggs so that they would remain safely edible for many months. 

The analogous considerations are valid for polymer systems as well. Indeed, amphiphilic monomer units also tend to occupy interfacial areas of macromolecular associates as it is normal for low molecular weight surfactants to adsorb at polymer-poor solvent boundaries. And, if such interfacial groups of the polymer associate catalyze chemical transformation of a compound which tends to adsorb at the associate interfaces, this can result in unusual kinetics effects. Okhapkin et al. [18] studied the influence of temperature-induced aggregation on the catalytic activity of thermosensitive... 

The unhomogeneous composition of the products generated by the photochemical reaction is due to another mechanism. While the thermal isomerization of 1,5-dienes proceeds via a cyclic transition state in a synchronous sense, the photochemically induced transformation causes a reorientation of the allyl radicals generated from the educts. Warming up the reaction mixture to 100°C activates a complete transfer from 4c to 5c) of all isomers. This step may be explained by a radical CC bond split of the 1,2-diphenylethylene unit. Since the isomerization of the diastereomeric compound 4c to 5c is activated at much lower temperatures than for the Cope rearrangement (from 3c to 4c), it is clear that the thermal transfer exclusively forms the twofold changed product. 

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