Collapse and Recrystallization

and Tr can be close together or are approximately in a 10% range, as shown in Table 1.13. For sorbitol, T is shown at a higher temperature (-43 °C) than T. (-57 °C), a fact which cannot be explained. 

All temperatures mentioned are influenced by the methods of their measurement [1.71], e.g. very thin samples in a cryomicroscope, very small amounts of product (mg 

Drying time (h) Average water content (g/100 g solid) Average content of 1-butanol (g/100 g solid) 

All temperatures mentioned are influenced by the methods of their measurement [ 1.711, e. g. very thin samples in a cryomicroscope, very small amounts of product (mg range) in an installation for differential scanning calorimetry (DSC) and some temperature gradients in the sample during the measurement of the electrical resistance (ER). Tg., Tc and Tr measured with pure substances can supply helpful information about the temperature range to 

Voilley et al. [1.75] confirmed the increasing retention with decreasing freezing speed and with the increasing number of carbon molecules of the alcohol (Fig. 1.94). 

Homologous series 2, 3-methyl-l-butanol 3, cyclopentanol (Fig. 1 from [1.75]). 

n-buty ric acid 2, isobutyric acid 3, lactic acid (Fig. 2 from [1.77]). 

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Abstract. After a brief introduction on zeolite constitution, structure and properties, the suitability of thermal analysis in characterizing the zeolite materials and in investigating their potential behavior in different application fields is analyzed. Kinetics and thermodynamics of water desorption, thermal stability, phase transformations, occluded phase decomposition and gas evolution, structure collapse and recrystallization, change in electrical properties, all in relation to thermal treatments, are the specific subjects reviewed. Use of thermal analysis in the evaluation of zeolite content in multicomponent mixtures and in the characterization of zeolite catalysts are the two additional main topics discussed. 

A comparison of literature values of subzero collapse transition temperatures (Tc) for an extensive list of water-compatible monomers and polymers has established the fundamental identity of Tg with the minimum onset temperatures observed for various structural collapse and recrystallization processes (1.2.115), for both model solutions and real systems of foods, as well as pharmaceuticals and biologicals (38.42.106.108.109). The Tc and Tr values of frozen or vitrified samples, tabulated elsewhere (1.2.22). are typically determined by cryomicroscopy, thermal analysis, or electrical resistance measurements (22 and refs, therein), on an experimental time scale similar to that for Tg by our DSC method (106.109). A comprehensive list of collapse processes (Table I), all of which are governed by Tg of frozen systems (or a higher Tg pertaining to low moisture systems processed or stored at T > 0 C) and involve potentially detrimental plasticization by water, has been identified and elucidated (1-5.18-... 

Just above the melting point the polymer is visually quite viscous and numerous observations have been made that the polymer exhibits a memory effect, that is to say, on recooling the melt crystallites will appear in the same sites where they had been before melting the polymer. Hartley, Lord and Morgan (1954) state It is reasonable to suppose that there will be a few localities in the crystalline polymer which have a very high degree of crystalline order, and therefore the melt can contain, even at considerable temperatures above the observed melting or collapse point, thermodynamically stable minute crystals of the polymer . Especially if the polymer has been irradiated so as to contain a few crosslinks as in irradiated polyethylene, then flow is inhibited and spherulites can be made to appear on recrystallization in the same sites that they had before the polymer was melted, Hammer, Brandt and Peticolas (1957). However, as mentioned above, the specific heat of irradiated polyethylene in the liquid state is identical with that of the unirradiated material, within the limits of experimental error. Dole and Howard (1957). 

Zhang and co-workers reported partial conversion of a mesoporous starting material (SBA-15) into a mesoporous aluminosilicate with zeolitic characteristics in a so-called vapour phase transport method.[82] In this process, Al is firstly introduced onto the mesoporous surface, followed by a filling of the mesopores with a carbonaceous species, and finally a partial recrystallization of aluminosilicate in the vapour of the SDA is conducted. The advantage of this method, compared with the hydrothermal recrystallization method of Kloetstra et al., lies in the fact that the mesopore structure collapses to a lesser extent as the crystallization is limited to the surface of the mesoporous precursor. 

It is also essential to consider the glass transition temperature of the freeze-dried product, particularly with a view to predicting physical and chemical storage stability. The relationship between the physical stability of freeze-dried formulations and Tg must be considered not only in terms of recrystallization but also of product collapse. For example, te Booy et al. (119) showed that storage of sucrose-containing freeze-dried formulations above Tg may result in product shrinkage, collapse, or excipient recrystallization. 

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