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Monotropic changes

The positional order of the molecules within the smectic layers disappears when the smectic B phase is heated to the smectic A phase. Likewise, the one-dimensional positional order of the smectic M phase is lost in the transition to the nematic phase. AH of the transitions given in this example are reversible upon heating and cooling they are therefore enantiotropic. When a given Hquid crystal phase can only be obtained by changing the temperature in one direction (ie, the mesophase occurs below the soHd to isotropic Hquid transition due to supercooling), then it is monotropic. An example of this is the smectic A phase of cholesteryl nonanoate [1182-66-7] (4), which occurs only if the chiral nematic phase is cooled (21). The transitions are aH reversible as long as crystals of the soHd phase do not form. [Pg.197]

The presence of three oxyethylene units in the spacer of PTEB slows down the crystallization from the meso-phase, which is a very rapid process in the analogous polybibenzoate with an all-methylene spacer, P8MB [13]. Other effects of the presence of ether groups in the spacer are the change from a monotropic behavior in P8MB to an enantiotropic one in PTEB, as well as the reduction in the glass transition temperature. This rather interesting behavior led us to perform a detailed study of the dynamic mechanical properties of copolymers of these two poly bibenzoates [41]. [Pg.396]

When a solid system undergoing a thermal change in phase exhibits a reversible transition point at some temperature below the melting points of either of the polymorphic forms of the solid, the system is described as exhibiting enantiotropic polymorphism, or enantiotropy. On the other hand, when a solid system undergoing thermal change is characterized by the existence of only one stable form over the entire temperature range, then the system is said to display monotropic polymorphism, or monotropy. [Pg.91]

It is worth noting that a monotropic polymorphic system offers the potential of annealing the substance to achieve the preferred form of the thermodynamically stable phase. The use of the most stable form is ordinarily preferred to avoid the inexorable tendency of a metastable system to move toward the thermodynamic form. This is especially important especially if someone elects to use a metastable phase of an excipient as part of a tablet coating, since physical changes in the properties of the coating can take place after it has been made. Use of the most stable form avoids any solid-solid transition that could... [Pg.93]

In the monotropic case the relative solubility of the two polymorphs does not change with temperature if Form I is the most stable and least soluble at low temperatures then it will also be the same at all other temperatures, Figure 3. An example of this behavior is the Cimetidine Form A and B relationship presented in the case study of section 6. [Pg.35]

Ammonium nitrate exists in four different forms, all of which are enantiotropic the change of white phosphorus into the red (or violet) variety is monotropic. Mercuric iodide exhibits a striking example of an enantiotropic transition. Above 126.3°, it is obtained in yellow rhombic crystals while below that temperature, a scarlet tetragonal modification appears. [Pg.149]

The relation between the octahedral and prismatic modifications has not yet been satisfactorily elucidated. The former is the stable form at ordinary temperatures and the latter at higher temperatures the transition point according to Rushton and Daniels is 250° C. and according to Smits and Beljaars 200° C., but the prismatic form is persistent at much lower temperatures and the change from octahedral to prismatic may be monotropic. Interesting information has been obtained from measurements of the vapour pressure of the oxide. The following values have been obtained ... [Pg.129]

The monotropic nature of mixture A created foreseen difficulties in its use as a medium for thermal or photochemical BN resolutions (9 ) sporadically, some mesophases of mixture A solidified during experiments. Mixture B at monotropic phase temperatures was well-behaved and its BN soTutions could be kept for hours without noticeable change. [Pg.155]

Whereas hot stage microscopy can be used to obtain qualitative information on polymorphic behaviour, thermal analysis provides quantitative information about the relative stability of polymorphic modifications, the energies involved in phase changes between them and the monotropic or enantiotropic nature of those transitions. The two techniques are best used in conjunction. [Pg.104]

In some cases, new phases that may not be detectable by other methods may be detected optically (Chang et al. 1995). Solid state conversions and their monotropic (Burger et al. 1997) or enantiotropic nature (Henck et al. 2000), or the products of desolvations may be easily recognized (Schinzer et al. 1997). Intimate processes of polymorphic behaviour, such as nucleation, crystal growth, habit transformation, sublimation and properties of the melt (e.g. degradation) may be readily observed and video recorded (de Wet et al. 1998). [Pg.249]

We shall first consider the extreme situations of Figures 2 and 3. When the perfect crystal of Figure 2 is reheated, it melts directly into the isotropic liquid at T. thus such a system displays the mesophase only on cooling, and is called "monotropic". On the other hand, the imperfect crystal of Figure 3 first changes back into the mesophase at T -lc an< fnt0 the isotropic... [Pg.313]

Metaboric acid is known in three crystalline modifications, which provide a good example of monotropism (Fig. 24.12) and of the increase in density with change from 3- to 4-coordination of B (Table 24.4). The monoclinic form is readily prepared by the dehydration of H3BO3 in an open vessel at 140°C quenching of the molten material gives a glass which later recrystallizes as the orthorhombic form. If the melt is held at 175°C the most stable (cubic) form is slowly precipitated, while complete dehydration at about 230°C yields B2O3. [Pg.854]

The thermodynamic activity of each crystalline form, represented by its solubility, may change quite differently as a function of temperature. Monotropic systems are dehned as systems where a single form is always more stable regardless of the temperature. Enantiotropic systems are dehned as systems where the relative stability of the two forms inverts at some transition temperature (Bym et al., 1999). [Pg.145]

Chang and Hung [83] have synthesised 22 below, in which an extra double bond is inserted into the N-(4-carboxyphenyl trimellitimide) unit. When copolymerised with an even chain alkane diol, an enantiotropic PEI is formed (MI=10) when the spacer is odd, only a transient monotropic phase is observed (MI=9.4). This is further evidence that the critical value for mesogenicity in PEIs is close to 9.5. It is of interest to note that the diacid employed in the copolymer, N-(car-boxyphenylacrylic) trimellitic anhydride acts as the complete mesogenic unit for the copolymer. [Pg.225]


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See also in sourсe #XX -- [ Pg.185 ]




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