Monotropic transition

The different phase behaviors are evidenced in the corresponding free energy diagrams, which have been estimated for both polymers [15]. These diagrams are shown in Fig. 10 (due to the different approximations used in the calculation of the free energy differences, these diagrams are only semiquantitative [15]). It can be seen that the monotropic transition of the crystal in... 

Most solid materials produce isotropic liquids directly upon melting. However, in some cases one or more intermediate phases are formed (called mesophases), where the material retains some ordered structure but already shows the mobility characteristic of a liquid. These materials are liquid crystal (LCs)(or mesogens) of the thermotropic type, and can display several transitions between phases at different temperatures crystal-crystal transition (between solid phases), melting point (solid to first mesophase transition), mesophase-mesophase transition (when several mesophases exist), and clearing point (last mesophase to isotropic liquid transition) [1]. Often the transitions are observed both upon heating and on cooling (enantiotropic transitions), but sometimes they appear only upon cooling (monotropic transitions). 

Note Monotropic transition temperatures are indicated by placing parentheses, (), around the values. 

Parentheses represent a monotropic transition temperature. SmC-N transition at 95 °C. 

Parentheses represent a monotropic transition temperature. Measured at a reduced temperature of 0.96 x Tn-i- Extrapolated value from a 10% concentration in a nematic mixture. 

Lehmann further reduced Mallard s structural crystallites to be aggregates of physical molecules . Then the structural crystallites could differ in the number or in the arrangement of the physical molecules of which they were composed, thereby constituting the difference between two polymorphs. These distinctions were then related to the transformation phenomena an enantiotropic transformation was characterized by Lehmann as a reversible polymerization that is, with an increase in temperature, elementary particles of a large size were transformed into elementary particles of a smaller size. In a monotropic transition, according to Lehmann, there is no such relationship between temperature and the mode of rearrangement. 

Where ( ) denotes a monotropic transition [ ] denotes a virtual transition (observed on fast supercooling) denotes a phase transition where the enthalpy was too small to be measured. 

It was used for the identification of the smectic B phase in another substance (4-n-hexyloxyphenyl-4 -n-decyloxybenzoate, HOPDOB). Figure 1.16 shows the corresponding phase diagram for mixtures of the two compounds. For the sake of simplicity, all lines for the monotropic transitions are removed. Prom the diagram it is clear that the nematic, smectic A, and smectic B phases (but not the 5c phase) are extended from pure HBAB to pure HOPDOB and the following sequence of phases for HOPDOB was established ... 

FIGURE 1.16. The isobaric phase diagram of mixtures HOPDOB-HBAB (a simplified version of [42]). Dashed lines correspond to monotropic transitions. 

Monotropic transitions describe mesophases that are not thermally reversible they are encountered on heating only or on cooling only. This phenomenon is exhibited by the nonlinear mesogen bis(p-heptyloxyphenyl)-2,5-thiophene dicar-boxylate (2) with = 6 ... 

Virtual temperatures in brackets and monotropic transitions in parentheses. For all extrapolated values the datapoint for the pure host is included unless specified otherwise. Typical precision of the extrapolated values 2 K. 

Values in parentheses represent monotropic transition temperatures. 

Monotropic transitions determined by hot stage polarizing microscopy enthalpies could not be measured. 

Colho = columnar hexagcmal ordered phase, = monotropic transition, observed by polarising mio-oscopy on cooling 2,4,7-trinitrofluorenone (TNF, 5a), 6 chiral (->2-(2,4,5,7-tetranitro-9-fluorenylideneaminooxy)propionic acid ((-)-TAPA, 5b, commwcially available) or (2,4,7-trinitro-9-fluorenylidene)malonic bidiexadecylester (5c) [18]. The transition temperatures of this doncx Nd phase are given in TABLE 3. Here, due to the diind TAPA an induced chiral nonatic phase. 

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