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Monotropic phenomenon

Fig. 10.6 Illustration of monotropic phenomenon on the cooling and heating curves of the metastable mesophases. (a) Free energy curves (b) DSC heating and cooling curves... Fig. 10.6 Illustration of monotropic phenomenon on the cooling and heating curves of the metastable mesophases. (a) Free energy curves (b) DSC heating and cooling curves...
Polymorphism is the ability of the same chemical substance to exist in different crystalline structures that have the same empirical composition [39,40]. It is now well established that DSC is one of the core technologies used to study the phenomenon. Polymorphic systems are often distinguished on the basis of the type of interconversion between the different forms, being classified as either enantiotropic or monotropic in nature. [Pg.91]

Phase diagrams with monotropic transformation or enantiotropic transformation have been discussed. " Quite interesting is the transformation of the racemic compound into a conglomerate since this phenomenon can be used for purification via crystallization, as described for nimodipine. DSC is applied for the establishment of phase diagrams, for the determination of thermodynamic data, for the purity determination, or for the monitoring of industrial resolutions. For the establishment of phase diagrams it is suitable to add spectroscopic or crystallographic methods. ... [Pg.3741]

Polymorphism. Polymorphic literally means multiform, but the term does not refer to variation in external shape. It indicates that crystals of the same molecules have different unit cells, be it of the same or of a different crystal system. The phenomenon is quite common. There are two types of polymorphism. Enantiotropic polymorphs each are stable within a certain range of temperature and pressure. Consequently, a phase diagram of the various polymorphs can be made. The prime example is ice (Section 15.3.1). If monotropic polymorphs exist, all but one of these are unstable. There is no phase diagram and, given time, only the most stable form will remain. The prime examples are compounds with long paraffinic chains, including most lipids (especially acylglycerols), where three main polymorphs exist (a, (F, and (3). [Pg.609]

Another interesting phenomenon found by Stevens et al. is the monotropic mesophase formed by the dimer and trimer of the polymer with m = 6. The dimer has a monotropic nematic phase, the trimer has a monotropic smectic phase. These metastable monotropic phases become stable enantiotropic phases with the increase of n by 1. At about the same time, Blumstein et al. (1984) found the low mass model compound of a main-chain type liquid crystalline polymer was monotropic while the mesophase of the polymer was enantiotropic. [Pg.150]

The mesophase state of liquid crystals is normally opaque due to relatively large sizes of ordered domains. Its transition point to the isotropic melt state is called the clear point T,. The DSC scanning curves of liquid crystals can exhibit either enantiotropic or monotropic phenomena. For the thermodynamically stable mesophases of liquid crystals, they occur between the melt and the crystal states during both cooling and heating processes, as illustrated in Fig. 10.5. When both the cooling and heating curves show two symmetric consecutive phase transitions, it is known as the enantiotropic phenomenon. In contrast, for the metastable mesophase... [Pg.191]

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 ... [Pg.328]

A melt to recrystallisation to melt phenomenon on a DSC curve may represent either monotropic or enantiotropic conversion. Small amounts of decomposition may favour transitions or the apparent production of a polymorph. Frequently thermogravimetry will detect this decomposition. An example of monotropic polymorphism is displayed by temazepam [11]. [Pg.930]


See other pages where Monotropic phenomenon is mentioned: [Pg.192]    [Pg.192]    [Pg.23]    [Pg.153]    [Pg.226]    [Pg.149]    [Pg.301]    [Pg.202]    [Pg.56]    [Pg.15]    [Pg.396]    [Pg.31]    [Pg.2172]    [Pg.255]   
See also in sourсe #XX -- [ Pg.192 ]




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