Supercooled Smectic

Dynamic magnetic resonance techniques do not suffer from this deficiency. In particular, the use of CSL probes [8, 101, 102] offers the chance of determining the micro- and macroorder unequivocall This is demonstrated in Fig. 16. The spectra refer to the side chain polymers 2 (M = 14000) and two different temperatures (T = 382 K, left column T = 263 K, right column). Drastic spectral changes are observed when the sample is rotated (top row, q = 0 central row, q = 50°, bottom row, Q = 90°). Comparison of the angular variation in the nematic (left column spectra) and supercooled smectic phase (right column spectra) reveals the crucial effect of the microorder on the spectral feature. 

Nanoparticles based on the supercooled smectic phase of cholesterol esters were introduced as a potential novel carrier system for lipophilic drugs. Due to the high viscosity of the smectic phase but mobility on the molecular level, advantages in... 

Figure 10.8. General phase behavior of cholesterol esters used for the preparation of supercooled smectic nanojrarticles. A dotted line indicates supercooling of the transition. Abbreviations C crystalline, Sm smectic, Ch cholesteric and I isotropic liquid phase.

Figure 10.9. DSC heating and cooling curves (5°C/min) of cholesteryl myristate in the bulk and in colloidal dispersion (5% CM, 2% PVA PCS z-average 172 nm, PDI 0.09). With kind permission from Springer Science + Business Media Fharm. Res. Supercooled smectic nanoparticles A potential novel carrier system for poorly water soluble drugs, 21, (2004), 1834-1843, J Kuntsche et al. Abbreviations DSC differential scanning calorimetry, PVA polyvinyl alcohol, PCS Photon correlation spectroscopy, PDI Polydispersity index.

Electron microscopic investigations (cryo-TEM and freeze-fracture ) indicate that supercooled smectic nanoparticles have a nearly cylindrical particle shape in most cases. This is in agreement with the cylindrical shape of LDL when studied at room temperature where the LDL are in a liquid crystalline state. ... 

Stability of file Supercooled Smectic Phase and Influence of Hie Nanoparticle Compo on... 

As the supercooled smectic state is metastable, the recrystallization tendency of the nanoparticles during storage is of high interest. Due to its completely reversible phase behavior and favorable crystallization temperature in the colloidal state, cholesteryl myristate was used as model cholesterol ester for investigations of the recrystallization tendency and influencing parameters like particle size and stabilizer system. 

Compared to lyotropic LCNP the thermotropic mesophase-based supercooled smectic nanoparticles are in a still much more early stage of development. Although structural aspects of these nanoparticles are not quite as complex as with lyotropic LCNP and the physicochemical properties of these nanoparticles as well as the influencing parameters on the phase behavior have been studied in some detail yet much remains to be done for further optimization of the formulations in particular with regard to the stability of the nanoparticles against recrystallization. Furthermore, application related studies have to be performed, especially with a focus on parenteral drug delivery which was the main driving force for the development of these particles. 

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. 

On heating from a crystalline phase, DOBAMBC melts to form a SmC phase, which exists as the thermodynamic minimum structure between 76 and 95°C. At 95°C a thermotropic transition to the SmA phase occurs. Finally, the system clears to the isotropic liquid phase at 117°C. On cooling, the SmC phase supercools into the temperature range where the crystalline solid is more stable (a common occurrence). In fact, at 63°C a new smectic phase (the SmF) appears. This phase is metastable with respect to the crystalline solid such phases are termed monotropic, while thermodynamically stable phases are termed enantiotropic. The kinetic stability of monotropic LC phases is dependent upon purity of the sample and other conditions such as the cooling rate. However, the appearance of monotropic phases is typically reproducible and is often reported in the phase sequence on cooling. It is assumed that phases appearing on heating a sample are enantiotropic. 

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