Liquid crystals, thermal stability

Borides have been prepared as single crystals by making use of gas-phase, liquid-phase and flux methods, depending on the thermal stability of the boride and on the required size and perfection of the single crystals. 

Liquid crystals based on aliphatic isocyanides and aromatic alkynyls (compounds 16) show enantiotropic nematic phases between 110 and 160 °C. Important reductions in the transition temperatures, mainly in clearing points (<100 °C), areobtained when a branched octyl isocyanide is used. The nematic phase stability is also reduced and the complexes are thermally more stable than derivatives of aliphatic alkynes. Other structural variations such as the introduction of a lateral chlorine atom on one ring of the phenyl benzoate moiety or the use of a branched terminal alkyl chain produce a decrease of the transition temperatures enhancing the formation of enantiotropic nematic phases without decomposition. 

It is interesting to note the influence of the counteranions on the thermal behavior. Irrespective of the isocyanide used, all the nitrate gold derivatives show low thermal stability and undergo extensive decomposition at relatively low temperatures (only the low melting trialkoxyphenyl derivative shows liquid crystal behavior). In contrast. 

Several mixtures of hexanethiol capped gold nanopartides and triphenylene based discotic LCs have been studied. These mixtures display liquid crystal behavior (columnar mesophases) and an enhancement in the DC conductivity, due to the inclusion of gold nanoparticies into the matrix of the organic LC [70]. Other studies of mixtures of gold nanoparticies with mesogens indude a series of cholesteryl phenoxy alkanoates. The inclusion of the nanopartides does not change the inherent liquid crystal properties of the cholesteryl derivative but the mesophases are thermally stabilized [71]. 

A review of the literature demonstrates some trends concerning the effect of the polymer backbone on the thermotropic behavior of side-chain liquid crystalline polymers. In comparison to low molar mass liquid crystals, the thermal stability of the mesophase increases upon polymerization (3,5,18). However, due to increasing viscosity as the degree of polymerization increases, structural rearrangements are slowed down. Perhaps this is why the isotropization temperature increases up to a critical value as the degree of polymerization increases (18). 

Figure 1.20 Encapsulation of microdroplets of liquid crystals in ORMOSIL matrices results in materials with better transparency and thermal stability than polymer-dispersed liquid crystals. Gel-glass dispersed liquid crystal device switched between the OFF and ON state (thickness 10 pm, 4 x 2 cm, Fp p = 90V). (Reproduced from ref. 45, with permission.)...

The thermal stability of metal nanoparticles (for the most part nanoparticles used in liquid crystal phases with high phase transition temperatures or nanoparticles decorated with functional molecules) should also be of significant importance,... 

Most reports over the past 4 years have dealt with the manipulation of display-related parameters such as electro-optic response and alignment, but increasingly also with thermal effects, pattern formation, nanoparticle-liquid crystal compatibility (i.e., enhancing the stability of dispersions), and to some degree with nanoparticle organization. 

The long-term stability of the nanomaterials used as well as the liquid crystal composites containing these are also very critical points that will need to be addressed with future research. Likewise, a complete characterization of all nanoparticles used is essential given the plethora of possible dissimilarities from one batch to another including size, size distribution, surface coverage, thermal stability, and ligand distribution on the nanoparticle surface (for mixed monolayer-capped nanoparticles), to name but a few. 

Differential Scanning Calorimetry (DSC) is a sensitive way of detecting phase transformations of a bulk material [85,86]. Monitoring the thermal behavior of a crystal or a powder as a function of its conversion to product can give important information. This technique can verify whether a reaction occurs in a purely solid phase or whether there may be liquid phases involved at a given temperature. Melting point depression can be monitored as product appears, and the characteristic melting of a new phase can be detected if one is formed. DSC can reveal whether or not a eutectic transition attributable to a mixture of phases is present. We have also used DSC in our lab to monitor the thermal stability of reactive crystals. 

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