Metastable hexagonal phase

Fig. 7 Electron micrographs of two different virgin UHMW-PE samples, (a) commercially synthesized Z-N (grade B) and (b) laboratory-synthesized Z-N (grade A). The samples were annealed in the hexagonal phase, at 1500 bars, 190 °C for 30 minutes. Under these conditions the samples were in the thermodynamically metastable hexagonal phase. (For details see references [27,28])...

The value of Xs tends to infinity as the denominator (7 0 - 7 h) -> 0 as T - Tt. Conversely, as (T 0 - T h) increases, i.e. the hexagonal phase becomes more metastable, A,s decreases as more surface contributions to the free energy, offset by the enthalpy of crystallization, are required, which according to the hypothesis must favour the hexagonal phase. 

While there have been efforts to polymerize other surfactant mesophases and metastable phases, bicontinuous cubic phases have only very recently been the subject of polymerization work. Through the use of polymerizable surfactants, and aqueous monomers, in particular acrylamide, polymerization reactions have been performed in vesicles (4-8). surfactant foams ), inverted micellar solutions (10). hexagonal phase liquid crystals (111, and bicontinuous microemulsions (121. In the latter two cases rearrangement of the microstructure occured during polymerization, which in the case of bicontinuous microemulsions seems inevitable b ause microemulsions are of low viscosity and continually rearranging on the timescale of microseconds due to thermal disruption (131. In contrast, bicontinuous cubic phases are extremely viscous in genei, and although the components display self-diffusion rates comparable to those... 

Neodymium falls into the first group in forming a cubic trihydride. However, if the hydride is prepared at < 350°C, a hexagonal Nd trihydride is formed , similar to those in the second group. This trihydride is metastable, however heating >350°C transforms it to the cubic phase, and subsequent cooling < 350°C does not reform the hexagonal phase. 

The thermodynamic properties of hexagonal AIN and cubic transition metal nitrides and carbides are known, but for the calculation of the ranges of metastability of the cubic and hexagonal phases in the AlN-MeN or AlN-MeC systems, the energy differences for the transitions from the stable to the metastable structures, e.g. MeNj i, —> MeN and AlNi, —> AlN b primary importance. In addition, the... 

Figure 11 Calculated ranges of the metastable cubic and hexagonal phases for a number of MeN-AlN systems at 500 K...

Triply hydrogen-bonded 1 1 liquid-crystalline complexes 23 are formed by molecular-recognition processes of uracyl and 2,6-diaminopyridine derivatives [27], The complex with R - CH3, k = 1 = 10, m = 11, and n = 16 exhibits a metastable columnar hexagonal phase below 72 °C, In this case, the complex contains four long aUcyl chains. An X-ray study has shown that two complex molecules form one column side-by-side. 

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