Virtual transition temperature

Note 1 A virtual transition temperature lies outside the temperature range over which the (meso) phase implied can be observed experimentally. 

Note 2 A virtual transition temperature is not well defined it will, for example, depend on the nature of the liquid-crystal components used to construct the phase diagram. 

Note 3 A virtual transition temperature is indicated by placing square brackets, [ ], around its value. 

Parentheses represent a monotropic transition temperature brackets represent an extrapolated virtual transition temperature Sma-N transition at 134 C. 

Virtual transition temperatures obtained from mixtures by extrapolation. 

Figure 19. (a) Pressure-temperature diagram of benzene hexa-n-hexanoate. The mesophase-isotropic transition line extrapolated to atmosphere pressure yields a virtual transition temperature of 89 °C. (From Chandrasekhar et al. [63], reproduced by permission of Academic Press), (b) Miscibility diagram of benzene hexa-n-hexanoate and the heptanoate. The virtual mesophase-isotropic transition temperature for the hexanoate is 89 °C, in agreement with the value obtained from the pressure-temperature diagram (a). From Billard and Sadashiva [64], reproduced by permission of the Indian Academy of Sciences). 

The study of binary mixtures of LMMLCs has attracted considerable attention since the earliest days of LC research. Mixtures have played an important role in the technology of LC display devices by extending the range of the nematic phase to room temperature. Moreover, extrapolation of the transition temperatures of binary systems involving a non-mesogen sometimes provides an estimate of the virtual transition temperature of the latter component. This is a quantity of theoretical interest and one which obviously cannot be determined experimentally from study of the non-mesogen as a single component. 

The effect of the lanthanide contraction on the metal and ionic radii of hafnium has already been mentioned. That these radii are virtually identical for zirconium and hafnium has the result that the ratio of their densities, like that of their atomic weights, is very close to Zr Hf = 1 2.0. Indeed, the densities, the transition temperatures and the neutron-absorbing abilities are the only common properties of these two elements which differ... 

Probably the best understood and most commonly used property of polymers, glass transition temperatures are important in virtually... 

The heat distortion temperature of impact-resistant polystyrene may also be improved by polymer blends. Those of impact-resistant polystyrene with poly-2,5-dimethylphenylene-1,4-oxide (PPO) are particularly interesting (90). Polystyrene and PPO are molecularly compatible and mixtures of them have glass transition temperatures which vary virtually linearly with composition. A further advantage of these compositions which should not be under-estimated is their better flame resistance. 

The isomeric bibenzoic acids (BBs), would appear to share similar structural features with naphthalene dicarboxylic acid. Like the PET-naphthalate copolymers, PET-bibenzoates have been demonstrated to possess moduli and glass transitions temperatures which increase with increasing levels of rigid comonomer [37-39], Unlike the PET/PEN copolymers, when the symmetrical 4,4 - I f I f monomer is substituted into a PET backbone, virtually every composition of PET-BB is semicrystalline the 2,4- and 3,4- isomers of BB, when... 

Often, however, there is insufficient molecular mobility in the solid state for asymmetric fluorine placement to significantly increase the dielectric constant. Below the glass transition temperature, only restrained local motions are possible, and below subglass relaxations such as the P relaxation in polyimides, even these limited motions are virtually eliminated, rendering orientation polarization negligible. 

Tphe complexing of virtually all purines with aromatic molecules seems - to have far-reaching biological significance. For example, it is known that caffeine affects the rates of many enzymatic reactions (e.g., 0.01, 0.05, and 0.10M caffeine will inhibit salivary amylase 29, 54, and 72% respectively) (12), and purine can decrease the helix-coil transition temperature of the proteins bovine serum albumin and lysozyme (2). It is not unreasonable to expect the involvement of caffeine-aromatic and purine-aromatic complexes because caffeine derivatives and purine complex with the aromatic amino acids tyrosine, phenylalanine, and tryptophan (2). (In fact tryptophan forms a stable 1 to 1 crystalline complex in 0.5M theophylline glycol.)... 

The three adjustable parameters are determined, A/kB = 90 K, Jo/kB = -36 K, and J /kB = 125 K, so as to reproduce the spin-crossover transition temperature Tc = 48 K, the virtual Jahn-Teller transition temperature rJT = 6 = 26 K, and the effective LS-HS gap in the LS phase Acff/kB = 340 K. (Note Aeff is approximated by A + 2Jx in this mean-field model.) This choice of model parameters gives a phase sequence from the LS to HS with increasing temperature, corresponding to the arrow path in Fig. 7. Temperature dependence of thermodynamic quantities (Fig. 8) is calculated along the path indicated by the arrow in Fig. 7, where the discontinuities arising from the first-order spin-crossover transition are recognized Ap0 = 0.99, AH = 0.64 kJ mol-1, and AS = 13.3 J K-1 mol-1 These theoretical... 

Fig. 8 Free energy per chain bond of the liquid (L), of the bulk mesophase (M), and of the crystalline (C) polymer. Txy is the transition temperature from X to Y (X, Y = L, M, C). a In this case the mesophase is stable between 7cm and 7ml while Tcl is virtual. (From [11]) b The mesophase is virtual, and so are 7cm and 7ml melting of the crystal into the liquid is only observable transition occurring at Tcl...

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