Order-disorder temperature

The ability of XPD and AED to measure the short-range order of materials on a very short time scale opens the door for surface order—disorder transition studies, such as the surface solid-to- liquid transition temperature, as has already been done for Pb and Ge. In the caseofbulkGe, a melting temperature of 1210 K was found. While monitoring core-level XPD photoelectron azimuthal scans as a function of increasing temperature, the surface was found to show an order—disorder temperature 160° below that of the bulk. 

The shape of the enthalpy-temperature curve is similar to the volume temperature curve through the order-disorder temperature range in the case of polytetrafluoro-ethylene, Fig. 14. The difference in temperature between the two curves at the inflection point may be due to a difference in heating rate or to a difference in the samples studied, probably the former. 

It should also be noted that the stability of the distinct mesophases can be quite different. It seems that there is a significant effect of molecular shape and topology, stabilizing SmA phases in the system 41/43 and Colhex phases in the system 35/37. In addition, the mesophase stability is often reduced close to the transition to another mesophase (see Fig. 15). Hence, the order-disorder temperatures can only be roughly estimated based on segmental solubility parameters [24, 25]. 

Fig. 17 Time evolution of the SAXS spectra of a 32.5 wt% solution of SI in toluene exposed to an electric field of 3 kV/mm during heating cycle. The order-disorder temperature (7odt) was 53.3°C. The dashed lines correspond to temperature above the 7odt and the solid lines to temperatures below 7odt- Data in black correspond to the pattern shown in Fig. 16. Reprinted with permission from Macromolecules [72], Copyright 2009 American Chemical Society...

The decomposition temperature of the CBA should be higher than the melting point (Tm) or glass transition temperature (Tg) of the polymer. The blending operation of CBA and the polymer should be conducted at a temperature sufficiently above (close to the order-disorder temperature Toa = or Tg + 55°C) but preferably below the decomposition temperature of the CBA. 

The order parameter is essentially a kinematic measure, describing the state of order within a system without any intrinsic reference to what factors drove the system to the state of interest. For example, in thinking about the transition between the ordered and disordered states of an alloy, it is useful to define an order parameter that measures the occupation probabilities on different sublattices. Above the order-disorder temperature, the sublattice occupations are random, while below the critical temperature, there is an enhanced probability of finding a particular species on a particular sublattice. The conventional example of this thinking is that provided by brass which is a mixture of Cu and Zn atoms in equal concentrations on a bcc lattice. The structure can be interpreted as two interpenetrating simple cubic lattices where it is understood that at high temperatures we are as likely to find a Cu atom on one sublattice as the other. A useful choice for the order parameter, which we denote by r], is... 

The order-disorder temperature of y , FeNi3 is lowered by the addition of Cu [195 IChe]. The Ni3Fe long-range order is destroyed at 9-10 at.% Cu [1970Goml, 1970Gom2]. 

It is energetically favourable for the SS and HS not to mix. Thus during cooling from above a critical order-disorder temperature, spontaneous segregation of SS and HS into separate soft (SS-rich) and hard (HS-rich) phases occurs by the process of spinodal decomposition. To achieve elastomeric performance, the SS must be the majority constituent by mass, and the phase structure then takes the form of discrete hard domains dispersed within a soft matrix. Such a phase structure impacts on mechanical properties, and a further structural parameter of importance, therefore, is the degree of phase separation. 

Hard segment (hs) transition temperature designated Tg, altmiatively described as an order-disorder temperature for die hard block. 

We start by discussing the structural phase behavior of symmetric diblock copolymers in selective solvent, which will be used as a point of comparison. Figure 13(a) plots the approximate structural phase behavior for the simple Lennard-Jones bead-spring model commonly used to study amphiphilic sys-tems as a function of volume fraction, temperature state point below the order-disorder temperature. Specifically, simulations were performed for a symmetric hStS... 

Using single crystals, [1995Pal] investigated the effect of C on the order-disorder transition temperature involving (aFe) and B2 phases. Their results are shown in Fig. 12. It is obvious that at a constant A1 content, C increases the order-disorder temperature. This was attributed to the fact that in 52 structure six Fe atoms create favorable sites for the C atoms. In addition, ab initio calculations show that C atoms prefer to occupy the Fe sublattice in 52-FeAl leading to an increase in cohesion [2006Kel]. 

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