Hexagonal-orthorhombic phase transition

Miyashita et al. [102] have proposed a possible model for the hexagonal-orthorhombic phase transition (Fig. 10). They proposed that since the hexagonal crystal contains many defects along the chain axis, such as kinks and jogs, on its phase transition to the orthorhombic phase, these defects are excluded from the crystal... 

Figure 13.18 Schematic picture showing temperature against reciprocal size of polyethylene. Subscripts refer to phases (o, orthorhombic h, hexagonal) or phase transition (tr, transition from hexagonal phase to orthorhombic phase) of polyethylene. Keller et al. [102]. Reproduced with permission of The Royal Society.

Conversely, the high-pressure hexagonal phase is one of high entropy for which the reverse inequality is true. In lower -paraffins, the occurrence of the rotator phase at atmospheric pressure is due to the fact that the orthorhombic-hexagonal (or rotator) transition temperature, is lower than the temperature of fusion of... 

Figure 10 Model for the phase transition of PE from the hexagonal to the orthorhombic phase, (a) In the hexagonal phase, there are many defects along C-axis (kinks, jogs), (b) On the phase transition, these defects are excluded to form boundaries between bands. (From Ref. 102.)...

Figure 18 Molecular weight dependencies of the phase transition temperature (T,) from orthorhombic to hexagonal phase and the melting temperature Tm) of the hexagonal phase of PE. O = phase transition from orthorhombic to hexagonal phase A A = melting of the hexagonal phase. (From Ref. 131.)...

Chatani42,43 has shown that if the hexagonal inclusion compounds are cooled many of them undergo a reversible transformation into a low-temperature form with the orthorhombic space group These phase transitions take place over a... 

An enormous number of phase transitions are known to occur in common solid compounds. For example, silver nitrate undergoes a displacive phase transition from an orthorhombic form to a hexagonal form at a temperature of approximately 162°C that has a enthalpy of 1.85 kj/mol. In many cases, the nature of these transitions are known, but in other cases there is some uncertainty. Moreover, there is frequently disagreement among the values reported for the transition temperatures and enthalpies. Even fewer phase transitions have been studied from the standpoint of kinetics, although it is known that a large number of these transformations follow an Avrami rate law. There is another complicating feature of phase transitions that we will now consider. 

Figure 20 shows the phase diagram of polyethylene119). The existence range of the condis crystals increases with pressure and temperature. The enthalpy of the reasonably reversible, first order transition from the orthorhombic to the hexagonal condis phase of polyethylene is 3.71 kJ/mol at about 500 MPa pressure 121) which is about 80 % of the total heat of fusion. The entropy of disordering is 7.2 J/(K mol), which is more than the typical transition entropy of paraffins to their high temperature... 

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