Hexagonal phase, high-pressure

A large number of experiments [22,68-71] have shown that the specific volume, vi, and specific entropy, Sh, of the high-pressure hexagonal phase of PE are closer to the corresponding values v , and s , of the melt than they are to those of the orthorhombic phase, and s. For instance, at 500 MPa, the ratio of entropies of the orthorhombic-hexagonal and hexagonal-melt transi-... 

Besides the thermodynamic modelings, the statistical mechanical analysis has also been used to study the high-pressure hexagonal phase of PE. Priest [73-75] re-... 

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... 

Properties. Thallium is grayish white, heavy, and soft. When freshly cut, it has a metallic luster that quickly dulls to a bluish gray tinge like that of lead. A heavy oxide cmst forms on the metal surface when in contact with air for several days. The metal has a close-packed hexagonal lattice below 230°C, at which point it is transformed to a body-centered cubic lattice. At high pressures, thallium transforms to a face-centered cubic form. The triple point between the three phases is at 110°C and 3000 MPa (30 kbar). The physical properties of thallium are summarized in Table 1. 

In addition to the occurrence in PE at high temperature and pressure, the hexagonal phase is frequently found in paraffin substances under various circumstances. The analysis of crystal structure and order of these hexagonal phases is not only interesting, but also important for the understanding of their origins. The circumstances in which hexagonal phases arise in paraffin substances can be summarized as follows ... 

A hexagonal phase appears at atmospheric pressure in ultra-drawn PE fibers containing highly extended chains when the fiber is heated above the normal temperature while the ends are held fixed to prevent retraction [89-92]. 

The presently described case of formation of PE hexagonal phase at suitably high temperatures and pressures. 

On heating a sufficiently high-dose (>500 Mrad) irradiated PE, a hexagonal phase can be induced to form prior to melting at atmospheric pressure [93,94]. 

The heat of fusion of the rotator phase in n-paraffms is 38-40 cal/g, while AHo- m for PE irradiated with 1000 Mrad is about 29 cal/g. The corresponding value of AHo->m for hexagonal PE at high pressure (500 MPa) is only around 15 cal/g. Thermograms of constrained PE fibers showed similarly low heats of fusion of the hexagonal phase. 

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