Polyethylene -the Hexagonal Phase |
| At atmospheric pressure typical linear polyethylene is normally found
in either the orthorhombic solid or, above its (practical) melting
point of around 130°C, the liquid phase. This melting point rises about
20°C with every kbar or 100MPa of pressure. Above about 3kbar, the
orthorhombic phase does not melt directly, but transforms into a hexagonal
phase, whose zone of stability expands by about 8° with every 1kbar
increase in pressure.
The process also occurs in reverse, either by cooling or stepping up the pressure, going from melt-hexagonal-orthorhombic. This can even occur at pressures somewhat below 3kbar, with metastable hexagonal crystals forming first. Under suitable conditions individual crystals can be watched in the diamond anvil as they grow for several minutes, until suddenly with a "visible pop" they transform to orthorhombic, after which their growth rate is imperceptiple. One way in which the hexagonal phase can be made more stable is by crosslinking PE with ionizing radiation. This comes about because the molten crosslinked PE cannot relax to an isotropic melt, thus increasing its entropy and moving the thermodynamic goalposts in regard to the phase diagram. A similar but transient effect is observed when melting highly oriented
fibres of high molecular weight PE. Here the molecules are highly entangled:
this acts like temporary crosslinks, and so if the heating is carried out
faster than the material can relax (order of minutes if the fibres are
firmly compacted or clamped) the effective melting point is raised by up
to 20°, allowing an orthorhombic-hexagonal transition to occur.
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The Diamond Anvil Cell |
| If a thin gasket or washer of metal is ferociously squashed between
the faces of two diamond, it starts to flow. If the hole is filled with
a soft material, this leads to very high and effectively hydrostatic pressures
at the centre of the hole. Since diamonds are transparent to light and
to X-rays, the material can then be studied at very high pressures (GPa)
either under the optical microscope or by X-ray crystallography.
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