Crystalline state cold drawing

Dilatometric studies have demonstrated the negative thermal expansivity for many oriented crystalline polymers 64,170 176). The results of these experimental studies may be summarized as follows. Cold-drawing of PE below Tm 172) and solid-state extrusion under elevated pressure 170 1711 lead to a monotonous decrease of the positive thermal expansion coefficient with increasing draw ratio. At a certain degree of orientation, dependent on temperature, PM becomes negative with Pi < Pell (Fig. 16). This is the second way of reaching negative expansivity applied, e.g. to POM (w = 63 % Tdr = 423 K) 173>. 

The behavior of hehum at low temperatures is quite different from that of all other materials. When cooled under atmospheric pressure, helium liquefies at 4.2 K, but it never solidifies, no matter how cold it is made. This behavior is inejqrlicable in terms of classical physics where, at a sufficiently low temperature, even the very weak interatomic forces that exist between helium atoms should be sufficient to draw the material together into a crystalline solid. Furthermore, the liquid in question has maity extraordinary properties. In particular, it undergoes a transition at 2 K to a state of superfluidity, such that it has zero viscosity and can flow without dissipation of energy, even through channels of vanishingly small dimensions. This frictionless flow of the liquid is closely analogous to the frictionless flow of the electrons in a superconductor. On Earth, superfluidity and superconductivity are exclusively low-temperature phenomena, but it is inferred that they probably also arise in the proton and neutron fluids within neutron stars. 

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