Amorphous state density

Dependence on Density.—If the density of a metal is increased by hammering, its specific heat is slightly decreased. The same change is observed if the change of density is due to a change of crystalline form, or to change from an amorphous state to a crystalline state, and with different allotropic forms (Wigand, loc. cit,). 

The density of the rubbery amorphous state is only slightly higher than that of the molten state, the difference being attributable to reduced thermal motion of its chains. In this loosely packed condition, the polymer incorporates a substantial amount of molecular scale void... 

Fig. 2-33. Electron energy and state density in amorphous semiconductors A and C = diffuse band tail states B = gap states, cmc = mobility edge level for electrons MV = mobility edge level for holes ...

The diffusion coefficient D is inversely related to the cross-link density of vulcanized rubbers. When D is extrapolated to zero concentration of the diffusing small molecules, it is related to the distance between the cross-links. Thus, as the cross-link density increases D becomes smaller, as expected. Further, the diffusion coefficient is less for crystalline polymers in comparison with the same polymer except in the amorphous state. In fact, this can be roughly stated as follows. 

There are three significant possible effects when radiation interacts with matter (5,6). First, the radiation can interact with the nucleus and induce radioactivity as in the case of neutrons. Second, displacement of atoms can occur. This has happened in a number of uranium- and thorium-containing minerals over geological periods. The outstanding example is zircon, which can contain over 10% Th and 2% U. The internal bombardment from these materials and their decay products over geological periods produces low or metamict zircon, where the disorder gives an amorphous state having a low density. 

Notice that transformation from a crystalline phase to presumably metastable amorphous phases is called amorphization. It is very promising to use for making of adjustable stores hydrogen fuel phenomena that is called polyamorphism. This term meaning that the pure material can exist in more than one amorphous state. In principle, the abovementioned mechanism of density jumps at polyamorphic transition of ice allows to obtain reversible accumulation of methane inside cellular nanostructures of cryogenic amorphous ice. It is important that the degree of accumulation can be sharp adjusted by pressure and temperature. 

Low-Density Amorphous Ice (LDA). Upon heating HDA to T > 115 K or very high density amorphous ice (VHDA) to T > 125 K at ambient pressure, the structurally distinct amorphous state LDA is produced. Alternatively, LDA can also be produced by decompressing HDA or VHDA in the narrow temperature range of 139-140 K to ambient pressure [153-155]. The density of this amorphous state at 77 K and 1 bar is 0.93 g/cm3 [152]. These amorphous-amorphous transitions are discussed in Sections III.C and III.D. 

Very High Density Amorphous Ice (VHDA). By annealing HDA to T > 160 K at pressures > 0.8 GPa, a state structurally distinct from HDA can be produced, which is called VHDA ice [152]. The structural change of HDA to a distinct state by pressure annealing was first noticed in 2001 [152]. Even though VHDA was produced in experiments prior to 2001 [170], the structural difference and the density difference of about 10% at 77 K, and 1 bar in comparison with HDA remained unnoticed. Powder X-ray diffraction, flotation, Raman spectroscopy, [152] neutron diffraction [171], and in situ densitometry [172, 173] were employed to show that VHDA is a structural state distinct from HDA. Alternatively, VHDA can be prepared by pressurization of LDA to P > 1.1 GPa at 125 K [173, 174] or by pressure-induced amorphization of hexagonal ice at temperatures 130 K < T < 150 K [170]. The density of this amorphous state at 77 K and 1 bar is 1.26 g/cm3 [152]. 

A polymer always has a higher average refractive index in the crystalline than in the amorphous state. However, since also the density of the crystalline polymer is higher, the molar refraction according to Lorenz-Lorenz and Gladstone-Dale remains practically constant. The molar refraction according to Vogel is not applicable to crystalline polymers, since it does not contain the polymer density. 

Commercially available polyethylene fiber has a degree of crystallinity between 70 and 80% and a density 0.97 gcm . There is a linear relationship between density and crystallinity for polyethylene. A 100% crystalline polyethylene will have a theoretical density, based upon an orthorhombic unit cell, of about lgcm . A totally amorphous polyethylene (0% crystallinity) will have a density of about 0.85gcm . Khosravi et al. (1995) used nitric acid attack on gel-spun polyethylene fibers to observe structural imperfections such as fold, molecular kinks and uncrystallized regions. Raman spectroscopy has been used to study the deformation behavior of polyethylene fiber. This technique gives peaks for the crystalline and amorphous states of polyethylene (see Chapter 9). 

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