Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 35

An important difference between crystals and liquids concerns the effects of pressure on their structures. The former are stable as long as the variations in their bond angles and distances induced remain consistent with their long‐range symmetry. A transition to a new phase takes place when this constraint is no longer respected. In contrast, the lack of long‐range order makes a wide diversity of densification mechanisms possible in a liquid, whose structure thus keeps constantly adjusting to varying pressures through changes in short‐range order characterized by shorter equilibrium distances and steeper slopes around the minima pictured in Figure 18. The compressibility is thus greater for a liquid than for its isochemical crystal. It is also made up of vibrational and configurational contributions. Because the shape of interatomic potentials determines the vibrational energy levels, compression is termed vibrational for the elastic part of the deformation. As for the configurational contribution, it is related to the aforementioned changes in the potential energy wells.

If a liquid is quenched as a glass at high pressure, the final glass recovered after decompression will be denser than its counterpart formed at room pressure because only the vibrational part of the compression is eventually recovered (Figure 19). But permanent densification can also be achieved at room temperature through compression of a glass at a few tens of kbar (Chapter 10.11). The effects of pressure and temperature on the properties of glasses are thus of a different nature since the kinetics of pressure‐ and temperature‐induced configurational modifications are markedly different for given frequencies or experimental timescales. This dissimilarity mainly originates in the fact that the shape of potential energy wells varies little with temperature, but significantly with pressure. If a high kinetic energy is needed to overcome potential barriers at constant pressure, the changes in these barriers with pressure can lead by themselves to new configurational states, at low temperatures, if the pressure is high enough.

Figure 19 Permanent compaction of polyvinyl acetate after compression at 800 bar (80 MPa) in the liquid state.

Source: Data from [44].