Biomedical Engineering Reference
In-Depth Information
Glass differs from what we think of as regular (crystalline) solids in
a number of ways. A glass does not ''melt'' in the way a crystalline
solid does. If we heat a pure single-phase crystalline solid, at some point
the solid will melt with a well-defined melting temperature. Impurities
will usually alter the melting point, and the presence of more than one
crystal phase will lead to multiple melting points. Nevertheless, when
melting occurs, there is an abrupt change from the solid to the liquid.
If we attempt the same experiment with a piece of glass, we will not
see a sudden change at a well-defined temperature, but we will see
the solid ''ease'' into the liquid, probably a quite viscous liquid. The
glass ''transition'' from the solid glass to the viscous liquid glass is an
important property. Basically, the glass is an elastic solid below this
transformation region and a viscous liquid above it. The structure of
the solid has all the attributes of a liquid, except that the solid does not
have the ability to flow on any meaningful time scale. The apocryphal
story of cathedral windows in Europe being thicker at the bottom than
they are at the top, having flowed due to gravity, is not true: the silicate
glasses in windows are only going to flow on something approaching a
geological time scale (unless things were really to heat up on Earth, in
which case we would not be worrying about cathedral windows). What
is even more curious about this glass transition (called
T
g
for short) is
that, unlike a melting point, the range over which it happens and the
temperature at which it starts very much depends on how the glass was
made in the first place, the rate at which the glass was cooled, whether
it had subsequent heat treatments, and so on. For most commercial
glass used in medicine and biotechnology, if the glass is cooled from the
melt faster, the overall glass structure will have a larger volume (lower
density) than one that is cooled slowly.
In terms of structure, solid glass and liquid glass look very similar.
However, if you were able to take a photograph of the atoms showing
their position, in a subsequent photograph of a liquid the atoms would
have all moved, whereas in the glass they would be in much the same
position as in the first photograph. Essentially, a glass is an elastic solid
without the structural periodicity and long-range order of a crystalline
material. It looks like a liquid but behaves like a solid.
Why are not all solids like this? After all, it seems that there is a lot
less rearranging involved in moving through the glass transition than
there is in melting a crystalline material. Thermodynamics provides the
clue: thermodynamically, systems are generally driven to the lowest
energy (stable) state, so most solids would adopt the inherent order of
the crystalline state, resulting in a lower potential energy for the solid.
