Page 27 - Materials Chemistry, Second Edition

14 2 Solid-State Chemistry

Table 2.1. Glass Transition Temperatures
Material Intermolecular bonding T g ( C)

Covalent 1,430
SiO 2
Borosilicate glass Covalent 550
Metallic 580
Pd 0.4 Ni 0.4 P 0.2
Ionic 570
BeF 2
As 2 S 3 Covalent 470
Polystyrene Van der Waal 370
Covalent 310
Se 1
Poly(vinyl chloride) Van der Waal 81
Polyethylene Van der Waal 30
disordered state, the formation of amorphous materials is favored in kinetically
bound processes (e.g., chemical vapor deposition, sol-gel, solid precipitation, etc.). [1]
Some materials featuring extended networks of molecules such as glasses may
never exist in the crystalline state. In these solids, the molecules are so entangled or
structurally complex that crystallization may not occur as the temperature is slowly
decreased. Due to the rigidity of the solid, but proclivity to remain in the amorphous
state, these compounds have been incorrectly referred to as supercooled liquids.
It was even thought that a slow flow of glass over hundreds of years has caused
nineteenth century stained glass windows to have a proportionately thicker base. [2]
However, it is now well understood that the glass structure remains in tact unless
its threshold transition temperature is exceeded. This parameter is known as the
glass transition temperature, T g , and corresponds to the temperature below which
molecules have very little mobility.
Other amorphous solids such as polymers, being rigid and brittle below T g ,
and elastic above it, also exhibit this behavior. Table 2.1 lists the glass transition
temperatures of common solid materials. It should also be noted that whereas
crystalline solids exhibit a discrete melting point, amorphous solids undergo a
solid–liquid phase transition over a range of temperatures. Although most solid-
state textbooks deal almost exclusively with crystalline materials, this text will
attempt to address both the crystalline and amorphous states, describing the
structure/property relationships of major amorphous classes such as polymers
and glasses.

2.2. TYPES OF BONDING IN SOLIDS

Every amorphous and crystalline solid possesses certain types of inter- and intramo-
lecular interactions between its subunits that govern its overall properties. Depend-
ing on the nature and strength of these interactions, a variety of physical, optical, and
electronic properties are observed. For example, intramolecular forces (i.e., atomic
separations/inter-atomic bonding energies) directly influence the conductivity,
thermal expansion, and elasticity of a material; in contrast, intermolecular forces
will govern the melting/boiling/sublimation point, solubility, and vapor pressure of a