Callister - An introduction - 8th edition

592 • Chapter 15 / Characteristics, Applications, and Processing of Polymers
15.11 MELTING
The melting of a polymer crystal corresponds to the transformation of a solid
material, having an ordered structure of aligned molecular chains, to a viscous
melting temperature
liquid in which the structure is highly random. This phenomenon occurs, upon
heating, at the melting temperature, T m . There are several features distinctive to the
melting of polymers that are not normally observed with metals and ceramics;
these are consequences of the polymer molecular structures and lamellar crystalline
morphology. First of all, melting of polymers takes place over a range of
temperatures; this phenomenon is discussed in more detail shortly. In addition,
the melting behavior depends on the history of the specimen, in particular the
temperature at which it crystallized. The thickness of chain-folded lamellae will
depend on crystallization temperature; the thicker the lamellae, the higher the
melting temperature. Impurities in the polymer and imperfections in the crystals
also decrease the melting temperature. Finally, the apparent melting behavior is
a function of the rate of heating; increasing this rate results in an elevation of the
melting temperature.
As a previous section notes, polymeric materials are responsive to heat treatments
that produce structural and property alterations. An increase in lamellar
thickness may be induced by annealing just below the melting temperature. Annealing
also raises the melting temperature by decreasing the vacancies and other
imperfections in polymer crystals and increasing crystallite thickness.
15.12 THE GLASS TRANSITION
The glass transition occurs in amorphous (or glassy) and semicrystalline polymers
and is due to a reduction in motion of large segments of molecular chains with decreasing
temperature. Upon cooling, the glass transition corresponds to the gradual
transformation from a liquid to a rubbery material and finally to a rigid solid.
glass transition
temperature
The temperature at which the polymer experiences the transition from rubbery to
rigid states is termed the glass transition temperature, T g . Of course, this sequence of
events occurs in the reverse order when a rigid glass at a temperature below T g is
heated. In addition, abrupt changes in other physical properties accompany this
glass transition: for example, stiffness (Figure 15.7), heat capacity, and coefficient of
thermal expansion.
15.13 MELTING AND GLASS TRANSITION
TEMPERATURES
Melting and glass transition temperatures are important parameters relative to inservice
applications of polymers.They define, respectively, the upper and lower temperature
limits for numerous applications, especially for semicrystalline polymers.
The glass transition temperature may also define the upper use temperature for
glassy amorphous materials. Furthermore, T m and T g also influence the fabrication
and processing procedures for polymers and polymer-matrix composites. These issues
are discussed in succeeding sections of this chapter.
The temperatures at which melting and/or the glass transition occur for a polymer
are determined in the same manner as for ceramic materials—from a plot of
specific volume (the reciprocal of density) versus temperature. Figure 15.18 is such
a plot, where curves A and C, for amorphous and crystalline polymers, respectively,