GTBL042-11 GTBL042-Callister-v3 October 4, 2007 11:59
2nd Revised Pages448 • Chapter 11 / Phase Transformationswherekandnare time-independent constants whose values depend on the crystalliz-
ing system. Normally, the extent of crystallization is measured by specimen volume
changes since there will be a difference in volume for liquid and crystallized phases.
Rate of crystallization may be specified in the same manner as for the transforma-
tions discussed in Section 11.3, and according to Equation 11.18; that is, rate is equal
to the reciprocal of time required for crystallization to proceed to 50% completion.
This rate is dependent on crystallization temperature (Figure 11.46) and also on
the molecular weight of the polymer; the rate decreases with increasing molecular
weight.
For polypropylene (as well as any polymer), the attainment of 100% crystallinity
is not possible. Therefore, in Figure 11.46, the vertical axis is scaled as “normalized
fraction crystallized.” A value of 1.0 for this parameter corresponds to the highest
level of crystallization achieved during the tests, which, in reality, is less than complete
crystallization.11.14 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 liquid in which
melting temperature the structure is highly random. This phenomenon occurs, upon heating, at themelting
temperature,Tm. There are several features peculiar 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 below. 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 Section 8.18 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.11.15 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. The
temperature at which the polymer experiences the transition from rubbery to rigid
glass transition states is termed theglass transition temperature,Tg. Of course, this sequence of
temperature events occurs in the reverse order when a rigid glass at a temperature belowTg
is heated. In addition, abrupt changes in other physical properties accompany this
glass transition: for example, stiffness (Figure 7.28), heat capacity, and coefficient of
thermal expansion.