AN INTRODUCTION TO PLASTICS - Plastics and Environmental Sustainability - page 88

quenched quickly in a cold fluid, will still yield an amorphous glass. Slow
cooling of the same polymer will often yield a semicrystalline material.
Figure 3.6 illustrates the embedded crystallites in a plastic matrix and a
schematic of a semi-crystalline polymer.
Figure 3.6 Left : An illustration of crystallites embedded in an amorphous
polymer matrix. Right : Crystallites in plastic crystals imaged by AFM.
Source: Reproduced with permission from Mouras et al. (2011).
In common with those of simple organic compounds, polymer crystallites
also have melting points defined by characteristic temperatures. When
melting occurs in the crystalline phase of the polymer, it is converted into
amorphous polymer. Under suitable conditions such as annealing, it can
slowly recrystallize. The crystalline melting point is characterized by its
thermal signature but generally does not result in a visible phase change
of the entire solid resin into a fluid as with organic compounds. Above
the melting point, 7 the modulus and the strength of the plastic drop very
significantly. Figure 3.7 illustrates these changes schematically. E is the
elastic modulus, a measure of stiffness of the material.