Mechanical Properties of Polymer Solids and Liquids - Polymer Science and Engineering - page 168

In a solid semicrystalline polymer, large-scale segmental motion occurs only
at temperatures between T g and T m and only in amorphous regions. At low
degrees of crystallinity the crystallites act as virtual cross-links, and the amor-
phous regions exhibit rubbery or glassy behavior, depending on the temperature
and time scale of the experiment. Increasing levels of crystallinity have similar
effects to those shown in Fig. 4.9 for variations in cross-link density. Schematic
modulus temperature relations for a semicrystalline polymer are shown in

Fig. 4.10 . As with moderate cross-linking, the glass transition is essentially unaf-
fected by the presence of crystallites. At very high crystallinity levels, however,
the polymer is very rigid and little segmental motion is possible. In this case the
glass transition has little practical significance. It is almost a philosophical ques-
tion whether a T g exists in materials like the superdrawn thermoplastic fibers
noted in Section 4.3.2.3 or the rodlike structures mentioned in Section 4.6 .
The modulus

temperature behavior of amorphous polymers is also affected
by admixture with plasticizers. These are the soluble diluents described briefly in
Section 6.3.2. As shown in Fig. 4.11 , the incorporation of a plasticizer reduces T g
and makes the polymer more flexible at any temperature above T g . In poly(vinyl
chloride), for example, T g can be lowered from about 85 C for unplasticized
material to 30 C for blends of the polymer with 50 wt% of dioctyl phthalate

plasticizer. A very wide range of mechanical properties can be achieved with this
one polymer by variations in the types and concentrations of plasticizers.

2

10

Increasing
crystallinity

8

Partially
crystalline

6

Amorphous

4

Temperature ( ° C)

FIGURE 4.10
Modulus temperature relations for amorphous and partially crystalline versions of the

same polymer.