Page 89 - Mechanical Behavior of Materials

88 Chapter 3 A Survey of Engineering Materials

Table 3.9 Typical Values of Glass Transition and
Melting Temperatures for Various Thermoplastics and
Elastomers
Transition Melting
◦
Polymer T g , C T m , C
◦
(a) Amorphous thermoplastics
Polyvinyl chloride (PVC) 87 212
Polystyrene (atactic) 100 ≈ 180
Polycarbonate (PC) 150 265
(b) Primarily crystalline thermoplastics
Low-density polyethylene (LDPE) −110 115
High-density polyethylene (HDPE) −90 137
Polyoxymethylene (POM) −85 175
Polypropylene (PP) −10 176
Nylon 6 50 215
Polystyrene (isotactic) 100 240
Polyetheretherketone (PEEK) 143 334
Aramid 375 640
(c) Elastomers
Silicone rubber −123 −54
Cis-polyisoprene −73 28
Polychloroprene −50 80
Source: Data in [ASM 88] pp. 50–54.

If the chain molecules are instead arranged in a random manner, the polymer is said to be
amorphous. Examples of amorphous polymers include PVC, PMMA, and PC. Polystyrene (PS) is
amorphous in its atactic form where the benzene ring substitution is randomly located within each
repeating unit of the molecule, but is crystalline in the isotactic form where the substitution occurs
at the same location in each repeating unit. This same situation occurs for other polymers as well,
due to the regular structure of the isotactic form promoting crystallinity. If the side groups alternate
their positions in a regular manner, the polymer is said to be syndiotactic, with a crystalline structure
being likely in this case also.
Amorphous polymers are generally used around and below their respective glass transition
temperatures T g , some values of which are listed in Table 3.9. Above T g , the elastic modulus
decreases rapidly, and time-dependent deformation (creep) effects become pronounced, limiting
the usefulness of these materials in load-resisting applications. Their behavior below T g tends to be
glassy and brittle, with the elastic modulus being on the order of E = 3 GPa. Amorphous polymers
composed of single-strand molecules are said to be linear polymers. Another possibility is that there
is some degree of branching, as shown in Fig. 3.14.
Crystalline polymers tend to be less brittle than amorphous polymers, and the stiffness and
strength do not drop as dramatically beyond T g . For example, such differences occur between the
amorphous and crystalline forms of PS, as illustrated in Fig. 3.15. As a result of this behavior, many
crystalline polymers can be used above their T g values. Crystalline polymers tend to be opaque to
light, whereas amorphous polymers are transparent.