Page 198 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 7.2 The Structure of Polymers |77

Glassy
A 100% crystalline A Increasing
2 S2 Glassy cross-linking
S 3
fl) (D $
E’ Leathery Increasing E’
jj crystallinity jg’ Leathery
3
E l 5
s 4, s
§ Fiubbery /bo/` E Flubbery
1: ,O B3
3 600 3 /l/O0
E 0 m /_OSS
Viscious Visfous W/>,(7h
9
rg Tm rm
Temperature Temperature
(8) (bl

FIGURE 1.6 Behavior of polymers as a function of temperature and (a) degree of
crystallinity and (b) cross-linking. The combined elastic and viscous behavior of polymers is
known as viscoelasticity.
Network Polymers. These polymers consist of spatial (three-dimensional) net-
works of three or more active covalent bonds (Fig. 7.5d). A highly cross-linked poly-
mer also is considered a network polymer. Thermoplastic polymers that already
have been formed or shaped can be cross-linked to obtain higher strength by sub-
jecting them to high-energy radiation, such as ultraviolet light, X-rays, or electron
beams. However, excessive radiation can cause degradation of the polymer.

Copolymers and Terpolymers. If the repeating units in a polymer chain are all of
the same type, the molecule is called a homopolymer. However, as with solid-solution
metal alloys (Section 42), two or three different types of monomers can be combined
to develop certain special properties and characteristics, such as improved strength,
toughness, and formability of the polymer. Copolymers contain two types of poly-
mers (for example, styrene-butadiene, which is used widely for automobile tires).
Terpolymers contain three types (for example, acrylonitrile-butadiene-styrene (ABS),
which is used for helmets, telephones, and refrigerator liners).

EXAMPLE 7.l Dental and Medical Bone Cement

Polymethylrnethacrylate (PMMA) is an acrylic poly- The powder consists of high-molecular-weight
mer commonly used in dental and medical applica- poly[(methylmethacrylate)-costyrene] particles about
tions as an adhesive and is often referred to as bone 50 /.tm in diameter and containing a small volume
cement. There are a number of forms of PMMA, but fraction of benzoyl peroxide. The liquid consists of a
this example describes one common form involving methyl methacrylate (MMA) monomer, with a small
an addition-polymerization reaction. PMMA is deliv- amount of dissolved n,n dimethyl-p-toluidine
ered in two parts: a powder and a liquid, which are (DMPT). When the liquid and powder are mixed, the
mixed by hand. The liquid wets and partially dissolves MMA wets the particles (dissolving a surface layer of
the powder, resulting in a liquid with viscosity on the the PMMA particles) and the DMPT cleaves the
order of 0.1 Ns/mz, similar to that of vegetable oil. benzoyl peroxide molecule into two parts to form a
The viscosity increases markedly until a “doughy” catalyst with a free electron (sometimes referred to as a
state is reached in about five minutes. The “dough” free radical). This catalyst causes rapid growth of
fully hardens in an additional five minutes. PMMA from the MMA mers, so that the final