Page 195 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

|74 Chapter 7 Polymers: Structure, General Properties, and Applications

'" `l"i”'i" `"ii"`“"i "`“”““ "`“" ““W "`”ii ’i"“`“”`f "“"i "i`“"‘ T"P "P" `“"'°i` feature that distinguishes them from most
Monomer Polymer repeating unit
other organic chemical compositions.
Polymers are long-chain molecules that are
H H H H
CZC _C_C_ Polyethylene formed by polynierization_(that is, by the link-
ing and cross-linking of different monomers).
H H H H n A monomer is the basic building block of a
polymer. The word mer (from the Greek
H H H H 6, ,,
rneros, meaning part ) indicates the smallest
C=C -C-C- Polypropylene repetitive unit; the use of the term is similar to
H CH H CH n that of unit cell in crystal structures of metals
(Section 1.3).
3 3
The term polymer means “many mers
H H H H
(or units),” enerall repeated hundreds or
C=C _C_C-_ POM/my' Cmonde (PVC) thousands of; times iln a chainlike structure.
H (;| H C| n Most monomers are organic materials in
which carbon atoms are joined in covalent
(electron-sharing) bonds with other atoms
H H H H
C=C -C-C- Polystyrene (such as hydrogen, oxygen, nitrogen, fluorine,
chlorine, silicon, and sulfur). An ethylene mol-
H CGHS H CGHS n ecule (Fig. 7.2) is an example of a simple
|:| |:| |:| |:| monomer consisting of carbon and hydrogen
C=C _ C__C_ Plrgygexgafluoroethylene (PTFE) atoms-

7.2.l Polymerization
Fl Fl Fl Fl H
Monomers can be linked into polymers in
FIGURE 7.2 Molecular structure of various polymers. These are repeating units to make longer and larger
examples of the basic building blocks for plastics. molecules by a chemical process called a
polymerization reaction. Polymerization
processes are complex; they will be described only briefly here. Although there are
several variations, two polymerization processes are important: condensation and
addition polymerization.
In condensation polymerization (Fig. 7.3a), polymers are produced by the
formation of bonds between two types of reacting mers. A characteristic of this
reaction is that reaction by-products (such as water) are condensed out (hence the
name). This process is also known as step-growth or step-reaction polymerization, be-
cause the polymer molecule grows step-by-step until all of one reactant is consumed.
In addition polymerization (also called chain-growth or chain-reaction poly-
merization), bonding takes place without reaction by-products, as shown in Fig. 7.3b.
It is called “chain reaction” because of the high rate at which long molecules form
simultaneously, usually within a few seconds. This rate is much higher than that in
condensation polymerization. In addition polymerization, an initiator is added to
open the double bond between two carbon atoms, which begins the linking process
by adding many more monomers to a growing chain. For example, ethylene
monomers (Fig. 7.3b) link to produce the polymer polyethylene; other examples of
addition-formed polymers are shown in Fig. 7.2.

Molecular Weight. The sum of the molecular weights of the mers in a representa-
tive chain is known as the molecular weight of the polymer. The higher the molecu-
lar weight of a given polymer, the greater the average chain length. Most commercial
polymers have a molecular weight between 10,000 and 10,000,000. Because poly-
merization is a random event, the polymer chains produced are not all of equal
length, but the chain lengths produced fall into a traditional distribution curve. The