Page 196 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Section 7.2 The Structure of Polymers

o o H H
\ /
II Il
Ci C CHQ CH2 CHQ CHQ C Ci + /N CH2 CH2 CH2 CHQ CH2 CH2 N\
Adipoyl chloride H Hexamethylene diamine H

if Ii T 'I
C CHQ CH2 CHQ CHQ C N CH2 CH2 CHQ CH2 CH2 CHQ N + HCl
Nylon 6,6 Condensate

(3)

H H H Hl_H"HlH H
Heat, pressure, I I I I
C C > C ||;| l:|| C Polyethylene
C~rC
C-1-C
in Catalyst
H H H H | H H : H H rr
l _____
Mer
(b)
FIGURE 1.3 Examples of polymerization. (a) Condensation polymerization of nylon 6,6 and
(b) addition polymerization of polyethylene molecules from ethylene mers.

molecular Weight of a polymer is determined on a statistical basis by averaging. The
spread of the molecular weights in a chain is referred to as the molecular weight
distribution (MWD). A polymer’s molecular weight and its MWD have a strong in-
fluence on its properties. For example, the tensile and the impact strength, the resist-
ance to cracking, and the viscosity (in the molten state) of the polymer all increase
with increasing molecular vveight (Fig. 7.4).
Degree of Polymerization. It is convenient to express the size of a polymer chain in
terms of the degree of polymerization (DP), which is defined as the ratio of
the molecular Weight of the polymer to the molecular Weight of the repeating :C0mmerCia|
unit. For example, polyvinyl chloride (PVC) has a mer weight of 62.5 ; thus, I pO|yn-,ers
the DP of PVC with a molecular Weight of 50,000 is 50,000/62.5 = 800. In
I
terms of polymer processing (Chapter 19), the higher the DP, the higher is the Tensile and
polymer’s viscosity, or its resistance to flow (Fig. 7.4). On the one hand, high Z, Impact ftrenglh
viscosity adversely affects the ease of shaping and thus raises the overall cost ‘Q I
of processing. On the other hand, high DP can result in stronger polymers. cg :
Bonding. During polymerization, the monomers are linked together by
covalent bonds (Section 1.2), forming a polymer chain. Because of their
strength, covalent bonds also are called primary bonds. The polymer
chains are, in turn, held together by secondary bonds, such as van der
' Vlscoslly:
Waals bonds, hydrogen bonds, and ionic bonds (Section 1.2). Secondary
104 107
bonds are weaker than primary bonds by one to two orders of magnitude.
Molecular weight, degree
In a given polymer, the increase in strength and viscosity vvith molecular
of polymerization
Weight is due (in part) to the fact that the longer the polymer chain, the
greater is the energy needed to overcome the combined strength of the
FIGURE 7.4 Effect of molecular
secondary bonds. For example, ethylene polymers having DPs of 1, 6, 35,
Weight and degree of polymerization
140, and 1350 at room temperature are, respectively, in the form of gas, on the strength and viscosity of
liquid, grease, wax, and hard plastic. polymers.