Advantages of polymers in terms of the following
characteristics:1- Relatively low cost and ease of manufacture2-
Corrosion resistance and resistance to chemicals3- Low electrical
and thermal conductivity4- Low density5- High strength-to-Weight
ratio (particularly when reinforced)6- Noise reduction7- Wide
choice of colors and transparencies8- Ease of manufacturing and
complexity of design possibilities9- Other characteristics that may
or may not be desirable (depending on the application), such as (a)
low strength and stiffness(b) high coefficient of thermal
expansion(c) low useful-temperature range-up to about 350C-(d)
lower dimensional stability in service over a period of
time.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, because the
polymer molecule grows step-by-step until all of one reactant is
consumed.
Condensation polymerization of nylon 6,6Addition polymerization
(also called chain-growth or chain-reaction polymerization),
bonding takes place without reaction by-products. 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 link to produce the polymer polyethylene;.
Addition polymerization of polyethylene molecules from ethylene
mersMolecular Weight: The sum of the molecular weights of the mers
in a representative chain is known as the molecular weight of the
polymer. The higher the molecular weight of a given polymer, the
greater the average chain length.The spread of the molecular
weights in a chain is referred to as the molecular weight
distribution (MWD). A polymers molecular weight and its MWD have a
strong influence on its properties. For example, the tensile and
the impact strength, the resistance to cracking, and the viscosity
(in the molten state) of the polymer all increase with increasing
molecular weight
Effect of molecular Weight and degree of polymerization on the
strength and viscosity of polymersDegree 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 unit. For example, polyvinyl chloride (PVC) has a mer
weight of 62.5; thus, the DP of PVC with a molecular Weight of
50,000 is 50,000/62.5 = 800.The higher the DP, the higher is the
Tensile and polymers viscosity, or its resistance to flow (Fig.).
On the one hand, high viscosity adversely affects the ease of
shaping and thus raises the overall cost of processing. On the
other hand, high DP can result in stronger polymers.
