Biomedical Engineering Reference
In-Depth Information
of initiating species for addition polymerization beside free radicals: cations, anions, and coordination
(stereospecific) catalysts. Some monomers can use two or more of the initiation processes but others can
use only one process as given in Table 3.3.
3.2.2 Basic Structure
Polymers have very long-chain molecules which are formed by
covalent bonding
along the backbone
chain. The long chains are held together either by secondary bonding forces such as van der Waals
and hydrogen bonds or primary covalent bonding forces through crosslinks between chains. The long
chains are very flexible and can be tangled easily. In addition, each chain can have
side groups
, branches,
and copolymeric chains or blocks which can also interfere with the long-range ordering of chains. For
example, paraffin wax has the same chemical formula as polyethylene (PE) [(CH
2
CH
2
)
n
], but will crys-
tallize almost completely because of its much shorter chain lengths. However, when the chains become
extremely long (from 40 to 50 repeating units [-CH
2
CH
2
-] to several thousands as in linear PE), they
cannot be crystallized completely (up to 80-90% crystallization is possible). Also, branched PE in which
side chains are attached to the main backbone chain at positions normally occupied by a hydrogen atom
will not crystallize easily due to the
steric hindrance
of side chains resulting in a more noncrystalline
structure. The partially crystallized structure is called semicrystalline which is the most commonly
occurring structure for linear polymers. The semicrystalline structure is represented by disordered non-
crystalline (amorphous) regions and ordered crystalline regions which may contain folded chains as
shown in Figure 3.1.
The degree of polymerization (DP) is defined as an average number of mers, or repeating units, per
molecule, that is, chain. Each chain may have a different number of mers depending on the condition
of polymerization. Also, the length of each chain may be different. Therefore, it is assumed that there
is an average DP or average molecular weight (MW). The relationship between MW and DP can be
expressed as
MW of a polymer = DP × MW of mer (or repeating unit)
(3.3)
The two average MWs most commonly used are defined in terms of the numbers of molecules,
Ni
, hav-
ing molecular weight,
Mi
; or
wi
, the weight of species with molecular weights
Mi
as follows:
1. The number-average molecular weight, Mn, is defined by
Σ
Σ
NiMi
NiMi
Wi
Wi Mi
Σ
1
(3.4)
Mn
=
=
Σ
niMi
=
=
Σ
(
/
)
Σ
(
wiMi
)
FIGURE 3.1
Fringed-micelle model of a linear polymer with semicrystalline structure.
