Polymers - Orthopaedic Biomaterials in Research and Practice - page 138

Polymer structural
terms

If a polymer is made from a single monomer, it is said to be homoge-
neous and referred to simply as a polymer. It is possible for there to
be differences in structure in a homogeneous polymer. These may arise
from branching and from asymmetry in the mer.
If the polymer has a single chain, with no branches, then it is called a
linear polymer. If, however, there are side chains or branches, as is com-
mon in polymers formed by condensation, then it is called a branched
or nonlinear polymer. In general, linear polymers possess higher density
than nonlinear ones since the chains can pack more closely together.
If the mer is asymmetric and can be present in two different orienta-
tions, then there are three other possibilities for chain structure. Suppose
we call the monomer “B” and the two possible orientations of the mer in
the chain “b” and “p.” Then, the three arrangements and their names are
as follows (Figure 6.4):

-bbbbb- (or -ppppp-): isotactic (single orientation)
-bpbpbpb-: syndiotactic (alternating)
-bbpbpppbp-: atactic (random)

As expected, isotactic and syndiotactic molecules can pack more
closely than atactic ones, thus forming higher-density solids.
Chain symmetry and the presence or absence of branching are
very important in determining the macroscopic structure of polymers.
Unbranched isotactic or syndiotactic polymers can form highly crystal-
line materials with properties that reflect their high density and internal
regularity. More irregular polymers tend to form amorphous, lower-
density materials with poorer mechanical properties. Most polymers are
a mixture, with crystalline regions in an amorphous matrix (Figure 6.5).
This aspect is described by the degree of or percent crystallinity : the
proportion of the volume that is crystalline. Higher degrees of crystal-
linity are associated with increased strength, stiffness, and light trans-
mission, but these are obtained at the expense of reduced toughness.

BBB

Linear

Copolymers (of A, B and C)

B

B
BB
B

ABC ABCABC

Regular

B

Branched A BB C A C BB

Random

If B is assymetric
('b” or “p”)
b

AAA BB B CCC AAABB

Block

bbbb Isotactic

Syndiotactic
Atactic

b pbp b p

b bpb p pp b

FIGUre 6.4 Polymer structures.