Polymeric Biomaterials - Biomaterials - page 63

networks (see Section 3.2.3 for more details). The chains also have “kinks” or “bends” in them which
straighten when a load is applied. For example, the chains of cis -polyisoprene (natural rubber) are
bent at the double bond due to the methyl group interfering with the neighboring hydrogen in the
repeating unit [ - CH 2 - C(CH 3 ) ∙ CH - CH 2 - ]. If the methyl group is on the opposite side of the hydro-
gen, then it becomes trans -polyisoprene which will crystallize due to the absence of the steric hin-
drance present in the cis form. The resulting polymer is a very rigid solid called gutta percha which
is not an elastomer. Below the glass transition temperature ( T g ; second-order transition temperature
between viscous liquid and solid), natural rubber loses its compliance and becomes a glass-like mate-
rial. Therefore, to be flexible, all elastomers should have T g well below room temperature. What makes
the elastomers not behave like liquids above T g is in fact due to the cross-links between chains which
act as pinning points. Without cross-links, the polymer would deform permanently. An example
is latex which behaves as a viscous liquid. Latex can be cross-linked with sulfur ( vulcanization ) by
breaking double bonds (C ∙ C) and forming C - S - S - C bonds between the chains. The more the cross-
links are introduced, the more rigid the structure becomes. If all the chains are cross-linked together,
the material will become a 3-D rigid polymer.

3.2.3 Effect of Structural Modification on Properties
The physical properties of polymers can be affected in many ways. In particular, the chemical composi-
tion and arrangement of chains will have a great effect on the final properties. By such means, the poly-
mers can be tailored to meet the end use.

3.2.3.1 Effect of Molecular Weight and Composition
The molecular weight and its distribution have a great effect on the properties of a polymer since its
rigidity is primarily due to the immobilization or entanglement of the chains. This is because the chains
are arranged like cooked spaghetti strands in a bowl. By increasing the molecular weight, the polymer
chains become longer and less mobile and a more rigid material results as shown in Figure 3.4. Equally
important is that all chains should be equal in length since if there are short chains they will act as
plasticizers. Another obvious way of changing properties is to change the chemical composition of the
backbone or side chains. Substituting the backbone carbon of a PE with divalent oxygen or sulfur will
decrease the melting and glass transition temperatures since the chain becomes more flexible due to the
increased rotational freedom. On the other hand, if the backbone chains can be made more rigid, then
a stiffer polymer will result.

Rubber

Viscous
liquid

T m

Liquid

Tough plastic

T g

Semi-crystalline
plastic

Crystalline
solid

10 1000 100,000 10,000,000

Molecular weight (g/mol)

FIGURE 3.4 Approximate relations among molecular weight, T g , T m , and polymer properties.