Polymers - Orthopaedic Biomaterials in Research and Practice - page 140

If the structure in C is actually achieved, then it is easy to see how
several of these chains might “fit together” better than those of A or B :

-AAABBBAAABBBAAA-
-BBBAAABBBAAABBB-
-AAABBBAAABBBAAA-

Thus, C would probably have a higher degree of crystallinity than the
other two polymers.

additional aspects
of polymer
composition and
structure

These structural features of polymer molecules, by themselves, produce
a great variety of possible materials. However, additional aspects fur-
ther complicate the production and resultant properties of real polymeric
materials. I will briefly summarize some of these topics here; a full treat-
ment is beyond the scope of this work.

Molecular weight distribution . Either addition or condensation
polymerization produces molecules with a wide distribution of
molecular weights (reflecting different numbers of mers per mol-
ecule). It is possible to tailor these distributions, to make them
broader or narrower, to assure a minimum or maximum molecular
weight, and so on. Each of these choices affects both density and
mechanical properties. In particular, low-molecular-weight mol-
ecules may act as plasticizers (see below).
Plasticization . Plasticizers are low-molecular-weight compounds
that are added to polymers and act as internal lubricants, making
relative molecular motion easier. These materials tend to increase
elongation and toughness at the expense of reduced melting point,
hardness, and ultimate tensile strength. Residual low-molecular-
weight species may act as “self”-plasticizers. In implant applica-
tions, absorbed water and low-molecular-weight lipids also serve
as plasticizers, particularly in hydrophilic and lipophilic low-
density materials such as polyamides (Nylon; DuPont).
Other additives . Numerous agents are added to polymers to pro-
duce changes in color (colorants), increases in strength (reinforc-
ing materials), resistance to attack by oxygen (antioxidants), easy
removal from molds (mold release agents), and so on. These mate-
rials, added either deliberately or inadvertently, may be a source of
adverse local tissue response. Manufacturers of polymers intended
for medical applications routinely screen new compositions and
individual production batches by in vitro techniques, usually cell
culture, to prevent such problems from arising.

In general, polymers that have proven useful as biomaterials
have few or no low-molecular-weight additives. The leaching of
such additives may promote significant adverse local host responses.
However, some additives have proven to be very beneficial. Of par-
ticular interest in orthopaedics, the antioxidant vitamin E may be
added to UHMWPE, which in turn, reduces the effects of oxi-
dative degradation after gamma irradiation for sterilization or