Biomedical Engineering Reference
In-Depth Information
Fig. 11.4
Left
:flat2D
representation of the
repeating unit of
polyethylene.
Center
:3D
representation;
arrow
:in
front of paper plane,
dashed
line
: back of the paper plane.
Right
: simulated polymer
structure
Fig. 11.5
Disastrous wear of a polyethylene prosthesis head after implantation of a couple years.
The dashed line gives the original contour of the head. The prosthesis was a product of Oscobal
(Selzach, Switzerland)
foreign body reaction with abundant giant cells and intra- and extracellular HDP
particles [358]. The use of this prosthesis was a disaster. This case is an illustration
of both the importance of MW on properties and the lack of sufficient mastering
materials properties.
In Fig.
11.4
, a simple linear nonbranched PE chain is represented but if it was
nothing but that the polymer world would lack its wonderful flexibility. Chains
in UHMWPE are less efficiently packed into a crystal structure, which results in
lower density but is tougher and better wear resistant. High density is generated for
a species with a low degree of branching and thus stronger intermolecular forces
(
cross-linking
). In this way, the thermoplast can be turned into an elastomer (PEX)!
While the polymerization of ordinary PE starts from pure ethylene, another set of
properties can be generated by
copolymerization
with other short chain olefins such
as I-butene, a process not yet mentioned thus far.
