Biomedical Engineering Reference
In-Depth Information
have insufficient thermal energy to slide past one another and the only way
for the material to respond to mechanical stress is by stretching (or rupture)
of the bonds constituting the molecular chain. in UHMWPe, the glass
transition occurs at around -160°c. As the temperature is raised above
T
g
,
the amorphous regions within the polymer gain increased mobility. When
the temperature of UHMWPe rises above 60-90°c, the smaller crystalline
lamellae begin to melt. the melting behaviour of semicrystalline polymers,
including UHMWPe, is typically measured using differential scanning
calorimetry (dsc).
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dsc measures the amount of heat needed to increase
the temperature of a polymer sample. the dsc trace for UHMWPe shows
the peak of the melting temperature (
T
m
), which occurs at around 137°c
and corresponds to the point at which the majority of the crystalline regions
have melted. The melting temperature reflects the thickness of the crystals
as well as their perfection. thicker and more perfect polyethylene crystals
will tend to melt at a higher temperature than smaller crystals.
in addition, the area underneath the melting peak is proportional to the
crystallinity of the UHMWPe. dsc provides a measure of the total heat
energy per unit mass (also referred to as the change in enthalpy, D
H
) required
to melt the crystalline regions within the sample. By comparing the change
in enthalpy of an UHMWPe sample to that of a perfect 100% crystal, one
can calculate the degree of crystallinity of the UHMWPe.
if the temperature is raised above the melt temperature, the polymer
may undergo a flow transition and become liquid. This phenomenon is only
observed for polyethylenes with a molecular weight lower than 500 000 g
mol
-1
. For UHMWPe, the entanglement of the immense polymer chains
prevents it from showing a flow transition (
T
f
).
8.2.4 Mechanical properties of UHMWPE
the mechanical behaviour of UHMWPe is closely related to its average
molecular weight, which is routinely calculated from its intrinsic viscosity
(iV).
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other methods for molecular weight measurement of UHMWPe are
sequential extraction
13
and gel permeation chromatography (GPc), a type
of size exclusion chromatography.
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the bulk impact strength and abrasive wear resistance of UHMWPe after
conversion to bulk form are related to the molecular weight and thus to the
iV, although the relationships are non-linear. the maximum impact strength
is found between 16 and 20 iV, which is equivalent to a molecular weight
range of 2.4 million to 3.3 million g mol
-1
. As the iV increases, abrasive
wear resistance increases, as measured by sand slurry testing, reaching a
plateau for iV greater than 20.
the static fracture response, as well as the mechanical behaviour at large
strains, is also influenced by the molecular weight of UHMWPE.
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For
