Biomedical Engineering Reference
In-Depth Information
resistance to abrasion of all the thermoplastic materials. Furthermore, it has
high impact and fatigue resistance, low friction coefficient, autolubricant
properties and high capacity to dissipate mechanical energy.
8
From the structural point of view, UHMWPe is presented in three different
phases: orthorhombic crystalline, approximately 60% in proportion, monoclinic
crystalline, around 10% of the material, and an amorphous phase, which
accounts for approximately 30%. in contrast to the relatively short chains
of HdPe, the long chains of UHMWPe hamper the formation of crystalline
layers, thus limiting the amount of crystalline material. the crystalline phase
is formed by lamellae with thicknesses ranging from 10 to 50 nm and 1 to
50 mm in length with tie molecules connecting these lamellae,
5
where the
amorphous phase is embedded (Fig. 8.2). the average spacing between
lamellae is 50 nm.
9
8.2.3 Thermal properties and transitions
one of the distinguishing characteristics of polymeric materials is the
temperature dependence of their properties. in general terms, polymers
undergo three major thermal transitions: the glass transition temperature (
T
g
),
the melting temperature (
T
m
), and the flow temperature (
T
f
).
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the glass transition (
T
g
) is defined as the temperature below which the
polymer chains behave like a brittle glass. Below
T
g
, the polymer chains
Crystalline
lamella
Tie
molecule
Chain
folds
Amorphous
region
Amorphous
region
Crystalline
lamella
8.2
Morphological features of UHMWPE showing the presence of
crystalline lamellae and the amorphous phase embedded in the
crystalline phase.
