Biomedical Engineering Reference
In-Depth Information
can then occur between adjacent chain segments and regions of crystallinity
can form. the complexity of the chain affects this, including the size of side
groups, any branching, and the tacticity. While both isotactic and syndiotactic
arrangements allow for crystallisation, atactic structures cannot crystallise.
Owing to the size and complexity of all polymer chains, it is virtually
impossible for them to be completely crystalline. thus polymers are either
amorphous or semi-crystalline.
the degree of crystallinity can be determined by the density of the material.
the more crystalline, the more compact the structure is and thus the more
dense the polymer is. in general, ductile semi-crystalline polymers have a
crystallinity of about 50%, whereas very brittle ones are 90-95% crystalline.
The degree of crystallinity can also be influenced by processing conditions.
A slower cooling rate can allow for more crystallisation, as the chains have
more time to arrange themselves in an orderly fashion. Along with the more
ordered, dense structure comes an improvement in both tensile modulus and
tensile strength.
Thermal transitions
Many polymers undergo two important thermal transitions - melting
and glass transition. A melting temperature (
T
m
) is only present in semi-
crystalline polymers. it occurs when the solid material with ordered, aligned
chains turns into a randomly oriented viscous liquid. Owing to the range of
molecular weights in the structure, it actually takes place over a range of a
few degrees rather than at one, clearly-defined temperature. Like the degree
of crystallinity, chain properties, like the size of side groups and ease of
rotation, affect the melting temperature. it takes more energy to unpack a
well-packed chain, resulting in a higher
T
m
. Branching, on the other hand,
decreases
T
m
, as branching leads to defects in crystallinity. the rate at which
the polymer is heated also affects the
T
m
; a higher rate results in a higher
melting temperature.
the glass transition temperature (
T
g
) marks the point at which a polymer
goes from rubbery behaviour to a rigid solid as it cools. As temperature
decreases, the motion of large segments of polymer chains is reduced. All types
of polymers experience a glass transition. the same molecular characteristics
that affect
T
m
affect
T
g
in a similar way. the two temperatures are typically
linked, as well; a change in one affects the other by the relationship
T
g
=
0.5-0.8
T
m
(Kelvin).
Several techniques can measure thermal transition temperatures in polymers.
Differential scanning calorimetry (DSC) is perhaps the most widely used.
DSC monitors heat flow to determine
T
g
and
T
m
(if present). Other techniques
include thermomechanical analysis and dynamic mechanical analysis and
are described in more detail elsewhere (Campbell
et al.
, 2000b).
