Biomedical Engineering Reference
In-Depth Information
the average total molar mass for one chain in a polymer. there are several
ways in which to calculate molecular weight, including a number-average
molecular weight and a weight-average molecular weight, the details of
which can be found elsewhere (Campbell
et al.
, 2000a). The degree of
polymerisation is a related concept and represents the number of mers in a
polymer chain. During synthesis of a polymer, the degree of polymerisation
can be followed over time using iR spectroscopy to measure the chemical
bonds present and their relative quantity over time (Stuart, 2004). Molecular
weight can be measured experimentally by osmotic pressure, gel permeation
chromatography and light scattering, among other techniques (Campbell
et al.
,
2000a). The higher the molecular weight, the more rigid is the polymer. Soft
waxes or resins have molecular weights on the order of 1000 g mol
-1
, while
solid, hard polymers have molecular weights on the order of 10 000-1 000 000
g mol
-1
. One of the most widely used polymers in orthopaedic applications,
ultra-high-molecular-weight polyethylene (UhMWPe), has a molecular
weight of 4 × 10
6
g mol
-1
.
Chains can arrange themselves in a variety of ways. Linear chains are
simply long chains of monomers joined end-to-end. Branched chains, on
the other hand occur when shorter chains grow off the longer chain. in an
extreme example, a network polymer consists of many chains and many
branches, bonded together at various points by crosslinks. tacticity refers
to the arrangement of side groups around a chain. An isotactic arrangement
has all side groups on the same side, a syndiotactic alternates sides, whereas
an atactic arrangement is completely random. When more than one type of
monomer is present in the polymer structure, it is called a copolymer. the
locations of the different monomers relative to one another results in different
types of structures, including alternating (the monomers alternate back and
forth), block (several monomers of each type join together and then alternate
with a group of another monomer type) and random (completely random
arrangement of monomer types).
the various components of the chemical structure of a polymer can be
determined experimentally by a wide array of characterisation techniques. iR
spectroscopy can identify the types of bonds present, while nuclear magnetic
resonance can determine chemical groups. Wide-angle X-ray scattering
determines the local structure of semi-crystalline polymers, while small-angle
X-ray scattering can identify if a polymer is multi-phase, a copolymer, or
an ionomer.
Degree of crystallinity
When polymer chains are arranged in a particularly compactable way, say
a linear chain with small side groups in an isotactic configuration, they are
better able to align and pack into an ordered structure. Secondary bonding
