Biomedical Engineering Reference
In-Depth Information
21.3.2 S
YNTHETIC
S
EGMENTED
P
OLYESTERS
AND
P
OLYURETHANES
Synthetic segmented polyesters and polyurethanes are interesting materials for AC repair because of
the versatility of the synthesis processes and useful intrinsic properties of a group of thermoplastic
elastomers (TPE) by which these polymers are classifi ed. The acronym TPE defi nes the polymers
of specifi c properties being a consequence of a unique structure showing the lack of miscibility
between discrete thermoplastic segments capable of forming rigid nanoscale domains or channels
(hard segments) and covalently bonded to rubber-like segments (soft segments) (Figure 21.9). The
hard domains render high mechanical strength. They also infl uence the processing conditions. The
soft domains are responsible for elasticity and capacity to accumulate large strains—typical char-
acteristic of elastomers [44].
Due to the covalent linkages between the chemically dissimilar segments, the rigid domains
can form a 3D network of cross-linked domains, which are thermally reversible. The key prop-
erty of TPE is their processability typical of the thermoplastics and the elasticity of vulca-
nized rubber. This is achieved by combining properties of the constituents—low glass transition
of the soft, elastomeric component and high melting point of the glassy or semicrystalline
thermoplastics.
The elastic properties and simultaneously thermoplastic character of TPE result from their
molecular and submolecular structures, and morphology [45]. Due to the possibility of controlling
the concentration of the hard and soft phases at the synthesis stage (polyester and polyurethane-type
TPE are synthesized by step growth polymerization reactions), it is possible to produce TPE varying
in their properties from hard thermoplastics to rubber-like materials [44].
Depending on the varying chemical nature of blocks, different combinations of semicrystalline
and amorphous phases are possible. Hard blocks are crystallizable, as demonstrated by segmented
(multiblock) copolymers of poly(butylene terephthalate) (PBT), with amorphous PEG forming soft
blocks (Figure 21.10).
When the concentration of the soft segment is very low (30 wt.%), PBT-based poly(ester-ether)
copolymers resemble cartilage material in terms of the mechanical properties [46-48]. Moreover,
they are biodegradable and can be used as scaffolds for cartilaginous matrix formation [49].
Polyurethane TPE were intensively developed and applied for many years in such areas as
biostable implantable devices (e.g., indwelling catheters, intra-aortic balloons, components of artifi -
cial heart or breast implants). Their chemical structure is presented in Figure 21.11.
Hard segments
Soft segments
FIGURE 21.9
Schematic representation of segmented (multiblock) polyurethane.
O
O
O
O
C
C
O
C
C
O
(CH
2
)
4
O
O
x
DP
Hard block
Soft block
x
= 22
FIGURE 21.10
Schematic representation of multiblock poly(butylene terephthalate) (PBT).
