Biomedical Engineering Reference
In-Depth Information
Figure 6.1
Schematic representation showing the two steps involved in the synthesis of
segmented polyurethanes.
so-called polyether-based urethanes, which have been the most common so far.
Nevertheless, in recent years polyester-based PURs have begun to be developed
due to their increased biodegradability. Selected macrodiols are all viscous liquids
with a number average molecular weight ranging between 400 and 5000 g/mol.
Polyester diols can be prepared by ring-opening polymerization of a cyclic lactone
[6] or condensation between a dicarboxylic acid and an excess of a diol. In some
cases, the polyester diol, which is characterized by a hydrophobic character, is
mixed with the more hydrophilic polyethylene glycol (PEG) before performing the
reaction with the corresponding diisocyanate. This way, PURs with an increased
biodegradation rate and enhanced cell attachment can be obtained. Note that these
characteristics can be easily tailored by a simple change in the composition of the
mixture [7] .
The reaction between the diol and the diisocyanate is carried out with an excess
of the latter (keeping the isocyanate/hydroxyl molar ratio usually close to 2:1) in
order to obtain a reactive prepolymer with isocyanate end groups. Catalysts (typi-
cally tertiary amines, stannous octoate, or dibutyltin dilaurate) and high tempera-
tures (60-90 °C) are required to increase the reaction rate. A thermoplastic PUR
material characterized by a segmented architecture is fi nally obtained by reaction
of the terminal isocyanate groups with a chain extender (Figure 6.1) which may
be either a diol or a diamine with low molecular weight [8]. In the fi rst case, ure-
thane bonds are formed and the fi nal polymer is usually thermally processable,
whereas in the second case new urea bonds are formed and the resulting
poly(urethane/urea) is usually only suitable for solvent casting.
Some secondary reactions, which generally result in branched or cross-
linked polymers, can also occur under certain conditions [9]. The most usual are
