Page 164 - Biodegradable Polyesters

142 6 Shape Memory Systems with Biodegradable Polyesters

bacterial PHA, namely, poly(3-hydroxyoctanoate-co-3-hydroxyundecanoate), can
be cross-linked by the POSS derivative, making use of UV-assisted thiol–ene cou-
pling. The thiol compounds were tetrathiol cross-linker and thiol-functionalized
POSS. A seldom used cross-linking technique is to create ionic clusters as net
points through incorporation of ionic monomers in the main chain [49]. They can
be, however, easily incorporated into polyesters in polycondensation reactions.
The ionic aggregates may restrict the crystallization of the main chain or its
segments. Consequently, creation of ionomers may be a reasonable approach to
tune the T of semicrystalline SMPs.
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6.2.3
Polyester-Containing Polyurethanes and Related Composites

PU-based systems are the most widely prepared and studied SMPs. They were first
developed by Hayashi in the 1990s and they are still of considerable industrial and
academic interest [50]. This is, in particular, due to their highly versatile PU chem-
istry. The reason why we have inserted this brief section is that the polyols used
for PU synthesis are typically biodegradable polyester-based ones. The interested
reader might have already noticed that the CL chemistry plays an important role in
the reported polyester-related SMPs. PCL diols are, however, traditional building
blocks of PUs. Recently, CL-based oligomers with various molecular architec-
tures became platform chemicals for PU and polyester syntheses [51]. The interest
behind this development is due to the complete biodegradability, low T and rela-
g,
tively low T of PCL. PCL-based PUs are segmented polymers with excellent SM
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behavior. The reaction of diisocyanate with low-MW diol chain extenders yields
the hard segments, whereas the PCL chains give the soft segments. It is generally
accepted that the exceptional SM behavior of such PUs is related to their phase-
segregated morphology. For the thermoplastic version, the hard phase is respon-
sible for memorizing the permanent shape, whereas the soft phase is responsible
for the temporary shape and its fixing [52]. As a consequence, T trans is linked to
the T of PCL (Figure 6.6). In cross-linked PUs, the cross-linked network guaran-
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tees the permanent shape and thus transitions related to the hard phase may also
be involved in SM programming. The research on SM PUs has many similarities
with that on SM polyesters. Particular attention was paid to the copolymer archi-
tecture. Efforts were dedicated to modify both soft and hard phases in order to
improve the SM performance. The ways followed cover the use of polyester diols
other than PCL (e.g., PLA-based ones [53, 54]), additional cross-linking via func-
tional POSS [55], incorporation of nanofillers [56, 57], creation of ionomers [58],
and blending [59, 60].

6.2.4
Blends and Composites

Polymer blends may exhibit SM behavior irrespective of the miscibility of the
blend components. One of the blend components should show the required