Page 161 - Biodegradable Polyesters

6.2 Shape Memory Polymer Systems 139

Bacterial poly[(3-hydroxybutyrate)-co-(3-hydroxyvalerate)] (PHBV), exhibiting
∘
∘
∘
T ∼−13 C and a broad melting range (37–115 C), peaked at T = 94 C
g
m
and showed also thermoresponsive SM behavior. For setting the temporary
shape, T > T , while for shape fixing ambient temperature were used [28].
trans m
Poly(trimethylene carbonate-co-D,L-lactide) random copolymers were synthe-
sized by Yang et al. [29] to adjust the mechanical properties and biodegradability
upon request. Poly(trimethylene carbonate) (PTMC) itself is an amorphous
∘
elastomer having a Tg ∼−15 C, which is much too low for many SM appli-
cations. By copolymerization, T could be enhanced. For these copolymers
g
∘
∘
T trans (= 37 C) > T (= 22 C) was used for shaping (elongation to 150%) and
g
∘
shape fixing occurred T = 0 C. R wasfound for83%.Several workswere
r
devoted to synthesize terpolymers composed of lactide, glycolide, and trimethy-
lene carbonate. For their copolymerization, usually stannous compounds are
used and these may be relatively toxic. Therefore, these catalysts were replaced
by low-toxicity zirconium-based compounds. The T of the terpolymers, which
g ∘
was considered for selecting T ,was between12and 42 C [30, 31].
trans
Linear, cyano-functionalized polynorbornene having long PCL side chains
showed good SM properties according to the report of Yang and coworkers
[32]. This polynorbornene grafted by PCL along with the cyano groups in the
main chain showed higher ductility and strength than the unmodified poly-
norbornene. Moreover, T of the phase-separated PCL could be well used for
m
T trans in SM tests. It is worth noting that polynorbornene is one of the oldest
SMPs.
Inorganic fillers such hydroxyapatite, carbon nanotubes (CNTs), Fe O ,and
3 4
TiO are often incorporated as micro- and nanoscale fillers in SMPs to enhance
2
the mechanical properties and trigger other types of actuation than the direct ther-
mal one. Lu et al. [33] succeeded in improving the SM behavior of PLACL using
surface-grafted TiO nanoparticles. However, both R and R went throughamax-
2 f r
imum as a function of the TiO content. Recall that in this amorphous PLACL,
2
T served for T trans and shape fixing was owing to the physical network of the
g
entanglement.
It is worth noting that filling and reinforcing with traditional materials usu-
ally improve the mechanical but often degrade the SM properties. The latter is
typically a loss in ductility that hampers the deformability of the corresponding
composite [34]. To overcome this problem, Wang et al. [35] reinforced PLACL
by in situ produced micro- and nanofibers from poly(glycolic acid) (PGA). The
authors generated the PGA fibers in PLACL through in-line extrusion stretch-
ing. Attention should be paid to the fact that this method basically follows the
microfibrillar composite concept of Fakirov and Bhattacharyya (see [36] and ref-
erences therein). The (LA) L-lactide/CL ratio of 82/18 resulted in an amorphous
∘
∘
PLACL with a T of 22 C. For the temporary shape, T = T + 15 C, and for
g ∘ trans g
its fixing, T = T − 15 C were selected. The PGA phase supported both shape fix-
g
ing (restraining the molecular chain movement) and recovery (acting as additional
net points).