Page 162 - Biodegradable Polyesters

140 6 Shape Memory Systems with Biodegradable Polyesters

6.2.2.2 Cross-linked
To prepare cross-linked systems, various functionalization methods and cross-
linking procedures may be chosen. Cross-linkable functional groups may appear
as end or side groups, and even in the main macromolecular chains. The
group of Lendlein functionalized oligomers composed of LA and glycolide with
UV-curable methacrylate end groups [37]. The comonomer ratio and length of
the chain segments were varied in the experiments. The T values, used to choose
g
∘
T trans , of the photocured random copolymers were in the range T = 50 − 55 C.
g
Schmidt [38] produced thermosets from oligomeric CL dimethacrylate and
butyl acrylate by peroxide cross-linking. Into the network, Fe O nanoparticles
4
3
were also embedded to trigger the SM function by electromagnetic activation.
The magnetite particles in this case worked for the “remote” control of the
temperature by transforming the electromagnetic energy to heat. The T m of
∘
the PCL segments (43–49 C) was selected as T trans . Garle et al. [39] modified
PCL homo- and copolymers by cinnamate compounds. The UV cross-linkable
cinnamoyl side groups resulted in a cross-linked gel content of about 70 wt%.
SM transformation was governed by the T m of the PCL segments. Kumar and
coworkers [40] elaborated a synthesis route for producing thermally and magnet-
ically activated triple-SMPs using methacrylate end-functionalized crystallizable
∘
∘
PCL (T = 55 C) and polyethylene glycol (PEG) (T = 38 C). The copolymer
m m
was cured by peroxide in the presence and absence of silica-coated magnetite
nanoparticles. The outcome was a magnetically active SMP. For programming
of the two temporary shapes, the above T values were considered by adapting
m
one- or two-step programming procedures (Figure 6.5).
The group of Nagata incorporated UV-curable moieties directly into the
main chain of the related polymers [41–43]. Thus, photocurable, biodegradable
multiblock SMPs were prepared by polycondensation from PCL diol, PEG,
and 5-cinnamoyloxyisophthalic acid [41]. The latter compounds acted as UV
cross-linker without any photoinitiator. The semicrystalline photosets exhib-
∘
∘
ited T ∼−60 C, and T in the range 35–47 C. Both R and R values were
g m f r
reported above 90% when selecting tensile deformation between 100 and 300%
∘
and T = 37 − 60 C. A similar strategy was followed to produce photo-
trans
cross-linked PCL [42] and copolymers [43] composed of CL and LA whereby
incorporating coumarin groups in the man chain. Coumarin is biodegradable
and nontoxic. Moreover, it participates in reversible cross-linking depending on
the wavelength. Multiblock copolymers were also synthesized from PCL diol
and PLA diol via polycondensation with cinnamic acid compound [44]. Again,
the latter was responsible for reversible photocuring that did not affect the
∘
crystallization of PCL but reduced that of PLA. Choosing T trans = 40 − 60 C,
∘
tensile elongations 100–500%, and fixing temperature 22 C, the R and R data
f r
between 88 and 100% were measured. The authors emphasized that reversible
photo-cross-linking may be a promising way to produce light-sensitive SMPs.
There are many possibilities to tailor the properties of SMPs via their
block (segmented) architecture. Zhang et al. [45] produced diacrylated
PCL–polydimethylsiloxane–PCL macromers that have been photocured