Page 157 - Biodegradable Polyesters

6.2 Shape Memory Polymer Systems 135

should be cross-linked in a suitable manner. Otherwise, the chain entanglement
serves to “keep” the permanent shape. PCL was cross-linked by sol–gel chemistry
by Pandini et al. [14]. The related pathway contained the alkoxysilane end grouping
of an α,ω-hydroxyl terminated PCL, followed by hydrolysis and polycondensation
reactions of the terminal alkoxysilane groups. The created chemical cross-links
maintained the crystallization ability of PCL. The T m and crystallization tem-
perature (T ) of PCL were, however, affected by the degree of cross-linking. The
c
resulting material showed both one- and two-way SM behaviors. Figure 6.2 dis-
plays the morphology changes during the one- and two-way SM cycles in this
sol–gel cross-linked PCL.
In a companion paper, the same authors [15] prepared peroxide cross-linked
PCL with different molecular architectures. PCL was terminated by methacrylate
and cross-linked in its melt phase. The typical thermomechanical behavior of the
corresponding SMP, showing two-way SM effect, is given in Figure 6.3.
Polyhedral silsesquioxane (POSS) containing PCL with acrylate end groups
were also synthesized and photo cross-linked. This approach allowed control-
ling the morphology of the resulting SMP in micro- and nanoscale whereby
“compromising” the crystallizations of PCL and POSS [16]. PCL can also be
cross-linked by peroxide in the melt. When the goal is magnetic field-induced
SM effect, PCL is usually filled with Fe O (magnetite) nanoparticles [17]. To
3 4
achieve electroactive SMPs, conductive fillers such as carbonaceous nanofillers
may be incorporated into the cross-linkable PCL. This was demonstrated
by Xiao and coworker [18] who incorporated multiwalled carbon nanotubes
(MWCNTs) in cross-linked PCL. Recall that although here the electric field
is the external stimulus, it is translated into heat by the Joule effect. So, the
corresponding nanofilled cross-linked PCL remains a thermoresponsive sys-
tem. R changed parallelly with that of the gel fraction for the cross-linked
r
PCL. This fact corroborates the prominent influence of cross-linking on the
SM performance.
Apart from photo and peroxide cross-linking, radiation cross-linking methods
canalsobeadapted.Thegroup of Zhu et al. [19, 20] used γ-irradiation to cross-link
PCL in the presence and absence of various sensitizers and studied the SM behav-
ior as a function of radiation dose and dosage rate. Recovery of the related PCLs
∘
was studied at T = 56 C. With increasing radiation dose, the recovery tempera-
ture could be decreased due to a similar decrease in T . The higher the molecular
m
weight (MW) of PCL, the higher was the cross-linking degree [19]. Polyfunctional
polyester acrylates proved to be suitable additives to enhance the cross-linking
degree of PCL upon γ-irradiation [20].
ε-Caprolactone (CL) diol oligomers can easily be converted into the related
dimethacrylates which can be efficiently photocured [21]. During their SM
programming, the T of the PCL segments served as T .Thelattercould
m ∘ trans
be changedbetween 30 and50 C via the MW of the PCL diol. The photosets
showed excellent SM properties in multiple cycles. Thus, R = 86–97% and
f
R = 92–97% data were measured after the fifth cycle.
r