Page 166 - Biodegradable Polyesters

144 6 Shape Memory Systems with Biodegradable Polyesters

transition (T -or T -related) and act as the “switching” phase. The other, being
m
g
elastic, is responsible for the permanent shape.
6.2.4.1 Linear
Because PLA is highly brittle, it has been blended with numerous polymers to
improve its toughness. A “by-product” of this research was the observation that
some blends, in fact, showed SM feature. Lai and Lan [61] studied the SM perfor-
mance of PLA/thermoplastic PU blends at 70/30 and 50/50 compositions. Ther-
moplastic PU was found in dispersed form at 70/30 ratio, while a bicontinuous
phase structure was concluded for the PLA∕PU = 50∕50. After deforming the
∘
specimens at T = 25, 80, and 120 C, the recovery was assessed in the tem-
trans ∘
perature range T = 20–160 C. Note that the selected T trans data are below and
∘
above that of the T of the PLA (about 80 C). R , R , and the recovery stress
g f r
strongly depended on T and recovery temperatures. R increased with increas-
trans f
ing T , while an opposite trend was observed for R .
trans r
Zhang et al. [62] demonstrated SM behavior for PLA toughened by a polyamide-
12-based elastomer that was incorporated up to 30 wt%. For T of the tensile-
trans
loaded specimens, room temperature was selected, which is in between the T of
g
∘
∘
the polyamide elastomer (T ∼−50 C) and that of the PLA (T = 75 C). Recov-
g
g
ery was triggered at temperatures above the T of PLA.
g
∘
Thermoplastic PU elastomer (Tg ∼−35 C) was blended with PLA in 10 wt%
with and without MWCNT with various surface treatments [63]. The latter was
introduced in 10 wt% to achieve electroresponsive SM. For temporary shaping,
the T of PLA was considered. R decreased with increasing number of the elec-
r
g
troactivated thermal cycles. This was attributed to the formation of “frozen-in”
crystals in the dispersed PLA phase.
PLA/PCL blends in the compositions range of 100/0 to 60/40 were produced
with and without additional MWCNT by Amirian et al. [64]. The phase-
segregated blends exhibited two T and two T values. The latter increased with
m
g
∘
increasing amount of MWCNT. For T trans = T (PLA)+ 15 C, while for shape
g
∘
∘
fixing T (PLA)− 15 C were chosen. R was measured at T = 70 C where the
g
r
melting of PCL is also involved. As a consequence, both R and R decreased with
r
f
increasing PCL content of the blends. R was marginally affected, while R went
f r
through a maximum as a function of the MWCNT content (0–3 wt%) during the
tensile deformation SM tests.
A novel approach should be credited to Luo and coworkers [65] to improve
the SM performance of PCL. They prepared inclusion complexes between
α-cyclodextrin and PCL. Through this host–guest complexation, a peculiar
physical network was created with “naked” PCL segments as “switching” phase
and cyclodextrin–PCL inclusion complex domains as net points. Both R and R
f r
were slightly reduced with the inclusion ratio (30–50%). The in vitro degradation
of this new type of blend was faster than the reference PCL.
PCL worked as an efficient switching phase also in styrenic thermoplastic rub-
bers, such as the styrene–butadiene–styrene block copolymer [66]. R increased
f
steeply before leveling off above 30 wt% PCL content. An opposite tendency,