Page 156 - Biodegradable Polyesters

134 6 Shape Memory Systems with Biodegradable Polyesters

and cross-linked versions. This subdivision is reasoned by the fact that the SM
properties are generally improved by various cross-linking mechanisms.

6.2.1
Homopolymers and Composites

6.2.1.1 Linear
Semicrystalline poly(L-lactic acid) (PLLA or PLA) exhibits T and T m in the
g
∘
ranges of 60–70 and 150–170 C, respectively. Therefore, in its un-cross-linked
form, the crystalline domains act as net points, whereas molecules in the
amorphous phase work as switching segments. Accordingly, T is usually
trans
slightly higher than T . Wong and Venkatraman [8] studied the shape recovery
g
and recovery stress as a function of stretching ratio (≤400%) and deformation
∘
temperature (T = 85–120 C). As expected, R decreased with increasing
trans r
stretching and T . The drop in the recovery was attributed to strain-induced
trans
crystallization. The latter hampered the chain mobility in the amorphous phase
yielding low R . To overcome this problem, two approaches have been followed.
r
Ghobadi et al. [9] enhanced the amorphous chain mobility by plasticization with
water (≤2 wt%). Radjabian et al. [10] used spun PLA filament, wound in helical
form, for SM testing. The filament itself has a complex supermolecular structure
which does not change substantially in the SM cycle. Thus, R did not change with
r
∘
T trans (70–90 C), but remained still modest (∼50%). By contrast, R deceased
f
with increasing T trans .
6.2.1.2 Cross-linked
It is intuitive that the net point function cannot be fulfilled properly by the
crystalline domains evolving in slow crystallizing polymers, such as PLA and
PCL. Slow crystallizing polymers are prone to cold crystallization above their
T (annealing), which is, in addition, prominently influenced by the deforma-
g
tion of the amorphous chains [11]. Therefore, research interest turned toward
cross-linked systems. In the corresponding cross-linked semicrystalline polymer,
T m can be used as T trans instead of the T . To use the melting/recrystallization
g
transition instead of the glass/rubbery one is beneficial as the former transition
is “sharper” (occurring in a smaller temperature range) and faster than the latter
one. Recall that chemical (covalent or thermoreversible) or physical cross-linking
(through phase segregated domains), and their combination, may replace those
“net points” which were initially responsible for the permanent shape.
PLA is often plasticized in order to decrease its T to ambient temperatures.
g
This, however, supports the cold crystallization that should be avoided. Moreover,
the plasticizer easily bleeds out. Both crystallization and plasticizer migration can
be circumvented by suitable electron beam irradiation of plasticized PLA. The
resulting elastomer-like material shows promising SM behavior [12].
Unlike PLA, the ductility of PCL is very high, which predestinates it for SM
∘
∘
applications. The T and T of PCL are at about −50 Cand 60 C, respectively
g m
[13]. In order to make use of its T as T (which is exclusively the case), PCL
m trans