Page 167 - Biodegradable Polyesters

6.2 Shape Memory Polymer Systems 145

almost a mirrored picture of the R course, was found for R as a function of
r
f
the PCL content that was varied in the whole composition range. This behavior
was traced to the actual morphology of the blends. As long as the thermoplastic
rubber phase remained continuous, excellent R and R data were measured.
r
f
Du et al. [67] investigated effects of layered architecture and blend morphol-
ogy on the SM behavior using thermoplastic PU and PCL. The PU/PCL ratios set
were 75/25, 50/50, and 25/75, respectively. The layered structure was achieved by a
special multilayer coextrusion technique. The layer thickness varied with the com-
position ratio whereby keeping the number of layers and the overall thickness of
∘
the multilayer film as constants. T of PCL served as T trans (= 70 C) during shape
m
∘
creation that was fixed at T = 21 C. The R of the multilayer film was the higher
f
the lower its PCL content was, and it changed as a function of the thermome-
chanical cycles only marginally. The PU/PCL blend at 50/50 ratio outperformed
the multilayer film with respect to R at the same PCL content. On the other hand,
f
all other blends showed inferior R data to the multilayer film. Interestingly, R
f r
increased with increasing thermomechanical SM cycles, at least up to the fifth
one. This may be an effect of recrystallization in confined space in both blends
and multilayer films. The PU/PCL multilayer films demonstrated SM behavior
comparable (at 50/50) to or better than those of the corresponding blends.
6.2.4.2 Cross-linked
Cross-linking is a useful tool to improve the SM behavior as already stated.
This technique has been adapted for PLA/PEG blends investigated in the range
of 100/0 to 70/30. Cross-linking occurred by adding blocked polyisocyanate.
The T of the amorphous phase, composed of both PLA and PEG, served
g
to select the T trans .Both R and R increased with increasing amount of the
f
r
polyisocyanate cross-linker. The cross-linking reduced also the recovery time of
the corresponding blends [68].
6.2.5
Polymers with Thermosets

Combination of cross-linkable resins with biodegradable polyesters may result in
various structures. Distribution of the polyester as dispersed phase in thermosets
is the usual prerequisite of toughness improvement. The dispersion is gener-
ated by phase separation upon curing. Such systems may show SM properties
although this is not yet reported for systems with biodegradable polyesters.
Far more interesting are, however, those systems which feature conetwork,
semi-interpenetrating networks (semi-IPNs), and full IPN structures.

6.2.5.1 Conetworks
Conetworks are chemically cross-linked networks in which none of the con-
stituents forms a continuous phase. This definition does not exclude however the
possible presence of homo-cross-linked domains. On the other hand, they differ
markedly from the grafted IPN structure in which both phases are continuous