Page 154 - Biodegradable Polyesters

132 6 Shape Memory Systems with Biodegradable Polyesters

or several (multi-shape) stable temporary shapes and recover their original or
permanent shape (or other temporary shapes when not dual-shape type) upon
the action of an external stimulus. The external stimulus may be temperature (set
by direct or indirect ways), pH, water, light irradiation, redox condition, and so on.
In most cases, however, the SMPs are thermosensitive or thermally activated ones.
The “switching” or transformation temperature (T ), enabling the material to
trans
return to its permanent shape, is either linked with the glass transition (T )orthe
g
melting temperature (T ). Therefore the SMPs are often subdivided on the basis of
m
their switch types into T -or T -based SMPs. As reversible “switches.”, however,
g m
other mechanisms such as liquid crystallization/melting, supermolecular assem-
bly/disassembly, irradiation-induced reversible network formation, formation and
disruption of a percolation network, may also serve [2]. The permanent shape
is guaranteed by physical (entanglement, interpenetrating networks (IPNs)) or
chemical network (composed of permanent or temporary covalent bonds) struc-
tures. The corresponding sites are also termed net points.Thetemporary shapeis
set by mechanical deformation above T . In exceptional cases, the deformation
trans
temperature may be below T . The macroscopic deformation applied is trans-
trans
lated to conformational changes of the molecular chains or its segments. They
are fixed by maintaining the deformation via cooling, causing either vitrification
(T -based version) or crystallization (T -based version). The strain energy, stored
g
m
in this way, is released when the material is unloaded and heated above T trans .Note
that T trans > T or T . This heating triggers conformational rearrangements of
g
m
the molecular chains or melting of the crystals, respectively, through which the
permanent shape is restored. All that is disclosed above is related to one-way
SMPs. This means that the external stimulus activates only the change from the
temporary to the permanent shape (dual-shape variant) or from one temporary to
the other one in multi-shape SMPs. Such one-way SMPs meet the requirements
of many applications as will be shown later. Nevertheless, considerable research
efforts are devoted to produce two-way SMP systems which feature a reversible
shape change on the basis of “on–off” switching of the external stimulus. A large
body of works is already dealing with different SMPs, and the related knowledge
is well summarized in recent reviews [2–6].
Shape memory (SM) properties are typically quantified by the shape fixity (R )
f
and shape recovery ratios (R ). R means the extent of fixing of the externally
r f
applied deformation in the temporary shape. Its value is 100% when the applied
deformation, introduced above T , is fully kept below T in the temporary
trans trans
shape. The usual deformation modes are tension, bending, and compression. R
r
is the percentage of the recovery of the original shape when the material is sub-
sequently heated above T . R = 100% when the original shape of the material
trans r
is fully restored. SM properties are usually determined in cyclic (one or more)
thermomechanical tests performed under stress- or strain-controlled conditions.
Figure 6.1 displays the course of a SM thermomechanical test. Apart from R and
f
R , further SM characteristics, such as the temperature interval of recovery,
r
recovery rate, and recovery force, can be measured. The latter is assessed in
strain-controlled mode, termed also as recovery.Anexcellent overview on the