smpp_compressed-7

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shape- memory effect, the chains stop moving in a par-
tially oriented conformation. In an entropy wheel41, a 
reversible contraction and elongation of a series of rub-
ber spokes generates mechanical energy from a ther-
mal gradient. The scale of the coordinated molecular 
reorientation of an amorphous domain provides a suf-
ficient driving force to generate macroscale deformation 
without the need for crystallizable actuation domains. 
The actuation is caused purely by entropic change. The 
function of geometry- determining domains, supply-
ing the network anisotropy required to link molecular 
orientation to macroscale movement, is substituted by 
constant external stress on the system.
This analogy raises the question of whether crys-
tallization is necessary for a free- standing reversible 
shape- memory actuator. In the entropy wheel and 
in numerous other LCE materials7,42,43, a sufficient 
energetic gain can be generated by macromolecular 
entropic changes without the need for crystallization 
and melting. However, the key challenge is to ensure 
that this energy release results in a macroscopic shape 
change. Orientation needs to direct the volume change; 
otherwise, crystallization does not result in coordinated 
network deformation. The entropy wheel uses external 
stress, which makes actuation neither programma-
ble within the same material nor free- standing. As an 
alternative to crystallizable geometry- determining 
domains, molecular switches may provide the network 
anisotropy necessary to guide the alignment of the actu-
ation domains while also enabling programmable and 
reprogrammable deformation.
Characterization
One- way shape- memory effect. Macroscale mechan-
ical testing can be performed to quantify the shape- 
memory effect. The shape- recovery ratio Rr and shape 
fixity ratio Rf are two of the most important character-
istic quantities for one- way shape- memory materials. 
The shape- recovery ratio Rr indicates the effectiveness 
of the recovery process, given by the amplitude ratio of 
the original shape to the recovered shape. The shape 
fixity ratio Rf provides an indication of the program-
ming efficacy, given by the amplitude ratio of the fixed 
deformation εu to the total deformation εm.
Cyclic thermomechanical testing is the most widely 
applied method for the determination of these values. 
Using a conventional mechanical testing apparatus 
equipped with a thermochamber, a single, fixed exper-
imental setup can be used to simultaneously control the 
stress, strain and temperature during programming and 
recovery5. Three approaches to material deformation, 
known as compression44,45, bending46,47 and tensile test-
ing, are the most prevalent for shape- memory materials. 
Combined with more complicated approaches, such as 
shear deformation48 and torsion49, these tests can provide 
a thorough thermomechanical description of a single 
material. The relative simplicity and unidirectional nature 
of tensile testing further make this method applicable to 
modelling50,51 and have led to its widespread use.
The results of cyclic thermomechanical testing are 
usually presented in a stress–temperature–strain graph, 
allowing the visualization of mechanical behaviour at 
a b
c d
One-way shape-memory effect
Time (min)
T 
(°
C
)
ε (
%
)
60
40
20
0
Time (min)
1,000
800
1,200
200
600
400
100
100 400300200
100 400300200
Recovered shape
T
reset
T
low
Temporary shape
Temperature (°C)
ε (
%
)
σ 
(M
Pa
)
σ
max
σ
m
σ
max
σ
Tσ,inf
Tσ,max
Tσ,max
ΔT
rec
ε
u
ε
ΔT
rec
T
SW
Temperature (°C)
Covalent network
Thermoplast
Covalent network
Thermoplast
ε
p
ε
p
T
low
T
reset
T→
2
3
4
ε
u
ε
m
ε
p
1
ε
Stress-free recovery Constant-strain recovery
1 2 3 41 2 3 4
T
low
T
reset
T
reset
T
reset
T
reset
T
low
T
low
T
low
Fig. 3 | Quantification of a one- way shape- memory effect. a | The stress 
(σ)–temperature (T )–strain (ε) graph of a cyclic thermomechanical tensile test 
illustrates the complete one- way shape- memory cycle, including programming, 
fixation and recovery , as obtained by macroscale measurements. For programming 
(shape- memory creation procedure (SMCP)), the sample is heated to Treset with an 
elongation- to-extension εm (resulting in stress σm) (1) and then cooled to Tlow, while εm is 
kept constant for fixation of the temporary shape (2); the fixed temporary shape at εu is 
obtained from the unloading of the sample to zero stress at Tlow (3); after the temporary 
shape is fixed, the recovery process can be initiated by heating the sample to Treset (4); 
and the recovery strain εp is obtained after full recovery of the sample. b | Cyclic 
testing. The reliability of the programming and recovery cycle for a one- way shape- 
memory effect can be tested by repetitive measurements. ε and T cycles of the 
temporary and recovered shapes are shown. c | A schematic programming procedure 
with stress- free recovery is shown in orange. The black arrows in the sample schemes 
indicate the applied strain ε. Schematic recovery curves are shown for a thermoplastic 
polymer and a covalent network with no external stress applied during the recovery 
process. The switching temperature TSW is the inflexion point, representing the highest 
recovery rate (ΔTrec). d | Schematic SMCP with constant strain is shown in green. 
The red arrows indicate the direction of recovery of the material. Constant strain is 
applied during the recovery process. The maximum strain σmax and the temperature at 
maximum strain Tσ,max can be determined from the temperature–stress diagram. Tσ,inf is 
defined as the temperature at the inflexion point of the stress curve. Panels a, c and d 
are adapted from Sauter, T. et al. Quantifying the shape- memory effect of polymers by 
cyclic thermomechanical tests. Polymer Rev. 53, 6–40 (2013) reF.128, by permission of 
the publisher (Taylor & Francis). Panel b is adapted with permission from reF.129, 
Wiley- VCH.
Nature reviews | Materials
R e v i e w s
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	Reprogrammable recovery and actuation behaviour of shape-​memory polymers
	Characterization
	One-​way shape-​memory effect. 
	Fig. 3 Quantification of a one-​way shape-​memory effect.