Civil Engineering Reference
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was necessary for achieving good recovery properties. h e authors found that at er
the second cycle, the recovery was higher than 90% and was able to increase further
by the i t h cycle. However, the reduction of the elastomer fraction inevitability pro-
duced an irreversible strain during the i rst tensile cycle, as illustrated by the decrease
of the recovery ratio from 90.5% for the nanocomposite with 1.1 wt% cellulose to 53.6%
for the i lm containing 29.6 wt% cellulose (cycle 2). h e explanation proposed by the
authors pointed out that the exposure to water produces a breakup of the CNW per-
colation network so that the l exible elastomer matrix can be deformed to the desired
shape. h e CNWs percolation network reforms at er drying, thereby providing enough
i xative force for the temporary shape. h e entropy contribution to the elasticity of the
TPU matrix then enabled the rapid shape recovery when the CNW percolation net-
work was disrupted again during wetting. h is completely water-sensitive shape mem-
ory mechanism is totally dif erent from the traditional ones, in which water or other
solvents are used as plasticizers to lower the glass transition temperature of the shape
memory polymers. h e reported work provides a novel and ef ective strategy to achieve
rapidly switchable shape recovery in a material by using a simple wetting process and
i xing through an easily applicable programmed drying process.
Regarding biodegradable polyurethanes, Luo
et al.
[35] synthesized polymeric
nanocomposites by incorporating cellulose nanowhiskers (obtained by the clas-
sic sulfuric acid hydrolysis) into shape memory polyurethane based on poly
(ε-caprolactone) diol (PCL, Mw = 65000), 4,4
′
-diphenylmethane diisocyanate (MDI)
(
a
)
(
b
)
(
c
)
10
9
8
7
6
5
4
3
2
1
0
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
0
100
200 300
Whisker Length (nm)
400
500
600
0
10
20
30
40
50
(
d
)
(
e
)
Whisker Diameter (nm)
Figure 3.2
(a and b) TEM images of cellulose nanowhiskers dispersed on a carbon i lm from the
DMF suspension, (c) TEM image of cellulose nanowhiskers dispersed in the elastomeric matrix (i lm
containing 20.6 wt% cellulose), (d and e) the length and diameter distributions of cellulose nanowhiskers
were measured by image analysis. Reproduced with permission from [40].
