Civil Engineering Reference
In-Depth Information
The 1
st
tensile
The 2
nd
to 5
th
tensile
Heating oven
Water tank
crosshead
Sample tape
Water immersion
for 10 min
Shape ixity
Observed when
crosshead
returns to
original position
Shape recovery
triggered by
water
(water immersion
for 10 min)
Drying at
75 °C
for 10 min
ti ix
ε
u
Stretching
in wet
m
is the
maximum strain applied in the cyclic tensile testing, ε
u
is the strain in the unloading dry state, and
Figure 3.3
Deformation cycle to demonstrate the water-sensitive shape memory ef ect.
ε
ε
m
is the residual strain values of the two successive cycles at er shape recovery was triggered by water
immersion. Reproduced with permission from [40] .
and 1, 4-butanediol (BDO). h e PCL segment and the cellulose percolation network
(CPN) in the composites were considered as the thermal-sensitive and the water-
sensitive switches, respectively. h e authors found that the combination of the crys-
tallized PCL and the dry-state CPN contributed to the shape i xity when the extended
composites were cooled down and dried under vacuum; however, the temperature
increase only triggered one part of the recovery, due to the PCL switch, while the
PCN switch was maintained. At er an extensively specii c exploration of the thermal-
aqueous process, the thermal-induced shape memory recovery of the composites in
the dry-state was found to be inversely proportional to the CNW content, which was
explained by assuming that the dry-state of the PCN partially restrained the entropic
elasticity of the polymer matrix.
In a later work [41], the same authors developed a novel smart nanocomposite
featuring a heterogeneous-twin-switch shape memory ef ect using cellulose whiskers
as reinforcement and segmented shape memory polyurethane as the matrix. Again
the polyurethane matrix was prepared from MDI, BDO, and PCL (Mn = 4000 g/mol),
but in this case, the weight percentage of PCL in the shape memory polymer was
60 wt%. h e composites maintained the thermal-induced switchable shape ef ect
originally existing in the polymer matrix (due to the crystallization/melting of the
PCL phase) and simultaneously acquired a water-induced shape memory ef ect due
to the percolation network of the cellulose whiskers, whose hydrogen bonding can be
regulated by water, reversibly. In this case, the dry-state CPN with strong hydrogen
bonding, via -OH groups of the cellulose, was called in the state of “switched on” or
“locked”, and this was associated with the shape i xity. h e CPN with disrupted hydro-
gen bonding by the absorbed water molecules was called in the state of “switched
of ” or “opened”, and this was associated with the shape recovery. h ese transitions
are schematized in Figure 3.4. h e conditions selected for testing composite samples
