Civil Engineering Reference
In-Depth Information
were: melting PCL phase at 65ºC in nitrogen atmosphere (state 2); immersion in water
at 65ºC for ten minutes (state 3); cooling down to room temperature and drying in
vacuum for 10 hrs (state 4). h e time-dependent elongation of the composites in the
process of the shape recovery was quantitatively measured, as shown in Figure 3.5.
h e composites recovered from the temporary shape to the original shape sequen-
tially following three separate steps. h e i rst recovery step occurred when the stress
was released (1). h e composites had a lesser percentage of recovery induced by stress
releasing with the increase of the CNW contents. h e SMP had the higher value of
14%, whereas the sample containing 23% cellulose (CP23) had the smaller one (1.6%),
which was considered consistent with the proposed explanation of CPN contributing
to the shape i xing when the extended samples were cooling down and dried. At er
heating at 65 ºC (2) the percentage of the thermal-induced shape recovery varied
upon the CNW contents pronouncedly. It was found that neat polymer recovered
more than 90% while CP23 recovered around 16% in this step, coni rming that the
strong hydrogen bonding partially restrained the entropic elasticity, resulting into the
uncompleted shape recovery. From Figure 3.5, it can be noticed that all the samples
nearly recovered to the original shape at er they were immersed in hot water for 2
to 3 min (3). It was proved that the addition of CNW slightly increased the i xity
ratio while maintaining the recovery ratio in comparison with the neat SMPU. Both
the i xity and the recovery ratio for all composites were higher than 90%, indicat-
ing that the CNW-SMPU composites maintained a heterogeneously twin-switchable
shape memory ef ect. h e contribution of the shape recovery by water pronouncedly
increased with the increase of the CNW content in the composites, providing other
strong evidence that the percolation network of CNW served as the water-sensitive
switch that was responsible for the water-induced shape recovery. h is was presented
as a new strategy to combine two dif erent types of switches into one material, being
the composite responsiveness dif erent from the conventional ones that are only able
to response to one external stimulus only.
Humidity
PCL is opened; PCN is
switched of
PCL is locked; PCN is
switched of
Wet
state 4
state 3
step (2)
step (1)
dry
state 1
state 2
PCL is locked; PCN is
switched on
PCL is opened; PCN is
switched on
hot
cool
Temperature
Figure 3.4
State transitions of the composites under dif erent humidity and temperature conditions. Step
(1): heating from room temperature to the temperature above the T
m
of PCL; step (2): immersion into hot
water at the temperature above the T
m
of PCL. From Luo
et al.
[41] , with permission.
