Civil Engineering Reference
In-Depth Information
3D network pervading the liquid suspension at a crystal concentration threshold close
to 1 wt%, as shown in Figure 3.1. Films that showed a good dispersion of the crys-
tals were prepared by casting and thermal curing of the stable suspensions. h ermo-
mechanical and mechanical properties of the i lms indicated that strong i ller-matrix
interactions were developed during curing as a result of a chemical reaction occur-
ring between the nanocrystals and the isocyanate component. h e authors showed
that the cellulose crystals signii cantly increased the tensile modulus of the PU i lms
at very small i ller loadings (i.e., 0.5-5 wt%). However, due to the i ller-matrix chemi-
cal interaction, there was no indication of i ller percolation in the cured i lms, but an
increase in the crosslink density of the PU matrix. Cryo-fractured surfaces showed that
higher energy was consumed in the fracture of the composite i lms as compared with
the unreinforced PU. h e fact that the incorporation of cellulose nanocrystals in this
polyurethane elastomer greatly af ected the properties of these materials was further
coni rmed by positron annihilation lifetime spectroscopy (PALS) [32]. PALS results
indicated that, for all samples, the free nanoholes volume remained constant, indicating
that nanoholes are formed in the neat polyurethane. Moreover, positron data indicated
that the fractional free volume was strongly dependent of the nanocellulose concentra-
tion; specii cally, the amount of nanoholes systematically decreased with respect to that
of the neat polyurethane as the concentration of cellulose nanocrystals was increased.
In other words, a good qualitative correlation was observed between the decreasing
fractional free-volume and the increasing rigidity (storage and Young moduli) of the
samples as a function of the cellulose concentration.
Juntaro
et al.
[30] prepared and characterized nanocomposite i lms of bacterial cel-
lulose (10-50 wt%) and polyurethane-based resin. h e authors indicated that bacterial
cellulose showed good compatibility with PU-based resin. Moreover, they observed
that the i ller swelled in ethanol, and that bacterial cellulose sheets prepared from i ber
10
3
G
′
(Pa)
10
2
10
1
(m - m
cG
′
)
β
G
′
G
′ α
10
0
1
10
%wt hydrolyzed cellulose
Figure 3.1
Storage modulus (G') at 1 Hz versus nanocellulose concentration i tted using the percolation
model. Reproduced with permission from [22] .
