Civil Engineering Reference
In-Depth Information
of cellulose nanoi brils considerably af ected the mechanical properties of the resulting
nanocomposites: the tensile modulus increased by 33% and 43% with the incorpora-
tion of 0.5 and 1 wt% of nanocellulose, respectively (Table 1), although these increases
were accompanied by a considerable decrease in stress at failure and extensibility. h ese
results were associated not only with the fact that the particles are more rigid than the
polyurethane matrix but also with their chemical reaction with the matrix through
hydroxyl groups that improved the interfacial bonding between the polyurethane
matrix and the i ller.
Cellulose nanocrystals prepared by acid hydrolysis are rarely dispersed in organic sol-
vents because of the high polarity that remains in the particles at er treatment. h us, in
order to improve their dispersion in organic solvents, such as tetrahydrofuran, Lin
et al.
[59] carried out the substitution of hydroxyl groups on the CN surface obtained by acid
hydrolysis of native linter by acetyl groups, generating acetylated cellulose nanocrystals
(ACN). h ese authors also used castor oil as the main polyol component in the polyure-
thane nanocomposites by preparing a prepolymer according to the method described
by Sperling, [60] using a toulene diisocyanate (TDI)/poliol ratio of NCO/OH = 2. At er
that, the prepolymer was mixed with the desired mass of ACN (from 0 to 25 wt%)
and 1,4-butanediol as the chain extender (the amount was adjusted to give a total of
NCO/OH = 1) in THF at room temperature. h e solutions were casted and cured at
room temperature for two days to form dried i lms that were about 0.5 mm thick.
As the ACN loading-level increased from 0 wt% to 25 wt%, the tensile strength and
Young's modulus of the nanocomposites increased from 2.79 MPa to 10.41 MPa and
from 0.98 MPa to 42.61 MPa, respectively. When the ACN loading-level was 10 wt%, the
elongation at break reached a maximum value of more than twice that of the neat poly-
urethane. h e enhanced mechanical performance was primarily attributed to the uni-
form dispersion of the ACN nanophase and the strong interfacial interactions between
the i ller and matrix that lead to the formation of a three-dimensional ACN network.
However, the glass transition temperature of the sot segments obtained by dif erential
scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) was lower for the
composites than that of neat PU, which was explained considering two opposing ef ects.
First, the motion of the sot segment could have been suppressed by the steric hindrance
of the rigid ACN nanophase and by hydrogen bonding onto the active ACN surface,
which could result in a shit of Tg to a higher temperature. Second, the incorporation
of ACN may have cleaved the original interaction between the hard and sot segments
leading to a decrease in the glass transition temperature (Tg) of polyurethane. h e
results suggested that the second ef ect dominated the i nal properties of the composites.
Table 1
Tensile properties of polyurethane-based nanocellulose composites.
Sample
Tensile Modulus, E
(MPa)
Stress at break, σ
b
(MPa)
Deformation at break,
ε
b
(%)
0 wt.% NC
479.5 ± 43.1
27.6 ± 0.8
23.5 ± 3.9
0.5 wt.% NC
636.4 ± 66.3
19.2 ± 4.0
4.9 ± 1.8
1.0 wt.% NC
682.9 ± 69.1
31.2 ± 4.3
11.5 ± 6.3
