Civil Engineering Reference
In-Depth Information
as reported in literature [10]. h ermal properties of CNCs depend mainly on source
origin, extraction methods and type of functionalization. h ermogravimetric analysis
(TGA) has been used for measuring thermal stability of the nanocrystals. Typically, for
CNCs the onset of thermal degradation temperature occurs between 200 and 300°C.
In axial direction, the coei cient of thermal expansion of crystalline cellulose has been
measured at about 0.1 K
-1
, which is comparable to the high modulus carbon i bers
[110, 111] .
15.4.4.2
Suspension Behavior
h e dispersion level of CNCs in aqueous or organic solvents determines their prop-
erties and corresponding applications for interfacial interaction between CNCs and
matrix. h e CNCs tend to agglomerate due to high surface area, hydrophilic nature,
and strong hydrogen bonding. Sulfuric acid-hydrolyzed CNCs have negatively charged
(surface) sulfate acid (OSO
3
-
/H
+
) groups, which induce the homogenous dispersion in
aqueous medium via electrostatic repulsions. In dilute regime (isotropic phase), CNCs
are randomly oriented and appear as spheroids or ovaloids and are similar to tactoids
in initial ordered domains, whereas at higher concentration these tactoids coalesce to
form an anisotropic phase showing unidirectional self-organization of rod-like CNCs.
Above critical concentration, it forms a chiral nematic phase and CNCs suspensions
produce shear birefringence and spontaneously separate into two ways as an upper
isotropic phase and a lower anisotropic phase [6, 101]. h is self-organization phenom-
enon was observed by polarized optical microscopy showing "i ngerprint" patterns
(chiral-nematic order) [56]. h is phase-forming ability of chiral nematic phases and
their properties depend on several parameters such as the aspect ratio, charge density,
osmotic pressure, medium, and processing.
15.4.4.2.1 Aqueous Medium
h e isotropic-to-anisotropic equilibrium is sensitive to the presence of electrolyte and
specii c nature of the electrolyte counterions. h e increasing amount of added elec-
trolyte decreases formation of anisotropic phase followed by a decrease in chiral nem-
atic phase and became more twisted phase. h e sulfated CNCs suspensions are also
strongly inl uenced by the nature and size of the counterions. For inorganic counter-
ions, increasing van der Waals radii increases the critical concentration for forma-
tion of ordered phase in the order H
+
< Na
+
< K
+
< Cs
+
, and for organic counterions
the critical concentration is inl uenced by the relative contributions of hydrophobic
attraction and steric repulsion of NH
4
+
, (CH
3
)
4
N
+
, (CH
3
CH
2
)
4
N
+
, (CH
3
CH
2
CH
2
)
4
N
+
,
(CH
3
CH
2
CH
2
CH
2
)
4
N
+
, (CH
3
)
3
HN
+
and (CH
3
CH
2
)
3
HN
+
. In this way, the chemical
nature of the counterions inl uences the stability, dependency of phase separation on
temperature, the chiral nematic pitch and redispersibility of CNCs made from the
desired suspensions. h e nature and charge density on CNCs surfaces also inl uence the
chiral nematic phase formation. When HCl-hydrolyzed CNCs were post-sulfated (hav-
ing one third sulfur content as compared to directly H
2
SO
4
-hydrolyzed CNCs), these
formed a birefringent glassy phase comprised of crosshatch pattern, whereas directly
sulfated CNCs form a i ngerprint pattern (chiral nematic phases) [112]. In this way,
carboxylated CNCs (TEMPO-mediated oxidation) formed homogeneous dispersions
