Page 190 - Book Hosokawa Nanoparticle Technology Handbook

FUNDAMENTALS CH. 3 CHARACTERISTICS AND BEHAVIOR OF NANOPARTICLES AND ITS DISPERSION SYSTEMS

10 2 10 2
55vol%
53 10 1
Viscosity η (Pa·s) 10 0 50 47 Storage modulus G′ (pa) 10 0 1.5 wt%
1

1.0
10
45 0.7 0.5
0.3
10 -1
10 -1 10 -2 10 -1 10 0 10 1 10 2
-1
10 -2 10 -1 10 0 10 1 10 2 Angular frequency ω (s )
·
-1
Shear rate γ (s ) Figure 3.7.2
Effect of flocculant concentration on the elastic behavior
Figure 3.7.1 of suspensions.
Effect of particle concentration on the viscosity behavior
for suspensions of non-interacting particles.
acts as a flocculant in the suspensions. Since the floc-
concentrated suspensions of non-interacting particles. culation is induced by a bridging mechanism in which
To explain the viscosity jump, two types of mecha- one polymer chain adsorbs onto two or more particles
nisms are proposed: rupture of two-dimensional lay- to bind them together, the flocculation level is consid-
ering and hydrodynamic clustering. ered to increase with PVA concentration. In the
absence of PVA, the suspension is electrostatically sta-
(2) Rheology of flocculated suspensions bilized and the elastic responses are not detected. The
additions of PVA cause the increase in storage modu-
(a) Dynamic viscoelasticity lus. Below 0.5 wt%, the storage modulus linearly
In many cases, particles dispersed in a liquid are bound decreases with decreasing angular frequency and
together to form flocs due to interparticle attraction. hence the huge flocculated structures are not expected.
The interparticle forces can be of three types [1]: As the particle concentration is increased to 0.7 wt%,
London-van der Waals attractive forces, electrostatic the storage modulus clearly indicates the existence of
repulsive forces, and those connected with adsorbed plateau, showing that the relaxation does not occur
polymer. At low particle concentrations, the floccu- even after long times. Because the suspensions begin
lated suspensions consist of a collection of discrete to respond elastically, one can accept that the
flocs. As the concentration is increased, the formation unbounded flocs are developed at this point. The flocs
of floc–floc bonds progresses and above some critical with finite sizes are regarded as independently distrib-
volume fraction three-dimensional network is devel- uted and have mobility to some extent, while the net-
oped through the system. When the network structure work structure of unbounded flocs hold a certain
is sufficiently strong to transmit the forces through shape. The former systems are characterized as liquids
floc–floc bonds, the flocculated suspensions behave and the latter as solids. Therefore, it is considered that
as solids, that is, the suspensions respond elastically the phase transition from liquids to solids takes place,
to small strains. Although the elasticity arises only at a point where the frequency-dependent curve of
from the attractive forces between two particles, the storage modulus shows a plateau at low frequencies. In
appearance of elasticity can be associated with the many flocculated suspensions, the interparticle bonds
structural changes from discrete flocs to unbounded generally have solid-like properties that are not broken
network. The extreme changes in material transport down by thermal energy and the viscoelastic behavior
properties can be discussed in relation to the long- can be basically understood through the same concept.
range interactions which are cooperatively generated
in the infinite network made up of contiguous (b) Viscosity behavior
sequences of primary bonds between the most neigh- The floc structures in ordinary suspensions are not
boring units [4]. broken down in a quiescent state. But they are easily
Figure 3.7.2 shows the effect of polymer concentra- ruptured in shear fields, since the interparticle bonds
tion on the frequency ( ) dependence of storage mod- are not very strong. For suspensions that show solid-
ulus (G’) for 30 vol% suspensions of latex particles like responses due to the unbounded network
with a diameter of 240 nm. The polymer is polyvinyl developed three-dimensionally, at high stresses the
5
alcohol (PVA) with a molecular weight of 1x10 and systems can flow as the result of structural ruptures.
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