Handbook of Solvents - George Wypych - ChemTech - Ventech!

12.1 Rheological properties, aggregation, permeability 687
Intrinsic viscosity, cm 3 g -1
80
70
60
50
40
30
20
MEK
toluene
10
0 20 40 60 80 100
PVAc, wt%
Figure 12.1.7. Intrinsic viscosity of PS/PVAc mixtures
in methyl-ethyl-ketone, MEK, and toluene. [Data from
H Raval, S Devi, Angew. Makromol. Chem., 227,
27-34 (1995).]
Viscosity, Poise
0
0 2 4 6 8 10 12 14
are considerable departures from the simple predictions of the above equations because,
based on them, viscosity should be a simple function of molecular weight. Figure 12.1.3
shows, in addition, that the relationship between viscosity of the solution and molecular
weight is nonlinear. 6 Also, the critical shear rate, at which aggregates are formed, is a nonlinear
function of molecular weight (Figure 12.1.4). 7
These departures from simple relationships are representative of simple solutions. The
relationships for viscosities of solution become even more complex if stronger interactions
are included, such as the presence of different solvents, the presence of interacting groups
within polymer, combinations of polymers, or the presence of electrostatic interactions between
ionized structures within the same or different chains. Figure 12.1.5 gives one example
of complex behavior of a polymer in solution. The viscosity of PMMA dissolved in
different solvents depends on concentration but there is not one consistent relationship (Figure
12.1.5). Instead, three separate relationships exist each for basic, neutral, and acid solvents,
respectively. This shows that solvent acid-base properties have a very strong
influence on viscosity.
Figure 12.1.6 shows two different behaviors for unassociated and associated block copolymers.
The first type has a linear relationship between viscosity and concentration
whereas with the second there is a rapid increase in viscosity as concentration increases.
This is the best described as a power law function. 8 Two polymers in combination have different
reactions when dissolved in different solvents (Figure 12.1.7). In MEK, intrinsic viscosity
increases as polymer concentration increases. In toluene, intrinsic viscosity
decreases as polymer concentration increases. 9 The polymer-solvent interaction term for
MEK is very small (0.13) indicating a stable compatible system. The interaction term for toluene
is much larger (0.58) which indicates a decreased compatibility of polymers in toluene
and lowers viscosity of the mixture. Figure 12.1.8 explicitly shows that the behavior of
400
350
300
250
200
150
100
50
pH
PEO/PAA
PEO
PAA
Figure 12.1.8. Viscosity of poly(ethylene oxide), PEO,
poly(acrylic acid), PAA, and their 1:1 mixture in
aqueous solution vs. pH. [Adapted, by permission, from
I C De Witte, B G Bogdanov, E J Goethals, Macromol.
Symp., 118, 237-46 (1997).