SOLUTION PROPERTIES OF ASSOCIATING POLYMERS
5.4 ASSOCIATING POLYELECTROLYTES IN SEMIDILUTE SOLUTIONS
In semidilute solutions, the aggregation of hydrophobic groups belonging to different macromolecules leads to the formation of an intermolecular network. The association of chains in a semidilute regime is more straightforward [1] than in the dilute regime because of the lower entropy losses of the counterions: in the semidilute regime, the counterions can displace throughout the whole volume of the system and not be con- fined in the restricted area of clusters [2].
In semidilute solutions, APs show much higher viscosity, as compared to the parent polymers without associating groups (Figure 5.3). This is due to the intermolecular crosslinking of polymer chains by hydrophobic domains. Each hydrophobic domain usually contains a few dozen associating groups belonging to different macromolecules [58], so that it acts as multifunctional crosslink in the network. When the concentration of APs increases, new hydrophobic domains are formed, but the aggregation number of each domain Nd (i.e., the number of
k k
Viscosity
Concentration of polymer 1
2 Viscosity 2
Concentration of polymer 1
(a) (b)
Figure 5.3 Typical dependences of viscosity on polymer concentration for associating poly- electrolyte (1) and corresponding polymer without associating groups (2) in the presence (a) and absence (b) of intramolecular aggregation in a dilute solution.
hydrophobic groups in it) remains unchanged. This is similar to the behavior of low molecular weight surfactants just above the critical micelle concentration: the increasing surfactant concentration induces the increase of the number of micelles, but their size and aggregation number do not change. Similar to the micelles of low molecular weight surfactants, the hydrophobic domains are “living” objects as they are being incessantly reformed: there is always an exchange of hydrophobic groups between different domains, and there are also some free nonaggregated groups present in the system [83]. At the same time, the dynamics of exchange of the hydrophobic tails between hydrophobic domains in APs is much slower [83]
than in the low-molecular-weight surfactants because they are linked to a polymeric chain and therefore keep this chain with them at the displacement.
The rheological properties of solutions of associating polymers including APs were recently reviewed by C. Chassenieux et al. [85]. In this section, I consider mainly the responsiveness of these properties to various parameters, which is due to the reversible character of hydrophobic domains formed by weak noncovalent interac- tions. The responsiveness opens some rich possibilities for fine-tuning the rheological properties of the solutions. Due to the reversible character of the junctions, the semidi- lute solutions of APs exhibit many peculiar phenomena. The first set of phenomena concerns shear-responsive behavior, which, depending on the conditions, can lead either to thinning or thickening of the solutions.
A pronounced shear-thinning behavior is very common for most, if not all, APs.
This behavior is due to the disruption of the labile crosslinks (hydrophobic domains) under the shear [86]. At the highest shear rates (above ca. 800–1500 s−1) the viscosity tends toward that of the corresponding parent polymer without hydrophobes [8, 86, 87]. An important point is that for APs the shear-thinning is completely reversible, and at rest, the solution recovers its initial high viscosity. This property is highly exploited commercially, in particular, in various fluids for oil recovery (low viscosity at pumping and gelation at rest) and for painting (low viscosity at the application of paint on a surface with further jellification preventing dithering). It should be noted that high viscosity at rather low polymer concentrations can be attained not only for
k k APs but also for very-high-molecular-weight polymers that do not contain associ-
ating groups. But in this instance, the shear-thinning is not fully reversible because of the possibility of chain destruction under the shear. This is why APs are more promising as viscosity modifiers in comparison with very-high-molecular-weight polymers.
In contrast to shear-thinning, an opposite effect—shear thickening—is rather rarely observed in APs [8, 27, 86, 88, 89]. Shear thickening is attributed to shear-induced switching from intra- to intermolecular interactions of associating groups [90]. At a certain shear rate, the shear forces extend so considerably the polymer coils that they disrupt the intramolecular hydrophobic associations.
The released hydrophobic groups associate intermolecularly, thus intensifying the viscosity [86]. A determining factor for a good thickening ability is the distribution of hydrophobic groups [8]. Shear thickening disappears with increasing concentration of polymer, which favors the formation of intermolecular hydrophobic domains in the initial solutions (before the application of shear); therefore, no significant switching from intra- to intermolecular interactions can be achieved under the shear.
In some APs, a small shear thickening is observed just before a wide shear-thinning region on the flow curves (Figure 5.4a) [8, 86]. But the most interesting is a giant shear-thickening effect, at which the viscosity of the solution of APs abruptly increases by few orders of magnitude at some critical shear rate (Figure 5.4b). Such a huge effect was observed in the aqueous solutions of a HM poly(N,N′-dimethylacrylamide-co-acrylic acid) [27, 88, 89]. This polymer has a high molecular weight and exhibits pronounced intramolecular aggregation in dilute solutions. It was shown that for the abrupt giant shear thickening to occur, the solution of the associating polymer would need to be on the threshold of phase separation and the concentration of the polymer be close to cac [89]. In fact, the giant shear-induced thickening is a gelation process occurring at a critical shear rate [89].
It was observed that reducing the stress after gelation leads to the stiffening of the gels. This behavior was explained [89] by the formation of additional intermolecular associations, when the bridging chains in a gel retract upon the reduction of stress.
Keeping in mind that the aggregation of APs is governed by the interplay of hydrophobic attraction and electrostatic repulsion, it appears that we can control the rheological properties by increasing or decreasing the attractive or repulsive forces in accord with content, size, and distribution of hydrophobic groups; content of the charged groups; or addition of low molecular weight salt or some organic co-solvents preventing aggregation. It was demonstrated that the higher the content of the hydrophobic groups and the longer their length (i.e., the higher the attraction energy), the more pronounced is the increase of viscosity (Figure 5.3). At a constant hydrophobe level, a microblocky distribution of hydrophobic groups along the chain enhances the viscosity. The longer the hydrophobic microbocks along the chain, the more pronounced is the increase of viscosity [8, 86, 87]. However, at too strong an attraction of hydrophobic groups (e.g., due to their high content), phase separation occurs.
As for the charged units, the electrostatic repulsion that they induce has two oppo- site effects on viscosity [8]. On the one hand, the viscosity may decrease because
k k of the lower degree of association. On the other hand, the viscosity may increase
because of the coil expansion. Also charged units may enable a larger number of associating groups to be introduced in the AP without losing its water solubility.
Therefore, for each type of AP, an optimum composition (content, size, and distribu- tion of hydrophobic groups; content of charged groups) capable of providing higher viscosity can be found.
Since APs contain charged units, their viscosity is highly sensitive to the addition of low-molecular-weight salt, which can screen electrostatic repulsion and strengthen the hydrophobic interactions. The screening of the electrostatic repulsion encourages similarly charged macromolecules to associate, and thus viscosity increases. At the same time, the screening of the repulsion causes the polyelectrolyte coils to shrink, and thus viscosity decreases. Therefore, salt can have different impacts on viscos- ity, depending on which of these effects dominates at given conditions (molecular structure of polymer, its concentration, temperature, etc.).
The rheological properties of APs can also be fine-tuned by adding some organic co-solvents that weaken the hydrophobic attraction between associating groups. This approach was exploited recently [91–95] to create a system that blocks selectively the water flow in the oil well. For this purpose, the authors used an organic co-solvent (e.g., ethanol) that is perfectly soluble in water but insoluble in hydrocarbons (oil).
Addition of this co-solvent to a semidilute aqueous solution of an HM partially hydrolyzed polyacrylamide had a pronounced effect on viscosity, reducing it as a result of a disruption to the hydrophobic domains and a shrinking of the polymer coils (ethanol is a poor solvent for polyacrylamide). When this fluid with low viscosity was brought in contact with water, ethanol, which is perfectly soluble in water, diffused out of the polymer solution. Once the substance preventing gelation was removed, instantaneously a strong gel formed on the interface between water and polymer solution. This gel served as a plug to prevent the water flow. However, when in contact with oil, the polymer solution maintained a low viscosity because ethanol, being insoluble in oil, remains in the polymer solution and continues
Viscosity
Shear rate
Viscosity
Shear rate
(a) (b)
Figure 5.4 Schematic representation of “ordinary” (a) and “giant” (b) shear thickening pre- ceding shear thinning in solutions of associating polyelectrolytes.
k k to prevent gelation. Thus, by exploiting the reversible character of hydrophobic
junctions, a smart system was created [91–95]. This system is able to recognize the place of water inflow in the well and to block it without hindering the flow of oil.
Clearly, the reversibility of hydrophobic associations can be exploited in tailoring the properties of AP solutions, and this is quite promising for various applications of these systems.