Polyampholytes viscosity behavior

Synthesis and Aqueous Solution Viscosity Behavior of Polyampholytes from Cationic-Anionic Monomer Pairs... 

This chapter reports the syntheses and viscosity behavior in aqueous salt solution of two recently prepared polyampholytes. These polymers, derived from cationic-anionic vinyl monomer pairs, exhibit viscosity behavior in salt solution that is contrary to that of normal polyelectrolytes. The intrinsic viscosity is found to increase with increasing salt concentration for one of the samples and remain virtually unchanged for the other. Also, a modified form of the EinsteinSimha equation is observed to correlate especially well with the experimental data, where, in some cases, the Huggins equation apparently is not appropriate. In the context of these findings, some previous results with regard to the behavior of a polyvinylimidazolium sulfobetaine are discussed. 

We investigated the syntheses and viscosity behavior of polyampholytes derived from cationic-anionic monomer pairs of the type shown in Figure 2. It should be noted that the cationic-anionic monomer pairs that are under consideration contain no nonpolymerizable ions. 

Polyampholytes. Ampholytic copolymers (i, 30-35) exhibit interesting viscosity behavior in electrolyte solutions that is quite opposite to the behavior of conventional polyelectrolytes. Recently we prepared high charge density copolymers from the matched, nonhydrolyzable comonomer pairs sodium 2-acrylamide-2-methylpropanesulfonate (NaAMPS) and 2-acrylam-ide-2-methylpropanedimethylammonium chloride (AMPDAC) (1, 33) see structures). 

Figure 3 represents the effect of added electrolyte concentration on the [nl obtained from the modified Huggins plot for poly(4VMP/pSS) and poly(MPTMA/AMPS), and the usual Huggins plot for poly(METMA/MES). The intrinsic viscosity increases with increasing salt concentration for all three ampholytic systems. Similar results are also reported for other polyampholyte-salt systems (6,13,27,28). This behavior may be rationalized on the basis of chain expansion which results in increased solute-solvent interaction. The [ri] is related to the hydrodynamic volume of macromolecules in solution (29). An expansion of the chain results in the viscosity increase due to an increase in effective hydrodynamic volume of the solute in the given solvent. It is expected that the added electrolyte would disrupt the intramolecular and intermolecular interactions and allow the polymers to behave more freely. Thus, the increase in [n] may be related to extended chain conformations resulting from the increased polymer-solvent interactions. 

This study confirms the concept that polyampholytes take an expanded conformation in aqueous salt solutions which is in contrast to typical polyelectrolyte behavior. Viscosity determinations in conjunction with light scattering studies has provided a general confirmation of the polyampholyte effect in the polymers derived from the ion-pair comonomer in aqueous salt solutions. This effect is related to the ion-binding capabilities of the added electrolytes. 

Solution Properties. The aqueous solution behavior of polyampholytes is dictated by coulombic interactions between the basic and acidic residues. Polyampholytes have the ability to exhibit both polyelectrolyte and antipolylelectrolyte behavior in aqueous media. Which type of behavior is exhibited depends on factors such as solution pH, copolymer composition, the relative strengths of the acidic and basic residues, and the presence/absence of low molecular weight electrolyte (239). A feature of polyampholytes—in particular those comprised of weak acidic and basic residues—is the so-called isoelectric point, or lEP. This is simply defined as the solution pH at which the polyampholyte is electrically neutral. Statistical polyampholytes often remain soluble at and around the lEP whereas block polyampholytes tend to be soluble above and below but insoluble at this critical pH. The lEP may be determined either by titration or by measuring the reduced viscosity as a function of pH—the lEP also represents the point at which the polyampholyte chain is in its most compact conformation and thus corresponds to the minimum in reduced viscosity (239,266). With a knowledge of the respective piiLa s and copolymer composition it is also possible to predict the lEP (267). 

The free radical polymerizations of 3-[(2-Acrylamido-2-mefliyl propyl)dimethylammonio]-l-propanesulfonate with acrylamide and/or acrylic acid, and 3-(i Tj -diallyl-i /-methylammonio)-l-propanesulfonate with A, iV diailyl-i, iV-dimethyl ammonium chloride or AT,A/-diallyl-iV,-methyl amine have been studied. Reactivity ratios indicate random incorporation of comonomers. Molecular weights range from 3.0 x 10 to 15.1 x 10 g mol for the acrylamido-based copolymers and from 2.2 x 10 to 6.0 x 10 g mol for the cyclocopolymers. Second virial coefficients and viscosities decrease as sulfobetaine content increa s for each of the copolymers. A transition from polyelectrolyte to polyampholyte behavior is observed with added NaCl for those copolymers with sulfobetaine monomer incorporation greater than 40 mol%... 

The modification of water soluble polymers for tailored solution behavior has been of interest to many research groups seeking different end uses. Enhanced oil recovery, fracturing fluids, flocculation, associative thickeners, personal care products and many other applications exist for polymers of unique solution behavior. Although the ultimate applications may differ, a common goal of researchers in the water soluble polymer field is a solution viscosity enhanced through polymer-polymer intermolecular associations, often with the stipulation that associations must remain intact in the presence of mono- and divalent salts. To this end our research group has extensively studied the properties of several classes of water soluble polymers. While the majority of our studies have involved ampholytic systems ampholytic ionomers [1], polyampholytes [2,3], polysulfobetaines [4,5] and liquid crystalline ionomers [6], we recently have been... 

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