Polyelectrolytes partially-ionized

At neutral pH, both a weak polyacid and a weak polybase are partially ionized, resulting in the formation of the polyelectrolyte complexes. 

A large number of macromolecules complementary to PMAA, namely polyvinylpyrrolidone, polyvinylpyridine, polyacrylamide, poly(vinyl alcohol), poly(ethylene oxide), oligoethylenimine, poly(sodium styrene sulfonate), polycations of the integral type ionen (2X) were used as P2 and P3. The pH of the media strongly influences the studied reactions of complex formation. For example, in PVPy + PVP + PMAA or OEI + PEO + PMAA systems in the add region, where weak polybases are completely protonized and PMAA does practically not dissodate, complexes with hydrogen bonds (PMAA-PVP or PMAA-PEO) are formed. In neutral medium weak polybases are partially ionizated and polyelectrolyte complexes (PMAA-PVPy, PMAA-OEI) are generated. In the alkaline medium formation of complexes has not been observed. 

The membranes used to measure the osmotic pressure are impermeable to polyions but permeable to counter-ions. In spite of this, the osmotic pressure of polyelectrolytes in pure water and for rather small concentrations (but in the semi-dilute regime), is huge. Let us assume that the polyelectrolyte has been put in cell I (see Fig. 5.1). It partially ionizes but both cells must remain practically neutral. The counter-ions which, theoretically, can cross the membrane, are retained in cell I and a contact-potential difference appears at the boundary between the cells. Actually, it looks as if the counter-ions contributed like polyions to the osmotic pressure. Let C be the polyion concentration. We may write approximately... 

A species (e.g., a colloidal particle or a flexible chain) that is partially ionizable when placed in a solution. The polyelectrolyte dissociates into a macroion and counterions. 

Experimental study of infinitely-dilute polymers Is difficult and few direct measurements have been reported for configurational properties of isolated polymers in solution. Molecular simulation offers an alternate powerful method for studying the properties of model polymeric systems, including infinitely-dilute polymers in solution. Computer simulations have been performed in several cases for examining the conformational behavior of isolated, uncharged polymers [28-30] and, on a more limited basis, for studying isolated, fu/Zy-zon/zec/polyelectrolytes [31, 32]. Hooper et al [14, 15] recently performed Monte-Carlo computer simulations for a lattice model of an isolated, partially-ionized polyelectrolyte. Here, we present some of the primary results from references 14 and 15, and discuss how these results can improve our understanding of phase behavior in aqueous/polymer systems. 

Figure 7. Lattice representation of an isolated, partially-ionized polyelectrolyte used in Monte-Carlo simulations of references 14 and 15. The potential Uy represents screened Coulombic repulsions between ail pairs of fixed charges on the chain e characterizes short-range interactions between non-bonded nearest-neighbor segments.

We first consider results for the hydrophilic polyelectrolyte limit. Figure 8 presents the effect of charge screening (denoted here by electrolyte concentration) on the reduced, end-to-end distance of partially-ionized 100-segment polyelectrolytes. The polymer end-to-end distance provides a measure of excluded volume, or space occupied by the polymer. The maximum of the vertical axis represents the fully-extended, or rodlike conformation of the polyion this conformation is approached for... 

Figure 8. Effect of ionic-strength on reduced end-to-end distance for 100-segment partially-ionized polyelectrolytes. Note the qualitative similarity of the single-chain behavior to the gel swelling equilibria shown in Figure 3. (Reproduced with permission from [14]).

Hydrophobic moieties were introduced along the macromolecular chains of a hydrophilic polymer, i.e. poly(acrylic acid) (PAA), through the partial grafting ( t = 10%) of L-phenylalanine (Phe) and some of its oligopeptides (Phefj) (l n 4). The overall conformational state of these new water-soluble polyelectrolytes was evaluated from potentiometric and viscometric data. Hydrophobic domains, mainly due to Phe aromatic moieties, lead, at low ionization, to the stabilization of compact states of the polymeric chains. 

From our own data, together with other data found in the literature, it is obvious that non electrostatic cohesive interactions are predominant at low degree of ionization and are responsible for the maintenance of very compact structures. In such ionization ranges, the coulombic repulsive interactions are therefore more or less completely screened, depending on the strength of cohesive forces as clearly indicated by the respective behaviour of PM A, partially hydrolyzed N,N-disubstituted polyacrylamide (COPy), PLL and related polyelectrolytes. 

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