Conformation of polyelectrolyte

Kitano, T., Taguchi, A., Noda, I. Nagasawa, M. (1980). Conformation of polyelectrolytes in aqueous solution. Macromolecules, 13, 57-63. 

In the limit of low salt concentrations, the conformation of polyelectrolytes is expected to be rod-like so that v = 1. In this limit, Eq. (2) is recovered. However, as the concentration of the added salt increases, the effective exponent v decreases toward 3/5. Therefore we expect from Eq. (6) that p will increase with M at higher salt concentrations, in contradiction with the experimentally observed result of Eq. (2). There are several erroneous claims in the literature... 

Theories of conformations of polyelectrolytes fall into two groups. In the first group [32-34] the chain is assumed to be a flexible chain and the consequence of electrostatic interaction is calculated. In the second category [35-42], the chain is assumed to be a stiff chain and calculations are performed to obtain the effect of the electrostatic interaction between charges on the chain backbone. To date, there is no satisfactory theory in the literature to describe the electrostatic effect on conformations of polyelectrolyte chains with arbitrary intrinsic stiffness. In the following we briefly outline the developments for both groups of theories. 

According to these data, one may draw a conclusion that at low concentrations PE molecules adsorb in flat conformation and at high concentrations more extended layer with loops and tails is formed. These data about conformation of polyelectrolyte molecules are in a good agreement with other experimental and theoretical works [21-23], Note that the curves are reversible and f potential values establish immediately at each pressure value after pressure rising and decreasing. This is the argument that the deformation of adsorbed layers but not desorption of macromolecules takes place on experimental time scale, since our measurements are carried out in polyelectrolyte-free solution. 

Fig. 10. Schematic representation of conformation of polyelectrolyte (a) with a low charge, and (b) high charge density with the charge spacing similar to that present on a surface. Reptinted with permission from Ref. [118]. 2002, American Chemical Society.

Mechanical properties of the chemically modified wood depend on the nature of the introduced side chain. If the introduced side chain is ionized, the interaction between wood component molecules is influenced by not only the free volume but also by electrostatic repulsion of ionized side chains. For example, the conformation of polyelectrolyte depends on the degree of ionization [23-28]. The structure is transformed from random coil to rod-like conformation with an increase in the degree of ionization. A screening effect also influences the interaction because electrostatic action is inhibited by electrolytes such as NaCl. 

Random Coil Conformations of Polyelectrolyte Chains in Aqueous Solution ... 

Figure 6. Schematic description of the effects of ionic strength (1) and pH on the conformations of polyelectrolytes such as humic molecules in solution and at a solid-water interface. rh denotes the hydrodynamic radius of the polyelectrolyte in solution and (bh denotes the hydrodynamic thickness of the adsorbed polyelectrolyte. (Adapted from reference 15. Copyright 1989 American Chemical...

Let us consider the conformation of polyelectrolyte macromolecules immersed in an infinite quantity of solvent. The counterions having high translational entropy are distributed over the whole volume of the solution their concentration in the vicinity of the macromolecules is extremely low, and their influence on the molecular conformation can be omitted completely. The conformation of the polyelectrolyte macromolecule is determined by rather strong unscreened repulsive interactions between charged groups attached to the chain. Due to this repulsion, the macromolecule assumes a strongly stretched conformation in the sense that its end-to-end distance R is a linear function of the degree of its polymerization m [14-18]. 

In view of the increasing interest concerning the existence of non electrostatic interactions and their influence on the conformation of polyelectrolyte chains, we have extensively investigated in our laboratory some synthetic polyelectrolytes showing a pH-induced conformational transition. 

Markarian MZ, Hariri HH, Reisch A et al (2012) A small-angle neutron scattering study of the equilibrium conformation of polyelectrolytes in stoichiometric saloplastic polyelectrolyte complexes. Macromolecules 45 1016-1024... 

The details of the transition and conformations of polyelectrolyte chains above the transition were described by Dobrynin et al Performing Monte Carlo (MC) simulations of the freely jointed uniformly charged polyelectrolyte chains with fractional charge on each monomer /, they showed that the critical number of charged monomers fN)cm on the chains at which charged globules becomes unstable is proportional to ViV (see Figure 5). 

Table 1 Typical conformations of polyelectrolyte chain with degree of polymerization 1V=304 and fraction of charged monomers f=1/3.

The effect of added salt on the conformation of polyelectrolyte chains in dilute and semidilute solutions was investigated by Stevens and Plimpton [151], At high salt concentrations the electrostatic interactions between charged monomers are screened and the chain conformations are similar to those observed for good solvent, R As the salt concentration... 

Figure 2J2 Schematic representation of the impact that the presence of salt ions have on the conformation of polyelectrolytes.

Due to the charged nature of the polymer chain, the conformation of polyelectrolytes is quite different from that of neutral polymers - the electrostatic repulsion between monomer units leads to a highly extended rod-like conformation. 

Viscometry is the most widely applied experimental method to determine the conformation of polyelectrolytes. The experiments are easily performed except for the measurements of the shear rate dependence, which is usually neglected (see, however. Sect. 2.3.3 for the importance at low ionic strength conditions). 

Cheng N, Azzaroni O, Moya S, Huck WTS (2006) The effect of [CuI]/[CuII] ratio on the kinetics and conformation of polyelectrolyte brushes by ahun transfer radical polymerization. Macromol Rapid Commun 27(19) 1632-1636... 

Angelatos, A. S. Wang, Y Caruso, F. Probing the conformation of polyelectrolytes in mesoporous silica spheres. Langmuir 2008,24,4224-4230. 

In general, the conformation of polyelectrolyte is determined by the total chain persistence length (Ip) which represents the effective rigidity of the polyelectrolyte chain. Ip is the sum of Iq and the electrostatic persistence length (le) [52] ... 

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