Contraction of polyelectrolytes

Another effect of the influence of the electric field on the properties of charged networks was described in a recent publication by Osada. He discovered the phenomenon of contraction of polyelectrolyte networks under the influence of direct current in a good solvent [55, 76, 77]. 

These results have a natural explanation if we assume that contraction of polyelectrolyte networks in the course of the passage of the electric current is due to the electroosmotic transport of water. The phenomenon of electroosmosis can be described in the following way Suppose that the electric current passes through a finely porous medium with electrolytic solution inside the pores. In this case, the moving ions of electrolyte carry with them all molecules of water from the pores through which the current passes. 

Kishi, R., Hasebe, M., Kara, M., Osada, Y., Mechanism and Process of Chemomechanical Contraction of polyelectrolyte Gels Under Electric Field , Polymers for Advanced Technologies, vol. 1, ppl9-25,1990. 

Random coil conformations can range from the spherical contracted state to the fully extended cylindrical or rod-like form. The conformation adopted depends on the charge on the polyion and the effect of the counterions. When the charge is low the conformation is that of a contracted random coil. As the charge increases the chains extend under the influence of mutually repulsive forces to a rod-like form (Jacobsen, 1962). Thus, as a weak polyelectrolyte acid is neutralized, its conformation changes from that of a compact random coil to an extended chain. For example poly(acrylic acid), degree of polymerization 1000, adopts a spherical form with a radius of 20 nm at low pH. As neutralization proceeds the polyion first extends spherically and then becomes rod-like with a maximum extension of 250 nm (Oosawa, 1971). These pH-dependent conformational changes are important to the chemistry of polyelectrolyte cements. 

Systems that develop contractile forces are very intriguing as analogues of physiological muscles. The idea for gel muscles was based upon the work of Katchalsky and Kuhn. They have prepared polyelectrolyte films or fibers which become elongated or contracted in response to a change in pH of the surrounding solution, and have estimated the induced force and response time. The contraction of gel fibers is also achieved by electric fields. Use of electric fields has the merit that the signals are easily controlled. 

As mentioned in Section 4.7c, the tertiary electroviscous effect is at least partly due to the expansion and contraction of particles arising from the conformational changes of the polyelectrolytes (adsorbed or chemically bound to the surface of the particles) with changes in... 

For /3 = 0, the contraction of the macromolecule to a state close to the state of the electroneutral molecule takes place at higher salt concentration and the transition is less sharp. It is interesting to note that in some cases the transition of a macromolecule to the globular state is a first-order phase transition for /3 0 and a continuous transition for /3 = 0. Thus in the case of a single molecule immersed in salt solution the coil-globule transition is sharper than in the case of a polyelectrolyte gel, whereas in the case of the salt-free solution the collapse transition of a single macromolecule is vice versa expected to be less sharp than the transition of the polyelectrolyte gel. 

Aseyev VO, Tenhu H, Klenin SI. Contraction of a polyelectrolyte upon dilution. Light scattering studies on a polycation in saltless water-acetone mixtures. Macromolecules 1999 32 1838—1846. 

Ouibrahim and Fruman (47) in 1980 found dilational flow in three distinct flow situations, which each involve an extensional component capillary tube flow, orifice flow, and pitot tube flow. They examined extensively hydrolyzed polyacrylamide (HPAA) and found that the dilatant effect was greatly reduced in the presence of excess salt. This finding was attributed to the effect of the salt ions in screening the charges on the polyelectrolytic HPAA and thus causing the contraction of the highly expanded molecule. 

Ellipsometry at noble metal electrode/solution interfaces has been used to test theoretically predicted microscopic parameters of the interface [937]. Investigated systems include numerous oxide layer systems [934-943], metal deposition processes [934], adsorption processes [934, 944] and polymer films on electrodes [945-947]. Submonolayer sensitivity has been claimed. Expansion and contraction of polyaniline films was monitored with ellipsometry by Kim et al. [948]. Film thickness as a function of the state of oxidation of redox active polyelectrolyte layers has been measured with ellipsometry [949]. The deposition and electroreduction of Mn02 films has been studied [950] below a thickness of 150 nm, the anodically formed film behaved like an isotropic single layer with optical constants independent of thickness. Beyond this limit, anisotropic film properties had to be assumed. Reduction was accompanied by an increase in thickness, which started at the ox-ide/solution interface. 

The solution properties of polyelectrolytes in general are markedly different from those of polyelectrolyte solutions with added salts. These differences are very strikingly revealed in their viscometric behaviors. Viscosity, as pointed out in the previous section, is related to the size of polymer molecules and therefore is affected by molecular expansion. When a small amount of a simple salt, such as sodium chloride, is added to a dilute polyelectrolyte solution, the ionic strength of the solution outside of the polymer coil is increased relative to the strength of the solution inside of the coil. Consequently, some of the mobile electrolyte diffuses into the polyion coil and the thickness of the ionic atmosphere around the polymer chain is reduced. This effect produces a significant contraction of the polyion coil and is reflected in decreased values of the viscosity. 

When a water-swollen polyelectrolyte gel is interposed between a pair of plate electrodes and a DC current is applied, it undergoes electrically induced chemo-mechanical contraction and concomitant water exudation in the air [13]. The electrically induced contraction of the gel is associated with electrokinetic transportation of hydrated ions and water in the network, and a one-dimensional electroldnetic model for the contractile phenomenon was postulated. 

When an ionizable polymer is chemically cross-linked to form a three-dimensional network, an increase in the ionization of the network brings about extensive swelling of the gel, which can be observed visually on a macroscopic level. The expansion of the conformation is due to an increase in the electrostatic potential appearing on the macromolecules. On the basis of swelling and contraction of a weak polyelectrolyte gel, Katchalsky, Kuhn, and co-workers [25-27J proposed a muscle model referred to as mechanochemical or later as a che-momechanical system [1,28]. 

If a water-swollen cross-linked polyelectrolyte gel is inserted between a pair of planar electrodes and a voltage difference is applied, the material can undergo anisotropic contractions and concomitant fluid exudations [197,198], Electrically induced contractions of the gel are caused by transport of hydrated ions and water in the network (electrokinetic phenomena). In fact, when an outer electric field is applied across a gel, both macro- and micro-ions are subjected to electrical forces in opposite directions. However, macro-ions are typically in a stationary phase, being chemically fixed to the polymer network, while counter ions are mobile and are capable of migrating along the electric field, dragging water molecules with them. 

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