Poly electrolytic expansion

The behaviour in solution of the first type of polyions compared with that of uncharged molecules is more involved in that the coil expansion is affected not only by the solvent but also by the electric field formed by the polyion itself, by the counterions and by the ions of other low-molecular-weight electrolytes, if present in the solution. Infinite charge dilution cannot be achieved by diluting the poly electrolyte solution, as a local high-intensity... 

A combination of the molecular polyelectrolyte theory with the methods of statistical mechanics can be used at least for the description of the chain expansion due to charges along the polysaccharide chain. The physical process of the proton dissociation of a (weak) polyacid is a good way to assess the conformational role of the poly electrolytic interactions, since it is possible of tuning poly electrolyte charge density on an otherwise constant chemical structure. An amylose chain, selectively oxidized on carbon 6 to produce a carboxylic (uronic) group, has proved to be a good example to test theoretical results. ... 

In this section, the behaviour of the electrolytic expansion in conducting polymers, especially polyaniline and poly(o-methoxyaniline) (PMAN) are described, with discussion of the basic redox reaction of polyaniline, the dependence of the expansion ratios on oxidation levels, the kind of anions, strain, the pH of the electrolyte and anisotropy. 

The dependence of the electrolytic expansion rates in poly(o-methoxyaniline) film on the type of anions is shown in Figures 8.7a and 8.7b for pH = 0 and pH = 2, respectively. It should be noted that at pH = 0, the expansion rate scarcely depends on the kind of anion, whereas at pH = 2 the remarkable dependence is observed. The result indicates that at pH = 0 or pH < 1.5 in poly(o-methoxyaniline) the electrolytic expansion and contraction are certainly driven by the change of polymer conformation and/or the electrostatic repulsion. 

Figure 8.7 The dependency of electrolytic expansions in poly(o-methoxyaniline) films on the kind of anions at (a) pH 0, and (b) pH2. TSA is toluene sulfonic acid.

The electrolytic expansion for the thickness direction in polyaniline cast film [20] shows an extremely large expansion ratio of more than 25% as shown in Figure 8.10, and is comparable to that of natural muscles [6]. A similar result was also obtained in the cast film of poly(o-methoxyaniline) for the thickness direction. The large expansion ratio for the thickness direction is conjectured to relate to the condensation process of the cast film. It may be remarked that the evaporation of NMP solution results in shrinkage only in the thickness direction, but not in the area. Therefore, the cast film has more freedom to expand in the thickness direction than that parallel to the film surface. 

Apart from polyaniline, other condncting polymers that are being studied for electrolytic expansion include polypyrrole [11, 15-17], poly(alkylthiophene) [26] and carbon nanotubes [5]. For example, electrochemically prepared polypyrrole films were used to study the qualitative movement of electrolytic expansion by fabricating a bimorph actuator. The movement of bending and stretching of the actuator was demonstrated in electrolyte solution [15]. Actuators fabricated by electrodeposition on gold-coated polyethylene films were studied [11] for the evaluation of expansion ratio and response time. Also, a microactuator of several tens of microns made from two layers of gold and... 

Most macromolecules when dissolved in salt solutions acquire charges that are shielded by an atmosphere of counterions. This ion atmosphere affects the diffusion coefficient of the macromolecule and hence the light-scattering time-correlation function. Electrolyte solutions are discussed in Chapters 9 and 13. Recent measurements of diffusion coefficients have been made by several groups. Lee and Schurr (1974) have studied poly-L-lysine-HBr. Schleich and Yeh (1973) have performed similar studies on poly-L-proline. Raj and Flygare (1974) have studied bovine serum albumin (BSA) and find that at high ionic strength and low pH the diffusion constant decreases. This they attribute to the expansion of the molecule. 

A composite actuator was constructed using a copolymer of PPy and poly(methoxyaniline) formed in p-phenol sulfonic acid (PPS). Higher actuation deformations were seen at low pH. Pure PPy(PPS) initially showed mixed cation and anion actuation when tested in 1 M NaCl, but the cathodic expansion was removed by the inclusion of 30% poly(methoxyani-line) and the extent of electrochemical creep was also diminished [144]. The inclusion of poly(2-methoxyaniline-5-sulfonate) (PMAS) into PPy(DBS) films improved the cathodic expansion of the films. The presence of the PMAS also led to some anodic film expansion in some electrolytes (e.g. KCl), but exclusively enhanced cation expansion in other electrolytes (TBAPF4) [145]. 

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. 

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