Michel Tricot

Electro-optical studies on synthetic polyelectrolytes—II: Electric birefringence of poly-N-butyl-4-vinylpyridinium salts in aqueous solution

Abstract The field strength dependence of the electric birefringence of poly- N -butyl-4-vinylpyridinium bromide in aqueous solution changes markedly with polymer concentration. Analysis of the orientation curves reveals that the mean electric polarizability −Δα sharply increases when the concentration decreases, as a result of the extension of the chain, while the negative optical anisotropy factor remains unchanged. An empirical relationship, similar to that found for the reduced viscosity, is derived for the variation of electric polarizability with concentration. Increasing the ionic strength produces a coiling of the polyelectrolytic chain and a decrease of − Δx . The replacement of the bromide ions by sulphate ions causes a conformational change evidenced by a decrease of −Δα and of the relaxation time. This effect is interpreted by the formation of electrostatic bonding between the pyridinium rings on non-adjacent segments of the chain, and to a limited extend, by intermolecular bonding. The dependence of the relaxation time, extrapolated to infinite dilution, on the molecular weight of the samples is analysed on the basis of a worm-like chain model. The persistent length obtained is found of the same order of magnitude as in the case of other semi-rigid polymers. The predominant influence of the segmental orientation on the optical, electric or conformational parameters is discussed in detail.

Electro-optical studies on synthetic polyelectrolytes. IV. Electric polarizability and conformation of poly-N-methyl-2-vinylpyridinium bromide in aqueous solution.

Measurements of the relaxation time on aqueous solutions of the title polyelectrolyte as a function of the concentration and of the molecular weight show that its conformation at very high dilution can be better accounted for by a weakly bending rod or wormlike chain model, with persistence length ranging from 200 to 400 A. The analysis of the field strength dependence of the electric birefringence yields electric polarizability values which increase sharply with the dilution and are not significantly dependent upon the molecular weight. This has been tentatively interpreted on the basis of the theories derived by Oosawa and by Mandel and Van der Touw. The partially flexible model proposed by the latter authors allow to identify the electric polarizability calculated from electro-optical data to the specific dielectric increment measured in the high frequency range; both parameters appear to be molecular weight independent. The electric polarizability obtained from the Kerr effect would be originated in the induced dipoles caused by the delocalization of the bound counterions along rigid subunits whose length seems however to differ from the persistence length.

Electro-optical studies on synthetic polyelectrolytes: I. Ultraviolet electric dichroism of poly-N-alkylvinylpyridinium bromides in aqueous solution

The electric dichroism of aqueous solutions of poly-2- and 4-vinylpyridinium bromides has been measured in the 220 to 300 nm region. From the knowledge of the assignments of the absorption bands, the orientation of the pyridinium ring with respect to the macromolecular axis was determined for the poly-N-butyl-4-vinylpyridium bromide. An estimation of the birefringence contribution in the visible region originating from these ultraviolet dichroic bands revealed that the negative birefringence observed mainly arose from electronic transitions at shorter wavelengths.

Electro-optical studies on synthetic polyelectrolytes—III: Electric birefringence of poly-n-alkyl-4-vinylpyridinium bromides in various solvents

Abstract The field strength dependences of the electric birefrigence of poly- N -butyl-4-vinylpyridinium (4-PVP · BuBr) samples have been measured in water and in various organic polar solvents. The observed change of sign of the birefringence, negative in water and positive in nitromethane, is explained by considering the respective influences of the intrinsic and form optical anisotropies. The lower degree of extension of the polyelectrolyte in some organic solvents is attributed to a lower degree of dissociation of the ionic sites. The influence of the dielectric constant and of the functional character of the solvent on the dissociation and solvation processes of the ionic sites is taken into account. Relaxation measurements in dimethylsulphoxide as a function of the molecular weight allow quantitative expression of the degrees of extension and of rigidity of the polyelectrolyte; the presence of aggregates has also been detected in DMSO solutions. A long alkyl chain, such as octyl, attached to the quaternary pyridinium groups strongly modifies the conformation of the polyelectrolyte and a polyelectrolyte-polysoap transition has been observed as a function of the solvent composition in water-ethanol mixtures.

Electro-optical and viscometric behaviour of sodium polystyrene sulphonate in formamide

Unusual electro-optical signals have been recorded on sodium polystyrene sulphonate in formamide. It was shown that such signals largely arise from the positive birefringence of the solvent being superimposed on the negative birefringence of the polyion. On the basis of conductimetric experiments, the residual content of ammonium formate caused by hydrolysis of formamide was estimated. Commercial formamide of analytical grade was shown to contain rather large amounts (0.02 M) of ammonium formate, sufficient to very strongly decrease the polyelectrolyte effect. The persistence length in these conditions is equal to 80A, quite similar to that determined in 2 × 10−2 M NcCl aqueous solution. At a lower ammonium formate content (7 × 10−4 M), the degree of extension of the polyion at infinite dilution, calculated from the relaxation time of the birefringence, is identical to that measured in 6 × 10−4 M NaCl. The influence of the nature of the solvent and the ionic strength on the electric polarizability and the optical anisotropy factor was also analysed and compared with the results obtained in water. The latter parameter was discussed in terms of the so-called form anisotropy of the solvent and the intrinsic optical anisotropy of the polyion.