Light-scattering studies on the polymer and polyelectrolyte from 2-vinyl pyridine

Several batches of 2-vinyl pyridine are polymerised using azo-bis-isobutyronitrile as initiatoro The polymer obtained is fractionated hy a precipitation method using benzene as solvent and n-hexane as precipitantViscosity and light-scattering measurements are made on ten fractionso The solvents are ethanol and a 6-solvent the composition of which is determined as n-propanol (volume fraction 0.365) and n-heptane (volume fraction 0.635) at 25.0 by precipitation measurementso Light"3C8.ttering measurements indicate that the mixed solvent used is slightly better than idealo Molecular weights and dimensions together with second virial coefficients and limiting viscosity numbers are reported for the polymer in the two solventso The molecular dimensions corresponding to those in a true 0-solvent are obtained by extrapolation of the measured datao The equations relating the solution properties of this polymer to the molecular weight are shown below Viscosity - Molecular eight s- Ethanol = 2,8 x 10-4 Mixed Solvent 2 x 10 j, 0,50 J Molecular Dimensions Molecular Weight Ethanol = 0.063 1.08 Mixed Solvent 0.49 M True G-solvent 1.1 N Z W Second Virial Coefficient : Ethanol B = 2.5 X 10 in the general equation H. KM 1-1- is calculated hy different methods from measurements in ethanolo The values obtained are not self-consistent" The term co30 in the equation = n 1 l--co l+cos l-co32r is calculated for three different solvents taking into account excluded volume. The values of cos (increase as the solvent becomes better indicating that there is less hindrance to rotation in the polymer chain at more extended configurations. Tests of current equations relating the Plory expansion factor to molecular weight are mads. The agreement, in accordance with that obtained by other workers is pooro J Part 2o Two fractions of poly-2-vinyl pyridine are converted to polyelectrolyte by a quaternisation reaction with n-propyl bromide. While the reaction is incomplete ca, 70 the products exhibit the solution properties of highly charged synthetic polyelectrolytes, Light-scattering experiments, using liquids of high dielectric constant as solvent eog, dimethyl sulphoxide and water, reveal unusual light-scattering behaviour including inconsistent molecular weights and variation in the angular dependence of scattered light with concentration leading to incorrect values of molecular dimensions, These effects are interpreted as being due to long range electrostatic forces existing in the solution even at very low concentration which alter both the intensity of light scattered and its angular distribution, Experiments made using aqueous solutions of potassium bromide as solvent corroborate the findings in the pure liquids. At high ionic strength the interactions are suppressed but at low ionic strength of solution interpretation of data is difficult due to the distortion of lightscattering by these interactions. By using very low concentrations of poly electrolyte (0.5 - 2.0 x 10 gm/ml.) reliable dimensions for the molecules are obtained over a range in ionic strength from 0.4 - 0.01 The effect of large intermolecular forces in solution on its light-scattering is verified by measurements using high concentrations of uncharged, high molecular weight polymer. The shape of the reciprocal intensity scattering envelope Is discussedo The variation of molecular dimensions of the high molecular weight sample of polyelectrolyte is examined quantitatively using the theory of Rice and Harris. Good agreement Is obtained between the calculated values and those obtained from light-scattering over a range in ionic strength from 0.01 to 0.4 It is concluded that, for reliance to be placed on measurements of molecular weights and dimensions of charged polymers obtained by light-scattering, the concentration of simple electrolyte must be great enough to eliminate large molecular interactions. This sets a lower limit to the range of ionic strength over which polyelectrolyte dimensions may be studied by light-scattering techniques