Temperature‐dependence of light scattering and intrinsic viscosity of hydrogen bonding polymers

Abstract

AbstractA group of chemically related polymers—polymethacrylic acid polymethacrylamide, polyacrylic acid, and polyacrylamide—having a large number of groups capable of forming hydrogen bonds along the chain, was investigated by studying the temperature dependence of the 90° scattering of light and of the viscosity of dilute aqueous solutions, mineral acid being added in the case of the polyacids in order to suppress ionization. The intercepts of the light scattering plot indicated no dependence of molecular weight on temperature, but the slope B of this plot and the intrinsic viscosity [η] were found to change markedly with temperature. The slope parameter B and the intrinsic viscosity decreased with increase in temperature in the case of polymethacrylic acid, but increased with increase in temperature in the other three cases. The exceptional behavior of polymethacrylic acid is correlated with its known tendency to form gels on heating in concentrated aqueous solution. A discussion of the light scattering plot slope parameter B and its temperature‐dependence is given, based on fluctuation theory and the experimental evidence. It is emphasized that B is a measure of the interactions between the kinetic units as they exist in the solution and that its temperature‐dependence gives information about the entropy and energy changes accompanying changes in concentration. These quantities are calculated and briefly discussed. The correlation between B and the intrinsic viscosity [η] is examined in the light of the Flory‐Fox‐Krigbaum treatment. Relationships involving, respectively, the heat and entropy of dilution are derived and used for comparison. Our results are found to conform well with these theoretical predictions. Calculations of the length of the statistical chain element show that these values in the case of polymethacrylic acid and amide are about half the value for polyacrylic acid under corresponding conditions. This is at variance with the fact that the presence of the methyl group should have a marked stiffening effect on the chain. The discrepancy is attributed to intramolecular hydrogen bonding, the polymethacrylic chain being assumed to be the more strongly bonded. The possibility of intramolecular bonding makes the agreement with Flory's treatment of coil expansion particularly interesting. A possible explanation is suggested.