A Thermodynamic Study of the Super Molecular Order in Aqueous Solutions of Polyvinyl Alcohol

Abstract

The present investigation was carried out to study the formation of the super molecular structure in aqueous poly (vinyl alcohol) solutions by viscometric and light scattering methods. The PVA polymer was subjected to these studies at various temperatures and in different solvents. The solvents used were water, 5% Propyl alcohol, 10% propyl alcohol, 20% propyl alcohol, 10% Dioxane and 20% Dioxane, and the temperatures range was 10-80OC. The main aim of this project was to find the super-molecular formation and the determination of thermodynamic parameters, second virial co-efficient, molecular weight and the unperturbed dimensions of the PVA mpolymer. The viscosity studies were divided into two groups i.e., normal viscosity studies and the shear viscosity studies. The normal viscosity studies were undertaken by using Ostwald viscometer, while the shear viscosity was measured by using roto viscometer. It was found that the viscosity of polymer decreases with temperatures, while it increased with concentrations. The intrinsic viscosity was determined for all the solvents at all temperatures. From this, the Huggins constants, radius of gyration, root mean square end distance and thermodynamic parameters like enthalpy y, free energy and entropy of activation of viscous flow were calculated. The values of intrinsic viscosity are in good agreement with literature values. From these viscosity studies it was found that the 20% propyl alcohol behaves as a good solvent, while water is a poor solvent for PVA. Further orientation and coiling of polymer molecules were observed to be more in water and in aqueous dioxane as compared to aqueous propyl alcohol solvents. On the basis of temperature dependence, it can be concluded that PVA system obeys lower critical solution temperature (LCST) and the polymer molecules were found to be less mobile in low temperature conditions. The values of heat and entropy of activation of viscous flow increase with concentrations but no generalization can be made due to its irregular pattern at various temperatures. The values of heat and entropy of activation show that solutions of the polymer in 20% propyl alcohol have high order, where as in water have low, but in no case, there is complete order. The shear viscosity studies reveal that flow of PVA molecules is Newtonian shear rates, while at low shear rates, they behave as non-Newtonian. The shear viscosity increases initially for high molecular weight samples at lower temperature, due to orientation effects, while this behavior is opposite in low molecular weight samples. Also, entanglement of polymer molecules was found o be higher in high molecular weight samples as compared to the low molecular weight samples. More flexibility end lower retardation of rotation of polymer molecules was found in low concentrated solutions. The zero-shear viscosity was found to decrease with rise in temperature for a given concentrations end this behavior was same for both the molecular weight samples of PVA studies. The effect of concentration on zero shear viscosity was also observed. The zero-shear viscosity increased with increasing concentrations, however, the increase in molecular weights was more pronounce than low molecular weight samples. The light scattering studies were also undertaken for the PVA-water system for un-known molecular weight sample. From these studies, specific refractive index increment, molecular weight second virial coefficient, etc. were determined. It was found that the optical constant and specific refractive index increment are high at 436nm as compared to the values at 546nm. It means that the wavelength of incident light also plays a role in these studies. The molecular weight was also determined for these wave length of incident light and was found to be higher at higher wave length.