Velocity of dissolution of polymers. Part II

Abstract

AbstractPolystyrenes of molecular weight 429 to 973,000 were prepared by fractionation of a pure thermal polymerizate. The velocity of dissolution ⋅ (in cm./sec.) of these fractions in a solvent follows the relation ⋅ = kM−A over a wide range of molecular weight. Only the parameter k is temperature‐dependent and changes with the speed of stirring in the bath. A is constant. ⋅ can be very accurately measured in the molecular weight range from 1000 to 20,000. This offers a new means for determining the molecular weight in a range which otherwise presents difficulties. The swollen surface layer δ increases with the square root of the molecular weight. It increases abnormally above M ≈ 150,000 due to the strong entanglement of large macromolecules. The velocity of dissolution is thereby considerably diminished. The mean mutual diffusion coefficient D is independent of the chain length of the polymer. Its value can be determined from the slope of the ⋅ vs. δ−1 curve and agrees well with those values calculated from the time lag and dissolution formulae data. The assumption that macromolecular coils are the units building up the swollen surface layer can be proven. The average number of coils in a line of the length δ seems not to depend upon molecular weight and is approximately 104 coils. A distinction between two fundamental definitions of the quality of a solvent is necessary. The thermodynamical definition depends upon the value of the second virial coefficient B in the expression for the osmotic pressure. The kinetic definition is simply based on the value of ⋅. High values of these qualities indicate good solvents. However both properties are independent of each other. Whether or not a solvent is suitable from the thermodynamic standpoint depends upon the chemical constitution of the solvent and its interaction with the polymer. Suitability from the kinetic standpoint is related to a small molecular size and viscosity which creates no difficulties for the penetration into the channel system of the polymer.