Abstract: The influences of the balloon effect, i.e.the elastic deformation of the semipermeable membrane and the adhesion of liquids on the capillary, wall, on the measurement of solvent permeation rate and osmotic pressure of polymer solution were investigated by dynamic method theoretically and experimentally.The balloon effect leads to the departure from linearity of the plot of permeation rate of solvent against the hydrbstatic pressure difference.This effect could be described quantitatively by a deformation factor which is closely related with swelling ratio of the membrane in the solvent.The adhesion of liquids on the capillary wall influences the reproducibility of measurements and is the main reason for the occurrence of the unsymmetry of ascending and descending lines.

Key words: Dynamic membrane osmometry, Balloon effect, Number-average molecular weight