37—Sorption of Methacrylic Acid by Nylon I. Equilibrium Sorption by Drawn Nylon 6.6 Fibres from Aqueous Solutions

Abstract

Sorption isotherms for the absorption of methacrylic acid from aqueous solutions by drawn nylon 6.6 fibres have been determined from 0° to 60°C. For low concentrations, the simple distribution law is followed, but, at higher concentrations, the isotherms pass through a maximum. Mainly, it is the undissociated acid which is sorbed, the anion being sorbed only weakly; hydrogen bonding of the undissociated acid to the main-chain amide groups is the most important sorption mechanism. Undrawn nylon shows the same behaviour as the drawn fibres at low concentrations of acid, but absorbs more methacrylic acid at higher concentrations. On repeated sorption and desorption , the isotherms for the fibres remain unchanged and so does their lateral swelling, but complicated changes in length take place owing to an initial relaxation on first sorption followed by a small reversible longitudinal swelling. X-ray diffraction patterns show that for the fibres there is no significant penetration of the crystallites. The thermodynamics of the sorption process are discussed and values of the distribution coefficient, the exothermic heat of sorption (730 cal. mole-1) and the small negative entropy of sorption ( - 1.67 cal. deg.-1) are given. Solution sorption theory is applied, and the Flory-Huggins theory of polymer solutions is found to describe the results. The effect of the swelling of the fibres on the sorption is treated, and it is found that the negative value of the entropy is not due to the swelling.