THE LIMITATIONS OF USING FLORY-HUGGINS EQUATION FOR THE STATES OF SOLUTIONS DURING ASYMMETRIC HOLLOW-FIBER FORMATION

The Flory-Huggins equation of solutions, derived in 1942, has been ext ensively used to study the phase-inversion process during the formatio n of asymmetric flat membranes. In this report, we point out the limit ations of using Flory-Huggins theory to describe the Gibbs free energy for the states of solutions during hollow-fiber formation. It is conc luded that the Flory-Huggins equation of solution may not be fully cap able of describing the Gibbs free energy for the state of as-spun holl ow-fiber solutions (nascent fibers) spun non-isothermally with tension . At least two additional terms have to be included in this equation i f the fiber is spun isothermally; one is a work done by the external s tresses on the as-spun nascent fiber and the other is an extra entropy change, Delta S-extra/RT, induced by these stresses. Experimental res ults have confirmed our analysis. SEM observation of as-spun 6FDA-poly imide fibers indicates that external stresses (work) probably have thr ee effects on the states of solution: (1) creating extra-phase instabi lity; (2) facilitating phase separation; and (3) inducing orientation. The first two will either shorten the time for a solution moving from the binodal to the spinodal boundary or reduce the distance of precip itation path between binodal and spinodal boundaries, while the last o ne results in an oriented fiber structure. Experimental results also s uggest that the effectiveness of external stresses (work) on the phase stability of a spinning solution depends on the chemistry of coagulan ts.