Calculation of the phase diagram of a btda-tdi/mdi co-polyimide/ N-methyl-2-pyrrolidinone/water system in the phase separation process

Abstract

Theoretical ternary phase diagrams with good agreement with experiment cloud point data were constructed for a btda-tdi/mdi (p84) co-polyimide/N-methyl-2-pyrrolidinone (NMP)/water system. The theoretical phase diagram was calculated based on the Flory–Huggins theory of polymer solution. To construct the theoretical phase diagrams, all the binary interaction parameters were determined accurately. The water–P84 co-polyimide interaction parameter (χ₁₃) and the nmp-p84 co-polyimide interaction parameter (χ₂₃) were calculated from solubility parameters theoretically and determined by equilibrium swelling experiment and intrinsic viscosity measurement, respectively. The water–NMP interaction parameter (g₁₂) was calculated by the universe sichemical functional group activity coefficient method. According to the theoretical binodals, spinodals and critical points of the p84 co-polyimide/nmp/water system at different temperatures, we conclude that the effect of temperature on the position of binodal curves was small; this phenomenon is similar to the results of the cloud point titration experiment at different temperatures. The study also found that the p84 co-polyimide fractions of both theoretical critical points and experimental critical points are lower (less than 4 wt%), that is, the spinning or coating solution will be demixed by nucleation and the growth mechanism at the early stage of the forming process.