Evaporation in the Water, Polyethylene Oxide, and Polypropylene Oxide System

Evaporation paths were calculated for aqueous solutions of polyethylene oxide (PEO) and polypropylene oxide (PPO) from the phase diagram. With the exception of solutions with a predominant fraction of one of the polymers the evaporation gave rise to phase separation for intermediate water contents. With similar restrictions for the fractions of the polymers the separated phase contained greater amounts of water and of PPO. In the two-phase region two consolute points were found, one at lower water content and greater PPO content; the other one with the opposite extremes. The difference in the composition of these two points was significantly greater for the polymers, approximately 0.4, than for the water content, less than 0.1. As a consequence there was a transfer of each polymer and of water between the phases during the evaporation. The evaporation of water took place preferentially from the phase with less water and more PEO, and in addition water as well as polymers was transferred from this phase to the one with less water and greater amount of PPO. The versatility of the phase diagram approach was demonstrated by displaying the variation in phase ratios and phase compositions as function of water weight fraction, water weight, and weight of the water evaporated. Another example of the same feature was a simple equation to obtain the amount of water to be added to a mutual solution of the two polymers in order to obtain the two phases in equal amounts.