TUAT succeeds in using optical microscopy to measure polymer concentration distribution in an aqueous solution during evaporation

Graduate Student Masahiko Tanaka at the Graduate School of Bio-Applications and Systems Engineering, and Professor Susumu Inasawa of the Division of Applied Chemistry of Tokyo University of Agriculture and Technology, announced that they have successfully determined the distribution of polymer concentration in an aqueous solution during evaporation using differential interference contrast microscopy. This technique is expected to be used as a new measurement method for concentration distribution in aqueous solutions during evaporation. The results were published in the international journal Physical Chemistry Chemical Physics on November 19.

The differential interference method of optical microscopy is a technique used for the stereoscopic observation of nearly colorless, transparent samples, which are difficult to observe in bright-field observation, by contrasting brightness and darkness based on the difference in the way light travels through the sample. The technique is used for the surface observation of materials such as glass, resins, and wafers. The technique of creating thin films by coating and drying a solution of dissolved polymers is widely used in manufacturing. However, there is no simple method to measure how the polymers are concentrated, and their concentration distribution changes during evaporation, leading to difficulty in controlling membrane structures. Furthermore, evaporation concentrates the polymers, significantly reducing the evaporation rate; however, the mechanism underlying this phenomenon is unknown.

In this study, the differential interference method of optical microscopy was used to record the evaporation of an aqueous solution of a water-soluble polymer as a series of photographs. Further, the change in the evaporation rate with respect to time from the start of evaporation was measured. The researchers used polyvinyl alcohol, polyvinylpyrrolidone, or polyethylene glycol polymers, which are all highly versatile. The effect of molecular size was also examined by changing the molecular weight (size of the polymer) of each type of polymer by three to four different molecular weights.

As a result, it was found that the distribution of luminance in the micrograph could be converted into a distribution of polymer concentration using two calibration curves (the calibration curve of luminance and optical path length gradient in the aqueous solution and the calibration curve of refractive index of the aqueous solution and polymer concentration) plotted in advance. Moreover, it was found that the diffusion coefficient of a polymer in the aqueous solution could not be determined by the molecular weight of the polymer alone. Furthermore, when polymers accumulated at the boundary between the solution and air, the evaporation rate decreased to about one-tenth of the initial value. The extent of this decrease was also found to be inversely correlated with the diffusion coefficient of the polymer in the aqueous solution.

Inasawa said, "The fact that we were able to demonstrate that differential interference contrast can be used for a different purpose (determination of concentration distribution) is groundbreaking. It is very difficult to measure the concentration of a solute in a solution during evaporation. Not much progress has been made in measuring the mobile properties, such as diffusion coefficient, of solutes, especially when the solute concentration increases. We will continue to examine whether this method is applicable at high concentrations."

Journal Information
Publication: Physical Chemistry Chemical Physics
Title: How does the polymer type affect the rate of water evaporation from polymer solutions?
DOI: 10.1039/D4CP03457K