Use of ultrasonic time-domain reflectometry for real-time measurement of thickness changes during evaporative casting of polymeric films

The dry-cast membrane-formation process is a principal technique by which asymmetric porous membranes as well as dense films can be obtained. Much insight into the fundamental nature of this process can be obtained via real-time measurements of mass and thickness changes. The former requires an equation of state for the density to obtain the average instantaneous film thickness. Optical techniques for the latter such as interferometry and reflectometry usually require transparent samples and good reflecting surfaces; in many cases, such characteristics cannot be obtained. In this article, we report the adaptation of ultrasonic time-domain reflectometry (TDR) for real-time, noninvasive measurement of thickness changes of polymeric films. Ultrasonic TDR does not depend on optical characteristics and is based upon the reflection of longitudinal plane waves at the interface between two media with different acoustic impedances. The technique utilizes the acoustic velocity, which, in general, is a function of temperature and composition. Representative ultrasonic-TDR thickness data are presented for the cellulose acetate (CA)-acetone, CA-acetone-water, and polysulfone (PSf)-tetrahydrofuran systems. Values are compared to those obtained independently using a direct micrometer measurement as well as mass-loss data. In general, the results are in excellent agreement with a maximum error of less than 5%. In addition to overall changes in thickness, preliminary data indicate that ultrasonic TDR can be used to locate the interface between the polymer solution and an opaque solidified region. The latter is a characteristic of polymer systems that undergo a phase separation during solidification. Ultrasonic TDR thus appears to be a promising technique that can be applied to a wide range of polymer systems.