Use of infrared thermography for temperature measurement during evaporative casting of thin polymeric films

In this paper we describe the development of a real-time, noninvasive technique using infrared (IR) thermography for measurement of the temperature of polymer solutions cast as thin films in which evaporation of a volatile solvent occurs. The technique requires the accurate determination of a single surface radiation property, the emittance, and its dependence on the casting solution composition. We report results obtained for the cellulose acetate (CA)-acetone system for which IR measurements indicate a relatively rapid and significant temperature decrease followed by a gradual temperature recovery toward the initial temperature. In general the thin film temperature response can be described by the parameters ΔT_max, the maximum degree of cooling; t_min, the time at which T_min is reached; and the recovery temperature, T_rec, at a time equal to 2 × t_min. For 155 and 258 μm thick films evaporatively cast from a 15 wt% CA solution, IR thermography indicated maximum temperature decreases of 18 and 26°C, respectively. The characteristics of the thin film temperature curves for these two cases compare favorably with those predicted by a first-principles model recently developed by the authors. Based upon these results, the general applicability of the infrared technique for the study of dense film- and membrane-formation processes is described, and the significance of thin film temperature measurements with respect to membrane morphology is discussed.