Development of novel thermochromic plastic films for optical temperature sensing

Abstract

The preparation of plastic film optical ‘CO₂-based’ temperature-sensing films that utilise the temperature-dependent acid–base equilibria of indicator dyes is described. In film formulations a suitable dye, such as phenolphthalein, in a hydrophobic base, tetraoctylammonium hydroxide, solubilised within a plasticised, hydrophobic polymer matrix, creates a system which is sensitive to ambient CO₂ levels. The resultant solution, when cast on to glass supports, yields ‘CO₂-based’ temperature-sensitive films which change colour in response to a highly temperature-dependent reaction between the deprotonated form of the dye and CO₂ dissolved in the film. The absorbance characteristics of these films display a fully reversible response to temperature over a temperature range which is largely determined by the pK_a of the dye and the ambient CO₂ concentration. The magnitude of the response is dependent on the dye concentration. The response time towards changes in temperature is typically ⩽2.2 min and the films show good stability under operational conditions. A simple mechanism of the reaction is suggested and an associated working equation has been derived and fitted to data obtained for a typical sensor functioning over the range 278–333 K. A ‘CO₂-based’ temperature-sensing film is used successfully alongside a standard CO₂ sensing film. This combination not only provides temperature information but also ensures that the response of the CO₂ sensor is corrected for any changes in temperature. In addition, both sensors use the same interrogating light and light intensity monitoring system because they contain the same phenolphthalein dye. The latter two features represent an improvement on the existing optical systems used to measure CO₂ and temperature.