Quantitative monitoring of cross-linking reactions using simultaneous optical fibre reflectometry and Fourier transform infrared spectroscopy

The optical fibre-based Fresnel reflection sensor is a low-cost device that is manufactured by cleaving an optical fibre to produce a perpendicular end-face. When this sensor (cleaved end-face) is dipped into a liquid, qualitative information on the changes in the refractive index can be obtained. In order to demonstrate the suitability of this sensor for practical and real-life applications, it was used to study the cross-linking of a high-performance thermosetting resin. The data from the sensor were modelled and correlated to that obtained simultaneously, and in real-time, from a conventional Fourier transform infrared spectrometer and an Abbe refractometer. The sensor was calibrated using reference refractive index oils where an excellent correlation was observed between the datasets obtained using an Abbe refractometer and the Fresnel reflection sensor. A series of simultaneous hyphenated experiments were conducted involving the sensor and spectrometer where isothermal cross-linking kinetic data were obtained between 20 – 50 °C. An excellent correlation was observed for the rate and extent of cross-linking between the outputs from the Fresnel reflection sensor and Fourier transform infrared spectrometer. The kinetic rate constants obtained from the Fresnel reflection sensor and the conventional infrared spectrometer were established to be comparable statistically. Moreover, the activation energies for the FTIRS and FRS measurements were 75.29 kJ mol-1 and 74.12 kJ mol-1 respectively. This study has demonstrated that the low-cost intensity-based Fresnel reflection sensor can be used to obtain cross-linking kinetic data that are comparable to that obtained from a Fourier transform infrared spectrometer.