Simultaneous multi-measurand analyses of cross-linking reactions within a differential scanning calorimeter using optical fibre sensors

The cross-correlation of cross-linking kinetic data obtained from different analytical equipment is not straightforward because the experimental conditions are seldom identical (temperature gradient, mass of the sample and substrate containing the analyte). Furthermore, the measurement principles can also be different (thermal, optical, rheological, etc). This paper reports on the design and implementation of a multi-measurand fibre-optic probe within a differential scanning calorimeter. The probe consisted of two components: (i) a low-cost and disposable cleaved-end of an optical fibre that served as a Fresnel refection sensor; and (ii) an array of non-contact optical fibres that enabled FTIR spectra to be acquired from within the calorimeter. The Fresnel reflection sensor was used as an immersion device to infer the evolution of the refractive index during the cross-linking of an epoxy/amine thermosetting resin. The array of optical fibres enabled FTIR spectra to be acquired during the cross-linking of the thermosetting resin. The motivation for developing this capability was to demonstrate cross-correlation between the quantitative FTIR spectral data, the qualitative Fresnel reflection sensor and the output from the differential scanning calorimeter. The cross-linking kinetics were determined for the two optical techniques and an excellent correlation was found between them and the data obtained from the calorimeter. The feasibility of using the same Fresnel reflection sensor, after cross-lining of the resin, for detecting the glass transition temperature was demonstrated.