Infrared (IR) spectroscopy is one of the most popular analytical techniques used to characterize sol-gel silica materials in their different stages. The method represents, in particular, a simple and versatile tool to investigate the microstructural evolution in gels and films, as a function of temperature and synthesis parameters. Several studies have shown that sol-gel IR absorption spectra exhibit, with respect to silica melt glass spectra, some specific features closely related to the peculiarities of sol-gel processing. Furthermore, because of the differences between silica bulk gels and films the spectra- microstructure correlation must be especially evaluated for thin films. IR spectroscopy has been used to evaluate residual porosity, Si-O-Si bonding rearrangements during drying and firing stages and to model the microstructure evolution during film processing. Some questions are still, however, arising around the interpretations of the IR spectra, in particular about the presence of cyclic species in the microstructure and disorder-induced vibrational modes. An attempt is made here to present an overview of the different relationships existing between IR spectra and microstructure of sol-gel silica films as they actually appear from current literature.

Infrared spectroscopy of sol–gel derived silica-based films: a spectra-microstructure overview

Photopolymerization kinetics of multifunctional monomers

Chemical and biochemical transformations in ionic liquids

https://www.lehigh.edu/operando/Publications/1999%20Titania-silica%20molec%20struc%20physico-chemical.pdf

Titania-silica as catalysts : molecular structural characteristics and physico-chemical properties

Analysis of organic polymorphs. A review

This paper describes a comparative study of in situ cure monitoring by three methods: (i) evanescent wave spectroscopy; (ii) refractive index change; and (iii) near-infrared spectroscopy. The cure characteristics of an epoxy/amine reaction were followed in real-time during the crosslinking reaction via the above-mentioned techniques. The evanescent wave spectroscopy technique was based on monitoring the characteristic infrared absorption bands of the resin system to compute the concentration of the amine hardener as a function of cure time. Good correlation was obtained between the evanescent wave spectroscopy data and a conventional method of studying cure reactions, i.e. infrared spectroscopy. During the cure reaction, the refractive index of the resin system increases as a function of the crosslink density. This increase in the refractive index was monitored using two optical fibre techniques. In the first case, a declad region of the optical fibre was immersed in the resin system and in the second method an optical fibre reflectometer was used to track the changes in the refractive index. Once again, good correlation was obtained between the optical fibre techniques and infrared spectroscopy cure data. The results obtained from the optical fibre sensor experiments were used to model the cure kinetics of the resin system. The cure kinetic models were found to predict the cure reaction up to approximately 60% of the reaction.

https://iopscience.iop.org/article/10.1088/0964-1726/5/4/005/pdf

In situ cure monitoring of epoxy resins using optical fibre sensors

Structural studies of gel phases—IV. An infrared reflectance and Fourier transform Raman study of silica and silica/titania gel glasses

This paper reports a comparative study undertaken for different types of optical fibre sensor developed to monitor the cure of an epoxy resin system. The optical fibre sensors used to monitor the cure process were based on transmission spectroscopy, evanescent wave spectroscopy and refractive index monitoring. The transmission sensor was prepared by aligning two optical fibres within a specially prepared sleeve with a gap between the optical fibre end faces. During cure, resin from the specimen flowed into the gap between the optical fibres allowing transmission spectra of the resin to be obtained. The evanescent wave sensor was prepared by stripping the cladding from a high refractive index core optical fibre. The prepared sensor was embedded in the sample and attenuated total reflectance spectra recorded from the resin/core boundary. Refractive index monitoring was undertaken using a high refractive index core optical fibre which had a small portion of its cladding removed. The prepared sensor was embedded in the resin specimen and light from a single wavelength source was launched into the fibre. Changes in the guiding characteristics of the sensor due to refractive index changes at the resin/core boundary were used to monitor the progress of the cure reaction. The transmission and evanescent wave spectroscopy sensors were used to follow changes in characteristic near-infrared absorption bands of the resin during the cure reaction over the range 1450-1700 nm for the evanescent sensor and 2100-2250 nm for the transmission sensor. Consequently, these techniques required tuneable wavelength sources covering specific wavelength ranges. However, the refractive index based sensor used a single wavelength source. Therefore the equipment costs for this type of sensor were considerably less. Additionally, the refractive index sensor did not require a single wavelength source at any particular wavelength and could be applied to any spectral region in which the optical fibre would transmit light. The advantages and disadvantages of these three methods are discussed.

https://iopscience.iop.org/article/10.1088/0964-1726/7/4/017/pdf

David N. Waters

Papers

In situ cure monitoring of epoxy resins using optical fibre sensors

Structural studies of gel phases—IV. An infrared reflectance and Fourier transform Raman study of silica and silica/titania gel glasses

In-situ cure monitoring using optical fibre sensors - a comparative study

Equations for isothermal differential scanning calorimetric curves

Raman spectroscopy of powders : effects of light absorption and scattering