Rheo-IR: A combined setup for correlating chemical changes via FTIR spectroscopy and rheological properties in a strain-controlled rheometer

Fourier-transform infrared (FTIR) spectroscopy is a common tool for determining, both qualitatively and quantitatively, the chemical composition of a material in the solid, liquid, or gas phase. It is additionally used as a technique to monitor the rate of chemical changes. These chemical changes can have rheological relevance, e.g., polymerization kinetics, rubber crosslinking, or epoxy curing, just to mention a few. We report the design of an IR transparent upper-plate rheological geometry used as an attenuated total reflectance (ATR) sampling tool. Additionally, we describe the technical and methodological adaption of the ATR crystal into an ARES G2 rheometer for combined FTIR and rheological measurements. In this new setup, a strain-controlled rheometer is combined with an ATR crystal and the IR beam is guided through two off-axis parabolic mirrors to the quasi-static upper plate of the rheometer to gain maximum IR sensitivity. Thereby online and directly correlated real-time FTIR spectra can be acquired while simultaneously conducting rheological measurements. This allows for in situ correlation of macroscopic rheological properties with microscopic and molecular chemical changes. These experiments are conducted for a material under controlled conditions having exactly the same sample time evolution for both measurements. To demonstrate its potential, this newly developed method was applied to correlate the polymer network formation for a free-radical copolymerization of acrylic acid and methylenebis(acrylamide) as a crosslinking agent via IR spectroscopy and the respective mechanical time evolution, in a dilute water-based solution.