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

Abstract

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.

Hydrogen Bonds under Stress: Strain-Induced Structural Changes in Polyurethane Revealed by Rheological Two-Dimensional Infrared Spectroscopy

“…The most straightforward way to investigate the molecular origin of polymer elastic response is to directly observe the changes in molecular structure induced by externally applied strain. Such experiments have used a range of different structural probing methods, notably X-ray diffraction − but also Raman, infrared (IR), and visible spectroscopy. − …”

“…Among the above-mentioned techniques, IR spectroscopy is very suitable to investigate such rearrangements, since the frequencies and line shapes of the vibrational transitions contain detailed information on the hydrogen-bond structure . Combining rheology with infrared spectroscopy (rheo-IR) to study the molecular changes in polymers under applied strain (Figure A) has provided a detailed molecular picture of the strain-induced molecular rearrangements in a broad range of polymers. − However, infrared absorption spectra are often rather congested, and it can be difficult to disentangle overlapping vibrational bands. Although the absorption frequency is often a good indicator of the local structure and/or environment, it is generally difficult to derive unambiguous conclusions from the frequencies alone, since they are determined by more than one effect (e.g., the conformation and solvent interactions).…”

A Chemometric Approach to Assess the Rheological Properties of Durum Wheat Dough by Indirect FTIR Measurements

“…The connection between rheological properties and IR measurements can be an effective tool in the material characterization field, as demonstrated in the recent literature (Chen & Zhen, 2021;Öztürk, 2021;Radebe et al, 2021). This work correlates the possible gluten protein structures on a molecular level to the macroscopic rheological properties of the dough.…”

Rheo-2DIR spectroscopy reveals strain-induced hydrogen-bond redistribution in polyurethane

“…1A) has provided a detailed molecular picture of the strain-induced molecular rearrangements in a broad range of polymers. [4][5][6][7][8][9] However, infrared absorption spectra are often rather congested, and it can be difficult to disentangle overlapping vibrational bands; and although the absorption frequency is often a good indicator of the local structure and/or environment, it is generally difficult to derive unambiguous conclusions from the frequencies alone, since they are determined by more than one effect (e.g. the conformation and solvent interactions).…”

“…Such experiments have used a range different structural probing methods, notably X-ray diffraction, [1][2][3] but also Raman and infrared (IR) spectroscopy. [4][5][6][7][8][9][10] In the case of hydrogen-bonded polymer networks, the strain-induced structural changes generally involve rearrangement of the hydrogen bonds between molecular groups of adjacent (bio)polymer chains. Among the above-mentioned techniques, IR spectroscopy is very suitable to investigate such rearrangements, since the frequencies and line shapes of the vibrational transitions contain detailed information on the hydrogen-bond structure.…”