Thermal Lens Measurements of Thermal Expansivity in Thermosensitive Polymer Solutions

The weak absorption of a laser beam generates in a fluid an inhomogeneous refractive index profile acting as a negative lens. This self-effect on beam propagation, known as Thermal Lensing (TL), is extensively exploited in sensitive spectroscopic techniques, and in several all-optical methods for the assessment of thermo-optical properties of simple and complex fluids. Using the Lorentz-Lorenz equation, we show that the TL signal is directly proportional to the sample thermal expansivity alpha, a feature allowing minute density changes to be detected with high sensitivity in a tiny sample volume, using a simple optical scheme. We took advantage of this key result to investigate the compaction of PniPAM microgels occurring around their volume phase transition temperature, and the temperature-driven formation of poloxamer micelles. For both these different kinds of structural transitions, we observed a significant peak in the solute contribution to alpha, indicating a decrease in the overall solution density-rather counterintuitive evidence that can nevertheless be attributed to the dehydration of the polymer chains. Finally, we compare the novel method we propose with other techniques currently used to obtain specific volume changes.

Automated summary

The weak absorption of a laser beam in a fluid generates an inhomogeneous refractive index profile, acting as a negative lens. This effect, known as Thermal Lensing (TL), is widely used in sensitive spectroscopic techniques and all-optical methods for assessing thermal properties of fluids. Using the Lorentz-Lorenz equation, we demonstrate that the TL signal is directly proportional to the sample's thermal expansivity alpha, enabling high sensitivity detection of density changes in a small volume using a simple optical setup. We applied this method to investigate the compaction of PniPAM microgels during their volume phase transition temperature and the temperature-driven formation of poloxamer micelles, observing a significant peak in the solute contribution to alpha for both structural transitions.