CC BY
10LMI-I-25
Laser technologies in the diagnostics of heterogeneous substances with supercritical fluidic components
D. Zimnyakov1, S. Yuvchenko1
1Yury Gagarin State Technical University of Saratov, Physics, Saratov, Russian Federation
Laser diagnostic technologies are an effective tool for characterizing the structure and dynamics of complex heterogeneous systems on a microscopic scale. Among such heterogeneous systems, two-phase and multiphase systems with supercritical fluid (SCF) components are of particular interest from the viewpoint of the synthesis of novel functional materials for biomedical, photonic and electronic applications. We present the theoretical and experimental results of applying various laser-based diagnostic technologies, such as the diffusing-wave spectroscopy, swept-source low-coherence reflectometry, and laser polarimetry, to analysis of the structural and dynamic properties of SCF-synthesized and modified highly porous polymer matrices. The fundamental features of the laser light transfer in such random multiple scattering systems are discussed; in particular, the remarkable correlations in the mutual positions of neighboring pores cause a significant decrease in the anisotropy of laser light scattering in the matrices. This effect appears when the volume fraction of the polymer in the matrix is in the range from 0.3 to 0.1. Further reduction of the polymer volume fraction in the SCF-synthesized porous matrices leads to the "optical inversion" of these multiple scattering systems (dramatic changes in the physical mechanism of laser light scattering related to transition from scattering by an unordered ensemble of voids in a polymer matrix to scattering by an ensemble of polymer films and their intersections in a gas matrix). The transport mean free path of propagation of the probe laser light in the synthesized matrices is strictly determined by the average size of pores and the volume fraction of the condensed phase (polymer). This makes it possible to evaluate these structural parameters with acceptable accuracy using analysis of the signal decay rate in the low-coherence reflectometric system with a frequency-modulated laser (the swept-source reflectometer). The study of dynamic multiple scattering of laser light in synthesized matrices allows the characterization of the kinetics of pore formation and development in the expanding polymer foams. The potential of laser diagnostic methods in characterizing unstable and quasi-stable heterogeneous two-phase systems is illustrated by experimental data on foaming of bioresorbable polymers (polylactides) using supercritical carbon dioxide.
In addition, supercritical fluids can be considered as efficient immersion agents easily controlled by variations in the temperature and pressure. This opens up possibilities for the practical use of the refractive index tuning (RIT) method in laser diagnostics of micro- and nanostructured composite materials.
This work is supported by the RFBR grant # 18-29-06024.
