Dynamic Scattering of Laser Radiation by Polylactide Foams during Synthesis by Supercritical Fluid Foaming: Analysis of Mobility of Interfaces

Multiple scattering of laser radiation by the structure of forming porous matrices gives rise to spatiotemporal intensity fluctuations in the field of scattered radiation (dynamic speckle modulation). Dynamic scattering of laser radiation in the volume of evolving foams was used to study the process of formation of highly porous polylactide matrices during their synthesis by the method of supercritical fluid foaming in a carbon dioxide medium. The average lifetime τ_lt of dynamic speckles, which were recorded by a high-speed CMOS camera, was taken as a parameter carrying information on the ensemble-averaged mobility of scattering centers (boundaries of developing pores). A phenomenological model was developed, which established the relationship between the current values of τ_lt, macroscopic parameters of the foam (volume and the rate of its change), and the ensemble-averaged microscopic mobility of interfaces (the first derivative of the average pore size with respect to time). The developed model was used to interpret experimental data on the dynamics of polylactide foam expansion during fast (0.03 MPa/s) and slow (0.006 MPa/s) depressurization of supercritical CO₂ in the reactor. It was determined that in the case of fast depressurization, structural rearrangements in the foam volume continue even after the foam reaches the maximum value of the expansion factor. This feature was qualitatively interpreted based on the concepts of thermodynamic nonequilibrium of porous matrices during their formation.