Revealing molecular diffusion dynamics in polymer microspheres by optical resonances

Understanding the diffusion of small molecules in polymer microsystems is of great interest in diverse funda-mental and industrial research. Despite the rapidly advancing optical imaging and spectroscopic techniques, entities under investigation are usually limited to flat films or bulky samples. We demonstrate a route to in situ detection of diffusion dynamics in polymer micro-objects by means of optical whispering-gallery mode reso-nances. Through mode tracking, interactions between solvent molecules and polymer microspheres, including sorption, diffusion, and swelling can be quantitatively analyzed. A turning point of mode response is observed, while the diffusion exceeds the sub-wavelength-thick outermost layer as the radial extent of resonances and starts penetrating the inner core. The estimated solubility in the glassy polymer is consistent with the predicted value using Flory-Huggins theory. Besides, the non-Fickian contribution is analyzed in such a glassy polymer -penetrant system. Our work represents a high-precision and label-free approach to describing characteristics in diffusion dynamics.

Automated summary

In this study, the diffusion dynamics in polymer micro-objects are detected in situ by utilizing optical whispering-gallery mode resonances. The interactions between solvent molecules and polymer microspheres, such as sorption, diffusion, and swelling, are quantitatively analyzed through mode tracking. A turning point of mode response is observed when diffusion exceeds the sub-wavelength-thick outermost layer and starts penetrating the inner core. The estimated solubility in the glassy polymer agrees with the predicted value using Flory-Huggins theory. Besides, non-Fickian contribution is analyzed in the glassy polymer-penetrant system. This work represents a high-precision and label-free approach to describing characteristics in diffusion dynamics.