Optimization of Hydroxypropyl Cellulose (HPC) Microgel Imaging and Characterization Using Scanning Electron Microscopy

Microgels are polymer-based spherical nanoparticles suspended in water that exhibit a volume phase transition at a particular temperature. The standard, noninvasive method for characterizing microgels is with dynamic light scattering (DLS), which measures the collective diffusion of microgels exhibiting Brownian motion in a sample volume. While DLS provides reliable estimates for particle structure and dynamics, more direct methods of imaging are useful for studying polydisperse samples. Traditionally, scanning electron microscopy (SEM) uses an electron beam under high vacuum to characterize individual particles of dried samples. The dry particle imaging suffers from two main drawbacks in the case of microgels: the dehydrated particles shrink from their expected size by a factor of three, and dynamics are not observable due to particle immobilization. Ionic liquid was used to wet image microgels under high vacuum. Particles were suspended in a thin film of ionic liquid on a copper grid, and still images and movies were recorded to analyze the sample for both size distribution and dynamics. The average SEM sizes of the two samples tested generally agreed with the sizes obtained by DLS for the same particles both in ionic liquid and water at room temperature. Variation was observed in individual particle sizes, but the average SEM sizes for both samples were close to their DLS sizes. Initial attempts at diffusion analysis using SEM particle tracking yielded mixed results as it requires the tracing of many particles and thus further optimization.