Measuring microgel swell ratio by cryo-SEM

The swell ratio is a key parameter characterizing the structure and properties of a hydrogel. In macroscopic gels, the swell ratio can be determined by straightforward measurements of the gel weight in the dry and hydrated states. However, measuring the swell ratio characteristic of microgels (gel particles with dimensions of 0.1–100 μm) is substantially more challenging because of their small size and polydispersity. We use cryo-scanning electron microscopy (cryo-SEM) to measure microgel diameter both in the frozen-hydrated and fully dry states using pH-responsive poly(ethylene glycol)-co-acrylic acid microgels. The volume swell ratios characteristic of the various microgels are essentially the same as those measured from otherwise-identical macroscopic gels. Hence we can conclude that, at least in this case where the macroscopic gel and microgel synthesis methods are similar, the simple measurement of a macroscopic swell ratio provides a reasonable approximation to the microgel swell ratio. [Display omitted] •Cryo-SEM is used to make direct measurements of microgel volume swell ratio.•The macroscopic gel swell ratio is a good approximation for the microgel swell ratio.•Microgel-bound ice sublimates much more slowly than the ice matrix (free water).